Influence of the alkyl substitution position on photovoltaic properties of 2D-BDT-based conjugated polymers

Three conjugated polymers based on thienyl-substituted benzodithiophene (BDT) and 4,7-bis-thienyl-benzothiadiazole (DTBT) with varied substitution positions of the alkyl side chains were synthesized to investigate the correlations between the structure and photovoltaic performance of the polymer photovoltaic materials. The three polymers named PBDTDTBT-p, PBDTDTBT-o and PBDTDTBT-m were characterized by a set of methods including absorption spectroscopy, cyclic voltammetry, thermogravimetric analysis, X-ray diffraction, density functional theory and photovoltaic measurements. The results show that the steric hindrance caused by the different substitution positions of the alky chains has a significant influence on the photovoltaic properties of the polymers. The open-circuit voltage (Voc) of the photovoltaic devices based on the three polymers could range from 0.67 to 0.90 V. Clearly, this finding provides us a feasible strategy to optimize the photovoltaic properties by simply changing the positions of the alkyl chains.