Hopping mobility of charge carriers in disordered organic host-guest systems: Dependence on the charge-carrier concentration

The hopping mobility of charge carriers in disordered organic host-guest systems with a bimodal Gaussian density of states is studied. Two semianalytical models are used, viz. a relatively simple Mott-type model and a more advanced but computationally less efficient effective medium model. The latter model has been generalized, in order to be able to include the effect of different wave function extensions of the host and guest molecules. It is shown that energetic disorder can result in a pronounced charge carrier concentration dependence of the mobility. This provides an explanation so far for unresolved issues concerning the guest concentration dependence of the measured hole mobility in some well-characterized host-guest systems. It is also argued that treating the mobility as a carrier concentration dependent quantity is highly relevant to the modeling of carrier transport in organic light emitting devices (OLEDs), consisting of an organic matrix material with embedded fluorescent or phosphorescent dye molecules.