Side Chain Engineering of Non‐fused Ring Electron Acceptors for Organic Solar Cells

Comprehensive Summary Featuring flexible design and facile synthesis, non‐fused ring electron acceptors (NFREAs) have attracted considerable attention over the past few years in the field of organic solar cells (OSCs). To date, the power conversion efficiency (PCE) of OSCs based on NFREAs has exceeded 19%, gradually approaching that of their fused‐ring counterparts. Given their low‐cost nature, NFREAs have emerged as highly promising candidates for future OSC commercialization, especially in the large‐scale fabrication scenarios. During their development, side chain engineering has stood out as a simple yet effective strategy that significantly enhances the photovoltaic performance of NFREAs. By tailoring the side chains on electron‐donating and/or electron‐withdrawing building blocks, researchers can precisely regulate the physical properties of NFREAs, such as material solubility and light absorption. More importantly, side chain engineering plays a crucial role in controlling crystal packing and film forming kinetics, which directly affect exciton diffusion, charge transport and recombination dynamics within devices. In this context, extensive studies have revealed critical structure–property relationships among side chain modifications, device performance and stability, providing valuable guidelines for future molecular design. Herein, we present a comprehensive review of side chain engineering in NFREAs. First, we individually focus on the side chain engineering of the electron‐donating and electron‐withdrawing units. We then systematically summarize the diverse side chain engineering approaches, including the regulation of side chain length, branching point, substitution positions, etc . The effects of these modifications on physical properties, film morphology, device performance, and stability are analyzed in depth. Finally, the key challenges and future perspectives for achieving efficient, stable, and low‐cost NFREA‐based OSCs via advanced side chain engineering are comprehensively addressed.