The influence of side chains on solubility and photovoltaic performance of dithiophene–thienopyrazine small band gap copolymers

Three small band gap copolymers based on alternating dithiophene and thienopyrazine units were synthesized via Yamamoto coupling and applied in bulk heterojunction solar cells as donor together with PCBM ([6,6]-phenyl C61 butyric acid methyl ester) as acceptor. The polymers have an optical band gap of about 1.3 eV in the solid state and only vary by the chemical nature of the solubilizing side chains. The nature of the side chain has a major effect on solubility and processability of the polymer. Using n-butoxymethyl side chains a soluble, easy to process polymer was obtained that gave the best photovoltaic performance. With short-circuit currents up to 5.2 mA/cm2 an efficiency of 0.8% was achieved under estimated standard solar light conditions (AM1.5G, 100 mW/cm2) with spectral response up to 950 nm.