Increasing thiophene spacers between thieno[3,2-b]thiophene and benzothiadiazole units in backbone to enhance photovoltaic performance for their 2-D polymers

In order to investigate the effect of thiophene spacers in backbone between electron-rich thieno[3,2-b]thiophene (TT) and electron-deficient benzothiadiazole (BT) units on the photovoltaic properties, novel two-dimensional (2-D) polymers with appending conjugated dioctylthiophene units in the 3,6-positions of TT were designed and synthesized, in which one or two thiophene spacers were embedded between TT and BT, respectively. The polymer with two thiophene spacers exhibited a broader absorption and higher power conversion efficiency in the bulk hetero-junction polymer solar cells. The best performance with efficiency up to 3.34% and short-circuit current values of 12.6 mA/cm² was obtained, which are 2.1 and 3.0 times higher than those corresponding values for the polymer with only one thiophene spacer. Our work indicates that increasing the numbers of thiophene spacers between TT and BT moieties in the backbone is an efficient method to enhance the photovoltaic performance of their 2-D polymers in polymer solar cells.