Efficient Nonfullerene Polymer Solar Cells Enabled by Small‐Molecular Acceptors with a Decreased Fused‐Ring Core

Two A1‐A2‐π‐D‐A2‐A1 structural small‐molecular acceptors with a decreased fused‐ring core, TIDT‐BT‐R2 and TIDT‐BT‐R6, are designed and synthesized by using thiophene‐indenothiophene (TIDT) as the core unit and benzothiadiazole‐rhodanine (BT‐R) as the end groups. The resulting small‐molecular acceptors exhibit the desirably complementary absorptions and well matched energy levels with the low‐bandgap donor copolymer of PTB7‐Th. Compared to TIDT‐BT‐R6, TIDT‐BT‐R2 with shorter side chains exhibits better crystallinity and higher charge carrier mobilities in its blend films. The nonfullerene solar cells (NFSCs) based on TIDT‐BT‐R2 also show higher exciton dissociation efficiency and lower charge recombination, which leads to higher J sc and fill factor values. As a result, TIDT‐BT‐R2‐based NFSCs show a large V oc of 1.04 V and a high power conversion efficiency of 8.7% with a very low energy loss of 0.55 eV, which is one of the best values for PTB7‐Th‐based binary blend devices.