Energy level alignment at organic/metal interfaces studied by UV photoemission

Abstract

Energy level alignment at various organic compound/metal interfaces was investigated by UV photoemission spectroscopy (UPS). The systems measured were: (i) model interfaces of organic electroluminescence (EL) devices, (ii) porphyrin/metal interfaces, and (iii) tetratetracontane/Au interfaces. At all the measured interfaces, a shift of the vacuum level at the interface 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 characters at the interfaces. For ZnTPP (5,10,15,20-zinctetraphenylporphyrin), H 2TPP (5,10,15,20-tetraphenylpotphyrin), and H 2T(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 linear functions of the work function of the metal substrate. The slope of the linear function depends on the compound. The existence of the shift of the vacuum level at tetratetracontane (TTC. n-C 44H 90/Au interfaces, where TTC molecules are physically adsorbed on Au substrate, suggests that the vacuum level shift is an almost general phenomenon at organic/metal interfaces even without strong chemical interaction at the interface. These findings clearly demonstrate the invalidity of the traditional model for energy level alignment with the assumption of vacuum level alignment at the interface.