Meticulous Construction of Internal Encapsulation Layer via In Situ Self‐Cross‐Linking Polymerization and Ring‐Opening Addition Reactions for Efficient and Environmental Perovskite Solar Cells

Capping an internal encapsulation layer (IEL) on the top surface of perovskite plays significant roles in enhancing perovskite quality and achieving high‐performance perovskite solar cells. Herein, a novel IEL is in situ synthesized by self‐cross‐linking polymerization of siloxane motifs and ring‐opening addition of ethylene oxide groups to overcome the long‐overlooked drawbacks of IEL, such as eliminating deterioration influences of byproducts, as well as the tradeoff between improving perovskite quality and minimizing Pb 2+ leakage. Comprehensive characterizations reveal that amidogen, hydroxyl, and carbon‐fluorine (C─F) bond in GPTFP synergistically stabilize grain boundaries, passivate surface defects, block oxygen and moisture, as well as minimize the Pb 2+ leakage of perovskite through hydrogen bond, oxygen–lead, fluorine–lead, and hydrophobic interactions. The resultant perovskite exhibits enhanced crystallinity quality, relieved residual strain, air‐stable black phase retained after 200 days of ambient aging, and undetectable leakage of Pb 2+ under simulated harsh conditions. Consequently, the resultant inverted device achieves an excellent efficiency of 26.83% (certified 26.57%) with a certified quasi‐steady‐state output of 26.51%. Notably, such a device retains >93 % of its initial efficiency after 2000 h of continuous 1‐sun illumination (AM1.5G, 100 mW cm −2 ) and 2000 h of ambient storage (30±5% relative humidity, 25 °C), respectively.