Surface‐Anchored Acetylcholine Regulates Band‐Edge States and Suppresses Ion Migration in a 21%‐Efficient Quadruple‐Cation Perovskite Solar Cell

Abstract Although incorporating multiple halogen (bromine) anions and alkali (rubidium) cations can improve the open‐circuit voltage ( V oc ) of perovskite solar cells (PSCs), severe voltage loss and poor stability have remained pivotal limitations to their further commercialization. In this study, acetylcholine (ACh + ) is anchored to the surface of a quadruple‐cation perovskite to provide additional electron states near the valence band maximum of the perovskite surface, thereby enhancing the band alignment and minimizing the V oc loss significantly. Moreover, the quaternary ammonium and carbonyl units of ACh + passivate the antisite and vacancy defects of the organic/inorganic hybrid perovskite. Because of strong interactions between ACh + and the perovskite, the formation of lead clusters and the migration of halogen anions in the perovskite film are suppressed. As a result, the device prepared with ACh + post‐treatment delivers a power conversion efficiency (PCE) (21.56%) and a value of V oc (1.21 V) that are much higher than those of the pristine device, along with a twofold decrease in the hysteresis index. After storage for 720 h in humid air, the device subjected to ACh + treatment maintained 70% of its initial PCE. Thus, post‐treatment with ACh + appears to be a useful strategy for preparing efficient and stable PSCs.