Low‐Dimensional Dion–Jacobson‐Phase Lead‐Free Perovskites for High‐Performance Photovoltaics with Improved Stability

1,4-butanediamine (BEA) is incorporated into FASnI₃ (FA=formamidinium) to develop a series of lead-free low-dimensional Dion-Jacobson-phase perovskites, (BEA)FA_n-1 Sn_n I_3n+1 . The broadness of the (BEA)FA₂ Sn₃ I₁₀ band gap appears to be influenced by the structural distortion owing to high symmetry. The introduction of BEA ligand stabilizes the low-dimensional perovskite structure (formation energy ca. 10⁶ j mol^-1 ), which inhibits the oxidation of Sn²⁺ . The compact (BEA)FA₂ Sn₃ I₁₀ dominated film enables a weakened carrier localization mechanism with a charge transfer time of only 0.36 ps among the quantum wells, resulting in a carrier diffusion length over 450 nm for electrons and 340 nm for holes, respectively. Solar cell fabrication with (BEA)FA₂ Sn₃ I₁₀ delivers a power conversion efficiency (PCE) of 6.43 % with negligible hysteresis. The devices can retain over 90 % of their initial PCE after 1000 h without encapsulation under N₂ environment.