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-quinoline) aluminum (Alq) and alkali metal (Li and K) was studied by ultraviolet and X-ray photoelectron spectroscopy (UPS and XPS). Condensed molecular solid films of Alq were doped by alkali metal deposited on the surface. The alkali metal atoms were found to distribute uniformly through out the depth to which the sample could be studied. A new feature of the density of states was observed in the band gap, 1.7eV higher than the HOMO of the pristine Alq. The binding energy of N(1s) core level of the quinoline ligand of Alq was shifted by doping to lower binding energies, of 1.6 eV and 1.8 eV from the original peak for Li and K doping, respectively. The intensity of the new shifted peak was saturated when Alq was fully doped. The full doping level, determined by chemical analysis (XPS), was Alq:metal=1:3.