Abstract
It is shown that the charge carrier mobility in a positionally and energetically disordered hopping system can be evaluated by averaging either the hopping rates or the hopping times over the thermal equilibrium energy distribution of localized carriers. However, at variance with averaging the hopping rates, averaging the hopping times can be correct only if the energy dependence of the carrier energy relaxation time is also taken into consideration. The temperature and concentration dependences of the equilibrium carrier mobility were calculated by averaging the hopping rates. The consideration was based on the variable-range hopping approach with full account for the interplay between jump distance and energy difference. However, the obtained results prove that, in good quantitative agreement with both Monte Carlo simulations and experimental data, the mobility can be approximated as a product of two functions. The first function depends almost solely upon the temperature and reveals only a weak concentration dependence while the second mainly governs the concentration dependence of the mobility and is almost independent of the temperature.