Different Sized Cycloalkyl Chains on Non‐Fullerene Acceptors Enhance Molecular Packing, Film Morphology and Charge Transport for 19.62% Efficiency Organic Solar Cells

Abstract This work addresses the challenge of achieving advanced fibril morphology of non‐fullerene acceptors (NFAs) in layer‐by‐layer organic solar cels (LBL‐OSCs) by cycloalkyl chain strategy, focusing on ta series of Y6‐type NFAs, namely BTP‐C6, BTP‐C8 and BTP‐C12, featured with cyclohexyl, cyclooctyl and cyclododecyl chains with increasing steric hindrance. These side chains influenced significantly molecular planarity, packing and film morphology, which are critical for device performance. BTP‐C6 exhibits optimal molecular packing and fibril network morphology, enabling efficient exciton dissociation, charge transport and balanced carrier mobilities, finally achieving PCEs of 19.28% and 19.62% with chloroform‐ and toluene‐cast acceptor layers, respectively. BTP‐C8 featuring enhanced planarity (dihedral angle 8.27°) showed the loosest packing (packing coefficient 49.6%) due to the increased steric hindrance of side chains, limiting intermolecular charge transport. Conversely, BTP‐C12 formed a high crystalline and tightly packed 3D network but suffered from reduced intramolecular charge transfer caused by severe molecular distortion (dihedral angle 27.27°). The findings in this work underscore the critical role of side‐chain engineering in governing molecular packing and morphology, offering a systematic understanding of the relationships between steric hindrance, crystallinity and device performance, while providing a rational design strategy for next‐generation NFAs to advance high‐performance LBL‐OSCs.