Generalized effective-medium model for the carrier mobility in amorphous organic semiconductors

Electronic transport through disordered organic materials is relevant in many applications, including organic light-emitting diodes and organic photovoltaics. The charge-carrier mobility is one of the most important material characteristics that must be optimized to make organic devices competitive. Here we introduce a general effective-medium model for the analytic calculation of zero-field mobilities on the basis of material-specific parameters that are obtained from extensive ab initio simulations. By means of kinetic Monte Carlo simulations, we generalize the model to also include the strong disorder limit. As a proof of concept the model is applied to two different disordered organic materials exhibiting medium and strong disorder, respectively. Surprisingly, even at strong disorder the hole mobilities computed with the effective-medium model in its original form are found to agree best with the experimental data. Seeking a possible explanation for this result, we investigate the strong dependence of the mobility on the connectivity of the model topology and show that the distribution of hopping matrix elements in the material is indeed much broader than assumed in simple lattice models. As the input parameters of the model can be computed on the basis of relatively small samples, this model may be used for materials' screening without adjustable parameters.