Synthesis and characterization of benzodithiophene-based copolymers for polymer solar cells

Two novel thieno[3,4-b]pyrazine-based copolymers of the donor-acceptor type, poly[4,8-bis(2-ethylhexyloxy)benzo[1,2-b:4,5-b']dithiophene-alt-5,7-(2,3-bis-(4-octyloxy- phenyl)-thieno[3,4-b]pyrazine)] (P1) and poly[4,8-bis(5-((2-ethylhexylthiophene)- 2-yl))benzo[1,2-b:4,5-b']dithiophene-alt-5,7-(2,3-bis-(4-octyloxy-phenyl)-thieno[3,4- b]pyrazine)] (P2), designed and synthesized by Stille polymerization for use in polymer solar cells. The synthesis, thermal stability, as well as the optical and photovoltaic properties of these polymers are systematically investigated. The polymers were thermally stable up to 290°C, and readily soluble in common organic solvent. Conventional polymer solar cells (PSCs) with the configuration ITO/PEDOT:PSS/polymer:PC71BM/Ca/Al are fabricated. Under optimized conditions, the polymers display power conversion efficiencies (PCEs) for the PSCs in the range 0.28-1.46% under AM 1.5 illumination. Among the studied thienopyrazine-based copolymers, P2 displays a PCE of 1.46% with a short circuit current of 7.61 mA/cm², an open circuit voltage of 0.43 V, and a fill factor of 0.45 under AM 1.5 illumination.