Polarization double barriers at the interfaces in organic multilayered structures and superlattices

Abstract

At the interfaces of organic multilayered molecular structures (OMSs) the specific type of potential profile for electrons and holes, called here the polarization double barrier (PDB), is shown to appear due to the difference in polarization energies P of charge carriers in each layer. Such a PDB originates from the solid state effect of opposite changes of bandgaps in each layer and corresponds to attraction of both electrons and holes to the interfaces in molecular layer B (with low P B) and repulsion of both carriers from the interface in layer A (with high P A). The PDB is quite different from p-n junctions in doped semiconductors due to its symmetric effect on charge carriers, independent of their sign. Depending on the properties of single molecules in each layer, the PDB can lead either to selective charge injection from one layer to another or cause charge confinement at the interfaces in molecular superlattices of various types and may influence significantly photophysical and optical properties of OMSs. Among various fields of organic molecular solids to which Professor Hiroo Inokuchi has greatly contributed, ultraviolet photoelectron spectroscopy (UPS) occupies a special place. The concept of polarization energy P of a charge carrier has been accepted by researchers in the field of organic solids only after careful analysis by Inokuchi and co-workers of UPS spectra of more than 100 organic solids and experimental determination of their P values. Since 1963 this long-term research has led to the understanding of how physical properties of single molecules, molecular arrangement and strength of intermolecular interactions control the P value. Their remarkable findings that some molecular solids have extremely large P (3–3.5 eV, e.g., p-iodanil while other solids (like rubrene and TTCn-TTF) may have very low P (0.9–1.0 eV) as compared to typical polycyclic aromatic hydrocarbons (all having nearly the same P (1.6–137 eV)) has inspired us to try to understand the electronic properties of the interface between organic solids with largely different P values.