Hopping transport in doped organic semiconductors:: A theoretical approach and its application to p-doped zinc-phthalocyanine

A detailed approach to the complex hopping transport in organic semiconductors is presented and used to describe experimental data from Maennig [Phys. Rev. B 64, 195208 (2001)] on the effect of doping on conductivity, mobility and thermopower. In this approach, the energetic distribution of the charge carriers in a Gaussian shaped density of states (DOS) is calculated under thermal equilibrium conditions and compared to the energetic distribution of the current. The description is based on the Miller-Abraham model for hopping in a disordered material and utilizes the so-called transport energy concept. To include also the case of higher electron concentrations in the tail states of the DOS the Fermi distribution was taken into account. Furthermore, additional trap states in the gap are considered to describe the experimental data at low doping concentration more correctly. In the framework of the model there is no indication of a thermally activated ionization of the dopants. In contrast to other descriptions, the position of the Fermi energy and transport energy are calculated from the model. It is demonstrated that the principal behavior of the transport parameter can be well explained in terms of classical semiconductor physics. (C) 2003 American Institute of Physics.