Stabilizing Mixed Halide Lead Perovskites against Photoinduced Phase Segregation by A-Site Cation Alloying

Mixed halide perovskites as top absorbers with relatively high bandgaps (Eg > 1.70 eV) are essential for tandem and multijunction solar cells. Light-induced phase segregation of mixed halide perovskites limits their commercialization, and it has been found that organic cation substitution in the A site can suppress photoinduced phase segregation. By substituting methylammonium (MA+) ions into formamidinium (FA)PbBr1.8I1.2 perovskites, we probed photoinduced segregation at different temperatures. In this study, we found that the segregation rate constant increased with MA+ content; however, significant suppression of the rate constant was observed in the case of 10% substitution. The activation energy for photoinduced phase segregation increased (by ?5 kJ mol-1) upon introduction of 10% MA+ into FAPbBr1.8I1.2, reflecting an increased energetic barrier for halide segregation. The effect of A-site cation alloying against photoinduced phase segregation in active devices such as phototransistors and photovoltaic cells is also demonstrated and discussed.