Improvement of photovoltaic performance of the inverted planar perovskite solar cells by using CH₃NH₃PbI_3−xBr_x films with solvent annealing

In this paper, the CH₃NH₃PbI_3−xBr_x films with various Br-doping contents were successfully prepared by solution processed deposition and followed by annealing process. This method simultaneously modified the morphology and composition of the CH₃NH₃PbI₃ film. The effects of annealing treatment of CH₃NH₃PbI_3−xBr_x films under N₂ and DMSO conditions on the microstructure of films and photoelectric properties of the solar cells were systematically investigated. The relationship of the component ratio of R_Br/I= CH₃NH₃PbI_3−xBr_x/CH₃NH₃PbI₃ in the resulting perovskite versus CH₃NH₃Br concentration also was explored. The results revealed that the CH₃NH₃PbI_3−xBr_x films annealed under DMSO exhibited increased grain sizes, enhanced crystallinity, enlarged bandgap and reduced defect density compared with that of the N₂ annealing. It also was found that the R_Br/I linearly increased in the resulting perovskite with the increased of CH₃NH₃Br concentration in the methylammonium halide mixture solutions. Furthermore, the photovoltaic performances of devices fabricated using DMSO precursor solvent were worse than that of DMF under N₂ annealing atmosphere. When CH₃NH₃Br concentration was 7.5 mg ml⁻¹, the planar perovskite solar cell based on CH₃NH₃PbI_3−xBr_x annealed under DMSO showed the best efficiency of 13.7%.