A Nonfused Ring Electron Acceptor with Aggregation‐Induced Emission Assistance Achieving Over 20% Efficiency in Organic Solar Cells

The development of non‐fused ring electron acceptors (NFREAs) with high electroluminescence quantum efficiency (EQEEL) is essential for advancing the commercialization of organic solar cells (OSCs), due to their low‐cost and minimal energy losses (Eloss). However, designing such NFREAs remains a significant challenge. Herein, we present a medium‐bandgap NFREA, TT‐TCBr, which exhibits a high EQEEL of 1.0×10‐3 and an excellent electron mobility of 1.5×10‐4 cm2 V‐1 s‐1. Interestingly, TT‐TCBr shows enhanced photoluminescence quantum yield from solution (9.24%) to film (20.7%). In OSCs, the D18:TT‐TCBr‐based OSC achieves a good power conversion efficiency (PCE) of 13.20% with a high open‐circuit voltage of 1.09 V and a low nonradiative energy loss of 0.177 eV. Importantly, TT‐TCBr shows good miscibility with L8‐BO, leading to an increased exciton diffusion length and EQEEL in the L8‐BO: TT‐TCBr blend. When TT‐TCBr is introduced as a guest molecule into the D18:L8‐BO‐based OSC, Eloss is reduced from 0.543 to 0.530 eV, resulting in a significantly increased PCE from 19.16% to 20.10%, which represents one of the highest PCEs reported for OSCs. This research will significantly contribute to the advancement of high‐performance guest materials for the rapid development of single‐junction OSCs.