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

Abstract 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.