Indium tin oxide (ITO, or tin-doped indium oxide) is a solid solution of indium(III) oxide (In₂O₃) and tin(IV) oxide (SnO₂), typically 90% In₂O₃, 10% SnO₂ by weight. It is transparent and colorless in thin layers while in bulk form it is yellowish to grey. In the infrared region of the spectrum it is a metal-like mirror.

Indium tin oxide is one of the most widely used transparent conducting oxides because of its two chief properties, namely electrical conductivity and optical transparency. Like other transparent conducting oxides, a compromise has, however, to be reached during its film deposition, as high concentration of charge carriers will increase the material's conductivity, but decrease its transparency.

Thin films of indium tin oxide are most commonly deposited on surfaces by electron beam evaporation, physical vapor deposition, or a range of sputter deposition techniques.

Common Uses

ITO is mainly used to make transparent conductive coatings for liquid crystal displays, flat panel displays, plasma displays, touch panels, electronic ink applications, organic light-emitting diodes, solar cells, antistatic coatings and EMI shieldings. In organic light-emitting diodes, ITO is used as the anode (hole injection layer).

ITO has been used as a conductive material in the plastic electroluminescent lamp of toy Star Wars type lightsabers.^[1]

ITO is also used for various optical coatings, most notably infrared-reflecting coatings (hot mirrors) for architectural, automotive, and sodium vapor lamp glasses. Other uses include gas sensors, antireflection coatings, electrowetting on dielectrics, and Bragg reflectors for VCSEL lasers.

ITO was used as a sensor coating in the later Kodak DCS cameras, starting with the Kodak DCS 520, as a means of increasing blue channel response.^[2] It is reportedly used as a sensor coating in the Canon 400D/XTi and Sony Alpha DSLR-A100^{[citation needed]}.

ITO thin film strain gauges can operate at temperatures up to 1400 °C and can be used in harsh environments, e.g. gas turbines, jet engines, and rocket engines.^[3]

Alternatives

Because of high cost and limited supply of indium, the fragility and lack of flexibility of ITO layers, and the costly layer deposition requiring vacuum, alternatives are being sought. Carbon nanotube conductive coatings are a prospective replacement. These coatings are being developed by Canatu, Eikos and Unidym as a more mechanically robust alternative to ITO. More recently, thin metal films are also seen as a potential candidate to replace indium based transparent conductors. Inherently conductive polymers (ICPs) are also being developed for some ITO applications. Typically the conductivity is lower for conducting polymers than inorganic materials, but they are also more flexible, inexpensive and environmentally friendly in processing and manufacture. The most established ICP vendors are AGFA and H.C. Starck, which supply PEDOT:PSS directly. PEDOT:PSS layers are in use (though they degrade when exposed to ultraviolet radiation and have other disadvantages). Fibron Technologies is manufacturing polyaniline, and other conducting polymer nanofibers and nanostructures with similar conductivities to HC Starck's EDOT based materials, with lower environmental impact. Other alternatives are e.g. aluminium-doped zinc oxide. Cambrios, founded in 2002 by Dr. Angela Belcher of MIT and Dr. Evelyn Hu of Harvard University, has an inorganic wet-processable transparent conductive film alternative for ITO,^{[citation needed]} as does Cima NanoTech.

Constraints and trade-offs

The main concern about ITO is the cost. ITO can be priced at several times that of aluminum zinc oxide (AZO). AZO is a common choice of transparent conducting oxide (TCO) because of cost and relatively good optical transmission performance in the solar spectrum. However, ITO does consistently defeat AZO in almost every performance category including chemical resistance to moisture. ITO is not affected by moisture and it can survive in a CIGS cell for 25–30 years on a rooftop. While the sputtering target or evaporative material that is used to deposit the ITO is significantly more costly than AZO, consider that the amount of material placed on each cell is quite small. Therefore the cost penalty per cell is quite small too.

Benefits

Surface morphology changes in Al:ZnO and i-/Al:ZnO upon dump heat (DH) exposure (optical interferometry)^[4]

The case for using ITO simply is that if moisture does penetrate, ITO will degrade less than AZO.

Research examples

ITO can be used in nanotechnology to provide a path to a new generation of solar cells. Solar cells made with these devices have the potential to provide low-cost, ultra-lightweight, and flexible cells with a wide range of applications. Because of the nanoscale dimensions of the nanorods, quantum-size effects influence their optical properties. By tailoring the size of the rods, they can be made to absorb light within a specific narrow band of colors. By stacking several cells with different sized rods, a broad range of wavelengths across the solar spectrum can be collected and converted to energy. Moreover, the nanoscale volume of the rods leads to a significant reduction in the amount of semiconductor material needed compared to a conventional cell.^[5]

See also

• Transparent conducting film

References

External links

• Spectroscopic studies of conducting metal oxides, with many slides about ITO

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