Regio-asymmetric polymers based on fluorinated benzothiadiazole–benzodithiophene for polymer solar cells with a high open-circuit voltage

Abstract

Two regio-asymmetric polymers PR2F and PR1F with three repeat units A–D–A, π–D–A, and π–D–π along the polymer backbone were obtained by polymerizing different fluorinated benzothiadiazole–thiophene units, FBT-T and 2FBT-T units (A–π), with a benzodithiophene unit (D), respectively. Here, the new regio-asymmetric polymers were different from traditional polymers featuring a D–A or D–π–A–π backbone. Both the regio-asymmetric polymers showed a low lying HOMO energy level in relation to that of the PC₆₁BM acceptor and then a high open-circuit voltage of ∼1 V was observed in both PR2F and PR1F devices. Bulk-heterojunction devices based on PR2F with difluorinated BT exhibited an appreciable power conversion efficiency (PCE) of 4.73% due to the relatively high and balanced charge carrier mobilities, while the mono-fluorinated BT polymer PR1F-based devices showed a lower PCE of 2.92%. These results indicate that the regio-asymmetric conjugated polymers are a promising class of materials for polymer solar cell applications, and it is significant to further understand the influence of the polymer structure on the optoelectronic properties.