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 SiO₂ 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 Ų, 2 = 44.4 Ų, 3 = 43.8 Ų) . 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 cm²/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.