Electronic structure of organic carrier transporting material / metal interfaces as a model interface of electroluminescent device studied by UV photoemission

Electronic structures of N,N′-diphenyl-N,N′-(3-methylphenyl)-1,1′-biphenyl-4,4′-diamine (TPD) / metal and N,N′-diphenyl-1,4,5,8-naphtyltetracarboxylimide (DP-NTCI) / metal interfaces were directly investigated as a model interface of organic electroluminescent (EL) devices using UV photoemission spectroscopy (UPS). At the organic / metal interfaces, abrupt shift of vacuum level was observed, in contrast to the traditional assumption of common vacuum level at the interface. For understanding EL devices, we see the necessity of the direct observation of the interfacial electronic structure by UPS or other techniques for understanding EL devices.