Synthesis of novel low bandgap random and block terpolymers with improved performance in organic solar cells

Novel conjugated random and block terpolymers composed of two electron-donating blocks - fluorene (Flo) and carbazole (Cz) - as well as one electron-accepting unit of thiophene-benzothiadiazole-thiophene (TBT) with the same (Flo:TBT:Cz) molar ratio of 0.5:1:0.5 were synthesized. The effects of their microstructure on physico-chemical properties and the performance of photovoltaic conversion were characterized. These conjugated terpolymers displayed two absorption peaks in UV–Vis analysis around 400 and 600 nm with a similar and narrow bandgap (1.9 eV, UV–Vis film), which is suitable to be applied as p-type semiconductors in an organic solar cell (OSC). Both synthesized terpolymers presented a high average molecular weight and degradation temperature above 400 °C. OSCs were assembled for determining the power conversion efficiency (PCE). Block terpolymer with a PCE of 1.3%, higher than that of 0.9% for the random terpolymer, was used as the electron donor in a conventional cell configuration. The device's optimization improved the photovoltaic parameters, mainly the short circuit current density, which allowed a PCE of 3.2% to be achieved. These results demonstrate that the synthesis of block terpolymers is a simple and practical approach for optimizing a conjugated polymer for an efficient OSC.