Drift-diffusion current in organic diodes
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’s 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’s 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’s 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’s space-charge-limited regime.