Organic Semiconductors - Charge Transport in Disordered Organic Semiconductors

Charge Transport in Disordered Organic Semiconductors

Charge transport in organic semiconductors is dependent on π-bonding orbitals and quantum mechanical wave-function overlap. In disordered organic semiconductors, there is limited π-bonding overlapping between molecules and conduction of charge carriers (electrons or holes) is described by quantum mechanical tunnelling. Charge transport depends on the ability of the charge carriers to pass from one molecule to another. Because of the quantum mechanical tunnelling nature of the charge transport, and its subsequent dependence on a probability function, this transport process is commonly referred to as hopping transport. Hopping of charge carriers from molecule to molecule depends upon the energy gap between HOMO and LUMO levels. Carrier mobility is reliant upon the abundance of similar energy levels for the electrons or holes to move to and hence will experience regions of faster and slower hopping. This can be affected by both the temperature and the electric field across the system.

A theoretical study has shown that in a low electric field the conductivity of organic semiconductor is proportional to T–1/4 and in a high electric field is proportional to e–(E/aT), where a is a constant of the material. Another study shows that the AC conductivity of the organic semiconductor pentacene is frequency-dependent and provided evidence that this behavior is due to its polycrystalline structure and hopping conduction.