Influence of diffusion, trapping, and state filling on charge injection and transport in organic insulators

We examine unipolar charge carrier injection in organic insulators containing traps and sandwiched between two equal Ohmic contacts. We show that diffusion of charge carriers from the contacts plays a key role in the electrical transport of thin organic films with a low concentration of traps. In these systems a significant fraction of trapping states can be filled by diffusion even at zero applied bias. The diffusion-induced filling of traps decreases with increasing thickness of the insulator, which produces a “detrapping transition” in the voltage-thickness curve at constant current. The observation of this transition can be useful to determine the trap density. We demonstrate that the density of free charge carriers at the contacts, compared to the density of trapping states in the solid, determines different regimes of diffusion injection. Finally, we discuss how the density of available conducting states in the organic insulator can constrain the charge carrier injection and review the consequences of this effect on the electrical properties of a device.