Nonfullerene Acceptor with "Donor-Acceptor Combined π-Bridge" for Organic Photovoltaics with Large Open-Circuit Voltage

In this work, a kind of "donor-acceptor (D-A) combined pi-bridge" based on the regioselective reactivity of monofluoro-substituted benzothiadiazole (FBT) to link a thiophene ring has been designed to construct a new A-pi-D-pi-A-type small molecular acceptor (IDT-FBTR) with indacenodithiophene (IDT) as a central core (D) and 3-octyl-2-(1,1-dicyanomethylene)rhodanine as an electron-withdrawing terminal group (A). Because of the strong intramolecular push-pull electron effect, the IDT-FBTR shows a strong and broad intramolecular charge-transfer absorption band in the range of 500-750 nm. Especially, as an electron-deficient FBT unit (A') and an electron-rich thiophene ring (D') in "D-A combined pi-bridge" exert an "offset effect" to regulate the highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) energy levels of the molecule, a relatively high LUMO energy level can be maintained for IDT-FBTR that is helpful to enhance the open-circuit voltage (V-oc) for highly efficient organic solar cells (OSCs). Therefore, the optimized OSC device based on IDT-FBTR as the acceptor and PTB7-Th as the donor shows a much high V-oc of 1.02 V with a relatively low E-loss of 0.56 eV and a best power conversion efficiency of 9.14%.