Chemical Bond Management of FA‐Based Mixed Halide Perovskites for Stable and High‐Efficiency Solar Cells

Abstract The unavoidable migration of organic cation within formamidinium (FA)‐based mixed halide perovskite leads to severe phase segregation and device degradation. The intrinsic weak chemical bond between organic cation and [PbI 6 ] 4− octahedra can easily break during device operation, resulting in the formation of cation vacancies and undesirable structural transformation. In this work, a pyrrolidine compound is incorporated, with a strong electron‐withdrawing fluorine substitution, which strengthened the lattice bond between organic cation and [PbI 6 ] 4− octahedra. Meanwhile, the 1D/3D heterojunction films are also achieved due to the chemical reaction between PbI 2 and pyrrolidine, successfully constructing a new 1D perovskite such as PYFPbI 3 . The resultant hetero‐perovskite films retained their photoactive‐α phase even after eight days of ambient exposure, demonstrating superior phase stability without any post‐encapsulation. More importantly, the ion‐migration channels inside the perovskite lattice are effectively blocked by 1D/3D heterojunctions. The resultant rigid and flexible solar cells exhibited an enhanced power conversion efficiency (PCE) from the initial 24.48% to 25.39%, as well as 23.86% to 24.26%, respectively, which are among the highest records in 1D/3D‐based works. Furthermore, the unencapsulated devices retained 90% of their initial PCE during maximum power point tracking for over 350 hours under continuous illuminations.