Role of intrinsic band-gap states for the energy level alignment at weakly interacting organic-conductor interfaces: gap states versus band dispersion in pentacene thin films

Abstract

We observed very small electronic density of states in the band gap of pentacene thin films deposited on inert surfaces using ultraviolet photoelectron spectroscopy (UPS) with ultrahigh sensitivity. We found, furthermore, that a pentacene film with less density of gap states gives a splitting of the HOMO band in UPS spectra with energy separation of about 0.45 eV due to the band dispersion even for ultrathin polycrystalline films. The results indicate that the gap states do not originate from electronic interaction between pentacene and the substrate surface but from imperfect molecular orientation/packing structure. We confirmed that the Fermi level pinning in the pentacene films originates from the intrinsic gap states depending on their density and energy distribution. The Fermi level position as well as appearance of the band dispersion in pentacene thin films therefore depends sensitively on perfectness of the molecular packing structure in each crystal grain.