Indium tin oxide

absorption of glass and ITO glass

ITO grains on glass substrates with a few nanoparticle impuries

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 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.

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

Common uses

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 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 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.^[1]

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.^[2]

Material and spectral properties

ITO is a heavily-doped n-type semiconductor with a large bandgap of around 4 eV.^[3] Because of the bandgap, it is mostly transparent in the visible part of the spectrum. In the ultraviolet, it is opaque because of band-to-band absorption (a UV photon can excite an electron from the valence band to the conduction band). In the near infrared, it also opaque, 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).

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.^[4][5] 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.^[6] Thin metal films are also seen as a potential replacement material. 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. Other, inorganic alternatives include aluminium, gallium or indium—doped zinc oxide (AZO, GZO or IZO).

Constraints and trade-offs

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.

Benefits

Surface morphology changes in Al:ZnO and i-/Al:ZnO upon dump heat (DH) exposure (optical interferometry)^[7]

The primary advantage of ITO compared to AZO as a transparent conductor for LCDs is that ITO can be precisely etched into fine patterns.^[8] AZO cannot be etched as precisely: It is so sensitive to acid that it tends to get over-etched by an acid treatment.^[8]

Another benefit of ITO compared to AZO is that if moisture does penetrate, ITO will degrade less than AZO.^[7]

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.^[9]

See also

• Transparent conducting film

References

External links

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