A Universal Solid‐Phase Reaction Strategy for Lead Iodide Management in Wide‐Bandgap Perovskites Across Multiple Fabrication Methods

In wide‐bandgap (WBG) perovskite solar cells (PSCs), residual lead iodide (PbI 2 ) is a dual‐edged sword: while bulk retention stabilizes the α‐phase and suppresses defects, surface accumulation hinders charge transport and accelerates degradation under environmental stress. Herein, a solid‐phase reaction modification (SRM) strategy using L‐cysteine (L‐Cys) capping layer is reported to precisely regulate PbI 2 distribution. Leveraging its highly polar functional groups, L‐Cys effectively eliminates surface PbI 2 residues, mitigates interfacial defects, and releases residual lattice strain. These synergistic effects enable inverted PSCs (1.68 eV) fabricated through one‐step (spin coating) and hybrid two‐step (evaporation‐spin coating) fabrication methods to achieve a champion power conversion efficiency (PCE) of 23.27% and 19.63%. Furthermore, four‐terminal (4T) WBG PSC/copper indium gallium selenide (CIGS) tandem devices reach an impressive PCE of 28.14%. The SRM‐treated perovskite films exhibit exceptional robustness, showing no degradation under thermal aging (85°C, 24 h) or high humidity (85% RH, 72 h). Unencapsulated devices maintain 82.1% of their initial efficiency after 1000 h of ambient storage.