Biaxially extended quaterthiophene-thiophene and -selenophene conjugated polymers for optoelectronic device applications

New biaxially extended quaterthiophene (4T) conjugated polymers, including poly(5,5′′′-di-(2-ethylhexyl)[2,3′;5′,2′′4′′,2′′′]quaterthiophene) (P4T) and their copolymers with thiophene(P4TT), bithiophene (P4T2T), selenophene(P4TSe) and biselenophene (P4T2Se) were synthesized by Stille coupling reactions under microwave heating. The effects of the ring number of thiophene and selenophene moieties on the physical properties and polymer structures were systematically investigated experimentally and theoretically. With the increased ring number of the unsubstituted thiophene and selenophene moieties, the band gaps and the main-chain torsional angles were reduced. However, the side-chain torsional angles were increased with increasing the ring number, and thus significantly affected the carrier transporting characteristics. Among these studied conjugated polymers, the field-effect transistor (FET) based on P4TSe showed the highest hole mobility of up to 4.28 × 10−2 cm2 V−1 s−1 and an on/off ratio of 1.12 × 104. The photovoltaic device prepared from P4TSe/PC71BM exhibited the highest power conversion efficiency (PCE) of 2.6%, which resulted from more balanced hole/electron mobility and a smaller band gap. The above results revealed that the conformation, charge-transporting and optoelectronic device characteristics of biaxially extended 4T-based conjugated copolymers could be manipulated by incorporating the heteroaromatic ring spacer.