Electronic structure of anode interface with molybdenum oxide buffer layer

The electronic structure at the alpha-NPD/MoO3/Au interfaces has been investigated with ultraviolet photoemission spectroscopy ( UPS), X-ray photoemission spectroscopy (XPS) and inverse photoemission spectroscopy (IPES). It was found that the MoO3 layer contains some number of oxygen vacancies prior to any treatment and gap states are induced by the partial filling of the unoccupied 4d orbitals of molybdenum atoms neighboring oxygen vacancies. The alpha-NPD thickness dependence of XPS spectra for the alpha-NPD/MoO3 system clearly showed that molybdenum atoms at the surface of the MoO3 film were reduced by alpha-NPD deposition through the charge-transfer interaction between the adsorbed alpha-NPD and the molybdenum atoms. This reduction at the alpha-NPD/MoO3 interface formed a large interface dipole layer up to -1.79 eV. The deduced energy-level diagram for the alpha-NPD/MoO3/Au interfaces describes the energy-level matching that explains well the significant reduction in the hole-injection barrier due to the MoO3 buffer layer. (C) 2009 Elsevier B.V. All rights reserved.