Uv Photoemission Study of 8-Hydroxyquinoline Aluminum (ALQ₃) / Metal Interfaces

Abstract

Electronic structure of tris(8-hydroxyquinolino) aluminum (Alq₃) / metal interface was directly investigated as a model interface of electroluminescent (EL) devices by UV photoemission spectroscopy (UPS). Simulated spectrum of Alq₃ calculated by a MNDO / PM3 molecular orbital calculation showed a good correspondence with the observed UPS spectrum. Highest occupied molecular orbitals (HOMOs) of Alq₃ was assigned to HOMO of the ligands. At Alq₃ / metal interfaces, shift of the vacuum level was observed, in contrast to the traditional assumption with common vacuum level at the interface. Interfacial energy diagrams determined by the results of UPS corresponds well to actual carrier-injecting character at the interface.