Ternary Strategy and Molecular Electrostatics Collaboratively Optimize Low-Molecular-Weight Polymer Donor Organic Solar Cells: Over 20% Efficiency and High Scalability.

Achieving consistent performance across polymer donor batches is crucial for the commercialization of organic solar cells (OSCs). Compared with high-molecular-weight PM6 (HWPM6), low-molecular-weight PM6 (LWPM6) has lower efficiency but better stress-dispersion characteristics and solution-processability, making its performance improvement vital for practical applications. Here, LWPM6-based OSCs are optimized by introducing a trimeric guest (TYT-S). TYT-S improves PM6:Y6 compatibility, achieving a finer phase separation and a favorable interpenetrating network morphology. A ternary strategy, leveraging molecular electrostatic potential differences, promotes LWPM6 pre-aggregation, extends film-formation time, and enhances molecular ordering. The LWPM6-based ternary system exhibits an optimized vertical phase distribution, with maximum exciton dissociation occurring near the cathode, resulting in a power conversion efficiency (PCE) of 19.23% (LWPM6-based binary with a low PCE of 17.35%). When BTP-eC9 replaces Y6, the LWPM6-based ternary devices achieve a PCE of 20.12% (LWPM6-based binary with a low PCE of 17.64%). Additionally, LW polymers can dissipate stress via segmental motion. After 3000 bending cycles, LWPM6-based flexible devices retain higher initial efficiency than HWPM6-based one, demonstrating better mechanical stability. In mini-modules, they also have good solution-processability. This work demonstrates that a trimer guest strategy can significantly enhance the photovoltaic performance of low-efficiency LWPM6, offering new insights for OSCs commercialization.