Conformational entropy tuning in nonfused-ring electron acceptors for low-cost and record-efficiency organic solar cells.

The commercial viability of organic solar cells (OSCs) is hindered by the trade-off between cost and performance. In particular, low-cost non-fused-ring electron acceptors (NFREAs) suffer from conformational disorder, limiting their photovoltaic performance. Herein, we strategically regulate conformational entropy (S conf.) of NFREAs through the rational combination of intramolecular noncovalent interactions (INIs). The optimal candidate 3TT-SeS, identified through comprehensive density functional theory calculations, exhibits balanced Se···N and S···O INIs and demonstrates a significant reduction in S conf. This strategy enables an exclusive single stable conformation and highly ordered molecular packing, thereby facilitating efficient charge transport. Consequently, the 3TT-SeS-based OSC achieved an outstanding power conversion efficiency of 19.26% (certified at 18.75%), setting a new benchmark for NFREA-based systems to date. More importantly, 3TT-SeS-based device demonstrates exceptional economic potential with an extremely low power generation cost of 0.77$ kW-1, much lower than several high-performance FREA-based systems. Our work demonstrates a low-S conf. design of NFREAs for cost-effective and high-performance organic photovoltaics.