Thin Film Transistors Based on Alkylphenyl Quaterthiophenes: Structure and Electrical Transport Properties

Thin films based on the akylphenyl-substituted quaterthiophenes 5,5‘ ‘‘-bis(4-methylphenyl)-2,2‘:5‘,2‘ ‘:5‘ ‘,2‘ ‘‘-quaterthiophene ( 1 ), 5,5‘ ‘‘-bis(4- n -propylphenyl)-2,2‘:5‘,2‘ ‘:5‘ ‘,2‘ ‘‘-quaterthiophene ( 2 ), and 5,5‘ ‘‘-bis(4-hexylphenyl)-2,2‘:5‘,2‘ ‘:5‘ ‘,2‘ ‘‘-quaterthiophene ( 3 ) were grown by vacuum deposition on thermally grown SiO 2 substrates and characterized in a thin film transistor (TFT) configuration. Atomic force microscopy and specular (θ−2θ) X-ray diffraction (XRD) revealed films with small, crystalline grains, in which the oligothiophenes were oriented “end-on” with respect to the SiO 2 substrate. Interplanar spacing increased ( 1 = 28.0 Å, 2 = 29.5 Å, 3 = 37.7 Å), consistent with increasing alkyl tail length. Grazing incidence X-ray diffraction (GIXD) of the films (ca . 350 Å thick) revealed nearly equivalent in-plane unit-cell areas ( 1 = 44.1 Å 2, 2 = 44.4 Å 2, 3 = 43.8 Å 2 ). The films functioned as p -channel semiconductors in a TFT configuration, exhibiting nearly equivalent hole mobilities (μ ≈ 0.05 ± 0.01, 0.04 ± 0.01, 0.06 ± 0.01 cm 2 /Vs for 1, 2, and 3, respectively). Variable-temperature measurements demonstrated that the activation energy of the mobility for thin films of 3 was ∼55 meV. The increasing alkyl chain length does not appear to improve molecular ordering; however, the addition of a phenyl end-substituent appears to greatly improve the on-to-off ratio in TFTs.