Complete description of ionization energy and electron affinity in organic solids: Determining contributions from electronic polarization, energy band dispersion, and molecular orientation

Abstract

Ionization energy and electron affinity in organic solids are understood in terms of a single molecule perturbed by solid-state effects such as polarization energy, band dispersion, and molecular orientation as primary factors. However, no work has been done to determine the individual contributions experimentally. In this work, the electron affinities of thin films of pentacene and perfluoropentacene with different molecular orientations are determined to a precision of 0.1 eV using low-energy inverse photoemission spectroscopy. Based on the precisely determined electron affinities in the solid state together with the corresponding data of the ionization energies and other energy parameters, we quantitatively evaluate the contribution of these effects. It turns out that the bandwidth as well as the polarization energy contributes to the ionization energy and electron affinity in the solid state while the effect of the surface dipole is at most a few eV and does not vary with the molecular orientation. As a result, we conclude that the molecular orientation dependence of the ionization energy and electron affinity of organic solids originates from the orientation-dependent polarization energy in the film.

Understanding the Impact of Film Disorder and Local Surface Potential in Ultraviolet Photoelectron Spectroscopy of PEDOT

“…It is however expected that when intermolecular interactions play an important role, new features in the UPS spectra coming from extended delocalization due to, e.g., hybridization of π‐orbitals or intermolecular polarization cannot be captured by the description based on the DFT calculations for a single molecule in the gas phase. Moreover, UPS spectra of the ordered molecular and polymeric systems showed a strong anisotropy and a pronounced orientation dependence with respect to the substrate or to mixing of differently terminated molecules on a molecular length scale .…”

Band structure engineering in organic semiconductors

“…In ordered thin films, large orientation dependencies of the IE were seen for molecules containing polar bonds (25). For highly ordered thin films of pentacene and perfluoropentacene, this was explained by an orientation dependence of the polarization energy originating from different charge-quadrupole interactions (26,27).…”

Electron Transport in Bathocuproine Interlayer in Organic Semiconductor Devices

“…We have demonstrated that the electron affinity of organic materials can be determined with the precision comparable to the ionization energy determined by UPS. [27][28][29][30][31][32] Using this technique, we determined the electron affinity of BCP to be 1.9 eV. 30 This value is more than 1 eV smaller than the previously assumed one suggesting that the electron transport through the LUMO level of BCP is unlikely.…”