High‐Efficiency Organic Solar Cells Enabled by Chalcogen Containing Branched Chain Engineering: Balancing Short‐Circuit Current and Open‐Circuit Voltage, Enhancing Fill Factor

Abstract The elaborate balance between the open‐circuit voltage (V_OC) and the short‐circuit current density (J_SC) is critical to ensure efficient organic solar cells (OSCs). Herein, the chalcogen containing branched chain engineering is employed to address this dilemma. Three novel nonfullerene acceptors (NFAs), named BTP‐2O, BTP‐O‐S, and BTP‐2S, featuring different peripheral chalcogen containing branched chains are synthesized. Compared with symmetric BTP‐2O and BTP‐2S grafting two alkoxy or alkylthio branched chains, the asymmetric BTP‐O‐S grafting one alkoxy and one alkylthio branched chains shows mediate absorption range, applicable miscibility, and favorable crystallinity. Benefiting from the enhanced π–π stacking and charge transport, an optimal power conversion efficiency (PCE) of 17.3% is obtained for the PM6:BTP‐O‐S‐based devices, with a good balance between V_OC (0.912 V) and J_SC (24.5 mA cm⁻²), and a high fill factor (FF) of 0.775, which is much higher than those of BTP‐2O (16.1%) and BTP‐2S‐based (16.4%) devices. Such a result represents one of the highest efficiencies among the binary OSCs with V_OC surpassing 0.9 V. Moreover, the BTP‐O‐S‐based devices fabricated by using green solvent yield a satisfactory PCE of 17.1%. This work highlights the synergistic effect of alkoxy and alkylthio branched chains for high‐performance OSCs by alleviating voltage loss and enhancing FF.