Effect of extended thiophene segments in small band gap polymers with thienopyrazine

Three new small band gap polymers were synthesized and applied in bulk heterojunction solar cells together with [60]PCBM and [70]PCBM. The polymers are based on quarterthiophene or quinquethiophene segments alternating with thienopyrazine units along the chain and were obtained via Yamamoto or Suzuki polymerization. In thin solid films, the band gaps vary from 1.3 to 1.5 eV. All polymers give a photoresponse up to 850 nm when combined with a methanofullerene derivative. The best cells gave a power conversion efficiency of 1.4% under simulated standard solar light conditions (AM1.5, 100 mW/cm2).