The elimination of interface charge density singularity in single layer organic semiconductor structures

Abstract

The arguments are presented that the assumption of a zero electric field at the charge carrier injection electrode/organic interface, as assumed by the current-voltage model of shallow traps occupying a single energy level within the organic semiconductor charge transport band as well as by the model of traps exponentially distributed in energy, is invalid in principle. In consequence of this fact, it is shown that the value of material parameters is strongly related to the bias dependent non-zero electric field at the charge-injecting interface. A set of published room temperature current-voltage data of a single layer, electron-only, metal/organic semiconductor structure is used in order to show explicitly that the singularity of free (or total) electron charge carriers at the injecting interface and the associated space charge limited current characteristics reflects an unrealistic assumption of the above two models. For the model of traps exponentially distributed in energy, the charge traps density and the associated non-zero interfacial electric field are uniquely determined under a subsidiary condition of minimal energy of charges in an external electric field. The resulting (bias dependent) spatial distribution of the internal electric field and the spatial dependence of the electron density turns out to be practically uniform throughout the organic sample in agreement with the predictions of the model of shallow traps occupying a single energy level. The internal electric field and the charge density within the narrow midway region of the organic are, at each bias, invariant to the variation of the trap density and the associated interfacial electric field. It is also shown that the drift-diffusion electron transport interpreted in terms of the model of shallow traps occupying a single energy level is incompatible with the published measurements.