Stable FAPbI₃ Perovskite Solar Cells via Alkylammonium Chloride-Mediated Crystallization Control

α-Phase formamidinium lead iodide (FAPbI₃) perovskite solar cells (PSCs) have garnered significant attention, owing to their remarkable efficiency. Methylammonium chloride (MACl), a common additive, is used to control the crystallization of FAPbI₃, thereby facilitating the formation of the photoactive α-phase. However, MACl's high volatility raises concerns regarding its stability and potential impact on the stability of the device. In this study, we partially substituted MACl with n-propylammonium chloride (PACl), which has a long alkyl chain, to promote the oriented crystallization of FAPbI₃, ultimately forming an δ-phase-free perovskite. The FAPbI₃ film containing PACl demonstrates an enhanced photoluminescence intensity and lifetime. Additionally, PACl's presence at grain boundaries acts as a protective layer for the PSCs. Consequently, we achieved a power conversion efficiency (PCE) of 22.4% and exceptional stability. It maintains over 95% of initial PCE for 100 days in an N₂ glovebox, over 85% after 100 h of maximum power point tracking, and over 80% after 60 °C thermal aging.