Regioregular Low Bandgap Polymer with Controlled Thieno[3,4-b]thiophene Orientation for High-Efficiency Polymer Solar Cells

The regioregular p-type copolymer PBDTTT-C-T composed of TT-BDT-TT-BDT repeating units (TT = thieno[3,4-b]thiophene, BDT = benzo[1,2-b:4,5-b′]dithoiphene) and perfectly controlled TT orientation was synthesized. The optical, thermal, and electrochemical properties of the regioregular PBDTTT-C-T were characterized and compared with the random PBDTTT-C-T without structural regioregularity. The regioregular PBDTTT-C-T showed lower optical bandgap (1.55 eV) and higher degree of crystallinity compared to the random PBDTTT-C-T. The inverted bulk heterojunction PSCs based on the regioregular PBDTTT-C-T exhibited a power conversion efficiency as high as 7.79%, which is 19% higher than the random PBDTTT-C-T-based PSCs. It was found that the improved photoabsorption and increase in charge carrier mobility due to high regioregularity of conjugated polymer backbones and effective ordering between polymer chains are the most likely reasons for enhancement of power conversion efficiency in PSCs.