Solar light harvesting with multinary metal ...

Chemical Society Reviews

Solar light harvesting with multinary metal-chalcogenide nanocrystals in photovoltaic, photoelectrochemical and photocatalytic systems

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Chemical Society Reviews CS-SYN-01-2018-000029.R1 Synopsis n/a Stroyuk, Oleksandr; L.V. Pysarzhevsky Institute of Physical Chemistry of National Academy of Sciences of Ukraine, Laboratory of Organic Photovoltaics and Electrochemistry; Technical University of Dresden, Physical Chemistry Raevskaya, Alexandra; L.V. Pysarzhevskiy Institute of Physical Chemistry of Ukr. Nat. Acad. Sci., Photochemistry Department Gaponik, Nikolai; TU Dresden, Physical Chemistry

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Received 00th January 20xx, Accepted 00th January 20xx DOI: 10.1039/x0xx00000x www.rsc.org/

Solar light harvesting with multinary metal-chalcogenide nanocrystals in photovoltaic, photoelectrochemical and photocatalytic systems Oleksandr Stroyuk*a,b, Alexandra Raevskayaa,b and N. Gaponikb The paper reviews the state-of-the-art in the synthesis of multinary (ternary, quaternary and more complex) metal chalcogenide nanocrystals (NCs) and their applications as a light absorbing or an auxiliary component of light-harvesting systems. This includes solid-state and liquid-junction solar cells and photocatalytic/photoelectrochemical systems designed for the conversion of solar light into electric current or the accumulation of solar energy in the form of the products of various chemical reactions. The review discusses general aspects of the light absorption and photophysical properties of multinary metal chalcogenide NCs, the modern state of the synthetic strategies applied to the multinary metal chalcogenide NCs and related nanoheterostructures, and recent achievements in the metal chalcogenide NC-based solar cells and the photocatalytic/photoelectrochemical systems. The review is concluded by an outlook with a critical discussion of the most promising ways and challenging aspects of further progress in the metal chalcogenide NC-based solar photovoltaics and photochemistry.

Introducton In the broadest terms a solar light-harvesting system can be defined as a combination of light-sensitive moieties (molecules, metal complexes, supramolecular complexes, bulk or nanodimensional inorganic/organic semiconductors, biomolecules and their assemblies) with various electron mediators, co-catalysts, etc. This system serves to absorb the incoming solar light and to convert the electromagnetic energy into its electrical or chemical form available for future utilization or for immediate chemical/electrochemical reactions. The solar light harvesting can be formally divided into two parallel and sometimes intertwined areas. One is the realization of the conception of artificial photosynthesis, that is, the endothermic light-assisted conversion of extensively abundant compounds – water, N2, CO2, etc. into chemical products that can be processed later releasing the accumulated energy or used as valuable chemical raw materials for synthetic purposes. Alternatively, the solar energy can be used directly as a driving force for various, mostly destructive, photocatalytic transformations of chemical species, for example, to decompose various persistent organic and inorganic contaminants and the harmful biota in air and waters. The second area is the solar light energy conversion into electric power that occurs in the so-called solar cells. The main component of a solar cell is a light absorber which is

photo-excited and supplies non-equilibrium electrons and electron vacancies – holes into an electric circuit, thus resulting in photocurrent generation. Numerous studies carried out worldwide in the last 3–4 decades showed that both types of light harvesting systems can be successfully realized by using photo-sensitive semiconductor materials. Indeed, some semiconductors show characteristics ideal for the solar light harvesting. The semiconductors have continuous absorption bands that cover the entire ultra-violet (UV), visible, and sometimes, near infrared (NIR) ranges of the solar spectrum. This absorption results from electron transitions between a continuously filled valence band (VB) and a completely vacant conduction band (CB) under the excitation with light of any energy higher than the difference between CB and VB, i.e. the bandgap Eg. Typically, the semiconductors are chemically robust and reveal incomparably higher photochemical stability than lightharvesting molecular species and metal coordination compounds. Finally, some of the semiconductor materials used to convert solar light are composed of extremely abundant components, such as silicon, iron oxide, or kesterite Cu2ZnSnS4. The semiconductor-based light-harvesting systems can be attributed to three major groups depending on the character of the utilization of the photogenerated charge carriers (Fig. 1). The first group encompasses the photovoltaic systems where the light absorption results in a spatial separation of the CB electrons and VB holes facilitated by the internal electric field of a single p/n heterojunction or a “cascade” of multiple heterojunctions. The photoelectrons are forwarded from the light-harvester into the electric circuit, while the VB holes get

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filled from the circuit reestablishing the original state of the photosensitive semiconductor. The p/n junction can be organized as a 2D contact between two (or more) adjacent thin films or as multiple microscopic (nanoscopic) p/n junctions in a mixture of a donor and an acceptor in the bulk heterojunction solar cells. Thus, the light absorption results in the generation of a photovoltage and, consequently, a photocurrent, but no chemical transformation takes place in the photovoltaic system.

Fig 1. Basic types of semiconductor-based solar-light-harvesting systems

The same principle of charge separation is realized in the photoelectrochemical systems but here the charge carriers participate in the electrode reactions. The system can be designed in such a way that a product of a cathodic reductive reaction is subsequently oxidized in an anodic reaction on the counter electrode and this process proceeds reversibly with no chemical changes in the electrolyte, thus producing only an electric current. This principle is realized in the photoelectrochemical solar cells with liqiud electrolytes, or the liquid-junction solar cells. A photoelectrochemical system can be used to yield chemical products with spatially separated anodic and cathodic semireactions, for example, water can be split into hydrogen and oxygen in such a photoelectrocatalytic system regime. In this case, the photoelectrochemical system is aimed at the accumulation of solar energy in the chemical form and an external bias can additionally be applied to the electrodes to facilitate the light-driven chemical reactions. Finally, the photoelectrochemical cycle can be confined to a single semiconductor crystal with both the anodic and cathodic reaction taking place on the same or different crystal faces, or separated spatially on different components of a composite heterostructure. If the rates of the anodic and cathodic reactions are balanced the semiconductor acts as a photocatalyst accelerating the reaction between an electron donor and an electron acceptor at the expense of the solar energy. Depending on the nature of the target process, the photocatalytic reaction can be regarded as a photosynthetic one, if it leads to the accumulation of the light energy in the energy-rich products (reduction of H2O, CO2, N2, decomposition of H2S, etc.), or as a photodestructive one, if

the light energy is used to decompose the donors/acceptors present in the system. From the historical perspective, the semiconductor-based photocatalytic, photovoltaic, and PEC systems stem from the same roots. For example, the photocatalytic water splitting can be realized in a combined photochemical/photoelectro1–3 chemical system in a water splitting solar cell . A breakthrough in the area was inspired by the work of A. Fujishima and K. Honda on the photoelectrochemical splitting 1–3 of water on a biased titanium dioxide electrode . However, titania is only sensitive to the UV and a tiny (around 5%) portion of the visible light. This utter limitation inspired and continues to inspire numerous concepts and methods for the sensitization of TiO2 to the visible light by coupling it with highly-absorbing species or by altering its band structure imparting the TiO2 crystals with sensitivity to visible light. Alternatively, new semiconductor materials with narrower bandgaps suitable for the visible-light-harvesting were constantly explored and tested, especially among the metal chalcogenide (MC) semiconductors. In the recent two to three decades, the photochemistry of semiconductor materials experiences an explosive growth associated with the miniaturization of the semiconductor crystals to nanometer dimensions. A transition from the microto the nano-scale opens tremendous perspectives and potential of tailoring/designing the properties of semiconductor light harvesters via variations in the nanocrystal (NC) size, shape, coupling mode, surface chemistry, doping, etc. NCs with a crystal dimeter d smaller than the double Bohr exciton radius (aB), (also named quantum dots, QDs) reveal pronounced dependencies of the electronic structure and properties on the crystal size. As a result, size dependencies of photophysical properties, such as the position and intensity of absorption and photoluminescence (PL) bands, the energy and rate of the interfacial transfer of charge carriers, etc. are observed. MC NCs with size-dependent properties are currently broadly used in various applications, including photodetectors and photovoltaics, light-emitting diodes, luminescent bio-sensors, photochemistry, etc.4–27. Specifically, large absorption coefficients (104 – 105 cm–1), broad and continuous absorption bands combined with the size-dependence of the absorption threshold and energy of the photogenerated charge carriers make the MC NCs very promising as light harvesters for solar cells. Despite the diversity of available metal-chalcogenide semiconductor materials, the studies of MC NCs were mostly confined to cadmium CdX and lead PbX chalcogenides (X = S, Se, Te) that exhibit photoresponses and PL in the visible (CdX) and NIR (PbX) ranges. The CdX and PbX QDs were successfully employed as highly luminescent bio-markers and components of solar cells, however, the lucrative optical and photovoltaic properties of CdX- and PbX-based materials are counterweighted by the acute toxicity of cadmium and lead. The toxicity problem exists also in the case of the other “rising star” of photovoltaics – hybrid organo-inorganic Pb-based 28–30 perovskites that exhibit an unrivaled efficiency of light

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conversion (higher than 20%) but suffer from “dark” and photochemical instability. In this view, a constant search for new non- or less-toxic MC semiconductor compositions including MCs is constantly continued. One of the most promising directions of the search is associated with ternary indium-based and quaternary Snbased MC NCs. The most typical multinary CuInS2 (CIS) and Cu2ZnSnS4 (CZTS) semiconductors have the bandgaps of around 1.5 eV and, therefore, they are perfectly suited for 31,32 solar light harvesting . Similar to cadmium and lead chalcogenides, the multinary MC exhibit quite strong quantum confinement effects when prepared in the form of NCs as small as several nanometers thus opening possibilities of a further tuning of the spectral response by varying the NC size. When passivated with protective shells (like ZnS) the ternary MC NCs emit quite intense PL with the reported quantum yields (QYs) reaching 60–70%. The multinary NCs are currently intensely studied as light harvesters in liquid-junction/solid-state solar cells showing very promising light conversion efficiencies. The highest efficiency reported for a certified ternary NC-based solar cell is as high as 11.91%16,20 and comparable with the best dye-sensitized Grätzel-type photoelectrochemical solar cells (~12%)4. The multinary MC NCs are studied and reported in an astounding versatility of compositions and morphologies. The basic In(Ga)-based carcass of the ternary MC NCs and the Sn(Ge)-framework of the quaternary MC NCs tolerates extremely broad deviations from the stoichiometry, the isomorphic substitution of the metal and chalcogenide constituents, as well as the abundant doping with “alien” metal ions. In the simplest classification, the multinary NCs can be assigned to three groups: A–EIII–X, A–EIV–X, and A–EV–X, where X is a chalcogene, E is the lattice-forming metal (In, Ga, Sn, Ge) and A is a variable metal or a combination of two metals (Fig. 2). In the first A–EIII–X group the lattice is formed by InIII or GaIII, much rarer – with AlIII and BIII, while the chalcogene X can be varied from S to Se to Te5,8,10,12,14–18,24,33–39. Typically, A is copper(I) or silver(I) or a CuI/AgI combination, however AII can also be introduced into the MC structure resulting in AIn2S4 compounds. The EIII elements can be mixed producing all kinds of alloyed solid-solution compounds with the molar ratio of the second EIII element varying in a continuous manner from 0 to 1. Finally, “alien” ions can be incorporated into the ternary MC NCs lattice, most often, Zn2+, modifying considerably the optical and electrophysical propertiers of the doped MC compounds but not compromising the general tetragonal chalcopyrite structure. The quatenary A–EIV–X NCs12,17,18,24,32,38,40–45 are typically based on EIV = Sn, rarer – on Ge and Sn/Ge mixtures, while Si-based compounds remain quite exotic (Fig. 2). The EIV lattice framework can be “filled” with A = Cu2Zn producing the most popular kesterite Cu2ZnSnS4 phase that also reveals a variety of non-stoichiometric compounds within the same lattice symmetry. Into this group fall also compounds with A = Ag2Zn (quite rare up to date) and Cu2M with M = Ba, Fe, Co, Ni, etc.

also manifesting the typical kesterite motif. A number of IV ternary compounds is based on the Sn carcass with M being 2+ 2+ 3+ bivalent, like Cu , Zn , or even trivalent as In . Such compounds have a different lattice symmetry than the quaternary kesterite-like formulations, but they also reveal the typical and distinguishable features of multinary MC NCs – a broad variability of compositions and the capability to accommodate copious dopants.

Fig. 2. Overview of the multinary MC NCs used for light-harvesting applications.

The same characteristic features allow to enlist into the V multinary MC family also a group of A–E –X compounds, mostly based on Sb and Bi, rarely on As, with complementary Cu(I) and Ag(I) ions in the lattice (Fig. 2). Of interest for the photovoltaic/photochemical applications are the MC with X = 12,34,38 S, Se as the tellurides have too low bandgaps for the efficient light conversion. Because of a vast diversity of possible compositions of the light-harvesting multinary MC NCs, some of them having quite bulky formulations, a broad utilization of abbreviations, such as CIS for copper indium sulfide or CZTS for copper zinc tin sulfide, is observed in the original reports on the multinary compounds. Typically, the abbreviations are composed from the first letters of the atoms forming a MC compound both in their English and Latin transcription (compare CIS and AIS for copper and silver indium sulfides). The usage of abbreviations seems to be helpful since in a major part of the cases nonstoichiometric compounds are examined with the NC composition varied for the determination of an optimal ratio of the constituents for the light harvesting applications. At that, some of the abbreviations became generally accepted, though not being systematic. For example, zinc-doped copper indium sulfide and silver indium sulfide compounds are typically referred to as CIZS and ZAIS (not AIZS), and, therefore, no specific meaning is associated with the order of letters in the abbreviations. Table 1 shows a collection of the abbreviations adopted in the present review and used to refer to metal sulfides. The abbreviations for the corresponding selenides and tellurides are typically derived by substituting S with Se or Te, or by using several chalcogene symbols in the cases of mixed MC compounds (for example, CIS, CISe, CITe, CISSe for copper

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indium sulfide, selenide, telluride and sulfo-selenide, respectively). The present review aims at encompassing the state-of-the-art in the synthesis of multinary (ternary, quaternary and more complex) MC NCs and their applications as a light absorbing or an auxiliary component of the light-harvesting systems, including solid-state and liquid-junction solar cells and photocatalytic/photoelectrochemical systems designed for the conversion of the solar light into electric current or the accumulation of solar energy in the form of the products of various chemical reactions. The review comprises four sections covering (i) the general aspects of the light absorption and photophysical properties of multinary MC NCs (relevant to the light-harvesting applications), (ii) the modern state of the synthetic strategies applied to the multinary MC NCs and related nanoheterostructures, (iii) the state-of-the-art in the MC NC-based solar cells, and (iv) the photocatalytic/photoelectrochemical systems with the multinary MC NCs as light absorbers and photocatalysts. We intentionally combined photovoltaic and photocatalytic (photoelectrochemical) systems based on multinary MC NCs in this review. The recent progress of both fields shows that frequently the photocatalysis “hints” to the photovoltaics which MC materials are the most suitable for light harvesting in terms of the charge carrier energy and charge generation/transport efficiency. At the same time, the photovoltaics often brings the focus on new materials, typically tested as microcrystalline thin-film light absorbers, which later find applications in the photocatalytic/ photoelectrochemical systems in nanocrystalline forms. The review is concluded by an outlook with a critical discussion of the most promising ways and challenging aspects of the further progress in MC-based solar photovoltaics and photochemistry.

1 Light absorption, photophysical and electrophysical properties of multinary metal chalcogenide nanocrystals 1.1 Absorption spectra – influence of composition and size The multinary MC NCs reveal an exceptionally broad variability of the light absorption range and the parameters of absorption spectra. The diversity of spectral properties stems from a 13,14,20 combination of various variable factors and effects , in particular (i) from the possibility of a very broad deviation from the stoichiometry; (ii) the quantum size effects for small NCs (d < 5–10 nm) affecting the band gap and oscillator strength and modifying the optical properties of MC NCs; (iii) the possibilities of introduction of various dopants, the most 2+ popular being probably Zn ; (iv) the possibility of alloying via the partial substitution constituents, such as S with Se, In with Ga, Sn with Ge, etc. One of the unique properties of ternary In (Ga) -based and quaternary Sn (Ge) -based MC NCs is the possibility of a broad variation of the NC composition, sometimes very far from the stoichiometry, with the preservation of the crystallinity and

the crystal lattice type. This feature is especially vivid for ternary CIS and AIS compounds. For example, the Cu-In-S NCs retain a tetragonal chalcopyrite structural motif, similar size and size distribution in a broad range of compositions and Cu:In ratios – from the stoichiometric 1:1 to copper-poor NCs with the ratio of 1:10. Recent findings made by the highresolved scanning transmission electron microscopy (TEM) showed that even for the stoichiometric CuInS2 NCs there exist In-enriched and Cu-enriched areas within each sole 46 nanocrystal . These results are discussed in more details in Section 2. Similar findings on the composition fluctuations were recently reported for CIGSe absorbers basing on the results of atom 47 probe tomography . A combined high-resolution TEM 77 (HRTEM) and nuclear magnetic resonance studies on Se and 119 Sn atoms of alloyed CZTS-CZTSe films showed that the lattice ordering and composition homogeneity are not longrange phenomena irrespectively on the film composition and 48 the temperature of the preliminary thermal treatment . A composition inhomogeneity of microcrystalline annealed CZTS films on the nano-scale was clearly visualized by using the lowtemperature cathodoluminescence in a scanning electron 49 microscopic mode . Therefore, one can speak about the stoichiometry or phase composition of multinary NCs only in the first approximation or as about the collective properties characterizing the NC ensemble as a whole but varying for each microscopic point of each separate NC. The analysis of reports on the light-harvesting systems based on multinary MC NCs shows that in many cases the highest efficiencies of light conversion are observed for the nonstoichiometric NCs and the dependence of the light harvesting efficiency on the NC composition (expressed as a molar fraction of one of the constituents) typically is dome-shaped. In the case of CIS and AIS compounds, the filled copper 3d and Ag 4d orbitals contribute into the top of the valence band and therefore, the variations in the copper/silver content have a drastic effect on the light absorption capabilities of CIS/AIS NCs. For example, the intensity and structure of the absorption band of CIS NCs stabilized in water by thioglycolic acid (TGA) changes considerably with an increase of the copper content (Fig. 3a). For the CIS and core/shell CIS/ZnS NCs synthesized at Cu:In:S = x:5:10 the integral absorbance of colloidal NCs 50 depends almost linearly on x (Fig. 3a, insert) . The direct relationship of the absorption intensity of the copper content is a clear indication of the direct involvement of copper ions into the interband electron transitions. A magneto-optical 51 study of the non-stoichiometric CIS NCs revealed the ground state of the NCs to be diamagnetic consistent with the + presence of Cu ions in the NC lattice. The photoexcitation 2+ results in the appearance of magnetically-active Cu centers clearly indicating the transitions of a copper d-electron into 51 the conduction band . Simultaneously with the growth of absorbance the increase in Cu content results in a red shift of the absorption threshold indicating the bandgap narrowing (Fig. 3a).

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Table 1. Abbreviations typically used to refer to the multinary metal sulfide compounds

Abbreviation AIS AIGS CAIS CBS CBTS CFTS CGS CIS CIGS CGZS CIZS CTS CZGS CZTS CZTGS ZAIS ZIS

Name Silver indium sulfide Silver indium gallium sulfide Copper silver indium sulfide Copper bismuth sulfide Copper barium tin sulfide Copper iron tin sulfide Copper gallium sulfide Copper indium sulfide Copper indium gallium sulfide Copper gallium zinc sulfide Copper indium zinc sulfide Copper tin sulfide Copper zinc germanium sulfide Copper zinc tin sulfide Copper zinc tin germanium sulfide Zinc silver indium sulfide Zinc indium sulfide

Fig. 3. (a) Absorption spectra of colloidal CIS NCs (a) prepared at a Cu:In:S ratio of x:5:10 with x = 0.2 (curve 1), 0.6 (2), 1.0 (3), 1.5 (4), 2.0 (5), and 2.5 (6). Insert in (a): integral absorbance of CIS NCs (curve 1) and CIS/ZnS NCs (curve 2) as a function of x. Adapted with permissions from ref. 50. (b) Absorption spectra of size-selected CZTS NCs (the average size is given in figure). Insert: spectral curves in the Tauc coordinates for the determination of Eg. (c) Diffuse reflectance spectra of CuGaxIn1–xS NCs with a varied x. (d) Absorption spectra of alloyed (AgIn)xZn2(1–x)S2 NCs with a varied x. (e) Bandgap of alloyed Cu2ZnSnS1–xSex as a function of x. Reprinted with permissions from ref. 59 (b), ref. 76 (c), ref. 77 (d), and ref. 78 (e). Copyright (2011, 2017) The Royal Society of Chemistry (a, b); (2016, 2017) Elsevier (c, d); (2013) Nature Publishing Group (e)

Similar spectral changes were interpreted by P.V. Kamat et al.51 in terms of the possibility of two optical transitions, one responsible for the spectral maximum and the second contributing to the absorption “tail” and arising from the transitions with the participation of subbandgap sates. As the Cu:In ratio increases the second transition dominates resulting in a red shift of the absorption band51. Therefore, the narrowing of the optical bandgap can originate not from the

Composition Ag-In-S Ag-In-Ga-S Cu-Ag-In-S Cu-Bi-S Cu-Ba-Sn-S Cu-Fe-Sn-S Cu-Ga-S Cu-In-S Cu-In-Ga-S Cu-Ga-Zn-S Cu-In-Zn-S Cu-Sn-S Cu-Zn-Ge-S Cu-Zn-Sn-S Cu-Zn-Sn-Ge-S Zn-Ag-In-S Zn-In-S

Most typical stoichiometric form AgInS2 Ag(In,Ga)S2 (Cu,Ag)InS2 CuBiS2, Cu3BiS3 Cu2BaSnS4 Cu2FeSnS4 CuGaS2 CuInS2 Cu(In,Ga)S2 – – Cu2SnS3 Cu2ZnGeS4 Cu2ZnSnS4 Cu2Zn(Sn,Ge)S4 – ZnIn2S4

size quantization effect but from the subbandgap state-related electronic transitions. A very similar spectral evolution is observed with an increase of the Ag content in aqueous TGAstabilized AIS and AIS/ZnS NCs52, indicating a general character of the relationship between the absorption intensity and the d10 metal content in ternary NCs. The ligand-free stoichiometric AIS NCs revealed a band edge at energies lower than the bulk bandgap of AgInS253. As the thermal treatment of such NCs resulted not in the expected red shift but, inversely, in a blue shift of the absorption edge, it was concluded that the band edge originates from the electron transitions with the participation of subbandgap states (the socalled Urbach absorption) gradually extincted at the thermal treatment53. A strong contribution of the subbandgap-staterelated absorption was recently reported for the aqueous sizeselected AIS NCs54: the Urbach “tail” masked the band edge and even made impossible the accurate Eg determination directly from the absorption spectra. A similarly strong contribution of the subbandgap states into the absorption band edge prohibiting the accurate Eg determination from the optical data was also observed for the kesterite CZTS and CZTSe films55–57 showing this phenomenon to be of general nature for the multinary MC NCs. At the same time, the CIS (AIS) NCs produced at different Cu:In (Ag:In) ratios reveal unchanged chalcopyrite crystalline structure and roughly the same NC size in a broad x range54,58. As discussed below, an increase in the Cu:In (Ag:In) ratio results in a decrease of absolute values of the CB and VB levels and, therefore, simply by varying the d10 metal content the optical properties and electron structure of CIS/AIS NCs can be changed fundamentally without noticeable changes in the NC morphology. Another way of tuning the absorption range of multinary MC NCs is in “coming down” to a few-nanometer-sized crystals for which the quantum confinement is observed. As the NC size

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becomes comparable to and smaller than the doubled Bohr exciton radius 2aB for a given semiconductor the photogenerated exciton as well as dissociated free charge carriers experience spatial confinement in the NC volume resulting in a series of effects, in particular, in a broadening of the bandgap with a corresponding change of the absolute values of the CB and VB potentials. For example, the bandgap of kesterite CZTS NCs increases from the bulk-like value of 1.5 eV to 1.8 eV as 59 the NC size is reduced from 7 nm to 2 nm (Fig. 3b) . The Bohr 59–62 exciton radius of 2.5–3.4 nm is reported for CZTS NCs and therefore, the 5–7-nm CZTS NCs are not expected to reveal appreciable size-dependence of the bandgap, which is vividly 59 observed for smaller 2.5 nm and 2 nm NCs (Fig. 3b, insert) . This example shows that the doubled Bohr exciton radius can be used as a quantitative measure of a threshold size below which the quantum confinement can be expected. Table 2 illustrates the aB values for selected MC compounds along with * * the effective masses of CB electron (m e) and VB hole (m h). For all listed substances the effective electron mass is comparatively low (below 0.2m0) and therefore it can be expected that the quantum confinement effects will strongly affect the properties of free photogenerated CB electrons, in particular the CB “bottom” energy (ECB) and the dynamics of the interfacial electron transfer to other components of the light-harvesting systems. The values of effective masses can be used to estimate the bandgap of MC NCs of a given size d using 2 the effective mass approximation envisaging Eg scaling as 1/d 75 , or, vice versa, the size of MC NCs can be estimated using a bandgap determined from the absorption spectra. However, in many cases the effective mass approximation results in an over-estimation of the NC size and, therefore, it should be applied with a caution. For example, the model predicts a bandgap of 2.3–2.4 eV for ~3.5-nm CIS NCs, while the experiments showed that such NCs are characterized by Eg 64 = ~2.1 eV . In this view, the effective mass approximation can be modified, in particular, by assuming a finite depth of the potential well thus making the estimations more realistic. The 63,70 Eg(d) calibration curves were reported for CIS NCs , CISe 69,70 65 NCs , and AIS NCs . In a close relation to the exceptional tolerance to the nonstoichiometry, the multinary MC NCs can also tolerate copious doping and substitution of the core elements, for example, In with Ga in CIS/AIS NCs, Zn with various transition metals in CZTS NCs as well as S with Se in all kinds of multinary MC NCs, thus providing an exceptionally broad variability of the optical and electrophysical properties at the more or less unchanged morphology of NCs (size, shape, phase, etc.). In particular, indium ions in ternary CIS/AIS NCs can be partially replaced with gallium(III) resulting in a blue shift of the absorption band edge proportional to the substitution 76 extent (Fig. 3c) . In this way the optical response of CIGS/AIGS NCs can be smoothly varied in a broad range (from ~900 nm to 500 nm) without changes in the lattice structure and NC size/shape. The Ga incorporation increases the CB and VB energies making the substituted NCs more active donors of the photogenerated electrons and holes. As a rule, the increased

driving force of the interfacial charge transfer of CIGS/AIGS NCs is counterbalanced by a narrowed spectral response resulting in dome-shaped dependences of the light-harvesting efficiency of the Ga content. The lattice of CIS/AIS NCs is abundant with In and copper/silver vacancies allowing another “alien” cations to diffuse into the NC forming solid solutions. This phenomenon is most often exploited to produce alloyed CIS-ZnS (CIZS) and AIS-ZnS (ZAIS) 2+ NCs. Similarly to the In-to-Ga substitution, the Zn introduction results in a gradual blue shift of the absorption band edge proportional to the dopant content providing unprecedented variability of the spectral sensitivity range (Fig. 77 3d) . Finally, the chalcogenide sublattice of all multinary MC NCs can be formed by a mixture of two chalcogenes, typically sulfur and selenium allowing to vary the bandgap of the alloyed NCs between the bandgaps of individual MC compounds. For example, kesterite CZTSSe NCs revealed a linear (Vegard-like) dependence of the bandgap on the S:Se 78 ratio (Fig. 3e) with the morphological properties of the alloyed NCs (size, shape, phase) unchanged throughout the entire x range. 1.2 Determination of CB and VB levels of MC NCs The applications of MC NCs in the light-harversting systems require accurate data on their photophysical, electrophysical, and electrochemical characteristics. Among them the positions of the conduction band “bottom” (ECB) and the valence band “top” (EVB) as analogs of the lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO) levels of molecular species are of paramount importance for the rational design of the photovoltaic and photochemical systems and devices. In particular, the accurate CB and VB positions of the MC NCs introduced into the bulkheterojunction solar cells either as a light absorber or as a transport layer are necessary to select/adjust the energy levels of the other components of the cell so that the charge separation and efficient electrons/holes flow to the electron/hole transport layers can be organized. The CB/VB positions of MC NCs introduced as spectral sensitizers into the liquid-junction photoelectrochemical solar cells are primary parameters for the selection of the sensitizer in such a way that the photogenerated CB electrons can be injected into the lower-lying CB of the wide-bandgap metal-oxide scaffold, while the photogenerated VB holes of MC NCs – filled with electrons from the redox-species in the electrolyte. Finally, the CB/VB levels of a nanocrystalline MC photocatalyst are the vital parameters allowing to predict and to explain the photoinduced events in a photocatalytic system with a set of given photocatalyst, electron acceptor, mediators, and an electron donor. Therefore, the accurate determination of CB and VB parameters is a general and crucial goal for the rational design of any kind of the light-harvesting systems based on the multinary MC NCs.

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Journal Name ARTICLE Table 2. Bulk bandgap, Bohr exciton radius and effective masses of charge carriers for selected multinary metal chalcogenides

MC CIS CISe CGS CGSe AIS AISe CZTS CZTSe AZTS AZTSe

Egbulk, eV 1.45–1.53 1.04–1.07 2.44 1.68 1.83 – 1.87 1.23 1.45–1.5 1.0 2.0 1.34

aB, nm 4.1 10.6 not reported (n/r) n/r 3.6–5.5 5.5 2.5–3.4 6.4–13.3 n/r n/r

The CB and VB positions of bulk metal chalcogenide crystals can be considered to be more or less constant depending weakly on the pH of surrounding aqueous electrolytes (the most often case for the photocatalytic sytems and liquidjunction solar cells) and showing Nernstian behavior in the 2– 2– 79 solutions of potential-determining ions, such as S or Se . However, for the MC NCs smaller than or comparable to 2aB in the given material the bandgap and CB/VB positions become 75,80,81 functions of the NC size . In theory, both the size and the NC NC size-dependent CB and VB levels (ECB , EVB ) can be estimated using the effective mass approximation from the NC optical bandgap Eg of the MC NCs determined from absorption spectra. In particular, the offsets of the CB/VB positions of MC NCs relative to the corresponding “bulk” NC bulk values, that is the differences ΔECB = ECB – ECB , and ΔEVB = * –1 NC bulk EVB + EVB can be calculated as ΔECB = ΔEg × m h and ΔEVB = * –1 NC bulk 80,81 ΔEg × m e , respectively, where ΔEg = Eg – Eg . In contrast to the “conventional” photoactive binary MC NCs, such as cadmium and lead chalcogenides, for ternary In-based and quaternary Sn-based MC NCs the accurate determination of the size-dependcent CB and VB levels becomes much more complicated and tricky. Indeed, the photophysical and electrophysical properties of NCs depend both on the NC composition and size and for the NCs experiencing the quantum confinement effects both dependences are typically intertwined and very hard to discriminate. Therefore, the above-discussed estimation approach can be applied only to the stoichiometric MC NCs, such as CuInS2 or AgInS2 with the well-reported “bulk” electrophysical constants, which is quite a rare case because typically the highest light-harvesting efficiencies are observed for the non-stoichiometric multinary compounds. In this view, the issue of the reliable determination of the CB and VB level positions of multinary MC NCs varying in compositions and size becomes an even more urgent and vital than it is for the binary MC NCs and can be resolved only by

m*e/m0 0.16 0.09 0.13 0.14 0.15 0.12 0.18 n/r 0.14–0.16 0.06–0.07

m*h/m0 1.3 0.73 0.69 1.20 1.36 0.39 0.71 n/r 1.16 0.89

Ref. 63–68 68–70 70 70 65, 71–73 67, 70 59–62 61 74 74

some direct or indirect experimental measurements. Basically, two different approaches are currently applied to assess the CB and VB positions of composition- and size-selected MC NCs, in particular, the electrochemical methods and photoelectron spectroscopic methods. The first electrochemical group includes studies of the current-voltage characteristics of MC NCs registered in dark or under the illumination and the analysis of the Mott-Schottky dependences combined with the bandgap determination from the absorption or photoaction 65–87 . The second group includes X-ray photoelectron spectra (XPS) and UV-light photoelectron (UPS) spectroscopic 54,88–94 studies . The most straightforward way to assess the CB/VB levels of MC NCs is by studying the current-voltage characteristics in a broad range of potentials. It is expected that the NCs attached to an electrode (for example glass carbon) exhibit oxidation peaks at positive potentials and reduction peaks at negative potentials corresponding to the hole injection into the VB and electron injection into the CB (Fig. 4a). This cyclic voltammetry (CV) approach has a general character and was successfully 95 applied to a variety of semiconductor NCs including Si , 96,97 98–100 99 101,102 PbS , CdSe , CdTe , and ZnO , indicating a broad applicability and credibility of the CV results. The onset potentials of the oxidation and reduction peaks of the size-selected CIS NCs (Eox, Ered, measured versus Ag/AgCl) can be converted into the vacuum scale by using the equations EHOMO = EVB = –Ip = –(Eox + 4.71 eV) and ELUMO = ECB = –Ea = – (Ered + 4.71 eV), were Ip and Ea denote the ionization potential 66 and electron affinity, respectively (Fig. 4b) . A 4.71 eV constant should be replaced with 4.50 eV if the potentials are expressed relative to the normal hydrogen electrode (NHE). Additionally, the exciton binding energy in MC NCs can be derived as a difference between the electrochemical bandgap NC (EVB – ECB) and the optical band gap Eg determined from the 66 NC absorption/photoaction spectra .

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Fig. 4. (a) Current-voltage curves produced by the cyclic voltamperometry of the sizeselected CIS NCs. (b, c) CB and VB levels for the size-selected (b) and compositionselected (c) CIS NCs. (d) (a, b) CB and VB potentials of size-selected CISe NCs. Reprinted with permissions from ref. 66 (a, b), ref. 103 (c), and ref. 82 (d). Copyright (2010, 2015, 2016) American Chemical Society.

A similar procedure can be applied to assess the CB and VB positions of more robust CIS NCs differing in composition, that is in the Cu:In ratio and exhibiting no quantum confinement 103 effects . As the copper content in the non-stoichiometric CIS NCs increases, the distance between the onset potentials of the oxidation and reduction peaks in the cyclic voltamperograms of the composition-selected CIS NCs corresponding to the VB/CB levels decreases proportionally to the reduction of the optical bandgap (Fig. 4c). The CV was also applied to evaluate the CB/VB positions of a 104 82 number of multinary MC NCs including CIS and CIZS , CISe , 105 106 107 86,87,108 ZISe , CTS , CTSe , CZTS , Cu2MSnS4 (M = Zn, Ni 85 78,109,110 Co) , CZTSSe . The size-dependence of CB/VB potentials of CISe NCs (Fig. 4d) determined by the CV was indispensable for the interpretation of their size-dependent 82 photocatalytic activity in the phenol oxidation . A CV study of polytypic CZTSSe NCs with different lattice symmetry showed the CB/VB levels to be phase-dependent and increasing from zinc blende to kesterite to wurtzite 110 modification . The variation of Se fraction in kesterite CZTSSe NCs from 0 to 1 was found to result in a considerable CB shift from –3.90 eV to –4.61 eV, while the VB position was affected by the chalcogene variation in a much moderate manner, 78 changing from –5.40 eV to –5.73 eV . The CB/VB positions of mixed CZTSSe NCs determined by the CV are in a satisfying accordance with the results of other alternative methods, in particular, the Mott-Schottky measurements, UPS and semiempirical calculations, the UPS systematically yielding 109 ~0.2 eV lower values as compared to other methods . The CB/VB positions can also be derived from the currentvoltage curves registered in a much narrow range under the illumination. For example, the size-selected AIS NCs produced by the photoetching technique exhibited a size-dependence of the photocurrent onset potential when illuminated on the 65 surface of FTO (fluorine doped tin oxide) electrode . The onset potential shifted from –0.60 to –1.00 V (versus Ag/AgCl) with the NC size decreasing from 5.1 nm to 2.7 nm. The

photocurrent onset potential of the n-conducting AIS NCs can be assumed to be equal to the Fermi potential EF and very 79,80 close to ECB as reported for many n-type semiconductors . The EVB potential can then be derived using the equation Eg = EVB – ECB + EC, where Eg is the bandgap determined from the absorption or the external quantum efficiency (EQE or photoaction) spectra of AIS NCs of a selected size d, and EC is the Coulomb energy of the electron-hole interaction, EC = – 2 –1 1.8e ×(2πεrε0d) , εr and ε0 are the electric permittivity of the 65 AIS and of the free space . The size-dependent CB and VB of AIS NCs evaluated by this approach are presented in Fig. 5a. The position of EF, which is close to ECB for the individual ndoped MC NCs but becomes a collective property in the binary and more complex heterostructures, can be assessed via the measurements of the capacitance–voltage dependences. The capacitance C of the NC/electrolyte interface is given by the equation 1/C2 = 2×(e0ε0εrND)–1×(E – EF – kT/e0), where e0 is the elemental charge, ND is the charge carrier density, E is the applied potential, k is the Boltzmann constant, T is temperature80. By plotting 1/C2 versus E, EF can be estimated as a cross point between the linear section of this MottSchottky dependence and the abscissa axis (Fig. 5b).

Fig. 5. (a) CB and VB potentials of the size-selected AIS NCs (solid lines drawn only as an eye-guide). (b) Mott-Schottky dependences for the bare TiO2 nanotube arrays and TiO2/CIS composites in the dark and under the illumination. Reprinted with permissions from ref. 65 (a) and ref. 111 (b). Copyright (2012) American Chemical Society (a); (2014) Elsevier (b). 2

The slope of 1/C –E dependence is proportional to the charge carrier density allowing to estimate the efficiency of the charge photogeneration from the Mott-Schottky curves. For example, illumination of a TiO2/CIS nanoheterostructure results in a shift of the Fermi level indicating the accumulation 2 of the photocarriers and an increase of the 1/C –E curve slope, showing that the TiO2/CIS composite is characterized by a considerably higher charge carrier density under the 111 illumination than bare titania nanotube array . The MottSchottky method was also applied to assess the CB position of 83 112 84 ZAIS , ZIS , and CdIn2S4 NCs . The photoelectron spectroscopy is a powerful alternative to the CV measurements of the CB and VB positions of MC NCs. Typically, the band structure of NCs is probed by UPS and less frequently – by XPS, the latter providing a much lower resolution. The incoming UV or X-ray irradiation results in the photoelectron ejection thus supplying the information on the HOMO (EVB) position. The LUMO (ECB) level can then be

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estimated by using bandgaps determined by the complementary absorption or EQE spectroscopies. An attractive version of this method is the photoelectron spectroscopy in air (PESA) allowing to study MC NCs in the ambient conditions and, therefore, probe the NCs with volatile ligands or solvent residuals that hamper the conventional XPS/UPS measurements. In this version, the VB top level (ionization energy) is determined as a threshold from the dependence between the photoelectron yield and excitation 88 photon energy . The PESA was successfully applied for the determination of the VB positions of size- and composition88 selected ZAIS NCs showing a steady increase in EVB with a decrease of the average NC size from 8.5 nm to 3.9 nm (Fig. 6a) and the x value – from 1.0 to 0.2 (Fig. 6b). In a similar way the VB positions of composition-selected ZAISe were also evaluated by the PESA in the whole range of x values89 as well as the VB/CB levels of CZTS NCs113.

Fig. 6. (a, b) VB and CB levels of the size-selected (a) and composition selected (b) ZAIS NCs determined by the PESA. (c) VB and CB levels of CIS NCs anchored to the TiO2 surface (the size-selected CIS NCs were prepared at a different temperature from 90 to 170 °C as indicated on the figure) . Reprinted with permissions from ref. 88 (a, b) and ref. 90 (c). Copyright (2015) American Chemical Society (a, b); (2016) The Royal Society of Chemistry (c).

In some cases, the results of the photoelectron spectroscopy of MC NCs supported on metal or semiconducting substrates should be taken with a caution because of the possibility of the so-called “pinning” of the NC Fermi level to the EF of the substrate with a much higher electron density. For example, the UPS studies revealed more or less the same VB level position for the size-selected CIS NCs anchored to the TiO2 surface114 and the authors concluded that the entire sizedependent bandgap increment comes into the negative shift of the CB level (Fig. 6c). A very similar situation was recently observed for the size-selected AIS and AIS/ZnS NCs supported on gold and on FTO plates54. Oppositely to the abovediscussed results on ZAIS and ZAISe NCs, both titaniasupported CIS NCs114 and gold-supported AIS NCs50 were capped with small ligands, formic acid and TGA, respectively. Therefore, a strong electron interaction between the NC and support can be envisaged resulting in the EF pinning and compromising the accurate EVB determination. Similar EF pinning phenomena were observed also for the 3.7–6.0-nm CdTe NCs on gold115 and the 2.1–4.2-nm CdSe NCs on the ZnO substrates116 indicating a general character of this effect. The EF pinning may be regarded as very favorable for the lightharvesting applications because the entire size-dependent ΔEg contributes into the increase of the absolute ECB value thus

potentially enhancing the interfacial electron transfer. However, the CB/VB values affected by pinning should be taken cautiously as a kind of effective properties of the NCsupport composite, not the individual NCs themselves. The pinning effect is expected to depend on various factors, such as the state of TiO2 surface, ligand type, the NC size and 91 composition, deposition mode, etc. For example, CIZS NCs 93 and CIGS NCs capped with mercaptopropionic acid (MPA) and anchored to the TiO2 surface revealed the expected upward shifts of the VB level due to the Zn and Ga insertion as 91 compared with pristine CIS NCs indicating that the pinning is absent or weak in these cases. 1.3 Photophysical properties of MC NCs The photogenerated charge carriers in MC NCs can participate in several competing processes, in particular in the interfacial charge transfer to electron/hole acceptors, the radiative electron-hole recombination with PL emission and the nonradiative recombination processes resulting in the thermal dissipation of the photoenergy without any chemical events in the light-harvesting system. Typically, the major efforts are directed at the inhibition of the latter processes as they are responsible for the losses of the photogenerated charge carriers. As the efficiency of both PL emission and the interfacial charge transfer are determined by the competing non-radiative recombination, the PL can serve as a diagnostic tool to assess the “quality” of MC NCs, that is, the efficiency of all the processes that compete with the non-radiative dissipation. Indeed, as discussed in the next sections, the PL QY quite often follows the efficiency of the light conversion – the higher is the PL QY, the more efficient light conversion can be achieved. The character and possible mechanisms of the PL emitted by various multinary MC have been discussed in details in a series of recent review papers (see, for example, refs. 8, 10, 13, 14, 18, 29, 35, 37, 38, 63, and 117) and, therefore, here we focus only on some special features especially relevant for the understanding of the MC NC-based light-harvesting systems. The ternary MC NCs typically emit in the visible/NIR spectral range in the broad bands characterized by a spectral width of more than 100 nm and a considerable Stokes shift. The PL decay of CIS, AIS and related doped/alloyed NCs occurs in several hundreds of nanoseconds revealing a dependence of the decay rate on the energy of emitted light quanta. Taking this into account, the PL of these NCs is attributed to the socalled “donor-acceptor” recombination, when one or both charge carriers are captured by “deep traps” – the lattice defects and vacancies with the corresponding electronic states lying in the forbidden band13,14,20,67,118. In terms of such model the spectral width of the PL band is attributed to several distributions – (i) a NC size distribution in a NC ensemble with size-dependent bandgaps; (ii) a distribution of the deep traps by energy (depth), and (iii) a distribution of distances between captured electrons and holes providing for a different Coulomb interaction between them and affecting the rate and energy of the emitted PL quanta. However, it was found that the donor-acceptor model meets some inconsistencies when

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applied to the ternary MC NCs. A precise control of the NC size and size distribution achieved by the hot injection/heating up syntheses (see next section) allowed to synthesize highly monodisperse CIS and AIS NCs still revealing broad PL 14,63 bands . The highly monodisperse AIS NCs produced by the size-selected precipitation showed the same (or even higher) 54 spectral PL band width as the original polydisperse ensemble . These facts allow to exclude the size distribution of ternary NCs as a major contributor to the boadband PL. Recent single119 120 paticle PL studies of CIS NCs and AIS NCs showed them to have more or less equally broad PL bands indicating the high PL spectral width to be an intrinsic property of each ternary NCs. Typically, the defect states in NCs participate both in radiative and non-radiative recombination and thus the QY of donoracceptor PL is expected to be quite low, in contrast to the direct interband (excitonic) radiative recombination. However, recent efforts aimed at the synthesis of brightly emitting CIS and AIS NCs showed the feasibility of the production of NCs with ~40–70% PL QY14,31,54,121, which is obviously not consistent with the model of defect-related emission. The donor-acceptor model with partially unevenly distributed e–h couples also contradicts to the reported temperature and excitation power dependences of the PL emission. The spectral PL parameters keep unchanged at low T122 and very small light intensities67,121, while a strong dependence of the PL band shape both of T and excitation power could be expected for the donor-acceptor model. In this view, alternative models are under development, in particular, the model assuming a strong electron-phonon interaction typical for the multinary MC NCs to be responsible for the large PL band width; the entire PL band is formed by phonon replicas of the interband electron transition67,118. This model seems to be more realistic and does not necessarily assume that ternary NCs are abundant with defects to emit the broadband PL. These findings are as striking as the capability of non-stoichiometric and highly doped CIS/AIS NCs to emit bright luminescence and, taken in ensemble, they evidence a unique character of such NCs differing drastically from “conventional” binary MC NCs, such as cadmium and lead chalcogenides, where even minor structural disordering or introduction of dopants typically deteriorates the PL properties. As a result, the broadband PL of ternary MC NCs appears to be as characteristic of the efficiency of competing electron/hole transfer events as does the excitonic PL in the case of CdX and PbX NCs (where X = S, Se). For example, the studies of the PL quenching of MC NCs contacting with metal oxide scaffolds58,123 supply indispensible information on the rate of the interfacial electron transfer and efficiency of the MC as spectral sensitizers in the lightharvesting systems. The interfacial electron/hole transfer rate constant ket can be derived from a comparison of the characteristic radiative life times of MC NCs attached to an electron acceptor (TiO2)58 or an electron donor (NiO)123, τrad1, and to some dielectric substrate with a similar surface chemistry, for 0 1 0 example, glass or silica, τrad , as ket = 1/τrad – 1/τrad (Fig. 7a). With this method, the rate constant of the electron transfer

from NiO scaffold to the Zn-doped CIS and CISSe NCs was 7 –1 7 –1 123 found to be 5.4×10 s and 8.2×10 s , respectively .

Fig. 7. (a) PL decay curves of Zn2+-doped CISSe NCs anchored to NiO and glass123 – Published by The Royal Society of Chemistry. (b, c) Time-resolved transient absorption spectra recorded following 387-nm laser pulse excitation of CIS NCs on SiO2 substrate at different delay times (b) and kinetic curves of the CIS NC bleaching decay (recorded at 590 nm) on SiO2 and TiO2 (c). (d) Dependence of the charge separation efficiency (ФCS) and PL QY on the average size of CIS NCs. Reprinted with permissions from ref. 92 (b, c) and ref. 116 (d). Copyright (2013, 2014) American Chemical Society.

This clearly shows that the enhanced photo-responce of the CISSe-based solar cell originates not only from the broader light absorption range of CISSe NCs, as compared to CIS, but also from the more efficient hole transfer to the NiO electrode. The rate constant of the electron transfer from the size6 – selected AIS NCs to TiO2 was found to increase from 5.6×10 s 1 6 –1 to 6.6×10 s as the size-dependent bandgap of AIS NCs 58 increased from 2.5 eV to 2.75 eV . It should be noted, that the PL quenching results provide only a lower range of ket because the radiative recombination in the MC NCs occurs 6 –1 14,54,59,63,118 with the constant rates of an order of 10 s . Only those electrons survived in earlier faster processes can contribute into the competition between PL and the charge transfer. To assess the faster events, the MC NCs are typically probed with the femtosecond transient absorption 51,91,92,124–127 spectroscopy . The laser photoexcitation of MC NCs with ~150 fs-long pulses results in the transfer of the VB electrons into the CB filling some portion of vacant states near the CB “bottom” and making them unavailable for the interband electron transitions till the CB electrons are not consumed in some secondary process – the recombination or interfacial charge transfer. As a result, the interband VB-to-CB electron transition in such excited NCs requires a higher energy, the fact reflecting in a transient blue shift of the absorption band edge observable as a negative transient bleaching band in the differential spectra (Fig. 7b)51,92,124. The bleaching decay reflects the dynamics of extinction of the photoexcited CB electrons allowing to assess the rate of the intefacial charge transfer similarly to the above-discussed PL

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quenching method. The possibility of the CB electron transfer to titania NCs in the TiO2/CIS composite results in a faster bleaching decay as compared with reference SiO2/CIS heterostructures (Fig. 7c) providing comparative data for the ket determination. The charge transfer rate constant ket = 11 –1 5.75×10 s was determined from the kinetic curves shown in the figure representing an upper limit of the charge injection 92 rates , similarly as the above-discussed PL data provide a lower rate limit for this process. A close constant rate of 11 –1 2.08×10 s determined from the transient bleaching decay 125 was reported for CIS NCs on TiO2/SiO2 scaffolds . As a ZnS 11 –1 shell is deposited on CIS NCs ket decreases to 0.35×10 s and 11 –1 further – to 0.14×10 s with the increasing shell thickness, indicating that the ZnS shell constitutes a barrier for the 125 interfacial electron transfers . However, the shell simultaneously blocks the surface defects on CIS NCs and thus the overall performance of TiO2/CIS/ZnS photoanodes appears to be higher than for the uncovered analog, despite the losses 125 in the charge transfer efficiency . A slowing of the transient bleaching decay of CIS/ZnS NCs in 127 the presence of an active electron donor – ascorbic acid was attributed to the efficient filling of the VB hole thus suppressing largely the electron-hole recombination, allowing the NCs to act as an active photocatalyst of the water reduction even without additional co-catalysts. The increase of the bleaching lifetime was accompanied with the PL quenching additionally supporting the conclusion of a suppressed e-h 127 recombination . The rate constant of the electron transfer from CISe NCs to a titania scaffold measured from the transient bleaching reco10 –1 10 –1 very was found to be 2.4×10 s and increasing to 9.1×10 s 2+ after doping the NCs with Zn ions as a result of the surface 91 trap states passivation in accordance with the above123 discussed PL quenching studies of Zn-doped CISSe NCs . The transient bleaching spectroscopy allows also to assess the efficiency of charge separation within individual size-selected MC NCs. P.V. Kamat et al. studied a series of 2.9–5.3-nm CIS NCs and found that the efficiency of charge stabilization, that is, the ratio of initial bleaching intensity to that measured in 50 ps after the photoexcitation depends in a non-monotonic way 124 on the NC size being maximal for the 4-nm CIS NCs (Fig. 7d) . The same size-dependent trend was exhibited by the PL QY indicating that the 4-nm NCs reveal an optimal balance between the charge trapping and recombination resulting in superior light-harvesting capacities of such NCs as compared 124 to smaller and larger CIS NC species .

2 Synthesis of multinary MC NCs The ideology and practice of the syntheses of multinary 8,10–18,22,35–38,42,45,128–130 nanocrystalline MC NCs stem directly from those well-established now for various binary MC compounds broadly used in the light-harvesting systems, such as CdS, CdSe, ZnS, PbS, PbSe, Bi2S3, Sb2S3, MoS2, etc. Similar to the binary compounds, the colloidal NCs of ternary and quaternary MC are most often synthesized by the well-known heating up and hot injection methods in a variety of the high-

boiling-temperature non-coordinating and coordinating organic solvents. To render such NCs soluble in the polar solvents as well as to avoid barriers for the photoinduced electron transfers between multinary MC NCs and other components of the light-harvesting systems, various postsynthesis ligand-exhange and solvent-excange procedures were developed for the multinary compounds. Simultaneously, the direct syntheses of MC NCs in water and other polar solvents provide a useful alternative to the “classic” heatingup/hot-injection approaches. The nanocrystalline films of multinary MC NCs, similar to their binary counterparts, are typically produced from the NC “inks” with appropriate modifications, such as additional sulfidation/selenidation, ligand exchange, and annealing aimed at an increase in the crystallinity. Additionally, a broad arsenal of physical and physical/chemical methods is applied for the film formation, including the vapor deposition and sputtering techniques, thermal treatment of molecular and metalcomplex precursors, electrochemical and electrophoretic deposition, etc. Finally, the nanocrystalline powdered MC samples are often produced by the solvothermal/hydrothermal methods allowing to tune the NC size, phase composition, the morphology of final products (NC shape, porosity, the shape of secondary NC aggregates, etc.). These methods typically yield the powdered products with a high crystallinity and surface area suitable for the photocatalysis and other light-harvesting applications. 2.1 Heating-up synthesis In the heating-up synthesis a single precursor or a mixture of metal precursors combined with a chalcogene source are subjected to heating in the high-boiling-point and stable organic solvents such as oleylamine (OLA), octadenece (ODE), or trioctyl phosphine oxide (TOPO). In many cases, in particular, with OLA and oleic acid, the solvent acts simultaneously as a surface-capping agent which controls the growth of MC NCs. Another popular medium for the heatingup synthesis of metal sulfide NCs is 1-dodecanethiol (1-DDT) acting simultaneously as a dispersive medium, a capping 92,131 ligand, and a sulfur source . In the simplest heating-up synthesis of CIS NCs, copper(I) iodide and In(ac)3, are heated at around 200 °C in 1-DDT resulting in the well crystallined and 131,132 quite monodisperse 1-DDT-capped NCs . In the case of the heating-up synthesis of AgInS2 and ZAIS NCs a variation of the 1-DDT content allows to tune the average size of resulting NCs 72,88,121,133 from 4 to 7–8 nm (Fig. 8a) . By adjusting the ratio of Ag and In precursors, the shape and composition of the ZAIS NCs can be influenced in a complex way. At a lowered Ag content the synthesis yields rod-like In-enriched NCs (3 nm × 3+ 10 nm) as a result of a higher In reactivity. With increasing Zn concentration the reactivities of both metal precursors equalize and the synthesis results in smaller 2–4-nm NCs 72 enriched with Zn . Along with the 1-DDT concentration, the heating duration plays a key role in the growth of the CIS NCs. For example, the average size of CIS NCs can be increased from 3.5 nm to 7.3

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nm by increasing the heating exposure time from around 20 to 134 120 min . Since the long alkyl chain of 1-DDT can hamper charge transfers from CIS NCs in the photovoltaic systems, smaller 135 ligands are probed as stabilizing agents such as octanethiol 136 or 2-mercapto-5-n-propyl pyrimidine . The above-duscissed general procedure with some variations was applied to 51,66,68,92,124,134,137,138 139 140–142 77,143 synthesize CIS , AIS , CIZS , ZAIS , 144 145 CIGS , CGS and CGZS NCs . The phase composition of CIS NCs produced by the heating-up can be controlled by the composition of the high-boiling-point dispersive medium. In particular, the metastable zinc blende CIS NCs form in ODE or oleic acid, while the synthesis in the coordinating solvents – OLA or TOPO yields thermodynamically stable wurtzite CIS NCs146. Such phase-selective approach was also extended to the synthesis of zinc blende/wurtzite CTS and CZTS NCs146. The multinary metal sulfide NCs are often synthesized by the heating up of a solution of metal diethyl dithiocarbamates (dedtc) in the high-boiling solvents. In this way, alloyed ZAIS wurtzite nanorods (NRs) were produced147. At first, the hexagonal Cu2S NCs form serving as nucleation sites for the following formation of ZAIS NRs147. The thermal decomposition of a mixture of Sn:In dedtc complexes in OLA at a Sn:In ratio of 1:4 was shown to yield 3.8-nm-thin In4SnS8 nanosheets (Fig. 8b, c)148.

Fig. 8. (a) Dependence of the average size of ZAIS NCs on the 1-DDT content during the heating-up synthesis. (b, c) AFM image and a roughness profile for In4SnS8 nanosheets. (d, e) TEM of rod- and rice-shaped ZAIS NCs. Reprinted with permissions from ref. 88 (a), ref. 148 (b, c), and ref. 154 (d, e). Copyright (2015, 2016) American Chemical Society (a, d, e); (2017) The Royal Society of Chemistry (b, c).

Similarly, the CIS and CIGS nanoribbons were produced from the corresponding dedtc complexes in a mixture of 1-DDT and 149 oleic acid . Here also Cu1.75S NCs form on the primary synthe+ sis stage. A high mobility of Cu generates vacancies in the Cu xS NCs that can be filled with In(III) and Ga(III) thus inducing the alloying and growth of CIS/CIGS nanoribbons. The thermal decomposition of single-molecule precursors containing all the necessary elements in a pre-defined proportion is a reliable way for the synthesis of MC NCs with a precise stoichiometry. In the case of metal sulfide NCs the precursor is typically formed by dithiocarbamates. In particular, ~5-nm ZAIS NCs were produced by the thermal

decomposition of (AgIn)xZn2(1–x)(S2CNEt2)4 complexes with a 65,150–153 varied x in octylamine . Depending on the precursor composition, the codecomposition of copper and antimony diethyl dithiocarbamates in OLA yields NCs with a variety of compositions and shapes, in particular rectangular CuSbS2, pyramidal 155 Cu12Sb4S13, and rhombic Cu3SbS3 . The thermal decomposition of a mixture of Cu, Zn, In, and Cd diethyl dithiocarbamates in toluene in the presence of oleic acid and OLA is a versatile approach to various multicomponent NCs, such as Zn2x(CuIn)1–xS2, (CuIn)1–xCd2xS2, 156 and (ZnS)x(CuInS2)y(CdS)z . Recently, OLA-capped 3-nm CIS NCs were reported to form even at room temperature as a result of the interactions between the amine and Cu(I) and In(III) dithiocarbamates in toluene157,158. Similar approaches were applied to synthesize CIGS NCs from the molecular complex bimetallic precursors159. Colloidal OLA-capped CISe and CIZSe NCs were synthesized by the heating-up approach at 180 °C using elemental selenium dissolved in diphenylphosphine91,160 or OLA68,161,162. Typically, CISe NCs evolve in coordinating solvents such as OLA via an intermediary step of the formation of CuSe and InSe NC phases while in the non-coordinating solvents (ODE) the intermediary Cu2xSe (0 < x < 0.5) and In2Se3 NCs were observed161. The CISe NCs were synthesized by reacting OLA complexes of Cu+ and In3+ with oleyl ammonium selenocarbamate formed in situ from Se and CO163. The alloyed ZAISe NCs with average sizes of 3.5–6.2 nm were produced through the heating-up of metal acetates with selenourea in hot OLA89. The anisotropic ZAIS NCs with a precise shape control were synthesized in OLA/1-DDT using a solution of S8 and dibuthyl thiourea in 1-DDT as a complex sulfur source154. The precursor composition was comparatively rapidly (with an increment of 1.2 °C in 1 s) heated up to 250 °C and maintained at this T. The short heating (less than 5 min) produced rod-shaped ZAIS NCs, while a longer heating-up procedure yielded a mixture of rod-shaped and rice-shaped NCs. The latter can be shape-selectively separated by adding methanol as a “poor” solvent inducing the preferential precipitation of nanorods and allowing to separate both fractions (Fig. 8d, e). Ternary Cu2SnS3 NCs were synthesized by the heating-up approach starting from CuI, SnAc4 and 1-DDT. The average NC size increased smoothly from 2.7 nm to 3.6 nm as the heating was prolonged from 5 min to 30 min106. Highly monodisperse CTS NCs were produced by the thermal treatment of copper and tin salts with diphenyl selenide in OLA/ODE164. By performing the heating-up synthesis in a flow tubular reactor instead of the conventional batch reactors the heating duration can be reduced to several minutes and the NC size distribution narrowed considerably (Fig. 9a). The latter effect results from a very efficient heat transfer typical for the flow reactors and shows good perspectives for the commercial scaling up of such approach. The heating-up protocol was also 165 adapted for the flow synthesis of CZTS NCs . The 17–18-nm silver tin sulfide Ag8SnS6 NCs were produced by the heatung-up method using ethylene glycol as a solvent and

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a capping agent at 160 °C . Similar approach was applied to 167 168 form CIS NCs at 200 °C and CZTS NCs at 450 °C .

Fig. 9. (a) Size distributions of CIS NCs synthesized using the heating-up approach in a conventional batch reactor (curve 1) and in a flow reactor (curve 2)132. (b) TEM of CuInTe2 nanosheets (insert: a photograph of colloidal CITe solution showing the scattering of a laser beam). Reprinted with permissions from ref. 170. Copyright (2017) The Royal Society of Chemistry.

The thermal treatment of a ODE/OLA solution of AgNO3, sulfur, and GeCl4 at 200 °C resulted in Ag8GeS6 NCs169. A fast heating up procedure was applied to produce ultrathin 1-DDTcapped CuInTe2 nanoplates (Fig. 9b)170. The nanoplates formed via the 2D assembly and recrystallization of primary CITe nanoseeds. Luzonite Cu2AsS4 NCs with an average size of 7 nm were prepared by the heating-up from AsCl3 and S/OLA at 175 °C171. The copper sulfide NCs that often form on the first stage of both heating-up and the hot-injection syntheses are abundant with copper vacancies which, in combination with a comparatively high mobility of Cu+ ions can be used to incorporate alien metal ions into the NC lattice and/or to exchange partially the copper(I) ions in the lattice thus producing various multinary MC NCs. For example, by the sequential ion incorporation/ion exchange with In3+ and Zn2+ in the original Cu2S NCs a series of core/shell CIS/ZnS and CIZS/ZnS NCs was produced104 (Fig. 10a). In this way Zn2+ ions introduced on the second stage mainly form a shell around CIS 2+ NCs, but a partial incorporation of Zn occurs inevitably as an inherent feature of both CIS and AIS compounds. Alternatively, 3+ 2+ if both In and Zn are simultabneously introduced to Cu2S 104 NCs the synthesis yields homogeneously alloyed CIZS NCs . By varying the heating duration the average size of CuxS NCs growing in ODE/TOPO/1-DDT mixtures was varied from 3 nm 172 to 8 nm . This size-selected copper sulfide NCs were then applied as a sacrificial prercursor for the synthesis of CIS NCs 3+ 3+ via the In incorporation/exchange. The In inclusion occurs already at room T, but can be accelerated substantially by 172 carrying the process at 125 °C . Triangular CuSe NCs were used as a sacrificial template to produce CIGS NCs via the incorporation of In ions in 173 hexadecylamine at 230 °C in an oxygen-free environment . The TEM studies of the probes taken at different growth moments exemplify very clearly the process of CuSe transformation into smaller CIGS NCs (Fig. 10b–f). Since the copper sulfide and CIS have almost the same atom arrangment the transformation of CuxS nanoplates into CIS (CIGS) nanoplates by introducing the In (In/Ga) precursor at 260 °C proceeds with the preservation of the original template

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morphology . In turn, CIS nanoplates can be used as a sacrificial template for the synthesis of CZTS nanoplates by the complete substitution of Cu ions with a combination of Sn and 174 Zn ions . The final CZTS nanoplates crystallize as a kesterite/stannite mixture and have a lateral size of around 150 nm and a thickness of ~12 nm. The gradual incorporation of Zn and Sn ions into the preformed CuxS NCs at 220 °C in ODE/OLA mixtures was used to 175 synthesize quaternary Cu-poor kesterite CZTS NCs . In the presence of 1-DDT the bullet-shaped wurtzite CZTS predominantly forms in this system. The synthesis can be upscaled to yield gram amounts of the CZTS NCs and thus is very perspective for the commercial implementations in the photovoltaics175.

Fig. 10. (a) A scheme of ion exchange synthesis of core/shell CIS/ZnS and alloyed CIZS NCs from Cu2S NCs. (b–e) TEM images of CuSe NCs transforming gradually into CIGS NCs as a result of Ga ion incorporation at 130 °C (b), 160 °C (c), 200 °C (d), and 230 °C (e). (f) A schematic of the transformation. Reprinted with permissions from ref. 104 (a) and ref. 165 (b–f). Copyright (2012, 2015) American Chemical Society.

The ion incorporation/ion exchange was applied to introduce In3+ ions into Ag2S NCs converting them into AIS NCs of roughly the same size176. The size of “template” Ag2S NCs can be relatively simply varied from ~3 to ~14 nm by adjusting the Ag:S ratio during the formation of silver sulfide NCs in OLA/toluene. On the example of the ion exchange with 4.1-nm Ag2S NCs converting into the 3.9-nm AIS NCs, it was shown that the NCs reduced in size for the expected ~14% as three Ag+ ions get exchanged with a sole In3+ ion176. A similar approach was developed for the transformation of Ag2Se NCs into the nanocrystalline ZAISe by the simultaneous diffusion of 2+ 3+ 177 Zn and In into the silver selenide nano-template . The heating-up approach is frequently used for the postsynthesis modifications of multinary NCs aimed at the deposition of various shells and the formation of functional composites. In particular, CZTS and Ga-doped CZTS NCs can be produced by the heating-up of metal acetylacetonate (acac)

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complexes in OLA as a solvent and capping agent , as well 78,184 as in combinations of OLA with glycerol and 185 186 thioacetamide (TAA) , and in benzyl alcohol . The synthesis yields NCs with an irregular shape that grow as the heating duration is increased (Fig. 11). The compositional studies of the NCs showed that the primary fast nucleation growth stage when the precursors are totally consumed lasts for around 0.5 h and then the Oswald ripening of the NCs becomes a dominating growth mechanism.

case may be attributed to a highly increased rate of the primary nuclei formation. A versatile single-reactor heating-up approach to synthesize 113 who produced CZTS NCs was proposed by Chesman et al. metal dithiocarbamate complexes of Cu, Zn, and Sn in situ via reactions between CS2 and OLA or 1-DDT. The reactions yield dithiocarbamate anions coupled to oleylammonia cations (Fig. 12c). The thermal decomposition of metal dithiocarbamates on the first synthesis stage at 100 °C results in 1–2-nm nuclei which then grow at 250 °C at the expense of the 1-DDT decomposition. The decoupling of the nucleation and nuclei growth stages allows to reach highly concentrated colloidal products as well as to exert a precise control of the NC parameters113. A similar approach was reported for the preparation of wurtzite CZTS nanocones194. The thermal decomposition of copper, tin, and zinc diethyl dithiocarbamates in a mixture of OA/ODE yields CZTS NCs in a relatively short synthesis finishing within 10 min195. By elevating the thermolysis temperature from 150 to 340 °C the average size of kesterite NCs can be gradually increased from around 2 to ~40 nm.

Fig. 11. TEM and size distribution of CZTS NCs produced by the heating-up with a different heating duration (indicated on figures). Reprinted and adapted with permissions from ref. 178. Copyright (2015) American Chemical Society.

Oleylamine was used as a universal medium for the heating-up synthesis of a family of CZTX (X = S, Se, Te) NCs from 187 corresponding elemental chalcogens . The compositiontuned Cu2ZnSn(S1–xSex)4 NCs were produced by the heating-up 61 in OLA/DTT with diphenylselenide as a Se source . A combination of 1-DDT and (C6H5)2Se yields exclusively wurtzite NCs, contrary to the similar synthesis with elemental S and Se 61 producing predominantly zinc blende CZTSSe NCs . Pure wurtzite 10-nm CZTS NCs were also produced from metal acetates by using thiourea and diethanolamine as a sulfur 188 source and a solvent, respectively . The heating-up synthesis of CZTS NCs can be made “green” and sustainable by using natural vegetable oils as a high-boiling-point solvent and a 189,190 simultaneous source of capping ligands . The detailed studies of the CZTS and CZTSSe NC growth in different mixtures of the high-boiling-T solvents revealed the phase and shape of the NCs being a complex function of the coordinating abilities of the solvent component, the nature of the copper precursor and the duration and temperature of the 191 synthesis . Thus, in the coordinating solvents such as OLA and TOPO multi-faceted cubic/hexagonal NCs can be produced, while the heating-up in the non-coordinating ODE yields mostly polygonal wurtzite NCs (Fig. 12a). Also, the stability of the Cu complex determined the Cu:Se ratio in the final CZTSSe NCs. By increasing the concentration of all the reactants by a factor of 6–8 higher than typically used in the heating-up method, 192 the highly monodisperse CZTS can be produced (Fig. 12b) . In view of the above-discussed results indicating a fast consumption of the reactants already in the first minutes of the synthesis, the narrow size distribution observed in this

Fig. 12. (a) A scheme depicting various geometries and phases of CZTS NCs produced by the heating-up synthesis. (b) TEM and size distribution of CZTS NCs synthesized at an increased starting concentration of the precursors. (c) Schemes of reactions between CS2 and OLA and OLA/1-DDT. (d) Absorption spectra and photographs of colloidal (Cu2Sn)x/3Zn1–xS NCs with a varied composition (x values given in the insert). Reprinted with permissions from ref. 191 (a), ref. 192 (b), ref. 113 (c), and ref. 193 (d). Copyright (2013–2016) American Chemical Society (a–c); (2010) The Royal Society of Chemistry (d).

The Zn-doped copper tin sulfide sphalerite (Cu2Sn)x/3Zn1–xS NCs can be synthesized by the heating-up of metal chlorides and thiourea in OLA at 200 °C in a broad compositional range of 0 < x < 0.75193. The variation of x from zero to 0.75 is accompanied by a narrowing of the bandgap from 3.48 eV (ZnS) to 1.23 eV and a shift of the absorption threshold down to ~1000 nm (Fig. 12d). The metal–CZTS nanoheterostructures can be produced by the heating-up with Au, Pd and Pt NCs196. In this case, the reaction in a non-coordinating ODE yields core/shell CZTS/metal NCs (Fig. 13a) while the utilization of the coordinating OLA results in dimer heterostructures (Fig. 13b).

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Fig. 13. (a, b) TEM images of the core/shell (a) and binary (b) CZTS/Pd nanoheterostructures. (c) TEM of CZTS-Pt nanoheterostructures. (d) Au/Cu2FeSnS4 core/shell nanoheterostructures. Reprinted with permissions from ref. 196 (a, b), ref. 197 (c), and ref. 198 (d). Copyright (2014–2017) American Chemical Society

CZTS NRs can be produced by the heating-up in a mixture of linear 1-DDT and tert-DDT (t-DDT) (Fig. 10a) acting as both 181,199–202 surfactants and sulfur sources . The formation of elongated (15 nm × 8 nm) wurtzite CZTS NCs was observed at the thermal decomposition of a mixture of Cu, Zn, and Sn– dedtc complexes in a mixture of trioctylamine and hexadecane 203 thiol . By changing hexadecane thiol to OLA the synthesis can be routed to the exclusive formation of the tetragonal zinc blende CZTS NCs. The monodisperse CZTS NCs synthesized by using the 1-DDT/tDDT mixture in OLA were used as seeds to form CZTS-Au and CZTS-Pt heterostructures. In particular, platinum deposition was performed by injecting a mixture of CZTS NCs and Pt(II)acac complex into phenyl ether at 200 °C. The procedure resulted in several ~2-nm Pt NCs deposited on each ~12-nm CZTS NC (Fig. 13c). Uniform kesterite ~10–15-nm CGZS and CGZSe NCs were synthesized by the heating-up from copper and zinc acetylacetonates, a GeCl2 complex with dioxane and 204 S(Se)/OLA solution at around 300 °C . In a similar procedure, CGZS NCs were produced using Cu and Zn dedtc complexes 205 and 1-DDT at 300 °C . The heating-up deposition of Cu2FeSnS4 at 140 °C on the surface of pre-formed Au NCs triggered by the injection of the 1-DDT/t-DDT mixture results in the core/shell Au/Cu2FeSnS4 nanostructures with the core size of 10 nm and shell thickness 198 of 10–20 nm (Fig. 13d) . A special version of the heating-up procedure was reported + 4+ 2+ 2+ when a precursor mixture containing Cu , Sn , Zn (or Co ), thiourea, and tetramethyl ammonium hydroxide was 206 introduced into the molten KSCN at 450 °C . The potassium – rodanide decomposition produces CN and sulfur followed by the reduction of sulfur to sulfide by cyanide ions and the formation of highly crystalline Cu2ZnSnS4 or Cu2CoSnS2 NCs. The CZTS NRs can be manipulated to produce self-assembled photosensitive layers. In particular, by addition of a polar 2propanol the side-by-side assembly of the NRs into 2D sheets 199,207 was triggered via the dipole-dipole interactions . The 2D aggregates can be then disassembled by modifying the NRs with tetradecyl phosphonic acid. Finally, both assembled and

individual CZTS NRs can be deposited by the electrophoresis as the NR layers with a different geometry (Fig. 14a). An alternative way to the assembly of CZTS NRs grown by the heating-up approaches in 1-DDT/t-DDT mixtures is in doping 3+ them with Sb that triggers the coupling and gradual fusing of 208 NRs into 2D sheets (Fig. 14b,c) . The aspect ratio of CTS and CZTS NRs formed in the presence of 1-DDT/t-DDT can be tuned from 3 to ~1.9 by decreasing the concentration of a copper (or copper + tin in the case of CZTS) precursor with the rest of 209 component concentrations kept constant . The phase of CTS NCs can also be controlled by the nature of sulfur precursor. The heating-up synthesis with S/OLA and 1DDT was reported to lead selectively to the zinc blende and wurtzite modifications of Cu2SnS3 NCs, respectively210. The CTS NCs was then converted into CZTS by the substitution of Cu with Zn/Sn with the preservation of the phase composition of the original CTS template210.

Fig. 14. (a) A scheme illustrating the assembling/disassembling of CZTS NRs. (b, c) Darkfield TEM images of separate CZTS NRs (b) and their 2D assemblies (c). Reprinted with permissions from ref. 199 (a) and ref. 208 (b, c). Copyright (2016) The Royal Society of Chemistry (a); (2017) American Chemical Society (b, c).

2.2 Hot-injection synthesis In the hot injection synthesis the metal precursors are typically dissolved in OLA, oleic acid and other media at around 100 °C, then the temperature is rapidly increased to 180–200 °C and a chalcogen source (for example sulfur solution in OLA – S/OLA or 1-DDT) is injected to trigger the nucleation of MC NCs. The solution is kept at this temperature for a certain period of time to allow the primary nuclei to grow to a desired size and then the temperature is swiftly reduced thus arresting the further NCs growth and ”freezing” the existing size distribution. Therefore, in the hot-injection method, the variation of the heating duration after the injection event is most frequently 71,118,211 used to tailor the average size of the resulting NCs . For example, the average size of AIS NCs growing in 1-DDT increases from 1.9 nm just after the S/OLA injection to 2.3 nm after a 30-s treatment and to 3.1 nm – after the heating for 20 211 min .

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CIS NCs are typically produced by the hot-injection technique + 3+ from Cu and In chlorides or acetates dissolved at 80–90 °C in Ar- or N2-saturated OLA or other coordinating solvent to yield amine complexes. The sulfide source is then rapidly injected at 240–280 °C and the mixture is kept at this temperature for a 125,212–214 given time to increase the NC size . After the CIS NC formation a protective ZnS shell is often deposited by the secondary hot-injection of zinc(II) acetate and an additional 213 heating cycle . AIS NCs were synthesized by the hot-injection procedure with 67,118,215,216 1-DDT as a capping ligand . The NCs formation was induced by the S/OLA injection at 110 °C. When the Ag-to-In ratio is elevated the synthesis yields AgIn5S8 NCs. An increase in the In:Ag ratio from 1:1 to 5:1 was reported to forward the hot-injection synthesis towards the preferential formation of cubic AgIn5S8 NCs with a well-defined stoichiometry217. Similar methods were applied to synthesize copper-diffused AIS NCs and CIS NCs. The 1-DDT ligands reduce in situ Cu(II) to Cu(I) which diffuses into the AIS lattice during the incubation of the reaction mixture at 180 °C218. The shape of CIS NCs formed by the hot-injection can be changed from polygonal to elongated bullet-like by adding OLA to the TOPO solvent (Fig. 15a,b). Oleylamine decreases the concentration of monomeric CIS available for the NC growth favoring to the elongated NC shape219. Additionally, phaseselected wurtzite and zinc blende CIS NCs can be synthesized with different sulfur sources – 1-DDT and S/OLA, respectively. By adjusting the composition of the dispersive media, the phase-selected tetragonal and orthorombic 7–25-nm AISe NCs were synthesized220. The type of crystalline lattice of AIS NCs grown by the hot-injection can be easily changed from orthorombic to tetragonal by elevating the after-injection temperature from 120 to 180 °C.217 In the intermediate T range, both phases were reported to co-exist.71 Recent findings made by the high-resolution scanning transmission electron microscopy coupled to the electron energy loss spectroscopy46 showed, however, that the highly crystalline and stoichiometric CIS NCs produced by the classical heating-up/hot-injection methods comprise inhomogeneities of copper and indium distribution within single NCs. The CIS NCs showed the presence of Cu-rich and In-rich areas (Fig. 15c) without any apparent discontinuity of the chalcopyrite lattice structure.46 These findings indicate that the unambiguous assignment of the CIS/AIS NC structure to some certain lattice type is tricky and obviously does not reflect the real picture on the atomic scale. The thiol ligands bind strongly to the MC NC surface presenting obvious difficulties for the post-synthesis ligand exchange and introduction of the NCs into the light-harvesting systems. In this view, various thiol-free hot-injection approaches are probed. In particular, the thiol-free CIS NCs were successfully prepared by hot-injecting bis(trimethylsilyl) sulfide into the solution of copper and indium halides in OLA/TOP/ODE.90,221 CuInSe2 (CISe) NCs form in a 1-DDT/ODE mixture from metal oleates after the injection of tributhylphosphine selenium at 69,222 200 °C. The average size of CISe NCs can be varied by the 222 post-injection heating duration in the range of 2.5–3.5 nm.

223

Similar syntheses of CISe NCs were realized with selenourea . A more reactive selenium precursor – diphosphine selenide 224 allowed to tune the CISe NC size from less than 2 to 7 nm . Mixed CISSe NCs with a varied composition were formed by combining 1-DDT and TOP-Se as sulfide and selenide source, 225 respectively.

Fig. 15. (a,b) TEM of CIS NCs produced by the hot-injection in different dispersive media. (b) Elemental maps of Cu and In for single CIS NCs. Reprinted with permissions from ref. 210 (a, b) and ref. 46 (c). Copyright (2011) Springer (a, b).

Alloyed CuInxGa1−xSe2 NCs can be produced in the entire x range from 0 to 1 by the hot-injection in hexadecyl aminebased mixtures by injecting Se/hexadecyl amide precursor 226 with metal-amine complexes at 240 °C . Alternatively, CIGS 227–229 NCs can be formed by introducing Ga(acac)3 and Se/OLA 93 or diphenyl phosphine selenium . Alloyed Cu2Ge(S3−xSex) NCs were synthesized by the injection of GeCl4/OLA into the OLA precursor of copper followed by the second injection of S/Se 230 dissolved in OLA . The average size of NCs increased from 11 nm to 19 nm as S was completely substituted with Se. Moreover, the bandgap was found to depend linearly on the 230 S:Se ratio . A general approach to the synthesis of CITe and AITe NCs was 231 developed utilizing TOP-Te complex as a telluride ion source. The 10-nm CdIn2S4 NCs were prepared by the hotinjection of S/OLA into the mixture of Cd(II) and In(III) OLA 232 complexes at 180 °C . The antimony-based Cu12Sb4S13 and Cu3SbS4 NCs were produced by injecting bis(trimethylsilyl)sulﬁde into the hot solution of copper(I) and Sb(III) 233 chlorides in ODE/oleic acid at 190 °C . 2+ 2+ By injecting S/OLA into a mixture of Cu and Sn OLA complexes at 240 °C, CTS NCs were synthesized with an 234 average size of 10–11 nm . Similarly, CTS NCs were produced by the hot-injection in TOPO/OLA mixtures with the elemental 235 sulfur dissolved in OLA or ODE as a sulfide ion source . The hot-injection synthesis in ethylene glycol at 190 °C yields CTS nanoflakes with a lateral size of around 100 nm236. Hexadecyl amine was used as a universal reaction medium for the

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formation of Cu and Sn precursor complexes, dissolution of 107 selenium and formation of ~20-nm Cu2SnSe3 NCs . A selection of the solvent composition and selenium precursor can forward the hot-injection synthesis of Cu2SnSe3 either to 110,237 hexagonal or to cubic CTSe NCs . In particular, the hotinjection of Sn-OLA complex into a mixture of diphenyl selenide and Cu-OLA at 240 °C yields predominantly wurtzite CTSe NCs, while the injection of separately prepared Cu- and Sn-OLA complexes into the hot Se/OLA solution produces cubic CTSe NCs. The time profiles of the NC evolution showed that in each case the ternary NCs form as a result of the Sn ions 237 insertion into the primary copper selenide NCs . The phase-selective hot-injection syntheses of kesterite, wurtzite and mixed CZTS NCs were realized by using respectively elemental sulfur, 1-DDT, and thioacetamide as sulfur sources108. The key stage of the kesterite phase formation is the H2S release in a reaction between elemental sulfur and OLA108. By adjusting the ratio of 1-DDT and sulfur the phase composition and purity of CZTS NCs can be controlled238. As the 1-DDT-to-S ratio increases, the CZTS NCs gradually evolve from kesterite nanoplates to wurtzite nanorods, the NCs becoming smaller and more uniform. A similar effect of the 1DDT/S ratio on the NC phase and morphology was observed in the hot-injection synthesis of CZTS NCs from Sn(II) ethyl hexanoate239. This precursor is stable in air and can be easily mixed with the solvents commonly used in the hot-injection procedure. The hot-injection approach is universal and can be extended to complex MC compositions. In particular, the CZTS NCs with a varied stoichiometry and a size of 4–5 nm were produced by the hot-injection using dioctyl amine complexes of metals and 1-DDT injected at 230 °C240. Similar syntheses of CZTS NCs were realized in OLA with S/OLA87,200,241–252 as well as in ethylene glycol253 and triethylene glycol254. The method was generalized to synthesize CIS254 and Cu2–xAgxZnSnS4 NCs254,255. A mixture of Cu, Zn, and Sn diethyl dithiocarbamates injected into the hot OLA at 290–320 °C serves as a universal source of both metal and sulfide ions for the CZTS NC growth256. Similarly to the heating-up approach, the injection results in the formation of primary CuxS NCs followed by the incorporation of Zn2+ and Sn4+ ions regardless of the injection temperature200. However, an increase in the heating T from 200 °C to 300 °C allows to tune the average NC size from ~5 nm to 8 nm200. At that, the variation of the bandgap of the resulting CZTS NCs with the injection/heating T does not necessarily reflect only the size variation of the NCs but can also originate from variations of the phase composition and stoichiometry of CZTS NCs241. Alloyed CZTSSe NCs can be prepared by the hot-injection by using mixtures of elemental S and Se dissolved/suspended in OLA257–261. Using solely Se/OLA results in ternary CZTSe NCs262. NaBH4 reducing agent is sometimes added to enhance the solubility of selenium in hot OLA and to ensure the 109,263,264 homogeneity of the mixed S/Se precursor . Alternatively, selenium can be reduced by 1-DDT to the OLAsoluble oleyl ammonium selenide that was used for the hot-

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injection synthesis of CISe and CZTSe NCs . Apparently, the NCs may contain some sulfur admixtures from the 1-DDT decomposition. The metastable wurtzite CZTS NCs were produced by the hotinjection of a 1-DDT/t-DDT precursor into a mixture of OLA 2+ 2+ 2+ 266 complexes of Cu , Zn , and Sn . A similar approach was 267 applied to form metastable wurtzite Cu2ZnGeS4 NCs . The post-synthesis heating of wurtzite CZTS NCs transforms them 110 into very stable cubic zinc blende modification . OLA was found to decrease the temperature of the thermal decomposition of metal dedtc complexes as a result of the coordination to thiocarbonyl carbon atoms of dedtc. This effect was exploited59 to produce small (2–7 nm) CZTS NCs from the mixtures of dedtc complexes of Cu, Zn, and Sn, that have different decomposition temperatures. The hot injection of OLA at a temperature lower than the decomposition T of any of the individual three complexes but higher than the decomposition T of such complexes dissolved in OLA triggers simultaneous decomposition of all three complexes and the formation of stoichiometric CZTS NCs. It was found that the concentration of primary nuclei and, therefore, the size of final NCs depends on the OLA concentration, varying from 2 to 7 nm as the OLA content is increased59. Kesterite Cu2FeSnS4 NCs with an average size of 14 nm were prepared by the hot-injection in OLA using acac complexes of metals and S/OLA (Fig. 16a)268,269. The obtained product contained admixtures of FeS and Cu2SnS3 phases and thus the synthesis conditions need to be further optimized. Similar hotinjection approaches were used to synthesize Cu2(FexZn1–x)SnS4 270, Cu2NiSnS4 271, Cu2CdSnS4 and Cu2MnSnS4 246 , as well as Cu2CoSn(S,Se)4 NCs 261.

Fig. 16. (a) TEM of Cu2FeSnS4 NCs (insert: the NC size distrtibution). (b) TEM of CZTS/Au-Ag nanoheterostructures produced with a different duration of the metal deposition (indicated on the figure). Reprinted with permissions from ref. 268 (a) and ref. 272 (b). Copyright (2012) The Royal Society of Chemistry (a); (2017) American Chemical Society (b).

Alloyed Cu2Zn(Ge,Sn)S4 (CZTGS) NCs revealed a dependence of the average NC size on the germanium precursor used in the hot-injection procedure273. While the CZTGS NCs synthesized using Ge-citrate complex have an unimodal size distribution centered at 15 nm, the NCs produced from GeCl4 or complexes of germanium(IV) with glycine and tartrate ions have a bimodal size distribution with maxima at ~5 nm and ~15 nm. The citrate-mediated synthesis, which is a preferable one due to a better control of the NC morphology, alows to tune a

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Sn:Ge ratio as well as the optical band of the produced NCs 273 from 1.6 eV for pure CZTS to 2.7 eV for CZGS . The mixed tingermanium CZTGS NCs were formed using two-step sequentional Ge-OLA and S-OLA injections at 130 °C and 280 274 °C, respectively . The hot injection of a mixture of 1-DDT and t-DDT into TOPO/OLA metal precursor yields wurtzite/stannite 275 18-nm Cu2CdGeS4 NCs . A general approach to a series of Cu2ZnAS4–x and CuZn2AS4 (A = 76 Al, Ga, In) NCs was developed based on the hot-injection of a mixture of 1-DDT/t-DDT into OLA complexes of corresponding metals. Similarly to the ternary MC NCs, this combination of sulfur precursors, acting simultaneously as capping agents and sulfur sources predominantly leads to the 50–60 nm long and 10-nm thick CZTS NRs. By two consecutive hot-injection procedures, first CZTS NCs and then binary CZTS/Au-Ag heterostructures can be 272 produced . At that, by varying the duration of the second hot-injection stage of metal deposition the average size of silver-gold alloyed NCs was smoothly increased from around 2 nm for 5 s heating exposure after the precursor injection to about 5 nm – for the 1 h thermal treatment (Fig. 16b). In a similar way, the PtCo-Cu2ZnGeS4 nanocomposites were 267 synthesized . The ternary CuSbS2 NCs were formed by hot-injecting S/OLA 276 into Cu(acac)2 + SbCl3 solution in OLA at 230 °C . The synthesis yields nanoplates with the length / width / thickness of 50–120 nm / 20–40 nm / 6–9 nm, respectively. 2.3 Ligand exchange The heating-up/hot-injection procedures are typically followed with ligand exchange aimed at replacing bulky organic capping agents with much smaller bifunctional species containing simultaneously carboxylic and thiol group, such as TGA or MPA, or a combination of thiol, COOH and amino groups as in the case of cysteine or glutathione (GSH). The new ligand adsorbs strongly on the surface of MC NCs while the carboxylic group becomes deprotonized rendering NC soluble in the polar solvents, specifically, in water and providing an electrostatic barrier for the NC agglomeration. Also, the carboxylic group serves as a “bridge” for the attachment of MC NCs to the surface of titania or zinc oxide. The substitution of OLA/1-DDT ligands with GSH or MPA in N,N-dimethyl formamide (DMF) yields water-dispersible CIZS NCs stable for months and showing a hydrodynamic size of less 277 than 10 nm . The process is scalable and can be upgraded to the gram-scale synthesis. The exchange of bulky OLA ligands with 1,2-ethanediol binding together the adjacent NCs allows 223 to form very compact films of ~7 nm CISe NCs and therefore such approach should have good perspectives for the preparation of NC inks for the jet-printed photovoltaics. The ligand exchange with MPA followed by the anchoring of MC NCs to the surface of titania was applied to produce the titania 278 125 160 heterostructures with CIS NCs , CIZS NCs , CISe NCs , and 91,279 CIZSe NCs . Similar ligand exchange procedures were performed with less stable to oxidation but smaller TGA 140 molecules yielding photoactive TiO2/CIZS nanocomposites . The above-discussed general ligand exchange methodology

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, AIS , was applied to produce water-soluble CIS 229,283 93 164 202,245,284–288 CIGS , CIGSe , CTSe , CZTS , and CZTSSe 258 NCs . The ligand exchange using 5-amino-1-pentanol yielded the concentrated and stable CZTS inks in ethanol or n-propanol 289 with the NC content of up to 200 mg/mL . OLA ligands on the CZTS NC surface were also substituted with smaller tert-buthyl 86 pyridine by the solvothermal treatment at 90 °C . Copper antimony sulfide NCs produced by the heating-up approach and deposited as a film on the ITO (indium-tin oxide) surface can be stripped from the surface OLA ligand by 290 contacting the film with the Meerwein’s agent (C2H5)3OBF4 , however, no details on the chemistry of this process were reported. As discussed in ref. 291, the contact of OLA-capped CZTS NCs with (C2H5)3OBF4 or (CH3)3OBF4 results in the amine – elimination and the modification of the NC surface with BF4 anions. Recenly a very promising synthesis strategy was developed by 292 D. Talapin et al. profiting from the ligand exchange of bulky organic moieties like OLA with charged metal chalcogenide 2– complexes (MCC), like In2Se4 . In this approach, the OLAcapped Cu2–xSe (x = 0 – 0.2) NCs were transferred from hexane to dimethyl sulfoxide (DMSO)/ethanolamine solvent by the 2– ligand exchange with In2Se4 and a charge-compensating + N2H5 cation. Such MCC-capped NCs are soluble in a variety of solvents including formamide, DMF, DMSO, ethylacetate, etc. The annealng at 500 °C transforms the MCC-capped NC films 292 into uniform CISe layers with only 2–3% of mass loss making such colloidal NCs an ideal ink for the jet-printing of the thinfilm solar cells. The approach is very versatile and can be universally used for the preparation of a variety of multinary 2– MC nanomaterials. In particular, by replacing In2Se4 with an equivalent amount of gallium selenide MCC, CIGS layers were 292 produced in a similar way . In the case of the CISe NC 2– precursor only 0.1 mol equivalent of In2Se4 or a more 3− complex [In2Cu2Se4S3] MCC is sufficient for the complete substitution of OLA ligands on the surface of 16-nm CISe NCs producing concentrated NC inks. Moreover, by combining 4− Cu2–xSe and ZnS NCs both stabilized by a Sn2S6 MCC, the highquality CZTS films can be fabricated by this approach. The composition of the CZTS layer can be easily adjusted by varying the molar ratio of MCC-capped NCs. The same procedure can 4− be reproduced for ZnSe and Cu2–xSe NCs stabilized by Sn2Se6 MCCs to synthesize phase- and composition pure CZTSe thin films, while the mixtures of the S- and Se-containing NCs yield 292 alloyed CZTSSe layers with a predefined S:Se ratio . 2.4 Precursor deposition and annealing This method is based on the heating-driven transformation (reduction, decomposition, hydrolysis, etc.) of a chalcogenide precursor in a solution containing simultaneously metal 293 ions . The nucleation occurs on the surface of a substrate – the optically transparent electrode (OTE) or a semiconductor substrate. The nucleation and growth can be controlled by the rate of the chalcogene precursor decomposition as well as by introducing complexing agents that bind metal ions and reduce the concentration of free cationic species. In this way,

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the Zn-, Sb-, and Ni-doped nanocrystalline CIS films were 294 formed at 80 °C as well as CuBiS2 NCs on the titania surface 295 at room temperature . The nanocrystalline multinary MC films can be relatively easily produced by spin coating of a liquid layer of molecular precursors on a chosen substrate, in particular, ITO, FTO, or the Mo layer on the soda lime glass. The spin coating is followed by the solvent evaporation and annealing resulting in the dense nanocrystalline MC films. In this way, the 296 nanocrystalline films of CTS were prepared. The CIS NCs with a size of around 25 nm can be encapsulated into a 3D carbon network by thermal decomposition of the chicken eggshell membraned impregnated by the copper and indium 297 salts and thiourea in the nitrogen atmosphere at 900 °C . A molecular precursor for the synthesis of CTS NCs was prepared by dissolving copper(I) and tin(II) sulfides in 1,2296 ethanedithiol . In a sol-gel version of this method the precursor sol is first prepared, then it is transformed into the gel and to the final NCs by the annealing. For example, a sol for the CTS NCs deposition was produced from Cu(II) and Sn(II) 298 chlorides, thiourea and methoxy ethanol . Similar sol was 299 used for the synthesis of nanocrystalline Cu2CdSnS4 films . The sol-precursor of Cu2FeSnS4 NCs based on thiourea/ methoxy ethanol combination with metal salts was spincoated onto the ZnO/ZnS nanorod array, preheated at 150 °C to produce a xerogel and then annealed at 250 °C. This procedure can be repeated several times to increase the thickness of the MC NC layer. The method was also applied to 300 form Cu2CoSnS4 and Cu2NiSnS4 NC layers . The spin coating of metal salts with thiourea in ethanol/water mixtures onto nanocrystalline titania followed by the annealing in S atmosphere was applied to produce TiO2/CZTS 301 nanoheterostructures . The films were subjected to the postsynthesis treatment with concentrated HCl selectively leaving intact only kesterite CZTS phase. In a similar way, ternary 302 303 ZnO/CdS/CZTS and ZnO/Ag/CIS nanoheterostructures were synthesized. The water/ethanol Cu-Zn-Sn-S NC precursor can also be spray-coated on conductive substrates and annealed sequentially in the N2 and Se vapor atmosphere to 304 yield CZTSSe films . The light-harvesting multinary MC films are often produced from the NC inks subjected to the high-temperature sulfidation/selenidation. This method was applied to 305–307 308 synthesize the films of CTZS, CZTSe , CZTSSe , Cu2FeSnS4 309 and Cu2CdSnS4 . Especially uniform and compact CZTSSe films were formed by the annealing of aqueous NC inks produced by the stabilization of copper and zinc sulfide NCs 310 4− 6− with Sn2S6 and Sn2S7 MCCs . The inks can be relatively easily prepared by dissolving elemental Sn and S in aqueous 2+ 2+ (NH4)2S solutions with the following addition of Cu and Zn ions. The ~10-nm CuxS and ZnS NCs are securely stabilized by the electrostatic repulsion of negatively charged MCCs that 310 have a strong affinity to the metal sulfide NC surface . The annealing of electrospun polyvinyl pyrrolidone (PVP) fibers soaked with a precursor solution yields highly crystalline 311 kesterite CZTS nanofibers .

The precursors for CTZSe films typically contain metal 312 chlorides, Na2SeO3 and a reducing agent, such as hydrazine . Alternatively, the selenide species in the precursor can be 313 generated during the annealing . By this method, the nanocrystalline CTZSe films with a grain size of 20–30 nm were 312 formed at moderate temperatures of ~180 °C . Alternatively, the CTZSe films can be produced using selenourea at much higher annealing temperatures (~500 °C in the Se 314 atmosphere) . The powdered CZTS NCs was synthesized by the sulfidation of a milled mixture of nanocrystalline CuxS, SnS, 62 and ZnS . The grain size of the final CZTS powders depends on the sulfidation temperature increasing from ~14 nm (300 °C) to 85 nm (450 °C). The sols containing metal dithiocarbamates were used to form 2+ 2+ Cu2MSnS4 (M = Co , Ni ) NC thin films subsequently converted to selenides by the thermal treatment in Se 315 atmosphere . 2.5 Hydrothermal/solvothermal synthesis Formally, this method is a version of the heating-up procedure but carried out under the much higher pressure, when the precursor mixture is confined into an autoclave and heated above the boiling point of the solvent for a proloned time. For example, the aqueous precursor solutions are typically heated at 180–220 °C for 12–48 h. As the reaction medium is characterized by high temperatures and pressures that cannot be reached in normal reaction conditions of the conventional heating-up process, the products of the hydrothermal/solvothermal synthesis can reveal quite unexpected morphology and phase composition. The chalcogens X are typically introduced in the form of stable compounds releasing chalcogenide ions as a result of the precursor hydrolysis and 2– 132 reduction to X during the treatment . The solvothermal treatment can also be adapted for continuous flow syntheses 316 of MC NCs . The hydrothermal treatment (HTT) and similar solvothermal (STT) procedures typically yield microscopic secondary agglomerates of primary MC NCs. To control the morphology (size, shape, porosity, etc.) of the secondary agglomerates various structure-directing agents are introduced during the 317,318 HTT/STT such as multi-functional molecular species (TGA , 319 320–324 143 325 MPA , cysteine , cystine , GSH ), organic polymers 326–331 such as PVP ), and surfactants (sodium dodecyl 332 333 sulfonate , cetyl trimethyl ammonium bromide , Triton X334 100 , etc.). Highly monodisperse microspheres composed of CIS NCs (Fig. 17a) were produced by the HTT of aqueous suspensions of Cu2O, In(OH)3 containing thioacetamide as a sulfide source and 335 PVP . Such scheme excludes the presence of any potential impurities such as halogenide ions and can be applied universally to synthesize various MC NCs. Using a similar composition of precursors, Cu2O, ZnO, and In(OH)3, mesoporous hierarchical CIZS microspheres composed of 7-nm 336 NCs were produced by the STT in ethanol . The synthesis was performed in the presence of CS2 and ethanolamine which in situ formed ethanol dithiocarbamic acid serving as a capping ligand, a structure-directing agent, and a sulfur source.

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Fig. 17. (a,b) Scanning electron microscopic (SEM) image of CIS microspheres produced by the HTT. (b) SEM of CIS, CISe, and CITe nanocubes produced by the HTT. (c) A unit cell of a crystal formed by [Cu5In30S56H4]13– cluster anions and HTEA+ cations. The cation is depicted with violet circles. (d) SEM of hierarchical ZnIn2S4 microspheres. Reprinted with permissions from ref. 335 (a), ref. 352 (b), ref. 353 (c), and ref. 354 (d). Copyright (2013, 2017) The Royal Society of Chemistry (a, c); (2014, 2017) Elsevier (b, d).

The HTT/STT is broadly applied to produce nanocrystalline CIS337–342, CIZS319,333,343, AIS151,344, ZAIS345, ZIS346,347, and CISe348. By performing the STT of copper and indium chlorides with thioacetamide in OLA or ethylenediamine, the selective formation of respectively zinc blende and wurtzite 10–20-nm NCs was observed340. The aggregates of CIS NCs were produced by the STT of a mixture of CuCl2, In2O3, oxalic acid and thiourea without additional solvents349. As the autoclave was kept at 230 °C for 5 days the molten thiourea served as a solvent, while oxalic acid neutralized releasing OH– ions inhibiting the formation of metal hydroxides. The growth of CIS NCs during the solvothermal synthesis can be restricted by introducing graphene oxide (GO) sheets that supplied the nucleation sites by adsorbing Cu2+ ions and anchored the growing CIS NCs thus inhibiting the agglomeration and growth processes350. Simultaneous reduction of GO occurs in the autoclave yielding final CIS nanocomposites with reduced graphene oxide (RGO). A similar method was applied for the synthesis of CZTS/RGO heterostructures188. A reliable control over the stoichiometry of CIX (X = S, Se, Te) materials produced by the HTT/STT can be achieved by using single-molecular precursors containing Cu, In and X in desired proportions. This approach was successfully applied to prepare CIS and CISe from [(Ph3P)2CuIn(SEt)4] and [(Ph3P)2CuIn(SePh)4] complexes, respectively351. The HTT of a suspension of elemental chalcogenes, metallic indium and CuCl2 is a universal method for the preparation of large (100–200 nm) CIX NCs crystallized in a cube-like 352 morphology (Fig. 17b) . The solvothermal synthesis of CIS NCs in OA with a very large excess of sulfur (Cu:In:S = 1:1:100) allows to reach a burst-like nucleation already at 110 °C and produce nearly stoichiometric CIS NCs with a rather small size

of 3.5 nm increasing to 4.8 nm as the treatment T is elevated 64 to 170 °C . The enhanced nucleation stage also results in a narrow NC size distribution of 7–11 %, quite untypical for the solvothermal syntheses. The HTT of an aqueous suspension CuI, In2O3, and triethylamine (TEA) at 160 °C for 5 days was reported to yield macrocrystals composed of atomically-resolved anionic 13– + pyramidal [Cu5In30S56H4] clusters compensated with HTEA 353 cations (Fig. 17c) . The strong binding between the cluster + and HTEA makes the crystals unsoluble in most solvents but an increase in the ionic strength by adding LiBr results in the fast dissolution of the crystals in DMF. Therefore, the results on further structural and optical characterizations of these ultra-small CIS particles can be expected to appear in the near future. Similar to the successful stabilization of multinary NCs with highly charged MCCs, it can be anticipated that such highly charged CIS anions can potentially be used for the stabilization of larger NCs allowing to produce dence nanocrystalline films for the solar light harvesting applications. The solvothermal synthesis of alloyed Cu(InxB1–x)Se2 NRs in diethylene triamine at 180 °C allows to vary the composition of the products from CISe to pure CuBSe2 and in this way to tune the bandgap of the NCs from 3.13 eV (x = 0) to 1.04 eV (x = 1)355. The hydrothermal/solvothermal approaches are broadly applied for the preparation of ternary ZIS materials as well as related compounds with other cations. In particular, the hierarchical ZIS and CdIn2S4 microspheres were prepared by the HTT with TAA as a sulfur source (Fig. 17d)354. Thiourea was used as a sulfur source for the HTT synthesis of nanocrystalline CaIn2S4 356. Copper-doped ZIS nanosheets were produced by the STT from layered Cu-doped NaInS2 nanosheets in the presence of zinc(II) chloride357. A partial Zn2+ ion exchange with Ag+ in the mesoporous ZnIn2S4 microspheres produced by the HTT results in a transformation of the surface layer of the ZIS microspheres into AgIn5S8 358 which seems to be a preferable stable phase for the Agdeficient Ag-In-S systems. According to the XPS results, the depth of Ag+ penetration into the ZIS, that is, the thickness of the AIS shell, depends on the starting Ag:In ratio and increases from around 90 nm for Ag:In = 0.5 to 160 nm for Ag:In = 1.25358. The samples preserve their hierarchical microspheroidal morphology during the ion exchange (Fig. 18a–c). As a result of the transformation the absorption threshold of the microspheres shifts from around 550 nm for ZIS to ~900 nm for the Ag-ZIS samples with the highest silver content (Fig. 18d). The morphology of AgGa1–xInxS2 nanoaggregates forming during the HTT at 180 °C is strongly affected by the introduction of alcohols – pentanol or heptanol resulting in the highly porous hierarchical AIGS microspheres composed of nanosheets and nanorods359. By adjusting the ratio of metal precursors x can be varied from 0 to 1 with the corresponding bandgap changing from 2.71 eV to 1.93 eV.

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Fig. 18. (a–c) SEM images of ZIS microspheres (a, insert shows a resemblance between + the microspheres and a flower) and the products of ion exchnage with Ag for Ag:In ratio of 0.5 (b) and 1.25 (c). (d) Absorption spectra of ZIS microspheres and ion exchange products (Ag:In = 0.5 (H1), 0.75 (H2), 1.0 (H3), and 1.25 (H4)). Reprinted with permissions from ref. 358. Copyright (2015) The Royal Society of Chemistry.

The HTT of zinc and indium acetates combined with GSH and TAA yields ZIS NCs with an average size increasing from 3.2 nm to 7.5 nm as the HTT temperature is elevated from 120 °C to 325 2– 160 °C . The authors assume that TAA serves not only as a S 2+ source but also forms various chelate complexes with Zn and 3+ In ions that gradually combine to form the nuclei and favor in this way the isotropic growth of spherical NCs with the lowest relative surface energy. The introduction of tartaric acid forwards the HTT process to the preferential formation of cubic ZIS films, while the hexagonal ZIS with an admixture of 360 In(OH)3 is typically obtained with no special additions . The latter process can be used for the formation of ZIS/In(OH)3 heterojunctions with the indium hydroxide selectively assemb361 ling on the rims of ZIS nanosheets . The self-assembling of MoS2 nanosheets on the surface of ZIS nanosheets during the solvothermal reduction of sodium molybdate in the presence 357 of thiourea was observed resulting in a very close contact between both layered compounds. The ZIS replicas of 3D structure of aquatic weed hydrilla that have a high specific surface area can be produced by the HTT 362 using the hydrilla as a sacrificial template . The cysteineassisted HTT was found to produce ultra-thin one-unit-cell ZIS 321 layers . An electron paramagnetic study of such nanolayers showed them to be rich with Zn vacancies when the synthesis is performed at 200 °C in the contrast to the similar samples 321 produced at a slightly lower temperature of 180 °C . The HTT of an inverted micellar aqueous Zn and In salt solution in an ionic liquid 1-butyl-3-methyl imidazolium hexafluorophosphate containing thiourea was found to yield the mesoporous ZIS microspheres composed of separate ZIS NCs334. A two-step HTT process was proposed to anchor cubic 3–6-nm ZIS NCs on the surface of hexagonal 5–8 μm ZIS microspheres320. The hexagonal carrier was first synthesized by the HTT in glycerol/ethanol solutions in the presence of cystein, and then the cubic ZIS NCs were formed by the second HTT round in aqueous TAA solutions320. Ternary Cu2SnS3 microspheres consisting of nanocrystalline platelets were synthesized in triethylene glycol using

elemental sulfur reduced during the solvothermal process 363 most probably by the solvent . By introducing PVP as a 327 structure-directing agent, the individual 3-nm CTS NCs , mesoporous 120–140 nm CTS nanospheres and hexagonal CTS 326 sheets with a lateral size of 0.5–2 μm were synthesized in the solvothermal regime. The spherical aggregates of Cu2Zn1−xFexSnS4 NCs were reported to form in the presence of 364 PVP during the HTT with thiourea as a sulfur source . The variation of iron content x from 0 to 1 resulted in a bandgap narrowing from 1.52 to 1.21 eV thus increasing the optical response of the kesterite NCs. Similar PVP-assisted solvothermal approach was applied to synthesize alloyed Cu2ZnSn(SxSe1–x)4 NCs with the bandgap varying from 1.11 eV (x = 0) to 1.49 eV (x = 1)331. Single-crystalline Cu4Bi4S9 nanoribbons were grown by the STT of toluene suspensions of CuCl, BiCl3, and dodecyl amine as a structure-directing agent365. To form the MC NCs, CS2 was injected into the autoclave and the mixture kept at 200 °C for 30 h. The nanoribbons are believed to grow via the layer-bylayer stacking of thin Cu4Bi4S9 sheets separated by the dodecyl amine linkers, the ribbon thickness depending on the amine concentration365. Similar to the ternary compounds, the hydrothertmal/solvothermal preparation of quarternary kesterite-type compounds also typically yields microspherical aggregates of nanocrystalline primary building blocks – petals, rods, wires, etc. In particular, CZTS microspheres were produced by the STT of metal precursors in ethylene glycol in the presence of thiourea366 and triethylene glycol with the elemental sulfur367. The microspheres are composed of micropetals which, in turn are built of 3–5 nm CZTS NCs (Fig. 19a,b). The 5–10-nm kesterite NCs were produced by the HTT in the presence of sodium dodecyl sulfonate as a structure-directing agent and capping ligand in water and water/ethylene diamine mixtures332. The resulting CZTS phase contains admixtures of SnS, Sn2S3, and ZnS (depending on the solvent composition) that can be removed by the annealing of the NC films at 500 °C in the sulfur vapor atmosphere332. The kesterite NCs were also synthesized by the STT of metal chlorides with thiourea in ethylene glycol, OLA, and ODE329. The choice of the solvent can be crucial for the morphology and phase composition of kesterite NCs produced by the solvothermal methods. In particular, the synthesis in triethylene tetramine yields predominantly 160 nm kesterite crystals, while the ethylene diamine favors to the formation of wurtzite/kesterite mixtures with a reduced grain size368. The treatment in ethylene glycol yields kesterite nanoplates with a thickness of around 45 nm, while the synthesis in oleic acid produced mostly the flower-like micro-aggregates of CZTS NCs368. Similar solvothermal syntheses were abundantly reported for of CZTS NCs86,330,369–382 as well as for CZTSe NCs331,383. The selenide kesterite NCs are typically produced using a Se/N2H4 complex383 or selenium solutions in 331 2– monoethanol amine as Se sources.

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Fig. 19. SEM of nanocrystalline CZTS microspheres (a, b) and nanosheets (c, d). Reprinted with permissions from ref. 367 (a, b) and ref. 384 (c, d). Copyright (2012) American Chemical Society (a, b); (2016) The Royal Society of Chemistry (c, d).

The phase composition of the products of the hydrothermal syntheses of nanocrystalline kesterites depends considerably on pH of the autoclaved solution. In particular, the formation of pure-phase CZTS NCs was observed only at pH 7, while at a lower pH (4.3–5) and a higher pH (9) additional CTS, Cu2–xS, 372 SnS2, and Sn2S3 phases appeared . The average size of CZTS NCs produced by the HTT can be tailored from 3 to 10–11 nm 60 by varying the treatment duration from 6 to 48 h . The introduction of TGA during the HTT results in a very uniform size distribution of final kesterite NCs. The thio-acid 2+ + reduces Cu to Cu and provides the primary Cu2–xS nuclei for the formation of CZTS NCs via the heating-assisted diffusion of 2+ 4+ 317 Zn and Sn ions into the bulk of copper sulfide particles . A surfactant-free hydrothermal approach to ~12 nm CZTS NCs was proposed based on the ammonia addition to the conventional aqueous solutions containing metal chlorides and 385 TAA as a sulfur source . The ammonia plays a crucial and manifold role. First, the presense of NH3 governs the kinetics 2– of TAA hydrolysis and enhances the S release thus providing 2+ abundant nuclei for the NC growth. Then, ammonia binds Cu 2+ and Zn ions into stable complexes and keeps to a minimum the concentration of free ions thus allowing for a slow NC growth. In this way, the combination of fast nucleation and the slow growth yield small and uniform CZTS NCs without 385 additional surfactants . A general approach to CuInS2, Cu2ZnSnS4, and Cu2CoSnS4 NCs 284,386 was found based on the STT in ethylene glycol, ethanol, or 2-propanol in the presence of thiourea and tetramethyl ammonium hydroxide. The post-synthesis ball milling of the products yields 150–250-nm aggregates of discrete 5–8-nm 386 MC crystals . Hierarchical Cu2FeSnS4 microspheres consisting of stannite ~100-nm-thick nanosheets were synthesized by the STT in ethylene glycol with L-cystine acting both as a capping 143 agent and a sulfur source . The HTT of FTO plates in a water/ethanol solution of CuAc2, ZnAc2, SnCl4 and thiourea at 200 °C for 24 h yields films of nanocrystalline CZTS 387 microplatelets composed of 10–30 nm kesterite NCs . The kesterite nanosheets were also produced by the STT of electrochemically deposited mixture of Cu, Sn, and Zn in ethanol 384 suspension of elemental sulfur at 200 °C (Fig. 19c, d) . The HTT of Cu2S nanowire arrays in the presence of InCl3 and 3+ TAA as a sulfur source results in the incorporation of In ions

into the lattice of copper sulfide nanowires and their transformation into CIS nanowires composed of nanosheets 388 (Fig. 20a,b) . The CuxS NCs grown in OLA were used as a template for the growth of CuGaS2 NCs by Ga ion 389 incorporation in the solvothermal regime . The procedure took place in OLA at 210 °C with the participation of Ga(acac)3 complexes and resulted in a change of the NC shape from roughly spherical for CuxS to elongated NR-like for CGS NCs (Fig. 20c,d). Hierarchical CIS microsperes grown by the solvothermal synthesis in ethylene glycol were also found to form from primary copper sulfide microspheres acting as a sacrificial template and translating the characteristic “microsphere-ofnanoplatelets” morphology to the resulting ternary copper indium sulfide390. The interaction of Cu2O nanocubes with tin-sulfide MCCs [Sn2S6]4– or [Sn2S7]6– converts the former into the hollow CuxS nanoboxes capped by the tin complexes391 which can be converted into the hollow kesterite (at pH 12) or wurtzite (at pH 7) CZTS nanoboxes upon the introduction of Zn2+ salts and the HTT at 190 °C.

3+

Fig. 20. (a, b) SEM of Cu2S (a) and CIS nanowires (b) produced by In incorporation during the HTT. (c, d) TEM of CuxS NCs (c) and CuGaS2 NCs (d) produced by the solvothermal incorporation of Ga ions. (e) TEM of CdS nanowires decorated with CdIn2S4 nanosheets. (f) Cross-sectional SEM of Si nanowire/TiO2/ZnIn2S4 nanostructure. Reprinted with permissions from ref. 388 (a, b), ref. 389 (c, d), ref. 399 (e), and ref. 400 (f). Copyright (2017) Elsevier (a, b); (2016) The Royal Society of Chemistry (c, d); (2017, 2015) Springer (e, f). 392

393

The ternary CuS/CdIn2S4/ZnIn2S4 and ZnS-In2S3-CuS composites synthesized respectively by the microwave381 393 assisted HTT and the STT in pyridine , most probably, also started to form from the primary copper sulfide NCs. This reaction appeared to be incomplete and yielded complex + nanoheterostructures. In a reverse way, Cu can be incorporated into the In2S3 nanoplates in the solvothermal regime and the optical properties of the resulting CIS nanoplates tuned by 103,394 varying the Cu:In ratio . The spinel In3–xS4 NCs were used as a sacrificial precursor of 5–10 nm CIS NCs in a solvothermal 395 synthesis with Cu(dedtc)2 at 180 °C . The synthesis showed a remarkable phase selectivity yielding wurtzite CIS, zinc blende CIS, and spinel CuIn5S8 NCs when performed in DDT, OLA, and a mixture of OLA with oleic acid, respectively.

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The hydrothermal/solvothermal processes are broadly used for the synthesis of various binary and more complex heterostructures. For this purpose, the formation of MC NCs is carried out in the presence of a prospective carrier, for example TiO2 or other wider-bandgap semiconductor. Alternatively, both components of the future heterostructure can be formed simultaneously in the autoclave. In particular, TiO2/CIS composites were synthesized by the STT of titania sols in ethanol suspension of copper and indium chlorides with 396 397 thiourea as a sulfur source . In a similar way, TiO2/CIS and 398 TiO2/AIS heterostructures based on the commercial nanocrystalline TiO2 were produced. The solvothermal treatment of CdS nanowires suspended in ethylene glycol with InCl3 and cysteine at 200 °C results in the modification of the nanowire surface with numerous thin 399 CdIn2S4 nanosheets (Fig. 20e) . A similar procedure was developed for the synthesis of CdS nanowires decorated with 401 ZnIn2S4 nanosheets . The HTT deposition of ZIS onto the nanocrystalline titania films results in a uniform cover composed of ZnIn2S4 nanosheets with a thickness of 20 nm and a lateral size of around 20 402 μm . Similar ZIS microflakes form during the HTT on the 400 112 titania-coated Si nanowires (Fig. 20f), TiO2 NRs , and 403 hollow titania microspheres , indicating that the sheet-like morphology is typical for the ternary In-based compounds with Zn and Cd in general, regardless of the substrate used for the deposition. The solvothermal approaches were applied to produce 404–407 numerous nanocomposites of multinary NCs with TiO2 , 408,409 318,410–414 415–421 ZnO , g-C3N4 , and carbon nanomaterials . The HTT of Zn/In/Ag salts and TAA in the presence of MoS2 flakes at 180 °C results in the ZAIS/MoS2 heterostructures with the MC component having a nominal composition of 83 Ag0.26In0.8Zn4S5.33 . A multi-stage HTT approach was developed to produce 422 ITO/ZnO NR/CAIS nanocomposites . The synthesis starts with the “seeding” of ZnO NC layer on the ITO surfaces by the solgel/annealing followed by the HTT in aqueous solutions of zinc nitrate and hexamethylene tetramine. The latter procedure is a generally accepted protocol for the synthesis of ZnO NR arrays. In the next step, the ITO/ZnO/ZnS heterostructure was produced by the HTT in the aqueous TAA solution followed by the ion exchange with silver(I) to yield ITO/ZnO/Ag2S composite. Finally, the films were subjected to the HTT in 3+ triethylene glycol for the In incorporation and exposed to the 2+ aqueous Cu solution to form the final ITO/ZnO/CAIS heterostructure. A similar multi-stage process was applied to 423 form ZnO/CIS (ZnO → ZnO/ZnS → ZnO/CuS → ZnO/CIS) , 424 ZnO/AgSbS2 (ZnO → ZnO/ZnS → ZnO/Ag2S → ZnO/AgSbS2) , and ZnO/Ag-Cu-Sb-S (ZnO → ZnO/ZnS → ZnO/CuS → 425 ZnO/CuS/Ag2S → ZnO/Ag-Cu-Sb-S) composites . Ultra-thin CIZS nanosheets with a varied Cu:In ratio were deposited onto the surface of TiO2 nanotube arrays by the STT 426 in triethylene glycol at 180 °C . The CZTS nanosheet arrays were formed exploiting the titania nanosheets as a structuredirecting template by the STT in ethylene glycol in the 328 presence of PVP .

A binary BiVO4-BiFeO3 heterostructure was sensitized with CuInS2 NCs the solvothermal deposition in ethanol in the pre427 sence of tuiourea and cetyl trymethyl ammonium bromide . The STT of lanthanum titanate in ethylene glycol suspension of Cu, Sn, and Zn chlorides with TAA yields La2Ti2O7/CZTS nanocomposites with ~50 nm kesterite NCs attached to the 10 428 nm thin titanate nanosheets . 2.6 Aqueous synthesis As discussed above, the conventional heating-up, hot-injection and solvothermal approaches typically produce MC NCs capped with protecting ligands and surfactants that can impede the interfacial electron transfer in the NC-based lightharvesting systems. To increase the efficiency of the photoinduced charge transfers as well as to render the MC NCs soluble in polar solvent the original bulky organic stabilizers such as OLA, 1-DDT and others are substituted with smaller multifunctional ligands, such as mercaptocarboxylic acids by the ligand exchange procedures. Summarily, the synthesis of MC NCs ready for the implementation as sensitizers and lightharvesting moieties includes also the procedures of postsynthetic purification and ligand exchange leaving behind a lot of wastes and chemicals to be recycled and/or disposed off. In view of the fact, alternative methods of the direct synthesis of MC NCs in polar media, in particular in water are incessantly searched and optimized. As the synthesis temperature in such sytems is naturally limited by the solvent boiling temperature (100 °C for aqueous syntheses) and the reactivity of various metal ions can be considerably changed by the partial hydrolysis or solvation effects, the preparation of crystalline products with a controlled NC size and a reasonably narrow size distribution becomes a challenge for many multinary MC compounds. Typically aqueous syntheses are based on the interaction between metal complexes with mercaptocarboxylic acids or other multifunctional molecules (cysteine, GSH, etc.) and a chalcogenide source. In this approach, CIS NCs were produced from mercaptoacetate (thioglycolate) complexes of Cu and In 429,430 and Na2S . Such synthesis typically yields relatively small, 2–3 nm, colloidal CIS NCs that show a crystalline motif of the 50,430 tetragonal chalcopyrite . As discussed in the previous section, the light absorption properties of CIS NCs are determined mostly by the copper content. For the nonstoichiometric NCs, which is the most often case for aqueous syntheses, the NC composition can be conveniently expressed by a molar ratio Cu:In:S = xCu:yIn:zS. The molar fraction of copper, xCu, can be varied in a broad range yielding a spectrum of non-stoichiometric CIS and CIZS NCs with a band edge shifting toward smaller wavelengths (and the bandgap – to 50 lower energies) as the xCu is elevated (Fig. 3a) . The light absorbance in the visible range increases almost linearly with an increase in the copper content (Fig. 3a, insert). Similar trends were observed for the TGA-capped AIS and ZAIS 54,58 NCs . The heating of the precursor solution results in a gradual growth of TGA-capped CIS and AIS NCs reflecting in a red shift of the absorption threshold and of the PL band (Fig. 21a). At that, the synthesis in mixed water/glycerol media

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allows to increase the post-synthesis heating temperature to ~120 °C thus achieving deeper crystallization and NC growth as 58 compared to pure aqueous AIS colloids .

fractions becomes lower (Fig. 21b) while the absorption band edge and the PL band maximum shift to shorter wavelength resulting in a spectacular change of the PL properties (photograph on Fig. 21b). It should be noted that the sizeselective precipitation allows separating NCs with very small differences in the average size. For example, the series of 10 fractions depicted in Fig. 21 shows an average size varying from around 2 to 3.5–4 nm (Fig. 21c). At the same time, the composition of such size-selected AIS (ZAIS) remains roughly the same, indicating that a homogeneous distribution of 54 elements in the original polydisperse NC solutions .

Fig. 21. (a) PL spectra and photographs of AIS NCs water/glycerol mixtures before (curve 1) and after heating at 120 °C for 5 (2), 10 (3), 15 (4), 20 (5), 25 (6), 30 (7), 35 (8), and 40 min (9). (b) Relative population of the size-selected AIS NCs in fractions produced by the size-selective precipitation (upper panel) and a photograph of sizeselected AIS/ZnS NCs under UV excitation (fraction numbers given under the image). (c) Size distribution of ZAIS NCs in two of the size-selected fractions. Reprinted with permissions from ref. 58 (a) and ref. 54 (b, c). Copyright (2015) Springer (a) and (2017) American Chemical Society (b, c).

Interaction between copper and indium iodides and Na2S in pyridine in the absence of any capping ligands yields amorphous CIS nanoparticles that were used as the precursor 114 inks for the preparation of CISe films . CIS and CIZS NCs were synthesized from the MPA-complexes of metal ion precursors with Na2S and kept at ~100 °C to finish the crystallization [431]. As a second step, a ZnS was deposited onto the NCs to enhance their stability toward the aggregation 431 and corrosion and to increase considerably the PL efficiency . An elegant way of the post-synthesis size control of AIS NCs 65 was proposed by T. Torimoto et al. based on the sizeselective photoetching. At that, the original ensemble of multisized NCs is illuminated in the oxygen-saturated water with a laser wavelength corresponding to the absorption band edge of the fraction of larger NCs (Fig. 22a). The oxidative photocorrosion, that can be described by a brutto-equation: + 3+ 2– AgInS2 + 4O2 = Ag + In + 2SO4 , results in the gradual dissolution of such NCs and then more shorter-wavelength laser light is applied to photocorrode the next fraction of smaller NCs. By repeating this procedure consecutively with a decreasing excitation wavelength (increasing quantum energy), several fractions of size-selected AIS NCs can be isolated with an average size varying from 5.1 (Eg = 1.8 eV, Fig. 65 22b) to 3.0 nm (Eg = 2.4 eV, Fig. 22c) . The post-synthesis separation of differently sized fractions of aqueous colloidal CIS and AIS NCs can also be performed by the size-selective precipitation. The method is based on the size dependence of the coagulation kinetics of colloidal NCs – upon the addition of a non-solvent, the larger NCs coagulate and precipitate faster than smaller NCs. By repeating additions 54 of the non-solvent (2-propanol in this particular case ) with the separation of the precipitated NCs and their re-dispersion in aqueous solutions, at least ten size-selected fraction of AIS and ZAIS NCs can be extracted from the original TGA-spabilized 54 aqueous AIS and ZAIS colloids . As the average size in the separated AIS/ZAIS fractions decreases, the population of the

Fig. 22. (a) Absorption spectra of original and photoetched AIS NCs (the laser wavelength used for the photoetching is displayed on the figure. (b, c) Size distribution of original (b) and the smallest photoetched AIS NCs (c). Reprinted with permissions from ref. 65. Copyright (2012) American Chemical Society.

Colloidal AIS NCs passivated by only the surface charge were produced via a reaction between silver(I) and indium(III) nitrates and (NH4)2S in formamide taking the sulfide ions in an 53 excess . ZAIS NCs in water were synthesized in the presence of GSH via prolonged heating of the precursor solution at ~100 432 °C . Similar to the heating-up and solvothermal syntheses, the ternary/quaternary Ag-based NCs can also be produced in water by the partial ion exchange. For example, colloidal Ag2S NCs stabilized by the bovine serum albumin were exposed to 3+ the heating at 90 °C in the presence of GSH and In resulting in the indium penetration into the lattice and formation of AIS 433 2+ NCs . By introducing Zn /GSH at room temperature it is possible to form a ZnS shell on the surface of AIS NCs, while the heating of such core/shell NCs at 100 °C results in the penetration of Zn into the core and the formation of alloyed 433 ZAIS NCs . The average size of original “template” Ag2S NCs can be varied from 2.5 nm to ~7 nm by adjusting the Ag/S ratio, thus opening the possibilities of the size-selective 433 synthesis of AIS and ZAIS NCs . Various multi-functional molecules were also successfully applied for the syntheses of quaternary Cu2ZnSnS4 NCs in aqueous solutions, including TGA, MPA, and GSH. In particular, aqueous colloidal 5–20 nm CZTS NCs were produced from a 2+ 2+ 2+ mixture of MPA complexes of Cu , Zn , and Sn via the mild 311 thermal treatment at ~90 °C at pH 10 for 24 h . Similar syntheses were reported with TGA (and (NH4)2S as an 434 435 additional sulfur source ) and GSH .

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2.7 Ion exchange The ion exchange method can be applied for the preparation of MC-containing heterostructures from zinc oxide nanoarchitectures. The broad utilization of zinc oxide in solar cells and other light harvesting systems is largely compromised by its chemical instability in acids and alkalis as well as in sulfide/polysulfide solutions typically used in the NC-sensitized liquid-junction solar cells. However, the instability of ZnO can be used to produce new composite nanoheterostructures by the partial or complete ions exchange of both oxygen anions and zinc cations with elements forming less soluble metal chalcogenides. For example, ZnO can be partially converted into ZnS by the HTT in the presence of thioacetamide. In this way ZnO NTAs were converted into ZnO/ZnS NTAs and then – into ternary ZnO/ZnS/Ag2S nanotubular composites by a partial Zn2+ exchange in zinc sulfide with Ag+ 436. Finally, by incorporating In3+ ions the ternary composite can be converted into ZnO/AgInS2 nanotube (NT) arrays preserving the morphology of the original zinc oxide substrate436. A partial exchange of Zn2+ with Cu2+ in 2D ZnIn2S4 nanosheets grown on the titania surface yields CIZS nanosheets with a much higher light sensitivity in the visible spectral range437. The CZTS-Ag2S heterostructures were produced by the partial Cu+-to-Ag+ exchange in the heating-up-synthesized CZTS NCs181. CdIn2S4 was synthesized from the nanocrystalline cadmium sulfide by the gas-phase cation exchange84.

can be applied to enhance the penetration of SILAR solutions into the nanotube voids and ensure a uniform coverage with 443 CIS NCs . To gain a better control over the MC NC size and shape the SILAR procedure is often performed after the preliminary decoration of the metal oxide surface with ex situ formed “seed” NCs. In this way, the CIS NCs were grown by the 278 SILAR on TiO2 seeded with 2.5-nm CIS NCs . The SILAR procedure was successfully used for the decoration 444 of TiO2 NT arrays with 15-nm AIS NCs (Fig. 23c) . At that, the Ag2S and In2S3 layers were consecutively deposited onto the titania NTs each in a single SILAR cycle and the AIS NCs formation apparently occured spontaneously as a result of the metal ion interdiffusion. Similarly, TiO2/AIS heterostructures can be produced from layered TiO2/Ag2S/In2S3 precursor with silver sulfide layer produced by the SILAR and the indium sulfide NCs formed by the chemical bath deposition. TiO2/In2S3/AIS composites form also as a result of Ag+ diffusion into the indium sulfide layer of the binary TiO2/In2S3 nanowires synthesized by the SILAR445.

2.8 Successive ionic layer adsorption and reaction The successive ionic layer adsorption and reaction (SILAR) is one of the most simple and, at the same time, potent and versatile methods of the preparation of various oxidechalcogenide heterostructures based on “conventional” binary MC NCs, such as CdS or PbS used in the photocatalysis and the liquid-junction semiconductor-sensitized solar cells (SSSCs). The method is based on alternating immersion of a metal oxide film into a solution of metal salt and a solution of a chalcogenide ions source. This procedure results in the formation of a metal chalcogenide layer on the metal oxide surface and is referred to as a SILAR cycle. By repeating the SILAR cycle many times, the content of MC can be controllably increased. Simultaneously, the size of MC NCs and their sizedependent optical properties change as well. The SILAR procedure was successfully adapted for the preparation of heterostructures based on multinary MC NCs by using several metal precursor solutions or a single solution containing a mixture of the metal ions in a pre-set proportion. In particular, layers of CIS NCs can be formed on the surface of 438 439,440 mesoporous titania (Fig. 23a) , TiO2 NT arrays (Fig. 23b) , 441 and TiO2 NRs by the SILAR procedure from aqueous precursor solutions. The 10-12 nm thick CIS nanosheets were produced by the SILAR on the surface of TiO2-coated FTO substrates followed by the annealing at 370–500 °C. The postSILAR annealing in the N2 atmosphere is a typical procedure 441,442 and aims at an increased crystallinity of CIS NCs . In the case of titania NT arrays the additional ultrasound treatment

Fig. 23. (a, b) TEM of CIS NCs deposited by the SILAR on the surface of mesoporous titania (a) and TiO2 NT arrays (b). (c) SEM of SILAR-deposited AIS NCs on the titania NT arrays. Reprinted with permissions from ref. 438 (a), ref. 440 (b), and ref. 444 (c). Copyright (2016, 2011) The Royal Society of Chemistry (a, b); (2015) Elsevier (c).

The post-synthesis alloying approach can be applied to other ternary NCs containing mobile Ag+ ions. In particular, TiO2/AgSbS2 films were produced by the annealing of the TiO2/Ag2S/Sb2S3 heterostructure with both metal sulfides deposited by the SILAR446. The alloying of SILAR-deposited Ag2S and Bi2S3 NCs into the nanocrystalline AgBiS2 layer on the titania surface occurs already at a moderate annealing temperature of 150 °C in ambient air conditions447. Three-layer ZnO NR/CuS/Sb2S3 structure prepared by the successive SILAR procedure was successfully converted into ZnO/CuSbS2 composite via the annealing at 350 °C necessary for the activation of a solid-state reaction 2CuS + Sb2S3 = 448 2CuSbS2 + S . 2.9 Electrodeposition The electrodeposition is a convenient method of the formation of nanocrystalline and bulk MC films on conducting substrates.

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The method is used in two versions – the electrodeposition of MC NCs or the electrodeposition of metals constituting the MC compounds followed by sulfidation/selenidation of the metallic deposits. In particular, CIS NCs were electrodeposited from aqueous solutions of Na2S2O3 and metal chlorides at –1 V versus Ag/AgCl and then annealed to increase the crystallinity 449 of the MC layer . The electrodeposition of CIS NCs from DMSO solutions containing elemental sulfur and GO is accompanied by the electrochemical GO reduction and yields 450 RGO/CIS heterostructures . Porous nanocrystalline CISe layers were electrodeposited from aqueous solutions containing copper and indium chlorides, selenite ions and additions of potassium hydrophtalate and sulfamic acid aimed at an equilibration of the redox-potentials of the reduction of all the constituents of CISe NCs451,452. The CISe nanowire arrays (Fig. 24a) were prepared by the electrochemical deposition of the MC NCs into the pores of anodized alumina membranes using H2SeO3 as a selenium source followed by the chemical etching of the original template membrane453. The pulse electrodeposition was shown to be more beneficial for the uniformity and quality of the deposited CIS layer as compared with the stationary electrodeposition454. The rectangular pulses with a frequency of 1 KHz with alternating deposition voltage and the relaxation voltage (V = 0) allow for the continuous nucleation/deposition and diffusion of fresh reactants, respectively. At that, both too high (1 MHz) and too low (1 Hz) pulse frequencies compromise the quality of the electrodeposited CIS layer of the surface of ZnO nanorod arrays (Fig. 24b) indicating that a balance between the electrochemical reaction and the reactant diffusion into the nanorod arrays should be maintained454. The pulsed electrodeposition was also reported to produce smooth layers of CIS NCs on the surface of TiO2 NT arrays used as a working electrode455,456. The amount of loaded CIS NCs depends on the number of applied half-second pulses. The pulse regime favors to the formation of uniform 20-nm CIS NCs and avoids clogging of the NT entrances (Fig. 24c). Similar effect was achieved by alternating stationary electrodeposition with ultrasound treatment of the TiO2 NT arrays in the precursor solution which favors the uniform penetration of the electrolyte into the voids of the titania NTs104. In both cases, Na2S2O3 was used as a sulfur source. The Ce-doped nanocrystalline tetragonal CIS with a grain size of 35–50 nm was electrodeposited from triethanol amine solutions with a cerium content varying up to 15 molar% with respect to In457. The nanocrystalline layer of Cu2SnS3 was electrodeposited from aqueous solution containing Na2S2O3 and trisodium citrate at room temperature on the surface of ZnO NR array on FTO serving in this systesis as a working electrode458. The citrate anions are introduced to balance the reduction rates of copper(II) and tin(II). Thin films of CdIn2Se4 were potentiostatically electrodeposited on the stainless steel from acidic 459 solutions containing H2SeO3 . The p-type CIS films were produced by the sulfidation of 460–462 electrodeposited copper and indium metallic layers . Sodium citrate is typically introduced into the electrolyte for

the electrodeposition to equalize the rates of copper and indium deposition, since the ions have different reduction 462 potentials . To exclude the presence of CuxS phases in the final CIS films they were additionally etched by an aqueous 462 KCN solution .

Fig. 24. (a) SEM of CuInSe2 nanowire arrays produced by the electrodeposition. (b) SEM of ZnO NR arrays with a CuInS2 layer electrodeposited in a pulse regime with a different pulse frequency (indicated on the figure). (c) SEM of titania NT arrays and electrodeposited TiO2/CIS NT arrays. Reprinted with permissions from ref. 453 (a), ref. 454 (b), and ref. 455 (c). Copyright (2017, 2011) Elsevier (a,c); (2015, 2016) The Royal Society of Chemistry (b).

The CZTS nanosheet films were prepared by the STT of the electrodeposited metal mix in ethanol suspensions of 384,463 sulfur . The deposition results in a ~350 nm layer of CZTS nanosheets with an average thickness of 30 nm. The nanocrystalline CZTS/CZTSe films were produced by the 464–467 co-electrodeposition of copper, zinc, and tin followed by the sulfidation at ~600 °C in the H2S/S atmosphere, or selenization via the STT in ethanol suspension of elemental Se 464 at 200 °C . Similar to the case of ternary compounds, sodium citrate was introduced in the precursor solutions to level off the rates of the Cu, Zn, and Sn electrodeposition. Polycrystalline dense Cu3BiS3 films were electrodeposited onto 468 Mo-covered glass using thiourea as a sulfur source . The CBS has a direct bandgap of 1.6 eV favorable for the lightharvesting applications and therefore future progess of the electrodeposition toward the formation of nanocrystalline films with a controlled morphology can be envisaged for this ternary semiconductor. 2.10 Microwave treatment The microwave heating of liquid dispersive media becomes a popular method of the preparation of nanocrystalline multinary MC compounds. The method is very fast as only a few-minute treatment is sufficient for the formation of well crystallized products as a result of a rapid and uniform heating of the dispersive medium in the microwave field. For example, 469 336 GSH-capped 2–3 nm CIS and AIS/ZAIS NCs were produced in water by the microwave heating of the precursor solution of corresponding metal complexes with GSH. The microwave heating was used as an assisting procedure in the aqueous 470 synthesis of cystein- and TGA-capped CIS NCs , CIGS NCs in 159 471,472 benzyl acetate , CZTS NCs in OLA and TOPO/OLA

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473

mixtures . In the latter case, the application of the microwave irradiation allowed to reduce the synthesis temperature and time to 165 °C and 10 min respectively. The microwave synthesis of CIS NCs in propylene glycol can be carried out with a variety of sulfide sources in particular 474 thiourea, TGA, Na2S2O3, cystine, thiosemicarbazid, etc. . Similar sulfur sources were probed for the microwave-assisted 475 synthesis of orthorhombic AIS NCs in ethylene glycol . The 3nm CIS and CISe NCs were prepared in water in the presence of TGA by the microwave heating of a solution of metal chlorides and a chalcogenide source – sodium sulfide or 476 selenide . The CZTS NC inks were produced by the microwave heating with thioacetamide as a sulfur source477–479. Because of the rapidity of the synthesis, it can be realized in a flow regime with methyl ethyl ketone/1-DDT mixtures as a solvent480. The introduction of TGA allows decreasing the size of flowsynthesized CZTS NCs down to 3–5 nm480. 2.11 Other methods A solid state reaction occurring in a mortar between Cu2S, Ag2S, Ga2S3, and In2S3 results in CuGa2In3S8 and AgGa2In3S8 powders with spectral responces extending to 600–650 nm481. Presumably, such reactions may lead to nanocrystalline products with unexpected properties when conducted in the presence of suitable surfactants or/and capping agents. The nanocrystalline CISe layers grown by the pulsed laser deposition482 are characterized by a gradient structure, the layer composition changing from In2Se3 to CISe along the film cross-section. Such gradient structure that can be varied by adjusting the deposition conditions482 is very favorable for the directed migration of the photogenerated charge carriers in the solar cells – the fact making the pulsed laser deposition a promising alternative to the other synthetic protocols. This method was also applied to produce quaternary CZTS NCs483,484. By combining the pulsed laser deposition with simultaneous electrophoresis, the CZTS NC films on Mo substrates can be produced by this method483. The stoichiometry of the kesterite phase produced from an amorphous CuS/SnS/ZnS precursor depends on the intensity (fluence) of the laser irradiation (Fig. 25a). Various evaporation/sputtering techniques are broadly applied for the deposition of both nanocrystalline and compact MC films for the photovoltaic applications, in particular CIGS [485], (ZnSe)0.85(CIGS)0.15486, CZTS487, Cu3SbS4488, and Cu2BaSnS4489,490. Cu2BaSnS4 kesterite was deposited in the form of a 100-nmthick film with a grain size of 100–200 nm (Fig. 25b) exhibiting a bandgap of around 2 eV and intrinsic Cu deficiency leading to the p-type conductivity489. Highly crystalline CISe nanowires were synthesized by using the molecular beam epitaxy (Fig. 25c)491. The stoichiometry of CISe nanowires was controlled by adjusting the evaporation rates of copper and selenium. ZnIn2S4 nanosheets assembled into the hierarchical microspheres were grown on {100} silicon substrates by the chemical vapor deposition using InP and ZnS as raw materials 492 heated at 1100 °C .

Fig. 25. (a) Composition of the CZTS films produced by the pulsed laser deposition as a function of the laser fluence. (b–d) SEM of CBTS NC film produced by the sputtering (b), CISe nanowires formed by the molecular beam epitaxy (c), and CuSbS2 film synthesized by the atomic layer deposition (d). (e, f) AFM image (e) and roughness profile (f) of single layer MnSb2S4 nanosheets. Reprinted with permissions from ref. 484 (a), ref. 490 (b), ref. 491 (c), ref. 494 (d), and ref. 499 (e, f). Copyright (2017) Elsevier (a); (2016, 2017) The Royal Society of Chemistry (b, c, e, f); (2017) American Chemical Society (d).

Bulk chalcostibite samples produced by the mechanochemical alloying of the elemental precursors were used as a source for the electron beam evaporation and deposition of thin CuSbSe2 5 –1 films with high absorption coefficients of more than 10 cm 493 and a bandgap of 1.18 eV . Similar films were also deposited 494 by the atomic layer deposition (Fig. 25d) and the radiofrequency magnetron sputtering from Cu2S and Sb2S3 495 targets . The CuSbS2 films can be formed in a two-stage process combining the thermal evaporation/re-deposition of a copper layer onto the surface of a Sb2S3 film produced by the chemical bath deposition followed by the annealing in the 496 sulfur atmosphere . Thin films of CIS NCs can be produced by the spray deposition of mixed aqueous solution of metal chlorides and thiourea 497 onto glass substrates followed by the heating at 300 °C . Nanocrystalline Cu2SnS3 films with a Cu:Sn ratio varying from 0 to 1 and a grain size of 10–30 nm were deposited by the spray 498 pyrolysis with thiourea as a sulfur source . The free-standing single layer MnSb2S4 nanosheets were produced by the thermal elimination of hydrazine molecules from the interlayer distances of the starting layered 499 MnSb2S4(N2H4)2 precursor (Fig. 25e,f) . The bandgap of MnSb2S4 nanosheets, 1.69 eV, is favorable for potential applications in the light-harvesting systems.

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The CIS NCs were synthesized by the high-energy mechanochemical treatment in a planetary ball mill from elemental Cu, 500 In, and S constituents in the argon atmosphere . The procedure yielded comparatively large 17–24-nm CIS particles. Nanocrystalline CTS powders with a grain size of around 45 nm can also be produced by the mechanical ball milling of a mixture of elemental copper, tin, and sulfur in ethanol in the 501 H2S atmosphere . By adding PVP and water the stable inks suitable for the jet –printing of Cu2SnS3-based light-harvesting layers were prepared. A gram-scale mechanochemical synthesis of CZTS NCs from elemental precursors with no 502 solvents or additives was proposed by D.K. Le et al. . The synthesis takes only 0.5 h to finish indicating that a selfpropagating solid-state chemical reaction is induced by the mechanochemical treatment.

3 Solar cells based on multinary MC NCs The attempts of the light energy conversion into the electric power stem from the photoelectric effect discovered by Henri 503,504 Bequerel in 1839 . It should be noted that Bequerel observed the illumination-induced electric current between two electrodes immersed in a liquid electrolyte. Later, the photoeffect was also observed for solid electrolytes and a focus of the solar cell studies was turned to solid photovoltaic materials for many decades to come, till the liquid-junction dye-sensitized solar cells (DSSCs) were introduced. The first to appear were the silicon-based solar cells. Silicon has a bandgap of 1.1 eV which is close to the optimal value of ~1.3 eV (peak of the solar irradiation spectrum) necessary for the achievement of the highest photoconversion efficiency. Moreover, it is a photostable, low-toxic and earth-bundant 5,505,506 material . The Si-based solar cells, typically named the first-generation semiconductor solar cells, though being unrivaled in terms of the ratio of conversion efficiency vs. production cost, have a number of shortcomings. First, the silicon solar cell technology requires a huge amount of very pure silicon. Typically, the photovoltaic technologies used rejected materials from the semiconductor industry and the necessary amount of raw materials can be maintained only if both industries are developed with the same rate, which is doubtful in view of a recent drastic growth of interest to the solar energy harvesting. The solar cells based on amorphous silicon, which is much less expensive, emerged in 1960s– 504 1970s . Another fundamental shortcoming of silicon is the indirect character of the interband electron transitions resulting in a comparatively low linear absorption coefficients. At least a 100-μm-thick silicon layer is required for the complete solar light absorption on the Earth surface thus putting limitations on the minimal thickness (and, therefore, the cost) of the solar cells. The above shortcomings stimulated a search for new lightharvesting materials, in particular, among the visible and NIR light sensitive MC semiconductors, such as binary cadmium, lead, and copper chalcogenides, ternary indium-based MCs, 6,14,17,21,22,39,41– and more complex quaternary compositions 44,504 . It was found already in mid-1980s that some of the

materials, in particular, Cu2S and CdTe (studied since 1960s), CuInSe2, CuGaSe2, CuInS2 and their alloys CIGS/CIGSe (studied from 1070s), and kesterite CZTS (studied from 1990s), sputtered as thin films can be used as the solar cell light 5–11,14,15,17,21,22,34,35,37,39–44,117,504,507 absorbers . Most of such compounds are direct-bandgap semiconductors with high absorption coefficients and, therefore, much thinner (1–2 μm) absorber layers are required for the efficient solar light 505 harvesting . Additionally, the alloying of several components can be used for a precise variation of the absorber bandgap. The thin-film cells are typically referred to as the secondgeneration semiconductor cells and showed efficiencies of 20% and higher39,41,42,44,508. However, the cell production puts very rigorous requirements to the purity of sources used for the thin-film sputtering, and therefore, the second-generation cells are very expensive and found applications mostly in the aerospace industry39,41,504,509, where the cell cost is not so critical. Recently, the thin-film solar cell technology gained a new impetus by discovering the possibility of using NC or molecular precursor "inks" for the preparation of nanocrystalline MC thin-film absorbers4–6,8,9,11,21,22,33,41,42. The ternary and quarternary compounds can be prepared by using the well-established methods of the colloidal chemistry and concentrated to the form of inks. In a similar way, other components of the solar cell (metal contact, n-type component, barrier layers, etc.) can also be prepared as the NC inks so that the entire solar cell can be produced by using the ink-jet printing technologies making such solar cells competitive to the conventional silicon-based devices4,22,42,510–513. In the simplest layout, the NC-based thinfilm solar cell comprises a layer of the light-harvesting MC NCs “squeezed” between an electron transporting layer and a hole transporting layer that facilitate the extraction and transport of the photogenerated charge carriers (Fig. 26a, upper structure).

Fig. 26. Outline of the NC-based solid-state (a) and liquid-junction (b) solar cells.

Alternatively, the MC NCs can be blended with another lightsensitive components, such as conjugated polymers, fullerene

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derivatives, hybride organo-inorganic perovskites, etc. to form the bulk p/n heterojunctions (Fig. 26a, lower structure). The photoexcitation of both components results in the generation of charge carriers spatially separated by the electric field originating from the p/n heterojunction. As shown below, in some cells the MC NCs can simultaneously act as a light absorber and a charge transporting layer. The idea of devising solar cell with two electrodes − a lightsensitive photoanode/photo-cathode and a catalytically-active counter electrode connected by a liquid electrolyte stems directly from the Bequerels experiments on the photoelectric 503,514 effect . This idea was first realized by A. Fujishima and K. Honda in their photoelectrochemical cell for the water splitting on a rutile single-crystal. The sensitization of wide-bandgap semiconductors, like TiO2, with external molecular absorbers is also one of the oldest conceptions of the solid-state photochemistry, introduced by Vogel in 1883 for silver halide emulsions 503,514 used in the photographic process . A combination of a liquid-electrolyte- ("liquid-junction"-) based PEC cell with the dye sensitization approach gave rise to the DSSCs belonging to the third-generation solar cells, according to the generally accepted classification. The sensitization effect was also observed long ago for the narrow-bandgap semiconductors deposited to/formed on the surface of the wider-bandgap semiconductors, for example, for Ag2S produced by an ion exchange on the surface of AgBr (AgI) [515]. It is, therefore, logical that the conception of DSSCs was later extended to the SSSCs with the same liquid-junction 4–9,11,14,16,19,21,22,25,34,35,37,516,517 architecture as in the DSSCs . The DSSC research was strongly stumulated by the fuel crisis of 1973 and over a thousand papers on DSSCs emerged in a few years from the start of the studies attesting to an explosive 503,514,518,519 growth of the DSSC field . The solar light is absorved in the DSSCs by a molecular sensitizer − a dye or a metal complex anchored to a wide-bandgap semiconductor, like TiO2 or ZnO with a highly developed surface area. The DSSC concept is a good example of a system, where the performance of the overall device is better than that of the separate 16 components . Indeed, the mesoporous titania cannot absorb efficiently the solar light and also does not conduct electric current. The conventional Ru-bipyridyl complex sensitizers degrade very quickly when illuminated in solutions without any oxide support and redox-shuttles. However, a combination of all the components into a united system results in a solar cell that can generate the electric current densities of up to 20 2 mA/cm and exhibits stable performance for more than 15 519 years in the outdoor solar illumination . The toughest challenge for the DSSCs still to be met is to surpass a threshold 519 of 15% efficiency . Indeed, quite spectacular efforts applied in the field of DSSC in two recent decades resulted in only ~11% efficiency for the best performing cells. However, we have to mention here a very promising 28.9% efficiency under ambient illumination (but only 11.3 % under the AM1.5G sunlight) achieved very recently by M. Grätzel and A. Hagfeld 520 et al. for a DSSC with two sensitizers ). A limited success in the efficiency imporvement observed in the recent years

stimulated the studies of other liquid-junction cell designs, in particular, the above-mentioned SSSCs with the semiconduc7–9,16,19,21,22,25,37,517 . tor sensitizer introduced in the form of NCs The working principle of the SSSCs is essentially based on a combination of the light-driven photocatalytic processes on a photoanode (photocathode) and the electro-catalytic processes on a counter electrode (Fig. 26b). The incoming light is absorbed by the NCs of a narrow-bandgap semiconductor, for example, CuInS2 (the photosensitizer NCs), resulting in the electron transfer to the wider-bandgap porous semiconductor metal oxide layer (TiO2 or ZnO). The VB hole of the sensitizer NCs is then filled at the expense of the oxidation of sulfide ions – one of the components of the redox couple present in the liquid electrolyte and having a very high adsorption affinity to the surface of sensitizer NCs. The elemental sulfur produced as 2– a result of the S photooxidation gets bound by the polysulfide species and diffuses to the CuxS counter electrode where it is reduced by the electrons arriving from the photoanode and with this the PEC cycle is finished. The SSSCs started with a modest few-percents efficiency of the light harvesting but showed an accelerated growth and achieved in 2015−2016 a promising eﬃciency higher than 11% and the potential of such cells is still to be realized to a full extent. This section reviews consecutively the solid-state solar cells based on the multinary MC NCs (both thin-film and bulkheterojunctionJ types) and the liquid-junction solar cells with MC NC-sensitized wide-bandgap photoanodes/photocathodes. 3.1 Solid-state solar cells based on MC NCs The simplest design of the multinary NC-based solid-state solar cells includes a MC NC layer sandwiched between an ETL and an HTL. Typically, CIS or CISe layers are formed on semitransparent Mo-coated glass as an ETL, and covered consecutively with a buffer CdS layer and a layer of intrinsically doped and aluminium doped ZnO NCs as a HTL. The structure is finished by a top metallic layer (Au, Ah, or Ni-Al). Such configurations showed the light conversion efficiencies of ~1% 158 521 340 for CIS NCs , 3.4% for CISSe NCs , 0.74% for CISe NCs . A similar design was also realized with the absorber layer formed 355 by mixed Cu(InxB1–x)Se2 NCs . Tables 3 and 4 summarize some of the reported solid-state solar cells based on various multinary MC NCs and placed in the order of increasing light conversion efficiency for each light absorber. The tables show the cell composition with the lightharvester marked by the bold font as well as the most important photovoltaic parameters of the cells, in particular, the short-circuit photocurrent density (Jsc), the open-circuit photovoltage (Voc), the fill factor of the current-voltage characteristic (FF), and the power conversion efficiency (η). The CIS/CISe NC absorber layer in the solid-state solar cells can be formed by the direct electrodeposition from molecular 451 precursor solutions or by the electrophoretic deposition of 339 the pre-formed NCs . The cells formed by the electrophoretic deposition of alloyed CIGS NCs showed the highest photocurrent conversion efficiency (PCE) of 5.57% at a 25% 524 molar fraction of gallium .

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Journal Name ARTICLE Table 3. Photovoltaic parameters of some solid-state solar cells based on A-BIII,V-X NC

MC

CIS

CIS/perovskite Cu–In–B–Se CAIS

AIS CISSe CISe

CIGS CIGSSe CIGSe Cu–Ge–S–Se Cu2SnS3 CuSbS2

Cell composition

glass/Mo/CIS/CdS/ZnO/Al-doped ZnO ITO/PEDOT:PSS/TMSC-CuInS2/Al FTO/TiO2/CIS/Au FTO/TiO2/CIS/ spiro-OMeTAD/Ag FTO/TiO2/CdS/CIS/Au ITO/ZnO/CIS/PTB7:PCBM/MoO3/Ag FTO/TiO2/CH3NH3PbI3/CIS/ZnS/Au Glass/Mo/Cu(InxB1–x)Se2/CdS/iZO/ITO ITO/PEDOT:PSS/P3HT+CAIS/Al ITO/ZnO/Cu0.7Ag0.3InS2/P3HT/Pt ITO/ZnO NRA/ZnS/AIS/P3HT/Pt Mo/CIS(Se)/CdS/IZO/AZO/Ni-Al Mo/CISe/CdS/IZO/ZnO/Ni/Al Mo/CISe/CdS/IZO/ZnO/Al Mo/CIGS/CdS/IZO/ZnO/Ni-Al glass/Mo/CIGSSe/CdS/IZO/ITO Mo/CIGS/CdS/TiO2/FTO Glass/Mo/CIGSe/CdS/IZO/AZO/Ni-Al Glass/Mo/SiO2 NMs/CIGSe/CdS/ZnO/AZO Au/MoO3/CGeSSe/CdS/FTO Mo/CTS/CdS/IZO/AZO/Al ITO/TiO2/CBS/Spiro-OMeTAD/Au ITO/CdS/CBS/Ag iZO/AZO/CdS/CBS/Mo ITO/ZnO/CBS/P3HT/Pt

Jsc, mA/cm2 13.59 5.48 4.32 5.34 17.80 15.65 18.6 25.9 6.74 6.93 7.5 14 39.2 35.2 20.9 28.8 18.34 23.9 27.5 1.31 20.12 0.04 1.35 5.20 5.87

Voc, mV 190 479 700 640 590 750 924 265 540 499 512 521 404 460 526 630 490 501 589 570 327 313 665 350 491

FF

0.28 0.377 0.38 0.41 0.64 0.72 0.487 0.34 0.31 0.53 0.55 0.463 0.632 0.67 0.489 0.657 0.62 0.54 0.703 0.28 0.42 0.31 0.62 0.55 0.56

η, % 0.74 0.99 1.16 1.41 6.72 8.51 8.38 2.34 1.13 1.80 2.11 3.4 10.01 10.85 6.09 12.0 5.57 6.5 11.4 0.2 2.77 0.02 0.60 1.02 1.61

Ref.

340 158 90 438 339 522 213 355 218 422 436 521 451 114 523 227 524 525 526 230 236 494 496 495 448

Notes: the accuracy of values is preserved as reported in the corresponding references; IZO – intrinsic conductive transparent ZnO, AZO – Al-doped ZnO; SpiroOMeTAD – N2,N2,N2′,N2′,N7,N7,N7′,N7′-octakis-(4-methoxyphenyl)-9,9′-spirobi[9H-fluorene]-2,2′,7,7′-tetramine; P3HT – poly-3-hexylthiophene; PTB7 – poly[[4,8-bis[(2ethyl-hexyl)-oxy]-benzo[1,2-b:4,5-b′]-dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)-carbonyl]thieno[3,4-b]-thiophene-diyl]]; PCBM – [6,6]-phenyl-C71-butyric acid methyl ester; PEDOT:PSS – poly-3,4-ethylen-dioxythiophen doped with polystyrene sulfonate; TMSC – trimethylsilyl cellulose.

The development of an efficient bulk-heterojunction solidstate solar cell with MC NCs is a multiple task when variations of any possible parameters of the cell (geometry, composition, mixing of the components, thickness of layers, etc.) affect the cell performance to a lesser or larger extent. Among the major factors are (i) the “right” cell geometry allowing for the efficient charge extraction and transport to the opposite electrodes and (ii) the matching of energy levels between the donating and accepting components of the cells. This conception is exemplified by the bulk-heterojunction solar 436 422 cells based on AIS and CAIS NCs mixed with P3HT, the mixture deposited onto the surface of ZnO NR array (Fig. 27a). The cell illumination can excite both CAIS NCs and P3HT and a “cascade” electron transfer can occur from P3HT to CAIS and further to ZnO, while the photogenerated holes can move in the opposite direction. In this way, the efficient charge separation and directed transport are organized, while the P3HT HTL actively participates in the photogeneration of charge carriers (Fig. 27b). The ZnO NR array acts as a perfect

electron carrier allowing for the directed electron flow to ITO 422,436 . The copper ions occupy and out into the electric circuit the vacant states in the AIS lattice and increase the NC conductivity resulting in a decreased internal resistance of the 218 bulk heterojunctions between CAIS NCs and P3HT polymer . The cells based on the CAIS NC/P3HT bulk-heterojunction reveal a higher PCE of 1.13%, than similar composites with 218 individual CIS and AIS NCs (0.25% and 0.47%, respectively) . The 1-DDT/OLA-capped CIS NCs produced by the hot injection can be easily mixed with solutions of conjugated polymers, for example, P3HT resulting in a NC/polymer bulk531 heterojunction . The heterojunction quality and the efficiency of the charge transfers between the bulkheterojunction components can be increased considerably by 531 the post-deposition ligand exchange . The composites of RGO with CIS NCs were applied as an ETL and, simultaneously, as a co-absorber in the bulk350 heterojunction with MEH-PPV . The RGO nanosheets increased the cell productivity as a result of the directed

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electron transfer along the RGO basal plane and a percolating 350 effect of the nanosheets .

film and thus each sensitizer NC potentially has a direct contact with the titania NCs (Fig. 28c, d). In such configuration the PCE of the cell increases to 1.16% as a result of the simultaneous increase in the Jsc (Fig. 28e), FF and a 10-fold rise 90 of the EQE . The ideality factor (FF) of the cell was found to be much higher for the porous layer-based bulk-heterojunction as compared with the flat-bilayer indicating strongly reduced electron-hole recombination in the porous bulkheterojunction.

Fig. 27. (a, b) Scheme (a) and current – voltage characteristics (b) of the solid-state solar cells based on ZnO NR array and ZnO/CAIS/P3HT heterostructures. (c, d) Schematic layout (c) and energy diagram (d) of the cell based on CZTS NC/MEH-PPV (poly(2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylene vinylene) bulk-heterojunction. Reprinted with permissions from ref. 422 (a, b) and ref. 86 (c, d). Copyright (2015) The Royal Society of Chemistry.

The bulk heterojunctions of 3–5-nm CZTS NCs with P3HT also have favorable mutual CB and VB positions allowing for the efficient electron transfer from the photoexcited polymer to NCs and the hole transfer from the photoexcited NCs to the polymer (Fig. 27c, d). As a result, they can efficiently absorb 86 the visible light at wavelengths shorter than 900 nm . The introduction of CZTS NCs into the bulk-heterojunction of P3HT with a fullerene derivative – PCBM increases the light absorption efficiency and the electrical conductivity of the bulk-heterojunction resulting in an improvement of the cell 382 performance from 3.30% to 3.65% . Mixed kesterite CZTSSe NCs can be used instead of the conjugated polymers in a bulkheterojunction with PCBM acting as a sole light absorber, but 257 the cell efficiency is low (0.01%) in this configuration . The light conversion efficiency of 9.51% was reported for “inverted” solar cells based on the bulk-heterojunction of PTB7 conjugated polymer with PCBM with an additional buffer single layer of 7-nm CIS NCs placed between a ZnO ETL and the 532 light-harvesting bulk-heterojunction . The CIS buffer layer plays multiple roles including the additional light absorption, the reduction of the roughness of ZnO/ bulk-heterojunction interface as well as the acceleration of electron transport 532 between the junction and ETL . The configuration and performance of the TiO2/CIS-composite based bulk-heterojunction solar cells depends considerably on 90,438 the morphology of the titania scaffold . In the case of a dense TiO2 layer the Zn-doped CIS NCs form a separate layer on top of the titania scaffold (Fig. 28a, b) and thus the junction forms only on the interface between the edge of titania and the closest layer of CIS/CIZS NCs, while other sensitizer NCs have no direct contact with the titania. Such cell demonstrated 90 a meager PCE of 0.30% . In the case of mesoporous titania scaffolds, the CIZS NCs penetrate the entire volume of the TiO2

Fig. 28. Cross-sectional SEM (a), element distribution maps (b,c), current-voltage characteristics (d), and EQE spectra in the Urbach coordinates (e) for flat-bilayer and porous TiO2/CIS composites. Reprinted with permissions from ref. 90. Copyright (2016) The Royal Society of Chemistry.

The charge transfer possibility between CIS and ZnO NCs coupled to the light scattering capabilities of anisotropicallyshaped zinc oxide nanoformations was quite elegantly used to enhance the performance of conventional silicon solar cells408. The ZnO nanopyramids attached to the silicon surface decreases the light reflectance to 10–15% as compared to 25– 40% of the bare Si, thus enhancing the cell performance from 4.5% to 6.6%. In the case of CIS NC-capped ZnO nanopyramids, the cell performance can further be impressively increased to 10% at the expence of the light-harvesting by CIS NCs and the charge transfers from CIS NCs to ZnO to silicon408. The comparison between compact and porous TiO2-based cells90 is an excellent illustration of another special feature of the light-harvesting systems based on multinary MC NCs differing quite drastically from the “conventional” binary MC light harvesters, such as cadmium and lead chalcogenide NCs. As discussed in section 1, the absorption band edge of CIS and AIS NCs is dominated by the absorbance on the subbandgapstates – lattice defects, vacancies and other irregularities abundant in multinary NCs. It was found that such subbandgap states can take active participation in the light conversion and thus the EQE spectra also show distinct absorption “tails” below the bandgap that can be linearized in the Urbach equation coordinates “ln(EQE) – (hv–Eg)” (Fig. 28f). Quite surprisingly, the characteristic Urbach energy EU being proportional to the density and distriburtion width of the midbandgap states is almost twice lower for the solar cells based on the porous TiO2/CIS layers as compared with the compact bilayer analog.

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Journal Name

Table 3. Photovoltaic parameters of some solid-state solar cells based on A–BIV–X NC

MC

CZTS

In:CZTS CZTS+CAIS Cu2NiSnS4 Cu2BaSnS4 CZTSe

CZTSSe

Ag:CZTSSe Cd:CZTSSe CZTGS

Jsc, mA/cm2 1.06 4.91 7.44 10.69 8.91 13.9 12.58 8.1 11.34 30.28 13.97 23.28 10.67 21.5 18.55 17.6 18.94 31.5 20.7 20.0 5.87 0.52 4.37 31.6 6.83 29.1 23.9 29.6 22.7 30.4 32.9 31.2 30.9 35.19 29.66 21.5

Cell composition

ITO/TO2/MEH-PPV/CZTS/PEDOT:PSS/Au ITO/PEDOT:PSS/P3HT-PCBM/CZTS/LiF/Al Mo/CZTS/CdS/IZO/ITO/Ni-Al ITO/PEDOT:PSS/CZTS/CdS/ZnO/Al Al foil/Mo/CZTS/ZnS/IZO/ITO/Al-Ni FTO/TiO2/CdS/CZTS/spiro-OMeTAD/Au Glass/Mo/CZTS/CdS/IZO/AZO/Al Glass/Mo/CZTS/CdS/IZO/AZO/Al Glass/Mo/CZTS/CdS/IZO/AZO/Ag Glass/Mo/CZTS/CdS/ZnO/Ni-Al FTO/ZnO/ZnO NRA/CdS/CZTS/MoO3/Au Mo/CZTS/CdS/IZO/ITO/Ni-Al ITO/ZnO/P3HT-CZTS-PCBM/MoOx/Ag Glass/Mo/CZTSSe/CdS/IZO/ITO Glass/Mo/CZTS/CdS/iZO/AZO/Ag Glass/Mo/MoS2/CZTS/CdS/ZnO/ITO/MgF Glass-Mo/CZTS/CdS/IZO/ITO Glass/Mo/CZTSSe/CdS/IZO/AZO/A ITO/CZTS/CH3NH3PbI3/PCBM/Ag Glass/Mo/CZTS/In2S3/ZnO/ITO/Al ITO/CZTS/CAIS/Ca/Al Glass/Mo/CNTS/CdS/AZO FTO/CBTS/CdS/ZnO/AZO Mo/CZTSe/CdS/IZO/AZO/Al Glass/ITO/TiO2/CdS/CZTSSe/Au Glass/Mo/CZTSSe/CdS/IZO/ITO Glass/Mo/CZTSSe/CdS/IZO/AZO Glass/Mo/CZTSSe/CdS/IZO/ITO/Ni–Al Glass/Mo/CZTSSe/CdS/IZO/ITO/Ag Glass/Mo/CZTSSe/CdS/ZnO/Ni-Al Glass/Mo/CZTSSe/CdS/ZnO/ITO/Al Mo/CZTSSe/CdS/ZnO/ITO/Ni-Al Mo/CZTSSe/CdS/ZnO/ITO Ag/ITO/i-ZnO/CdS/ CAZTSSe/Mo/glass glass/Mo/Cd-CZCTSSe/CdS/iZO/ITO/Ag Mo/CZGSSe/CdS/iZO/ITO/Ni-Al

Voc, mV 650 520 493 420 484 312 471 450 460 272 500 431 610 532 513 616 556 448 920 621 460 424 522 423 363 360 478 360 424 420 410 401 460 448 460 640

FF

0.37 0.369 0.452 0.385 0.451 0.462 0.41 0.70 0.44 0.352 0.484 0.356 0.55 0.428 0.517 0.479 0.567 0.644 0.81 0.545 0.417 0.43 0.49 0.457 0.447 0.428 0.45 0.507 0.622 0.527 0.569 0.628 0.60 0.657 0.604 0.49

η, % 0.26 0.95 1.66 1.73 1.94 2.0 2.44 2.5 2.29 2.89 3.34 3.6 3.65 4.9 4.92 5.2 6.0 9.08 15.4 6.9 1.126 0.09 1.60 6.11 1.11 4.48 5.14 5.4 6.0 7.23 7.68 7.9 8.6 10.36 8.11 6.8

Ref.

86 373 195 87 330 301 253 329 250 210 245 249 382 313 477 484 266 307 291 527 288 271 528 502 262 473 310 305 308 260 289 243 304 529 530 274

Notes: the accuracy of values is preserved as reported in the corresponding references. 90

This phenomenon was explained by a partial filling of the mid-bandgap states near VB of CIS NCs with electrons coming from titania. It is well known that n-conducting TiO2 has an excess of electrons that can cause, for example, “dark” reductive processes even without the photoexcitation 533 events . The partial filling of the mid-bandgap states in porous TiO2/CIS heterostructures lowers the energy barrier for the electron extraction and can be the reason for the enhanced PCE in such devices as compared with the compact 90 architecture . The photovoltaic devices produced via the spray-coating deposition of CISe NCs (the cell configuration is Au/CISe/CdS/ZnO/ITO) showed a large and size-dependent Voc reaching around 850 meV for the NC with Eg = 1.46 eV, that is, 65–70% of the open-circuit voltage theoretically possible for 224 such bandgap . It is claimed to be the highest Voc reported for the NC-based solar cells exceeding by far the values reported for CdSe and PbS NC-based solar cells. Contrary to the above-discussed TiO2/NC-based architectures, the large

size-dependent Voc is considered to show that the midbandgap 224 states of CISe NCs do not dominate the cell performance . It can be speculated from these contradictory results, that the surface chemistry of MC NCs may play a crucial role in the charge transfer dynamics thus allowing for or excluding the possibility of the participation of the NC midbandgap states in the photocurrent generation. The CIS NCs incorporated into the solid-state solar cells with ZnO NCs as a charge extraction layer (the cell configuration is ITO/PEDOT:PSS/CIS/ZnO/Al) reveal a “light soaking effect” when the cell efficiency increases under the illumination (Fig. 534 29a) . The effect was found to be considerably enhanced with the UV light being added to the illumination spectrum indicating the involvement of ZnO NCs. Moreover, the effect is reversible indicating a dynamic character of the photoinduced changes. A similar, but much more modest light-soaking enhancement effect was observed for the CZTSe-based solidstate solar cells where the PCE increased during the 260 illumination from 6.7% to 7.2% . In contrast, for the above-

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discussed ZnO/CIS heterostructure-based solar cell the lightsoaking-effect-induced enhancement of the light conversion efficiency is tremendous as the PCE increases during the illumination from around 2 to 8% (Fig. 29b) indicating that the photoinduced changes affect mostly the wide-bandgap ZnO scaffold. A detailed analysis of possible losses in a CIS/ZnO 534 heterojunction revealed two major loss sources: (i) a barrier for the charge extraction at the CIS/ZnO interface lowered by the light-soaking effect and (ii) the recombination of the charge carriers in CIS NCs via the trap states. The latter is 534 unaffected by the incoming light and needs to be addressed in other ways, in particular by adjusting the NC synthesis conditions and NC surface passivation. A typical design of kesterite-based solar solid-state cells195,249,250,304,308,310,502 includes a semi-transparent molybdenium layer sputtered on the soda lime glass, on top of which a kesterite (CZTS, CZTSSe, CZTSe) micro-layer is formed by the deposition of various precursors followed by sulfidation/ selenidation at 400–500 °C. Then, a thin (30–50 nm) buffer CdS layer is typically deposited by the chemical bath deposition, followed by the deposition/sputtering of a layer of intrinsically doped ZnO NCs (IZO) and, optionally, an AZO layer. The final top layer of the solar cell is made of ITO NCs covered by the vapor-deposited/sputtered metal (Au, Ag, Ni-Al, etc.) layer.

The bulky insulating ligands can be efficiently removed from the surface of CZTS NCs by reacting them with strong alkylating agents, such as methyl iodide or the Meerwein’s agent (C2H5)3OBF4. The treatment converts primary amines (OLA, dodecylamine, etc.) into the corresponding quaternary amines that have a much lower affinity to the NC surface while 179 not affecting the crystalline structure of the CZTS NCs . 271 528 Cu2NiSnS4 NCs and Cu2BaSnS4 NCs were reported as lightharvesting layers of solid-state solar cells with a general configuration Mo/MC/CdS/IZO/AZO. The nanocrystalline CZTS layer with a thickness of around 400 nm deposited by the pulsed laser deposition were applied as a light-harvesting component of the solid-state cells with a PCE of 5.2%484. A similar cell with CZTS NCs synthesized by the liquid-phase pulsed laser/electrophoretic deposition revealed a PCE of 4.77%483. Solar cells produced from the NC inks. One of the most attractive and promising directions in the solid-state photovoltaics based on the multinary MC NCs is the ink-jet printing of the cells on various substrates (both rigid and flexible) by using pre-formed NC inks. The method allows for unprecendented flexibility in the variation of the ink composition and rheology, substrate nature as well as the conditions of the post-printing annealing in sulfur- or selenium-rich atmosphere required for the formation of compact and high-quality MC absorber layers.

Fig. 29. Current – voltage characteristics (a) and EQE spectra (b) of CIS/ZnO composite based solid-state solar cells subjected to the illumination with a mixture of “white” light (WL) and UV light. Insert in (b): PCE as a function of the light soaking duration. Reprinted from ref. 534 – Published by the PCCP Owner Societies.

The kesterite Cu2MSnS4 (M = Fe, Co, Ni) NCs deposited onto the ZnS-coated ZnO nanorod array (Fig. 30a) act as spectral sensitizers enabling the entire visible light harvesting in simple solid-state ITO/ZnO/ZnS/kesterite/Au solar cells with an efficiency of 2.73%, 3.23%, and 2.71% for Fe-, Co-, and Ni300 based NCs, respectively . The CB levels of the cell components are properly aligned to induce the cascaded photoelectron transfer from the kesterite NCs to the intermediate ZnS layer, further to the ZnO scaffold and finally 300 to ITO (Fig. 30b) . The kesterite CZTS NCs deposited onto the titania NR arrays from water/ethanol suspensions and coupled with a CdS buffer layer and a spiro-OMeTAD HTL exhibited 2% efficiency 301 of light conversion . The efficiency of electron transfers in the bulk-heterojunction of CZTS and inorganic oxide semiconductors expectedly depends on the NC ligand nature. By 2– exchanging bulky OLA ligands with S , the light conversion efficiency of completely inorganic ZnO/CdS/CZTS/MoO3/Au cell 245 was increased from 1.83% to 3.34% .

Fig. 30. (a) SEM of CFTS NC-coated ZnO/ZnS NR array. (b) Energy diagram of a CFTS NCbased solar cell. (c) Cross-sectional SEMs of a glass/Mo/CZTS NC layer at different stages of the selenidation. The scale bar in (c) is 500 nm. Figures denote thicknesses of the layers. Reprinted with permissions from ref. 300 (a, b) and ref. 289 (c). Copyright (2016) The Royal Society of Chemistry (a, b); (2014) American Chemical Society (c).

The inks composed of amorphous CIS NC with copper and indium monoethanol amine complexes were used to form CISe-based solar cells with a 10.85% light conversion 114 efficiency . The selenidation of CIS NC inks for the cell formation was recently established as a reproducible method 214 of thin-film solar cells production . The solid-state solar cells with CIGS absorber layers were successfully formed by the ink-jet printing of CIGS NCs 525,535 followed by the rapid annealing in selenium vapors . The film uniformity and the performance depend crucially on the

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ink formulation requiring a combination of a low-boiling-point (hexane thiol) and a high-boiling-point (dodecane thiol) solvent in a specific proportion providing a trade-off between the 525 viscosty and volatility of the CIGS NC inks . By using three different CIGS inks with a varied In/Ga ratio a compositiongraded absorber layer can be easily formed by the consecutive 523 jet-printing procedures . The graded Mo/CIGS/CdS/IZO/nZnO/Ni/Al solar cell showed almost twice higher efficiency 523 (PCE of 7.29%) as a non-graded analog . The doctor blade deposition of a layer of CIGS NC inks in alkane thiols followed by the selenidation was applied to 227 produce CIGSSe absorber layers . The phase of the nanocrystalline inks can be varied from chalcopyrite to sphalerite. Also, NaCl additions can be introduced into the NC inks resulting in a uniform distribution of Na+ ions in the final CIGS films. The presence of Na+ positively affects the CIGSSe grains crystallization producing larger and more densely packed absorber grains and increasing the cell performance from 7.7% up to 12.0%227. In a similar way, CZTSSe absorber thin films were produced by the doctor blade deposition of CZTS NC inks and the annealing in the presence of selenium243,306. The incorporated Na+ ions were assumed not only to affect the CZTS crystallization kinetics but also to passivate grain surface defects and enhance the charge carrier concentration thus resulting in around 50% enhancement of the solar cell performance247. A typical approach for the formation of light-harvesting kesterite layers of the solid-state solar cells is the annealing of CZTS NC inks in sulfur250,253,266 or selenium260,289,304,305,307,313 atmosphere at 400–500 °C. The sulfidation/selenidation results in the formation of compact micron grains with the size d increasing with the annealing time and temperature253,305. In the case of CZTSe-based cells the highest PCE of around 5% was observed in intermediate ranges of both anealing duration (20 min, d = 0.2–0.5 μm) and T (500 °C, d = 0.5–1.0 μm)305. Uniform precursor CZTS NC films on Mo-coated glass (Fig. 30c, fragment 1) were produced by the layer-by-layer deposition with the annealing at 350 °C after each consecutive spin coating of the NC ink solution289. As the selenidation starts, the compact CTZSe layer forms with the front between new CZTSe phase and the precursor CTZS NC phase moving from the outer NC film surface to the Mo layer (Fig. 30c, fragments 2,3). This movement resulted in the formation of a dense 550–600-nmthick CZTSe layer over the residual nanocrystalline Cu- and Serich CZTS layer (Fig. 30c, fragment 4) that resists to further selenidation. In this way, the composition and morphology of the absorber layer can easily be controlled by varying the Se treatment duration. The annealing of the CZTS wurtzite NC ink precursor in a mixed sulfur/SnS vapor atmosphere yielded an absorbing layer with 6.0% efficiency of light conversion266. “Green” aqueous CZTS NC inks were developed by J. Tang et al.310 that can easily be deposited onto Mo-glass substrates and converted into the compact CZTSSe layers by annealing at 400 °C in the presence of a mixture of elemental S and Se. The composition of the final absorber layers depends on the molar S/Se ratio during the annealing. The highest PCE of 5.15% at

Voc = 478 mV and a FF of 45% (Fig. 31a) was achieved for CZTSSe layers with a bandgap of 1.22 eV, intermediate 310 between the pure CZTS and CZTSe phases . The cell can harvest the visible and NIR light to ~1020 nm (Fig. 31b). The colloidal CZTS NCs produced in water/ethanol mixtures were used as inks sprayed nonpyrolytically onto Mo-glass substrates and then annealed either in N2 or in the Se-saturated atmosphere producing compact CZTS and CZTSSe layers with a PCE 304 of 5.0% and 8.6%, respectively . The ITO/TiO2/CdS/CZTSSe/Au cells produced from the ethoxy cellulose/CZTSe NC inks showed a higher uniformity of the absorber layer and light conversion efficiency than similar devices formed from CZTSe NC dispersion in ethanol262. The ethanol suspension of CZTS NC ink was designed for the rollto-roll printing of NC layers on the flexible Mo-coated Al foil330. After consecutive procedure of the annealing with sulfur vapour, the chemical bath deposition of a CdS layer and the sputtering of IZO, ITO, and Ni-Al layers, a flexible solar cell with a PCE of 1.94% was produced (Fig. 31c)330. All of the cell components can be printed opening perspectives for the industrial production of such devices. The uniformity and crystallinity of CZTS thin films (and therefore their light-harvesting capability) can be enhanced considerably by combining a NC ink with a molecular precursor ink, as discussed earlier for CIS NCs, the latter ink containing acetates/chlorides of copper, zinc and tin in methoxy ethanol477. The sulfidation of the films produced by the consecutive spin-coating of 10 layers of the NC ink with 5 layers of the molecular ink was found to produce the most active light-harvesting layers surpassing by the activity (4.92%) both the similar films formed exclusively from the NC inks (2.10%) and from the molecular inks (2.80%)477.

Fig. 31. (a, b) Current–voltage characteristic (a) and EQE spectrum (b) of the CZTSebased cell produced from the aqueous CZTS NC inks. Insert in (b): EQE spectrum presented in the Tauc coordinates to assess the bang gap of the light absorber layer. (c) Cross-sectional SEM and photographs of flexible CZTS-based cell constructed on the Al foil. (d) Atomic composition of In-difused CZTS/In2S3 film as shown by the Auger spectroscopic analysis (the etching rate was ~4 nm×min–1). Reprinted with permissions from ref. 310 (a, b), ref. 330 (c), and ref. 527 (d). Copyright (2014) American Chemical Society (a, b, d); (2012) The Royal Society of Chemistry (c).

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By combining two NC inks containing CZTS NCs and SnS/Ag2S NCs, the Ag-doped CZTSe films were formed by the rapid 536 + selenidation at 500 °C . The introduction of Ag dopant resulted in accelerated growth of the kesterite grains, a higher charge carrier density and life time as well as the suppressed sub-bandgap absorption indicating reduced formation of 529,536 defects . The light conversion efficiency of CZTSSe absorber layers produced by the partial selenization of CZTS NC inks can be enhanced considerably by the post-synthesis etching treatments aimed at the elimination of impurity CuxS(Se) phases. The procedure is typically performed by using KCN etching agent but can be made “greener” and safer by applying a combination of ethylene diamine and 2-mercaptoethanol which selectively dissolves copper chalcogenide phases while etches only slightly the kesterite and wurtzite 537 CZTS NCs . The inks composed of separated sulfide NCs (CuS, SnS, ZnS, Cu7S4, Cu2SnS3, etc.) were also applied as precursors for the light-harvesting kesterite layers produced by sulfidation/ 308 selenidation under annealing conditions . The selenidation of binary NC mixture yields compact alloyed CZTSSe layers with a bandgap of around 1.1 eV corresponding to the light sensitivity 308 threshold of around 1100 nm . The annealing of a layer of spin-coated OLA-capped CZTS NC inks in sulfur or selenium atmosphere yields CZTS/CZTSSe lightabsorbing layers containing the residuals of the ligand pyrolysis, such as graphitic carbon and thiophene/selenophene 244 moities that enhance the electrical conductivity of the films . A combination of the doctor blading of a NC ink and the selenidation was applied to form multinary Cu–Zn–Sn–Ge–S– Se light-harvesting layers with the bandgap determined by the 274 Sn/Ge ratio in the starting Cu2Zn(Sn1–xGex)S4 NCs . By preparing the NC inks with a deficiency of Cu and an excess of Zn at Ge/(Ge+Sn) = 0.7, the composition-gradient (in terms of the Ge distribution) CZTGSSe films were produced showing a much higher PCE of 6.8% as compared to 0.51% for the 274 stoichiometric absorber . The reported efficiencies for the kesterite solar cells produced from the NC inks vary from 195,249,250,253,266,329,527 1.66% to 6.0% for CZTS NCs , from 4.48% to 289,307,310,473 9.08% for the alloyed CZTSSe NCs , and reach 6–7% 260,502 for for CZTSe NC-based cells . Alternatively to the jet-printing, the inks can be sprayed upon the substrates followed by the sulfidation/selenidation 340 256,473 process. In this way the CIS NCand CZTS NC-based solid-state solar cells were fabricated. The quality of the CZTS coating was found to depend on the size of CZTS NCs, being 256 optimal for the 15 nm NCs but deteriorating for larger NCs . Other techniques were also applied for the CZTS NC inks 329 deposition, including radio frequency magnetron sputtering . The electrodeposition coupled to different post-deposition thermal treatments was found to be quite efficient, scalable and reproducible method for the formation of light-harvesting kesterite layers. For example, the electrodeposited CZTS films with a top In2S3 barrier layer subjected to the post-synthesis 527 heat treatment showed a PCE of 6.9% . The treatement was found to induce the In diffusion into the CZTS layer. The

surface of the In-doped CZTS sample was etched for different times and the atomic composition was determined by the Auger photoelectron spectroscopy thus providing a “depth” profile of the In distribution (Fig. 31d). The In incorporation results in a narrowing of the CZTS bandgap from 1.47 eV to 1.40 eV, thus enhancing the photoresponse and PCE of the cells. The heat treatment of the CZTS film without the In2S3 layer does not affect the PCE clearly indicating the role of the In diffusion for the cell performance. The In atoms filling the voids between the CZTS grains allow for more efficient charge transport increasing the charge density of the p-conductive 527 CZTS layer . Two types of multinary MC NCs differing in the conductivity type – p-conducting CZTS NCs and n-conducting copperdiffused AIS NCs were coupled to form a p/n junction with the CB/VB levels allowing for the efficient separation of the 288 photogenerated charge carriers . A depletion region forming on the p/n junction creates an electric field favoring to the drift of the minority carierrs across the junction. The CZTS and CAIS layer thickness can be adjusted to match the exact length of the depletion region, which can be determined from the 288 capacitance-voltage characteristics of the junction . In this way, the undepleted parts of the junction that can contribute to the internal resistance are eliminated. Along with the main light-harvesting function, the multinary MC NCs were found to be capable of charge transport and, therefore, can combine the charge photogeneration and charge transport functions, thus allowing to exclude an additional organic hole transporting layers. The ZnS-coated CIS NCs were applied to form a hole transporting layer on top of the hybrid CH3NH3PbI3 perovskite 213 (Fig. 32a) . The mutual positions of CB/VB layers of TiO2, perovskite and CIS NCs favor to the electron extraction into TiO2 and the hole extraction into the CIS layer (Fig. 32b) allowing to achieve a PCE of 8.38% without additional organic hole transporting materials. 280 490 The p-conducting kesterite CZTS and CBTS NCs are promising HTL materials for the CH3NH3PbI3 (MAPbI3)-based solar cells. The CBTS NCs combine a high chemical stability with a 2 –1 –1 high carrier mobility of around 10 cm ×V ×s . For the comparison, the carrier mobilities of two typical HTLs – organic –5 Spiro-OMeTAD and inorganic CuSCN are 4×10 and 0.01–0.1 2 –1 –1 490 cm ×V ×s , respectively . The insertion of a 100-nm-thick CBTS layer between FTO and the light-harvesting perovskite layer (Fig. 32c) allows to reach a PCE of ~10%. The hole transporting properties were reported for Cu2CdSnS4 NCs introduced as a HTL in a P3HT/PCBM bulk-heterojunction 299 solar cell . This nanocrystalline HTL enhances the cell performance from 3.08% to 3.63%. The kesterite CZTS nanospheres introduced as a buffer layer into the bulk-heterojunction solar cells with mixed P3HT/PCBM absorbing layers can act both as an additional light absorber and as an ETL that accepts the photogenerated charge carriers from P3HT/PCBM heterojunction and relays them to the metal 373 electrode (Fig. 32d) . Although the overall light conversion efficiency of the ITO/PEDOT:PSS/P3HT–PCBM/CZTS/LiF/Al cell is rather small ( 0.95%) it is almost 8 times higher than PCE of a

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similar cell produced without NCs (0.12%) reflecting a high potential of kesterite NCs in the organic bulk-heterojunction cells. One of the current trends in MC photovoltaics is to trade-off the cost of the cell components against the absorber efficiency. It is assumed that the utilization of an abundant, low-toxic and low-cost absorber materials, such as the abovediscussed kesterite can levell off a comparatively low light conversion efficiency. With this aim, new compositions and phases are constantly probed as light-harvesting components of the solid-state solar cells. In particular, ternary CTS NCs are considered as a very affordable alternative to the conventional 236,298 MC absorbers . The cells produced from CTS NC inks subjected to additional annealing in the sulfur vapor revealed a reasonable PCE of 2.77% and a high stability to degradation during a 50-day testing in ambient air conditions236. The composition-tunable CGSSe NCs were also introduced into the solid-state solar cells operating with a 0.2% efficiency230. Alternatively, CGS NCs can be used as a hole transporting layer for the perovskite-based bulk-heterojunction solar cells538.

Fig. 32. (a, b) Schematic layout (a) and energy diagram (b) of the TiO2/MAPbI3/CIS/Au solar cell. (c) Cross-sectional SEMs of a FTO/CBTS/MaPbI3/PCBM solar cell. (d) Energy diagram of a P3HT/PCBM-based cell with a CZTS transporting layer. Reprinted with permissions from ref. 213 (a, b), ref. 490 (c), and ref. 373 (d). Copyright (2014, 2015) American Chemical Society (a–c); (2017) The Royal Society of Chemistry (d).

A CuSbS2 NC-based thin-film solar cell prepared by the spincoating of the NC ink precursor exhibited a PCE of 0.01%290. Alternatively, CuSbS2 NCs can be grown on titania by the atomic layer deposition494. Despite low efficiencies achieved to date, the copper antimony sulfide materials attract growing attention due to a bandgap of around 1.5 eV that matches perfectly the solar spectrum and high absorption coefficients (104 – 105 cm–1). Such properties, along with a broad abundancy of constituents and a lost raw material and production costs494–496 allow to anticipate a progress with this light absorber in the near future. Despite a considerable difference in the lattice type as compared to chalcopyrite and kesterite NCs, the chalcostybite CuSbS2 also reveals a tendency to the non-stoichiometry, in particular, showing a tunable VB 16 18 –3 hole density of 10 – 10 cm depending on the Sb2S3 excess 495 during the synthesis . Recently, a CuSbS2 NC-based solar cell with a PCE of 0.6% produced from a bilayer precursor Cu/Sb2S3

496

composite was reported . The cell composed of ZnO/CuSbS2 and Pt electrodes connected by the solid P3HT electrolyte 448 exhibited even higher PCE of 1.61% . Both mentioned results show a distinct and fast progress with this light harvester material. Similarly, the preliminary studies of the nanocrysta365 539 line Cu4Bi4S9 (Eg = 1.14 eV) and Cu3BiS3 (Eg = 1.47 eV) attested to a high potential of these semiconductors for the solid-state photovoltaics. 3.2 Liquid-junction photoelectrochemical solar cells based on multinary MC NCs The heterostructures of MC NCs with the wide-bandgap semiconductors like TiO2 and ZnO are typically formed either by the ex situ and in situ methods. Table 5 summarizes the light conversion efficiencies of some of the reported liquidjunction NC-sensitized solar cells. In the ex situ option, the mesoporous wide-bandgap oxide is decorated by separately prepared MC NCs stabilized by the surface ligands capable of covalent of electrostatic binding to the oxide surface, most often TGA and MPA. Sodium or potassium salts of the acids are typically used for the phase transfer of hydrophobic MC NCs from organic solvent into polar media (water, DMF, etc.) or used for the direct synthesis of MC NCs in aqueous solutions, as discussed in section 2. The sulfur atom of –SH group of the acid binds to a cationic site on the surface of NCs while ionized carboxyl group remains unbound and available for further interaction with other species present in the system. It can readily attach to the positively charged sites on the surface of titanium or zinc oxides thus enabling strong interaction between the oxide surface and the sensitizer NCs while the electrostatic repulsion between adjacent negatively-charged NCs favors the uniform coverage of the wide-bandgap semiconductor surface. As the TGA anion is prone to oxidation by the air oxygen to disulfides as well as to the hydrolysis, MPA is a more often used option. An alkyl chain longer than –(CH2)2– hinders strongly the electron transfer from the photoexcited MC NCs and the oxide surface and so the choice of available mercaptocarboxylic “linker” molecules is typically confined either to a less-stable but shorter TGA or to a longer but more stable MPA. This method was applied to produce titania heterostructures with CIS NCs125,540, CIZS NCs541, ZAISe NCs177, etc. The TiO2/CIS NC photoanode produced from the MPA-capped CIS NCs and combined into a liquid-junction cell with a CuxS counter electrode (CE) and polysulfide electrolyte showed a certified PCE of 6.66% with a champion device reaching 7.04% efficiency125. Alternatively, the titania scaffolds can be modified by bifunctional molecules, like TGA that adsorbs via the carboxylic groups and can serve as a binder for the sensitizer NCs when the TiO2 film is immersed into the NC colloid. This way was applied to prepare TiO2/CIS64,163,542 and TiO2/AIS 133 heterostructures . The PCE of TiO2/CIS photoanodes prepared from the ex situ synthesized cysteine-capped CuInS2 NCs can be enhanced quite drastically by using thiols – co-adsorbents on the stage of 470 the soaking TiO2 films in the NC solution . By adding GSH or

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TGA, the PCE of resulting TiO2/CIS photoanode can be increased from 0.29% (no additives) to 2.37% for GSH and to 4.20% for TGA. A PL quenching study showed that the rate constant of electron transfer from CIS NPs to TiO2 is the 10 –1 highest for the cysteine linker, 9.5×10 s , decreasing to 10 –1 470 7.1×10 s for a more bulky GSH . In the similar AIS NCbased system the PCE increases from 0.19% to 2.72% in the presence of TGA indicating a general character of the additive 470 influence . The effect was attributed to the inhibition of the cysteine oxidation to disulfides on the NC surface resulting in an increased NC loading and enhanced electron transfer from NCs to TiO2 (accounting for a Jsc increase) as well as in a shift of the titania band edges causing an increase in Voc 470. A photoanode can be prepared by soaking with a concentrated NC solution followed by removal of excessive NCs on the stage of washing. In such approach the colloid concentration should be carefully adjusted to avoid a too high loading of the sensitizer NCs that can block the mesopores of TiO2 (ZnO) and hinder the electron transport from outer layers of the photoanode toward the OTE. In this way, the titania electrodes were decorated with the MPA-capped CIS NCs522, CISe and CIZSe NCs91,279. The incorporation of Zn ions into the lattice of CISe induces an upward shift of the CB level (Fig. 33a) favoring to a more efficient transfer of the photogenerated CB electron to the titania scaffold. As a result, the electron transfer rate constant increases from 2.4×1010 s–1 for CISe NCs to 9.1×1010 s–1 for CIZSe NCs, while the PCE of corresponding cells grows from 9.54% (CISe) to 11.66% (CIZSe). An even more drastic enhancement of the photoinduced electron transfer rate induced by Zn2+ incorporation was recently reported for TiO2/CIS nanostructures – from ~0.3×1010 s–1 for pristine CIS NCs to ~3×1010 s–1 for Zn-doped CIS NCs541. The partial substitution of In with Ga in CISe results in a considerable increase of the NC CB level – from –3.88 eV to –3.63 eV versus the vacuum level for the 50 mol.% gallium fraction93. As a result of the increase of the driving force of the photoinduced electron transfer from CIGSe NCs to the TiO2 scaffold, the liquid-junction solar cells with the TiO2/CIGSe photoanode, and CuxS or Ti/carbon CEs showed unprecedentedly high PCEs of up to 11.49% under the AM1.5G illumination versus 9.46% for the undoped CISe NCs93.

Fig. 33. (a) Energy diagram of TiO2/CISe and TiO2/CIZSe nanocomposites. (b) Absorption spectra of TiO2/(ZnSe)1–x(AISe)x photoanodes with a different x (given on the figure). (c) Absolute positions of CB and VB levels of the composition-selected ZAISe NCs relatively to the CB level of TiO2. Reprinted with permissions from ref. 91 (a) and ref. 89 (b, c). Copyright (2014, 2016) American Chemical Society.

charge by the electrophoretic deposition, when a relatively –1 strong electric field (more than 200 V×cm ) is created between the oxide electrode and a CE forcing the NCs to adhere to the oxide surface. For example, the method was 92,131,522 successfully applied to form TiO2/CIS photoanodes . Probably the most straightforward of the ex situ approaches is the direct synthesis of multinary MC NC in aqueous solutions resulting in TGA, MPA, or GSH-capped NCs that can be subsequently adsorbed or dropcasted onto the surface of metal oxide scaffolds. In this way TiO2/CIS photoanodes were 429,430 prepared by using aqueous TGA-capped CIS NCs . The TGA-stabilized CIS/ZnS 3–5 nm NCs synthesized directly in aqueous solutions50 penetrate uniformly the volume of mesoporous TiO2 films revealing a homogeneous composition of the resulting TiO2/CIS both along the cross section of the films (Fig. 34a, b) and across the outer film surface (Fig. 34c). The stability and photoactivity of CIS NCs both increase upon the deposition of a thin ZnS shell on the CIS NC surface. The TiO2/CIS/ZnS composites act as visible-light-sensitive photoanodes in the SSSCs with polysulfide electrolyte and copper sulfide-based CEs with a total conversion efficiency of around 8%50. The 5-nm ZAIS NCs were tested as a spectral sensitizer for the ZnO scaffolds manifesting a PCE of 0.72% in the cells with a Pt CE and iodide/triiodide redox-couple150. The liquid-junction cells with the I–/I3– shuttle were also built on ZnO NR photoanodes sensitized with AIS and ZAIS NCs152. The TiO2/AIS photoanodes produced by soaking titania with the ligand-free AIS exhibited a PCE of 0.8% in a cell with the polysulfide electrolyte and a gold CE53. The solid-solution ZAISe NCs immobilized on porous titania scaffolds modified with MPA act as a spectral sensitizer extending the light sensitivity range of the photoelectrode into the visible range to 900 nm89. The CB and VB energies of the sensitizer NCs grow continuously as the molar fraction of ZnS is increased (and the x value decreased), thus providing higher driving forces for both the photoinduced electron transfer to titania and the hole transfer to the electrolyte species (Fig. 33c). At the same time, the light absorption range of the sensitizer NCs is reduced for smaller x and thus an optimal ZAISe composition exists, corresponding to ZnSe:AISe = 1:1, at which the light harvesting efficiency η reaches the highest value of 1.9%. A balance between the CB/VB levels and the light-harvesting range always exists in series of solar cells based on differentlysized MC NCs. As the NC size d is decreased in a critical range roughly corresponsding to d < 2aB, the CB and VB energies of MC NCs increase as a result of the quantum confinement and, therefore, the driving force of the interfacial electron/hole transfer to other cell components increases as well. However, a concomitant size-dependent broadening of the bandgap reduces the visible-light-harvesting capacity of the NCs similarly as it happens in the above-discussed case of the solidsolutions of a broad-bandgap and a narrow-bandgap MC semiconductors.

The sensitizer NCs can be deposited onto the surface of oxide scaffolds regardless of their ligand shell nature and surface

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Table 5. Photovoltaic parameters of some MC NC-sensitized liquid-junction solar cells

MC

CIS

CIS/CdS CIS/CdSe CISe CISSe CIZSe CIZSSe CIGSe AIS

ZAIS

ZAISe ZAISe CdIn2S4 ZnIn2S4 AgSbS2 CuBiS2 AgBiS2 CZTS

ZnSe/CZTS

Cell configuration and preparation details PA: TiO2 NTA/CdS+CIS, SILAR, CE/RC: Pt/Na2S PA: TiO2/CIS/CdS, dip-coating, CE/RC: Carbon/Sx2– PA: TiO2/CIS/ZnS, dip coating+SILAR, CE/RC: CuxS/Sx2– PA: TiO2/CIS/ZnS, ED/SILAR, CE/RC: CuxS-RGO/Sx2– PA: TiO2, ED of CIS NCs (~3 nm), CE/RC: RGO-CuxS/agarose gel with Sx2– PA: TiO2/CIS/CdS, ED/SILAR, CE/RC: CuxS-RGO/Sx2– PA: TiO2/CIS/In2S3/ZnS, dip-coating/SILAR, CE/RC: Carbon/Sx2– PA: TiO2/CIS/CdS, dip-coating/SILAR, CE/RC: PEDOT-Carbon/S2– PA: TiO2/CIS/ZnS, dip-coating, CE/RC: CuxS/Sx2– PA: TiO2/CIS/CdS/ZnS, dip coating/SILAR, CE/RC: CuxS/Sx2– PA: TiO2/CIS/CdSe/ZnSe, SILAR, CE/RC: CuxS/Sx2– PA: TiO2/CISe/Mn-CdS, dip-coating/SILAR, CE/RC: CuxS/Sx2– PA: TiO2/CISe/ZnS, soaking with NC colloid/SILAR, CE/RC: CuxS/Sx2– PA: TiO2/CISSe/CdSSe, dip-coating, CE/RC: CuxS/Sx2– PA: TiO2/CIZSe/ZnS, drop casting+SILAR of ZnS, CE/RC: Ti-Carbon/Sx2– PA: TiO2/CIZSe/ZnS, drop casting+SILAR of ZnS, CE/RC: CuxS/Sx2– PC: NiO/CIZSSe, adsorption from colloid, CE/RC: CuxS/Sx2– PA: TiO2/CIGSe/ZnS/SiO, drop-casting, CE/RC: Carbon on Ti/Sx2– PA: TiO2/AIS/In2S3/ZnS, ion exchange/SILAR, CE/RC: Pt/Sx2– PA: TiO2, drop-casting of AIS NCs (size 5–6 nm) in FA, CE/RC: Au/Sx2– PA: TiO2/AIS/ZnS, adsorption from colloid, CE/RC: CuxS/Sx2– PA: ZnO NRs, ZAIS NC (~5 nm) adsorption from octyl amine, CE/RC: Pt/(I2/I–) PA: ZnO/ZAIS, dip-coating, CE/RC: Pt/(I2/I–) PA: TiO2/ZAISe/CdS, adsorption from colloid+SILAR of CdS, CE/RC: CuxS/Sx2– PA: TiO2, dip-coating with ZAISe NCs (size 3.2–3.5 nm), passivation with ZnS (8 SILARs), amorphous TiO2 and SiO2 layers; CE/RC: CuxS/Sx2– PA: FTO/CdIn2S4, CE/RC: Au/Sx2– PA: TiO2/ZnIn2S4, CE/RC: Au/Sx2– PA: TiO2/AgSbS2, SILAR, CE/RC: Au/Sx2– PA: TiO2/CuBiS2,CBD, CE/RC: CuxS/Sx2– PA: TiO2/AgBiS2, SILAR, CE/RC: Pt/Sx2– PA: TiO2/CZTS, drop casting, CE/RC: Pt/tetrabutyl ammonium perchlorate PA: TiO2/RGO-CZTS, adsorption from colloid, CE/RC: Pt/(I2/I–) PA: ZnO/ZnSe/CZTS, ion exchange + spin coating of CZTS, CE/RC: CuxS/Sx2–

Jsc, mA/cm2 5.18 8.12 7.72 10.10

Voc, mV 710 489 570 501

0.38 0.37 0.42 0.47

η, % 1.42 1.47 1.84 2.38

10.75

584

0.47

2.97

131

15.65 9.76 16.07 9.73 16.9 17.0 16.10 26.93 10.5 25.18 25.97 9.13 25.01 7.87 4.62 9.75

529 860 912 580 560 575 590 528 550 742 752 350 740 320 450 432

0.47 0.524 0.356 0.58 0.45 0.465 0.41 0.57 0.604 0.624 0.644 0.39 0.621 0.28 0.39 0.646

3.91 4.39 5.15 6.66 4.2 4.55 3.96 8.10 3.45 11.66 12.57 1.25 11.49 0.70 0.80 2.62

92 429 542 125 540 543 160 163 225 91 279 123 93 544 53 470

3.8

540

0.35

0.72

152

3.8

540

0.35

0.72

150

8.8

500

0.43

1.9

89

16.03

381

0.59

3.57

177

3.5 2.55 2.42 6.87 7.61

331 430 320 250 180

0.415 0.385 0.436 0.361 0.386

0.48 0.42 0.34 0.62 0.53

232 402 446 295 447

0.36

283

0.286

0.03

193

0.411

560

0.576

0.133

182

10.46

490

0.43

2.2

183

FF

Ref. 439 430 278 124

Notes: the accuracy of values is preserved as reported in the corresponding references; PA – photoanode, PC – photocathode; RC – redox couple, FA – formamide; ED – electrophoretic deposition; CBD – chemical bath deposition.

Fig. 34. Cross-sectional SEM images of FTO/TiO2/CIS/ZnS photoanode (a,b); elements distribution across the photoanode (b) and on the outer surface (c). Adapted and reprinted from ref. 50 – Published by The Royal Society of Chemistry.

As a result, there exists an optimal NC size at which the photoanodes exhibit the highest performance. For example, the largest photoresponse of TiO2/CISSe photoanodes was observed in the case of 4.2-nm sensitizer NCs, being smaller for 3.9-nm and 4.9-nm NCs (Fig. 35a)162.

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For a series of 2.9–5.3-nm CIS NCs deposited electrophoretically onto the porous TiO2 photoanodes the highest PCE and the broadest light harvesting range (Fig. 35b; 35c, blue bars) 124 were observed for the intermediate-size 4.3-nm NCs . The cell studies were coupled to the laser flash photolysis and PL investigations that showed such CIS NCs accumulating a higher charge under illumination and revealing a larger rate of the radiative recombination than the smaller and larger NC mates. These results show that the above-discussed intuitively simple balance between the light-harvesting capability and the sizedependent CB/VB levels of the sensitizer NCs is accompanied by a balance of the size-dependent electron-hole recombination. The charge accumulation properties of CIS NCs, and the latter balance can be decisive for the defect-rich and non-stoichiometric multinary CIS NCs124. The size-dependence of η for the TiO2/CIS photoanodes follows the size variation of PL emission intensity of original colloidal CIS NCs (Fig. 35c, red bars)124 because both the light emission and the photocurrent generation compete with the non-radiative electron-hole recombination. It should be noted that a distinct correlation between compositional dependences of the PL intensity and photocurrent generation efficiency was observed for the non-stoichiometric TGAstabilized CIS and CIS/ZnS NCs (Fig. 35d)50. A good agreement between the efficiencies of PL and light conversion for a variety of size and compositions of CIS NCs indicates that the PL spectroscopy is a reliable diagnostic tool for assessing/predicting the light-harvesting capability of the ternary MC NC-based photoanodes. The modification of TiO2/CdS photoanodes with an atomicallyresolved tetrahedral [Cu5In30S56H4]13– cluster with a triethanol ammonium counter cation exhibiting the light sensitivity down to 700 nm results in a drastic increase of the photocurrent density showing a great potential of such molecular-like sensitizers353, especially if they will be combined with conventional multinary NCs by electrostatic interactions. The harvesting efficiency of titania NT array-based photoanodes loaded with complementary light harvesters – CdS and CIS NCs, can be increased considerably by using double-sided array with NTs formed on both sides of the ITO plates439. The kesterite CZTS NCs were applied as a co-sensitizer together with the N719 complex dye-sensitizer to enhance the visible-light response of mesoporous titania electrodes synthesized by the annealing of a precursor metal-organic framework545. Both sensitizers can participate in the photoinduced electron transfer into the TiO2 CB, as the electron can come via CZTS NCs in the case of the photoexcitation of the dye adsorbed on the kesterite NC surface. The cell exhibited a light conversion efficiency of 8.10% outperforming the similar devices containing only the N719 complex545. CZTS NCs were attached to the titania electrodes by using thiolated RGO as a bridge instead of conventional MPA/TGA182. The most efficient liquid-junction cell with the TiO2/RGO/CZTS photoanode, I–/I3– shuttle and a Pt CE exhibited a PCE of 0.133%, which is around 30% higher than for a similar photoanode with the MPA 182 linker .

Fig. 35. (a) Current – voltage characteristics of TiO2/CISSe photoanodes with the sensitizer NC of a different size. (b, c) Size-dependence of the EQE spectra (b), PCE (c, red bars) and PL quantum yield (c, blue bars) for TiO2/CIS photoanodes with sizeselected CIS NCs. (d) Photocurrent density (curves 1 and 2) obtained for TiO2/CIS (1) and TiO2/CIS/ZnS (2) photoanodes and PL intensity of colloidal CIS/ZnS NCs (curve 3) as a function of the CIS NC composition Cu:In:S = xCu:5:10. Reprinted with permissions from ref. 162 (a), ref. 124 (b, c), and ref. 50 (d). Copyright (2014) American Chemical Society (a–c); (2017) The Royal Society of Chemistry (d).

The arrays of ZnS- and ZnSe-decorated zinc oxide NRs were sensitized by CZTS NCs and introduced as photoanodes in the liquid-junction cells with a CuxS CE and polysulfide electrolyte exhibiting a PCE of 0.85% and 2.2% for the ZnO/ZnS and 183 ZnO/ZnSe scaffolds, respectively . Copper bismuth sulfide NCs were applied as a spectral 295 sensitizer for the titania photoelectrodes . When combined with the aqueous polysulfide electrolyte and CuxS CE, the TiO2/CBS photoanode showed PCE of 0.62%. Cu3SnS4 NCs deposited from the gas-phase on the surface of mesoporous titania revealed a bandgap of 1.2 eV and photocurrent 488 generation capacity under the visible-light illumination . The in situ methods of the sensitizer NCs deposition are used to a much lesser extent for multinary MC NCs as compared to the solar cells based on binary cadmium and lead chalcogenides. In particular, TiO2/AIS NC composites prepared in situ 3+ via the In incorporation into the TiO2/Ag2S heterostructure were applied as photoanodes in a combination with a Pt CE 544 and polysulfide electrolyte revealing a top PCE of 0.70% . A similar two-stage approach was applied to form TiO2/AgBiS2 nanocomposite and the feasibility of using this material as a light-harvesting component of a liquid-junction solar cell with polysulfide electrolyte and Au or Pt CE was shown446,447. The best AgBiS2-based cells showed a PCE of 0.34% at AM1.5G illumination446 and a spectral sensitivity range extending down to 800 nm447. A NaSbSe2 NC-sensitized TiO2 photoanode with a bandgap of ~1.5 eV produced by SILAR manifested a PEC of 2.22% with an Au CE and aqueous poilysulfide electrolyte546. The ZnIn2S4 nanosheets deposited in situ onto the TiO2 surface by the HTT extend the light sensitivity range of the titania photoanode to 600 nm and exhibit a PCE of 0.42% in a liquidjunction solar cell with polysulfide electrolyte and Au CE402.

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A combined ex situ/in situ approach to TiO2/CIS photoanodes 278 was developed including the adsorption of primary small CIS NCs on the titania surface followed by the SILAR deposition of additional CIS layers to increase the sensitizer loading. The primary nuclei provide the nucleation sites forcing the growth of pre-adsorbed CIS NCs and preventing the formation of new nuclei. For the same CIS content, the ex situ/in situ photoanode manifested an increased PCE (1.84%) as compared with two analogs produced exclusively by the deposition of the ex situ sensitized CIS NCs (0.64%) and solely 278 by the in situ SILAR deposition (1.27%) . Depending on the composition and synthesis conditions the multinary MC NCs can inherit both n- and p-type electrical conductivity. The n-conductive NCs are typically used as photoanode component that fills the photogenerated VB hole 2– at the expense of the oxidation of S species in the electrolyte. In its own turn, the p-conducting NCs can be applied as sensitizers of photocathodes, where the NC photoexcitation results in the transfer of a photogenerated VB hole into the electric circuit, while the CB electron is 123 transferred to the sulfur species in the electrolyte . Correspondingly, the wider-bandgap metal-oxide scaffold should also have the p-type electrical conductivity, while the counter electrode should be catalytically active towards the 2– oxidation of S species to short-circuit the PEC cycle in the cell. Such “inverted” solar cells can be combined with regular solar cells with the MC NC-sensitized photoanodes to obtain cumulatively increased Jsc and Voc values in the tandem systems and in this way, the Shockley–Queisser limit of the single cell performance can theoretically be overcome. In recent years, an extensive search for transparent and stable p-type metal oxide scaffolds for the photocathodes is performed with nickel oxide being currently the most active and promising material. The CE materials used in such “inverted” liquid-junction solar cells are similar to those applied in the usual cells and include Pt, CuxS, carbon allotropes, etc. Indeed, typically the CEs reveal simultaneously the catalytic activity for both sulfide oxidation and sulfur reduction, as, for example, shown by catalytic experiments for 547 copper sulfide NCs . However, contrary to the binary MC NC probed quite extensively as the photocathode sensitizers, in the case of multinary MC NCs only scarse reports can be found. In particular, a stable and efficient photocathode for the liquid-junction solar cell can be constructed with a mesoporous NiO layer sensitized by CIS or CISe NCs and combined with the aqueous polysulfide electrolyte and a CuxS 123,548 CE . The rates of hole injection from the sensitizer NCs to 8 –1 the NiO scaffold are comparable (on the order of 10 s ) with the electron injection rates in conventional NC-based 123 photoanodes and the maximal achieved PCE is 1.25% . Another concept used broadly for the rational design of the binary MC NC-based photoelectrodes but still to be realized to the full extent for the multinary MC NCs is the cascade design of the light-harvesting photoelectrodes. In most cases, the light-harvesting efficiency of the liquid-junction cell photoanodes is limited by the photoinduced charge separation between the wide-bandgap metal-oxide scaffold and narrow-

bandgap MC sensitizer NCs. To achieve the efficient charge separation, both components are typically chosen so that the CB level of the sensitizer NCs is more negative and the VB level – less positive than ECB and EVB of the metal-oxide, respectively. Such conditions favor the transfers of the photogenerated electrons from MC NC to titania and the VB holes – from titania to the sensitizer NCs. Therefore, the final charge-separated state of the composite does not depend on which component absorbs the incoming light. This principle can be extended for multi-sensitizer systems, where several light-harvesting components are used with the CB energies descending (by the absolute value) from the outer components of the photoanode to the inner components and so the photogenerated electrons flow from the photoanode/ electrolyte interface to TiO2, further to OTE and, finally to the electric circuit. Such “cascade” design can be realized in several ways, in particular, (i) by using different semiconductor photoelectrode materials with favorable CB and VB level offsets; (ii) by using alloyed solid solution compounds with a varied or even spatially gradient structure to create a CB (VB) offset from the outer surface of the photoelectrode toward OTE, and (iii) by combining NPs of the same semiconductor but of different size that reveal a strong size-dependence of CB and VB levels and placing them in the order of decreasing CB energy from the outer photoelectrode surface toward OTE. Only the first of these approaches seems to be probed for the multinary MC NCs. For example, the deposition of a CdSe layer on top of TiO2/In2Se3/CIS photoanodes was reported to result in a increase of the light conversion efficiency from 1.35% to 3.15% as a result of the formation of a cascade CdSe → CuInS2 543 → In2Se3 → TiO2 → OTE structure . This cascade can be extended by the deposition of an outer ZnSe layer with the highest CB level that serves simultaneously as a protector agains the charge leakage into the electrolyte and as an additional light absorber, enhancing the PCE of the corres543 ponding cell up to 4.55% . The protection of TiO2/CIS photoanodes with a CdS layer not only passivates the surface of CIS NCs but also creates conditions for the cascaded 92 electron phototransfer from CdS to CIS and further to TiO2 . A CIS NC layer placed between the titania NT array and a layer of CdS NCs on the NT surface acts simultaneously as a cosensitizer supplying the photogenerated charge carriers to the titania layer, transferring the photogenerated electrons from the adjacent CdS NC layer to the TiO2 nanotube array and serves as a barrier for the electron recombination with 440,540 electrolyte species (Fig. 36a) . The co-sensitization was found to increase the cell PCE up to 4.2% as compared to 1.8% 540 for the binary TiO2/CdS heterostructure (Fig. 36b) . It should be noted that despite the above-discussed broad variability of the stoichiometry and composition of multinary MC NCs, the photoelectrodes of the liquid-junction solar cells with the cascaded charge transfers between the alloyed NCs of a different composition were still not reported. Also, a great potential of the size-dependent variability of the CB/VB levels of multinary MC NCs in the regime of quantum confinement was also not explored to the true extent for the formation of cascaded sensitizer layers.

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Fig. 36. (a) Energy diagram of a TiO2/CIS/CdS photoanode. (b) Current-voltage characteristics of the liquid-junction solar cells with TiO2/CdS, TiO2/CIS and combined TiO2/CIS/CdS photoanodes. (c) Current-voltage characteristic of a TiO2/CISSe photoanode-based liquid-junction solar cell. (d) Light conversion efficiency η and opencircuit voltage Voc in the cells based on TiO2/CIS photoanodes with different MC buffer layers. Reprinted/adapted with permissions from ref. 440 (a), ref. 540 (b), ref. 225 (c), and ref. 543 (d). Copyright (2011, 2012) The Royal Society of Chemistry (a, b); (2013) American Chemical Society (c, d).

The photocurrent generation efficiency of both solid-state and liquid-junction NC-based solar cells is limited to a far extent by the electron-hole recombination in the sensitizer MC NCs. In the case of the liquid-junction photoelectrochemical solar cells, the additional possibilities of the charge losses exist due to the recombination on the interfaces between the sensitizer NCs and the wide-bandgap scaffold as well as between the photoanode as a whole and the electrolyte species. To suppress the recombination and minimize losses of the photogenerated charges various approaches are developed, one of the most simple and, at the same time, efficient being the formation of additional “protective” (or “buffer” or “barrier”) semiconductor layers either on the sensitizer NC surface, or between the sensitizer and the oxide scaffold, or 19 between the metal oxide and OTE . The recombination in the sensitizer NCs occurs predominantly via the structural defects introducing additional states in the NC bandgap and allowing for the thermal or radiative dissipation of the light excitation energy. These defects (unsaturated bonds, cation/anion vacancies, etc.) can be passivated by appropriate ligands or a shell of other metal chalcogenide semiconductor, typically with a larger bandgap, such as ZnS. The passivating layers can be very conveniently deposited by the SILAR procedure allowing for a quite precise control over the thickness of the barrier layer. For example, 50,278 TiO2/CIS photoanodes are routinely covered with a ZnS or 92,430 CdS shell to enhance the chemical/photochemical stability of the sensitizer NCs and to suppress the non-radiative 278 recombination by passivating NC surface defects . The modification of TiO2/CIS photoanodes with a CdS layer increases the PCE of a cell with the aqueous polysulfide 92 electrolyte and CuxS CE from 1.14% to 3.91% . The SILAR deposition of a CdS layer on ZAISe NCs immobilized on the TiO2

scaffold resulted in a more than an order of magnitude 89 increase in the light conversion efficiency . The passivation of CISe NCs by a layer of Mn-doped CdS was shown to enhance 160 the light conversion efficiency from 0.8% up to 3.96% . 3+ 2– The successive adsorption of In and S on the surface of TiO2/CIS photoanodes reduces the number of surface vacancies in the sensitizer NCs resulting in a cathodic shift of 429 the Fermi level of the photoelectrode . The SILAR deposition of a ZnS layer was found to affect considerably the light conversion efficiency of TiO2/CISe NC 163 photoanodes resulting in a PCE of 8.10% . The impedance studies revealed that the ZnS coating does not alter the energy characteristics of the photoanode, yet enhances the kinetic characteristics of the charge carrier transfers, resulting in the strong reduction in the efficiency of both non-radiative recombination in the CISe NCs and the recombination on the photoanode/electrolyte interface163. At the same time, the ZnS shell does not impede the interfacial electron transfer from CISe NCs to titania as confirmed by the cell performance and the efficient PL quenching after the CISe/ZnS NCs deposition onto the titania scaffold. A thin CdSxSe1–x layer forming as a result of the copper-tocadmium ions exchange on the surface of CISSe NCs enhances the PCE of the passivated TiO2/CISSe photoanode up to 3.45% (Fig. 36c)225. It should be noted that this photoanode revealed a fill factor of more than 60%225 indicating the high efficiency of the passivating procedure. Similarly high FFs, 58% and 62%, observed for a Zn-doped CIS NC-based photoanode125 and for a Ga-doped CISe NC-based photoanode93, respectively, are rather an exception, than a steady tendency. In most cases and as opposite to the DSSCs, the NC-sensitized solar cells reveal modest fill factors amounting to around 40%, much rarer – to 50%. The reasons for this moderate performance are considerable recombinative losses in the sensitizer NCs and on the interfaces between the MC NCs and the metal oxide scaffolds, as well as between the photoanode and electrolyte. Provided the CB levels of the sensitizer NCs and TiO2 (ZnO) are favorable for the electron transfer from NCs to oxide, the photoexcitation of sensitizer NCs results in extremely fast electron transfer to neighboring TiO2 (ZnO) layer, leaving a hole in the NC valence band. Potentially, the electron in the titania CB can recombine with the hole in the sensitizer VB, similarly as it occurs in the DSSCs after the electron injection from the photoexcited dye-sensitizer to the titania scaffold. To prevent such charge losses, a blocking layer is often introduced between the sensitizer NCs and TiO2 (ZnO) constituting a potential barrier for the injected electron on its way back to MC NCs. For example, a screening study of potential interfacial buffer layers for TiO2/CIS heterostructure among various MC materials (Fig. 36d, orange bars) showed In2Se3 to be the most efficient buffer layer enhancing the photocurrent generation by about 120% and increasing the PCE from 0.58% for binary TiO2/CIS photoanode to 1.35% for TiO2/In2Se3/CIS 543 heterostructure . An increase in the open-circuit voltage for the cells having an intermediary MC layer (Fig. 36d, green bars) indicates quite clearly that the PCE enhancement effect comes not from (or not only from) the additional light absorption by

J. Name., 2013, 00, 1-3 | 41





the buffer layer, but from the inhibition of the recombination on the scaffold/sensitizer interface. After the injection from the photoexcited sensitizer NCs to the adjacent mesoporous TiO2 (ZnO) layer, the electron starts to migrate through the network of contacting metal oxide NCs till it reaches OTE and comes into the electric circuit. An efficient design of the photoanode of SSSCs requires a high contact area between the photoanode surface and the electrolyte, however, the coverage of the TiO2 surface with the sensitizer NCs is typically not complete. As a result, a possibility exists for the electron to be captured by the components of the 0 + electrolyte, for example, by water, S or H3O ions that can reduce the photoanode performance considerably. To avoid such losses, the TiO2/NCs heterostructures can be covered with an additional protective layer of a wide-bandgap material, most often, zinc sulfide that creates a barrier for the electron to reach the electrolyte. The protective ZnS layer also serves to provide a stronger contact between sensitizer NCs and TiO2 (ZnO) as well as to protect the light-harvesting MC NCs from corrosive processes during the photoelectrochemical events in the SSSC. For example, the leakage of the photogenerated charge carrier was efficiently suppressed by the deposition of ZnS layers onto TiO2/CIS429,540 and TiO2/ZAISe177 photoanodes. In the case of ZAIS NCs, the photoanode was additionally passivated with amorphous TiO2 and SiO2 layers. This final treatment resulted in a PCE increase from 3.41% to 3.57%177. The insulation of a TiO2/CIS/CdSe photoanode with an outer ZnS or ZnSe layer results in a PCE increase from 3.15% to 3.27 % or to 4.55%, respectively543. A higher efficiency of the ZnSecoated photoanode is explained by the contribution of ZnSe into the total light absorption. 3.3 Multinary NCs in the counter electrodes of the solar cells Multinary MC NCs emerged recently as relatively active electrocatalytic materials for the counter electrodes of both DSSCs and SSSCs25,516,517. For example, CIS NCs supported by a three-dimentional carbon fiber network produced by the egg shell carbonization revealed an activity as a DSSC counter electrode outperforming the bare carbon fiber network and producing photocurrents comparable to those in a similar DSSC with a Pt CE297,549. Modification of the carbon fibers with RGO resulted in a better dispersion of CIS NCs over the fiber surface and further increased the catalytic properties of the CE549. Reasonably efficient DSSC CEs were also prepared from CIS NCs on FTO103,281, CIS nanoflakes on Mo substrates335, CIS nano-sheets grown on CuS442, composites of CIS NCs with PEDOT:PSS212. The exchange of 1-DDT ligands with S2– opened the surface of CIS NCs to the electrolyte species resulting in a drastic increase of the catalytic activity of CIS NC/FTO films toward the triiodide ions reduction281. The carbon/CIS NC composites were found to be more efficient CEs for the liquid-junction solar cells with TiO2/CdS/CdSe photoanodes than both carbon (a mixture of 341 carbon black and activated carbon) and CIS NCs taken separately. The catalytic activity of a CE can be quite straightforwardly assessed from the slope of the dark current – voltage

charactertistics in a three-electrode scheme with the same electrolyte/redox-couple as used in the solar cell. At that, the steeper is the J-V curve the higher is the catalytic activity of a CE material (Fig. 37a). Another informative method of the assessment of the CE activity is the impedance spectroscopy. A more active CE material is characterized by a lower charge transfer resistance (Rct) and a smaller radius of the semicircular Nyquist plot. The carbon/CIS example shows that the maximal activity as a CE can be expected with a composite having CIS:carbon weight ratio of 1:1 (Fig. 37b), in accordance 341 with the results of the solar cell trials . Tetragonal Cu2SnS3 NCs revealed “universal” catalytic activities toward the redox-couples traditionally used both in the dyesensitized and NC-sensitized solar cells – I–/I3–, Co(phen)33+/Co(phen)32+ 296, and S2–/Sx2– 363. Remarkably, the CTS NCs outperformed both Pt CE in DSSCs (10.26% for CTS versus 9.31% for Pt)296 and CuxS microspheres in NC-sensitized solar cells (4.05% for CTS versus 3.65% for CuxS)363. The kesterite CZTS and CZTSe NCs were proposed as alternatives for the conventional Pt CEs in the DSSCs with I–/I2 redox-couple464. The kesterite-based CEs slightly outperform the platinum electrode (Fig. 37c) and exhibit a long operation life-time. The CZTS nanofiber-based DSSC CE revealed almost doubled activity as compared to FTO/Pt CE (PCE of 3.9% versus 1.72% for platinum)550. Similarly, the Cu2MSnS4 NCs (M = Fe, Cd309, and Co261) showed the catalytic effect toward the triiodide anions reduction, close to that of Pt CEs. The high catalytic activity in the DSSC CE was also found for CZTS(Se) NCs on FTO261,264,314,383,387,370, CZTS nanoplate arrays384, CZTS NCs anchored on the multi-wall carbon NTs551, the wurtzite/stannite Cu2CdGeS4 NCs275, Ag8GeS6 NCs169, as well as for CZTS/RGO552 and Pt-Co/Cu2ZnGeS4 267.

Fig. 37. (a) Dark current – voltage characteristics of CIS NCs, carbon and carbon/CIS composites. (b) Nyquist plots for CIS NCs and carbon/CIS with a different component ratio. (c) Current–voltage characteristics of DSSCs based on Pt, CZTS, and CZTSe CEs. (d) “Dark” current-voltage characteristics of FTO/CZTSSe films immersed into the aqueous polysulfide solution. Reprinted with permissions from ref. 341 (a, b), ref. 464 (c), and ref. 258 (d). Copyright (2013) American Chemical Society (a, b, d); (2017) Elsevier (c).

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The alloyed kesterite Cu2ZnSnSxSe1–x NCs exhibited a catalytic 2– 0 activity toward the S /S redox couple transformations when 78,258 coupled to a NC-sensitized TiO2 photoanode . The catalytic current of the CE immersed into the polysulfide solutions (Fig. 37d) and the overall solar cell performance depend on the S/Se ratio in the kesterite NCs, being maximal for x = 0.5. The enhanced catalytic activity of CZTSSe NCs was explained by 258 their increased porosity as compared to other compositions . The catalytic activities as CEs in the polysulfide-based NCsensitized solar cells were also reported for the hierarchical 367 CZTS microspheres . It should be noted that in most cases the catalytic activity of kesterite and related nanomaterials is compared with bare FTO and Pt, which is not an optimal electrode for the polysulfide electrolyte. At the same time, comparisons with the most frequently used and one of the most active CE materials for the polysulfide electrolytes – copper sulfide are typically not reported, thus making impossible the evaluation of the efficiency and perspectives of such materials for the applications in the NC-sensitized solar cell with the polysulfide electrolytes.

4 Photocatalytic and photoelectrocatalytic processes with multinary MC NCs A photocatalytic process starts when a semiconductor NC absorbs a light quantum with the energy hv equal to or (typically) higher than the bandgap energy Eg (process 1 in Fig. 38a). The photogenerated charge carriers rapidly (in tenshundreds fs) dissipate the energy excess hv – Eg via the electron-phonon interactions acquiring the chemical potentials equal to the ECB and EVB levels. A major part of the CB electrons and VB holes recombines either via the radiative pathway 2 emitting PL or in nonradiative processes 3 providing a vibrational energy to the NC lattice. Some portion of the charge carriers migrates and reaches the NC surface prior to the recombination (processes 4) where the carriers get “trapped” by various lattice defects (vacancies, adatoms, undercoordinated atoms, etc.) as well as by the adsorbed species. The photogenerated VB hole typically has a high oxidation potential and gets filled with an electron from various donor species present in the system, for example from water molecules or from a sacrificial electron donor introduced for this purpose such as sulfide ions (Fig. 38a). The photogenerated CB electrons can transfer directly to the electron accepting species through the NC/electrolyte (NC/gas) interface or accumulated on a co-catalyst coupled to the semiconductor NCs (Pt NCs in Fig. 38a). Many vital photocatalytic reactions, such as the reduction of water, CO2, or N2, are multi-electron processes and characterized by 553– relatively high over-voltages of the rate-determining stages 557 . The co-catalyst is typically required to accumulate the photogenerate charge carrier and to reduce the overpotentials thus increasing tremendously the efficiency of the 79,554–556 photocatalytic transformations . As both CB electron

and VB hole are consumed the semiconductor NC regains its original state. Provided that fresh donors and acceptors are constantly supplied to the photocatalyst surface it works continuously in a cyclic manner resembling a “pump” transferring electrons from the donors to the acceptors at the expense of the solar light energy. To avoid the electron-hole recombination, the visible-lightharvesting nanocrystalline MC photocatalyst is typically coupled to another semiconductor having a less negative ECB and a more positive EVB levels (Fig. 38b). In this way the CB electron photogenerated in MC NCs can transfer across the interface to another semiconductor (process 5 in Fig. 38b). The interfacial transfer is typically very fast (occuring in the range of ps or even down to tens-hundreds fs) and results in a lowering of the CB electron energy thus making impossible (or very improbable) the reverse transfer to MC NCs. The VB hole remains separated from the transferred CB electron due to a potential barrier between the valence bands of both components and in this way the electron-hole recombination is efficiently suppressed. Typically, the binary and multinary MC NCs are coupled to metal oxide, such as TiO2 or ZnO or metallate (titanate, tungstate, vanadate, etc.) semiconductors with a larger bandgap and a lower CB level. The direct photoexcitation of such wide-bandgap semiconductor also results in a charge-separated state, i.e. the VB hole transfers irreversibly into the less positive VB of the MC NCs (process 5/, Fig. 38b). In this way, the same charge-separated state is produced irrespectively of which heterostructure components is photoexcited.

Fig. 38. Scheme of the working principle of (a,b) photocatalytic systems based on a MC NC/metal NC composite (a) and a binary NC heterostructure (b), and (c) photoelectrochemical system with a MC NC sensitized photoanode.

Alternatively, the components of the photocatalytic system (semiconductor and metal or two semiconductors) can be physically separated in space and connected via an electric circuit. Thus, the reactions with the participation of electrons and holes take place in different electrolytes connected via a bridge/membrane (Fig. 38c) and the photoexcitation results in the photocurrent generation. A photoelectrochemical mode is especially beneficial for the processes where recombination can occur between the intermediates or final products of the reaction as, for example, in the water splitting. The twoelectrode photoelectrochemical system can also be externally biased allowing to reduce the potential barriers and increase the efficiency of photocatalytic transformations. Pioneer reports on the semiconductor-mediated photocatalytic processes appeared as early as in 1920s–1930s dealing

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mostly with the photobleaching of dyes in the presence of titania crystals. In the late 1960s Fujishima and Honda reported on the water photoelectrolysis on externally-biased rutile TiO2 single-crystal electrode coupled to Pt and 79,554,555 illuminated with the UV light . Afterwards, the minituarization of semiconductor crystals to several microns and the direct deposition of Pt NPs on the titania surface allowed to compose the suspension-based photocatalytic systems where both water reduction and oxidation took place on the suspended microparticle aggregates. Such systems can function without an external bias as the photogenerated electrons can migrate through the net of contacting crystals thus avoiding the electron-hole recombination. As titania can harvest less than 5% of the solar light various ways of spectral sensitization of TiO2 micro- and nanocrystals were developed including doping with metal ions and non-metals (N,C,S) as well as coupling to molecular dyes and metal complexes and to narrow-bandgap MC and metal-oxide NCs. The latter studies stimulated the development of the photochemistry of individual MC NCs and showed a great potential of these semiconductors for a variety of important processes, including the reduction of water and CO2 with sacrificial electron donors and transformation of ionic inorganic species and various organic compounds aimed both on the total decomposition and on the formation of some specific synthetic products. In recent years, the nanocrystalline multinary MC semiconductors and related heterostructures showed an everincreasing contribution into the development of various photocatalytic systems, including the reduction of water to molecular hydrogen and destructive degradation of various 11,23,24,33,35,37,38 organic species . The multinary MC NCs are quite broadly studied as light-harvesting components of the photoelectrochemical systems, operating similarly to the liquid-junction solar cells but aimed not at the photocurrent production but on the photooxidation/photoreduction of selected organic/inorganic species. 4.1 Photocatalytic hydrogen evolution with MC NCs Typically, the MC NC-based photocatalytic systems for the hydrogen evolution contain noble metal co-catalysts, such as Pt NCs, that can be formed in situ via the photocatalytic reduction of metal precursors on the surface of MC NCs. This process occurs quite fast resulting in the MC/Pt heterostructure which then acts as a photocatalyst of the water reduction. Both Pt precursor and water are typically reduced at the expense of sacrificial electon donors, the most efficient ones being sulfide and sulfite ions that can strongly adsorb on the NC surface and fill the VB with a very high rate inhibiting the oxidative photocorrosion of semiconductor NCs. The role of a sacrificial donor is often delegated to methanol, ethanol, triethanolamine and other organic substances. The efficient systems for the hydrogen production were organized using MPA-capped CIS NCs without additional co127 342 catalysts and decorated with Ru NCs , AIS NCs with the 344 photodeposited Pt , ZAIS NCs coupled to MoS2 432 nanosheets , powdered CZTS NCs with the photodeposited

285

197

Pt , CZTS NCs coupled to Au and Pt NCs , and p/n558 heterojunction CZTS/CdS nanostructures . Early studies of the photocatalytic properties of “classical” semiconductor photocatalysts, such as TiO2, showed that the nanocrystalline semiconductors typically reveal a much higher photocatalytic activity than the bulk counterparts even after the normalization of the H2 evolution rate to the surface area 554–556 of the photocatalyst . This effect originates from a balance between the rates of the photogenerated charge carriers migration to the crystal surface and their recombination which is much more favorable for the photocatalytic 79,80 reactions in the case of the nanocrystalline matters . At the same time, among the nanocrystalline materials the highest efficiencies of the photocatalytic H2 evolution were obserbed for various loosly aggregated materials, such as mesoporous microaggregates of NCs, hollow microspheres, mesoporous NTs, etc. The advanced photocatalytic activity of such materials stems from the possibility of the migration of the photogenerated electrons among the network of contacting NCs that reduces considerably the probability of recombination events. Moreover, the effects of the multple light scattering and absorption within the mesoporous NC aggregates as well as easy accessibility of the loosely aggregated NCs to the electron donor solution also have contributions to the increased photoactivity. The arsenal of the above-discused approaches is broadly and fruitfully applied for the multinary MC NCs. In particular, the photocatalytic hydrogen evolution from aqueous donor solutions under the illumination of the visible light was reported for mesoporous spherical microaggregates of CIS 390 393,559 359 346 NCs , CIZS NCs , AgGa1–xInxS2 NCs , and ZnIn2S4 . Similarly to the case of the binary semiconductor-based photocatalytic systems for the water splitting, steady and considerable efforts are applied to replace the noble metal cocatalysts with less expensive and, at the same time, available and abundant species. The successful examples are 83,357,381,432 415–418,420 421 2+ MoS2 , RGO , carbon NTs , and [Co(bpy)3] 77 complex . Both molybdenium disulfide and RGO nanosheets combine a high activity in hydrogenation/dehydrogenation processes with a high available surface and excellent electronaccepting capabilities as co-catalysts of the photocatalytic 560–563 processes . The activity of RGO as a hydrogen evolution co-catalyst was found to be far superior to that of carbon 420 nanoparticles and carbon NTs . Typically, the dependence of the photocatalytic H2 evolution rate on the RGO content is 416,418 dome-shaped with the maximal rate observed at around 418 1 wt.% RGO (rel. to the photocatalyst load) , most probably because of the light-shielding effect of RGO at higher loadings. 2+ The [Co(bpy)3] complex can strongly bind to the MC NC surface and has a slightly more negative potential of the II I Co /Co transition (–0.95 V versus NHE) than the potential of water oxidation (–0.41 V at pH 7). Thus, this complex acts as an electron relay accepting CB electrons from the NCs and transferring them to the water molecules in the outer 77 coordination sphere . The quaternary Cu2MSnS4 (M = Zn, Co, Ni) NCs were reported to serve as a co-catalyst of the hydrogen evolution in the

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presence of Eosine Y as a light-harvesting species and 85,564 triethanolamine as a sacrificial donor . The photoexcited dye has a redox-potential E = –1.1 V (NHE) and can inject an electron into the CB of kesterite NCs positioned at –0.88 V (Cu2NiSnS4), –0.83 V (Cu2CoSnS4), and –0.65 V (Cu2ZnSnS4) versus NHE. The NCs act as an electron relay transferring the 85,564 charge to adsorbed water molecules (–0.53 V, pH 9) . Alternatively, the amount of the noble metals in the cocatalysts can be reduced by a partial substitution of Pt or Pd with catalytically-active transitions metals, such as cobalt. The Co/Pt NC-modified CZTS NCs showed twice as high photocatalytic activity in the water reduction as the CZTS/Pt composites with the same nominal metal content (Fig. 39a)202. Another strategy stemming from the binary semiconductor photocatalysts is the application of various anisotropic NCs, such as nanorods and nanowires. Single-crystalline character of such NCs coupled with the enhanced mobility of the photogenerated charge carriers along a long NC axis allows for a more efficient charge separation and related reactivity, than in the case of isotropic (spherical) NCs. The efficient hydrogen evolution was reported for Pt-decorated CIZS NRs141, ZAIS NRs154,565, CdS nanowires decorated with 2D CdIn2S4 nanosheets399, CZTS NRs and nanoplates566, ultra-thin CZTS nanosheets567 and the single-layer MnSb2S4 nanosheets494. The latter compound has a layered structure with a [MnS2]2– sublayer sandwiched between two [SbS]+ sublayers (insert in Fig. 39b). The calculations showed that the VB of MnSb2S4 is predominantly formed by d-orbitals of Mn and p-orbitals of sulfur, while the CB is composed mainly of the p-orbitals of Sb and localized on the nanosheet surface494. Therefore, the photoexcitation directly supplies photoelectrons to the adsorbed water molecules and protons providing for the efficient hydrogen evolution. The VB holes are simultaneously extracted from the middle [MnS2]2– layer by Ce3+ acceptor. The photocatalyst is composed of broadly available and relatively low-toxic elements, it has a bandgap of 1.69 eV, very appropriate for the solar light harvesting, and can evolve H2 when illuminated by the visible light to 650 nm (Fig. 39b). Ternary MC NCs are often combined with other, typically, wider-bandgap semiconductor NCs in composites with balanced CB and VB levels. These heterostructures possess a much higher photocatalytic/photoelectrochemical activity than the separate components taken summarily. For example, the graphitic carbon nitride (GCN) decorated with CIS and Pt NCs revealed a high photocatalytic activity in the hydrogen evolution from aqueous Na2S/Na2SO3 solutions, which is 3.3 and 6.1 times higher than the activity of GCN and CIS NCs taken alone, respectively410. The rate of H2 evolution reaches ~1.3 mmol×g−1×h−1 in this system. Both components of the heterostructure can be excited in different sections of the visible spectrum resulting in the charge separation between CIS and GCN (Fig. 40a) and consequent spatial separation of the water reduction and the oxidation of sacrificial donors. A similar activity enhancenent was observed for GCN/ZnIn2S4 411–413,568 heterostructures , where the photogenerated CB electron is transferred from GCN (ECB = –1.12 V, NHE) to ZIS NCs (ECB = -0.68 V, NHE), while the VB holes can decrease their

energy by coming from the ZIS VB (+1.62 V, NHE) to the GCN 406 VB (+1.57 V, NHE) .

Fig. 39. (a) Rate of the photocatalytic hydrogen evolution in the presence of CZTS NCs and their composites with Pt and PtCo NCs. (b) Photoaction and absorption spectra of the single-layer MnSb2S4 nanosheets, insert: a scheme for the photocatalytic hydrogen production over the MnSb2S4 monolayer. (c). Photoaction and absorption spectra of TiO2/CuGa2In3S8 nanoheterostructure. (d) Efficiency and selectivity of the photocatalytic NO3– reduction to N2 over CIS NCs. Reprinted with permissions from ref. 202 (a), ref. 494 (b), ref. 228 (c), and ref. 337 (d). Copyright (2015) American Chemical Society (a); (2016) The Royal Society of Chemistry (b, d); (2016) Elsevier (c).

Similarly to cadmium sulfide, ZnIn2S4 can crystallize in cubic and hexagonal forms having different CB/VB positions (hexagonal – ECB/EVB = –1.1/1.5 V, cubic – ECB/EVB = –1.5/0.9 V, 362 NHE ) and thus a heterostructure with the offset CB and VB 320,362 levels forms between cubic/hexagonal ZIS NCs (Fig. 40c) . The composite can evolve hydrogen without additional cocatalyst, the efficiency of this photocatalytic process being 7.3 times and 3.2 times higher than in the case of sole cubic and 320 hexagonal ZIS NCs, respectively . The efficient hydrogen evolution from the solutions of sacrificial donors under the visible light excitation was 342,397 405 569 reported also for TiO2/CIS and TiO2/AIS , BiVO4/CIGS 417 361 357 and BiVO4/RGO/ZIS , In(OH)3/ZIS , ZIS/MoS2 , 392 322 ZIS/CdIn2S4/CuS , ZnFe2O4/ZIS and ZnFe2O4/ZIS/CdS , 570 381 CZTS/MoS2 , and CZTS NCs loaded onto silica nanospheres . Recently, combination of ZIS NCs with metal-organic frameworks was demonstrated to be a promising strategy for 571 the design of new H2-evolving photocatalysts . To increase the driving force of the interfacial CB electron transfer from CIS NCs various methods of “band design” can be applied, in particular, a partial substitution of In with Ga that results in a bandgap increment proportional to the gallium 229,569 content . As a result, the CIGS NCs revealed a higher photocatalytic activity in the hydrogen evolution from aqueous Na2S/Na2SO3 solutions, as compared to pure CIS NCs. A similar 481 effect was also observed for AIGS compounds . At the same time, the bandgap increase shifts the light sensitivity threshold of CIGS NCs to shorter wavelengths as the Ga fraction is increased, thus reducing the visible-light-harvesting capabilities of the alloyed NCs. Both tendencies produce a dome-

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shaped dependence of the hydrogen evolution efficiency on the composition of CIGS NCs (Fig. 41a) illuminated with the “white” light (λ > 400 nm). The highest photoactivity is 229 observed for CuIn0.3Ga0.7S2 (Eg = 1.75 eV) . The modification of CIGS NCs with 1 wt.% Pt results in a 3-fold increase of the H2 −1 −1 evolution rate reaching ~1.7 mmol×g ×h .

Fig. 41. (a, b) Photocatalytic hydrogen evolution rate as a function of (a) the molar Ga fraction x in CuIn1–xGaxS2 NCs and (b) the molar Zn fraction x in (CuIn)xZn2(1–x)S2 microspheres. Reprinted with permissions from ref. 229 (a) and ref. 333 (b). Copyright (2014) The Royal Society of Chemistry (a), (2010) Elsevier (b).

Fig. 40. Energy diagrams of the photocatalytic systems for (a, b) the hydrogen evolution based on GCN/CIS (a) and cubic/hexagonal ZnIn2S4 (b) composites; (c) the CO2 reduction based on CZTS/TiO2 heterostrtucture; (d) the organics oxidation based on TiO2/CaIn2S4. Reprinted with permissions from ref. 410 (a), ref. 320 (b), ref. 251 (c), and ref. 407 (d). Copyright (2017, 2015) American Chemical Society (a, d); (2016, 2017) The Royal Society of Chemistry (b, c).

A dome-shaped dependence of the hydrogen evolution QY on the solid solution NC composition is therefore a general phenomenon observed for a variety of MC compounds, in 141,343,393,559 particular CIZS , (AgIn)xZn2(1−x)S2 (peak activity 77 154 reported at x = 0.5 and at x = 0.35–0.45 ), and AgGa1–xInxS2 359 NCs (peak at x = 0.1). The noble metal co-catalysts for CIGS can be replaced with 283 0 Ni(II) , which is, most probably, reduced in situ to Ni NCs as 563 it happens in CdS and CdxZn1–xS NC-based systems . The composition-optimized CuGa2In3S8 NCs showed a maximal QY of the H2 evolution from aqueous ascorbic acid solutions of 12% at 540 nm and the spectral sensitivity to 600–650 nm. It is 2+ also notable, that CIGS/Ni system displayed the steady H2 evolution for a prolonged term of around 50 hours while the noble-metal-containing photocatalysts gradually lost the photoactivity due to a possible poisoning with the products of 283 the ascorbic acid oxidation . The highest photocatalytic activity in the water reduction for CIGS NCs deposited into the nanocrystalline titania was observed for CuGa2In3S8 NCs228. The MC acts in this system as a spectral sensitizer that absorbs the visible light and supplies the photogenerated electrons to titania where the hydrogen formation occurs with the participation of Ru NCs. The sensitization effect is unambiguously confirmed by a close matching of the absorption spectrum of CIGS NCs and the photoaction spectrum (the wavelength-dependence of the apparent QY) of the TiO2/CIGS nanoheterostructure (Fig. 39c).

A similar dome-shaped dependence of the photocatalytic hydrogen evolution rate on the composition was observed for the alloyed (CuIn)xZn2(1–x)S2 solid solution microspheres with x 333 varied in a range of 0.01–0.5 (Fig. 41b) . The hybridized 4s4p 2+ orbitals of Zn contribute to the CB of the alloyed NCs and, therefore, the introduction of zinc ions increases the bandgap and the photogenerated CB electron energy thus favoring the water reduction to H2. At the same time, the Eg increase reduces the capability of CIZS microspheres to absorb the visible light resulting in a compromise composition, x = 0.20 (Eg = 1.84 eV), at which the highest QY of the H2 formation, ~15.5% at 420 nm, is observed. The in situ photodeposited Ru NCs acted as a co-catalyst enhancing the photoactivity of CIZS microspheres by more than an order of magnitude (Fig. 333 41b) . A combination of CuGaS2 NCs with ZnS creates a p/n heterojunction with enhanced separation of the photogenerated 389 charge carriers between the components . As a result, the CGS/ZnS heterostructure is a 15-times more active photocatalyst of the hydrogen evolution than sole CGS 389 approaching the photoactivity of “classical” CdS . The formation of a p/n heterojunction at the CZTS/ZnS core/shell interface imparted such composite with the photocatalytic activity in the hydrogen evolution exceeding by an order of magnitude the activity of a mechanical mixture of the kesterite 379 and ZnS NCs . Colloidal (AgIn)xZn2(1–x)S2 NCs revealed distinct dependences of the photocatalytic activity in the hydrogen evolution from 88 aqueous Na2S solutions both on the NC size and composition . The authors selected a synthetic protocol yielding NCs of roughly the same size for different compositions allowing to analyze both types of dependences separately. The dependence of the photoactivity on the composition is “classical” for MC NCs and similar to that discussed above for copper-based NCs. As the Zn content is increased, the photocatalytic activity of ZAIS NCs increases, reaches a summit at x = 0.5 and then decreases due to the balance between an increase of the CB electron energy and a loss of the lightharvesting capability (Fig. 42c)88. Almost for any studied x the dependence of the H2 evolution rate on the ZAIS NC size also features a maximum explained by another balance between the size-dependent driving force of the photoinduced CB electron transfer to water molecules and an increase in the surface disorder and charge trapping with a decreased NC size

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[88]. In general, the highest photoactivity was observed for the ZAIS NCs with an average size of 4.2–5.5 nm and a bandgap of 2.3–2.4 eV (Fig. 42d). The calculations of the electron parameters of multinary Ag2ZnSnS4 and Ag2ZnSnSe4 compounds showed them to have a combination of suitable bandgaps and small electron * * effective masses (m e = 0.14m0 ÷ 0.16m0 for sulfide, m e = 74 0.06m0 ÷ 0.07m0 for selenide, m0 is the electron rest mass) . These values are very beneficial for the photocatalytic water splitting, though the feasibility of using such compounds for the hydrogen production is still to be experimentally verified. The introduction of Ag2ZnSnS4 into a ternary Cu2ZnSnS4– Ag2ZnSnS4–ZnS heterostructure was found to impart it with the n-type conductivity and to facilitate the in situ photodeposition of Ru NC co-catalyst572. The incorporation of ZnS results in an upward shift of the CB level of the heterostructure entailing it with the photocatalytic activity of H2 evolution from aqueous S2–/SO32– solution under the AM1.5G illumination572.

insulating SiO2 shell prevents the direct electron transfer from ZAIS NCs to Au NCs the latter act not as typical metallic cocatalyst, but rather by enhancing the light absorption and charge separation in ZAIS NCs subjected to the electromagnetic field created in Au/SiO2 NCs as a result of the photoinduced surface plasmon resonance (SPR). The Auinduced enhancement effect becomes stronger as the overlapping between the SPR band of gold NCs and absorption band of ZAIS NCs is increased via the tuning of the NC 153 composition . Noteworthy, a mechanical mixture of ZAIS and Au/SiO2 NCs did not show any enhancement of the photocatalytic activity indicating the crucial role of the immobilization of the ZAIS NCs on the surface of silica-coated gold NCs. The near-field enhancement of the photocatalytic hydrogen evolution caused by the SPR excitation was observed for Au/Cu2FeSnS4 composites with a gold core and a 20–30-nmthick kesterite shell198 as well as for the core/shell Au/CZTS composites formed by the kesterite NRs and nanoplates566. An effect of plasmonic enhancement of the photocatalytic hydrogen producton was also observed for ternary CaIn2S4/Au/Pt heterostructures573. In this case, the enhancement effect was explained by the transfer of “hot” electrons from the SPR-excited Au NCs into the conduction band of CaIn2S4 and then – to Pt NCs, where the water reduction to H2 took place573. The term “hot” electrons refers to the electrons with the energy higher than the Fermi level of the CaIn2S4/Au heterostructure generated by the plasmonic pumping of the light energy. The assumption on the participaton of the hot electrons is supported by the fact of hydrogen evolution under the illumination of the CaIn2S4/Au/Pt composite, while no H2 was observed at the same conditions for the non-plasmonic CaIn2S4/Pt heterostructure (Fig. 43b)573.

Fig. 42. (a, b) Dependence of the photocatalytic H2 evolution rate R(H2) on the average size (a) and bandgap (b) of ZAIS-x NCs prepared at different x. (c, d) Efficiency of the photocatalytic phenol decomposition as a function of the bandgap/size of CISe NCs (c) and the driving force of the photoinduced CISe CB electron transfer to O2 (d). Reprinted with permissions from ref. 88 (a, b) and ref. 82 (c, d). Copyright (2015, 2016) American Chemical Society.

The nanocrystalline AIS and ZAIS microassemblies coupled with the sheet-like MoS2 serve as efficient photocatalysts of 83 the hydrogen evolution from aqueous lactic acid solutions . As shown by the Mott-Schottky measurements, the CB potentials of the studied ZAIS compounds and MoS2 are at 83 –0.6 V and –0.1 V (NHE) . Thus, the molybdenium disulfide nanosheets can accept the photogenerated ZAIS CB electrons acting as an electron-collecting co-catalyst that facilitates the water splitting and H2 formation. The photocatalytic activity of ZAIS NCs in the hydrogen evolution from water/2-propanol mixtures can be enhanced by the coupling with Au NCs covered with a protective SiO2 153 shell . The enhancement factor depends on the thickness of the silica shell and can be higher than 1.5 (Fig. 43a). As the

Fig. 43. (a) SPR-induced enhancement factor fenhance of the photocatalytic activity of ZAIS NCs by Au/SiO2 particles as a function of the silica shell thickness. (b) Kinetic curves of the photocatalytic hydrogen formation in the presence of sole CaIn2S4 NCs and their composites with Au and Pt NCs. Reprinted with permissions from ref. 153 (a) and ref. 573 (b). Copyright (2013) American Chemical Society (a); (2016) The Royal Society of Chemistry.

The CZTS NCs revealed a phase-dependence of the photocatalytic activity in the water reduction, being almost 3 times higher for the kesterite modification than for the 574 wurtzite NCs . The phenomenon was attributed to differences in the facet population on the NC surface. The dominantly exposed kesterite facets were {112} with an unbalanced charge of exposed anions and cations tending to compensate at the expense of the anions/cations adsorption.

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At the same time, the wurtzite showed much less polarized {100} facets selectively exposed. The preferential adsorption of protons and sulfide ions on the polar {112} facets of the kesterite CZTS NCs is considered to be the main reason for 574 their enhanced photocatalytic activity . 4.2 Photocatalytic reduction of CO2 and various ionic species In general, the reported examples of such reactions are quite rare for multinary MC NCs but a growing interest for such photoinduced transformations can be envisaged in the light of the abatement of CO2 emissions and related environmental issues. The mesoporous CZTS coupled to the titania was reported to be a photocatalyst of the CO2 reduction to CH4 under the solar irradiation251. The CZTS NCs act as a spectral sensitizer excited by the visible light and transferring the CB electrons on the lower-lying CB level of the nanocrystalline TiO2 (Fig. 40c). The photocatalytic CO2 reduction to CO was reported for the one-unit-cell-thick ZnIn2S4 nanosheets321. The key point defining the efficiency of the CO2 reduction is the presence of Zn vacancies on the nanosheet surface. When the HTT conditions are adjusted to decrease the number of Zn vacancies in the ZIS nanosheets the photocatalytic activity of those decreased by a factor of almost 4321. The photocatalytic activity of ZIS nanosheets can be further enhanced by coupling to the TiO2 nanobelts to form cascaded heterostructures575. The alloying of ZnIn2S4 with CaIn2S4 results in the variations of both CB and VB levels producing the most negative ECB for a Zn0.4Ca0.6In2S4 compound94. The photocatalytic reduction of CO2 to methane and CO with the participation of this compound is respectively ~17 and ~8 times faster than for ZnIn2S4 and ~7 and ~4 times faster than for CaIn2S4 94. The CIS NCs simultaneously loaded with Pt, Ru, and Au NCs were found to be an excellent UV- and visible-light-driven photocatalyst of the nitrate anion reduction with oxalic acid337. The process is relatively fast and not accompanied by side processes of ammonia formation or H2 evolution. The highest apparent QY, ~24% was observed at 500 nm. The highest activity was observed for the Pt co-catalyst, while the highest selectivity – for Pd NCs (Fig. 39d). The photocatalytic activity of the nanocrystalline CIS in the nitrate reduction by oxalic acid was reported to be equal to that of the nanocrystalline TiO2 Evonik P25349, which is broadly used as a reference sample for comparing the photoactivity of various semiconductor nanomaterials. The surfactant-free aqueous CZTS NCs revealed photocatalytic properties in the reduction of Cr(VI) to Cr(III) with only ~5% loss in activity after ten consecutive cycles of the photocatalytic test380. The TiO2/CIS/ZnS composites were used as a photocatalyst of Cr(VI) reduction in aqueous solutions. The stability of the heterostructure was considerably increased by the covering with an amorphous TiO2 layer576,577. 4.3 Photocatalytic decomposition of organic dyes and other compounds The photodecomposition of organic dyes is often used as a test reaction to assess the photocatalytic properties of MC nanomaterials and compare them with other semiconductor

materials. Typically, persistent dyes are used in these studies, that is, the dyes stable to the photochemical and thermal oxidation in the absence of photocatalysts, in particular, methylene blue, methyl orange, Rhodamine B, etc. The photocatalytic properties in the dye degradations were 138 142 147 and CIZS NRs , AIS reported for CIS/ZnS , CIZS/ZnS NCs 435 334,392 164 and ZAIS NCs , ZnIn2S4 (ZIS) , CTS , 168,196,197,366,371,377,380,381,578 205,579 CZTS and CZGS NCs , Fe-, Mn-, 366 143 148 Co-, Ni-doped CZTS , and CFTS NCs , In4SnS8 nanosheets 166 and Ag8SnS6 NCs , and a variety of nanoheterostructures – 137 580 68,167 409 TiO2/CIS and TiO2/CIZS , ZnO/CIS and ZnO/CIS/GO , 458 358 68 401 ZnO/Cu2SnS3 , ZnO/CISe , CdS/ZIS , ZIS/AgIn5S8 , 392 414 354 ZIS/CdIn2S4 and ZIS/CdIn2S4/CuS , GCN/BiVO4/ZIS , carbon NT/CaIn2S4421 and Au/CaIn2S4356, GCN/CdIn2S4318, RGO/Cu2SnS3419, TiO2/CZTS328 and La2Ti2O7/CZTS428. The deposition of noble metal NCs (Pd, Au) on the surface of CZTS NCs was found to accelerate the photocatalytic degradation of methylene blue196,581. This study showed that the presence of a sole co-catalyst NC on the surface of a CZTS NC is preferential than the deposition of multiple smaller metal NCs (Fig. 44a). The most probable reason for this phenomenon is the enhanced electron-hole recombination on the multiple boundaries between CZTS and metal NCs in the latter case. In the studies of the photocatalytic degradation of organic dyes the reported data on the photoactivity of semiconductor materials can be compromised by a self-sensitization effect, when the dye decomposes as a result of the photoexcitation and the electron transfer to the semiconductor NCs present in the system582. In some cases this pathway can even be dominating, as for the Rhodamine B decomposition over a ZAIS/MoS2 heterostructure, where the photoexcitation of the adsorbed dyes molecules results in the cascade electron transfers from Rhodamine B to ZAIS to MoS2 and, finally, to oxygen345.

Fig. 44. (a) Methylene blue conversion efficiencies with CZTS and CZTS/Pd(Au) heterostructures. (b) A scheme of NIR light upconversion by the NaYF4:Yb,Tm phosphor coupled to CZTS NCs. Reprinted with permissions from ref. 196 (a) and ref. 578 (b). Copyright (2017) American Chemical Society (a); (2017) Elsevier.

Similar to the hydrogen evolution systems, the dyes decomposition processes often reveal a dome-shaped dependence on the MC NC composition. A typical example is the photocatalytic degradation of Rhodamine B over TiO2/(ZnS)x(CuInS2)1–x composites where the highest activity was observed for NCs with x = 0.8 corresponding to Eg = 2.14 580 eV . The dependence originates from a lower light-harvesting capability at smaller x and a lower bandgap at higher x.

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A proof-of-concept experiment on the expansion of the lightharvesting range of CZTS-based photocatalysts into the NIR domain by the up-converting approach was reported by W. Qe 578 et al. . The CZTS NCs were deposited onto the nanocrystalline ZnO support and coupled to the NaYF4:Yb,Tm microplates. The NIR light absorption by the microplates populates higher-lying terms of the dopant atoms followed by the discharging of much more energetic visible and UV light quanta that can be absorbed by the neighbouring CZTS NCs (Fig. 44b) acting as a photocatalyst of the methylene blue 578 degradation . The water-soluble CISe NCs capped with differently-sized polyethylene glycol (PEG) molecules with the terminal –SH groups revealed photocatalytic properties in the oxidation of a set of organic compounds – phenol, N,N-dimethyl-4phenylenediamine, methylene blue, and thiourea under the 82 illumination with the visible light . The universal action of the CISe NCs stems from the same primary and rate-determining step of the photoinduced CB electron transfer to molecular •– oxygen converting it to the superoxide anion-radical O2 . The superoxide-radical is well-known for its ability for the • protonation to HO2 and the recombinative decomposition yielding hydrogen peroxide that readily decomposes to very • 554,555,583,584 aggressive HO radicals : • • HO2 + HO2 = H2O2 + O2, • H2O2 = 2HO . The rate of oxygen reduction depends on the CB position of CISe NCs and increases as the size of NC is decreased due to the quantum confinement effect. In particular, the photocatalytic phenol conversion increased from 30% to 60% with a decrease of the average CISe NC size from 5.3 nm to 1.8 nm and an concomitant increase of the electrochemical (determined by the CV) bandgap from 1.4 eV to around 1.85 eV (Fig. 42a). At that, the phenol conversion efficiency follows in an almost linear manner the size-dependent increase of the thermodynamic driving force (free Gibbs energy difference, – ΔG) observed for the CISe NCs of a decreasing size (Fig. 42b). Another, but less important, factor limiting the photocatalytic activity of CISe NCs is the size of capping ligand. The phenol conversion was found to be slightly (by around 10%) decreased when the number n of elementary (OCH2CH2) fragments in the 82 thiolated PEG ligand was increased from 6 to 18 . At that, the photoactivity of PEGylated (n = 6) CISe NCs was at least 4 times higher than for the OLA-capped NCs. The composites of nanocrystalline titania with microspherical aggregates of CaIn2S4 NCs revealed photocatalytic properties in the oxidation of two pharmaceuticals – isoniazid and 407 metronidazole under the illumination with the visible light . The CB and VB levels of both components allow for the spatial electron and hole separation (Fig. 40d). The electrons localized on TiO2 NCs reduce oxygen, while the holes are consumed in the direct oxidation of organic compounds and water to •– + • hydroxyl radicals. All three intermediates (O2 , h VB, and HO ) were confirmed to be active oxidizing species by the radical 407 trapping experiments . The metronidazole was also photocatalytically destructed over the composites of hollow 403 TiO2 microspheres with ZIS NC microaggregates .

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and CdIn2S4 The composites of GCN with ZnIn2S4 NCs showed photocatalytic properties in the oxidation of antibiotic tetracycline under the visible light illumination. The GCN/ZIS nanoheterostructures were also reported to act as a 411 photocatalyst of methyl orange and phenol oxidation . The ternary BiVO4/BiFeO3/CIS nanoheterostructures revealed an increased photocatalytic activity in the degradation of 2,4427 dichlorophenol . The photoactivity enhancement (as compared to separately used CIS and BiVO4/CIS) is associated with the inner polarization introduced by the bismuth ferrite NCs facilitating the photoinduced electron transfer from CIS to bismuth vanadate. The introduction of RGO into the TiO2 NTA/CIS heterostructures enhances their photocatalytic activity in the 2,4-dichlorophenol degradation by around 456 40% . The TiO2/CIS nanoheterostructure immobilized on glass can be used as an active and stable photocatalyst of the 585 4-nitrophenol oxidation . The photocatalytic properties in the 2-nitrophenol degradation were also reported for AIS 586 nanoplates . The TiO2 NT/CIS heterostructures can be applied as a photocatalyst of 2,4-dichlorophenoxyacetic acid degradation 455 under the visible light illumination . The TiO2/CIS composites were used as a photocatalyst for the degradation of soil 140,587 fumigant 1,3-dichloropropene . To protect the composite in the aggressive conditions of the photoprocess it was covered with a ~1.3-nm-thick layer of Al-doped ZnS. A difference between the CB levels of titania (–0.4 V, NHE) and Ag-In-S NCs (–0.57 V, NHE) coupled into a TiO2/AIS nanoheterostructure enables efficient transfer of the photogenerated electrons from AIS to TiO2 resulting in a high photocatalytic activity of such composite in the visible-light398 driven gas-phase degradation of 1,2-dichlorobenzene . Selective oxidation of aromatic alcohols to the corresponding aldehydes driven by the visible light was reported for 347 ZnO/Zn3In2S6 heterostructure . The effect of selectivity is attributed to a cooperative catalytic action of zinc oxide that weakens the C–H and O–H bonds of the alcohols with a photocatalytic effect of ZIS NCs supplying oxidants – the + •– photogenerated h VB and O2 . The Mo-doped ZIS NCs were reported to be an efficient photocatalyst of the photocatalytic cyclization of aromatic 588 amines into imidazoles . 4.4 Photoelectrochemical/photoelectrocatalytic processes with multinary MC NCs The photocurrent generation in a photoelectrochemical systems based on MC NCs results from two simultaneous processes – the oxidation of water or additional sacrificial electron donor on the NC-sensitized photoanode and the reduction of water (or other acceptor species) on a counterelectrode (Fig. 38c). The photocurrent density is proportional to the amount of the H2 evolved on the counter electrode and can be adopted as a quantitative measure of the water splitting efficiency. An effect of the spectral sensitization of photocurrent generation was observed for titania NT arrays decorated with 443 426 444 CIS NCs , CIZS nanosheets , and AIS NCs ; TiO2 NR arrays

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404

189

445

with CIS NCs and CZTS NCs , TiO2 nanowire/In2S3/CIS , 406 TiO2/ZnIn2S4/CdS , ZnO nanowire arrays decorated with CIS 303 423,454 458 and Ag NCs , ZnO NRs with CIS NCs , Cu2SnS3 , and 424 425 AgSbS2 NCs , ZnO NTs with Ag-Cu-Sb-S NCs , as well as for 84,399 CdS nanowire/CdIn2S4 heterostructure . The TiO2 nanowire arrays covered with a buffer In2S3 layer and decorated with CIS NCs revealed unprecedentedly high stability during at least twenty consecutive current–voltage sweeps in aqueous KOH electrolyte, that is in the absence of 445 typical sulfide-containing sacrificial donors . This is possible, most probably, as a result of very efficient charge separation and transfers. A similar effect of fast hole transfer was deemed to be responsible for a very high photocorrosion stability of 84 CdS/CdIn2S4 photoelectrodes . The multinary MC NC-based photoanodes are often modified with additional MC shells both to enhance the light-harvesting efficiency and to protect the MC layer from the chemical/ photochemical corrosion. For example, a protective ZnS shell 437 was deposited onto the TiO2/CIZS photoanode , while the deposition of a Bi2S3 layer over the TiO2/CIS heterostructure increases the photoelectrochemical activity by a factor of 1.5 as a result of an increased light absorbance and a possibility of 441 the cascade charge separation . A buffer layer of zinc oxysulfide placed between the TiO2 nanorod array and a layer of the CZTS sensitizer NCs resulted in a photocurrent density of 2 ~15 mA/cm (at 1.23 V versus NHE) which is a record for the 189 CZTS NC-based photoelectrodes . The ZnIn2S4 nanosheets were tested as light-harvesting components of a photoelectrochemical system without additional semiconductors and showed a promising light-to-current 360,492 conversion efficiency . With the n-type conductivity and a bandgap of 2.6 eV this material can be a good alternative to toxic cadmium sulfide which is broadly applied in the photocatalytic and photoelectrochemical light-harvesting systems. The deposition of a layer of ZnIn2S4 nanosheets on the surface of Si nanowires increased the light conversion efficiency by a 400 factor of 64 . Apart from the water splitting, many other processes can be realized in a photoelectrochemical regime with physically separated reduction and oxidation stages, for example, the visible-light-driven 2-chlorophenol oxidation on the surface of 111 TiO2 NT/CIS photoanode . Conventional binary photoactive MC NCs, such as cadmium, zinc and lead chalcogenides reveal usually the n-type conductivity and pareticipate, therefore, in the oxidation reactions with the electrolyte components, while the photogenerated electrons are transferred to a metal oxide scaffold and then – via the circuit to a CE. As opposite, the multinary In, Ga, and Sn-based MC compounds typically have the p-type conductvity that opens the possibilities of constructing photocathodes releasing the photogenerated electrons to the electrolyte species and holes – into the external circuit and on a CE. For exampe, a p-conductive CIS NC layer were used as a photocathode in a photoelectrochemical system for the selective reduction of CO2 to 450,461 methanol with pyridinium ions as a co-catalyst . The process requires an external bias of 20 mV to compensate for

the CO2 reduction overpotential and shows a Faradaic efficiency of 97%. The photoelectrochemical activity of the nanocrystalline CIS is affected by the stoichiometry and the presence of “alien” ions. In particular, the Cu-rich CIS NCs showed a much higher / activity in the presence of methylviologen (4,4 -dimethyl bipyridyl bication) as an electron acceptor as compared to the 136 Cu-poor CIS NCs . The highest photoelectrochemical activity of the non-stoichiometric CIS deposited on ITO was observed 449 at a Cu/In ratio of 0.28 . 2– The Zn-, Sb-, or Ni-doped CIS films evolve H2 from aqueous S 2– 294 /SO3 solutions when illuminated with the visible light with the highest activity observed for the Zn-doped films. Doping with Ce was also found to increase the photoelectrochemical activity of the nanocrystalline CIS films; the highest light 457 conversion efficiency was reported for 10 mol.% Ce . A composite GCN/CIS photocathode evolves hydrogen from 0.1 M H2SO4, the presence of polymeric GCN sheets enhancing considerably the stability of CIS toward the photocorrosion 589 and revealing steady performance for at least 22 h . A similar stabilizing effect of S-doped GCN was observed for the CIS/CdS 590 photocathodes . The photoelectrochemical efficiency of CIS NC-based photo462,591 cathodes can be multiplied in the presence of Pt , Au or 589 462 Ag NCs , and additional protective layers, such as In2S3 and 388,462,591 CdS . The introduction of RGO sheets into the CISbased photoelectrode results in a 2-fold increase of the 450 photogenerated current density . The deposition of n-type CdS and TiO2 NCs into the surface of porous CIS results in the formation of p/n junctions enhancing the cathodic photocurrent and increases the apparent light-to460 hydrogen conversion to 1.82% . CIGS is a popular MC material for the photocathodes with a broad spectral response covering the entire UV, visible and 589,592,593 partially NIR range to ~1200 nm . The CIGS films on the optically transparent electodes can generate the cathodic photocurrent as a result of the reduction of dissolved molecular oxygen with the simultaneous water (or sacrificial donor) oxidation on a counter electrode. The photoactivity of CIGS layers can be enhanced drastically by the deposition of Pt NCs acting as relays of the electron transfer to O2 (Fig. 45a). A further acceleration of the cathodic process as well as an enhancement in the photocathode stability can be achieved by the deposition of a ZnS shell onto the CIGS surface prior to the 592 anchoring of Pt NCs (Fig. 45a) . The generation of cathodic photocurrents in the absence of additional sacrificial donors 486,594 was also reported for the alloyed ZnSe-CIGSe electrodes . An enhancement of the photocurrent generation on the CIGS photocathodes was also observed after the deposition of CdS, TiO2, and ZnxSnyOz layers as a result of the formation of p/n 485 junctions . The best CIGS/CdS/TiO2/Pt photocathode 485 exhibited a light-to-current conversion efficiency of 2.63% . However, this composite gradually lost the photoelectrochemical activity as a result of the interdiffusion of Pt atoms into the film and the CIGS constituent – in the outer layer as well as the degradation of the protective oxide layer.

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Fig. 45. (a) Current–voltage curves registered under the chopped illumination for CIGS photocathode, as well as CIGS/Pt and CIGS/ZnS/Pt heterostructures. (b) An energy level scheme of Mo/CIGS/CdS/ZnO photocathode for the water splitting. (c) SEM/scheme of TiO2 NR/ZnIn2S4/CdS photoanode. (d) Energy diagram of FTO/TiO2/ZnIn2S4/CdS photoanode. Reprinted with permissions from ref. 592 (a), ref. 595 (b), ref. 596 (c), and ref. 112 (d). Copyright (2014, 2015) American Chemical Society (a, b); (2017) Elsevier (c); (2015) The Royal Society of Chemistry (d).

A ternary photocathode was formed on the surface of coevaporated CIGS layer on the Mo-covered glass by depositing consecutively the layers of CdS and ZnO (Fig. 45b)595. The CIGS and CdS phases having the p- and n-type conductivity, respectively, form a p/n heterojunction enabling the efficient electron/hole separation and electron transport in the CdS layer bulk. Then, due to a CB level offset between CdS and ZnO the electron is forwarded to the photocathode/electrolyte interface where it can reduce water to hydrogen. The electrodeposition of a Pt layer onto ZnO enhances the latter process producing the cathodic photocurrents as high as –32.5 mA/cm2 595. Simultaneously, the water oxidation to O2 occurs on the Pt CE thus making this system similar to the natural photosynthesis systems. A photoelectrochemical cell with Ru-modified CGZS photocathode and CoOx-modified BiVO4 photoanode, both absorbing in the visible spectral range, can split water without the application of an external bias569. Recently, a tandem cell with a ZnSe-CIGSe photoanode and a BiVO4 photoanode was reported to yield 1% efficiency of the water splitting in an 597 unstirred electrolyte . One of the rapidly developing directions of NC-based photoelectrochemistry is the development of various photodriven sensors, in particular immunosensing platforms. For example, a photoelectrochemical immunosensor exploiting simultaneously semiconductor photoanode and photocathode was 598 reported by J.R. Zhang et al. The sensor comprised an ITO/TiO2/CdS:Mn photoanode and an ITO/CIS photocathode modified by an antibody complementary to a prostate-specific antigen. The photocathode surface free from the antibody is blocked by a layer of chitosan/bovine serum albumine. After the introduction of target species that specifically interact with the photocathode-anchored antibody, the photocathode surface gets completely blocked by the bulky organic molecules that impede the photocathodic oxygen reduction

(Fig. 46a). The blocade decreases the photocurrent proportionally to the logarithm of the target concentration allowing the determination in a concentration range of 1 pg/mL to 100 598 ng/mL with a detection limit of 0.32 pg/mL . The design of the sensor allows to solve the problem of a low selectivity of typical photoanode-based sensors caused by indiscriminative oxidation of all organic species by the photogenerated holes. In the present system the molecular oxygen is the sole electron acceptor and the photocathodic process is not interfered by any side reactions. The ZnIn2S4 NCs reveal the n-type conductivity, similarly to binary ZnS and CdS and can be combined with other n-type semiconductor NCs into the heterostructured photoanodes. For example, the titania NR arrays were decorated by the nanocrystalline ZnIn2S4 nanosheets and converted by the partial ion exchange into the ZIS/CdS heterostructure596 (Fig. 45c). The light-harvesting layer was then decorated with an efficient hole acceptor – Ni(OH)2 that enhanced the electron/hole separation and increased the anodic photocurrent. The fast withdrawal of the photogenerated VB holes by nickel hydroxide imparts the photoanode with a high stability toward the oxidative photocorrosion596.

Fig. 46. (a) Scheme of the CIS-based photoelectrochemical immunosensor, Ab – complementary antibody, Ag – target antigen, BSA – bovine serum albumin. (b) Energy diagram of the FTO/ZnO/CdS/CZTS photoanode. (c) Chronoamperograms registered for FTO/TiO2, FTO/TiO2/CZTS and FTO/TiO2/CZTS/Ag2S photoanodes with a different size of Ag2S NCs (explanation of H1–H3 designations provided in the text). (d) Energy level diagram of the FTO/CZTS/ZnSe/CdS/CdSe photocathode. Reprinted with permissions from ref. 598 (a), ref. 302 (b), ref. 181 (c), and ref. 248 (d). Copyright (2014–2016) American Chemical Society (a, c, d); (2015) The Royal Society of Chemistry (b).

A combination of TiO2, ZIS and CdS with the CB levels at –0.3 V, –0.4 V, and –0.5 V (NHE), respectively, creates a cascade structure with the CB potential decreasing from the outer CdS layer to the inner TiO2 scaffold allowing for the directed flow of the photogenerated electrons from the electrolyte species through the photoanode components and further – into the 112 external circuit . Simultaneously, the VB offsets of the components allow for a directed flow of the photogenerated VB holes in the opposite direction – toward the electrolyte species (Fig. 45d). As a result, the photoelectrochemical

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activity of the ternary TiO2/ZIS/CdS composite exceeds that of binary TiO2/CdS and TiO2/ZIS heterostructures by 2 and 3 times, respectively. A similar cascaded charge transfers occur also in the FTO/ZnO/CdS/CZTS photoanodes allowing for the efficient 2– photocurrent generation at the expense of the oxidation of S 302 2– /SO3 and the hydrogen evolution on the Pt CE (Fig. 46b) . The latter reference reported an original “two-storey” photoanode composed of a ZnO/CdS/CZTS film with an upper layer of stainless steel grid covered with the same ternary heterostructure. The grid absorbs both incident light and the irradiation reflected/scattered from the bottom FTO/ZnO/CdS/CZTS layer and thus the combined structure 2 produces a much higher photocurrent (~12.6 mA/cm ) than 2 2 the separate layers (~6.3 mA/cm and ~9.6 mA/cm for the 302 mesh- and FTO-supported heterostructures, respectively) . Nanocrystalline kesterite CZTS can be used as a photocathode material for the water reduction after the deposition of protective layers of CdS, Al-doped ZnO, and TiO2 and the 465 decoration with co-catalyst Pt NCs . The best Mo/CZTS/CdS/AZO/TiO2/Pt photocathodes showed a photo2 current density of ~1 mA/cm under the AM1.5 illumination 465 and a long-term stability . CZTS NCs decorated by the in situ photodeposited Pt NCs revealed PEC activity in the water 2– 385 2– reduction in aqueous S /SO3 solutions . The PEC activity of alloyed Cu2(FexZn1–x)SnS4 NCs can be modulated by varying the NC composition, the highest photocurrents observed at x = 270 0.4 . A nanocrystalline CZTS photocathode was coupled to different 466 photoanodes made of TiO2, WO3 and BiVO4 NCs . In such Zscheme the photocathode is capable only of water reduction, the VB potential being insufficiently positive for the water oxidation to O2. The coupling to a photoanode allows to combine the H2 evolution on the CZTS photocathode with the O2 evolution on the photoanode when both are connected by 2+ 3+ 466 a salt bridge and a Fe /Fe electron-shuttling redox-couple . The CZTS NC-based Mo/Cu2ZnSnS4/CdS/Pt photocathode was applied to reduce toluene to methyl cyclohexane under the 467 illumination with the visible light . This process, along with the benzene reduction to cyclohexane, is considered to be very promising way of the hydrogen storage in the form of liquid molecular hydrides, because H2 can be recovered relatively easily from these compounds under mild conditions and a 467 corresponding metal catalyst. Binary CZTS/Ag2S NCs can be used as a spectral sensitizer for 181 the nanocrystalline TiO2 photoanodes . Silver sulfide NCs were produced by the partial cation exchange in the parent CZTS NCs and the size of Ag2S NCs was varied from 2 nm to 4 nm and to 7 nm (samples H1, H2, and H3 in Fig. 46c) by + increasing the Ag precursor amount. The peak photocurrent was observed for the 4-nm Ag2S NCs, being by an order of magnitude higher than for the pristine CZTS sensitizer NCs (Fig. 46c). The photoefficiency boost was attributed to the the cascaded electron transfers from CZTS to Ag2S and to TiO2 coupled to the directed hole transfer in the opposite 181 direction. The deterioration of the photocurrent generation efficiency for larger Ag2S NCs is explained by possible changes

+

in the crystalline structure of CZTS NCs due to Ag inclusions as well as by the depletion of CZTS phase (transforming into Ag2S during the ion exchange) acting as a perfect electron/hole relay in the ternary TiO2/CZTS/Ag2S composite. In the design of composite MC NC-based photocathodes, a reverse cascade of CB positions is typically organized. The CB level decreases from the inner components of the photocathode to its outer components to facilitate the electron transport to the acceptor species in the electrolyte. The CZTS/ZnSe/CdS/CdSe heterostructure reported by K. Sivula 248 et al. serves as a perfect example of this strategy (Fig. 46d). The transfer of the photogenerated electrons to the accepting 3+ Eu species in solution is facilitated by a declining CB in the CZTS/CdS/CdSe cascade, while for the ZnSe layer the authors suggest a midgap-state-mediated electron transfer. The quaternary structure exhibits a higher photocurrent than any other possible binary or ternary combination of the 248 components . Nanocrystalline Cu2BaSnS4 with a bandgap of around 2 eV was reported to be a promising material for the water-reducing 528 photocathodes , a similar cascade effect being realized in the PEC water splitting system with the Cu2BaSnS4 photocathode 489 decorated by consecutive layers of CdS, ZnO, and TiO2. The p-conducting nanocrystalline Cu12Sb4S13 and Cu3SbS4 films on ITO revealed a PEC activity when coupled with a Pt CE and 3+ 233 immersed in aqueous Eu acceptor solution . The modification of Cu2SbS4 films with a layer of In2S3 creates a p/n junction resulting in an enhancement of the photocathodic hydrogen evolution over the Pt-decorated Cu3SbS4/In2S3 film 468 coupled to a Pt CE . The PEC activity was also observed for 276 the p-type CuSbS2 NCs (Eg = 1.40 eV) on FTO and for CuAsS2 171 NCs (1.25 eV) deposited on the Mo/glass substrates . Cu2SnS3 NCs formed a Schottky junction with Au NCs allowing for the transfer of the photogenerated CB electrons to the 164 metal NCs and favoring the photocurrent generation .

Conclusions and outlook Numerous reports discussed in this review show unambiguously that the multinary MC NCs differ in many aspects from their binary MC “relatives” when applied as the light-sensitive and auxiliary components of various lightharvesting systems. These distinctions stem mostly from a unique capability of multinary MC NCs to tolerate a broad variability of compositions and stoichiometries while exhibiting the same and unified structural motif. This feature is multiplied by two other distinctive characteristics of multinary MC NCs, in particular, the unprecedented capability of forming solid solutions by the partial substitution of either metal or chalcogenide ions as well as the unique tolerance to doping. The tolerance to the non-stoichiometry allows to vary optical, photophysical and electrophysical properties of ternary and quaternary MC NCs in a broad range providing potent means of tailoring both their optical response and the properties of the photogenerated charge carriers. Apart from the energy characteristics, the conductivity type, the charge carrier density and the structure of the surface double electric layer

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can be affected by varying the constituent ratios in the multinary MC NC within a certain set of elements – Cu–In–S, Cu–Zn–In–S, Cu–Zn–Sn–S, etc. Along with the tolerance to the non-stoichiometry, the optical and electrophysical properties of multinary MC NCs can be affected in a decisive manner by alloying several types of cations/anions. The NC characteristics change gradually and in a continuous manner with the partial substitution of the “central” metal ions, e.g. In with Ga in CIS or Sn with Ge in CZTS. Similar trends are observed if a combination of two chalcogenide ions is introduced simultaneously in a multinary NC compound. Finally, the doping can be applied to achieve various aims, in particular, to affect the energies of the photogenerated charge carriers and to fill the lattice vacancies like in the case of AIS NC doping with zinc or copper ions. Moreover, doping can 3+ induce controlled aggregation, as in the case of Sb in CZTS NCs or affect the charge transport properties as in the case of indium doping of CZTS NCs. These three features – the non-stoichiometry, the alloying and the doping taken together define a specific landscape of the studies of the light-harvesting systems based on the multinary MC NCs making it quite different from those based on the “conventional” binary NC such as cadmium or lead chalcogenides. However, despite the discussed differences the basic principles of the light-harvesting system design, the ideology of enhancing the efficiency of the light conversion and minimizing the looses of the photogenerated charge carriers as well as the protocols for the synthesis of multinary MC NCs are generaly the same as devised and tested for the binary MC NCs. From the synthetic viewpoint, the multinary NC sensitizers for the solar cells and photocatalysts for the visible-light-driven redox-reactions are produced most often via the heating up/hot injection syntheses and the hydrothermal/ solvothermal treatments, respectively – similarly as it happens with the binary MC NCs. However, a much more intricate chemistry lies behind the formation of ternary, quaternary and even more complex MC NCs as compared to the binary counterparts, arising from a different reactivity of metal ionconstituents, the possibilities of ions migration and intermixing, the formation of different phases (even within the single NCs) and solid solutions, etc. In this view, a precise control of the multinary MC NC size, shape, composition, surface chemistry and other parameters becomes a much more difficult task. The need of the post-synthesis ligand exchange and phase transfer procedures makes handling of these NCs even more difficult. Therefore, even the case of “conventional” heating up/hot injection syntheses cannot be considered as a “done and closed” one. Indeed, further studies on the mechanism of the multinary NC nucleations and growth, the parameters governing the NC shape, phase and composition as well as the routes of more efficient ligand exchage, which is vital for the light-harvesting applications, are still necessary and will undoublty be continued in the future. Also, a broad field for new methods is open for the syntheses of multinary NCs. In particular, the syntheses of concentrated

MC NC “inks” directly in polar solvents with relatively small multifunctonal capping ligands and a defined composition/size can boost the development of both bulk-heterojunction and liquid-junction solar harvesters and make them much more competitive relative to the organic bulk-heterojunction and dye-sensitized liquid-junction cells. For both cell types, the self-assembly of light-absorbing MC NCs into highly ordered and dense layers with a pre-defined thickness and symmetry is a crucial task requiring the development of new approaches to the NC surface modification. A very promising step in this direction was made by stabilizing the multinary MC NC with highly charged metal chalcogenide complexes that allow for the self-assembly of the NCs and the conversion of the capping MCC ligands into a conducting interconnecting layer after the 22 thermal treatment . A further progress in the chemistry of NC-MCC composites may have far-reaching consequences for the progress of the NC-based solar cells as well as for other 27 fields dealing with the self-assembled NC-based solids . The multinary MC NCs were successfully introduced into the solid-state bulk-heterojunction solar cells both as light absorbers and charge transport layers. As the multinary MC NC are intrinsically rich with lattice vacancies and/or defects with the corresponding electron states lying in the bandgap, the solar light can be harvested not only in the “classical” range of hv ≤ Eg, but also below the bandgap as a result of the light absorption involving these defect-related states. This feature opens the possiblities of the NC design aimed at an optimization of such light absorption and an expansion of the light harvesting range. The morphology of wide-bandgap metal-oxide scaffolds housing the multinary MC NCs in the solar cells can affect not only the charge separation and transport efficiency but also 90 the character of such below-bandgap absorbance and, therefore, the further optimization/design of the oxide scaffold is expected to be one of the major factors affecting the efficiency of MC NC-based bulk-heterojunction solar cells 28–30 as it is in the case of hybrid organo-inorganic perovskites . Additionally to the light harvesting, the multinary MC NC can also perform as a hole transport layer when introduced in combinations with conjugated polymers and hybride perovskites. The latter combination seems especially attractive because the MC NC acts as a second light absorber that enhances the light harvesting of the cell and even extends it to the NIR range. A hint at the perspectives of combining the hybride perovskites with the multinary MCs was provided in 599 2014 by a report of T. Todorov et al. where the solar cells with adjacent layers of the microcrystalline CIGSe or CZTSSe and the hybride CH3NH3PbI3 perovskite manifested light conversion efficiencies exceeding 20%. It is expected that by extending this strategy to the bulk heterojunctions of nanocrystalline multinary MC materials even higher lightharvesting efficiencies can be achieved. The development of the technologies of ink-jet printing of some components of the bulk-heterojunction solar cells or even the entire cell including the ETL, HTL and conducting contact layers is of paramount importance for the progress of such devices. In view of special features of the multinary MC

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NCs, this method is expected to yield especially highly efficient cells when applied for the consecutive deposition of NC differing in composition and size. This will produce gradient structures with the CB/VB levels of the absorbing layers inclined to one or other charge transport layer thus enhancing the primary separation and migration of the photogenerated charge carriers as compared to “conventional” devices with a constant composition of the NC absorber layer. The simplicity and efficiency of the jet printing can be combined with broadly available multinary NC compounds, such as copper/silver bismuth sulfide to produce inexpensive solar cells. Such compounds manifested up to now quite modest light-to-current conversion efficiencies, however a trade-off between the low production cost, low toxicity, high availability and moderate light-conversion efficiency can make such devices a real competitor to both high-production-cost thin-film MC solar cells and the inherently toxic leadcontaining perovskite solar cells. The multinary MC NCs were found to be quite efficient spectral sensitizers for the wide-bandgap oxide photoanodes in the photoelectrochemical liquid-junction solar cells, similarly to the case of binary CdX/PbX-based cells. The broad variations of the CB/VB levels and the spectral sensitivity range by adjusting the composition and size of MC NCs promised unprecedently versatile possibilities of the design of such cells. These designs may be further extended if coupled to the accumulated knowledge of the basic principles of the optimal cell morphologies and the prevention of charge losses by various additional blocking/buffer layers gained previously for the SSSCs with binary CdX/PbX NCs. However, the “classical” design of such cells implies the utilization of a photoanode and therefore the sensitizer NCs should preferably have an n-type electrical conductivity along with the correspondence between the CB levels of the MC and metal-oxide NCs. The combination of these requirements de facto restricted the scope of available MC sensitizers to the nconducting CIS/AIS compounds and their derivatives, while the highly promising kesterite family did not show acceptable activities in the liquid-junction SSSCs. Nevertheless, the hope of overwhelming this restriction remains real and associated with an alternative cell design, where the light is harvested not with a NC-incorporating photocathode, where the sensitizer supplies the VB holes to the p-conducting metal oxide scaffold and accepts electrons from a redox-shuttle in the electrolyte. This design was shown to be efficient for lead chalcogenide absorbers, but only pioneer works appear to be published starting from 2016 on the multinary CIS/CISe NC-based 123 photocathodes of the liquid-junction SSSCs . It may be expected that a further development of such cell design and broad involvement of other MC compositions, such as kesterite CZTS/CZTSe NCs will allow to realize more completely a huge potential of multinary MC NCs as the light harvesting components of the liquid-junction SSSCs. The perspectives of this direction are vividly exemplified by a variety of photocathodes applied in the photoelectrochemical light-harvesting systems for hydrogen generation as well as in the PEC immunosensors described in the present review.

This strategy can also be extended to the cells combining a photocathode and a photoanode both absorbing the solar light and contributing to the photocurrent generation. A carefull optimization of such tandem cells (a successful example can be 600 found for a PbS-based tandem liquid-junction SSSC ) is expected to add the photovoltages of both electrodes and bring the cell efficiency above the Schockley-Queisser limit. Among the photocatalytic light-harvesting systems based on multinary MC NCs the most vivid progress was achieved in the area of water splitting/hydrogen evolution systems both in photocatalytic and photoelectrocatalytic versions. The multinary MC compounds are tested as the water-splitting photocatalysts for over three decades now and the photocatalyst crystal size coming into the nanometer range brought a new dimentionality and impetus to these studies. Similar photosynthetic systems for CO2 and nitrogen reduction are not as high developed as for the binary MC photocatalysts but a surge of interest for such systems is expected in view of recent apprehension of the global risks and challenges posed by the over-abundant anthropogenic CO2 emissions and 601 corresponding global climate regulation initiatives . Various photocatalytic reactions with multinary MC NCs can also be used as a test for new and emerging compounds as potential light-harvesters for the solar cells. Indeed, the MC materials typically used in the solar cells, such as chalcopyrites CIS/AIS and kesterites CZTS/CZTSe appeared to serve as excellent photocatalysts both in the individual state and when introduced into the heterostructures with other nanocrystalline semiconductors, 2D materials, and metals. At that, the assortment of MC materials tested as photocatalysts is even broader that that of light-harvesting solar cell components. In this view, the photocatalytic reactions with MC NCs should be viewed upon as an inspiration for the development of new solar cells, as the photocatalytically active MC NCs will most probably appear to serve also as efficient light absorbers. Some of the multinary compounds gained a reputation as efficient and quite universal photocatalysts with the activity close to or exceeding those of “classical” semiconductor photocatalysts – TiO2 and CdS, but many of such MC compounds were only scarsely tested in the solar cells; the layered zinc indium sulfide is probably the most vivid example. It is expected that the simultaneous development of MC NCbased photocatalysts and the photovoltaics will be a source of mutual enrichment with new materials and ideas for both of these neighboring disciplines.

Complete list of abbreviations and symbols AM1.5G – solar spectrum near the Earth surface 2D/3D – two-/three-dimensional AIS – silver indium sulfide (Ag-In-S) AIGS – silver indium gallium sulfide (Ag-In-Ga-S) AZO – aluminium-doped zinc oxide CAIS – copper silver indium sulfide (Cu-Ag-In-S) CB – conduction band CBS – copper bismuth sulfide (CuBiS2) CBTS – copper barium tin sulfide (Cu-Ba-Sn-S)

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CE – counter electrode CFTS – copper iron tin sulfide (Cu-Fe-Sn-S) CGS – copper gallium sulfide (Cu-Ga-S) CIS – copper indium sulfide (Cu-In-S) CIGS – copper indium gallium sulfide (Cu-In-Ga-S) CGS – copper gallium sulfide (Cu-Ga-S) CGZS – copper gallium zinc sulfide (Cu-Ga-Zn-S) CIZS – copper indium zinc sulfide (Cu-In-Zn-S) CTS – copper tin sulfide (Cu2SnS3) CV – cyclic voltamperometry CZTS – copper zinc tin sulfide (Cu-Zn-Sn-S) 1-DDT/t-DDT – 1-/tert-dodecanethiol DMF – N,N-dimethyl formamide DMSO – dimethyl sulfoxide DSSC – dye-sensitized solar cell ETL – electron transport layer EQE – external quantum efficiency FTO – F-doped tin oxide GCN – graphitic carbon nitride GSH – gluthathione GO – graphene oxide HOMO – highest occupied molecular orbital HRTEM – high-resolution TEM HTL – hole transport layer HTT – hydrothermal treatment ITO – indium tin oxide IZO – intrinsically doped zinc oxide LUMO – lowest unoccupied molecular orbital MC – metal chalcogenide MCC – metal chalcogenide complex MEH-PPV – with poly(2-methoxy-5-(2-ethylhexyloxy)-1,4phenylene vinylene) MPA – mercaptopropionic acid NR – nanorod NC – nanocrystal NIR – near infrared NT – nanotube OLA – oleylamine ODE – octadecene OTE – optically transparent electrode P3HT – poly-3-hexyl thiophene PCBM – [6,6]-phenyl C71 butyric acid methyl ester PCE – power conversion efficiency PEDOT:PSS – poly(3,4ethylenedioxythiophene): poly(styrene sulfonate PEG – polyethylene glycol PESA – photoelectron spectroscopy in air PL – photoluminescence PTB7 – poly[[4,8-bis[(2-ethyl-hexyl)-oxy]-benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]-thiophene-diyl]] PVP – polyvinyl pyrrolidone RGO – reduced graphene oxide SEM – scaning electron microscopy SILAR – successive ionic layer adsorption and reaction Spiro-OMeTAD – N2,N2,N2′,N2′,N7,N7,N7′,N7′-octakis-(4methoxyphenyl)-9,9′-spirobi[9H-fluorene]-2,2′,7,7′-tetramine

SPR – surface plasmon resonance SSSC – semiconductor-sensitized solar cell STEM – scanning transmission electron microscopy STT – solvothermal treatment TAA – thioacetamide TEA – triethylamine TEM – transmission electron microscopy TGA – thioglycolic (mercaptoacetic) acid TMSC – trimethyl silyl cellulose TOPO – trioctyl phosphine oxide UPS – UV photoelectron spectroscopy UV – ultraviolet VB – valence band QY – quantum yield XPS – X-ray photoelectron spectroscopy ZAIS – silver zinc indium sulfide (Zn-Ag-In-S) ZIS – zinc indium sulfide (ZnIn2S4) _________ acac – acetyl acetonate aB – exciton Bohr radius dedtc – diethyl dithiocarbamate Et – ethyl – eCB – conduction band electron + hVB – valence band hole hv – light quantum energy Ea – electron affinity Eg – bandgap ECB – conduction band potential EF – Fermi energy EVB – valence band potential FF – fill factor of a current-voltage characteristic Jsc – short-circuit photocurrent density Ip – ionization potential ket – electron transfer rate constant * m e – effective electron mass * m h – effective hole mass η – total light conversion efficiency Rct – charge transfer resistance Voc – open-circuit photovoltage

Conflicts of interest There are no conflicts to declare.

Acknowledgements O.S. acknowledges the financial support of the European Union’s Horizon 2020 research and innovation program (Marie Skłodowska-Curie grant agreement No 701254).

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