Determination of the orbital lineup at reactive organic semiconductor interfaces using photoemission spectroscopy

We determined the orbital lineup of the tris (8-hydroxyquinolinato) gallium (Gaq(3))/Mg interface using combined x-ray and ultraviolet photoemission s pectroscopy (XPS and UPS) measurements. The Gaq(3)/Mg system is a prototypi cal model structure for organic electron/low work function electrode transp orting materials interfaces found in organic light emitting diodes (OLED). A Gaq(3) thin film was grown in 15 steps on a previously sputter-cleaned Mg substrate starting at a 1 Angstrom nominal thickness up to a final thickne ss of 512 Angstrom. Before, and in between the growth steps, the sample sur face was characterized by XPS and UPS. The results indicate the formation o f a reaction layer of about 12 Angstrom thickness at the Mg interface, whic h resulted in a 0.96 V interface dipole potential. At Gaq(3) coverages high er than 256 Angstrom, a strong charging shift occurred in the overlayer rel ated UPS-emission lines, which was identified by measuring the high binding energy cutoff (secondary edge) of both the XP and UP spectra. The several magnitudes different x-ray and ultraviolet source photon intensities allow pinpointing charging shifts with high sensitivity. Due to the low work func tion of the reacted interface layer, the Gaq(3) electronic states are align ed at a binding energy below the substrate Fermi edge that exceeds the magn itude of the optical gap between the highest occupied and lowest unoccupied molecular orbitals (HOMO and LUMO). This allowed the conclusion that the g round state exciton binding energy of Gaq(3) needs to be larger than 0.43 e V. Based on these considerations, the lowest possible electron injection ba rrier matching the experimental data was estimated to be 0.15 eV. (C) 2001 American Institute of Physics.