B . UNIPOLAR CARRIER TRANSPORT IN THE MEAN MEDIUM APPROX IMATION FOR A GAUSSIAN DOS As motivated in part A of this section, we derive here an expression for the mobility within the MMA. Within the MMA, the current density is proportional to the sum of contributions of all possible hops between pairs of sites, with a weight that is equal to the product of the resulting displacement in the held direction and the hopping rate. With the Miller-Abrahams model for the hopping rate, the MMA on a discrete lattice leads to the following expression for the current density J : Here N$_{t}$ is the molecular site density, J¾ is the distance between the initial and final sites during one hopping event, z$_{k}$ is the hopping distance along the direction of the electric held F , g(E) is the density of states (DOS), / is the Fermi-Dirac distribution function, is the Fermi energy, and the definitions of all other quantities are the same as those in the main text. In the limit of F — 0, the current density becomes and the charge-carrier mobility becomes