Charge-carrier transport in disordered organic solids

An analytic theoretical description of transport processes based on the concept of transport energy is suggested for disordered organic solids. It gives not only the natural explanation of experimental data but also accounts for the results of computer simulations considered so far puzzling. In particular, this approach accounts for the strong difference between the temperature dependence of the carrier drift mobility and that of the relaxation time. Experimental data for the low-field drift mobility display the temperature dependence in the form $\mu \propto \exp \{-{(T}_0{/T)}^2\}.$ It is believed that the characteristic temperature $T_0$ is determined solely by the scale of the energy distribution of localized states, and such a temperature dependence of μ is widely used to determine this energy scale from experimental data for various materials. We show that this temperature dependence is not universal and that parameter $T_0$ depends also on the concentration of localized states and on the decay length of the carrier wave function in the localized states. The suggested theory provides a general basis for the treatment of transport processes in disordered organic media.