Structure and electrical properties of unsubstituted oligothiophenes end-capped at the β-position

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 beta-position (beta nT; n = 5, 6). To reveal the molecular geometry and molecular packing of beta nTs, X-ray single-crystal analysis was carried out. For the electrical characterization, organic field-effect transistors (OFETs) based on the beta nT 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 cm(2) V-1 s(-1) for beta 6T at a substrate temperature of 120 degrees C and 0.020 cm(2) V-1 s(-1) for beta 5T at a substrate temperature of 90 degrees C, which are slightly smaller than those of alpha-oligothiophene isomers with the same number of thiophene rings. beta 6T OFET characteristics depend on the film morphology such as grain size, while the dependence of beta 5T OFET on the morphology is slight. The different behavior of beta 6T and beta 5T can be interpreted by their different molecular orientation with respect to the film surface.