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Terry Chien-Jen Yang,1,* Peter Fiala,1 Quentin Jeangros,1 and Christophe Ballif1,2. High-bandgap ...... 3690–3698. 31. Eames, C., Frost, J.M., Barnes, P.R.F.,.

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High-Bandgap Perovskite Materials for Multijunction Solar Cells Terry Chien-Jen Yang,1,* Peter Fiala,1 Quentin Jeangros,1 and Christophe Ballif1,2

High-bandgap (>1.7 eV) mixed halide perovskites for multijunction solar cells are usually affected by photoinduced phase segregation, which triggers subbandgap defects that are detrimental to the open-circuit voltage. While this effect may be reversed, e.g., when leaving the cells in the dark, new perovskite compositions that exhibit enhanced stability may be required. In this Perspective, the compositional space beyond the conventional methylammonium- and formamidinium-based mixed halide compounds is reviewed in light of multijunction applications. These alternative absorber compositions include: (1) layered or quasi-2D perovskites, where larger organic cations are incorporated into the structure; (2) inorganic perovskites (i.e., when the organic components are removed altogether); and (3) lead-free structures, where the toxic lead is substituted by one or more elements. The development perspectives of highefficiency and stable perovskite materials based on these compositions are discussed in view of an integration in multijunction solar cells. Introduction Perovskite solar cells (PSCs) have drawn intense scientific interest in recent years due to their potentially low cost,1 high carrier diffusion lengths,2 high absorption coefficient,3 and bandgap tunability.4–7 The power conversion efficiency (PCE) of PSCs has increased rapidly from 3.8%8 (2009) to an initial steady-state value of 22.7%9,10 (April 2018). State-of-the-art perovskite materials consist of a chemical structure with the AMX3 formula (also commonly denoted as ABX3), where A is a cation (typically methylammonium [MA], formamidinium [FA], and/or cesium), M is another cation (typically lead and/or tin) and X is a halide (iodide, bromide, and/or chloride). It was shown that by varying the ratio between I and Br in MAPb(IxBr1x), the bandgap (Eg) of the perovskite material can be continuously tuned from around 1.6 to 2.3 eV.4 Also if Sn compositions are included, it can be decreased down to 1.2 eV.11 This particular aspect of perovskites makes them especially attractive for multijunction or tandem solar cells.12–17 Figure 1A shows the design of a monolithic double-junction cell and the corresponding modeled theoretical bandgap/efficiency map obtained from simulations using realistic material properties.18 This device architecture has an AM1.5G efficiency potential of >32% when combining a bottom cell with a bandgap of 1.12 to 1.2 eV (either silicon or a perovskite) with a 1.7- to 1.8-eV top perovskite cell.18 When switching to a triple-junction configuration, this efficiency is predicted to rise to >35% when using 1.45- and 1.95-eV perovskite intermediate and top cells on silicon as shown in Figure 1B. At the experimental level, perovskite/silicon tandem solar cells achieve currently the highest efficiencies when compared with other perovskite-based multijunctions but their performance does not come close to these theoretical predictions. Figure 1C shows a schematic view of all the layers of the monolithic 2-terminal perovskite/silicon tandem device that achieves the highest reported efficiency at 23.6% (April 2018). Its 1  R (where R is the reflectance) and external quantum efficiency

Context & Scale Crystalline silicon solar cells, today’s mainstream photovoltaics technology, are quickly approaching their efficiency limit of 29.4%. To further decrease the cost of solar energy in $/m2 or $/W terms, an improvement in efficiency is needed. The most promising and proven approach to surpass the single-junction efficiency limit is to stack absorbers of different bandgaps in a multijunction cell. While traditional multijunction solar cells use costly III–V materials, perovskite solar cells have emerged as a promising alternative, especially when combined with crystalline silicon or copper indium gallium selenide bottom cells. Perovskite materials exhibit high efficiency, potentially low processing costs, and, most importantly for multijunction devices, tunable bandgaps from 1.2 up to 2.3 eV. The standard methylammonium lead iodide (MAPbI3) perovskite (1.6 eV) is usually modified by substituting different cations on the MA site (e.g., with formamidinium and/or Cs) and anions on the I site (e.g., with Br and/or Cl) to widen the bandgap to desired values for multijunctions. Yet stability issues affect existing compositions, primarily with respect to photostability. Open-circuit voltages are lower than expected due to the formation of

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bottom cell Eg (eV)


front contact top cell n junction recombinatio bottom cell rear contact

PCE (%) 31.39 29.79

1.2 1.1

Device (C-D)

Si bottom cell (1.12 eV)

28.19 26.60

1.0 0.9


1.7 1.8 1.9 top cell Eg (eV)

front contact top cell n bination junctio om rec middle cell n ctio n jun recombinatio Si cell t rear contac


middle cell Eg (eV)




PCE (%) 34.30 32.28





sub-bandgap states under illumination, reducing any advantages of high-bandgap materials in multijunction devices. These issues highlight the need to explore the compositional space beyond conventional perovskite compositions.

1.4 1.8

1.9 2.0 2.1 top cell Eg (eV)






Perovskite NiO

70 60


Perovskite 1.63 eV 18.9 mA cm-2

Silicon 1.12 eV 18.5 mA cm-2




c-Si (n)

a-Si:H (i) a-Si:H (p+) ITO SiN Ag

Reflection losses


ic loss

NiO ITO a-Si:H (n+) a-Si:H (i)




500 nm LiF ITO

1−R and EQE


500 nm


20 10


0 300

900 600 Wavelength (nm)


Figure 1. Examples of Multijunction Solar Cell Configurations Featuring Perovskites (A) Theoretical bandgap/power conversion efficiency (PCE) map of dual-junction devices obtained using realistic material optoelectronic properties. 18 The bandgap combination used in Device (CD) is shown by a star. (B) Predicted power conversion efficiencies as a function of middle and top cell bandgaps for the case of a triple-junction perovskite/perovskite/silicon device. (C) Schematic view of the 2-terminal perovskite-on-silicon tandem solar cell that achieves a current record efficiency of 23.6% and corresponding scanning electron microscopy images of the device. (D) Its 1  R (reflectance) and EQE curves. 19 (A) and (B) were adapted from Ho¨ rantner et al. 18 with permission from the American Chemical Society, while (C) and (D) were adapted from Bush et al.19 with permission from Springer Nature.

(EQE) spectrum, which are reported in Figure 1D, highlight the contribution from each subcell.19 It is worth mentioning at this step that this device features a 1.63-eV top cell perovskite, a value about 0.1 eV (50 nm) lower than optimum values inferred from theoretical predictions.18,20 However, as discussed in the next paragraph, increasing the bandgap to ideal values comes with detrimental side effects. The above examples give an idea of the practicality of high-bandgap (Eg > 1.6 eV) mixed iodide-bromide perovskite materials for tandems and multijunction applications. However, one pressing issue is that mixed halide perovskites suffer notably from a lightinduced phase-segregation mechanism, as first reported by Hoke et al.21 This means that under illumination, mixed iodide-bromide perovskite materials segregate into iodide-rich and bromide-rich domains. Photoluminescence measurements capture this phase-separation process: a red-shifted peak appears over time as the initial peak disappears. This photoinduced effect is metastable and can be reversed if the


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1E ´ cole Polytechnique Fe´de´rale de Lausanne (EPFL), Institute of Microengineering (IMT), Photovoltaics and Thin-Film Electronics Laboratory (PV-Lab), Rue de la Maladie`re 71b, Neuchaˆtel 2002, Switzerland 2CSEM,

PV-Center, Jaquet-Droz 1, Neuchaˆtel 2002, Switzerland *Correspondence: [email protected] https://doi.org/10.1016/j.joule.2018.05.008

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mixed halide perovskite is then left in the dark. A detrimental consequence is that during illumination, the operational PSCs suffer from a deficit in open-circuit voltage (i.e., Vdeficit = Eg/q  VOC, where q is the electron elementary charge and VOC the open-circuit voltage) for many standard mixed halide perovskite compositions, as shown in a recent review by Unger et al.5 They show that this photoinduced segregation effect only occurs with mixed halide compositions, but not with pure iodide or bromide compositions. This deficit results from the appearance of sub-bandgap defect or ‘‘trap’’ states21 during this photoinduced phase segregation. This voltage deficit is currently a major roadblock to high-efficiency perovskite-based multijunction solar cells. As mentioned when discussing Figure 1, this photoinduced degradation mechanism has so far hindered the application of high-bandgap materials in tandem solar cells, even in record devices, which opt to use 1.55- to 1.63-eV perovskite top cells on silicon instead of higher, more optimal bandgaps.19,22 Recent investigations by various authors have shed some light on this metastability of mixed I/Br compositions, as reviewed notably in Noh et al.5 and Brennan et al.23 Excess charge-carrier density is likely at the origin of this phase-segregation phenomenon, given that the effect also occurs during electroluminescence measurements in the absence of light.24 Furthermore, it appears that it is the gradient in charge carriers generated across the absorber that may drive this segregation by inducing an inhomogeneous lattice strain.25 The decrease in bandgap associated with the formation of these iodide- and bromide-rich regions may counterbalance the unfavorable formation energies of these iodide and bromide domains, as quantitatively rationalized by Draguta et al.26 In the dark, entropy then drives the perovskite back to its homogeneous initial state. To cluster under illumination, iodide and bromide need to diffuse. In this regard, the presence of a large concentration of halide vacancies (on the order of 0.1% at room temperature according to simulations27) within these perovskite films, which are processed at low temperatures and often from solution, plays a critical role in determining the kinetics of this phase segregation.23,25 In addition to being already present in the as-deposited films, the concentration of such halide vacancies increases under illumination, presumably as some of the holes generated may activate halide vacancies by displacing neutral halide atoms from a lattice site to an interstitial position.28 On a side note, these halide defects are at the origin of many of the challenges faced by perovskite-based devices: these do not only control the dynamics of this phase segregation but also the hysteresis observed when measuring current-voltage characteristics of PSCs and other degradation mechanisms triggered by the interaction of halides with the carrier-selective contacts and metal electrodes.29–37 All these effects are, hence, interlinked. Several approaches have been demonstrated to hinder this phase-segregation process. Barker et al. suggested strategies to mitigate this process by acting either on the mechanism that drives it (by controlling the charge-carrier generation gradient by using colloidal perovskite nanocrystals) or on the mechanism that sustains it (by reducing the halide vacancy density).25 Similarly, a study by Bischak et al.38 suggested that lowering electron-phonon coupling (to reduce the strain on the lattice induced by the generation of carriers) and the concentration of defects should mitigate this photoinduced degradation effect. Given that the migration of halides is known to be more facile at grain boundaries, increasing the perovskite grain size and improving crystallinity were also shown to reduce this degradation process.24,39,40 In addition, switching from a solution-processed to a fully evaporated perovskite may be beneficial, as discussed by Longo et al.41 Indeed, the differences in affinity between Pb and I or Br and in solubility between PbI2 and PbBr2 may already result in a phase segregation during solution processing. Furthermore,

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Figure 2. Literature Survey of Quasi-2D, Inorganic, and Pb-Free Perovskites (A and B) Relationships between bandgap and the (A) V OC and (B) PCE of various non-standard perovskite compositions shown alongside record and notable results for standard Cs/FA/MA-Pb compositions. Half-filled symbols indicate PSCs made with pure halide compositions, while filled symbols show mixed halide results. Generally these segregate into distinct regions of bandgap, but other ions such as chloride, larger organic cations (LOCs), tin, and other Pb substitutes can also strongly influence the bandgap. The gray regions indicate the bandgap range that is difficult to reach by Cs/FA/MA-Pb perovskites without halide mixing. This region contains the compositions with ideal bandgaps for tandem and multijunction solar cells, i.e., compositions that exhibit a significant V OC deficit. The references can be found in Tables S1–S4.

traces of solvent may remain trapped in the film when using the former synthesis route, traces that may create defects and enhance the diffusivity of halides. The authors furthermore demonstrated that a low-temperature treatment (90 C for 15 min) also reduced this phase-segregation issue. Another approach is to mix (or replace) the MA A-site cation of the AMX3 structure with the less polar cesium cation, (Cs+),24,40,42 as well as other alkali cations such as potassium (K+)43 and rubidium (Rb+).44 Abdi-Jalebi and co-workers demonstrated that incorporating K+ quenched mobile halide species at the grain boundaries and suppressed the separation into I- and Br-rich domains, which enabled an open-circuit voltage of 1.23 eV for a bandgap of 1.78 eV.43 While progress has been made in mitigating this phase separation, the open-circuit voltage deficit to the bandgap still remains significant for these mixed halide compositions, especially when compared with standard MAPbI3 (which has a record VOC of 1.17 V for an Eg of 1.6 eV)45 or pure MAPbBr3 compositions (which has an Eg of around 2.2 eV and a VOC up to 1.61 V).46,47 This is illustrated in Figure 2, which notably compiles the VOC and efficiency as a function of bandgap for a selection of standard compositions (including pure iodide, pure bromide, and mixed halides). These examples of standard compositions and their solar cell parameters are listed in Table S1. Alternative stabilization strategies and/or compositions not prone to this effect hence appear essential to fully harvest the potential of high-bandgap materials. Long-term stability of organic-inorganic perovskite compounds, including highbandgap mixed halide compositions, is another key issue that has impeded the rapid commercialization of PSCs. Existing photovoltaic technologies such as crystalline silicon, cadmium telluride, copper indium gallium selenide, or even amorphous silicon solar cells, if encapsulated well, can pass the toughest standard testing conditions and have been shown to last over 25 years in the field.48 This is usually not the


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case for perovskite compounds. The optoelectronic quality of these perovskite absorbers often degrades irreversibly at a timescale of a few hours depending on electrical bias, temperature, and/or atmosphere conditions49,50; therefore, great care and best practices should be used when characterizing these perovskite materials.51 As there are many up-to-date reviews49,52–59 that discuss the topic of stability in PSCs, this will not be covered in great detail here. Conventional cesium-formamidinium-methylammonium lead iodide-bromide compositions are hence prone to several degradation mechanisms, some irreversible and common to all organic-inorganic perovskite compositions, and some reversible that are specific to high-bandgap mixed iodide-bromide materials. While stabilization strategies of current compositions have shown some promise, especially the latest reports on K+ incorporation or when switching to an evaporation-based synthesis route, new compounds have also emerged, offering hope that these alternative materials with suitable bandgaps may not be prone to the same degradation processes as mixed halides. This exploration of the compositional space of highbandgap perovskites offers potential for solving some of the pressing problems that are currently holding back perovskite materials from their potential applications in solar energy. Some prominent examples of recent research on new compositions will be reviewed throughout this Perspective. The examples discussed below include: (1) quasi-2D organic cations, (2) inorganic compositions, and (3) Pb-free materials, which may also be fully inorganic. Quasi-2D Perovskite Materials as Additives One of the ways in which operational and environmental stability are being addressed is through the role of the A-site cation, which as mentioned above is typically MA+, FA+, Cs+, or a combination of these cations. These cations are chosen because of their optimal size, given the Goldschmidt tolerance factor, which allows them to form a stable cubic perovskite lattice.60,61 However, recent research has started to use larger cations. These larger organic cations (LOCs) do not fit into the vacancies of the cubic structure, and thus can form new materials and modify perovskite properties in unique ways.62 In abundance, LOCs will cause the formation of a 2D Ruddlesden-Popper perovskite (RPp) structure.63,64 RPps can be used directly in PSCs,65,66 but their properties are significantly different from their standard 3D counterparts, with distinct charge transport, bandgaps, stability, and interfaces, as well as other properties.64 In lower concentrations, the LOCs can become interspersed within the bulk 3D perovskite, modifying the properties less dramatically, but still in potentially useful ways.67–71 Together, these LOC methods/materials open up a wide area of compositional space, modifying standard 3D perovskites to maximize the best properties of each.67–77 Figures 2A and 2B list the VOC and efficiencies of various PSCs based on LOCs, with more information on their architecture from the literature listed in Table S2. The main ways in which LOCs are being used in research is to modify the environmental stability of the main perovskite absorber layer by using these materials as additives to standard compositions. This can be achieved in two ways, as shown in Figure 3. The first is to form a protective layer of RPp on top of the 3D perovskite absorber layer. Compared with standard MAPbI3, RPps have higher environmental stability.62 Their stability comes from the layers of organic ligands acting as a protective barrier.63,64 Optimization of this structure involves making the RPp layer thin, so as to minimize alterations of the absorber’s electrical properties, while still being thick enough to offer sufficient protection. This method has allowed for efficient and durable PSCs to be realized,72–76 optimizing the best properties of each

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Figure 3. Examples of How LOCs Can Fit into a Perovskite Structure (A–B) A typical 3D perovskite structure (A) with mixed cations and 2D layer incorporation, shown with examples of popular organic cations (B) along with the standard compositional space (C). LOCs may become interspersed, filling in the gap at the grain boundary or forming a RPp layer on top.

perovskite form and even aiding in charge selection.74,75 Specifically, Bai et al.74 demonstrated a phenethylammonium-based 2D graded top layer on a standard MAPbI3 absorber. They observed significantly enhanced ambient stability, an efficiency of 19.98%, and a VOC of 1.17 V (a record for p-i-n devices using a NiOx hole-selective layer). The second way in which LOCs are commonly used is interspersed in the bulk 3D absorber layer67–71 (Figure 3). The LOCs are kept at a low enough concentration that they do not form a separate RPp layer. It is not completely clear how the LOCs interact and situate themselves within the 3D structure: the LOCs could form RPp layers dispersed in the 3D layer,68 form RPp layers on the surface,69 fill vacancies at interfaces,71 or coat the individual grains in a protective layer.70 All of these situations are possible, but the truth will await more advanced characterization. Regardless, this method has shown excellent potential. Jodlowski et al.77 used interspersed guanidinium cations in MAPbI3 and observed remarkably enhanced photostability and thermal stability, along with efficiencies up to 20.15%. Alternatively, pure RPp cells have been made, with a maximum efficiency of 12.52% being reached by Tsai et al.65 in 2016. In general, perovskites that incorporate LOCs have demonstrated efficiencies lower than their standard 3D counterparts. This is often attributed to reduced charge mobility due to the LOCs.78 Another important aspect of this class of materials is their significantly different bandgap range, when compared with more standard Cs/FA/MA-based compositions. RPps have large and tunable bandgaps from 2.04 to 3.64 eV.63 This, along with their environmental stability, makes RPp good potential candidates for highbandgap applications, notably in multijunction devices.62 While tuning over this range is possible without using multiple halides, mixing I and Br is usually necessary to tune the bandgap more finely and reach specific desired bandgap values.63 Unfortunately, there is a lack of research on how the halides migrate under illumination when constrained in the 2D RPp structure. Still, the potential remains for RPps to perform well in roles that require high-bandgap, operationally stable, and environmentally robust absorbers. Additionally, LOCs can improve the halide segregation issue with the interspersion method discussed above. Xiao et al.71 were able to demonstrate that photoluminescence peaks of thin films with I/Br ratios remained stable under illumination when LOCs were incorporated into the bulk 3D absorber


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layer. This suggests that the iodide and bromide mixed regions did not segregate and the absorbers retained their specifically tuned bandgaps. While research would need to be expanded to more diverse compounds, higher efficiencies, and specific bandgaps, LOC incorporation may enable enhanced stability of high-bandgap perovskite compositions, under illumination and other stresses. Inorganic Perovskites In general, perovskites display low thermal decomposition temperatures due to their less stable organic monovalent cation. For instance, MAPbI3 has been shown to degrade at 85 C in an inert atmosphere79 and rapidly at 150 C in air.6 As their name implies, inorganic perovskites remove this organic cation altogether, providing another promising pathway to improve the overall stability of perovskites. To satisfy the Goldschmidt tolerance factor, a suitably sized inorganic monovalent cation could be used to replace the organic cations. The Cs+ cation turns out to be a very suitable candidate and has already shown excellent stability improvements when mixed with MA+ and/or FA+ organic cations.42,80–83 As discussed below, it can also be used as the sole cation on the A site. Figures 2A and 2B list the VOC and efficiencies of such Cs-based inorganic perovskites. A full list of the inorganic PSCs and their architecture from literature is provided in Table S3. The CsPbBr3 structure is a good starting point for inorganic perovskites, despite its undesirable high bandgap of 2.3 eV (even for multijunctions). Kulbak et al.84 demonstrated working CsPbBr3 PSCs of varying architectures and later directly compared the performance and stability of the high-bandgap CsPbBr3 with MAPbBr3-based PSCs.85 It was shown, via thermogravimetric analysis, that CsPbBr3 was more thermally stable than MAPbBr3 with the weight loss onset occurring at 580 C versus 220 C, respectively. Even though the efficiency of the CsPbBr3 (6.2%) PSCs were slight lower than their MAPbBr3 counterparts (6.5%), the former displayed much better stability over a 2-week period when stored in a dry air atmosphere in the dark. Furthermore, under electron beam-induced current analysis, CsPbBr3 was found to be more stable than MAPbBr3, which degraded and exhibited decreased collection efficiency. Eperon et al. started in the other direction by investigating the lower bandgap material ideal for tandems, CsPbI3 (1.73 eV).86 Through careful process control, they managed to synthesize this material in its black perovskite phase at room temperature. As this material is unstable and rapidly reverts to its more thermodynamically stable yellow orthorhombic phase,86,87 they added a small amount of hydroiodic acid to the precursor solution to stabilize the CsPbI3 black phase at room temperature. Even though their CsPbI3 solar cell only had an efficiency of 2.9%, they were able to conclude that hysteresis is not induced by a ferroelectric phenomenon as previously thought, given the presence of hysteresis when measuring current-voltage characteristics even in the absence of the organic polar cation. While additives are required to yield a cubic CsPbI3 phase at room temperature when depositing thin films, it was shown that this phase could be stabilized by reducing the dimension down to the nanocrystal size, 5 nm. In particular, Swarnkar and co-workers88 were able to synthesize CsPbI3 quantum dots (QDs), which could retained their cubic phase for months in ambient air and also at cryogenic temperatures. Using these CsPbI3 QDs, they fabricated PSCs with a PCE of 11.77% and a remarkable VOC of 1.23 eV for an Eg of 1.75 eV, a value among the highest reported for PSCs, even surpassing what has been achieved through the addition of K+ to standard mixed halide compositions.43 Due to the quantum confinement effect and other enhanced properties such as stability or resistance to the photoinduced

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halide segregation effect, perovskite nanocrystals or QDs open up a whole new area of research for perovskites in both photovoltaics and other optoelectronics.89–95 Another compromise between stability and lower bandgap can be obtained using mixed halide CsPb(IxBr1x) compositions, with existing work showing higher efficiencies than the pure bromide or iodide compositions. Sutton et al.7 used a onestep solution processing method to form films ranging from CsPbI3 to CsPbI2Br. The latter composition demonstrated enhanced stability when enclosed in a nitrogen chamber with a low air flow at 50% relative humidity. CsPbI3 reverted to its yellow phase after about an hour, while CsPbI2Br remained stable with a bandgap of 1.9 eV. Using this CsPbI2Br, they produced a PSC with a VOC, short-circuit current (JSC), a fill factor, and PCE of 1.11 V, 11.89 mA/cm2, 75%, and 9.84%, respectively, which was the most efficient inorganic PSC at the time (early 2016). Similarly, elsewhere around the same time, Beal et al.96 were able to show with the same composition a CsPbI2Br solar cell (Eg of 1.9 eV) with a PCE of 6.5%. Using the same material, Liu et al. demonstrated the current highest reported efficiency for an all-inorganic PSC with a VOC and a PCE of 1.14 V and 13.3%, respectively.97 In addition, Ma et al.98 were able to produce uniform CsPbIBr2 (Eg of 2.05 eV) films via dual-source thermal evaporation to fabricate an all-inorganic PSC (i.e., with inorganic hole/electron transport layers as well) with a PCE of 4.7%. These results have sparked further investigations into mixed halide inorganic Cs-based PSCs, especially by doping/substituting the A-site99 or M-site100,101 cations, which is a common trend in the perovskite community. Liang et al., who also published a comprehensive review on this topic,102 fabricated a tin-doped CsPb0.9Sn0.1IBr2 solar cell with a VOC of 1.26 V and a remarkable PCE of 11.33%.100 The partial substitution of Pb2+ for Sn2+ ions shifted the conduction band minimum toward a more negative level, allowing for a slightly more desirable lower bandgap of 1.79 eV. An alternative approach to yield the cubic photoactive perovskite phase is to alloy a small fraction of Pb with Bi, as demonstrated by Hu and co-workers.103 With a CsPb0.96Bi0.04I3 composition, a PCE of 13.21% was demonstrated (VOC of 0.97 V for Eg similar to the standard MAPbI3 compound). But in general, inorganic lead mixed halides also appear prone to phase segregation under illumination,104 indicating that the mechanisms triggering this process are not linked to the presence of the organic cation(s). On a final note, current efficiencies of inorganic PSCs are still low compared with traditional organic-inorganic PSCs, although it must be said that this is a relatively new research area and efficiencies are improving rapidly. Significant progress has been made to tackle existing synthesis issues and stabilize photoactive perovskite phases. However, extensive research is still required to reach the same performance levels as organic-inorganic compounds and many question marks remain about whether these compositions can eventually exhibit an enhanced long-term operational stability, including an improved resilience to sub-bandgap defects under illumination. Lead-free and Double Perovskites Despite the intensive development of perovskite materials, it seems that Pb on the M site appears to be a necessary component in almost all high-performance PSCs. However, Pb is toxic and environmentally harmful.105 The crystalline silicon photovoltaics industry is moving toward Pb-free soldering106 due to the mounting pressure from the European Union’s RoHS2 directive,107 which states that the maximum concentration for the value of lead tolerated by weight in homogeneous electronic materials is 0.1%. Unfortunately the concentration is >10 wt% for the most effective


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photovoltaic devices based on perovskites demonstrated so far.108 Therefore from a commercialization point of view, Pb-free perovskites are attractive and a necessity for research in the future, and this has led to the publication of many Pb-free reviews/perspectives.108–112 The most obvious elements to replace the Pb+ on the M site of the AMX3 structure are other group 14 elements, in particular Sn. The replacement of Pb by Sn reduces the bandgap, which does open up the possibility for all-perovskite multijunction solar cells.113–117 The first Sn-based PSCs were fabricated as early as 2014.118 The latest efficiency record is now at 8.12% for a VOC of 0.61 V (for an Eg of about 1.3 eV).119 However, Sn is also toxic and is not stable in its 2+ oxidation state as it can readily oxidize to 4+ upon exposure to moisture or oxygen (leading to a severe self-doping mechanism, which decreases carrier lifetimes).120 Due to these issues, Sn-based perovskites exhibit lower stability than their Pb counterpart. Given this instability, efforts have turned to computational screening across the periodic table for other suitable Pb-free perovskite compositions,121–125 with some examples shown in Figures 4A–4C. Figures 2A and 2B list the VOC and efficiencies of various Pb-free PSCs, while the full list of Pb-free perovskites can be found in Table S4. The next group involves Sb- and Bi-based perovskites, which incorporate on the M site of the A3M2X9 structure. Saparov et al. examined the ternary halide semiconductor Cs3Sb2I9 through computational methods.121 A density functional theory analysis indicated that this compound may exhibit a nearly direct bandgap of 2.05 eV. As a first experiment, they managed to demonstrate a working Cs3Sb2I9 PSC, albeit with PCE of 100 nm. In terms of thermal stability, thermogravimetric analysis showed that the decomposition temperature for Cs2TiBr6 is about 400 C, which is higher than MA-based perovskites. Furthermore, their Cs2TiBr6 films showed much higher tolerance to both humidity and illumination versus MAPbI2Br references. Finally, their best corresponding PSC exhibited a VOC of 1.02 V and a PCE of 3.28%. Similar electron and hole diffusion lengths >100 nm have been reported for Cs2AgBiBr6 compounds (Eg of 2 eV and a PCE currently just above 1%).136 In summary, among the Pb-free perovskites, Sn-based perovskites work relatively well and have demonstrated the highest efficiencies, but their major drawback is the instability of Sn2+ in addition to their toxicity. Sn-based perovskites also have the advantage of lowering the bandgap, which is promising for bottom cells in


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perovskite-perovskite tandem configurations. With respect to high-bandgap perovskites, the subject of this Perspective, it should be stated that the other Pb-free examples presented here have much lower efficiencies. Overall, these Pb-free compositions have drawn significant interest recently as they open a whole new area for compositional engineering in the quest for a high-efficiency non-toxic perovskite light absorber. However, many question marks remain on whether they can catch up to their Pb-based mixed halide counterparts in terms of optoelectronic quality, stability, and efficiency when integrated in PSCs. As more and more elements are added to yield photoactive compounds that are Pb-free, the family of defects that can potentially form also increases. This may affect not only the optoelectronic properties of these materials in their as-deposited state but also their stability under different stresses, notably light. Significant research, primarily at the fundamental materials science level, is hence required before these high-bandgap Pb-free materials may be incorporated successfully in multijunction devices. Summary and Conclusions This Perspective discussed several perovskite compositions, some well established, some discovered only recently, which exhibit high bandgaps (>1.6 eV) suitable for tandem and multijunction solar cells in general. However, a photoinduced halide segregation process reduces the VOC of the conventional mixed iodide and bromide perovskite compositions. This reversible degradation process is triggered by the generation of charge carriers in the layer when illuminated and is kinetically controlled by the presence of halide vacancies that act as diffusion pathways. Looking forward, it seems that these existing compositions may not be ideal on their own for implementation in multijunction devices. Indeed, modifications to the synthesis route to reduce the concentration of halide vacancies, the use of additives to immobilize mobile halide species, or even other compositions where this photoinduced phase segregation is absent will probably be necessary in the long term to achieve high efficiencies. After reviewing current stabilization strategies of standard compositions based on I and Br mixtures, the use of large organic cations was discussed. In their pure 2D layer morphology, RPps have shown improved environmental stability, high and varied bandgaps, and easy manufacturing. Their efficiencies are still too low, but the potential of combining them with 3D absorbers shows significant potential. When mixed into 3D layers, the use of these cations has shown improvements to both environmental and operational stability, potentially solving the problem of halide segregation and the corresponding appearance of sub-bandgap defects under illumination. This is a promising direction of research in perovskite materials, with cells already topping 20% in efficiency. Due to their performance, stability, and synthesis routes compatible with, e.g., crystalline silicon bottom cells, these compositions could be readily incorporated in multijunction devices and are hence the most promising short-term option among the alternative approaches detailed herein. Inorganic perovskites generally exhibit higher thermal stability. The main drawback is that current efficiencies are lower than those of traditional organic-inorganic perovskites, although they have been rapidly improving since inorganic PSCs were first reported around 3 years ago. Perovskite nanocrystals or QDs have seen a surge in research efforts, given their quantum confinement effects, which allow a broader bandgap range and other advantageous properties. Finally, lead-free perovskites are seen as an important research area, as lead is a toxic and environmentally harmful element. The continued use of lead in perovskites may impede any potential commercialization of PSCs. Unfortunately the most common substitution for lead is tin, which is also toxic and not stable. So far, other existing lead-free perovskite

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alternatives based on Ge, Sb, or Bi have not been promising in either performance or stability. However, computational screening efforts have led to new double perovskite compounds, which open up the compositional space for more perovskite variations. First experimental PSC demonstrations using these compositions have been made but significant research effort is still required to demonstrate not only high efficiencies but also stability. With all the intensive research efforts across many institutions around the world, it is foreseen that a suitable perovskite material that is stable, low-cost, and ideally non-toxic, can be incorporated into highly efficient multijunction solar cells in the future.

SUPPLEMENTAL INFORMATION Supplemental Information includes four tables and can be found with this article online at https://doi.org/10.1016/j.joule.2018.05.008.

ACKNOWLEDGMENTS T.C.-J.Y acknowledges the support of a Marie Skłodowska-Curie Individual Fellowship from the European Union’s Horizon 2020 research and innovation programme (grant agreement no: 747221, action acronym: POSITS). This work was funded by the Nano-Tera.ch Synergy project (20NA21_150950), the Swiss Federal Office of Energy under grant SI/501072-01, the Swiss National Science Foundation via the Sinergia Episode (CRSII5_171000), and NRP70 Energy Turnaround PV2050 (407040) projects. The authors wish to thank B. Niesen, J. Werner, F. Sahli, M. Bra¨uninger, F. Fu, B. Bissig, O. Dupre´, B.A. Kamino, S.J. Moon, and A. Walter for fruitful discussions.

AUTHOR CONTRIBUTIONS Writing – Original Draft, T.C.-J.Y. and P.F.; Writing – Review & Editing, T.C.-J.Y., P.F., Q.J., and C.B.; Supervision, Q.J. and C.B.; Funding Acquisition, T.C.-J.Y., Q.J., B.N., and C.B.

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