Regulating secondary phase of Rb‐based aggregates for efficient and stable perovskite solar cells

Passivating defects in perovskite films using pseudohalides has proven to be an effective strategy for reducing nonradiative recombination losses in perovskite solar cells (PSCs). Herein, we use rubidium acetates (RbAc) as additives to modify quaternary cation perovskites, targeting to disclose the underlying mechanisms of this additive regulating the perovskite crystallization and thereby the photovoltaic performance. The acetate anions (Ac − ) coordinate with Pb 2+ while associating with Rb + , leading to their preferential accumulation at grain boundaries. The pseudohalide (Ac − ) intercalates into the δ phase, which allows for increased crystal plane spacing of this intermediate phase. This occurrence enables a delayed δ ‐to‐α phase transformation, thereby enhancing the crystal quality and phase purity of the film. This synergistic interaction effectively passivates defect states and suppresses the formation of the intermediate δ ‐phase in the final film. As a result, PSCs based on RbAc achieve a power conversion efficiency of 25.40%, and exhibit excellent stability maintaining performance for over 2000 h.