Taming Crystallization Kinetics via Side‐Chain Steric Engineering Enables Over 21% Efficiency in Non‐Halogenated‐Solvent‐Processed Organic Solar Cells

The development of high-performance, non-halogenated organic solar cells (OSCs) is essential for sustainable commercialization. However, the high boiling points of halogen-free solvents often trigger excessive pre-aggregation of non-fullerene acceptors (NFAs) and unfavorable phase separation, severely limiting device performance. Herein, a side-chain steric engineering strategy is employed to develop a weakly crystalline NFA, TPD-Y, featuring a thieno[3,4-c]pyrrole-4,6-dione unit. The reduced crystallinity of TPD-Y effectively suppresses aggregation-caused quenching (ACQ), yielding a high photoluminescence quantum yield (PLQY) of 8.19% and significantly minimized non-radiative voltage loss. Furthermore, TPD-Y acts as a potent crystallization-kinetics modulator in multi-component blends with the benchmark acceptor BTP-eC9. It forms an alloy phase that simultaneously promotes nucleation and inhibits excessive grain growth in non-halogenated solvents, accelerating film formation while refining oversized domains. This kinetic modulation yields an optimized nanoscale morphology with enlarged interfacial areas, synergistically enhancing exciton dissociation and charge collection. Consequently, quaternary OSCs processed from a non-halogenated o-xylene/carbon disulfide mixture achieve a remarkable fill factor of nearly 82% and a record power conversion efficiency exceeding 21%. This work provides a robust molecular design to overcome the efficiency bottleneck of chlorinated-solvent-free OSCs, marking a significant step toward the industrialization of high-performance, low-toxicity photovoltaics.