Grain-Size-Dependent Stability and Crystallographic Orientation Effects in MAFA Perovskite Thin Films

This study investigates the impact of grain size and crystallographic orientation on the stability and optoelectronic properties of MA_0.01FA_0.99Pb(I_0.99Br_0.01)₃ (MAFA) perovskite thin films. Grain size was tuned by varying the additive amount of methylammonium chloride (MACl) during fabrication. Macroscopic experiments demonstrated that larger grains with a dominant {100} orientation degrade rapidly under ambient conditions. In contrast, films with random orientation remain stable throughout the experiments. A maximum photoluminescence quantum yield (PLQY > 8%) was observed at an MACl molar concentration range from 0.28 mol dm^-3 to 0.38 mol dm^-3, which corresponded to the largest grain size with random crystallographic orientation. Beyond this range, the PLQY decreased significantly, indicating a high defect density and consequently increased nonradiative recombination, which leads to a reduced open-circuit voltage (<i>V</i>_OC,nonrad). These findings uncover a fundamental tradeoff between grain size and long-term material stability, highlighting the critical role of crystallographic orientation control in the development of durable, high-performance perovskite solar cells.