Backbone Twisting and Terminal Overlapping via π-Bridge Engineering for Highly Efficient Non-Fused Ring Electron Acceptors with Balanced J_SC-V_OC

High and balanced open-circuit voltage (V_OC) and short-circuit current density (J_SC) are crucial for the efficiency of organic solar cells (OSCs). Generally, the π-bridge strategy serving as an effective molecular functionalization route with the potential to balance the V_OC-J_SC pair. Herein, the study designs and synthesizes three non-fused ring electron acceptors (NFREAs): 2T-T-EH, 2T-T-2EH, and 2T-TT-2EH, by systematically regulating the π-bridge at size, number, and position of the lateral alkyl chains. Introducing inner alkyl side chains result in twisted backbones, which elevated the lowest unoccupied molecular orbital (LUMO) energy levels, and reduced energy loss, facilitating a higher V_OC. Single crystal analysis also reveals that the π-extending in 2T-TT-2EH can effectively relieve the congestion of dual lateral chains, leave more space for the terminal overlapping, which promotes efficient charge transport and enhancing J_SC. Consequently, a compromise between V_OC (0.916 V) and J_SC (21.21 mA cm⁻²) is accomplished in the binary OSCs. The elevated LUMO energy level and V_OC provides 2T-TT-2EH to serve as a third component in ternary OSCs, achieving an impressive power conversion efficiency (PCE) of 19.07% in the D18:BTP-eC9-4F:2T-TT-2EH-based device. These findings in this study suggest that fine-tuning the π-bridges is a simple method for optimizing photovoltaic performance in NFREAs, ensuring a well-balanced V_OC and J_SC.