Controlling Molecular Packing via Side-Chain Engineering of BDTT-Based Guest Donors for High-Performance Ternary Organic Solar Cells.

Applying the ternary bulk heterojunction (BHJ) strategy in organic solar cells (OSCs) has emerged as an effective approach to enhance photovoltaic performance. In this work, two newly designed wide-bandgap polymer donors, BDT-TS-6 and BDT-TS-20, were developed by incorporating a diester-functionalized terephthalate unit into a common BDTT-based conjugated backbone and introducing linear C6 or branched C20 ester substituents on the terephthalate-derived units. The short linear C6 and bulky long-branched C20 side chains were selected to provide contrasting steric and molecular packing environments while maintaining solution processability. Accordingly, the two polymer guest donors exhibited distinct aggregation behavior, relative miscibility with the PM6:Y6 host components, and molecular ordering. Both BDT-TS guest donors showed complementary absorption and well-aligned energy levels with the PM6:Y6 host system, supporting extended light harvesting and charge generation in the ternary blends. Notably, BDT-TS-6 exhibited more favorable relative miscibility and more ordered molecular packing, whereas BDT-TS-20 showed stronger aggregation and predominantly amorphous character. The optimized PM6:BDT-TS-6:Y6 and PM6:BDT-TS-20:Y6 ternary OSCs achieved PCEs of 17.55% and 16.92%, respectively, outperforming the PM6:Y6 binary OSC (15.92%). Overall, this work demonstrates that side-chain length, branching, and steric bulk collectively regulate aggregation, miscibility, molecular ordering, and the resulting photovoltaic performance of ternary OSCs.