Interface analysis of naphthyl-substituted benzidine derivative and tris-8-(hydroxyquinoline) aluminum using ultraviolet and x-ray photoemission spectroscopy

Abstract

We have studied the interface between naphthyl-substituted benzidine derivative (NPB) and tris-8-(hydroxyquinoline) aluminum (Alq3). Ultraviolet photoemission and x-ray photoemission spectroscopy (UPS, XPS) are used to distinguish contributions from NPB and Alq3 and reveal an interface formation region of approximately one to two monolayers. The UPS results show the highest occupied molecular orbital (HOMO) level offset is 0.3 eV, whereas the lowest unoccupied molecular orbitial offset is 0.8 eV, which confines electrons in the emissive Alq3 layer of the heterostructure. From the UPS difference spectrum, the gradual modification of the HOMO levels are revealed. Within the interface region, the energy levels bend by more than 0.3 eV. The Alq3 and NPB HOMO level increases are consistent with the vacuum level shift as well as the observed core level shifts from XPS. Further, the XPS and UPS results show no chemical interactions or wave function overlap at the interface.