In situ determination of interface dipole energy in organic light emitting diodes with iridium interfacial layer using synchrotron radiation photoemission spectroscopy

The interface dipole energies between 4,4(')-bis[N-(1-naphtyl)-N-phenyl-amino]biphenyl and Ir interfacial layers with different thicknesses (2 and 20 nm) coated on indium tin oxides (ITOs) were measured in situ using synchrotron radiation photoemission spectroscopy. In 20 nm Ir coated ITO, the work function increment of 0.15 eV due to O-2 plasma treatment was accompanied by an increase of interface dipole energy. In 2 nm Ir coated ITO, no change in the interface dipole energy was found. Thus, the work function increase (0.45 eV) in the 2 nm Ir by O-2 plasma treatment reduced the hole injection barrier by about 0.45 eV.