Enhancing Binary Organic Solar Cell Performance by Manipulating Molecular <i>J</i> ‐aggregation to Broaden Absorption and Reduce Energy Loss

Abstract The efficiency of organic solar cells (OSCs) is fundamentally constrained by the trade‐off between enhanced light absorption and voltage loss, driven by strong non‐radiative recombination as dictated by the energy gap law. To address this limitation, a co‐additive treatment strategy utilizing solid additive 4‐bromochlorobenzene (BCB) and liquid additive 1,8‐diiodooctane (DIO) is introduced to modulate the molecular aggregation in PM6:BTP‐eC9 active layers. DIO promotes acceptor aggregation, reducing the optical bandgap, while BCB stabilizes molecular stacking through J ‐aggregation, mitigating blending‐induced bandgap shifts. The co‐additive treatment enhances molecular ordering and aggregation simultaneously, leading to broader absorption and reduced non‐radiative recombination losses. Consequently, the OSCs processed by the co‐additive treatment strategy achieve a power conversion efficiency (PCE) of 19.72%, an exceptionally high fill factor of 81.3%, and a short‐circuit current density ( J SC ) of 28.61 mA cm −2 , while effectively suppressing voltage loss. Anti‐reflective layer MgF 2 further increases J SC to 29.62 mA cm −2 and PCE to 20.34% for the OSCs, which is among the highest J SC and PCE reported for binary OSCs. This work underscore the effectiveness of BCB and DIO co‐treatment in optimizing molecular arrangement and charge dynamics, providing a practical pathway for overcoming efficiency limitations and advancing organic photovoltaics toward higher performance.