Photoelectron spectroscopy studies on the electronic structures of Al/RbF and Al/CaF2 cathodes for 8-hydroxyquinoline aluminium-based organic light-emitting devices

Abstract

The electronic structures of Al/RbF/8-hydroxyquinoline aluminium (Alq3) and Al/CaF2/Alq3 interfaces were investigated using x-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS). For both systems, the UPS showed that a significant valence band shift occurred by evaporating the thin fluoride layer on Alq3. However, the gap state formation and the evolution of N 1s core level spectra showed rather different trends, suggesting that the alkali fluoride and alkali–earth fluoride interlayer have different reaction mechanisms at the interface with the Al cathode. In addition, the deposition of Al has considerably less effect on the valence band shift compared to the deposition of both RbF and CaF2. These results suggest that the complex charge transfer mechanism across the interface has considerably less effect on the enhancement of organic light-emitting device performance than the significant valence band shift as a result of deposition of interlayer materials.