Energy level alignment at organic/metal interfaces studied by UV photoemission: Breakdown of traditional assumption of a common vacuum level at the interface

Electronic structures of model interfaces of organic electroluminescent (EL) devices and porphyrin/metal interfaces were investigated by UV photoemission spectroscopy (UPS). At all the measured interfaces, shift of the vacuum level was observed, showing the formation of an interfacial electric dipole layer. For Alq(3) (tris(8-hydroxyquinolino) aluminum), TPD (N,N'-diphenyl-N,N'-(3-methylphenyl)-1,1'- biphenyl-4,4'-diamine), and DP-NTCI (N,N'-diphenyl-1,4,5,8-naphthyltetracarboxylimide)/metal interfaces, interfacial energy diagrams determined by UPS correspond well with the actually observed carrier-injecting character at the interfaces. For ZnTPP (5,10,15,20-zinc-tetraphenylporphyrin), H-2 TPP (5,10,15,20- tetraphenylporphyrin), and H2T(4-Py)P (5,10,15,20-tetra(4-pyridyl) porphyrin)/metal interfaces, the shifts of the vacuum level as well as the energies of the levels in porphyrins could be expressed as a linear function of work function of the metal substrate. The slope of the linear function depended on the compound. These findings are in contrast to the traditional assumption of common vacuum level at the interfaces, For ZnTPP/metal interfaces, sample exposure to oxygen induced energy level shift in close relation with the change of the substrate work function at oxygen exposure. The present results have clearly demonstrated that direct observation of the interfacial electronic structure by microscopic method such as UPS is necessary for understanding the organic electronic devices such as EL devices and organic solar cells.