Adaptive Barrier Layer Strategy for Suppressing Ion Migration Toward Efficient and Stable Semi‐Transparent Perovskite Solar Cells

Semi‐transparent perovskite solar cells (ST‐PSCs) often employ mixed halides, which can induce significant phase segregation and thus impair device performance. Defects at the film surface and grain boundaries are primary factors driving this segregation. In this study, we present a novel “adaptive barrier layer” strategy through the introduction of a benzalkonium chloride (BAC) mixture. BAC molecules with varying n values demonstrate selective adsorption to distinct defect sites, dynamically forming a tailored barrier layer. This multifunctional layer effectively passivates both surface and grain boundary defects while inhibiting ion migration, thereby mitigating halide phase segregation. When applied to ST‐PSCs, this strategy enabled power conversion efficiencies of 20.46% and 18.16% on the front and rear sides, respectively, with a bifaciality of 88.27%, and the semi‐transparent device retains 93.8% of its initial efficiency after 1080 h of continuous exposure. This study provides a critical optimization pathway for significantly enhancing the efficiency and operational stability of ST‐PSCs.