  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.
