Core level energy differences between the surface and bulk of organic semiconductor films: The effect of electrostatic polarization energy

The difference in the C1s core level energies between the surface and bulk regions of an organic semiconductor thin film is examined for pentacene and perfluoropentacene (F-pentacene). The experimental method we have recently developed allows us to determine precisely the differences as well as the thickness of the surface region. The C1s binding energies in the outermost layer turn out to be larger than those in the bulk by 0.32 eV (pentacene) and 0.25 eV (F-pentacene). The results are successfully explained by the interaction energy between the point charge and induced dipole moments suggesting the electrostatic polarization energy plays a main role in the difference of the core level energy.