Buried Interface Modification for Reduced Open‐Circuit Voltage Loss in Perovskite Solar Cells With Efficiency Exceeding 25.8%

In n–i–p perovskite solar cells (PSCs), the buried interface of the perovskite layer is crucial for boosting both performance and stability. Here, multifunctional small molecule potassium trifluoromethanesulfonate (TFSK) is employed as an interlayer to efficiently bridge SnO 2 and the buried perovskite film, simultaneously regulating interfacial energetics and morphology. This strategy provides several advantages: (1) TFSK passivates oxygen vacancy defects and surface hydroxyl groups on SnO 2 , while also improving energy level alignment; (2) TFSK modification induces a loose and porous morphology in PbI 2 , facilitating the diffusion of ammonium salts and promoting sufficient ionic reactions to high‐quality FAPbI 3 films; (3) TFSK interacts strongly with perovskite through Lewis acid–base interaction (between S=O groups and uncoordinated Pb²⁺) and hydrogen bonding (between F − and formamidinium cations), significantly suppressing non‐radiative recombination. Consequently, the quality of both SnO 2 and perovskite films is significantly improved, which greatly boosts the power conversion efficiency of small‐size PSCs to 25.82%, with a high open‐circuit voltage of 1.19 V, a minimal voltage loss of 0.341 V, and negligible hysteresis. Moreover, the optimized SnO 2 /TFSK‐based PSCs demonstrate improved storage, humidity, and thermal stability.