Ultraviolet photoelectron spectroscopy as a new method to determine energy gap values of molecular materials

摘要

Energy gap is one of the fundamental energetic characteristics of a material. In this paper a new method for the determination of the energy gap values in molecular materials from ultraviolet-photoelectron spectroscopy (UPS) is proposed. In a first part it is shown how UPS spectra are affected by intermolecular and intramolecular vibrations and polarizations. In a second part, from experimental data concerning radical phthalocyanines, and by comparison with results concerning non-radical phthalocyanines, fullerenes and polyacenes, it is established that the photoemission spectra give us information on the insulating or semiconducting behavior of these materials. In the case of undoped materials the energetic difference between the upper electron and the Fermi level allows us to determine the energy gap value. The UPS data are compared to those issued from other well known techniques. A limitation appears when extrinsic electronic levels move the Fermi level out of the middle of the gap.