Improved morphology and charge carrier injection in pentacene field-effect transistors with thiol-treated electrodes

The influence of chemical surface modifications of gold electrodes on the morphology and the electrical properties has been studied for pentacene based thin-film transistors with channel lengths of $L⩽4μm$⁠. Self-assembled monolayers (SAMs) of various aliphatic and aromatic organothiols have been used to selectively modify the metallic source and drain electrodes and are further compared with reference samples with untreated electrodes. For all SAM-treated devices a reduced roughness of the pentacene film is observed which is accompanied by a reduction of the threshold voltage from about $VT=2V$ for untreated transistors to $−0.9V$ for transistors with SAM modified electrodes. Using aliphatic SAMs a poor on/off ratio of about $102$ was obtained which is attributed to their low conductivity. In contrast, the on/off ratio is enhanced by four orders of magnitude if the surface is modified by an aromatic SAM. In this case a subthreshold swing as low as $0.55V$/decade is achieved which corresponds to a trap density reduced by one order of magnitude compared to the reference sample.