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

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 MgF₂ 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.