Regulating Donor Aggregation via Alkyl Side-Chain Engineering for Green-Solvent Processed Organic Solar Cells

Scalable fabrication of organic solar cells (OSCs) demands the replacement of toxic halogenated solvents with environmentally benign alternatives. However, processing with nonhalogenated solvents often leads to uncontrolled donor aggregation and suboptimal morphology, severely limiting device performance. Here, we report a molecular design strategy that enables efficient morphology control under green processing conditions through rational side-chain engineering. By introducing long even-numbered alkyl chains via random copolymerization of D18-Cl with DTBT-HD units, we synthesized a series of terpolymers (D18-Cl-<i>x</i>HD) with finely tuned solubility and aggregation behavior. The optimized terpolymer, D18-Cl-10HD, achieves a power conversion efficiency of 18.1% when processed from <i>o</i>-xylene, representing a >60% improvement over the D18-Cl benchmark. Morphological characterization reveals that D18-Cl-10HD forms a well-ordered bicontinuous network with enhanced molecular packing and domain purity, facilitating efficient exciton dissociation and charge transport. This study not only offers a broadly applicable strategy for green-solvent-compatible material design but also establishes a deeper mechanistic understanding of side-chain-regulated phase behavior, bridging the gap between high efficiency and sustainable OSC manufacturing.