EFFECTS OF THE DIPOLE-MOMENT ON CHARGE TRANSPORT IN DISORDERED MOLECULAR-SOLIDS

Hole mobilities have been measured in a series of donor compounds with different dipole moments. The results have been described by the disorder formalism, due to Bassler and co-workers. The formalism is based on the assumption that charge transport occurs by hopping through a Gaussian manifold of states with superimposed positional disorder. Key parameters of the model are the width of the density-of-hopping states and the degree of positional disorder. The width of the hopping site manifold increases with increasing dipole moment. This results in a sharp decrease in mobility with increasing dipole moment. The origin of this effect has been attributed to random internal fields associated with the dipole moments. The positional disorder has been attributed to packing constraints.