Surface preparation and characterization of indium tin oxide substrates for organic electroluminescent devices

Abstract

The cleanliness of indium tin oxide (ITO) sub- strates used in organic light-emitting diodes (OLEDs) is in- vestigated by contact angle measurement and by X-ray pho- toemission spectroscopy (XPS). It was found that ultraviolet (UV) ozone treatment is quite effective in removing organic contamination on the ITO surface. The degree of surface con- tamination was checked by changes in contact angles and by XPS. Strong correlation can be established between these two techniques. OLEDs fabricated from UV-irradiated ITO sub- strates exhibit low turn-on voltage and superior brightness. There are now widespread interests in using organic or poly- meric fluorescent thin films for fabricating electroluminescent (EL) devices (1, 2). The basic structure of an organic EL de- vice consists of one or more layers of organic fluorescent materials sandwiched between an anode and a metal cathode. In most cases, a thin film of indium tin oxide (ITO), with a thickness in the order of 0:1 mm, is used as the anode ma- terial. The ITO layer can be prepared by standard sputtering techniques onto a glass plate or a plastic substrate. Since ITO is a transparent conductor, it also functions as the viewing side for the EL device. Although there are now numerous re- ports on EL devices fabricated by many different materials, few reports have been devoted to the preparation of ITO for EL devices. Since organic EL devices are thin film devices, a small amount of contamination on the surface of the an- ode can severely alter the work function, or the interfacial barrier height between the organic layer and the anode. Un- less the anode is thoroughly cleaned, the electrical and optical characteristics can be highly unstable and unpredictable. It is, therefore, of utmost importance that the ITO substrate is carefully cleaned before deposition of the organic layers. A variety of methods (3-8) have been developed in the preparation of ITO surfaces for organic light-emitting diodes (OLEDs). A summary of these methods is shown in Table 1.