A comparative study of the effect of fluorine substitution on the photovoltaic performance of benzothiadiazole-based copolymers

Abstract

Two conjugated alternating copolymers to be used as the donor materials of the active layers in polymer solar cells have been designed and synthesized via a Stille coupling reaction. The alternating structure consisted of 4,8-bis((2-ethylhexyl)oxy)benzo[1,2-b:4,5-b′]dithiophene (BDT) as the donor unit and benzo[c][1,2,5]thiadiazole (BT) or fluorinated benzo[c][1,2,5]thiadiazole (FBT) as the acceptor unit, along with a thiophene group as the π-bridge between the donor and acceptor units. Since the donor units have attached alkoxy pendant chains, both polymers were soluble in common organic solvents. UV-vis spectra of both copolymers exhibited broad absorption bands in the range of 325–900 and 380–900 nm, respectively, and corresponding low band gaps of 1.82 and 1.80 eV. After fluorination of the BT unit, the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels of the polymer were lowered and estimated to be −5.51 and −3.71 eV, respectively. It was found that substitution of an F atom into the BT units facilitated the intramolecular charge transfer. In comparison with the nonfluorinated polymer, the photovoltaic performance of the fluorinated polymer was significantly improved due to the enhanced J_sc and V_oc. Based on the ITO/PEDOT:PSS/polymer:PC₆₁BM/LiF/Al device structure, the optimal device efficiency was obtained from a device with a blend of PBDTFBT and PC₆₁BM at a weight ratio of 1 : 1. For this blend ratio, the values of J_sc and power conversion efficiency (PCE) obtained at room temperature are 7.98 mA cm⁻² and 3.62%, respectively, under the illumination of AM 1.5 (100 mW cm⁻²).