Indium tin oxide
|Melting point||1800–2200 K (1526-1926 °C) (2800–3500 °F)|
|Density||7120–7160 kg/m3 at 293 K|
|Color (in powder form)||Pale yellow to greenish yellow, depending on SnO2 concentration|
|Values vary with composition. SI units and STP are used except where noted.|
Indium tin oxide (ITO) is a ternary composition of indium, tin and oxygen in varying proportions. Depending on the oxygen content, it can either be described as a ceramic or alloy. Indium tin oxide is typically encountered as an oxygen saturated composition with a formulation of 74% In, 18% O2, and 8% Sn by weight. Oxygen saturated compositions are so typical, that unsaturated compositions are termed oxygen deficient ITO. 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 acts as a metal-like mirror.
Indium tin oxide is one of the most widely used transparent conducting oxides because of its two chief properties, its electrical conductivity and optical transparency, as well as the ease with which it can be deposited as a thin film. As with all transparent conducting films, a compromise must be made between conductivity and transparency, since increasing the thickness and increasing the concentration of charge carriers will increase the material's conductivity, but decrease its transparency.
ITO is often used to make transparent conductive coatings for displays such as liquid crystal displays, flat panel displays, plasma displays, touch panels, and electronic ink applications. Thin films of ITO are also used in 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 films deposited on windshields are used for defrosting aircraft windshields. The heat is generated by applying voltage across the film.
ITO is also used for various optical coatings, most notably infrared-reflecting coatings (hot mirrors) for automotive, and sodium vapor lamp glasses. Other uses include gas sensors, antireflection coatings, electrowetting on dielectrics, and Bragg reflectors for VCSEL lasers. ITO is also used as the IR reflector for low-e window panes. ITO was also 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.
ITO is a heavily-doped n-type semiconductor with a large bandgap of around 4 eV. Because of the bandgap, it is mostly transparent in the visible part of the spectrum and its extinction coefficient, k, in this wavelength range is zero. In the ultraviolet (UV), it is opaque, so that k is non zero in the UV spectral range, because of band-to-band absorption (a UV photon can excite an electron from the valence band to the conduction band). It is also opaque in the near infrared (NIR) and infrared (IR), because of free carrier absorption (an infrared photon can excite an electron from near the bottom of the conduction band to higher within the conduction band). In this wavelength range k is non-zero, and reaches its maximum value in the IR regime, similar to the behavior of k for metals.
Alternative Methods of Deposition and Alternative Materials
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 and lower cost methods of preparing ITO are being investigated. For example, using conventional methods but varying the ambient gas conditions to improve the optoelectronic properties.  Most promising, however, from a cost perspective is to eliminate the need for vacuum deposition equipment. This can be accomplished using a sol-gel method, which several groups are working on.
Alternative materials can also be used. Doped binary compounds such as aluminum-doped zinc-oxide (AZO) and indium-doped cadmium-oxide have been proposed as alternative materials. Other, inorganic alternatives include aluminum, gallium or indium—doped zinc oxide (AZO, GZO or IZO).
In addition, more experimental non-traditional materials have been proposed. Carbon nanotube conductive coatings are a prospective replacement. As another carbon-based alternative, films of graphene are flexible and have been shown to allow 90% transparency with a lower electrical resistance than standard ITO. Thin metal films are also seen as a potential replacement material. A hybrid material alternative currently being tested is an electrode made of silver nanowires and covered with graphene. The advantages to such materials include maintaining transparency while simultaneously being electrically conductive and flexible. Inherently conductive polymers (ICPs) are also being developed for some ITO applications. Typically the conductivity is lower for conducting polymers, such as polyaniline and PEDOT:PSS, than inorganic materials, but they are more flexible, less expensive and more environmentally friendly in processing and manufacture.
In order to reduce indium content, decrease processing difficulty, and improve electrical homogeneity, amorphous transparent conducting oxides have been developed. One such material, amorphous indium-zinc-oxide maintains short-range order even though crystallization is disrupted by the difference in the ratio of oxygen to metal atoms between In2O3 and ZnO. Indium-zinc-oxide has some comparible properties to ITO.  The amorphous structure remains stable even up to 500 °C, which allows for important processing steps common in organic solar cells. The improvement in homogeneity significantly enhances the usability of the material in the case of organic solar cells. Areas of poor electrode performance in organic solar cells render a percentage of the cell’s area unusable.
Constraints and trade-offs
|This section does not cite any references (sources). (April 2011)|
The main concern about ITO is the cost. ITO can be priced several times more highly than aluminium 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.
The primary advantage of ITO compared to AZO as a transparent conductor for LCDs is that ITO can be precisely etched into fine patterns. AZO cannot be etched as precisely: It is so sensitive to acid that it tends to get over-etched by an acid treatment.
Another benefit of ITO compared to AZO is that if moisture does penetrate,ITO will degrade less than AZO.
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.
- KODAK PROFESSIONAL: Technical Information Bulletin: Increasing the Blue Channel Response
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- Spectroscopic studies of conducting metal oxides, with many slides about ITO