Criteria for ITO (indium–tin-oxide) thin film as the bottom electrode of an organic light emitting diode

Abstract

TCO (transparent conductive oxide) thin films such as ITO (indium tin oxide) have important roles in fabrication of OLED (organic light emitting diode) displays. Although ITO is the most widely used transparent conductor in the field of FPD (flat panel display) industry, there are some unique requirements from the viewpoint of OLED, which is different from other display devices such as LCD (liquid crystal display) and PDP (plasma display panel). Especially surface roughness and work function are very important to enhance the stability and efficiency of OLED. Because all functional organic layers which act as injection, transportation and emission layers are deposited on ITO, surface morphology of ITO is directly transferred to them and uneven interface is not desirable for the efficiency and stability of OLED. On the other hand, there are several different terminologies— R a (average roughness), R r.m.s. (root-mean-square roughness) and R pv (peak to valley roughness)—which are used to interpret the surface roughness, are now all used altogether. In this work, we tried to define which parameter is the most suitable to characterize ITO for OLED and to explain the relationship between surface and I– V characteristics of OLED. To characterize the effect of surface morphology of the ITO electrode, light emitting diodes were fabricated using various types of ITO films which have different surface morphology and their dynamic characteristics were compared.