Relationship between the ionization and oxidation potentials of molecular organic semiconductors

A relationship between the energy of the highest occupied molecular orbital (HOMO) and the oxidation potential of molecular organic semiconductors is presented. Approximating molecules as dipoles consisting of a positively charged ion core surrounded by an electron cloud, the HOMO energy (E-HOMO) is calculated as that required to separate these opposite charges in a neutral organic thin film. Furthermore, an analysis of image charge forces on spherical molecules positioned near a conductive plane formed by the electrode in an electrochemical cell is shown to explain the observed linear relationship between E-HOMO and the oxidation potential. The E-HOMO's of a number of organic semiconductors commonly employed in thin film electronic devices were determined by ultraviolet photoemission spectroscopy, and compared to the relative oxidation potential (V-CV) measured using pulsed cyclic voltammetry, leading to the relationship E-HOMO = -(1.4 +/- 0.1) x (qV(CV)) - (4,6 +/- 0.08) eV, consistent with theoretical predictions. (c) 2005 Elsevier B.V. All rights reserved.