Low-field charge-carrier hopping transport in energetically and positionally disordered organic materials

Abstract

The influence of superimposed energetic and positional disorder on the field dependence of drift mobility in disordered organic solids is considered for the first time analytically by an effective medium theory using jump rate expressions based either on Miller-Abrahams or Marcus models. We find that the negative field dependence of drift mobility at low fields is a genuine property of the hopping transport in disordered solids rather than an artifact and it can be theoretically reproduced for certain parameters of the investigated material and specific measurement conditions. We demonstrate that the recently suggested influence of the diffusion-controlled transport at low fields on the measured time-of-flight mobility plays no role in explaining the observed phenomena. Furthermore the frequently used practice of extrapolating field dependence of mobility to zero field has no theoretical justification. The results of the calculations are found to be in good agreement with both appropriate computer simulations and experimental results.