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.