Syntheses of PCDTBT containing tetrafluorobenzene as electron-withdrawing group with deep HOMO energy level and applications for photovoltaics

Abstract

Conjugated polymers using tetrafluorophenylene unit were synthesized and deployed for photovoltaics. The tetrafluorophenylene unit which has the strong electron-withdrawing ability, was utilized for the copolymers of push-pull type with low bandgaps for organic solar cells. 9H-Carbazole as electron-rich unit and di-2-thienyl-2,1,3-benzothiadiazole (DTBT) or tetrafluorophenylene unit as electron-deficient unit were utilized for the syntheses of donor-acceptor (D-A) types of conjugated polymers with different ratios of tetrafluorophenylene unit (PCDTBTF-1, PCDTBTF-3 and PCDTBTF-10).

By introduction of tetrafluorophenylene unit in the backbone of PCDTBT, the energy level of the highest occupied molecular orbital (HOMO) and the bandgap were decreased as compared to those of PCDTBT. The higher amount of tetrafluorophenylene unit, the lower energy levels of the HOMO of the polymers were observed. The device of PCDTBTF-1: PC71BM (1: 1) with additive of diphenyl ether (DPE) showed an open-circuit voltage (V-OC) of 0.82 V, a short circuit current (J(SC)) of 9.66 mA/cm(2), and a fill factor (FF) of 0.44, which yields PCE of 3.44%.