ELECTRONIC-STRUCTURE OF 8-HYDROXYQUINOLINE ALUMINUM (ALQ(3)) METAL INTERFACES STUDIED BY UV PHOTOEMISSION/

The electronic structures of tris(8-hydroxyquinoline) aluminum (Alq(3) )/metal (Au, Al) interfaces as a model interface of organic electrolum inescent (EL) devices were investigated by ultraviolet photoemission s pectroscopy (UPS). We found abrupt shifts of the vacuum level of ca. 1 eV at the interfaces in contrast to the traditional assumption with a common vacuum level at the interface. The shift indicates the formati on of interfacial dipole with the metal side negatively charged. At Al q(3)/Al interface, the estimated energy position of the lowest unoccup ied molecular orbital (LUMO) was very close to the Fermi level of the substrate metal, corresponding to the electron-injecting nature of the interface. This is in contrast to the poor electron-injecting charact er expected by assuming a common vacuum level where Fermi level of the substrate should be around the center of the gap. The electronic stru cture of Alq(3) as a solid is also discussed in comparison with the re sults by semi-empirical molecular orbital calculation.