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

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.