  For hopping transport in disordered materials, the mobility of charge
carriers is strongly dependent on temperature and the electric field. Our
numerical study shows that both the energy distribution and the mobility of
charge carriers in systems with a Gaussian density of states, such as organic
disordered semiconductors, can be described by a single parameter - effective
temperature, dependent on the magnitude of the electric field. Furthermore,
this effective temperature does not depend on the concentration of charge
carriers, while the mobility does depend on the charge carrier concentration.
The concept of the effective temperature is shown to be valid for systems with
and without space-energy correlations in the distribution of localized states.
