Charge carrier transport in disordered organic materials: dipoles, quadrupoles, traps, and all that

The reason for the Poole-Frenkel (PF) mobility field dependence in disordered organic materials is believed to be a long range spatial correlation in the distribution of energy levels of transport sites. This correlation is produced by molecules with significant permanent dipole moments. However, experimental data offer a strong evidence that in transport layers containing no molecules with high dipole moment essentially the same PF dependence is observed. We suggest the model of randomly oriented quadrupoles as a solution of this puzzle. We also calculate the effect of local sterical ordering on the transport properties of organic matrices. Using this calculation we discuss the dependence of transport parameters on the mean intersite separation. By means of computer simulation and analytic calculation we consider the influence of deep traps on the charge carrier mobility in organic materials.