Lead-based perovskite solar cells have attracted a huge amount of attention due to their rapid rise in efficiency, with CH3NH3PbI3 solar cells recently achieving certified efficiencies in excess of 22%.6 These materials can be solution processed and have a variety of promising characteristics including high absorption coefficients, large charge-carrier diffusion lengths, and relatively low exciton-binding energies,13,14 closer to the binding energies reported for inorganic materials (6–15 meV), than those reported for organic solar cells (0.5 eV or larger).15 

Still, commercialization of lead perovskite solar cells is hindered by their short lifetimes and the use of toxic lead.16 Some hope that these limitations can be overcome by replacing lead with other elements like tin,17 antimony,18 or bismuth.11,16,19 Bismuth-based analogs to lead perovskite photovoltaic materials offer great promise for future development, although they have been scarcely explored.

Bismuth perovskites generally have the chemical formula A3Bi2X9, where A is a monovalent cation (i.e., Cs+ or CH3NH3+) and X is a halogen anion (i.e., Cl−, Br−, and/or I−). (CH3NH3)3Bi2I9 and Cs3Bi2I9 are two bismuth perovskite materials that have been investigated thanks to their similarities to their high-efficiency lead counterparts CH3NH3PbI3 and CsPbI3, respectively.

The crystal structure of Cs3Bi2I9 was initially studied in the 1960s.20 Almost 50 years later, Park et al. first incorporated bismuth perovskite materials into solar cells, demonstrating power-conversion efficiencies of 1.09% for Cs3Bi2I9 and 0.12% for (CH3NH3)3Bi2I9.21 These modest efficiencies were attributed to high exciton binding energies (70-300 meV compared to 8-20 meV for lead perovskites22–24), significant non-radiative recombination due to defect states,25 non-optimal charge-extraction layers, and high bandgaps (2.1-2.2 eV). Despite their modest efficiencies, these materials exhibited high absorption coefficients and were much more air-stable than their lead counterparts.

The good stability of bismuth perovskites in both dry and humid air has since been repeatedly demonstrated. For instance, (CH3NH3)3Bi2I9 exhibited stable photovoltaic performance during 10 weeks in ambient air26 and 21 days in air with an average humidity of ∼50%.27 In another study, x-ray diffraction of (CH3NH3)3Bi2I9 after 25 days in ambient air demonstrated almost no change other than the formation of a thin, protective oxidation layer that likely prevents further degradation (Fig. 2).28 This is in stark contrast to the lead analog, CH3NH3PbI3, which almost fully converted to PbI2 during the same time period. Likewise, the bismuth perovskite exhibited only minimal visual changes after 26 days, whereas the lead analog changed from dark brown to light yellow after 5 days.

(a) Photographs of methylammonium bismuth iodide (MBI) and methylammonium lead iodide (MAPbI3) overtime in ambient air. [(b) and (c)] Normalized XRD patterns of MBI over time with air exposure. (d) The relative change in the normalized intensity of the diffraction peaks of MBI (day 25 vs. day 1). Reproduced with permission from Hoye et al., “Methylammonium bismuth iodide as a lead-free, stable hybrid organic-inorganic solar absorber,” Chem. - Eur. J. 22, 2605–2610 (2016). Copyright 2016 Wiley-VCH Verlag GmbH and Co. KGaA.

(a) Photographs of methylammonium bismuth iodide (MBI) and methylammonium lead iodide (MAPbI3) overtime in ambient air. [(b) and (c)] Normalized XRD patterns of MBI over time with air exposure. (d) The relative change in the normalized intensity of the diffraction peaks of MBI (day 25 vs. day 1). Reproduced with permission from Hoye et al., “Methylammonium bismuth iodide as a lead-free, stable hybrid organic-inorganic solar absorber,” Chem. - Eur. J. 22, 2605–2610 (2016). Copyright 2016 Wiley-VCH Verlag GmbH and Co. KGaA.

Further investigation of these and other bismuth perovskites has led to improvements in the material properties. In particular, many studies have attempted to tune the bandgaps of bismuth perovskites since unaltered bismuth perovskites tend to have bandgaps above 2 eV, which is higher than desirable for optimal solar-cell performance. For example, one study demonstrated the ability to tune the bandgap of Cs2AgBiBr6 through pressure-induced changes in its crystal structure.29 Others have shown that sulfur doping can decrease the bandgap of Cs3Bi2I9 to a much more desirable value of 1.45 eV.30,31

Several groups have also improved the film quality and consequently the solar-cell performance of (CH3NH3)3Bi2I9 by replacing standard solution-processing methods with alternative deposition methods that result in smoother, more compact films with fewer pinholes. For instance, Ran et al. used a two-step method that combines evaporation and spin coating to push the power conversion efficiency of (CH3NH3)3Bi2I9 solar cells to 0.39%.32 Zhang et al. later employed a two-step vacuum deposition method to fabricate (CH3NH3)3Bi2I9 solar cells with a power conversion efficiency of 1.64% (0.83 V open-circuit voltage, 3.0 mA/cm2 short-circuit current, and 0.79 fill factor).33 The vacuum-processed solar cells exhibited charge-carrier diffusion lengths, trap densities, and absorption coefficients on par with many lead perovskite materials.

Likewise, new deposition techniques have resulted in improved Cs2AgBiBr6 film quality and consequently higher efficiencies. Cs2AgBiBr6 and its precursors exhibit low solubility in most common solvents, resulting in porous films full of cracks and pinholes. Dimethylsulfoxide (DMSO), on the other hand, has proven to be a good solvent for Cs2AgBiBr6 and its precursors: AgBr, CsBr, and BiBr3. Gruel et al. therefore dissolved the precursors in DMSO, heated the solution, and spin coated it onto a heated substrate.34 A subsequent annealing step at 250 °C was required to complete the formation of Cs2AgBiBr6 and maximize solar-cell performance. The best devices exhibited power-conversion efficiencies approaching 2.5% and an open-circuit voltage of 1.06 V, the highest value reported thus far for a bismuth-based perovskite. In a subsequent experiment, Gao et al. demonstrated the ability to deposit smooth films by dissolving Cs2AgBiBr6 in DMSO and spin coating the solution using the anti-solvent dropping method with isopropanol (IPA) as the anti-solvent.35 Films deposited without the anti-solvent were rough and frosted in appearance, whereas Cs2AgBiBr6 films deposited using the anti-solvent dropping method were very smooth and achieved efficiencies up to 2.2% and open-circuit voltages in excess of one volt. Again, a post-annealing treatment at 250 °C was required to produce high-quality, crystalline films.

Several related bismuth halides have also been explored as promising photovoltaic materials. For example, Kim et al. created dense, pinhole-free AgBi2I7 films by spin coating silver and bismuth precursors and subsequent annealing. The resulting air-stable material exhibited a bandgap of 1.87 eV and solar-cell efficiencies up to 1.22%.36 Bismuth triiodide (BiI3) has also been shown to be air-stable, to have a bandgap of ∼1.8 eV, and to exhibit efficiencies up to 1.0%.37–39 

One important distinction between bismuth perovskite and halide absorbers and their lead counterparts is the dimensionality of their octahedral networks. Figure 3 shows illustrations of 0D, 1D, 2D, and 3D octahedral networks and a few examples of bismuth perovskites and halides with each type of network. The octahedral network dimensionality can affect solar-cell performance by altering relevant material properties. Although there are notable exceptions, lower dimensional perovskites are usually associated with larger bandgaps, higher exciton binding energies, lower carrier mobilities, and better moisture stability due to more spatial confinement.40–45 

Illustrations and examples of some bismuth perovskite and halide absorbers with 0D, 1D, 2D, and 3D octahedral networks.

Illustrations and examples of some bismuth perovskite and halide absorbers with 0D, 1D, 2D, and 3D octahedral networks.

The CH3NH3PbI3 perovskite structure contains a 3D network of PbI6 octahedra that share corners in all three octahedral directions.46 Bi3+ cannot directly replace Pb2+ in this 3D-perovskite structure due to its higher charge.47 Charge neutrality forces the bismuth counterpart (CH3NH3)3Bi2I9 into a 0D structure with face-sharing octahedra of (Bi2I9)3 dimers that are separated by (CH3NH3)+ ions.48 The lack of a network between the (Bi2I9)3 dimers has been blamed for lower carrier mobilities and a larger bandgap of (CH3NH3)3Bi2I9 and Cs3Bi2I9.42 Double perovskites, which contain two different cations, offer one route to forming higher dimensional bismuth perovskites.34,49 For example, Cs2AgBiBr6 has a 3D octahedral network and has demonstrated power-conversion efficiencies up to 2.43% as discussed previously.

Although the efficiencies of bismuth perovskites and halides are currently modest, their demonstrated stability in humid air and their avoidance of toxic lead encourage further investigation. As many of these materials have yet to be extensively investigated, there remains considerable hope that the efficiencies can be significantly improved.