Influence of the Synthetic Procedures on the Structural and Optical Properties of Mixed-Halide (Br, I) Perovskite Films

The influence of thermal treatments on the properties of mixed bromide-iodide organolead perovskites (MAPbI3-xBrx, MA = CH3NH3) is investigated in films prepared in air by single-step solution processes based on different precursor solutions. Initially, the bandgap energy (EG) dependence on composition is reconsidered on films obtained by mixtures of trihalide solutions. An EG(x) relation is obtained that is expected to be independent of the film properties and can be used to assess perovskite composition. In these samples, recombination centers are observed whose energy depth increases with x, likely involving the simultaneous presence of iodide and bromide, while the Urbach energy increases with the grain surface-to-volume ratio, which points out that the defects giving sub-bandgap absorption originate from grain boundaries. Trihalide mixtures allow perovskite synthetic processes suitable for solar cell production, being fast and reproducible. A slight MABr excess in the solution made of MABr and PbI2 gives MAPbI2Br films free of PbI2 phases and with a high compositional stability, but nonradiative recombination channels can make the material not appropriate for high efficiency solar cells. Finally, the solution made of MAI and PbBr2 (3:1 molar ratio) is little promising for solar cell production because its nonstoichiometric nature makes synthesis reproducibility an issue.