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Vertical phase-separated active layer morphology is essential for organic solar cells (OSCs), which can be effectively achieved through layer-by-layer (LbL) processing, enabling independent optimization of donor and acceptor layers. Here, we present a novel strategy to optimize the active layer morphology of D18/L8-BO-based OSCs by incorporating polyfluoroquinoxaline-type polymer additives. Three quinoxaline-based polymers with varying fluorination contents, namely, P2FQx, P3FQx, and P4FQx, were synthesized and evaluated. Although these polymers showed limited performance as standalone donor materials in bulk heterojunction (BHJ) devices, their use as additives in LbL-OSCs significantly enhanced device efficiency. These polymers promoted D18 aggregation, enhanced L8-BO penetration, and facilitated the formation of a vertically phase-separated interpenetrating donor/acceptor network. Among these additives, P2FQx demonstrated the best performance, enabling an optimized morphology and achieving a champion power conversion efficiency (PCE) of 20.13% as well as a high fill factor (FF) of 80.13%. Our results highlight the potential of rationally designed polymer additives to address morphology-related challenges in LbL-OSCs and provide a potential pathway for further development of high-performance and scalable organic photovoltaic devices.
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