Modeling organic thin-film transistors based on the virtual source concept: A case study

Abstract

We apply the virtual source concept to describe the DC current-voltage characteristics of organic thin-film transistors. The draining of charge carriers from the virtual source is calculated using emission-diffusion theory. The electrical characteristics of organic thin-film transistors are commonly analyzed employing a drift mobility that is enhanced by the transistor overdrive. One possible origin of such a mobility model is charge trapping. We show that the direct parametrization of the fraction of mobile to total charges including trapped ones allows for a straight forward adaptation of the virtual-source emission-diffusion theory to organic electronics capable of describing measured transfer and output curves with a small number of parameters. The resulting model offers an alternative parametrization of well-known bias and temperature dependences. Beyond others, a limited charge injection does not arise from the high-field bulk saturation velocity but from the unidirectional thermal velocity and the charge carrier mean free path is a critical model parameter. Moreover, diffusivity replaces the drift mobility as the current scaling factor, which would lead to slightly different predictions for the current-voltage curves in the case that the Einstein relation is not valid.