Phenyl-Alkyl Inner Side Chain-Based Polymerized Small-Molecule Acceptor with Improved Miscibility and Crystallinity Enabling High-Performance All-Polymer Solar Cells

All-polymer solar cells (all-PSCs) are regarded as potential candidates for practical applications. However, their performance lags behind that of devices based on small-molecule acceptors. A critical challenge lies in achieving appropriate donor–acceptor miscibility and high molecular ordering in the active layer. In this work, we designed and synthesized a polymerized small-molecule acceptor, PY-HDB, featuring phenyl-alkyl inner side chains. Compared to PY-HD with conventional alkyl chains, PY-HDB possesses tighter π–π stacking behavior and improved miscibility with the high-performance polymer donor D18, promoting a favorable nanoscale phase-separated morphology in the D18:PY-HDB blend. Consequently, the corresponding device delivers an excellent power conversion efficiency (PCE) of 17.49% with a remarkable fill factor of 75.46%, ranking among the top values for binary D18-based all-PSCs. This work provides a promising side-chain design strategy for developing high-performance polymer acceptors.