Toward commercially viable non-fullerene organic solar cells: a critical review of recent developments in high-performance non-fused ring electron acceptors

To achieve commercial viability, organic solar cells (OSCs) must balance efficiency, stability, and cost. Currently, the most efficient OSCs are still based on fused ring electron acceptors (FREAs). However, their synthesis is complex, laborious, and requires numerous purification steps, which hinders the scaling and commercialization of OSCs based on them. Non–fused ring electron acceptors (NFREAs) offer the advantage of flexible chemical modulation, which significantly reduces synthetic complexity. Currently, the power conversion efficiency (PCE) of NFREA-based OSCs is approaching record highs, already exceeding 17%. However, to develop efficient and affordable OSCs, a clear understanding of the molecular design principles of successful NFREAs is critical. This review focuses on the comparative analysis of the best small molecule symmetric and asymmetric NFREAs with different molecular designs that showed PCEs above 14% and 12%, respectively. In addition, novel high–performance polymer NFREAs are analyzed. Furthermore, we discuss efficient strategies to achieve planar and rigid molecular structures of NFREAs, analyze their stability and calculate the synthetic complexity and the figure–of–merit (FoM) values for the most promising NFREAs compared to well-known FREAs. Finally, we evaluate the relationships between the structure, properties, and performance of NFREAs, and consider their prospects.