Electronic properties of the interface between the organic semiconductor α-sexithiophene and polycrystalline palladium

The interface properties of α-sexithiophene (α-6T) and polycrystalline, atomically clean and contaminated palladium have been studied by combined core level X-ray photoemission spectroscopy and valence band ultraviolet photoemission spectroscopy. Our data indicate for the atomically clean palladium substrate a chemical reaction between the α-sexithiophene molecules and the substrate during the formation of a monolayer of flat lying α-6T molecules. We find an interface dipole of −1.2 eV and an injection barrier for holes of about 0.7 eV. In the case of ex-situ treated, contaminated palladium as metal electrode material we find a reduced interface dipole −0.4 eV and hole injection barrier 0.5 eV. By the comparison to the results of α-sexithiophene on gold we demonstrate the importance of the strength of chemical reactions at the interface.