Toward Perovskite Solar Cell Commercialization

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Toward Perovskite Solar Cell Commercialization: A Perspective and Research Roadmap Based on Interfacial Engineering Adharsh Rajagopal, Kai Yao,* and Alex K.-Y. Jen* (PVs) contribute only to a miniscule (1 ns to 1 µs), and (>1 µs to 1 ms), respectively.

techniques (charge extraction by linearly increasing photovoltage and time of flight), and steady-state methods (Hall effect, fieldeffect transistor, and space charge–limited current) have been employed to understand the charge carrier dynamics at all levels from neat films to complete solar cells.[127,128] Here, we provide a contextual discussion to elucidate the importance of interfacial charge dynamics in PV functionality, categorized into three parts: charge extraction governing photo current, charge recombination governing photovoltage, and charge accumulation governing hysteresis behavior.

3.3.1. Charge Extraction Governing Photocurrent Perovskite semiconductors are semi-intrinsic in nature due to the doping arising from several intrinsic (defect formation chemistry) and extrinsic (film processing environment and interlayer interaction) mechanisms.[35,37] As the unintentional intrinsic self-doping (Figure 4a) is difficult to control in perovskite films, interlayers (p- and n-type) sandwiching the perovskite absorber play a crucial role in the origin of the photovoltaic action. Interlayers establish the built-in electric field (Vbi) necessary to provide the driving force for charge separation (drift of electron and holes in the opposite directions toward interfacial contacts). The competition between interfacial charge transfer and recombination processes determines the fate of photogenerated carriers, and a large discrepancy in the dynamics of these processes is thus desired for efficient photocurrent extraction.[122,129,130] Dynamics of charge transfer and transport can be boosted to outcompete the recombination processes through suitable alignment of interfacial energy levels with perovskite, high conductivity

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of interlayers, and low levels of interfacial defects. A high carrier-collection efficiency is favored in PVKSCs because of the ultrafast charge generation, separation, and transfer processes compared to the free-carrier lifetime.[122] Diffusion of carriers to contacts preceding the interfacial charge transfer takes ≈200 ps to a few nanoseconds depending on the thickness of perovskite films.[131] In the common mesoporous-type architecture, the interfacial charge transfer (electron and hole injection from perovskite to TiO2 and spiroOMeTAD, respectively) occurs on picosecond timescales, which is ≈106 times faster than the pertinent recombination processes (µs range).[122] Compared to spiro-OMeTAD and TiO2, charge transfer from MAPbI3 to other p-type (P3HT, PTAA) and n-type (C60, PCBM) organic transport materials is 3–4 orders of magnitude slower, just sufficient enough to overcome the recombination processes for charge extraction.[132,133] For oxide CTMs (TiO2, NiOx), interfacial defects and poor bulk conductivities

can limit the charge extraction despite the high charge transfer rates (sub-picosecond).[134,135] Higher interfacial contact area (Sections 4.1.1 and 4.2.1), surface passivation (Section 4.3), doping and energy level tailoring (Sections 5.1.1–5.1.4) are approaches that have been employed to improve the charge extraction yield for interlayers with limitations. An illustration of the extraction yield as a function of ETL carrier density is shown in Figure 6a. Mitigation of the heterogeneity in interfacial charge extraction and the associated recombination losses are key for future performance improvements.[80,136] 3.3.2. Charge Recombination Governing Photovoltage The thermodynamic limit for Voc considering 100% radiative recombination is

Voc,rad =

 kBT  J sc In  + 1 ; Φem,0 = e  eΦem,0 

∫ a (E ) ϕ

BB

(E ) dE ,

where Voc,rad is the radiative limit for Voc, a(E) is the material

Figure 6. Effect of interfacial charge dynamics on solar cell device performance. a) Extraction yield as a function of charge carrier density for different electron transport materials. Reproduced with permission.[133] Copyright 2017, Wiley-VCH. b) EQEEL as a function of applied forward bias for PVKSCs with different ETMs; circles indicate values at injection current densities equivalent to Jsc under 1 Sun illumination. The inset shows device Voc versus average fluorescent lifetime, with linear line as a reference. Reproduced with permission.[142] Copyright 2017, Wiley-VCH. c) Illustration of energetic landscape at the TiO2/Perovskite interface at four different stages; band bending results from an increase in the hole concentration at the interface associated with ionic accumulation, which forms an electrostatic potential (added to Vbi) due to the electric field across the interface. Reproduced with permission.[149] Copyright 2016, Elsevier Inc.

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absorptance as a function of photon energy E, and ϕBB(E) is the black body spectrum. Using the reciprocity between absorption and emission, a solar cell Voc can be expressed as Voc = Voc,rad − ∆Voc,non-rad ; ∆Voc,non-rad =

kBT −1 In EQEEL , where ΔVoc,non-rad e

accounts for losses due to the nonradiative recombination and is quantified by the electroluminescence quantum yield (EQEEL).[36,137] The correlation between EQEEL and Voc for PVKSCs with different ETLs is shown in Figure 6b. To make the solar cell highly luminescent and maximize Voc, the quality of absorber material and interfacial contacts is crucial. Defects in the perovskite (dependent on the film composition and morphology) and the surface recombination (dependent on the interfacial contacts) are common nonradiative recombination channels that limit PVKSC performance.[138] The intimate link between surface recombination and photovoltage is evident in HTM- and/or electron transport material (ETM)-free PVKSCs, where the less regulated interfaces result in severe Voc loss.[139] In PVKSCs, Voc originates from the photogenerated electrochemical potential (quasi-Fermi level splitting) in perovskite, which is dependent on the bandgap and charge carrier densities sustained under illumination.[36] Significantly higher Voc loss is therefore observed when the surface recombination is dominant, as in the case of PVKSCs employing PEDOT HTL.[36,140] Carrier losses by surface recombination can be controlled by tuning the interfacial energetics, charge extraction, and defect passivation. Interfacial energy level alignment is important in the sense that they are strongly correlated with the Vbi driving charge carrier separation and the recombination losses due to back charge transfer at contacts. Improved charge selectivity and reduced interfacial recombination enabled by tailoring the interlayer or introduction of ultrathin insulating layers have successfully improved Voc.[141–144] Recently, it has also been identified that dopants employed to improve HTL (spiroOMeTAD, PTAA) conductivity act as recombination centers and are detrimental to Voc. Accordingly, the reduction of dopant concentration increases Voc up to 1.23 V, which is inching closer to the corresponding Voc,rad limit (1.33 V).[145] Furthermore, contact passivation mitigates electronically active interfacial defects to suppress nonradiative surface recombination and provides significant Voc gain (as discussed in Section 4.3).[146–148] 3.3.3. Charge Accumulation Governing Hysteresis Behavior The mixed electronic–ionic nature of hybrid perovskites has strong implications on their functional properties.[150,151] Facile vacancy-assisted ion migration with low activation energies (0.1–0.6 eV) results in significant ionic conductivities under conditions relevant to the solar cell operation.[63,151] Sandwiching perovskite between CTMs as in PVKSCs facilitates the electronic transport but blocks the ionic transport across interfaces, inevitably causing ionic accumulation and excess low-frequency capacitance. This significantly influences the electrode polarization (piling up of space charge near interfaces[152,153]), interfacial energetics (doping or dipole formation by ionic defects[153–155]), built-in electric field (compensating ion-induced electric field[156]), and functionality (tunneling mediated by ions[157,158]). Impacts associated with the charge accumulation are illustrated

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in Figure 6c using TiO2/perovskite interface as an example. The resultant PVKSCs are plagued by complex hysteresis behavior, where I–V characteristics are distorted by the additional voltagedependent capacitive (due to light and electric field–driven ionic/electronic charge redistribution) and noncapacitive (due to interfacial reactivity) current components with a slow (seconds to minutes) transient response.[8,61,62,139,159] They manifest as the change in PV performance metrics and shape of I–V characteristics with scan direction as well as the operational instability at maximum power point. In addition, the ionic migration and accumulation makes PVKSCs strongly sensitive to environmental exposure, illumination, and biasing history.[160,161] Interfaces play an important role in mediating the ion accumulation and hysteresis behavior, which are inherently tied to the interfacial interaction, diffusivity, charge extraction, recombination, and perovskite structural/chemical changes.[8,139] In this regard, fullerene-based interlayers are most influential because of multiple aspects such as passivation of defects,[111,117] inhibition of ion migration,[118] minimization of charge accumulation,[162] and improvement of charge extraction,[112] facilitated by the chemical reaction with perovskite (anion-induced electron transfer).[163,164] Hysteresis instabilities in PVKSCs have been mitigated through fullerene incorporation in numerous ways: interlayers, interfacial modifications, and blends with perovskite.[113,114,165] Engineering interfaces to reduce the charge accumulation by control of the interfacial electric field, ionic permeability, and ionic reactivity also suppresses hysteretic effects.[166–168] On the whole, improved interfacial charge dynamics (fast charge transfer, low nonradiative recombination, and less charge accumulation) alleviates hysteresis and improves performance of PVKSCs.[8,62,139]

3.4. Device Stability and Durability The promising rise of PVKSCs is hampered by the unstable nature of hybrid perovskites. Tailored charge transporting interlayers have added capabilities of a barrier layer to protect the perovskite from external contaminants and interfacial modifications modulate the chemical reactivity of perovskite with adjoining layers. External factors such as humidity, temperature, pressure, light, electric field, and chemical environment strongly influence the perovskite absorber characteristics and its functionality. Overcoming impediments in the material and device stability is thus crucial for commercial deployment. Schematic illustrations of different degradation pathways in PVKSCs are provided in Figure 7. The degradation of hybrid perovskites is intrinsically related to their structural instability and soft nature. Presence of volatile organic components and prevalence of mobile ionic defects result in sensitivity to external factors and interaction with associated layers, which are ultimately responsible for majority of degradation pathways (Figure 7a–d). Reactivity with H2O, O2, and I2 vapors also permanently renders the perovskite material less useful for photovoltaic conversion.[65,169,170] Detailed account of degradation mechanisms and approaches for perovskite design to circumvent stability issues can be found elsewhere.[8,65,171] Beyond intrinsic material nature of hybrid perovskites, interfaces also play a crucial role in influencing the device stability and durability (Figure 7e,f).

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Figure 7. Top figure schematically illustrates six representative degradation pathways induced by moisture, heat, and light in regular (n–i–p) architecture PVKSCs. Bottom figures((a)–(f)) provide descriptive information pertinent to each of those degradation pathways: a) hydration scheme for MAPbI3: initial hydration results in formation of monohydrate and subsequent hydration results in formation of dihydrate along with PbI2. Reproduced with permission.[170] Copyright 2015, American Chemical Society. b) Variation of photoluminescence peak wavelength and absorption band-edge wavelength (energy) as a function of temperature. Reproduced with permission.[178] Copyright 2015, Wiley-VCH. c) Modeled maximum power conversion efficiency (with fast halide and slow cation vacancy migration) over three cycles of light and dark shows similar (nonquantitative) reversible performance losses. Reproduced with permission.[160] Copyright 2017, The Royal Society of Chemistry. d) Schematic depiction of dissociation mechanism of MA+ due to the capture of a photogenerated electron. Reproduced with permission.[161] Copyright 2017, Wiley-VCH. e) Oxygen-induced formation of deep trap states at mesoporous TiO2 surface/interface under UV illumination. Reproduced with permission.[65] Copyright 2016, The Royal Society of Chemistry. f) Reconstructed elemental 3D maps for Au− and I− ions traced in the depth profile (by ToF-SIMS) demonstrate ionic diffusion across the device. Reproduced with permission.[187] Copyright 2016, American Chemical Society. First four pathways (a)–(d) of degradation can be slowed down by tailored interlayer choices, whereas the last two pathways (e) and (f) of degradation can be eliminated via interfacial modifications.

In this section we discuss critical roles of interfaces in modulating instabilities induced by intrinsic perovskite degradation, ionic migration, and other device components. The discussion

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summarizes interface-pertinent degradation pathways and points out the promise of interfacial engineering strategies for improving the PVKSC durability.

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3.4.1. Instability Induced by Intrinsic Perovskite Degradation Studies have shown that even though alternate perovskite formulations with organic cation modifications show better resilience than typical MAPbI3, the mechanism and principles of degradation are fundamentally similar to those established with earlier studies based on MAPbI3.[28,169,172,173] Degradation of MAPbI3 results in disintegration of the material into PbI2 and volatile products. Boundary conditions of the degrading perovskite determine the specific degradation pathway and the nature of volatile products (CH3NH2, HI, NH3, I2, and CH3I). Humidity exposure causes severe degradation due to the aqueous solubility of organic component in hybrid perovskites.[174,175] Water molecules diffused into perovskite films form hydrogen bonds with the volatile organic components and result in a reversible monohydrate phase; however, excessive moisture exposure subsequently results in an irreversible dihydrate phase and permanent loss of organic molecules (Figure 7a).[170,174,176,177] When coupled with light/heat, the degradation due to moisture ingress is faster because of the accelerated formation of PbX2 phases (Figure 7b).[178] Application of an electric field in the presence of water also quickens degradation due to the drift of loosely bound cations in the hydrated phase that results in destabilization of the perovskite structure.[160] Utilization of hydrophobic interlayers is an internal line of defense that resists the attack of moisture and alleviate the above-mentioned impacts of moisture.[179] Besides moisture, a combination of light and oxygen initiates a much severe degradation due to the formation and reaction of superoxide (O2−) with the organic moiety in perovskites (Figure 7c,d).[180–183] Since O2− species result from the interaction of photoexcited electrons with the molecular O2, efficient interfacial charge extraction is crucial for mitigating the impact of light- and oxygen-induced degradation.[183] Charges trapped at interfaces and grain boundaries are another irreversible degradation trigger, where the induced local electric field results in the deprotonation of organic cations and permanent decomposition of perovskite.[172] Appropriate selection of electron- and hole-transporting interlayers that possess hydrophobic properties and facilitate efficient charge extraction is thus imperative for preventing intrinsic perovskite degradation.

3.4.2. Instability Induced by Ionic Migration Degradation of CTMs and electrodes happen due to their undesirable reactivity with profoundly migrating ionic point defects (both within the perovskite and across interfaces). Factors such as electric field, moisture, and thermal stress prevalent during device operation have a catalytic effect on the ionic migration and result in a faster decay of PVKSC performance. Chemical reaction between spiro-OMeTAD+ and the migrating I− ions progressively reduces conductivity and hinders functionality of the HTL.[184] Migration and accumulation of I− ions at the internal surface of Ag electrode leads to the generation of a AgI barrier for charge collection. The oxidation of Ag to AgI (yellow coloration) and the associated performance decay are not only observed when exposed to air, but also observed in N2 atmosphere when the PVKSC is stressed under operating conditions.[185] Other

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metal electrodes such as Al, Cr, and Yb induce device degradation due to the redox reactions between Pb2+ in perovskite and the neutral metal contact.[186] When Au electrodes are employed, diffusion of Au through spiro-OMeTAD into the perovskite (Figure 7f) deteriorates all PV parameters due to the introduction of shunting pathways and deep trap sites.[187] Employment of robust interlayers and barrier layers at electrode contacts has minimized the ionic reactivity and alleviated the associated degradation pathways (Section 4.3.3). Beyond degradation of contacts, ionic migration and accumulation at interfaces is the major culprit behind widely observed I–V hysteresis instabilities in PVKSCs (Section 3.3.3).

3.4.3. Instability Induced by Other Device Components Degradation of other device components (interlayers, electrodes) and permanent damage triggered by their reactivity with the perovskite absorber also result in PVKSC instability. In terms of ETL, the commonly employed TiO2 suffers from photoinstability due to the photocatalytic activity under ultraviolet (UV) light exposure (Figure 7e). The unoccupied deep surface trap sites in TiO2 (generated by the reaction between photogenerated holes and oxygen radicals) induce oxidation of the perovskite layer and promote formation of PbI2, resulting in increased recombination losses.[188,189] The ZnO ETL suffers from thermal instability due to the basic nature of its surface. Proton-transfer reactions at the ZnO/perovskite interface result in loss of the organic cation and decompose the perovskite to PbI2.[190–192] Alternate ETL materials (Sections 4.1 and 4.2) and different interfacial modifications (Section 4.3.1) have been employed to alleviate instability at the ETL/perovskite interface. On the other hand, for HTL, the most popular candidate spiro-OMeTAD is often doped with 4-tertbutyl pyridine (tBP) and bis(trifluoromethane)sulfonimide lithium salt (Li-TFSI) to improve the formability and conductivity. However, the sophisticated oxidation procedure associated with the doping process, the corrosive action of tBP on perovskite, and the redistribution of Li-TFSI under the action of electric field or ambient exposure make PVKSCs with spiro-OMeTAD extremely unstable and not reliable in the long term.[193,194] Alternate organic HTL choices such as poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] and PEDOT:PSS are also not intrinsically stable because of their sensitivity to moisture and oxygen. Development of next-generation organic or inorganic p-type interlayers with better intrinsic properties (Sections 5.1.1–5.1.3) will be pivotal to mitigate the HTL instability.

4. Variants of Interfaces Driving Evolution of Perovskite Solar Cell Performance PVKSCs device configuration can be broadly classified into two types: mesoporous and planar. The mesoporous device configuration commonly features a perovskite-infiltrated and capped mesoporous scaffold with interfacial contacts on the either side. The planar device configuration encompasses perovskite layer sandwiched between the flat electron- and holeselective contacts. Elimination of the mesoporous scaffold is beneficial for the commercial scale production and reducing

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manufacturing costs. However, a major challenge in the mesoporous layer free planar devices is realizing high-quality perovskite films with suitable CTMs to facilitate efficient light absorption and charge extraction. In both mesoporous and planar devices, tailored ETL, HTL, and interfacial modifications are pivotal for developing high-performance, stable PVKSCs. Both mesoporous and planar PVKSCs can be fabricated in a regular “n–i–p” (Figure 8a) or an inverted “p–i–n” (Figure 9a) architecture, defined based on whether electrons (n) or holes (p) are collected at the bottom transparent conducting oxide (TCO) electrode. A regular (inverted) device architecture PVKSC has ETL (HTL) at the bottom and HTL (ETL) on the

top. Accordingly, the basic requisites for these architectures are as follows: 1) ETL (HTL) should be robust to function as an able growth platform for the perovskite absorber and 2) HTL (ETL) should be compatible in terms of the processing and reactivity with the underlying perovskite as well as possess desirable barrier properties to function as an intrinsic encapsulant. Initial progression of n–i–p and p–i–n PVKSCs originated based on principles of dye-sensitized and organic solar cells, respectively. However, with the continued evolution over time, both architectures have significantly diversified with unique characteristics. In this section, we present a comprehensive review of interfacial advancements, tailored to be useful for both nonspecialists

Figure 8. a) Schematic of regular (n–i–p) architecture PVKSCs in mesoporous and planar configurations. b) PCE of regular architecture PVKSCs over past 4 years using different ETLs (n) and HTLs (p). a = [204]; b = [205]; c = [198]; d = [206]; e = [196]; f = [207]; g = [208]; h = [209]; i = [210]; j = [211]; k = [148]; l = [212]; m = [213]; n = [172]; o = [214]; p = [215]. Note: spiro- is abbreviation of spiro-OMeTAD. c) Energy level diagram for representative hybrid perovskites and CTMs relevant for n–i–p architecture PVKSCs; dotted lines correspond to material work function.

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Figure 9. a) Schematic of inverted (p–i–n) architecture PVKSCs in mesoporous and planar configurations. b) PCE of inverted architecture PVKSCs over past 4 years using various ETLs (n) and HTLs (p). a = [249]; b = [250]; c = [248]; d = [251]; e = [252]; f = [253]; g = [254]; h = [255]; i = [256]; j = [257]; k = [258]; l = [147]. Note: PEDOT is abbreviation of PEDOT:PSS. c) Energy level diagram for representative hybrid perovskites and CTMs relevant for p–i–n architecture PVKSCs; dotted lines correspond to material work function.

and specialists in the field. Sections 4.1 and 4.2 discuss the evolution of different ETL and HTL materials used in PVKSCs, classified based on the device architecture. Section 4.3 provides a summary of interfacial modifications though a discussion centered on their impact and potential to overcome key challenges in PVKSCs.

4.1. Charge-Transporting Materials for Regular Device Architecture The evolution of PCE for regular (n–i–p) architecture PVKSCs over the past 4 years using various ETLs (referred to as “n-type”)

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and HTLs (referred to as “p-type”) is shown in Figure 8b. Energy levels for the relevant CTMs are summarized in Figure 8c.

4.1.1. n-Type Scaffolds The large internal surface area of mesoporous scaffolds allows effective perovskite infiltration and photocarrier extraction. Right from the pioneering work of PVKSCs reported by Kojima et al. in 2009,[195] mesoporous TiO2 (m-TiO2) has been the most prevalent n-type material for fabricating PVKSCs and has provided a platform to realize record efficiencies up to 22.1%.[196]

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As shown in Figure 8b, most of the highest efficiencies reported in regular PVKSCs employ m-TiO2 layers. Typically, the m-TiO2 is realized by spin-coating TiO2 paste onto a compact layer (cl) followed by annealing at temperatures >500 °C. With the evolution of PVKSCs, the thickness of m-TiO2 has reduced from 8–12 µm to 150 nm in the recent state-of-the-art devices.[196–198] Advantage of the rapid charge transfer within scaffolds is evidently apparent when employing perovskite absorbers with short diffusion lengths, such as MASnI3.[199,200] Although m-TiO2 is the most commonly employed scaffold to realize high device efficiencies, they suffer from stability issues under UV exposure. Other n-type metal oxides, such as ZnO[201] and BaSnO3,[202] have been alternatively explored for mesoporous scaffolds, but PCEs are limited to 15–16%. Functionality of PVKSCs was not significantly perturbed when the m-TiO2 was replaced with insulating materials such as Al2O3[203] or ZrO2,[88] which revealed exceptional optoelectronic properties of hybrid perovskites and sparked development of planar devices.

4.1.2. n-Type Interlayer Materials for Regular Device Architecture TiO2: TiO2 is a well-investigated ETL material for PVKSCs owing to its favorable energy levels and high transparency.[216] Uniform and dense compact-TiO2 (c-TiO2) layers are usually formed using sol–gel strategies, where the solution deposition (of titanium diisopropoxide bis(acetylacetonate) in ethanol) through spin coating[208] or spray pyrolysis[204] is followed by a hightemperature annealing step (>400 °C) for decomposition and oxidization. A c-TiO2 layer is typically used in both mesoporous and planar devices for efficient hole blocking. The significant drawbacks include accelerated perovskite degradation due to the photocatalytic activity of TiO2 and dominant interfacial charge recombination due to the low electron mobility of c-TiO2 films.[217] The resulting charge extraction issue contributes to a dominant hysteresis behavior (Section 3.3) in n–i–p devices with c-TiO2. Limitations in charge transport and transfer dynamics with TiO2 ETL are alleviated by tuning its electronic structure via substitutional dopants (see Component-Tuning: Doping and Multi-components in Section 5.1.2).[218] TiO2-triggered degradation and charge recombination pathways are mitigated by interfacial modifications (Section 4.3.1). ZnO: ZnO is another transparent n-type oxide semiconductor that has been used as ETL in n–i–p PVKSCs and is attractive because of the low crystallization temperature and a much higher electron mobility compared to TiO2. Numerous processing methods (sol–gel, hydrothermal, nanoparticle, RF sputtering, atomic layer deposition (ALD), electrospraying, and chemical bath deposition) have been used for the ZnO deposition.[191] Among them, the nanoparticle route allows facile preparation of an ultrathin ZnO ETL at room temperatures and PCEs up to 15.7% have been realized for encompassing n–i–p PVKSCs.[212] Beyond simple compact films, ZnO can also be formed in a wide range of nanostructures, such as nanowires and nanorods, which are beneficial for improving light harvesting.[219] The primary concern for ZnO ETL is its chemical and thermal instability, which is a result of its basic nature and associated deprotonation of the adjoining perovskite.[190] Substantial research efforts have been devoted to improve the stability of ZnO-based

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n–i–p devices, such as modifying the ZnO/perovskite interface (Section 4.3.1), changing chemical properties of ZnO, doping, and composite blending (Section 5.1.2).[191] SnO2: SnO2 ETL has been gaining popularity in the recent years since it has a deeper conduction band than TiO2 and facilitates better energy level matching for mixed perovskite compositions (such as (FAPbI3)0.85(MAPbI3)0.15).[213] c-SnO2 films can be fabricated via low-temperature processing routes using the as-prepared nanoparticles,[215,220] sol–gel-derived tin precursors (typically tin chloride),[221] or ALD technique.[213] A high PCE of 20.9% (certified) have been realized for n–i–p PVKSCs using SnO2 (prepared using nanoparticles).[215] Spin-coated SnO2 with a post-treatment chemical bath deposition utilized as ETL yielded remarkably high Voc (1.21 V using a perovskite with Eg = 1.62 eV).[222] PVKSCs with SnO2 ETL are typically free of hysteresis instabilities because of the efficient charge extraction (attributed to the high electron mobility and a better alignment of interfacial energetics).[220] Unlike TiO2 and ZnO, the SnO2 is not chemically active and yields better device stabilities.[216] Other Oxides: Other binary metal oxide compact layers, such as WOx[223] and Nb2O5,[224] are wide bandgap semiconductors with appropriate energy levels and have also been employed as ETL to fabricate efficient n–i–p planar PVKSCs. Besides the binary metal oxides, ternary metal oxides have also been investigated for achieving superior electronic properties with better chemical stability. Zn2SnO4 and BaSnO3 are of particular interest for ETL.[214,225] Recently, PVKSCs fabricated with (La)-doped BaSnO3 show a steady-state PCE of 21.2% with high photostability (93% of the initial performance was retained after 1000 h of 1 Sun illumination).[214] Component-Tuning: Doping and Multi-components in Section 5.1.2 provides a detailed discussion regarding possibilities for component tuning in metal oxides. Organic Materials: Organic materials are attractive because of their extensive tunability of functional properties (conductivity and frontier energy levels) by structural material design.[179] Fullerenes (specifically C60) are most representative organic ETL used in n–i–p PVKSCs. Thin films of C60 are densely packed with high electron mobility (1.6 cm2 V−1 s−1), easily deposited by thermal evaporation or low-temperature solution processing, and provide a robust platform for growth of perovskite on the top.[226,227] Compact C60 ETLs have been successfully used as bottom interlayer in fabrication of both solution and vacuum-deposited planar n–i–p PVKSCs, which are hysteresis free and show high efficiencies (over 18–20% PCE).[84,226,227] Fullerenes have also been used in cooperation with metal oxides in the form of bilayer ETLs for performance improvements (see Carbon-Derived Composites in Section 5.1.3).[228] Beyond fullerenes, several other organic semiconductors (polymers and small molecules) have also been explored for ETL application in planar PVKSCs (as discussed in Non-Fullerene Organic ETMs in Section 5.1.1).[229] Molecular crosslinking strategies (see Crosslinked Organic Interlayers in Section 5.1.1) are useful to enhance the inherent stability of fullerene and nonfullerene organic interlayers.[230,231]

4.1.3. p-Type Interlayer Materials for Regular Device Architecture Small Molecules: Small molecules offer structural versatility and are pervasively used as HTL in n–i–p PVKSCs. Among them,

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spiro-OMeTAD represents the most commonly used HTL (Figure 8b).[232] Low conductivity of the pristine spiro-OMeTAD results in a large series resistance and recombination losses. Conductivity issue is typically alleviated by using Co(III) complexes as a p-dopant in cooperation with Li-TFSI and tBP additives.[233] Using doped spiro-OMeTAD as HTL, n–i–p PVKSCs with certified PCE over 21.0% have been realized.[198] Hygroscopic nature of additives and their chemical interaction with underlying perovskite layer however raise long-term stability concerns. Increasing hole conductivity of spiro-OMeTAD HTLs through alternate stable dopants is currently a hot research topic.[232,234] Another major concern regarding spiro-OMeTAD is its high material cost due to the complex and multistep synthesis with low yields. Fluorene–dithiphene core substituted by donor groups is an attractive alternative, which can be prepared by a simple three-step synthesis from the commercially available materials (1/5th of the cost of spiro-OMeTAD) and can yield a high PCE of 20.2%.[206] Besides spirobifluorene core structure, other simple derivatives of thiophene, porphyrin, triphenylamine, and heteroacene are currently being extensively investigated (see Dopant-Free Hydrophobic Organic HTMs in Section 5.1.1).[179,235] Polymers: Compared to small molecules, conjugated poly mers exhibit a better film forming ability on the top of perovskite by solution deposition routes. They have a higher viscosity to obtain uniform films of desired thickness and the solubility can be easily tuned by grafting solubilizing groups to the polymer backbone. Polythiophene-based,[236] polyfluorenebased,[237] polytriarylamine-based[204] p-type polymers have been employed as HTL in n–i–p PVKSCs. Among them PTAA is the most successful polymeric HTL, which is used in highefficiency PVKSCs.[238] A thin layer (≈50 nm) of PTAA (with Li-TFSI and tBP additives) deposited on the top of perovskite functions efficiently as the HTL and results in the highest efficiencies for both mesoporous (22.1% PCE) and planar (21.2% PCE) device architectures.[196,214] Besides PTAA, other polymeric HTMs do not yield such high efficiencies and require further optimization. P3HT is one such example, where only 13.7% PCE was obtained using an undoped P3HT,[239] and was subsequently improved to 16.2% PCE by using the doped P3HT with a higher molecular weight.[240] Promising molecular design strategies (Donor–acceptor type and branched structure) for improving performance of polymeric HTLs are highlighted in Dopant-Free Hydrophobic Organic HTMs in Section 5.1.1. Inorganic Materials: Inorganic materials possess high intrinsic stability and are particularly attractive for n–i–p devices. A robust inorganic HTL is effectively a functional internal encapsulant that alleviates degradation pathways and protects the underlying perovskite layer. Processing prerequisites (low temperature and orthogonal processability) for solution deposition of HTL on the top of perovskite limit the choice of applicable p-type inorganic materials. Copper gallium oxide (CuGaO2),[241] copper phthalocyanine (CuPC),[242,243] copper thiocyanate (CuSCN),[207,244] and tungsten oxide (WOx)[245] are notable inorganic HTLs that have led to stable, highly efficient n–i–p PVKSCs. We show that solution-processed CuGaO2 nanoplates function efficiently as HTL. The high hole mobility enables use of a thick (345 nm) CuGaO2 layer to impart excellent moisture blocking without

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compromise in the PCE (≈19%).[241] CuPC HTL is another interesting candidate that has an excellent thermal stability and interfacial bonding properties. Solution processability enabled by CuPC derivative with bulky alkyl moieties has facilitated intimate interfacial coverage on the top of perovskite. The encompassing PVKSCs show high PCE (≈18%), maintain 97% of their initial PCE after 1000 h of thermal annealing at 85 °C, and survive 50 thermal cycles from −40 to 85 °C.[243] Doped CuPC used in cooperation with spiro-OMeTAD have resulted in PVKSCs with PCE >20%.[242] Compact and highly conformal CuSCN layers facilitate efficient hole extraction and yield stabilized efficiencies exceeding 20%.[207] Recently, the promising potential of tantalum-doped WOx (Ta–WOx) has been demonstrated by their use in n–i–p planar PVKSCs, which are well suited for commercialization because of their merits in terms of the high efficiency (21.2% PCE), excellent stability (stable under 1000 h illumination), and facile scalability (low cost without complex ionic doping).[245]

4.2. Charge-Transporting Materials for Inverted Device Architecture The evolution of PCE for inverted (p–i–n) architecture PVKSCs over the past 4 years using various HTLs (referred to as “p-type”) and ETLs (referred to as “n-type”) is shown in Figure 9b. Energy levels for the relevant CTMs are summarized in Figure 9c.

4.2.1. p-Type Scaffolds The development of mesoporous p–i–n PVKSCs requires p-type scaffolds to effectively function as the HTL. So far in terms of the material choice, besides NiOx no other p-type semiconductors have shown success for scaffold building. In a demonstration of inverted PVKSCs, mp-NiOx functioned efficiently as a scaffold and also facilitated sufficient loading and morphological control of the perovskite; mp-NiOx also enabled charge selectivity for efficient hole extraction at the HTL/perovskite interface.[246] A hybrid HTL composed of ultrathin clNiOx/mp-Al2O3 minimized interfacial recombination losses because of the “dual blocking effect.”[247] Performance of the encompassing devices was however limited because of the inherently low conductivity of NiOx nanoparticles.[135] Encouragingly, Cu-doped NiOx nanoparticles with enhanced electrical conductivity recently demonstrated by Jen and co-workers showed an enhanced charge collection and improved performance in mesoporous inverted PVKSCs (stabilized efficiency of 19.5%).[248] This has opened up a new avenue to develop more efficient p-type scaffolds for inverted PVKSCs.

4.2.2. p-Type Interlayer Materials for Inverted Device Architecture Organic Materials: The first inverted architecture PVKSC employed PEDOT:PSS as the HTL.[259] Relatively shallow work function (−4.8 to −5.1 eV) of PEDOT:PSS led to poor contact with the perovskite, induced severe interfacial losses, and limited Voc (20% have been accomplished using PTAA HTL.[262] Further optimization of the PTAA/perovskite interface results in an exceptional fill factor of 84% and PCE >20%.[147] Several conjugated small molecules and crosslinked interlayers (Section 5.1.1) have also been successfully used as HTL for inverted PVKSCs and resulted in PCEs up to 19%.[109,231,263–265] Inorganic Materials: Nickel oxide (NiOx) is a large-bandgap (3.4–3.8 eV) p-type semiconductor with appropriate valence band position (−5.2 to −5.4 eV) and is the most preferred HTL candidate for inverted PVKSCs. Several processing approaches like sol–gel,[266,267] nanoparticles,[268,269] and physical vapor deposition[250] have been used to deposit NiOx thin films. The NiOx HTL is often plagued by poor conductivity and the performance of associated p–i–n devices is limited by poor FF and Jsc. In the early demonstration by Jen and co-workers, Cu was used as dopant to increase conductivity of NiOx (Cu:NiOx) to facilitate efficient charge extraction and resulted in a PCE of 15.4%.[251] Subsequently, a low-temperature (150 °C) combustion method for preparation of Cu:NiOx was reported with an improved PCE of 17.8%.[270] Han and co-workers codoped NiOx with Li+ and Mg2+ and demonstrated a certified efficiency over 18% for 1.02 cm2 inverted devices.[119,253] Advancements made in nanoparticle route (see Low-Temperature Solution-Processable Metal Oxides in Section 5.1.2) have facilitated ambient, scalable room temperature solution processing of NiOx interlayers and PCE around 15–19%.[268,269] A PCE of 20.6% realized using the NiOx HTL is among the best reported inverted PVKSCs.[255,271] Copper oxide (CuOx) is another inorganic HTL used to achieve PCE up to 19%.[258] Several other p-type inorganic materials including CuSCN,[257,272] CuI,[273] vanadium oxide (VOx),[274] and reduced graphene oxide (rGO)[275] have also been explored and applied as HTL in inverted PVKSCs with modest success (14–16% PCE).

4.2.3. n-Type Interlayer Materials for Inverted Device Architecture Fullerenes: Fullerene or its derivatives are employed as ETL in the majority of inverted PVKSCs and they play an integral part in realizing high PCEs.[113,139,165,271,276] Outstanding merits that make fullerene-based interlayers imperative are defect passivation (Section 3.2), efficient charge extraction, and alleviation of hysteresis instabilities in encompassing PVKSCs (Section 3.3).[114,165] The inherent low-temperature solution processability using hydrophobic organic solvents without damaging the underlying perov skite layer favors the application of fullerene-based ETLs in p–i–n PVKSCs. A systematic comparison of three fullerene variants (C60, PC61BM, IC60BA) in the initial work by Jen and co-workers

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demonstrated an important correlation between the conductivity of fullerene-based ETLs and the resultant device performance of inverted PVKSCs.[112] Several n-type dopants have been designed to enhance the conductivity and formability of fullerene ETLs. This improved charge dynamics at the perovskite/ETL interface and increased the Jsc of encompassing PVKSCs.[277–279] The reduction of energy disorder in fullerene ETL by solvent annealing process enhanced the Voc in inverted PVKSCs.[280] The ability of fullerenes to block ingress of water and moisture is favorable for stability of p–i–n devices. Fullerenes functionalized with crosslinkable silane molecules on the hydrophobic tail groups greatly improved the barrier property of ETL and associated device stability.[281] Alternately, a bilayer ETL composed of fullerene and n-type metal oxide (such as SnO2) effectively enhances the performance and stability of inverted PVKSCs.[254] Other Materials: Other n-type organic and inorganic materials have also been explored to replace the expensive fullerene interlayers.[271,276] Several nonfullerene small molecules and polymers (see Non-Fullerene Organic ETMs in Section 5.1.1) specifically tailored for the ETL function have resulted in highefficiency inverted PVKSCs (17–19% PCE).[108,231,282–284] Inorganic alternatives such as low-temperature processible ZnO or SnO2 ETL (see Low-Temperature Solution-Processable Metal Oxides in Section 5.1.2) demonstrate great promise for fabricating n–i–p PVKSCs with all inorganic CTLs and show a greater resilience to degradation pathways.[252,285]

4.3. Interfacial Modifications Interfacial modifications are pivotal for successful integration of charge-transporting layers (CTL) in the development of high-performance PVKSCs and have led to improvements on multiple fronts: facilitate seamless coupling between layers as well as provide compatibility in terms of structural integrity, energetic landscape, and robustness pivotal for effective functionality. A wide range of modifications employed in PVKSCs can be grouped into three major categories, namely, CTL, perovskite, and electrode modifications. The representative variants are summarized in Table 1.

4.3.1. Charge-Transporting Layer Modifications Modification of the CTL surface onto which perovskite is deposited is a powerful strategy to tailor the perovskite growth, energetic landscape, interfacial charge dynamics, and compatibility. The deposition of self-assembled fullerene monolayers was initially found to greatly improve the interfacial charge transfer and alleviate the hysteresis in TiO2 based PVKSCs.[208] Since then, different types of self-assembled monolayers (SAM) have been successfully employed to improve the quality of interfacial contact between perovskite and metal oxide interlayers (ZnO, SnO2, NiOx, and WOx).[87,90,286,287] SAM modifications directly influence the nature of chemical interaction at interfaces and has been applied to circumvent the chemical incompatibility at the ZnO/perovskite interface.[90] Further, functionalization of metal oxides and organic interlayers (PEDOT:PSS) by SAM has been beneficial for growing a smoother and high-quality

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Table 1. Representative variants of interfacial (charge transport layer, perovskite, and electrode) modifications employed in PVKSCs. Bold text corresponds to the interfacial modifier employed. Interfacial variant Charge transport layer modifications

Self-assembled Benzoic acid based monolayer Amino acid based

Device architecture with modifier

Main functions

Ref.

FTO/c-TiO2/C60-SAM/MAPbI3/spiro-OMeTAD/Au; ITO/NiOx NPs/4-bromobenzoic acid/MAPbI3/PC61BM/Bis-C60/Ag

Reduce hysteresis, surface defect passivation

[208,87]

ITO/ZnO/3-aminopropanoic acid/MAPbI3/ spiro-MeTAD/MoO3/Ag

Tune work function and perovskite morphology

[90]

Facilitate efficient charge extraction

[287]

FTO/c-TiO2/ALD-Al2O3/mp-TiO2/MAPbI3/PTAA/Au

Suppress interfacial recombination

[292]

FTO/c-TiO2/CsBr/MAPbI3/spiro-OMeTAD/Au

Inhibit UV light-induced degradation

[294]

FTO/SnO2/PC61BM/MAPbI3/spiro-OMeTAD/Au

Defect passivation, improve charge transfer

[228]

FTO/c-TiO2/mp-TiO2/PMMA:PCBM/mixed perovskite/ spiro-OMeTAD/Au

Suppress interfacial recombination

[146]

FTO/c-TiO2/MAPbI3/thiophene or pyridine/spiro-OMeTAD/Au

Alleviate traps via surface passivation and reduce nonradiative recombination

[110]

Phosphonic acid ITO/WOx NPs/mixed C60-organic SAM/MAPbI3/spiro-OMeTAD/Ag based Thin layer

Inorganic buffer

Fullerene derived

Perovskite modifications

Surface functionalization

Lewis base

Ammonium halide

ITO/PTAA/MAPbI3/choline chloride/C60/BCP/Cu

[262] [299]

ITO/NiOx/MAPbI3/C8-diammonium iodide/PC61BM/Bis-C60/Ag FTO/c-TiO2/MAPbI3/tetraethyl ammonium/spiro-OMeTAD/Au

Thin layer

Electrode modifications

Thin layer

Neutral amine

ITO/c-TiO2/FAPbI3/benzylamine/spiro-OMeTAD/Au

[308] [306]

Perovskite QD

FTO/c-TiO2/MAPbI3/MAPbBr3−xIx QDs/spiro-OMeTAD/Au

Enhance hole extraction

[305]

Insulating polymer

ITO/PTAA/MAPbI3/polystyrene/C60/BCP/Cu

Selective extraction and resist moisture attack

[309]

Interfacial dipole

ITO/PEIE/Y-TiO2/MAPbI3−xClx/spiro-OMeTAD/Au; ITO/ PEDOT:PSS/MAPbI3/C60/Bis-C60/Al

Tune work function

[218,112]

Chemical inhibitor

FTO/c-TiO2/mp-TiO2/MAPbI3/spiro-OMeTAD/Cr/Au

Inhibit ion migration-induced degradation

[187]

FTO/c-TiO2/mp-TiO2/MAPbI3/CuSCN/rGO/Au

Prevent metal-induced degradation

[207]

ITO/PEDOT:PSS/MAPbI3/PC61BM/Bis-C60@F-C60/Al

Reduce moisture penetration

[319]

Buffer

perovskite films.[288] Other types of interlayer surface treatments such as titanium tetrachloride (TiCl4) and ethanolamine reduce the interfacial energy barrier through dipole formation as well as facilitate the growth of compact and highly crystalline films.[86,289,290] The introduction of ultrathin functional layers is another viable approach that has led to improvements on both device performance and stability. Thin layer (10–30 nm) of fullerene or its derivatives greatly reduce the surface trap density on metal oxides and improve the electronic quality of the above grown perovskite absorber.[228,291] Ultrathin layer (1–5 nm) of insulating materials such as Al2O3, PMMA, polyvinylpyrrolidone (PVP) also has beneficial passivation effect and reduces the interfacial carrier recombination at metal oxide interfaces.[292,293] Capitalizing on these merits, White and co-workers have recently showed that using a PMMA:PCBM composite ultrathin layer between TiO2-perovskite, significant improvements in Voc can be achieved (champion cell shows a high Voc ≈ 1.18 V with a steady-state PCE of 20.4%).[146] Beyond

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Mitigate moisture penetration and enhance device stability

the efficiency, modification of the TiO2 interface by cesium halides (CsBr or CsCl) has improved the stability of TiO2-based planar PVKSCs under UV light exposure.[294,295] Insertion of thin buffer layers (Al2O3, polyethyleneimine) at the ZnO/perovskite interface prevents the chemical interaction and improves the thermal stability of corresponding PVKSCs.[296,297]

4.3.2. Perovskite Modifications Surface modifications of solution-processed perovskite thin films have proved to be crucial for overcoming the deleterious impact of trap sites (surface dangling bonds, point defects) that are inevitably introduced during film processing. Chemical reactivity of surface defects has been capitalized to attach the suitable ligands for facilitating passivation of nonradiative recombination centers and enabling effective transport of photogenerated charge carriers across the interface. Anchoring

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ligands are typically introduced postfilm fabrication by a simple immersion or spin coating of the ligand solution onto perov skite films. Treatment of the perovskite film surface by Lewis bases (thiophene, pyridine, and trioctylphosphine oxide)[110,298] or diammonium iodides[299] passivates positively charged defects (undercoordinated lead and halide vacancy) and results in an improved photoluminescence quantum yield, carrier lifetime, and corresponding device performances. On the other hand, employment of thin fullerene layers on top of the perovskite passivates the negatively charged defects (undercoordinated halide and Pb–I antisite defects) and is beneficial for improving electron coupling at the interface.[111,112,117,254] Combined passivation of both types of charged defects can be realized through the treatment of perovskite films by quaternary ammonium halides with negative- and positive-charged components (NR +4 X−, where R is an alkyl or aryl group and X is an halide) and results in PCEs up to 21%.[262] Graded heterojunction formed by the incorporation of fullerene,[119,300] bulky organic cations,[301,302] or engineering a compositional gradient (MAPbBrxI3−x, FAPbBrxI3−x, and CsPbBrxI3−x)[303–305] at the perovskite/CTL interface are promising strategies to tailor the interfacial energy level alignment, enhance the charge extraction, reduce interfacial recombination losses, and increase the Voc of PV devices. Functionalization of the film surface with hydrophobic tertiary, quaternary, or phenylalkyl ammonium cations (Section 5.2.1) can effectively block the ingression of water molecules and enhance the moisture tolerance of perovskites.[306–308] One such demonstration by Yang et al. employing functionalization with tetra-ethyl ammonium cations showed that perovskite thin films can be protected under high relative humidity (≈90%) over 30 d[308] and elucidated the potential of surface modifications to improve long-term ambient stability of PVKSCs. The introduction of ultrathin layers (1–5 nm) of insulating buffer layers (Al2O3, polystyrene, PMMA) at the rear perovskite/CTL interface can serve as an inherent encapsulant to resist the damage of perovskite films from moisture penetration and chemical reactivity at interfaces,[186,244,309] as well as realize Voc improvement due to the reduced interfacial recombination losses.[142]

4.3.3. Electrode Modifications Electrodes (cathode and anode) are integral components of a PVKSC controlling the extraction of photogenerated charge carriers (electrons and holes) and the flow of electricity in the external circuit.[310] Typically in PVKSCs, the front electrode is a transparent conducting oxide (ITO, FTO) and the back electrode is an opaque and reflective metal (Ag, Au, and Al). Electrode polarities depend on the employed device configuration and architecture. Regardless of the choice of electrode, tailored modifications of the electrode contact are critical to minimize the contact resistance losses, extract photogenerated charges effectively, and alleviate instabilities due to chemical reactivity of the electrode with the adjoining layers. Cathode modifications are ubiquitous in PVKSCs for lowering the electrode work function to match with the ETL LUMO or conduction band minimum (CBM) and has a significant impact on all the PV performance metrics (Voc, Jsc, and FF). Polyethyleneimine ethoxylated (PEIE) modification of ITO electrode lowers the work function from

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−4.7 to −3.9 eV and is crucial for enhancing the electron extraction and charge transport from the ETL to the electrode in regular planar architecture devices.[218,311] On the other hand, several different organic and inorganic materials have been employed to modify the metal electrode work function via an interfacial dipole, for formation of Ohmic contact at the cathode interface in inverted planar architecture devices. Thin layers (0.5–10 nm) of bathocuproine (BCP),[312] bathophenanthroline (BPhen),[313] fulleropyrrolidine (N-C60),[314] fullerene bis-adduct (Bis-C60),[112,315] lithium fluoride (LiF),[316] n-doped zirconium oxide (ZrO2),[317] and metal acetylacetonates (MAcAc, where M = Ti, Zr, or Hf)[318] are typically used interfacial modifiers or surfactants and are beneficial for maximizing the realizable quasi-Fermi level splitting, reducing the electron recombination, and improving the electron extraction in inverted architecture devices. Beyond tailoring the energetic landscape and charge dynamics, electrode modifications also provide an opportunity for inhibiting the moisture penetration and chemical interaction, which are the common degradation pathways hindering long-term stability of PVKSCs. A tailor-designed fullerene derivative with a perfluoroalkyl side chain was incorporated by Jen and co-workers as the surfactant at the top electrode interface, which enhanced the ambient moisture stability of PVKSCs due to the increased hydrophobicity.[319] Alternatively, an ALD-deposited layer (10 nm) of amorphous-TiO2 (a-TiO2) at the electrode interface also enhanced stability and made PVKSCs less vulnerable to degradation by liquid water and thermal soaking at 100 °C.[320] Insertion of barrier layers like Cr or r-GO beneath the metal electrode hindered ionic diffusion across the perovskite/ metal electrode interface as well as alleviated chemical reactivity and intrinsic instabilities in the device stack.[187,207] One such modification has led to the recent impressive demonstration of PVKSCs with >95% of the initial efficiency retained even after being aged under the MPP condition for 1000 h at 60 °C.[207]

5. Prospects for Interfacial Engineering in Emerging Areas of Research Interest In this section, we evaluate the progress in different areas of research interest and pinpoint the future research directions (italicized) based on interfacial engineering for specialists in the field. Topics are grouped into two categories: material development (Section 5.1) and application (Section 5.2).

5.1. Material Development Perspective Here, we outline approaches for the design of organic chargetransporting materials (Section 5.1.1), novel inorganic chargetransporting materials (Section 5.1.2), interlayer composites (Section 5.1.3), interfacial modifiers (Section 5.1.4), and materials for optical/electrical field management in PVKSCs (Section 5.1.5).

5.1.1. Molecular Design of Organic Charge Transport Materials The tuning of chemistry, structure, and charge dynamics of organic CTMs by molecular engineering enables desired

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interfacial properties and improves performance of PVKSCs. For practical applicability, besides the high charge mobility and suitable energy levels, organic CTMs should also be intrinsically stable, nonreactive, compact, and hydrophobic for efficient protection of the perovskite absorber and realization of a longterm stable device stack. Figure 10 shows the chemical structure of representative organic CTMs discussed in this section, which are classified into three categories: dopant-free HTMs, nonfullerene ETMs, and crosslinkable CTMs. Materials developed in our group (Jen et al.) are highlighted. Dopant-Free Hydrophobic Organic HTMs: The commonly used organic HTMs, spiro-OMeTAD, and PTAA require undesired chemical doping process (increased complexity and additional degradation pathways) for efficient hole transport. Alternately used conjugated polyelectrolytes are hygroscopic in nature and degrade PVKSCs due to the water invasion. Development of novel dopant-free hydrophobic organic HTMs with efficient hole transporting properties is thus highly desirable

but challenging. They are realizable through two major design strategies: donor–acceptor (D–A) type and branched structure. The D–A design strategy can enable both high hole mobility and compatible highest occupied molecular orbital (HOMO) level through a rational integration of different electron–donor and electron–acceptor blocks. Among D–A copolymer HTMs, benzodithiophene (BDT) unit is a popular donor choice for the backbone chain. Based on the BDT donor, a series of acceptor units thienothiophene (TT), benzothiadiazole (BT), benzobistriazole (BBTa) were employed to prepare D–A-type polymeric HTMs PBDTTT-C,[321] PBTBDT,[322,323] and PBBTa-BDT,[324] respectively. In the representative structures of PBTDBT (PTGE and P3), both side chains of BDT and BT are valuable to tune the physical, optical, and electronic properties.[323,325,326] Asymmetric alkyl substituents on the BT unit enable PCE up to 19.8%.[327] BDT in combination with other hole-transporting moieties, such as DPP, has been employed to form D–A copolymers (PDPPDBTE).[328] The BDT donor unit is also a

Figure 10. Chemical structure of representative dopant-free HTMs, nonfullerene ETMs, and crosslinkable CTMs; materials developed by Jen et al.[229,231,264,265,283,337,342] are listed in the right side of the figure. Literature references corresponding to every molecular structure are provided in the text.

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favorable choice for building A–D–A structures. For example, rhodamine and BT units were used as electron-withdrawing (acceptor) groups to build A–D–A structure with the BDT group (DERDTS-TBDT).[329,330] Other donor units, such as S,N-heteropentacene, were also reported to prepare A–D–A-type HTMs.[331] Rational design of branched structures, including butterfly- shaped and star-shaped, provide another possible strategy to prepare dopant-free HTMs because it can induce better π–π interactions and improve the hole transport. The triphenylamine (TPA) unit with a propeller-like core structure represents one of the most used building blocks (basis for the spiro-OMeTAD structure) due to its nonplanar geometry that regulates the molecular packing in the solid state. A series of star-shaped HTM with a core based on TPA and fused-TPA were designed, including butterfly-like chemical structure with three TPA moieties,[332] star shape with rigid quinolizino acridine as the central unit (FA-CN),[333] and di-TPA derivate (TAPC).[263] A newly designed Trux-OMeTAD, consisting of a C3h Truxene core with three TPA terminals, has been demonstrated by Jen and co-workers to enable devices with a high PCE of 18.6%.[265] A dipolar D–A-type chromophore BTPA-TCNE combining TPA (donor) and tricyanovinylene (acceptor) has also been demonstrated recently by the same group as an efficient dopant-free HTM.[264] Learning from PTAA, the TPA unit was also used to construct branched-structure polymer.[334] Besides TPA, the spiro-structure has also been applied to prepare dopant-free HTMs. A thiophene unit was used to modify the spiro-core (PPyra-XA) for enhancing the intermolecular contacts.[335,336] A novel heteroaromatics based spiro-core HTM has also been shown to enable a PCE of 11.7%.[337] Nonfullerene Organic ETMs: Compared with efforts dedicated in developing organic HTMs, organic ETMs are less explored. Fullerene and its derivatives are the most commonly used organic ETM for planar PVKSCs. However, the difficulty in tuning chemical structure and energy level of fullerenes, high cost, and poor stability will limit their application in PVKSCs. Therefore, it is important to develop efficient nonfullerene organic ETMs with desired electrical conductivity and tunable energy levels. For n-type materials, perylene diimide derivatives (PDI)[108,229,338] have been used as an efficient building block for both conjugated polymers and small molecules. For example, a conjugated D–A polymer composed of fluorene and naphthalene diimide units (PFN-2TNDI)[108] as well as small molecules based on coronene diimide (CDIN)[229] has been developed and show superior electronic properties. Particularly, functionalized-alkylamine groups were incorporated in both cases to improve the interface interaction and form a desired interfacial dipole. Encouraged by these results of expanded π-conjugated plane in coronene diimide, cove-edge graphene nanoribbons based on PDI (hPDI3-Pyr-hPDI3) were employed as efficient ETMs, which possessed improved electron mobility and hydrophobicity.[284] Besides PDI blocks, hexaazatrinaphthylene derivatives (HATNASOC7-Cs) have attracted attention as promising ETLs because of their high electron mobility and energy level tunability.[282,283] Sulfur species are commonly introduced into the molecular structure of nonfullerene organic ETMs to take advantage of the S-Pb coordination for defect passivation (through interaction at the perovskite/ETL interface).

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Alternately, a nonfullerene organic ETM can be uses in cooperation with a fullerene ETM to passivate traps in perovskite films[106] and improve the interfacial energy level alignment.[339] Crosslinked Organic Interlayers: The structure of small mole cule CTMs, such as PC61BM and spiro-OMeTAD, is too loose to prevent the moisture permeation. Cross-linking is an attractive approach to build more intrinsic robustness of organic materials. Crosslinked CTMs can act as a protective barrier and enhance the UV-light stability and thermal resistance of PVKSCs.[340] Crosslinked CTMs with a 3D network can also potentially stabilize the radical cations for efficient charge extraction. The representative crosslinked building block for HTL is arylamine derivatives and different polymerization routes facilitate crosslinking. For example, triphenylamine dimer with oxetane ring (OTPD) was crosslinked through ring-opening polymerization to replace PEDOT:PSS.[76,341] Arylamine derivatives with styryl group have been crosslinked by the thiol-ene reaction (TCTA-BVP)[342] or thermally induced polymerization (VNPB)[343] to replace spiro-OMeTAD. On the other hand, fullerenes bearing functional groups for crosslinking have also been developed. In situ crosslinking of fullerene derivative with styryl units (PCBSD) was employed in combination with c-TiO2.[344] Thermal ring-opening process was applied on PCBM containing a benzocyclobutene moiety (PCBCB) to replace c-TiO2.[230] Another effective approach for ETL is incorporating a crosslinking agent into the fullerene domain, such as introducing silane molecules with hydroxyl groups to be bonded onto the carboxyl group (COOH) of a fullerene derivative (CLCS).[281,345] Recently, a nonfullerene crosslinked ETL based on HATNA with methacrylate group (HATNASOENE) has been developed by Jen and co-workers to demonstrate PVKSCs using all-crosslinked CTLs with high stability and efficiency.[231] The above results present a great progress in the field of hydrophobic organic CTMs and provide guidelines for material engineering toward highly efficient PVKSCs. For organic HTMs, PBTBDT type and branched TAA appear to be very promising for future designs, although more modifications on side groups should be tested to ensure the conjecture. For organic ETMs, more exploration of efficient building blocks for nonfullerene molecules is important. Continued development of crosslinked organic interlayers has immense potential. More investigations are needed to understand how various chemical structures interact with the perovskite surface and affect charge extraction, for developing a comprehensive set of design rules to enable design of desired organic CTM by molecular engineering.

5.1.2. Novel Inorganic Charge-Transporting Materials In Section 4, we have demonstrated how development of metal oxide CTMs has continuously driven improvement of PVKSC performance. Here, we discuss opportunities for further advancement of metal oxide CTMs, which are categorized into two parts: low-temperature solution-processable metal oxides (attractive for processing convenience) and component tuning (effective for modification of metal oxide properties). Low-Temperature Solution-Processable Metal Oxides: The traditional sol–gel approach used in fabricating metal oxide CTMs

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requires a high-temperature annealing procedure, which is not only energy intensive but also inhibits roll-to-roll processing on flexible substrates that cannot withstand temperatures >150 °C. Low-temperature solution processing of metal oxide can be realized by using either novel metal precursors or colloidal oxide nanocrystals. In precursor approach followed by sol–gel hydrolysis, the crystallization is induced in situ. Whereas in colloidal oxide nanocrystal approach, the material is crystallized prior to the deposition.[346] We have listed representative lowtemperature solution-processable metal oxides along with their formation mechanisms in Table 2. Both nonaqueous sol–gel approaches[347] or combustion reactions[348] lower the required annealing temperature in precursor approach and result in metal oxides with high crystallinity and uniform film morphologies. Hydrolysis of various Ti-precursors in ambient air has enabled fabrication of TiO2 at low temperature with excellent performance.[349,350] Low- temperature sol–gel approaches have also been applied to fabricate other ETLs, such as WOx[351] and ZnO.[90] In terms of combustion reactions, Jen and co-workers used nickel nitrate hexahydrate and acetylacetone as oxidizer and fuel to effectively drive the deposition of (Cu-)NiOx films at a much lower temperature (150 °C).[270] A similar combustion method has also been successfully adapted to decrease the processing temperature to 140 °C for SnO2 films.[352] Reactions based on various mechanisms, including hydrolysis, alcoholysis, and hydrothermal, were employed to synthesize crystallized oxide nanocrystals with low defect traps. Precursors and reaction parameters control reaction kinetics and the crystallization process. Since the nanocrystal approach decouples the crystallization of oxide material from the filmdeposition process, optimization of the film deposition process is key to obtain a dense and pinhole-free film with uniform coverage. For n-type oxides, Snaith and co-workers first used TiO2 nanoparticle from a nonhydrolytic sol–gel route to replace the high-temperature sintered TiO2 compact layer.[353] Besides TiO2, other highly conductive crystalline ZnO, SnO2, and Zn2SnO4

nanocrystals have also been reported. ZnO NPs synthesized by hydrolyzing zinc acetate can be deposited both underneath and above the perovskite film as compact ETL.[212,252] Compact Zn2SnO4 and SnO2 film can also be prepared via crystalline colloidal solution at low temperature.[220,225] Jen and co-workers prepared SnO2 nanocrystals by a hydrothermal method and have successfully applied as ETL in the inverted planar PVKSCs.[254] On the other hand, for p-type metal oxides, Jen and co-workers synthesized (Cu-)NiOx nanocrystals by a chemical precipitation method and applied as HTL in inverted planar PVKSCs with large-area uniformity.[268,269] Hydrothermal synthesis–derived CuGaO2 nanoplates have also been demonstrated by Chen and co-workers as HTL for making efficient and stable n–i–p PVKSCs.[241] For low-temperature processed oxide nanocrystals, efficient passivation strategies are needed to mitigate defects at highly reactive nanocrystal surfaces. The synthetic chemistry of p-type metal oxide is not as mature as that of the n-type counterparts and more efforts are required for designing alternate p-type nanostructured metal oxides with exquisite control. Component Tuning: Doping and Multicomponents: Doping metal oxides with other elements is often employed to tailor metal oxide properties. For instance, an isovalent substitutional doping can modify the bandgap of oxides while an aliovalent substitutional doping can generate additional free carriers in the metal oxide. Taking TiO2 modification as an example, a low level of Nb[253] and Li[355] doping passivates the oxygen defects, decreases the number of deep trap states in TiO2, and improves the FF and Voc. Heavily Mg-doped TiO2 (10%)[356] forms a better energy level alignment with perovskite due to an upward shift of the CBM and results in higher values of Voc and FF. Y-doped TiO2 enhances the conductivity of TiO2 through an increased carrier density and mobility[218] and correspondingly pushes up the Fermi level in the doped material. In certain cases, doping could be multifunctional as well. A small amount of aliovalent dopants with valency +3 (such as Al+3 and In+3 ions[357,358]) in TiO2 can effectively passivate the trap states,

Table 2. Representative low-temperature solution-processable metal oxides applied as CTL in PVKSCs. Composition

Function

Film-processing solvent/reagents

Post-treatment conditions

Synthesis mechanism

Ref. [349]

TiO2

Compact ETL (n–i–p)

Ti(OiPr)4

None

Hydrolysis

TiO2

Compact ETL (n–i–p meso)

TiO2 NPs in ethanol with 20% TiAcAc

150 °C for 30 min in ambient air

Alcoholysis

[353]

Doped-TiO2


TiO2 NPs in ethanol


Alcoholysis

[148,218]

WOx


WCl6


Hydrolysis

[351]

Al2O3

Scaffold (n–i–p)

ZnO NPs in isopropanol

150 °C for 1 h

–

[354]

Zn2SnO4


Zn2SnO4 in methoxy ethanol

100 °C for 1 h in ambient air

Aminolysis

[225]

ZnO


ZnO NPs in n-butanol

None

Hydrolysis

[212]

ZnO

Compact ETL (p–i–n)

ZnO NPs in chlorobenzene

None

Hydrolysis

[252]

SnO2


SnO2 NPs in H2O


–

[220] [254]

SnO2

Compact ETL (p–i–n)

SnO2 NPs in isopropanol

None

Hydrothermal

NiOx

Compact HTL (p–i–n)

NiOx NPs in H2O

None

Chemical precipitation

[268]

Cu:NiOx

Compact HTL (p–i–n)

Ni(NO3)2·6H2O + Cu(NO3)2·3H2O + acetylacetone

150 °C for 1 h in ambient air

Combustion chemistry

[270]

Ta–WOx

Compact HTL(n–i–p)

Ta–WOx NPs in ethanol or isopropanol

None

–

[245]

CuGaO2

Compact HTL (n–i–p)

CuGaO2 NPs in isopropanol

100 °C for 10 min

Hydrothermal

[241]

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Table 3. Doped metal oxide CTL in PVKSCs: contributing mechanisms, improved PV parameters (Voc: open-circuit voltage; Jsc: short-circuit current density; FF: fill factor), and additional merits. Mechanism Passivate electronic defects/deep trap states

Shift band-edge energy

Increase carrier density

Increase conductivity; shift band-edge energy

Increase carrier density; passivate defects/ remove trap states; raise work function

Composition

Function

Improved PV parameters

Additional merits

Ref.

Li–TiO2

Scaffold

Voc and FF

Suppression of hysteresis

[355]

Nb–TiO2

Scaffold and compact ETL

Voc and FF

Suppression of hysteresis

[253]

Mg–TiO2

Scaffold and compact ETL

Voc, FF, and Jsc

Better hole blocking

[356]

Y–SnO2

Scaffold

Voc, FF, and Jsc

Well-aligned nanostructure

[361]

Y–TiO2

Compact ETL

Jsc

Low-temperature processing

[218]

Cu–NiOx

Compact HTL

FF and Jsc

Deeper valence band

[251] [360]

Al–ZnO

Compact ETL

Voc and FF

Improved acid resistance

La–BaSnO3

Compact ETL

Voc and FF

Enhanced photostability

[214]

Li(Mg) –NiOx

Compact ETL

Voc, FF, and Jsc

Pinhole and defect free ETL

[253]

Ta–WOx

Compact HTL

FF

Quasi-Ohmic contact; increased stability

[245]

Al–TiO2 In–TiO2 Sb–SnO2

Compact ETL Compact ETL Compact ETL

Jsc Voc, FF, and Jsc Voc and FF

Increased stability Suppression of hysteresis Low-temperature processing

[358] [357] [359]

increase the conductivity, and raise the work function. Similar doping principles have also been demonstrated useful for modifying the electronic properties of other oxide CTLs. For instance Sb-doped SnO2,[359] Al-doped ZnO,[360] and La-doped BaSnO3[214] have all been demonstrated to produce highly efficient PVKSCs. Doping is also beneficial for enhancing the electronic performance of p-type oxide HTLs. Low conductivity and associated poor charge transport and extraction in NiOx films can be partly fixed by employing Cu as a substitutional dopant.[251] For further improvement, Chen et al. used high codoping with Ni and Mg.[253] Overall, an efficient charge extraction and collection can be achieved by properly doping the metal oxide CTLs to modify critical aspects such as carrier concentration, trap states, and work function. A detailed summary of doped metal oxide CTLs used in PVKSCs is provided in Table 3. Alternative to doping of binary oxides, multication systems such as ternary and quaternary oxides offer more possibilities to change bandgap energy, work function, and electrical resistivity. For ternary oxides, heavy metal cations with (n − 1) d10ns0 (n ≥ 4) electronic configurations are attractive compounds because of their lower cost and good thermal stabilities. The most relevant cations can be grouped as monovalent (Cu+, Ag+), divalent (Cd2+ and Zn2+), trivalent (Ga3+ and In3+), and tetravalent (Sn4+ and Ti4+). For n-type ETLs, Zn2SnO4, SrTiO3, BaSnO3, and BaTiO3 have wide bandgap values with favorable CBM position and excellent charge transport properties. Shin et al. prepared a well-dispersed crystalline Zn2SnO4 colloidal solution for low-temperature processability (100 °C) and demonstrated a PCE of 14.85% on flexible substrate.[225] In terms of perovskite-type ternary oxides, meso-SrTiO3 have been used to replace TiO2 in regular structure PVKSCs.[362] Alternatively, BaSnO3 and its analogs BaTiO3[363] have also been investigated as n-type material in PVKSCs. To further improve the electron conductivity, lanthanum (La)-doped BaSnO3 compact film was prepared from superoxide colloidal solution and when applied as ETL showed a steady-state PCE

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of 21.2%.[214] On the other hand for p-type CTMs, ternary oxide copper(I) delafossites (CuMO2, where M is Al, Ga, Cr, or other) are promising candidates because of their higher mobility and deeper valence bands. Solution-processed inorganic nanoplates CuGaO2 have been demonstrated as an efficient HTL in the n–i–p PVKSCs (a PCE of 18.51% with excellent ambient stability).[241] Recently, Hou et al. reported the use of Ta–WOx to realize quasi-Ohmic contact with the conjugated polymers and reduce the injection barrier at the HTL interface, which resulted in n–i–p PVKSCs with an impressive 21.2% PCE and 1000 h of light stability.[245] A detailed understanding of the underlying mechanisms for component tuning will be of key importance for generalizing the strategy. Rigorous protocols that specify details such as doping concentration and element information must be established, and the relation between component and performance of different oxides should be developed as a guideline. Alternately, a heterostructure assembly using bilayer structure[364,365] or core–shell nanostructure[366,367] is also quite promising. A heterostructure assembly of two oxides exhibits synergistic advantages of cascade charge transfer for high charge extraction, traps passivation for low interfacial recombination, and avoids phase separation prevalent in simple blending approaches.[223]

5.1.3. Interlayer Composite Engineering Here, we discuss the potential of carbon- and polymer-derived composite interlayers. Carbon materials and polymers possess remarkable electronic properties to function as standalone CTMs. Their combination with common CTMs provides several additional benefits: 1) better film formation, 2) accelerates charge transport, 3) enhances selective charge injection with cascade band alignment, and 4) prevents environmental degradation of the perovskite layer. Carbon-Derived Composites: Carbon materials, such as 0D fullerenes, 1D carbon nanotubes (CNTs), 2D graphene and

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graphdiyne, and 3D graphite (Figure 11), have been applied as CTL in PVKSCs. In 0D carbon materials, fullerene and its derivatives are among the most commonly used bilayer ETL, especially in inverted PVKSCs.[111] In metal oxide–fullerene bilayer systems, fullerene promotes electron transfer from perovskite to TiO2, ZnO, or SnO2 and passivates interfacial trap states, whereas metal oxide can work as a protection layer for high stability.[253,368] Besides fullerene, graphene quantum dots (GQDs) are another interesting carbon derivative for ETL composites. In cooperation with TiO2, GQDs act as an ultrathin glue for enhancing the electronic coupling and accelerating the electron transfer from perovskite to ETLs.[369,370] With SnO2, photogenerated electrons in GQDs can effectively fill the electron traps and improve the conductivity of SnO2.[371]

CNTs with outstanding electrical, optical, chemical, and mechanical properties possess both p-type and n-type behavior.[372] Insertion of an SWCNT film between spiroOMeTAD and perovskite reduces charge accumulation at the interface (Figure 11a).[130] SWCNTs can also work as an excellent p-type dopant for common HTM, such as spiro-OMeTAD and P3HT, either as a blend or in a stratified structure.[373,374] CNT can also be integrated with n-type materials for electron transport, such as the composite of cross-stacked carbon nanotube/SnO2.[285] Snaith and co-workers presented a novel approach to increase the stability of PVKSCs by replacing spiroOMeTAD with a composite of P3HT and SWCNT, in which P3HT solved the solubility problem and SWCNT solved the conductivity problem (Figure 11b).[375]

Figure 11. Overview of carbon materials (0D to 3D) used in composite CTMs and associated merits. a) Electronic aspects: a1) rapid hole extraction from perovskite with extremely slow back-transfer and recombination. Reproduced with permission.[130] Copyright 2016, The Royal Society of Chemistry; a2) microscopic interaction between GD and P3HT is favorable for hole transportation due to π–π stacking and p-type doping. Reproduced with permission.[376] Copyright 2015, Wiley-VCH; a3) schematic of hot electron transfer from GQDs to SnO2 under illumination. Reproduced with permission.[371] Copyright 2017, American Chemical Society. b) Stability aspects: b1) PVK interface with an HTL composed of P3HT-functionalized SWNTs embedded in an insulating polymer matrix. The inset photograph shows a complete PVKSC employing a composite HTL under the water flow directly on top of the device. Reproduced with permission.[375] Copyright 2014, American Chemical Society. b2) Comparison of water contact angles of hexa-peri-hexabenzocoronene (TSHBC)/graphene and spiro-OMeTAD. Reproduced with permission.[377] Copyright 2015, American Chemical Society. b3) Schematic of diffusion processes within the carbon-composite ETL. Reproduced with permission.[378] Copyright 2017, Macmillan Publishers Limited.

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Graphene-based carbon materials have been incorporated with both metal oxide and organic CTLs. If TiO2 is anchored onto graphene, a continuous 2D conductive framework reduces the formation of energy barriers at the interface through a cascade conduction band structure.[379] N-doped graphene and F-substituted graphene have been combined with PCBM for mitigating penetration of iodide ions or water (Figure 11b).[378,380] In additional to graphene, graphdiyne also provides better percolation paths for charge transport within CTMs (Figure 11a).[277,376] Other graphene derivates, GO (WF ≈ 4.9 eV), and rGO (WF ≈ 5.0 eV) are suitable for combining with traditional HTMs. Particularly, the rGO with enhanced conductivity and less oxygen functionalities is favorable for blending with P3HT[381] and spiro-OMeTAD.[382] The r-GO can also work in conjunction with PTAA in a bilayer structure and strongly absorbs the near UV light to improve stability.[85] Lithium-neutralized graphene oxide (GO-Li) has a WF ≈ 4.3 eV and matches well with the CB of n-type oxides. Insertion of GO-Li between TiO2 and perovskite passivates surface oxygen defects and enhances electron injection.[383,384] Graphite possesses high conductivity and surface area. Composites of graphite and carbon black function effectively as counter electrode in solar cells and protect perovskites from the attack of moisture.[385,386] Reduced graphene with superior surface area has been incorporated as an interface layer between TiO2 and perovskite absorber to improve charge extraction.[387] Carbon materials with high conductivity could enable metal-free hybrid CTMs for highly stable, low cost, and upscalable PVKSCs.[388] Polymer-Derived Composites: Polymer-based CTLs are attractive because of their adjustable properties and easy processability. Advantages in terms of energy alignment, charge selectivity, and morphology modification render polymers as a suitable compatibilizer for composites. Doping interaction between inorganic metal oxide layer (MoO3 and Ta–WoO3) and polymeric HTMs results in quasi-Ohmic contacts and low-loss hole transport paths.[245,343] PCBM can be electrically doped by blending with PFN-OX (polyfluorene-based semiconducting polymers) to improve electron injection at the perovskite/ETL interface.[389] Blending of ZnO NPs with crosslinkable PFN-OX provides a robust hybrid composite electron selective layer.[390] Besides, polymers can also be used as an interfacial compatibilizer with inorganic and organic CTMs to influence the perovskite film formation as well (Sections 3.1 and 4.3). Preferential electrostatic interaction between ZnO and conjugated polyelectrolyte polyethylenimine (PEI) improves infiltration of the perovskite film.[201] Blending of high-molecular-weight polymer improves film formation of CTMs and serves as a water-resistant layer to protect the perovskite from water damage.[389,391] For polymerderived composites, there is still much scope for synthetic chemists to enrich the toolbox of polymer compatibilizer to accommodate for different requirements of polarity and crystallinity at interfaces. Diversity of carbon materials and polymers in terms of their chemical composition, stoichiometry, and functionalities offers versatility to build novel CTMs; it however also results in poorly controlled chemistry contributing to difficulty in batch-to-batch reproducibility. Theory-assisted analysis is important to understand key factors in synthesis. Model for charge transport in composites needs to be built to establish design rules and matching principles for CTMs.

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5.1.4. Interfacial Modifiers Manipulating perovskite growth, charge dynamics, and instabilities at interfaces via interfacial modifiers has played a pivotal role in the evolution of PVKSCs thus far (Section 4.3). Interfacial interaction and reactivity arising from the ionic nature of perovskites is a critical aspect that needs to be precisely controlled for alleviating undesirable interfacial properties. Different extents of electrostatic and chemical interactions of perovskite with the adjoining interlayers have been identified to strongly influence interfacial properties.[117,163,286,288,392,393] Electrochemical redox interaction at interfaces and permeability of interlayers strongly determine the extent of ionic accumulation controlling the hysteresis and reverse bias characteristics in PVKSCs.[157,158,168] Ionic diffusion across interlayers and reactivity with electrodes contribute to deterioration of charge transport and degradation of perovskite.[186,394,395] Development of suitable interfacial modifiers with these considerations is important to further improve processability, charge dynamics, and long-term stability of PVKSCs. Lee et al.[396] have introduced an amphiphilic conjugated polyelectrolyte (hydrophobic backbone with hydrophilic ionic functional groups) as an interfacial compatibilizer to improve the wettability of perovskite precursor solution, which facilitated scalable formation of high-quality perovskite films over large areas. Yang et al.[210] have optimized the surface of TiO2 ETL using a special ionic liquid (1-butyl-3-methylimidazolium tetrafluoro-borate) to improve the interfacial contact with perov skite; the bonding of anion group of the ionic liquid to TiO2 and interaction of cation group of the ionic liquid with adjacent perovskite provided a better growth platform for perovskite with improved interfacial charge dynamics. Lim et al.[260] have developed a self-organized hole extraction layer (composed of a conducting polymer (PEDOT:PSS) and a perfluorinated ionomer (PFI)), whose work function can be tailored easily by varying the molecular ratio of PFI to remove the interfacial energy offset with perovskite and maximize the Voc. Agresti et al.[397] have illustrated that graphene interface engineering at the perovskite/ETL interface synergistically improves charge transfer kinetics and stability. Hou et al.[167] have revealed that surface functionalization of the TCO electrode with phosphonic acid–based mixed C60/organic SAMs enhances perovskite film growth to suppress density of mobile ions and their accumulation at interfaces, which has led to elimination of the unstable diode behavior and hysteresis characteristics in the corresponding PVKSCs. Back et al.[185] have demonstrated that an amine-mediated titanium suboxide (AM-TiOx) incorporated between the ETL and metal electrode acts as a chemical inhibition layer to prevent the metal-electrode induced degradation and greatly improves the shelf lifetime of PVKSCs; amine functional groups in AM-TiOx effectively neutralized the migrating corrosive iodide ions and eliminated the formation of deleterious insulating compounds (AgI) at the electrode interface. The above-highlighted examples portray the tremendous breadth of impact accomplishable through tailored design and implementation of versatile interfacial modifiers. Further studies to understand the nature of interactions between different state-ofthe-art interlayers with perovskite and their correlation to device efficiency and stability will unveil critical limiting factors as well as design requirements for new interfacial modifiers.

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5.1.5. Materials for Optical and Electrical Field Management Beyond the commonly discussed impacts of interfacial engineering (Section 3), interfacial materials also provide an opportunity to manipulate the optical and electrical field in PVKSCs for enhancing the efficacy of light harvesting. This aspect of interfaces is less exploited and is gaining an increasing attention recently. State-of-the-art PVKSCs employ 1.5–1.6 eV bandgap perovskites that harvest light efficiently between 400 and 700 nm[196,214] and there is still significant room for absorption improvement in the ultraviolet (700 nm) regime. Additionally, strengthening of the built-in electric field could further enhance the charge transport and extraction. Here, we discuss three intriguing research directions that are reliant on creative material design and device approaches to enable such improvements. Integration of NIR Absorbing Organic Bulk-Heterojunction Blends: Direct integration of an organic bulk-heterojunction blend with NIR light harvesting capabilities into PVKSCs (inset of Figure 12a) is an elegant way to extend its photoresponse without intricacies of a tandem architecture. Even though certain neat p-type or n-type organic materials employed as interlayers absorb NIR light, they do not contribute to the photocurrent because of insufficient driving force for exciton separation. Learning from organic photovoltaics (OPV), exciton separation in organic interlayers can be facilitated by blending them with suitable materials to provide the necessary energy offset. Initial demonstrations of this concept employed typical donor–acceptor (D–A) blends composed of low bandgap polymer or small mole cule and fullerene on the top of perovskite absorber. Despite an extended photoresponse, the integrated systems suffered from low efficiencies (PCE