Anion Mediated Self‐Assembly of Ammonium Salt for Buried Interfacial Engineering in Efficient and Stable Perovskite Solar Cells

Abstract The SnO ₂ /perovskite interface plays a crucial role in enhancing the efficiency and stability of n‐i‐p perovskite solar cells (PSCs). However, the presence of iodine and oxygen vacancies, along with lattice distortion at the perovskite's buried interface, critically impairs carrier dynamics and leads to severe ion migration. Herein, a novel ammonium salt, ethanaminium 4‐methylbenzenesulfonate (EATsO), is introduced to self‐assembles at the buried interface in n‐i‐p PSCs, thereby enhancing efficiency and stability. The strong dual chemical interactions between TsO ⁻ and Pb ²⁺ /Sn ⁴⁺ at the buried interface drive the self‐assembly process during the perovskite two‐step fabrication process, which simultaneously passivates the oxygen vacancy and unbonded Pb ²⁺ . Meanwhile, EATsO regulates perovskite crystal growth by forming 2D perovskite seed at the buried interface, resulting in enhanced crystallinity, reduced lattice stress, and lower defect density. Therefore, EATsO‐doping significantly improves the power conversion efficiency (PCE) from 22.63% to 25.51%. Additionally, incorporating EATsO effectively inhibits the iodide migration within perovskite. The EATsO‐doped PSC retains 91% of its initial PCE after 1280 h of maximum power point tracking. This study introduces a novel self‐assembling ammonium salt for buried interface modification during the two‐step fabrication process, offering a straightforward approach to bolster the durability of n‐i‐p structure PSCs.