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

We have studied the interface between naphthyl-substituted benzidine deriva tive (NPB) and tris-8-(hydroxyquinoline) aluminum (Alq(3)). Ultraviolet pho toemission and x-ray photoemission spectroscopy (UPS, XPS) are used to dist inguish contributions from NPB and Alq(3) and reveal an interface formation region of approximately one to two monolayers. The UPS results show the hi ghest occupied molecular orbital (HOMO) level offset is 0.3 eV, whereas the lowest unoccupied molecular orbitial offset is 0.8 eV, which confines elec trons in the emissive Alq(3) layer of the heterostructure. From the UPS dif ference spectrum, the gradual modification of the HOMO levels are revealed. Within the interface region, the energy levels bend by more than 0.3 eV. T he Alq(3) 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. (C) 1999 American Vacuum Society. [S0734-2101(99)02206-X].