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

Abstract

This study investigates the impact of grain size and crystallographic orientation on the stability and optoelectronic properties of MA0.01FA0.99Pb(I0.99Br0.01)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 (VOC,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.