MACROTRAP MODEL FOR CHARGE-CARRIER TRANSPORT IN LOW-MOBILITY SOLIDS

The neutral macrotrap model of charge carrier trapping has been developed to explain the electric field (F) and temperature (T) dependence of the effective charge carrier mobility (mu(eff)) in low mobility solids. The potential barrier for the carriers localized in the neutral macrotraps (spatially extended trapping domains) can be effectively lowered by an external electric field accessible in the experiment, making the mobility and its thermal activation field dependent. The potential of a neutral macrotrap is approximated by phi(r) = (3kT/e-sigma) ln (r0/r), where r is the distance from the center of the macrotrap, r0 stands for its radius, e is elementary charge and sigma is a characteristic parameter of the exponential energy distribution of point traps composing the macrotrap. The lowering of the barrier is proportional to F at low fields and to ln F at high fields. The field behavior of the effective mobility is governed by this lowering of the barrier and by its field-dependent position which, therefore, can be reached by the thermally activated carriers with a field-dependent probability in the process of carrier diffusion. The latter introduces a factor exp (-3kT/sigma-el(D)F) (l(D)-diffusion length of the carriers) responsible for the self-consistency of the field dependence of the mobility and the Arrhenius temperature term exp [-E(F,T)/kT]. This model provides a consistent description of representative experimental data reported for low-carrier mobility crystalline and disordered solids.