Electronic properties of the interfaces between the wide bandgap organic semiconductor para-sexiphenyl and samarium

Ultraviolet photoelectron spectroscopy was used to determine the energy level alignment at the interfaces formed by either the deposition of para-sexiphenyl (6P) onto samarium thin films, or by growing samarium on 6P films. We find that for both cases (6P on Sm, and Sm on 6P) the interaction at the interface is weak (physisorption), and the interfacial dipole is smaller than 0.2 eV. This weak interaction enabled us to demonstrate that small morphological changes in the substrate Sm film lead to a reversal in the direction of the interfacial dipole. For native silicon oxide substrates the appearance of a photoemission signal more than 1 eV above the Fermi level is observed, and explained by the energy level alignment process for wide bandgap organic semiconductors. Additionally, we demonstrate that large lateral vacuum level shifts (ca. 1 eV) can be locally realized on an inhomogeneous sample.