Function and fatigue of oxide electrodes in organic light emitting diodes
Prof. Dr. A. Klein (Div. Surface Science)
During the first period of the project, extensive investigations on ITO surfaces and its interfaces to organic materials were carried out using in-situ photoelectron spectroscopy. It could be clearly shown, that the major contribution to the variation of the ITO work function is a shift of the Fermi level position rather than a modification of the surface dipole, an aspect which has been widely neglected in literature, yet. It was furthermore demonstrated that the decrease and increase of Fermi level position of ITO is related to the incorporation and removal of oxygen, respectively. This is enhanced by the special defect structure of ITO, providing a large number of so-called structural vacancies and by the electronic structure of the ITO surfaces. During interface formation with organic molecules, oxygen is relieved from the ITO, leading to a decomposition of the organic material. At the same time, the Fermi level position at the ITO surface is shifted in such a way, that the barrier height for injection of holes is increased (reduction of work function). This effect could be accounted for the increase of voltage at constant current during operation of organic light emitting diodes. Thus, the degeneration of the organic molecules as well as the change of barrier height contribute to the electrical fatigue of organic optoelectronic devices. In the current period of the project the oxygen exchange, in particular the kinetics, at ITO surfaces will be investigated systematically. For this purpose, time-dependent conductivity measurements in different gas atmospheres (relaxation experiments), tracer diffusion measurements using the isotope 18O and photoemission at high pressure will be conducted. This methodology will be subsequently used in order to characterize modified ITO surfaces regarding variations of oxygen exchange. As modification of ITO for processing of organic light emitting diodes, various surface treatments and inorganic coatings are considered. In order to demonstrate the influence of the oxygen exchange and the benefit of surface modifications, organic light emitting diodes will be continuously fabricated during the project. Furthermore, SnO2 based electrode materials will be investigated with regard to their possible use in organic light emitting diodes.