Pseudo-Arch Bridge-Inspired Stress Modulation at Buried Interface for Stable High-Efficiency Perovskite Solar Cells

Thermal instability remains a key barrier to the commercialization of perovskite solar cells (PSCs), largely due to severe thermomechanical mismatch at the buried interface between the perovskite and transport layers. This mismatch induces interfacial strain, triggering deep-level defects, ion migration, and phase segregation that severely impair device stability. Here, a thermomechanical stress engineering strategy is introduced via rational molecular interface design. Specifically, a novel molecule, 4-(5,6-difluoro-2-(pyridin-2-yl)-1H-benzo[d]imidazol-1-yl)butan-1-ammonium iodide (FBI-PyAI) is synthesized, that anchors at the TiO₂/perovskite interface likely in a unique "molecular bridge" configuration. This soft interface yields an extremely low modulus and significantly reduces the interfacial stress energy from 0.554 to 0.178 eV, thereby suppressing defect formation and minimizing phase segregation. Meanwhile, the functional groups in FBI-PyAI passivate defects and induce vertically oriented perovskite crystallization, forming compact films with fewer voids and improved structural uniformity. As a result, the modified devices achieve exceptional thermal stability, which maintains 88% of initial efficiency after 50 thermal cycles (-15 to 65 °C). Moreover, the modified PSC delivers a competitive efficiency of 25.01% and outstanding photostability (95% retention after 800 h illumination under ISOS-L-1 protocol). This work offers mechanistic insight into interfacial stress modulation and underscores its importance for thermally stable PSCs.