Surface potentials of operating pentacene thin-film transistors (TFTs) with two different contact geometries (bottom or top) were mapped by Kelvin probe force microscopy (KFM). The surface potential distribution was used to isolate the potential drops at the source and drain contacts. These potential drops were converted to resistances by dividing by the appropriate drain current values. The bottom contact TFTs were contact limited at large gate voltages, while the top contact TFTs were not contact limited. In both geometries, the contact and the channel resistances decreased strongly with increasing (negative) gate bias but did not depend strongly on the drain bias. This study demonstrates the utility of KFM for visualizing charge transport bottlenecks in operating pentacene devices and for correlating electrical behavior with device structure by comparison of surface potential and topographic maps.

We describe gated four-probe measurements designed to measure contact resistance in pentacene-based organic thin-film transistors (OTFTs). The devices consisted of metal source and drain electrodes contacting a 300-Å-thick pentacene film thermally deposited on Al2O3 or SiO2 dielectrics with a p-doped Si substrate serving as the gate electrode. Voltage-sensing leads extending into the source-drain channel were used to monitor potentials in the pentacene film while passing current during drain voltage (VD) or gate voltage (VG) sweeps. We investigated the potential profiles as a function of contact metallurgy (Pt, Au, Ag, and Ca), substrate chemistry, VG, and temperature. The contact-corrected linear hole mobilities were as high as 1.75cm2∕Vs and the film sheet resistance and specific contact resistance were as low as 600kΩ∕◻ and 1.3kΩ-cm, respectively, at high gate voltages. In the temperature range of 50–200K, the pentacene OTFTs displayed an activated behavior with activation energies of 15–30meV. Importantly, the activation energy associated with the contact resistance showed no dependence on contact metal type at high gate voltage. Also, the activation energies of the contact resistance and film resistance were approximately the same. Above approximately 200K and below 50K, the mobility was essentially temperature independent.

We describe variable temperature contact resistance measurements on pentacene organic thin-film transistors via a gated four-probe technique. The transistors consist of Au source and drain electrodes contacting a pentacene film deposited on a dielectric/gate electrode assembly. Additional voltage sensing leads penetrating into the source-drain channel were used to monitor potentials in the pentacene film while passing current between the source and drain electrodes during gate voltage sweeps. Using this device structure, we investigated contact resistance as a function of film thickness (60–3000Å), deposition temperature (25 or 80°C), gate voltage, electrode geometry (top or bottom contact), and temperature. Contact resistance values were approximately 2×103–7×106Ωcm, depending on film thickness. In the temperature range of 77–295K, the contact resistance displayed activated behavior with activation energies of 15–160meV. Importantly, it was observed that the activation energies for the source and drain resistances were nearly identical for all device configurations. Contact resistance was found to be dependent on the film mobility in a power law fashion with exponents in the range of −0.58 to −1.94. The activation energy and the dependence of resistance on mobility suggest that contact resistance is not determined by a barrier at the metal-pentacene interface, but rather, drift/diffusion of carriers near the metal-pentacene interface. Two-dimensional device modeling of gated four-probe structures was performed to examine the validity of the source and drain resistance determination.