Universal diffusion-limited injection and the hook effect in organic thin-film transistors

Abstract

The general form of interfacial contact resistance was derived for organic thin-film transistors (OTFTs) covering various injection mechanisms. Devices with a broad range of materials for contacts, semiconductors, and dielectrics were investigated and the charge injections in staggered OTFTs was found to universally follow the proposed form in the diffusion-limited case, which is signified by the mobility-dependent injection at the metal-semiconductor interfaces. Hence, real ohmic contact can hardly ever be achieved in OTFTs with low carrier concentrations and mobility, and the injection mechanisms include thermionic emission, diffusion, and surface recombination. The non-ohmic injection in OTFTs is manifested by the generally observed hook shape of the output conductance as a function of the drain field. The combined theoretical and experimental results show that interfacial contact resistance generally decreases with carrier mobility, and the injection current is probably determined by the surface recombination rate, which can be promoted by bulk-doping, contact modifications with charge injection layers and dopant layers, and dielectric engineering with high-k dielectric materials.

Nonlinear Contact Effects in Staggered Thin-Film Transistors

“…However, this procedure leads to a deviation between model and experiment in the steepness of the curve for absolute drain-source voltages above 0.4 V and in the level of the saturation regime (see the Supplemental Material [51]). We observe that the slope of the curve slightly increases for curves measured with a large overdrive voltage jV GS − V th j, i.e., for gate-source voltages between about −2 and −3 V. Such a behavior leads to an overshoot in differential output conductance, as reported by Liu et al [47]. By contrast, a simulation with purely linear contact resistances produces a continuous decrease of the slope of the output curves towards the saturation regime, consistent with the gradual channel approximation.…”

“…The reason for this is that the properties of the interfaces between the contacts and the semiconductor, especially in the case of organic semiconductors, critically depend not only on the choice of material but also on a large number of process parameters [42,43]. As an alternative, various analytical models have been developed for treating the injection of charges into organic semiconductors [44][45][46][47][48], but these models are unable to predict the charge flow at the metal-semiconductor interfaces a priori. Parameters such as the energy barrier height partially lose their physical meaning and are instead used to adjust resistances to the correct levels during the fitting of experimental data, which means that they become or behave like phenomenological parameters.…”

Perylene-Diimide Molecules with Cyano Functionalization for Electron-Transporting Transistors

“…First, it is evident that the contact resistance phenomenon in the investigated devices is not simply related to the presence of a Schottky junction at the source electrode, as expected by considering the nominal values of the E F ≈ −5 eV) and of PDI8-CN 2 /PDIF-CN 2 LUMO level (−4.3/−4.5 eV, respectively). If this were the case, the value of the contact resistance (R S ) at the source electrode should be considerably larger than that at the drain electrode (R D ), as described also by the general diffusion-limited thermionic model, which was recently invoked to explain the contact resistance occurrence in several organic transistor configurations [64]. For our bottom-contact PDI_CY devices, R S and R D were at least comparable.…”

A Mobility Model Considering Temperature and Contact Resistance in Organic Thin-Film Transistors

“…(16) as the approximation of (15). Based on Liu et al [13], [28], the contact resistance is related to V gs , and it can be calculated by Eq. (10).…”