Precise Methylation Yields Acceptor with Hydrogen‐Bonding Network for High‐Efficiency and Thermally Stable Polymer Solar Cells

Utilizing intermolecular hydrogen‐bonding interactions stands for an effective approach in advancing the efficiency and stability of small‐molecule acceptors (SMAs) for polymer solar cells. Herein, we synthesized three SMAs (Qo1, Qo2, and Qo3) using indeno[1,2‐b]quinoxalin‐11‐one (Qox) as the electron‐deficient group, with the incorporation of a methylation strategy. Through crystallographic analysis, it is observed that two Qox‐based methylated acceptors (Qo2 and Qo3) exhibit multiple hydrogen bond‐assisted 3D network transport structures, in contrast to the 2D transport structure observed in gem‐dichlorinated counterpart (Qo4). Notably, Qo2 exhibits multiple and stronger hydrogen‐bonding interactions compared with Qo3. Consequently, PM6:Qo2 device realizes the highest power conversion efficiency (PCE) of 18.4%, surpassing the efficiencies of devices based on Qo1 (15.8%), Qo3 (16.7%), and Qo4 (2.4%). This remarkable PCE in PM6:Qo2 device can be primarily ascribed to the enhanced donor‐acceptor miscibility, more favorable medium structure, and more efficient charge transfer and collection behavior. Moreover, the PM6:Qo2 device demonstrates exceptional thermal stability, retaining 82.8% of its initial PCE after undergoing annealing at 65°C for 250 hours. Our research showcases that precise methylation, particularly targeting the formation of intermolecular hydrogen‐bonding interactions to tune crystal packing patterns, represents a promising strategy in the molecular design of efficient and stable SM‐acceptors.