Cross‐Layer Molecular Design for Coherent Interface Passivation in Rigid and Flexible Perovskite Solar Cells

Interfacial defects and inefficient charge extraction at the SnO 2 /perovskite junction remain critical bottlenecks in perovskite solar cells (PSCs). Here, thiazol‐5‐ylmethanamine hydrochloride (TMACl) is introduced as a cross‐layer modifier that simultaneously stabilizes and passivates both sides of the interface. The thiazole moiety interacts with undercoordinated Sn 4 + in SnO 2 and Pb 2 + in FAPbI 3 , with the terminal –NH 3 + group forming N─H···O hydrogen bonds with oxygen atoms in the SnO 2 layer and N─H···I/Cl hydrogen bonds with the FAPbI 3 layer. The chloride ion in TMACl results in the formation of Sn–Cl species by substitution of –OH groups on the SnO 2 surface and passivates undercoordinated Pb 2+ ions. Through these cooperative interactions across the buried interface, TMACl reduces defect density, enhances α‐FAPbI 3 crystallinity, improves energy‐level alignment, and enables more efficient carrier extraction. As a result, rigid PSCs employing TMACl deliver a champion power conversion efficiency (PCE) of 26.44% (certified 26.12%) with 90.43% retention of the initial PCE after 2,500 h, and flexible PSCs achieve a champion PCE of 24.72%, demonstrating the broad applicability of the TMACl in interfacial engineering.