Field dependence conductivity in disordered systems

We present a model to describe the field dependencies of the hopping conductivity in disordered systems. In literature, the universal electric field dependence observed in the organic disordered systems has been interpreted by three models: Gaussian disorder model (GDM) which neglects the charge–dipole interaction, the correlated disorder model (CDM) valid in disordered materials with permanent dipole moments, and Geometry fluctuation model (GFM) recently proposed for conjugated polymers without permanent dipole moments. In our model, we show that the Poole Frenkel behaviour is due to the effect of the intensity of the electric field on the dimension of the transport path, that becomes one-dimensional in the high-fields region, or to the effect of the field on the tunnelling probability between the localized states. For the field dependence of the conductivity we find, lnσ ∝ Eγ, with γ = 2 in the low field region, γ = 1 in the moderate field region and γ = 1/2 in the high field region. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)