Achieving 20.5% Efficiency in Organic Solar Cells via Co-Crystallizable Small Molecule Acceptors

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-BO and incorporates short alkyl chains to enhance π-π stacking and extend absorption. The structural similarity between host and guest promotes strong intermolecular interactions, giving rise to pronounced intermolecular electronic coupling that enables continuous tuning of energetic alignment and absorption profiles. Consequently, D18:L8-BO:Y18-C3 devices exhibit enhanced molecular ordering and suppressed charge recombination, delivering a high power conversion efficiency of 20.50% with simultaneously increased short-circuit current and fill factor while maintaining a high open-circuit voltage. Notably, the devices achieve high performance (> 19.6%) even when the Y18-C3 fraction reaches 50% of the total acceptor content, indicating a high tolerance to composition variations. These results highlight that constructing a BC through a shared conjugated backbone, combined with compatible intermolecular interactions and crystallization tendencies, provides an effective molecular design principle for achieving high-performance, composition-tolerant OSCs.