High‐Efficiency Ternary Organic Solar Cells Using Isomer‐Free Mono‐Fluorinated Terminal Group‐Based Fused‐Ring Acceptor as the Third Component

Abstract This study synthesizes two small‐molecule acceptors, C9‐1F and C9‐1F‐random, differing in the position of the terminal F atom. C9‐1F exhibits higher crystallinity and superior device performance compared to C9‐1F‐random when combined with donor polymers. To investigate the impact of acceptor crystallinity on the dual‐fiber network structure of organic solar cells (OSCs), the high‐performance binary system D18:L8‐BO is selected, and C9‐1F is introduced as a third component. The findings reveal that the high crystallinity of C9‐1F influenced the film‐forming kinetics of the active layer, leading to an optimally sized nanofiber network. The ternary blend film exhibits superior crystallinity and larger acceptor phase domain sizes compared to the binary system, facilitating charge transport and inhibiting carrier recombination. Consequently, the D18:L8‐BO:C9‐1F ternary bulk heterojunction OSC achieves a remarkable power conversion efficiency (PCE) exceeding 20%, among the highest reported levels. This study underscores the importance of fine‐tuning the nanoscale dual‐network structure of the active layer through third‐component design to optimize crystallinity and phase domain size for excellent photovoltaic performance.