Spontaneous Compositional–Interfacial Co‐Modification Engineering via Ion Exchange Reaction Between Perovskite and Electron‐Transporting Layer for Exceptionally Long‐Term Stability of Photovoltaics

Abstract The long‐term stability of perovskite solar cells (PSCs) is still challenging for commercialization and mainly linked to the life span of perovskite films. Herein, a spontaneous compositional–interfacial co‐modification strategy is developed based on the ion exchange reaction by introducing ammonium hexafluorophosphate (NH 4 PF 6 ) into antisolvent to form gradient structures through a simple one‐step solvent engineering. With the assistance of the ion exchange reaction, NH 4 PF 6 forms a multifunctional structure to protect perovskite films from both internal and external factors for the exceptionally long‐term stability of photovoltaics. The reason for this is linked to the high hydrophobicity of NH 4 PF 6 for preventing H 2 O invasion, suppressing ion migration by forming hydrogen bonding, and reducing perovskite defects. The resulting unencapsulated devices show exceptionally long‐term stability under standardized the International Summit on Organic Photovoltaic Stability (ISOS) protocols, with over 94%, 81%, and 83% retained power conversion efficiencies after aging tests under N 2 (ISOS‐D‐1I), ambient air (ISOS‐D‐1), and 85 °C (ISOS‐D‐2I) for 14016, 2500, and 1248 h, respectively. These performances compare well with the state‐of‐the‐art stability of inverted PSCs. Further investigations are conducted to study the evolution of macroscopic morphology and microscopic crystal structure in aged perovskite films, aiming to provide evidence supporting the aforementioned improvements in stability.