Gated nonlinear transport in organic polymer field effect transistors

Abstract

We measure hole transport in poly(3-hexylthiophene) field effect transistors with channel lengths from 3 μm down to 200 nm, from room temperature down to 10 K. Near room temperature effective mobilities inferred from linear regime transconductance are strongly dependent on temperature, gate voltage, and source–drain voltage. As T is reduced below 200 K and at high source–drain bias, we find transport becomes highly nonlinear and is very strongly modulated by the gate. We consider whether this nonlinear transport is contact limited or a bulk process by examining the length dependence of linear conduction to extract contact and channel contributions to the source–drain resistance. The results indicate that these devices are bulk limited at room temperature, and remain so as the temperature is lowered. The nonlinear conduction is consistent with a model of Poole–Frenkel-like hopping mechanism in the space-charge limited current regime. Further analysis within this model reveals consistency with a strongly energy dependent density of (localized) valence band states, and a crossover from thermally activated to nonthermal hopping below 30 K.