Effect of carbonitrile and hexyloxy substituents on alternated copolymer of polythiophene–Performances in photovoltaic cells

A novel series of regioregular bithiophene disubstituted with a carbonitrile group as electron acceptor and a hexyl or hexyloxy chain as electron donor, have been synthesized. The position and the regioregularity of substituents on bithiophene have been controlled via a head-to-head (HH) or head-to-tail (HT) coupling reaction to give four monomers: the HH and the HT carbonitrilehexylbithiophene monomers and the HH and the HT carbonitrilehexyloxybithiophene monomers. Each of them lead, by chemical polymerization, to four polymers constituted by a polythiophene backbone containing alternating electron-donating and electron-withdrawing units directly connected to the polythiophene. Optical and electrochemical characterizations reveal that all these polymers have a low band gap and absorb at longer wavelengths in comparison with a poly(3-hexylthiophene) (P3HT) film. The planarity of the polythiophene backbone was increased by introduction of sulphur–oxygen (S–O) interactions between hexyloxy chains and thiophene units. In addition, the introduction of carbonitrile groups allows to stabilize the LUMO and the HOMO energy levels of polymers at around −3.10 eV for the LUMO and −5.0/−5.3 eV for the HOMO, which were lower than the ones of P3HT (−2.19 eV for the LUMO and ∼−4.9 eV for the HOMO). Bulk heterojunction photovoltaic devices were fabricated using blends of these polymers with PCBM ([6,6]-phenyl-C61-butyric acid methyl ester). Power conversion efficiency of 1.07% was obtained under solar illumination AM 1.5 (100 mW cm−2) from a device containing the polymer synthesized from HT 3-carbonitrile-4′-hexyloxyl-2,2′-bithiophene monomers and PCBM as the active layer.