Highly Efficient Photovoltaic Polymers Based on Benzodithiophene and Quinoxaline with Deeper HOMO Levels

We present the synthesis and photovoltaic application of four conjugated polymers composed of benzo[1,2-b:4,5-b']dithiophene (BDT)-based and 2,3-diphenyl-5,8-di(thiophen-2-yl)quinoxaline (DTQx)-based units. Fluorination of the DTQx units and the conjugated side groups of the BDT unit shows synergistic effect on molecular energy level modulation of the polymers, and as a result, the polymer PBQ:4 exhibits the deepest HOMO and LUMO levels in these four polymers. The characterizations of the photovoltaic properties of the polymer solar cells (PSCs) based on these four polymers reveal that the fluorination has little influence on short-circuit current density (J(SC)) and fill factor (FF) but is very helpful to enhance open-circuit voltage (V-OC) of the devices. Benefiting from the synergistic effect of the fluorination, the device based on PBQ4 shows a high V-OC of 0.90 V, which is 0.26 V higher than the polymer without fluorine and ca. 0.10 V higher than the other two polymers with less fluorine. As a result, a power conversion efficiency (PCE) of 8.55% was recorded in the PBQ-4 based device, which is much higher than those of the other three polymers and also the highest one for the BDT-Qx-based polymers.