Exfoliated Graphite Electrodes for Organic Single-Crystal Field-Effect Transistor Devices

In this work, exfoliated graphite was used as an electrode for organic single-crystal field-effect transistor (FET) devices, and the electrode performance was demonstrated. The reduced rigidity of exfoliated graphite allows good physical contact with organic single crystals in various morphologies. Because the work function of graphite matches well the HOMO energy level of most p-type organic molecules or the LUMO level of some n-type molecules, efficient charge injection from graphite electrodes to organic crystals is expected to result in high device performance. Indeed, the rubrene single-crystal FET devices fabricated using graphite electrodes exhibit high mobility (average of 2.38 cm2/V·s) with low threshold voltage and a high on/off current ratio. This is comparable to the FET devices that we fabricated using gold electrodes. Moreover, the graphite electrode devices of phenothiazine, C60, and PTCDA crystals also exhibited similar or improved device performance compared to devices with conventional metal electrodes. Another advantage of using graphite is the availability of chemical and physical treatments by which the work function of graphite can be tuned to a certain degree. We demonstrated improved performance of picene single-crystal FET devices employing oxygen-plasma-treated graphite electrodes. The work function change of graphite upon oxygen-plasma treatment was confirmed using Kelvin probe measurements. We thus confirmed that the improved device performance was achieved by a reduced charge injection barrier when the oxygen-plasma-treated graphite electrode was used. These results indicate that exfoliated graphite should be considered a potential candidate electrode material for organic crystal devices.