ABSTRACT Despite the remarkable efficiency enhancement enabled by the ternary strategy in organic solar cells (OSCs), yet a molecular‐level understanding of electronic coupling and its role in energetic alignment remains lacking. Here, we report a well‐defined bimolecular co‐crystal (BC) formed between the widely used acceptor L8‐BO and a newly designed benzotriazole‐based guest, Y18‐C3, as confirmed by single‐crystal x‐ray diffraction analysis. Y18‐C3 adopts a conjugated backbone similar to L8‐