Molecular Engineering of Alkylammonium Interfaces for Enhanced Efficiency in Perovskite Solar Cells

Power conversion efficiency (PCE) improvements in perovskite solar cells (PSCs) are increasingly constrained by nonradiative recombination at interfacial defects. In this study, we demonstrate a systematic interface engineering strategy using alkylammonium iodide salts with varying chain lengths from methylammonium (C1) to dodecylammonium (C12) to modulate the interface between the mixed‐cation perovskite absorber (FAPbI 3 ) 0.97 (MAPbBr 3 ) 0.03 and the hole‐transport layer. Surface treatment with these salts significantly reduces interfacial recombination, as evidenced by enhanced photoluminescence and a strong chain‐length‐dependent increase in open‐circuit voltage ( V OC ) and fill factor (FF). Our champion device, passivated with dodecylammonium iodide, achieves a PCE of 24.6% with V OC = 1.166 V and FF = 81.5%, marking a > 12% relative increase over the untreated control. Structural, optical, and electrical ( J–V , SCAPS modeling) analyses collectively reveal that longer‐chain cations form ultrathin 2D interfacial layers that suppress defect‐mediated recombination without impeding charge transport. Additionally, these passivation layers impart enhanced stability under continuous illumination, ambient air exposure, and elevated temperature, with DDAI‐treated devices maintaining over 88% of their initial performance after thermal aging at 65°C for 500 h. This work establishes alkylammonium chain length as a powerful tuning parameter for optimizing PSC interfaces and advancing high‐efficiency, stable perovskite photovoltaics.