Organic semiconductor interfaces: Discrimination between charging and band bending related shifts in frontier orbital line-up measurements with photoemission spectroscopy

Abstract

Gaq 3 is a promising luminescent organic semiconductor for applications in organic light emitting diodes. The frontier orbital alignment at the tris (8-hydroxyquinolinato) gallium (Gaq3)/Pt organic Schottky contact was determined by combined x-ray and ultraviolet photoemission spectroscopy (XPS, UPS) measurements. A Gaq3 thin film was deposited in several steps on a previously Ar+ sputtered pure Pt foil. After each growth step, the sample was characterized by XPS and UPS. The combination of XPS and UPS measurements allows the precise evaluation of the interface dipole independent from the simultaneously occurring band bending at the interface and charging artifacts. The measurements show that the Pt/Gaq3 interface has a strong dipole of 0.71 eV indicating the transfer of negative charge from Gaq3 to Pt. Due to the large work function difference between Pt and Gaq3, strong band bending occurred. At Gaq3 coverages higher than 128 Å strong charging shifts occurred in the overlayer related emission lines which were identified by measuring the high binding energy cutoff (secondary edge) of both the XP and UP spectra. Due to the several magnitudes difference between the x-ray and ultraviolet source photon intensities, differences between the high binding energy cutoff positions of both measurements allow to pinpoint charging shifts with high sensitivity. In order to investigate the possible influence of radiation damage to the Gaq3 film, a single growth step control film of Gaq3 of the same thickness as in the multi-step orbital line-up procedure was deposited. The comparison of XPS peak intensities and positions suggest that no significant chemical alteration occurred during the photoemission spectroscopy measurements of the multi-step deposition experiment. However, the doping level of the layer may have changed due to the radiation exposure during the measurements.