Influence of dipole moment of the transport agents on the carrier mobility in photorefractive polymers

In this report, we demonstrate the effect of the dipole moment of the charge transport agent on carrier mobilities in photorefractive polymer systems. The charge carrier mobility measurements are presented as a function of applied field and temperature in several model systems. In these model systems, 30% by weight of a hole or an electron transport agent covering a range of dipole moments from 0.8 to 4 Debye, is doped into a polystyrene (PS) polymer matrix containing 25% by weight of the nonlinear optical chromophore 2,5-dimethyl-4-(p-nitrophenylazo)phenol (EHDNPB) having a dipole moment of 7.6 Debye. The results are described by the Gaussian disorder model (GDM) based on hopping through a manifold of states with superimposed energetic and positional disorder, with the addition of a dipolar disorder contribution. We conclude from the results that the dipole moment of the charge transport agent has a strong influence on carrier mobility even in the presence of strongly polar nonlinear optical chromophores. Transport agents with low dipole moments significantly increase the carrier mobility and thus provides a useful alternative approach to improve photorefractive speeds.