Molecular Bridging of Buried Interface Flattens Grain Boundary Grooves and Imparts Stress Relaxation for Performance Enhancement and UV Stability in Perovskite Solar Cells

Abstract The limitations imposed by interfacial voids and residual stress fundamentally constrain the stability and performance ceiling of perovskite solar cells (PSCs). Herein, the study engineers a molecular bridge by the placement of ectoine (Ec) at the SnO 2 /perovskite interface. The experimental investigations coupled with first‐principles density functional theory (DFT) calculations reveal that the carboxyl group preferentially passivates uncoordinated Sn 4+ defects and oxygen vacancies in SnO 2 , while the imine group establishes robust coordination with Pb 2 ⁺ ions in the perovskite to passivate uncoordinated Pb 2+ defects. The bi‐anchoring molecular bridging mechanism facilitates the residual stress release, flattens the grain boundary grooves, and significantly suppresses the nonradiative recombination. In turn, the Ec‐modified PSCs achieve a power conversion efficiency (PCE) of 24.68% (vs 22.56% for control). Significantly, the unencapsulated PSCs with the Ec show improved UV stability, retaining 80.12% of the initial PCE after 130 h (equivalent to 1412 h of solar irradiation) under 365 nm ultraviolet irradiation (50 mW cm −2 ). The study uncovers the role of Ec as a molecular bridge to optimize the buried interface for effective yet stable solar cell fabrication.