Four‐Arm Polymer as a Seeding Agent for High‐Performance Polymer Solar Cells

Comprehensive Summary Excessive intermolecular aggregation and crystallinity mismatch between donor and acceptor materials in binary blend system give rise to incompatible crystallization kinetics and insufficiently interconnected domains, which impede charge transport and limit the improvement of power conversion efficiency (PCE). Herein, a novel multi‐arm polymer was developed based on BDT and BDD units. Characterized by a large conjugated plane and a three‐dimensional molecular conformation, H21 not only complements the energy levels and absorption profile of the host system but also enhances charge generation and transport. More importantly, H21 can function as an effective seeding agent, regulating crystallization processes, suppressing over‐aggregation, and strengthening donor‐acceptor interactions within the active layer. As a result, the optimized ternary blend containing 10 wt% H21 demonstrates refined nanoscale phase separation and improved molecular ordering, leading to an excellent PCE of 19.70% in the D18:H21(10%):L8‐BO‐based ternary device. This work demonstrates that judiciously introducing multi‐arm molecular seeding agents represents a promising strategy for modulating morphology and intermolecular interactions toward high‐performance PSCs.