title: Impact of Molecular Orientation and Packing Density on Electronic Polarization in the Bulk and at Surfaces of Organic Semiconductors.
abstract: The polarizable environment surrounding charge carriers in organic semiconductors impacts the efficiency of the charge transport process. Here, we consider two representative organic semiconductors, tetracene and rubrene, and evaluate their polarization energies in the bulk and at the organic-vacuum interface using a polarizable force field that accounts for induced-dipole and quadrupole interactions. Though both oligoacenes pack in a herringbone motif, the tetraphenyl substituents on the tetracene backbone of rubrene alter greatly the nature of the packing. The resulting change in relative orientations of neighboring molecules is found to reduce the bulk polarization energy of holes in rubrene by some 0.3 eV when compared to tetracene. The consideration of model organic-vacuum interfaces highlights the significant variation in the electrostatic environment for a charge carrier at a surface although the net change in polarization energy is small; interestingly, the environment of a charge even just one layer removed from the surface can be viewed already as representative of the bulk. Overall, it is found that in these herringbone-type layered crystals the polarization energy has a much stronger dependence on the intralayer packing density than interlayer packing density.
authors.affiliation: Solar and Photovoltaics Engineering Research Center, Division of Physical Science and Engineering, King Abdullah University of Science and Technology , Thuwal 23599-6900, Kingdom of Saudi Arabia.|School of Chemistry and Biochemistry, Center for Organic Photonics and Electronics, Georgia Institute of Technology , Atlanta, Georgia 30332-0400, United States.
authors.forename: Sean M
authors.initials: SM
authors.lastname: Ryno
authors.affiliation: Department of Chemistry, Center for Applied Energy Research, University of Kentucky , Lexington, Kentucky 40506-0055, United States.
authors.forename: Chad
authors.initials: C
authors.lastname: Risko
authors.affiliation: Solar and Photovoltaics Engineering Research Center, Division of Physical Science and Engineering, King Abdullah University of Science and Technology , Thuwal 23599-6900, Kingdom of Saudi Arabia.
authors.forename: Jean-Luc
authors.initials: JL
authors.lastname: Brédas
keywords: bulk; intermolecular interactions; molecular packing; organic semiconductors; polarization energy; surface