See also Eqs. (2) —(5) and the definitions of all quantities above. The recombination rate at site i is a result of electrons hopping from site j to (if there is a hole on site i ) and holes hopping from site j to (if there is an electron on site i). These local rates are modified (in general enhanced) by the effect of the exciton binding energy. We note that the net effect of this enhancement on the recombination rate is only moderate, because the local and neighboring electron and hole densities will be reduced when increasing the local rates by increasing the binding energy, providing (in part) a compensating effect. Neglecting a small intrinsic (thermal-equilibrium) carrier concentration, the local recombination rate is from Eq. (7) above given by where the summation is over all nearest-neighbor (NN) sites. This expression has the general form of the Langevin formula, which for a system with equal electron and hole concentrations, and a constant and equal electron and hole mobility reads e$_{r}$ is the relative dielectric constant and is the vacuum permittivity. In the absence of energetic disorder, the unipolar mobility (for small fields) on a simple cubic lattice is given by