Origin of the highest occupied band position in pentacene films from ultraviolet photoelectron spectroscopy: Hole stabilization versus band dispersion

The electronic structure of pentacene/graphite, pentacene/Cu-phthalocyanine (CuPc)/graphite, and pentacene/SiO2/Si(100) was studied by ultraviolet photoelectron spectroscopy as a function of the thickness of the pentacene film. We observed that the binding energy position of the highest occupied molecular orbital (HOMO) of pentacene becomes significantly lower on CuPc and SiO2 surfaces than that on graphite. Furthermore, a splitting of the UPS band was observed only for the HOMO of thicker crystalline films on CuPc and SiO2 surfaces, where the molecules are oriented with their long axis nearly perpendicular to the substrate surface. The splitting is ascribed to the intermolecular band dispersion, and the decrease in the threshold ionization energy on CuPc and SiO2 surfaces originates from the HOMO-band dispersion as well as the increase in the relaxation/polarization energies, which may be caused by the better molecular packing structure with a nearly standing molecular orientation.