New low band gap thieno[3,4-b]thiophene-based polymers with deep HOMO levels for organic solar cells

Two new soluble alternating alkyl-substituted benzo[1,2-b:4,5-b']dithiophene and ketone-substituted thieno[3,4-b]thiophene copolymers were synthesized and characterized. We found that grafting 3-butyloctyl side chains to the benzo[1,2-b:4,5-b']dithiophene unit at C4 and C8 afforded the resulting polymer (P1) a high hole mobility (similar to 10(-2) cm(2) Vs(-1)) and a low-lying HOMO energy level (5.22 eV). Preliminary experiments in bulk heterojunction solar cells using P1 as the electron donor demonstrated a high power conversion efficiency of 4.8% even with PC61BM as the electron acceptor. The introduction of an electron-withdrawing fluorine atom into the thieno[3,4-b]thiophene unit at the C3 position (P2) lowers the HOMO energy level and consequently improves the open circuit voltage from 0.78 to 0.86 V. These values are about 0.1 V higher than those reported for their analogues based on alkoxy-substituted benzo[1,2-b:4,5-b']dithiophene. This work demonstrates that the replacement of the alkoxy chains on the benzo[1,2-b:4,5-b']dithiophene unit with less electron-donating alkyl chains is able to lower the HOMO energy levels of this class of polymers without increasing their band gap energy.