Indium tin oxide

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Indium tin oxide (ITO) is a

ternary composition of indium, tin and oxygen in varying proportions. Depending on the oxygen content, it can be described as either a ceramic or an alloy
. Indium tin oxide is typically encountered as an oxygen-saturated composition with a formulation of 74% In, 8% Sn, and 18% O 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 gray. In the infrared region of the spectrum it acts as a metal-like mirror.

Indium tin oxide is one of the most widely used

optical transparency, but also for the ease with which it can be deposited as a thin film, as well as its chemical resistance to moisture. 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
increases the film's conductivity, but decreases its transparency.

electron beam evaporation, or a range of sputter deposition
techniques.

Material and properties

Absorption of glass and ITO glass.

ITO is a mixed oxide of

touch-screen applications such as mobile phones
.

Common uses

Thin film interference caused by ITO coating on an Airbus
cockpit window, used for defrosting.

Indium tin oxide (ITO) is an optoelectronic material that is applied widely in both research and industry. ITO can be used for many applications, such as flat-panel displays, smart windows, polymer-based electronics, thin film photovoltaics, glass doors of supermarket freezers, and architectural windows. Moreover, ITO thin films for glass substrates can be helpful for glass windows to conserve energy.[2]

ITO green tapes are utilized for the production of lamps that are electroluminescent, functional, and fully flexible.[3] Also, ITO thin films are used primarily to serve as coatings that are anti-reflective and for liquid crystal displays (LCDs) and electroluminescence, where the thin films are used as conducting, transparent electrodes.[4]

ITO is often used to make transparent conductive coating for displays such as

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 a voltage across the film. ITO is also used to reflect electromagnetic radiation. The F-22 Raptor's canopy has an ITO coating that reflects radar waves, enhancing its stealth capabilities and giving it a distinctive gold tint.[5]

ITO is also used for various

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.[7]

ITO thin film strain gauges can operate at temperatures up to 1400 °C and can be used in harsh environments, such as gas turbines, jet engines, and rocket engines.[8]

Silver nanoparticle–ITO hybrid

ITO has been popularly used as a high-quality flexible substrate to produce flexible electronics.

nanoparticles (AgNPs) instead of homogeneously to create a hybrid ITO has proven to be effective in compensating for the decrease in transparency. The hybrid ITO consists of domains in one orientation grown on the AgNPs and a matrix of the other orientation. The domains are stronger than the matrix and function as barriers to crack propagation, significantly increasing the flexibility. The change in resistivity with increased bending significantly decreases in the hybrid ITO compared with homogeneous ITO.[12]

Alternative synthesis methods

ITO is typically deposited through expensive and energy-intensive processes that deal with physical

vapor deposition (PVD). Such processes include sputtering, which results in the formation of brittle layers.[citation needed
] Because of the cost and energy of physical vapor deposition, with the required vacuum processing, alternative methods of preparing ITO are being investigated.[13]

Tape casting process

An alternative process that uses a particle-based technique, is known as the tape casting process. Because it is a particle-based technique, the ITO nano-particles are dispersed first, then placed in organic solvents for stability.

polyvinyl butyral binder have been shown to be helpful in preparing nanoparticle slurries. Once the tape casting process has been carried out, the characterization of the green ITO tapes showed that optimal transmission went up to about 75%, with a lower bound on the electrical resistance of 2 Ω·cm.[3]

Laser sintering

Using ITO

nanoparticles can be converted easily into a conductive metal film under the treatment of laser, laser sintering is applied to achieve products' homogeneous morphology. Laser sintering is also easy and less costly to use since it can be performed in air.[15]

Ambient gas conditions

For example, using conventional methods but varying the ambient gas conditions to improve the optoelectronic properties[16] as, for example, oxygen plays a major role in the properties of ITO.[17]

Chemical shaving for very thin films

There has been numerical modeling of

photovoltaic (PV) cells. A problem that arises for plasmonic-enhanced PV devices is the requirement for 'ultra-thin' transparent conducting oxides (TCOs) with high transmittance and low enough resistivity to be used as device top contacts/electrodes. Unfortunately, most work on TCOs is on relatively thick layers and the few reported cases of thin TCO showed a marked decrease in conductivity. To overcome this it is possible to first grow a thick layer and then chemically shave it down to obtain a thin layer that is whole and highly conductive.[18]

Constraints and trade-offs

A major concern with ITO is its cost. ITO costs several times more than

copper indium gallium selenide solar 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, the amount of material placed on each cell is quite small. Therefore, the cost penalty per cell is quite small, too.

Benefits

optical interferometry)[19]

The primary advantage of ITO compared to AZO as a transparent conductor for

LCDs is that ITO can be precisely etched into fine patterns.[20] AZO cannot be etched as precisely: It is so sensitive to acid that it tends to get over-etched by an acid treatment.[20]

Another benefit of ITO compared to AZO is that if moisture does penetrate, ITO will degrade less than AZO.[19]

The role of ITO glass as a cell culture substrate can be extended easily, which opens up new opportunities for studies on growing cells involving

electron microscopy and correlative light.[21]

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.[22][23] Recent studies demonstrated that nanostructured ITO can behave as a miniaturized photocapacitor, combining in a unique material the absorption and storage of light energy.[24]

Health and safety

Inhalation of indium tin oxide may cause mild irritation to the

respiratory tracts and should be avoided. If exposure is long-term, symptoms may become chronic and result in benign pneumoconiosis. Studies with animals indicate that indium tin oxide is toxic when ingested, along with negative effects on the kidney, lung, and heart.[25]

During the process of mining, production and reclamation, workers are potentially exposed to indium, especially in countries such as China, Japan, the Republic of Korea, and Canada

nanoparticles existed in improved ITOs have been found in vitro to penetrate through both intact and breached skin into the epidermal layer. Un-sintered ITOs are suspected of induce T-cell-mediated sensitization: on an intradermal exposure study, a concentration of 5% uITO resulted in lymphocyte proliferation in mice including the number increase of cells through a 10-day period.[28]

A new occupational problem called

endotoxin to workers handling the wet process if in contact with endotoxin-containing liquids. This can be attributed to the fact that sITOs have larger diameter and smaller surface area, and that this change after the sintering process can cause cytotoxicity.[30]

Because of these issues, alternatives to ITO have been found.[31][32]

Recycling

Process of indium-tin-oxide (ITO) etching wastewater treatment

The

etching water used in the process of sintering ITO can only be used for a limited numbers of times before it has to be disposed. After degradation, the waste water should still contain valuable metals such as In and Cu as a secondary resource as well as Mo, Cu, Al, Sn and In, which can pose a health hazard to human beings.[33][34][35][36][37][38][39][40]

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, alternative materials are being investigated.[13] Promising alternatives based on zinc oxide doped with various elements.[41]

Doped compounds

Promising alternatives based on zinc oxide doped with various elements.[42]

Several transition metal dopants in indium oxide, particularly molybdenum, give much higher electron mobility and conductivity than obtained with tin.

aluminum, gallium
or indium-doped zinc oxide (AZO, GZO or IZO).

Carbon nanotubes

Carbon nanotube conductive coatings are a prospective replacement.[44][45]

Graphene

As another carbon-based alternative, films of

nanowires and covered with graphene. The advantages to such materials include maintaining transparency while simultaneously being electrically conductive and flexible.[47]

Conductive polymers

Inherently

PEDOT
:PSS, than for inorganic materials, but they are more flexible, less expensive and more environmentally friendly in processing and manufacture.

Amorphous indium–zinc oxide

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

organic solar cells. Areas of poor electrode performance in organic solar cells render a percentage of the cell's area unusable.[51]


See also

References

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External links
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