Carboxyl‐Functionalized Sulfonium Additive for Improved Crystallization and Defect Passivation in Ternary Cation Perovskite Solar Cells

This study explores the use of 2‐(carboxyethyl) (dimethyl)sulfonium bromide (CDMSBr), a carboxyl‐functionalized derivative of trimethylsulfonium (TMS + ), as an additive in ternary cation, (Cs 0.05 FA 0.90 MA 0.05 Pb(I 0.95 Br 0.05 ) 3 [CsFAMA]), perovskite solar cells (PSCs) to enhance both stability and photovoltaic performance. In solution, it exhibits a zwitterionic form that controls nucleation and growth of perovskite crystals. It further protonates into CDMS + during crystallization to facilitate the formation of larger and more uniform grains with better crystallinity. Optimized CsFAMA‐based device achieves power conversion efficiency (PCE) of 21.02% (enhancement of 6.54%) at 1‐sun condition and 38.79% (enhancement of 9.21%) under low‐intensity indoor lighting (1000‐lx, LED 5000 K). The dual role of the additive in defect passivation and grain size enhancement contributes to reduced trap density, promoting increased stability of the PSCs. Devices with CDMSBr maintain 88.53% of their initial PCE after 960 h in ambient conditions. These findings highlight the potential of carboxyl‐functionalized sulfonium additives, like CDMSBr, to enhance perovskite morphology and stability, advancing the performance and operational durability of PSCs.