Molecular potentials in organic thin films for electronic devices

Abstract

Molecular potentials are responsible for significant effects in various systems of organic thin films. Molecular potentials are combined with self-consistent electronic polarization calculations to determine layer-dependent ionization potentials in representative organic films. The results are used to model x-ray or ultraviolet photoemission spectra of organic films. Calculations are applied to standing and lying films of pentacene and fluoropentacene. Similar polarizabilities and oppositely oriented quadrupoles of pentacene and fluoropentacene make these films ideal candidates to identify effects of molecular potentials separate from electronic polarization. Ionization energies are calculated for standing and lying films of both molecules and are shown to agree with experiment. Surface and subsurface dopants in upright pentacene films lead to ion pairs and molecular potentials different from pure pentacene films. Ion pairs cause changes in local ionization energies and electron affinities in doped pentacene films measured by scanning tunneling microscopy. The difference in potential between pure pentacene films and films with tetrafluorotetracyanoquinodimethane (F4TCNQ) dopants is calculated and related to experimental data.

Other models are developed to account for specific effects in organic films. This is particularly useful when there is no structural information available. The polarization energy of a dipole in a lattice of randomly oriented polarizable point dipoles helps account for unusual luminescence observed in 1,1-bis[(di-4-tolylamino)phenyl]-cyclohexane (TAPC) films. Vacuum level shifts due to interface dipoles of sub-monolayer films are also modeled. The non-linearity of these shifts is emphasized and related to collective charge transfer