Mitigating Face‐Sharing Octahedral Impurity Phases for Efficient FA‐Based Perovskite Photovoltaics

Abstract Formamidinium (FA) based perovskites have emerged as one of the most promising light‐absorber layers for both single‐junction and advanced top‐cell tandem photovoltaics, owing to their precisely engineered electronic bandgap and exceptional stability. However, because of the mismatch FA cation and intricate crystallization of FA‐based perovskite, the formation of an impurity phase is inevitable, which reduces efficiency and stability. Herein, a N‐Phenyl‐bis(trifluoromethanesulfonimide) (NPTFSI)‐assisted crystallization method is presented to mitigate the formation of impurity phase, i.e., face‐sharing octahedra, and achieve phase pure and stable FA‐based perovskite. Comprehensive characterization shows that the addition of NPTFSI increases the formation energy of face‐sharing octahedra while reducing the formation energy of corner‐sharing. This effectively suppresses the impurity phase in the FA‐based perovskite films. Suppressing these face‐sharing octahedral impurity phases not only enhances the stability of perovskite films under heating or humidity conditions but also improves the carrier dynamics. Finally, the champion devices deliver a significantly enhanced efficiency from 23.23% to 25.74%. Moreover, these PSCs exhibit excellent stability: retain 96% of their initial efficiency after over 500 h maximum power point test.