ABSTRACT
Recent technological advances in organic electroluminescence devices have triggered intensive research on the electronic structure at the interfaces between organic molecules and inorganic materials. Since the charge injection characteristics in the devices are governed by the electronic level alignment at interfaces, much effort in the surface scientific studies has been focused on the determination of the electronic structures. It has been done by various electron spectroscopic methods such as ultraviolet photoelectron spectroscopy, X-ray photoelectron spectroscopy, inverse photoemission, and scanning tunneling spectroscopy. Through experimental studies by several groups it has been found that dipole layers are formed at various organic-inorganic interfaces [1]. However, the precise mechanism has not been elucidated so far, and it is still impossible to predict the strength of dipole layers of a given interface. Suggested mechanisms include charge transfer, molecular orbital hybridization, polarization, and influence of metal-induced gap states, which are interesting by themselves as subjects in quantum chemistry.
