A thiophene-fused benzotriazole unit as a “π-bridge” in A-π-D-π-A type acceptor to achieve more balanced J_SC and V_OC for OSCs

As known, to balance the open-circuit voltage (V_OC) and short-circuit current density (J_SC) is one of the effective strategies to improve the photovoltaic performances of OSCs. In this work, a thiophene-fused benzotriazole (BTAZT) unit acts as a “π-bridge” to build an acceptor (A3) with A-π-D-π-A structure for OSCs. Compared with a thiophene-fused benzothiadiazole (BTT) unit as a “π-bridge” in our previous acceptor (A2), BTAZT unit exhibits relatively weak electron-withdrawing ability that results in a significant uplift of LUMO energy level to favor for enhancement of V_OC. Also, the fusion of a thiophene ring onto BTAZ unit strengthens the intramolecular charge transfer to maintain the relatively high J_SC. Therefore, the PTB7-Th:A3 OSC device shows higher PCE of 11.0% with more balanced device parameters (V_OC = 0.959 V, J_SC = 17.30 mA cm⁻², FF = 66.2%) as well as smaller energy loss (E_loss) of 0.63 eV than the PTB7-Th:A2 device with PCE of 8.20% (V_OC = 0.701 V, J_SC = 19.47 mA cm⁻², FF = 60.1% and E_loss = 0.67 eV). This work indicates that the rational structural modification of a “π-bridge” for A-π-D-π-A type acceptor is an effective approach to achieve the balanced V_OC and J_SC for the OSCs.