Facile Side Chains Optimization of Y‐series Acceptor Enables High Performance Binary Non‐halogenated Solvent‐Processed Organic Solar Cells with Excellent Fill Factor of 79%

Most cutting‐edge performance binary bulk heterojunction organic solar cells (OSCs) with high power conversion efficiency (PCE) over 18% generally use harmful halogenated solvent systems to guarantee the optimal morphology of active layers, urging more investigations on the non‐halogenated solvent processed OSCs and their further large‐scale devices fabrication. Herein, two BTP‐eC9 analogs, BTP‐eC9‐EH and BTP‐eC9‐HD, were synthesized to investigate the effect of alkyl chain length on the solubility and photovoltaic properties systematically. Despite the subtle side‐chain engineering, the PM6: BTP‐eC9‐HD based device exhibited an outstanding PCE of 17.5% and a remarkable fill factor of 79% in the toluene processing binary blend system, which precedes the toluene processing PM6: BTP‐eC9 based binary blend system (17.1%). The combined investigation provides a valuable insight into the modulation of branched alkyl chains, which can serve as an effective approach for adjusting the aggregation and molecular packing of A‐DA′DA‐type Y‐series based non‐fullerene acceptors to balance the materials solubility and solvents selection, thereby achieving high‐performance large‐scale OSCs processed from non‐halogenated solvents.