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

Abstract

We determined the orbital lineup of the tris (8-hydroxyquinolinato) gallium (Gaq3)/Mg interface using combined x-ray and ultraviolet photoemission spectroscopy (XPS and UPS) measurements. The Gaq3/Mg system is a prototypical model structure for organic electron/low work function electrode transporting materials interfaces found in organic light emitting diodes (OLED). A Gaq3 thin film was grown in 15 steps on a previously sputter-cleaned Mg substrate starting at a 1 Å nominal thickness up to a final thickness of 512 Å. Before, and in between the growth steps, the sample surface was characterized by XPS and UPS. The results indicate the formation of a reaction layer of about 12 Å thickness at the Mg interface, which resulted in a 0.96 V interface dipole potential. At Gaq3 coverages higher than 256 Å, a strong charging shift occurred in the overlayer related 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 function of the reacted interface layer, the Gaq3 electronic states are aligned at a binding energy below the substrate Fermi edge that exceeds the magnitude of the optical gap between the highest occupied and lowest unoccupied molecular orbitals (HOMO and LUMO). This allowed the conclusion that the ground state exciton binding energy of Gaq3 needs to be larger than 0.43 eV. Based on these considerations, the lowest possible electron injection barrier matching the experimental data was estimated to be 0.15 eV.