Electron spectroscopy studies of interface formation between metal electrodes and luminescent organic materials

Using x-ray and ultraviolet photoelectron spectroscopy (XPS and UPS), we have studied the formation of metal/organic interfaces in organic electroluminescent devices. Oligo(p-phenylenevinylenes) (OPV) and tris-(8-hydroxyquinoline)aluminum (Alq3) were used as the organic materials and Ca was used as the metallic layer. Interfaces are formed differently by depositing organic layer on Ca and Ca on organic substrate. For Ca/OPV, UPS revealed a clear evidence for interface state formation upon Ca deposition. The evolution of XPS core level peak as a function of Ca layer thickness was consistent with the energy level bending picture. The XPS and UPS spectra for OPV/Ca as a function of organic layer thickness also confirmed the energy level bending. The data obtained allowed us to deduce the energy level diagram near the interface. Similar data for Alq3/Ca indicated that no electron injection barrier exists at this interface if the Alq3 optical band gap in the literature was used for estimating the energy position of the lowest unoccupied state.