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

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 [PbI₆]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 [PbI₆]⁴⁻ octahedra. Meanwhile, the 1D/3D heterojunction films are also achieved due to the chemical reaction between PbI₂ and pyrrolidine, successfully constructing a new 1D perovskite such as PYFPbI₃. 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.