Oxidation of LiF –Coated Metal Surfaces: Multilayer Cathode Structures as Used for Organic Optoelectronics

X-ray photoelectron spectroscopy was used to study the growth of oxides on the surface of Al and Mg films with and without a thin LiF coating under ambient conditions. At thicknesses typically used in optoelectronic device cathodes, LiF does not completely cover the surface, likely forming islands on the metal surface. On Al, 10Å LiF (61% coverage) is sufficient to significantly decrease oxidation. The passivation of Al surfaces is enhanced due to a diffusion dominated oxidation mechanism, with metal ions diffusing through the LiF islands. LiF coated Mg, on the other hand, shows preferential oxidation to form MgCO3 on the surface. These changes in the oxidation of the surface due to the introduction of a LiF layer can be used to explain the recent results for organic light-emitting devices. Bulk lattice constants can be used as a guide to predicting oxidation resistance, with matching interlayers providing better resistance in devices than nonmatching ones.