Effect of interfacial layer thickness on the formation of interface dipole in metal/tris(8-hydroxyquinoline) aluminum interface

We determined the interface dipole energies between interfacial layers with different thicknesses coated on indium tin oxides (ITOs) and tris(8-hydroxyquinoline) aluminum using ultraviolet and synchrotron radiation photoemission spectroscopy. After O2 plasma treatment on 20-nm thick metal coated ITO, the work function and interface dipole energy increased. In 2-nm thick metal coated ITO, no change in the interface dipole energy was found though the work function increased. According to the valence band spectra, 2-nm thick metals are fully oxidized, but 20-nm thick metals are partially oxidized after O2 plasma treatment. Therefore, it is considered that the contribution of the surface dipole by the deposition of Alq3 on 2-nm thick metal is lower, resulting in a lower interface dipole. Thus, the thickness of interfacial layer has a great impact on the formation of interface dipole.