Thin film transistors based on alkylphenyl quaterthiophenes: Structure and electrical transport properties

摘要

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 SiO2 substrates and characterized in a thin film transistor (TFT) configuration. Atomic force microscopy and specular (theta-2 theta) X-ray diffraction (XRD) revealed films with small, crystalline grains, in which the oligothiophenes were oriented " end-on" with respect to the SiO2 substrate. Interplanar spacing increased (1 = 28.0 angstrom, 2 = 29.5 angstrom, 3 = 37.7 angstrom), consistent with increasing alkyl tail length. Grazing incidence X-ray diffraction (GIXD) of the films (ca. 350 angstrom thick) revealed nearly equivalent in-plane unit-cell areas ( 1 = 44.1 angstrom(2), 2 = 44.4 angstrom(2), 3 = 43.8 angstrom(2)). The films functioned as p-channel semiconductors in a TFT configuration, exhibiting nearly equivalent hole mobilities (mu approximate to 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 similar to 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.