Sulfonated Bathocuproine Derivatives as Chemically Bonded Buried‐Interface Engineers for n–i–p Perovskite Solar Cells

ABSTRACT Buried‐interface engineering has emerged as a central strategy for advancing the efficiency and stability of halide perovskite solar cells. Bathocuproine (BCP) and its derivatives have been employed as ultrathin insulating interlayers that modulate interfacial junctions and suppress recombination; however, their role has remained limited to passive interface control, and BCP‐based designs have rarely been explored at the electron‐transport‐layer/perovskite buried interface in n–i–p architectures. Here, we elevate the BCP family from a passive junction‐modulating interlayer to a chemically bonded buried‐interface engineer by introducing disodium bathocuproine disulfonate (Na‐BCS) between chemical bath deposited SnO ₂ and FAPbI ₃ perovskite films. Unlike pristine BCP, sulfonate‐functionalized Na‐BCS establishes strong chemical coupling across the buried interface through Na ⁺ coordination, sulfonate anchoring to SnO ₂ , and pyridinic nitrogen interactions with the perovskite lattice. This molecular bridging simultaneously passivates ETL defects and promotes highly crystalline perovskite growth. Consequently, Na‐BCS incorporation markedly suppresses interfacial recombination, reduces surface recombination velocity, and prolongs carrier lifetime, leading to a 26.02% enhancement in power conversion efficiency and 86% retention of the initial efficiency after 762 h of operation. This work establishes a BCP‐based interfacial design paradigm in which the molecules function as chemically bonded buried‐interface engineers, providing a generalizable strategy for high‐performance n–i–p perovskite solar cells.