Comparative carrier transport characteristics in organic field-effect transistors with vapor-deposited thin films and epitaxially grown crystals of biphenyl-capped thiophene oligomers

Carrier transport characteristics in organic field-effect transistors were compared for vapor-deposited thin films and epitaxially grown needle crystals of biphenyl-capped thiophene oligomers with different lengths of the thiophene units. The hole mobility of the films deposited on Si/SiO2 substrate was improved up to 0.17 cm(2)V(-1)s(-1) by formation of platelet crystallites with a domain size of a few micrometer. The hole trasnsport in the epitaxial needle crystals grown on the KCl surface depended upon the molecular orientation with respect to the channel direction. The orientation of the needle axis bridging over the source-drain electrodes increased the mobility since pi-electronic interaction through the parallel stack of the linear molecules enhanced the carrier transport along the needle. The deposition condition and electronic energy levels of the oligomers, depending on the length of the thiophene units, also affected their characteristics.