High‐Efficiency Quasi‐2D Perovskite Solar Cells with 18.28% Efficiency and Improved Stability

Abstract 3D organic–inorganic metal halide perovskite solar cells (PSCs) have demonstrated remarkable power conversion efficiencies (PCEs) exceeding 27%, yet their susceptibility to degradation under environmental stressors like moisture, heat, and light hinders their commercial viability. Quasi‐2D perovskites offer enhanced stability but suffer from inferior photovoltaic performance due to the insulating nature of organic cations and random phase distribution. This study first introduces a new multifunctional sulfonate additive, formamidinium 4‐methylbenzenesulfonate (FATsO), to address these challenges. FATsO simultaneously passivates iodide anions and lead ions through hydrogen bonding and Lewis acid‐base interactions, promoting a uniform phase distribution and efficient energy transfer. The ─NH 2 + group in FATsO stabilizes the [PbI 6 ] 4 − framework, while the ─SO 3 − group passivates uncoordinated Pb 2+ defects. Additionally, hydrogen bonding between PEA+ and ─SO 3 − restricts PEA+ diffusion, enhancing phase uniformity. As a result, FATsO‐modified PSCs achieve a PCE of 18.28%, which is a champion PCE in the reported values of MA‐based quasi‐2D PSCs with n ≤ 4 up to now. Meanwhile, when the PSC worked as a light‐emitting diode (LED) under forward bias, the electroluminescence external quantum efficiency (EQEEL) reached 2.24%. In addition, the optimized devices exhibit enhanced stability under thermal and humidity environments.