Structure and Electrical Properties of Unsubstituted Oligothiophenes End-Capped at the β-Position

Abstract

The relationship between molecular structure, molecular packing in bulk, film structure, and electrical characteristics were investigated for the unsubstituted oligothiophene pentamer and hexamer, both of which have terminal thiophene units linking at the β-position (βnT; n = 5, 6). To reveal the molecular geometry and molecular packing of βnTs, X-ray single-crystal analysis was carried out. For the electrical characterization, organic field-effect transistors (OFETs) based on the βnT thin films were fabricated at various substrate temperatures. Film structure was investigated by X-ray diffraction and atomic force microscopy. Highest field-effect mobility of OFET was 0.050 cm2 V-1 s-1 for β6T at a substrate temperature of 120 °C and 0.020 cm2 V-1 s-1 for β5T at a substrate temperature of 90 °C, which are slightly smaller than those of α-oligothiophene isomers with the same number of thiophene rings. β6T OFET characteristics depend on the film morphology such as grain size, while the dependence of β5T OFET on the morphology is slight. The different behavior of β6T and β5T can be interpreted by their different molecular orientation with respect to the film surface.