Synthesis, properties, and photovoltaic characteristics of p-type donor copolymers having fluorine-substituted benzodioxocyclohexene-annelated thiophene

The incorporation of an acceptor unit into pi-conjugated systems is an effective approach to tune both the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels of organic semiconducting materials. We report on the design of a series of fluorine-substituted benzodioxocyclohexene-annelated thiophene acceptor units and the synthesis of donor-acceptor (D-A) type copolymers based on these acceptor units and dithienosilole as a donor unit. Physical measurements of the copolymers indicate that the D-A characteristics are increased by increasing the number of introduced fluorine atoms and the HOMO and LUMO energy levels of the copolymers are fine-tuned depending on the acceptor units. Organic photovoltaics based on blend films of these D-A copolymers and [6,6]-phenyl-C71-butyric acid methyl ester show photovoltaic responses with a power conversion efficiency of up to 7.30%. Investigation of the device physics shows that the performance is mainly limited by the hole transport, which provides insight in the direction of material design toward the improvement of OPV performance. These results demonstrate the potential of fluorine-substituted benzodioxocyclohexene-annelated thiophene as an acceptor unit in organic semiconducting materials.