Quaterthiophene-benzobisazole copolymers for photovoltaic cells: Effect of heteroatom placement and substitution on the optical and electronic properties

Three new donor-acceptor conjugated polymers were synthesized by combining electron-donating 3-octylthiophenes with various electron-accepting benzobisazoles. The influence of the structural differences of the three benzobisazoles on the electrochemical, optical, and photovoltaic properties of the polymers composed from them was investigated. According to our results, changing the arrangement of the oxygen atoms of the benzobisoxzoles from the cis to trans orientation slightly stabilized both the HOMO and LUMO levels, whereas replacing the oxygen atoms in trans-BBO with sulfur atoms only stabilized the HOMO level. Bulk heterojunction photovoltaic devices were fabricated by using the copolymers as electron donors and PC ₇₁BM ([6,6]-phenyl C ₇₁-butyric acid methyl ester). It was found that open-circuit voltages (V _ocs) as high as 0.86 V, and power conversion efficiencies (PCEs) up to 1.14%, were obtained under simulated AM 1.5 solar irradiation of 100 mW/cm ². Field-effect transistors based on these polymers exhibited hole mobilities as high as of 4.9 × 10 ^-3 cm ²/(V s) with the trans-BBO polymer giving the best performance in both devices.