Molecular Synergy‐Enabled Buried Interface Engineering Toward Highly Efficient and Operationally Stable Flexible Perovskite Photovoltaics

Self‐assembled monolayers (SAMs) have become pivotal hole‐selective layers for efficient inverted perovskite solar cells (PSCs), yet conventional single‐component SAMs suffer from severe intermolecular aggregation, insufficient thermal anchoring, and weak crystallization templating, which severely limit efficiency and operational stability, especially in flexible configurations. Herein, we demonstrate a rationally designed molecular engineering approach for constructing robust buried bottom interfaces via multifunctional SAMs featuring bidentate phosphonic acid anchors, electron‐donating methoxy groups. Such elaborate molecular design enables strengthened interfacial binding, optimized energy‐level alignment, suppressed self‐aggregation, and oriented perovskite crystallization with relieved residual tensile strain. As a result, rigid PSCs achieve a champion efficiency of 27.15% (certified 26.51%), and flexible PSCs exhibit a remarkable efficiency of 25.37% with outstanding mechanical robustness. Moreover, the optimized devices deliver exceptional operational stability, retaining 97.5% of initial performance after 1000 h of continuous operation under ISOS‐L‐2 protocols. This work provides a universal molecular engineering paradigm toward high‐performance and ultra‐stable flexible perovskite photovoltaics via robust buried interface engineering.