Effect of -CsPbI₃ Phase Separation in Cs_xFA_1-xPbI₃ under Ambient Conditions

Formamidinium-based metal halide perovskites (FAPbI3) have gained prominence as materials for high-efficiency solar cells owing to their superior optoelectronic properties compared to those of traditional methylammonium-based perovskites. However, it is difficult to maintain the photo-active alpha-phase of FAPbI(3) owing to the structural instability at room temperature. Although this issue can be addressed by partially substituting cesium (Cs), most reported CsFAPbI(3) perovskites are produced in inert environments because of their low stability under ambient conditions. In this study, ethyl acetate, which protects the perovskite wet film from moisture during synthesis, was used as an antisolvent to optimize the Cs concentration in Cs(x)FA(1-x)PbI(3) under ambient conditions. Despite the formation of delta-CsPbI3 with increasing Cs concentration, the carrier lifetime was enhanced, resulting in improved power conversion efficiency. Highest reproducibility of solar cell was observed at a Cs concentration of 22 %.