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

Abstract

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 SiO2 substrate. Interplanar spacing increased (1 = 28.0 A, 2 = 29.5 A, 3 = 37.7 A), consistent with increasing alkyl tail length. Grazing incidence X-ray diffraction (GIXD) of the films (ca. 350 A thick) revealed nearly equivalent in-plane unit-cell areas (1 = 44.1 A2, 2 = 44.4 A2, 3 = 43.8 A2). The films functioned as p-channel semiconductors in a TFT configuration, exhibiting nearly equivalent hole mobilities...