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 thermallygrown SiO
2 substrates and characterized in a thin film transistor (TFT) configuration. Atomic forcemicroscopy and specular (
![](/images/gifchars/theta.gif)
-2
![](/images/gifchars/theta.gif)
) 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 spacingincreased (
1 = 28.0 Å,
2 = 29.5 Å,
3 = 37.7 Å), consistent with increasing alkyl tail length. Grazingincidence X-ray diffraction (GIXD) of the films (ca
. 350 Å thick) revealed nearly equivalent in-planeunit-cell areas (
1 = 44.1 Å
2,
2 = 44.4 Å
2,
3 = 43.8 Å
2). The films functioned as
p-channel semiconductorsin a TFT configuration, exhibiting nearly equivalent hole mobilities (
![](/images/entities/ap.gif)
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 theactivation energy of the mobility for thin films of
3 was ~55 meV. The increasing alkyl chain lengthdoes not appear to improve molecular ordering; however, the addition of a phenyl end-substituent appearsto greatly improve the on-to-off ratio in TFTs.