Transparent ZnO Electrode for Liquid Crystal Displays

Abstract

Recently, the scarcity and toxicity of indium, a major constituent element of ITO, has become a concern. Indium is a rare element that ranks 61st in abundance in the Earth’s crust (Kempthorne & Myers 2007). In addition, the major amounts of indium consumed by the industries produceing the electronic devices such as liquid crystal displays (LCDs), touchscreens and solar cell systems are supplied by only a few countries. Furthermore, indium has also been suspected to induce lung disease, and particularly indium-related pulmonary fibrosis should be paid attention (Homma et al., 2005). Transparent conductive oxides have become the focus of attention as a substitute material for ITO currently used for optically transparent electrodes in electronic devices. In particular, transparent conductive ZnO films are expected to be suitable materials to achieve such purposes because, in contrast with indium as a major constituent element of ITO, Zn is an element that the human body requires and is a component of some marketed beverages, in addition to having being used for years in cosmetics and as a vulcanization accelerator for rubber products such as tires. Furthermore, conductive and transparent ZnO films have low electrical resistance and high optical transmittance comparable with those of ITO films reported by some authors (Wakeham et al., 2009; Shin et al., 1999). We have developed the technology to form transparent conductive ZnO films with low resistance (2.4 m for a 100 nm thick film (Yamada, et al., 2007)), optical transmittance exceeding 95% (film-only transmittance without that of the glass substrate) and high heat-resistance (thermally stable until 300-450 °C (Yamamoto, N. et al., 2010)). The technology of transparent conductive ZnO films applied as alternatives to ITO electrodes for LCD panels is described in this chapter.