Influence of interfacial trap states on injecting and extracting of charges across a metal-organic interface

The processes of injecting and extracting holes across a metal–organic interface were examined. The height and the width of the corresponding Schottky barriers existing at the metal–organic interfaces were determined from the current–voltage characteristics using the Simmons approximation. The measured temperature dependence of the I–V characteristics suggest that the injection process occurred in two steps. Charge carriers were initially injected into interfacial trap states, from where they eventually hopped to the transport states of the highest occupied molecular orbital (HOMO) band of the organic p-semiconductor. Such a two-step injection process was dominant at low voltages. For large voltages, where the Fermi level of the metal electrode was located near the HOMO level, injection is dominated by direct tunneling of holes from metal to the HOMO level. The voltage ranges, for which either a twostep process or direct tunneling dominated, depended on the height and width of the Schottky barrier, but also on the depth and distribution of the interfacial trap states. For the extraction of holes from organic to metal, an increase of the extracting resistance with increasing voltage could be observed at low voltages, which is identified as a consequence of the filling of the interfacial trap states with holes from the HOMO band. For large voltages, holes were extracted from the HOMO to the Fermi level of gold in two ways: (a) in a direct transition and (b) in a two-step transition involving trap states at the interface.