Modulation of the molecular geometry of carbazolebis(thiadiazole)-based conjugated polymers for photovoltaic applications

Two novel conjugated copolymers PTBDTCBT and PDTSCBT are prepared by alternating copolymerzation of N-alkyl-carbazole[3,4-c:5,6-c]bis[1,2,5]thiadiazole (CBT) with alkylthienyl benzodithiophene (TBDT) and dithenosilole (DTS), respectively. The energy levels and molecular geometry of all of the CBT-based polymers are compared by theoretical calculation and experimental observation. It has been found that the band gap and energy levels of all of the CBT-based polymers are well modulated by various building blocks, and the molecular geometry of polymers varies with the block structures as well. Among these CBT-based polymers, PDTSCBT shows the lowest band gap (1.53 eV), which matches the solar flux well, but the low degree of crystallinity and absence of preferential alignment of the π–π stacking result in a relative low PCE of 1.52%. However, its 2-D structure endows PTBDTCBT with favorable molecular packing to achieve the PCE of 1.71% under illumination (AM 1.5G 100 mW cm−2) without considerable optimization, although its band gap is larger than PDTSCBT. These results indicate that a good balance between energy levels and molecular microstructure arrangement is crucial for the performance improvement of photovoltaic with CBT-based polymers.