Interfacial electronic structures of organic/metal interfaces studied by UV photoemission

The electronic structures of model interfaces of organic electroluminescent (EL) devices were investigated with UV photoemission spectroscopy (UPS). At model interfaces of an Al/Alq(3)/TPD/Au device (Alq(3):tris(8-hydroxyquinolino) aluminum, TPD: N,N-1-diphenyl-N,N-1-(3-methylphenyl)-1,1(1)-biphenyl-4,4(1)- diamine) and DP-NTCI/metal interfaces (DP-NTCI: N,N-1-diphenyl-1,4,5,8-naphtyltetracarboxylimide), the vacuum level shifts were observed, indicating the invalidity of the traditional energy level alignment model where a common vacuum level is assumed at organic / metal interface. On the other hand, the results for Alq(3)/TPD interface showed little vacuum level shift, leading to apparent applicability of the traditional model. The observed interfacial electronic structures corresponds well to the injecting nature of the interface. The effect of air exposure to the interfacial electronic structure and the relation between the vacuum level shifts and the substrate work function were discussed, together with the results for porphyrin/metal interfaces.