Frontier electronic structures in fluorinated copper phthalocyanine thin films studied using ultraviolet and inverse photoemission spectroscopies

Ultraviolet photoemission and inverse photoemission spectroscopy was used to compare the electronic structure of copper phthalocyanine, copper octafluorophthalocyanine, and copper hexadecafluorophthalocyanine thin films. A rigid shift to lower energy was observed for both the occupied and unoccupied electronic states as the number of fluorine atoms around the phthalocyanine ring increased. The spectral features of the fluorine-substituted derivatives were otherwise very similar to copper phthalocyanine, and no change in the transport gap energy was observed. Vacuum level shifts were observed at the interface with polycrystalline gold substrates of sufficient magnitude to consistently pin the substrate Fermi level near the middle of the HOMO-LUMO gap. The calculated barrier heights for electron and hole injection across the interface were therefore equal, and no correlation with fluorine substitution was found.