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 π-bridge” based on the regioselective reactivity of monofluoro-substituted benzothiadiazole (FBT) to link a thiophene ring has been designed to construct a new A−π–D−π–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 π-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 (Voc) 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 Voc of 1.02 V with a relatively low Eloss of 0.56 eV and a best power conversion efficiency of 9.14%.