Polymer Acceptor BO-C20-BDTCl for Improving Efficiency and Stability in PM6/Y6-Based Ternary Organic Solar Cells

Organic solar cells (OSCs) deliver high efficiency but suffer from stability losses due to small molecule acceptor (SMA) aggregation, which hinders their commercial application. Incorporating polymer acceptors (PAs) into ternary blend systems has emerged as a promising strategy. By leveraging complementary advantages, ternary blend systems with a PA enable optimized energy level alignment, enhanced charge transport, and improved thermal and mechanical stability. In this work, we report BO-C20-BDTCl, a donor–acceptor copolymer engineered with structural units compatible with both PM6 and Y6 to create a well-controlled cascaded energy structure in PM6/Y6/PA ternary blend systems. This molecular design strategy enables the regulation of backbone regularity and side-chain structure, allowing the systematic optimization of energy alignment, charge transport, and morphological stability. At just 0.05 wt % addition, BO-C20-BDTCl promotes exciton dissociation and suppresses trap-assisted recombination, yielding a PCE of 17.9% under AM 1.5 G illumination (Voc = 0.876 V, Jsc = 26.7 mA cm–2, FF = 76.7%), 0.6% higher than PM6/Y6. Notably, devices exhibit enhanced stability, retaining 60% of the initial PCE after 500 h at 100 °C and 80% after 500 h under ambient conditions (25 °C, RH 20%). These results confirm that trace BO-C20-BDTCl substantially enhances both the efficiency and operational stability in ternary blend systems.