Drift-diffusion current in organic diodes

TL;DR

This paper analyzes the drift-diffusion current in organic diodes using the metal-insulator-metal model, revealing the limits of Schottky theory and the transition to space-charge-limited conduction.

Contribution

It provides a comprehensive analysis of the MIM model for organic diodes, clarifying the validity of Schottky theory across different injection regimes.

Findings

Schottky theory is valid only for high injection barriers and poor injection.

In weak injection, Schottky theory matches the MIM model below the diffusion potential.

In strong injection, the current aligns with the space-charge-limited regime.

Abstract

Because the conductivity of organic semiconductors is very low, a useful model for the organic diode consists of treating the organic layer as an insulator, an approximation often referred to as the metal-insulator-metal (MIM) model. Moreover, the dominant charge carrier injection process is diffusion, so that a modified Schottky theory can be used to derive a simple analytical equation for the current voltage curve of the diode. Here, we carried out a full analysis of the MIM model for the organic diode. We show that Schottky theory is only valid when charge injection is poor, that is, for high injection barriers. When the injection barrier is lowered, the current given by Schottky theory is still valid in the weak injection regime, when the applied potential is lower than the diffusion potential. However, it becomes largely overestimated in the strong injection regime. We also show that in the strong injection regime, the current given by the MIM model merges with Mott-Gurnery space-charge-limited regime.