Photoemission study of the electronic structures of tris-(8- hydroxyquinoline) aluminum/ Li2 OAl interfaces

The electronic structures of tris-(8-hydroquinoline) aluminum (Al q3) Li2 OAl interfaces were studied using in situ ultraviolet and x-ray photoelectron spectroscopies (UPS and XPS). The UPS and XPS spectra allowed us to evaluate the complete energy level diagrams and to analyze the chemical interactions at the interfaces. Inserting Li2 O between Al and Al q3 led to the highest occupied molecular orbital (HOMO) of Al q3 shifting to a higher binding energy compared to that without Li2 O, which resulted in an improved electron injection. We also observed that the magnitude of the secondary cutoff shift was almost identical to that of the HOMO shift with the insertion of Li2 O. This implies that the energy level alignment depends on the interface dipole and ionization energy of the adsorbate. Additionally, a gap state was observed in the gap of Al q3, which is related to the interfacial reaction. The N 1s spectra revealed that there were destructive chemical reactions between Al q3 and Al, which could be prevented by inserting Li2 O between them.