Determination of the electronic structure of organic Schottky contacts by photoemission spectroscopy

The alignment of the highest occupied and lowest unoccupied molecular orbitals (HOMO, LUMO) of the organic luminescent semiconductor Gaq₃ relative to the Fermi level of Au was determined by depositing a Gaq₃ thin film in a multi-step growth procedure on an Au foil. Before growth and after each deposition step the sample was characterized by combined x-ray and ultraviolet photoemission spectroscopy (XPS, UPS) measurements. Such measurements offer a direct way to determine the electronic structure of the interface. Our experiments demonstrate that this method allows distinguishing between band bending, charging and interface dipole related shifts in the UP-spectra. The additional XPS measurements allow the precise determination of the band bending occurring across the interface while comparison between XPS and UPS work function measurements allows one to pinpoint the organic film thickness dependent onset of charging phenomena. Our results show that the interface dipoles at Gaq₃ Schottky contacts with Au, Pt and Ag amount to 0.6-0.7 eV. Our experiments also show that final state screening shifts can be dismissed as insignificant in such orbital line-up measurements. This was shown at the chloroindium phthalocyanine (ClInPc)/highly oriented pyrolytic graphite (HOPG) interface where no such shifts were observed.