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 (EHOMO) 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 EHOMO and the oxidation potential. The EHOMO'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 (VCV) measured using pulsed cyclic voltammetry, leading to the relationship EHOMO = −(1.4 ± 0.1) × (qVCV) − (4.6 ± 0.08) eV, consistent with theoretical predictions.