Precise Morphology Control via Backbone‐Derived Solid Additive Enables Binary Organic Solar Cells to Achieve 20% Efficiency and Thick‐Film Compatibility

Precise control of active layer morphology is essential for advancing organic solar cells (OSCs) toward practical applications. Here, a generalizable morphology regulation strategy is proposed using a structurally tailored crystallization template derived from the polymer donor backbone. A small molecule, BDD-C6, is designed by extracting the benzodithiophene (BDD) unit from PM6 and appending an alkylated thiophene terminal. When incorporated into PM6:L8-BO and PM6:BTP-eC9 binary blends, compatibility studies and cross-sectional absorption analysis reveal a favorable vertical phase distribution on alcohol-soluble substrates. Thermodynamic evaluations confirm the good thermal stability and crystallinity of BDD-C6. Crystallization kinetics and morphological characterizations investigations further show that BDD-C6 delays film formation and promotes polymer ordering, thus extending exciton diffusion length. As a result, PM6:L8-BO-based binary device delivers a power conversion efficiency (PCE) of 19.81% in thin films and maintains a PCE of 16.93% even at a 400 nm thickness. Moreover, PM6:BTP-eC9-based OSCs present an efficiency of over 20%. Extending this strategy to D18 systems using a DTBT-C6 small molecule similarly enhances morphology and device performance, realizing a PCE of 20.18% in D18:L8-BO binary device. This work offers a backbone-derived crystallization template strategy for the scalable fabrication of high-efficiency OSCs.