End-group and Side-chain Dual-engineering of Selenium-fused Acceptors for 20.08% Efficiency Polymer Solar Cells with Low Energy Loss

Although near-infrared (NIR)-absorbing acceptors tend to capture more photons to boost the photocurrent (J_SC) of polymer solar cells (PSCs), they often suffer from significant energy loss (E_loss), resulting in a low photovoltage (V_OC), which limits further improvement of the power conversion efficiency (PCE). Herein, we developed four selenium-fused NIR-absorbing acceptors (Y-SeNF, Y-SeNF-2ClO, Y-SeBNF, and Y-SeBNF-2ClO) by regulating the molecular photoelectric properties via end-group and side-chain dual-engineering. Among them, Y-SeNF-2ClO with linear side-chains and asymmetric end-groups showed favorable molecular packing and energy levels, achieving an optimized active layer morphology and suppressed energy loss. Therefore, among the binary PSCs with a polymer donor D18, the Y-SeNF-2ClO device achieved the minimized E_loss and the highest product of V_OC×J_SC, leading to a champion PCE of 17.41%, outperforming other devices based on acceptors with branched side-chains and/or symmetric end-groups. Encouraged by the above success, NIR-absorbing Y-SeNF-2ClO was also introduced into the classic D18:L8-BO host system to fabricate efficient ternary PSCs. Notably, the D18:L8-BO:Y-SeNF-2ClO device offered a further improved PCE of 20.08%, ranking among the highest values reported for asymmetric acceptors. This work provides a feasible molecular design strategy for end-group and side-chain dual-engineering to develop NIR-absorbing acceptors for constructing efficient PSCs. (Figure presented.)