Through introducing six fluorine atoms onto quinoxaline (Qx), a new electron acceptor unit-hexafluoroquinoxaline (HFQx) is first synthesized. On the basis of this unit, we synthesize a new donor-acceptor (D-A) copolymer (HFQx-T), which is composed of a benzodithiophene (BDT) derivative donor block and an HFQx accepting block. The strong electron-withdrawing properties of fluorine atoms increase significantly the open-circuit voltage (Voc) by tuning the highest occupied molecular orbital (HOMO) energy level. In addition, fluorine atoms enhance the absorption coefficient of the conjugated copolymer and change the film morphology, which implies an increase of the short-circuit current density (Jsc) and fill factor (FF). Indeed, the HFQx-T:ITIC blended film achieves an impressive power conversion efficiency (PCE) of 9.4% with large short-current density (Jsc) of 15.60 mA/cm2, high Voc of 0.92 V, and FF of 65% via two step annealing (thermal annealing (TA) and solvent vapor annealing (SVA) treatments). The excellent results obtained show that the new copolymer HFQx-T synthesized could be a promising candidate for organic photovoltaics.
Keywords: hexafluoroquinoxaline; high efficiency; nonfullerene; photovoltaic polymer; thermal annealing.