Energy level alignment and band bending at model interfaces of organic electroluminescent devices

The basic concepts of common vacuum level and band bending in Mott–Schottky (MS) model was experimentally examined for the model interfaces of organic electroluminescent devices by using UV photoemission spectroscopy (UPS) and Kelvin probe method (KPM). We found that interfacial dipole cannot be neglected at most organic/metal interfaces and that the potential shift at the interface due to such dipole sometimes reaches over 1 eV in contrast to the assumption of the common vacuum level in MS model. Based on the observed data, possible origins of the interfacial dipole and general trends of the potential shift against the work function of the electrode metal were proposed. The band bending and Fermi level alignment were also examined at the interfaces between metals (Au, Cu, Ag, Mg, Ca) and TPD(N,N′-bis(3-methylphenyl)- N,N′- diphenyl-[1,1′-bisphenyl]-4,4′-diamine).