Preparation and characterization of the ITO surface and the Al/Alq3/ITO heterostructure for OLEDs

We have made a detailed investigation of structure and chemical composition in thermally grown Al/Alq3/ITO layer structures intended for light emission. The different parts of the structure were studied by X-ray photoelectron spectroscopy and microscopical techniques. The indium tin oxide substrate surface consisted of grains 20–50 nm in diameter and ∼5 nm in height, which also showed molecular sized sub-grain features. Due to the ambient the ITO surface was covered by a ∼1 nm thick hydrocarbon layer which serves as the actual surface on which an organic layer is deposited for device fabrication. A structural model for the indium tin oxide surface, on which the small molecules were deposited, was suggested. The substrate surface was treated to study the relation between the surface manipulation and the hole injection barrier. Such surface modification significantly improved the performance of a single layer Alq3 organic light emitting diode. Depth profiling of the complete structure, revealed that the aluminium contact mostly consisted of aluminium oxide with some intermixing of carbon. Further, variations of the atomic concentration of the elements In, Sn, C and O and a minor shift in their binding energies were measured. Both, position and shape of the oxygen and indium peaks were changed during argon ion sputtering. At the Alq3/ITO interface an indium–oxygen compound, different from that on the original surface, had formed. Various effects of the compositional findings on the light emission are discussed.