Indium Tin Oxide (ITO)
Library ArchiveIndium Tin Oxide (ITO)
By Dr. Thomas Jansseune Product Manager Displays
Indium Tin Oxide "ITO" is a Umicore product with many diverse applications. Indium Tin Oxide has some peculiar properties that make it applicable as a thin film material in flat panel displays, as well as solar cells, touch screens, architectural glass with solar control or low emissivity, antistatic glass, smart windows, electromagnetic shielding, and even DNA-selecting chips (1).
In this article, we will shortly present two electro-optic applications that have shown tremendous growth over the last few years: flat panel displays and solar cells. Between 1996 and 2001, the total installed photovoltaic power increased with annual growth rates of between 25 and 40 % (2). The projected growth rate for flat panel displays is shown in Figure 1.
Transparent and conductive
Indium Tin Oxide is a TCO or transparent conductive oxide. As the name indicates these materials, when applied as a thin coating, are both transparent to visible light (wavelengths between 0.4 and 0.8 µm) as well as electrically conductive. This opens up a whole range of possibilities. For both applications discussed here the Indium Tin Oxide is used to establish an electric current over the device and to pass light through it.
Flat panel displays
Although many types of flat panel LCDs (liquid crystal displays) are available, the function of the Indium Tin Oxide layer essentially remains the same. The active layer – the liquid crystal – is sandwiched between two polyamide orientation layers and Indium Tin Oxide layers (Figure 2). These liquid crystals that are long molecules have the property of changing the plane of the polarized light, depending on their molecular orientation. The surface structure of the orientation layers keeps the long liquid crystal molecules aligned in a certain direction close to the surfaces of the glass pane. In the off state, the light entering the cell is polarized by the entrance polarizer. The liquid crystal molecules, twisted in the orientation layers by 90°, redirect the light so it can pass the exit polarizer that is positioned perpendicular to the entrance polarizer. This is the bright state. In the on state, the ITO layer applies an electric field over the liquid crystal layer. The liquid crystal molecules are forced in one plane, no longer changing the polarization of the light coming into the cell. As a result the light cannot pass the second polarizer. This is the dark state. Switching between dark and bright states makes the images appear on the screen.
In solar cells the function of the Indium Tin Oxide is somewhat different. The cross section of a typical thin film CdTe solar cell (apart from the glass layers) is shown in Figure 3. The sunlight passes through the top glass surface and the ITO layer and generates electricity in the active CdTe/CdS layers that act as a p/n junction.
Using the electrical conductivity of the ITO and the metallic back contact, a current can be generated through the cell. So – why ITO? Apart from the intrinsic properties of Indium Tin Oxide, such as low resistivity, that make it suitable for flat panel displays and solar cells, also processing parameters determine which TCO is chosen. Traditionally good etch ability and low deposition temperatures have been favoring Indium Tin Oxide (3). For both flat panel displays and solar cells ITO is applied by sputtering from ceramic targets.
Indium Tin Oxide "ITO" is one of the most important applications of indium. About 45% of all indium is used in Indium Tin Oxide (4). On numerous occasions where the success of e.g. flat panel displays is celebrated, the availability of indium has been questioned.
Virgin indium is only recovered as a by-product of metals like lead and zinc and «indium mines» have never been operated. In 2002 the world indium supply was still plentiful. However, the closure of Metaleurop’s 65 mt/y capacity indium refinery in France marked just the beginning of a run for indium. Moreover, indium, as a by-product of zinc production, suffers from historically low zinc prices and cut backs in zinc production. This has lead to shortages in indium concentrates, especially in China. Additionally, severe mining accidents caused the shut down of some Chinese operations. As a result, even Chinese indium producers are now contemplating the future of their indium refinery, and Liuzhou Zinc Group is even considering a total stop, which would take another 25 mt/y capacity off the market (5). On the free indium market, this has lead to considerable price increases and the unavailability of sizeable quantities of indium. Although this undoubtedly will have some effect on Indium Tin Oxide, the changes in the indium market are not expected to have a large impact on global availability in the near future. Since only a small part of the ITO target is effectively used during sputtering, up to 80 % of it can be recycled.
This forms an important source for secondary indium. Shifts in the indium market place, however,
will necessitate a better control over the total value chain. The ability to manage this complete cycle puts Umicore in a unique position. The Belgian indium refinery at the Umicore Precious Metals plant in Hoboken counts among the most important ones in the world (Table 1).
Additional recycling facilities are available at Thin Film Products in the USA. As such, Umicore is able to guarantee all its customers the reliable supply they expect, today and in the future.
1. G. A. Miller, Y.Y. Belosludtsev, T. H. Murphy and H.R. Garner Biomedical Microdevices 2:3, 215-220, 2000
2. Data: International Energy Agency
3. R. G. Gordon MRS Bulletin, vol. 25, no. 8, 52-57 (2000)
4. U.S. Geological Survey, Mineral Commodity Summaries, January 2003
5. Reuters News Service, May 9, 2000