Multitechnique Approach for Determining Energy Levels and Exciton Binding Energies of Molecules for Organic Electronics

The energies of the frontier orbitals and their electronhole binding energy are key quantities in organic electronics, such as organic light-emitting diodes (OLEDs) and solar cells. A method is developed for obtaining these quantities from ultraviolet photoelectron spectroscopy (UPS) and optical absorption spectroscopy, combined with calculations of the ionization energy (IE) and the electron affinity (EA). This method provides a more efficient and accurate determination of the lowest unoccupied molecular orbital (LUMO) together with the exciton binding energy. The concept is extended to element-sensitive core-level spectroscopies, such as X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS), in order to obtain core exciton binding energies. Two prototypical organic molecules for OLED applications are used as examples, i.e., N,N'-di(1-naphthyl)-N,N'-diphenyl-(1,1'-biphenyl)-4,4'-diamine (NPD) and dipyrazino[2,3-f:2',3'-h]quinoxaline-2,3,6,7,10,11-hexacarbonitrile (HAT:CN). These methods have the potential to facilitate the systematic tailoring of molecules for applications in organic electronics. The method devised for determining the LUMO is consistent with previous reports using conventional techniques.