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 similar to -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 FIT 3-carbonitrile-4'-hexyloxyl-2,2'-bithiophene monomers and PCBM as the active layer. (C) 2010 Elsevier B.V. All rights reserved.