Dimensionality of charge transport in organic field-effect transistors

Abstract

Application of a gate bias to an organic field-effect transistor leads to accumulation of charges in the organic semiconductor within a thin region near the gate dielectric. An important question is whether the charge transport in this region can be considered two-dimensional, or whether the possibility of charge motion in the third dimension, perpendicular to the accumulation layer, plays a crucial role. In order to answer this question we have performed Monte Carlo simulations of charge transport in organic field-effect transistor structures with varying thickness of the organic layer, taking into account all effects of energetic disorder and Coulomb interactions. We show that with increasing thickness of the semiconductor layer the source-drain current monotonically increases for weak disorder, whereas for strong disorder the current first increases and then decreases. Similarly, for a fixed layer thickness the mobility may either increase or decrease with increasing gate bias. We explain these results by the enhanced effect of state filling on the current for strong disorder, which competes with the effects of Coulomb interactions and charge motion in the third dimension. Our conclusion is that apart from the situation of a single monolayer, charge transport in an organic semiconductor layer should be considered three-dimensional, even at high gate bias.

Investigation of the dimensionality of charge transport in organic field effect transistors

“…This confirms the finding in Ref. [13] that confinement effects can have either a positive or a negative effect on the mobility, depending on disorder. For high disorder, the positive effects of state filling that are associated with an increased confinement (thinner active layer or higher gate field) dominate over the negative effects of enhanced Coulomb scattering and a reduced number of pathways in the z direction.…”

“…S5(b) of the Supplemental Material [17]. This has been observed before and attributed to an increased effect of state filling at higher disorders [13]. However, a Gaussian DOS does not reproduce our experimental findings and was therefore discarded for the remainder of this paper.…”

“…The interplay between these factors has been discussed extensively in Ref. [13] and will therefore, in the context of the present paper, only shortly be addressed in the following.…”

“…S5(a) of the Supplemental Material [17]. As mentioned earlier, in previously published numerical investigations of OFETs a Gaussian distribution was assumed for the DOS [12,13]. Although this assumption might be correct for ODSCs in general, it is not commonly used for analyzing experimental investigations of OFETs [14][15][16]19].…”

“…[7], i.e., a gate field-independent accumulation layer thickness that only spans across the first few MLs; however the effect of the spatial confinement on the carrier mobility in a thin film was not addressed [12]. A numerical analysis taking into account various thicknesses concluded that, whereas carriers are in fact confined to the first few MLs, charge transport in OFETs is generally a three-dimensional (3D) process [13]. However, in both cases an energetic disorder of the localized hopping states in the form of a Gaussian distribution is assumed, whereas commonly an exponential distribution is used in analyses of experiments.…”