Quasi-One-Dimensional Charge Transport Can Lead to Nonlinear Current Characteristics in Organic Field-Effect Transistors

Nonlinearity in the current characteristics of organic field-effect transistor (OFET) devices has become a serious issue for accurate evaluation of the charge-carrier mobilities in organic semiconductors. In particular, in the case of several high-mobility materials, a kink appears in the transfer curves, and this nonlinearity has been generally interpreted as the result of poor contacts. Here, we describe another possible origin for the appearance of such a kink. Extensive molecular-scale device simulations indeed demonstrate that the quasi-1D nature of charge transport often encountered in organic crystals or highly oriented polymers can lead to significant transport through the bulk and result in nonlinearity of the transfer current characteristics if the actual charge injection is away from the channel. When this is the case, the low-gate voltage regime in fact does not overestimate the charge mobility along the channel direction.