In Situ Ring‐Opening Polymerization for a Reactive Polymer Interlayer Enables Efficient and Stable Perovskite Solar Cells

ABSTRACT The migration of indium ion (In 3+ ) severely impedes the performance and operational stability of typical n‐i‐p perovskite solar cells (PSCs). Herein, we propose a novel reactive polymer (RP), chlorinated α,ω‐Dibenzoxazine‐Poly(dimethylsiloxane) (P(Cl‐DiBz‐Si)), which acts as an ion diffusion barrier at SnO 2 /perovskite interface. This polymer is formed via an in situ ring‐opening polymerization during perovskite thermal annealing. The multifunctional and ordered RP interlayer bridges SnO 2 and perovskite through synergistic coordination of hydroxyl groups with Sn 2+ /Pb 2+ , passivating buried interface defects, enabling effective carrier extraction. More importantly, this stabilized configuration robustly inhibits the generation and diffusion of In 3+ induced by the acidic by‐product (H + ) of perovskite degradation. Furthermore, the polysiloxane chains can enhance the interfacial adhesion, effectively releasing residual stress. Consequently, the optimized n‐i‐p PSCs incorporating this RP interlayer achieve a champion power conversion efficiency (PCE) of 26.10% with notably reduced hysteresis. The suppression ion migration promotes the stability of unencapsulated RP‐assisted devices, enabling them to retain 91.6% of their initial efficiency after 1320 h storage at 65 ± 10% RH (ISOS‐D‐1). These devices exhibit excellent illumination and thermal stability, maintaining 90% of their initial efficiency after 1200 h of MPP tracking under illumination (ISOS‐L‐1I) and 88.8% after 240 h of thermal aging at 85°C (ISOS‐T‐1).