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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.