Po-Kai Kung, Farhan Yousuf, Hong‐Hsueh Chen, Ming-Hsien Li, Chen‐Fu Lin, Peter Chen
High Resolution Image Download MS PowerPoint Slide Perovskite solar cells approach the efficiency of Si devices but remain limited by ion-migration-driven instability. Here we show that steric tuning of the A-site cation, CsPb(I 0.85 Br 0.15 ) 3 → FA 0 .78 Cs 0.22 Pb(I 0.85 Br 0.15 ) 3 → FA 0.765 MA 0.15 Cs 0.085 Pb(I 0.85 Br 0.15 ) 3 → 1% DMA + -doped FA 0.765 MA 0.15 Cs 0.085 Pb(I 0.85 Br 0.15 ) 3, systematically suppresses photoinduced halide migration. In-situ photoluminescence spectra reveal reduced red-shift with increasing tolerance factor, while operando electrochemical impedance spectroscopy under AM 1.5G decouples recombination and ionic transport. The activation energy extracted from Warburg admittance rises from 0.037 to 0.199 eV, and heat-accelerated recombination is mitigated across devices. Under continuous illumination at 300 K, unencapsulated cells retain performance more effectively, with a 54% improvement in operational stability for the DMA + -doped device. Our results provide a composition-agnostic guideline: A-site steric expansion elevates the migration barrier, limits light/heat-driven stoichiometric drift, and stabilizes device operation.