Mixing of interface dipole and band bending at organic/metal interfaces in the case of exponentially distributed transport states

Abstract

The interface dipole for organic adlayers on metal substrates, as determined by photoemission spectroscopy, is often almost as large (>80%) as the built-in potential determined from the ionization potential and the transport gap. Based on our experimental data and the formal description of the band bending in the thin layer, it is shown that the generally accepted view on the interface of thin organic adlayers with a metal substrate must be refined. First, besides band bending one has in the layer also floating of the potential expressed by the finite value of the potential at the outer surface of the layer. Second, for the usually observed large interface dipole, band bending is negligible as long as the electronic states are well defined in energy. It is demonstrated that an exponential distribution of the transport states, with a width of the distribution that is large compared to the thermal energy, leads to a drastic modification of this picture. For CuPc and two fluorinated CuPc’s a band bending of the magnitude of the measured interface dipole is caused within the first 2 nm of the organic adlayer by a width of the distribution of εa≈300 meV. Even for a much narrower distribution (εa=100 meV) a considerable part of the interface dipole arises from the band bending for cases with a large built-in potential. Consequently, one has to expect that the measured interface dipole within a layer of about 2 nm from the interface has a considerable contribution from the normal band bending mechanism in a system with exponentially distributed tails of the transport states. Beyond this layer of some atomic distances the band bending will be small.