Sulfur Locking and Chlorination Synergistically Enable Efficient Charge Transport and Low Nonradiative Energy Loss in Organic Solar Cells

ABSTRACT Achieving efficient charge transport while maintaining low nonradiative energy loss remains a key challenge in the molecular design of small‐molecule acceptors (SMAs) for organic solar cells (OSCs). Herein, we report a stepwise molecular engineering strategy based on sulfur locking and chlorination to regulate intermolecular packing and excited‐state properties simultaneously. Sulfur locking modifies molecular conformation and intermolecular packing, leading to enhanced exciton diffusion and charge transport. Subsequent chlorination further strengthens intermolecular interactions through additional Cl···S contacts, resulting in tighter π–π stacking and improved crystalline ordering. Binary OSCs based on D18:Qx2Ph‐SCl achieve an efficiency of 20.0% with a high open‐circuit voltage ( V OC ) of 0.951 V, while maintaining a low nonradiative energy loss of ∼0.19 eV. Owing to its intrinsically lower reorganization energy and weaker electron–phonon coupling than L8‐BO, Qx2Ph‐SCl further serves as an effective third component in ternary OSCs, reducing nonradiative recombination and increasing the V OC . As a result, the ternary device delivers an efficiency of 21.3%. This work demonstrates that stepwise sulfur locking and chlorination provide an effective molecular design strategy for simultaneously optimizing charge transport and voltage loss in high‐performance OSCs.