The properties of
organic thin-film transistors (TFTs) and thus
their ability to address specific circuit design requirements depend
greatly on the choice of the materials, particularly the organic semiconductor
and the gate dielectric. For a particular organic semiconductor, the
TFT performance must be reviewed for different combinations of substrates,
fabrication conditions, and the choice of the gate dielectric in order
to achieve the optimum TFT and circuit characteristics. We have fabricated
and characterized organic TFTs based on the small-molecule organic
semiconductor 2,7-diphenyl[1]benzothieno[3,2-b][1]benzothiophene
in combination with an ultrathin hybrid gate dielectric consisting
of aluminum oxide and a self-assembled monolayer. Fluoroalkylphosphonic
acids with chain lengths ranging from 6 to 14 carbon atoms have been
used to form the self-assembled monolayer in the gate dielectric,
and their influence on the TFT characteristics has been studied. By
optimizing the fabrication conditions, a turn-on voltage of 0 V with
an on/off current ratio above 106 has been achieved, in
combination with charge-carrier mobilities up to 0.4 cm2/V s on flexible plastic substrates and 1 cm2/V s on silicon
substrates.