XPS and UPS study of charge transport material electrode interface of light emitting diodes.

The electronic structure of the interfacial region between tris(8-hydroxy-q uinoline) aluminum (Alq) and alkali metal (Li and K) was studied by ultravi olet and X-ray photoelectron spectroscopy (UPS and XPS). Condensed molecula r solid films of Alq were doped by alkali metal deposited on the surface. T he alkali metal atoms were found to distribute uniformly through out the de pth to which the sample could be studied. A new feature of the density of s tates was observed in the band gap, 1.7eV higher than the HOMO of the prist ine Alq. The binding energy of N(ls) core level of the quinoline ligand of Alq was shifted by doping to lower binding energies, of 1.6 eV and 1.8 eV f rom the original peak for Li and K doping, respectively. The intensity of t he new shifted peak was saturated when Alq was fully doped. The full doping level, determined by chemical analysis (XPS), was Alq:metal=1:3.