Investigation of band bending and charging phenomena in frontier orbital alignment measurements of para-quaterphenyl thin films grown on highly oriented pyrolytic graphite and SnS2

Thin films of the polyparaphenylene (PPP) molecule para-quaterphenyl (p-4P) were grown in ultrahigh vacuum in a multi-step growth procedure on in situ cleaved highly oriented pyrolytic graphite (HOPG) and single crystals of the layered semiconductor SnS2. Prior to growth and after each growth, combined X-ray and ultraviolet photoelectron spectroscopy (XPS, UPS) measurements were carried out to determine the electronic structure of the interfaces. PPP organic interfaces are of interest because of the emissive properties of PPP molecules and their potential use in organic light emitting diodes and displays. The large difference between the SnS2 and HOPG work functions (SnS2: 5.38 eV; HOPG: 4.65 eV) allowed the quantitative examination of band bending occurring in the p-4P layer due to the equilibration of the substrate and p-4P Fermi levels. The combination of UPS and XPS measurements allowed for separation of band bending, charging, and interface dipole-related shifts of the high binding energy cutoff (secondary edge) of the UP-spectra which are needed for the precise determination of the interface dipole and the highest occupied and lowest unoccupied molecular orbital alignment relative to the substrate electronic structure.