Correlation Between Outer Side Chain Design in Y‐Series Small Molecule Acceptors and the Performance of Organic Photovoltaics

Organic photovoltaics (OPVs) have witnessed remarkable advancements, with power conversion efficiencies (PCEs) now exceeding 20%, largely driven by innovation in non-fullerene acceptors (NFAs), especially Y-series acceptors. Side chain engineering is a powerful strategy for tuning the properties of NFAs. In particular, the engineering of outer side chains plays a critical role in optimizing active layer morphology and interfacial behavior, thereby ultimately determining device performance. However, a comprehensive understanding of the precise structure-performance relationships of outer side chains remains elusive, posing a significant obstacle to the rational design of high-performance acceptors. This review aims to elucidate the intricate relationship between outer side chain design and device efficiency, providing a systematic summary of recent breakthroughs in this specific field. Notably, emerging machine learning predictions corroborate empirical trends and help identify optimal configurations, such as branched and conjugated motifs. A key conclusion underscores that iterative molecular optimization through the synergistic integration of asymmetry, conjugation, heteroatom incorporation, and chain-length tuning is indispensable for enhancing OPV performance. Future research must therefore prioritize such multi-faceted synergy to simultaneously achieve superior efficiency, stability, and scalability, firmly establishing side chain engineering as a cornerstone for next-generation organic solar cells.