Energy level alignment and exciton binding energies using model interfaces between metals and evaporable organic electroluminescent materials

We report on a scanning probe spectroscopy study of the electronic properties of model organic/metal interfaces. The experiments allow us to determine parameters that are critical in charge carrier injection and transport, as are the energy gap between positive and negative polaronic states and the height of the barrier for charge carrier injection at metal/organic interfaces. In combination with optical absorption measurements, we gauge the exciton binding energy, a parameter determining energy transport and electroluminescence efficiency. The study was performed on thin films of tris(8-hydroxyquinolato)aluminum (Alq(3)) deposited on clean and LiF-covered Au(111), and on N,N'-di(naphthalen-1-yl)-N,N'-diphenylbenzidine (NPB) on Ni(111) and substrates.