Indium tin oxide (ITO) is a ternary composition of

indium Indium is a chemical element with the symbol In and atomic number 49. Indium is the softest metal that is not an alkali metal. It is a silvery-white metal that resembles tin in appearance. It is a post-transition metal that makes up 0.21 par ...

, tin and

oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as we ...

in varying proportions. Depending on the oxygen content, it can be described as either a

ceramic A ceramic is any of the various hard, brittle, heat-resistant and corrosion-resistant materials made by shaping and then firing an inorganic, nonmetallic material, such as clay, at a high temperature. Common examples are earthenware, porcelai ...

or an

alloy An alloy is a mixture of chemical elements of which at least one is a metal. Unlike chemical compounds with metallic bases, an alloy will retain all the properties of a metal in the resulting material, such as electrical conductivity, ductilit ...

. Indium tin oxide is typically encountered as an oxygen-saturated composition with a formulation of 74% In, 18% Sn, and 8% 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 transparent conducting oxides because of its electrical conductivity and optical transparency, the ease with which it can be deposited as a thin film, and 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. Thin films of indium tin oxide are most commonly deposited on surfaces by

physical vapor deposition Physical vapor deposition (PVD), sometimes called physical vapor transport (PVT), describes a variety of vacuum deposition methods which can be used to produce thin films and coatings on substrates including metals, ceramics, glass, and poly ...

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

Material and properties

ITO is a mixed oxide of

and tin with a melting point in the range 1526–1926 °C (1800–2200 K, 2800–3500 °F), depending on composition. The most commonly used material is an oxide of a composition of ca. In₄Sn. The material is a n-type semiconductor with a large bandgap of around 4 eV. ITO is both transparent to visible light and relatively conductive. It has a low electrical

resistivity Electrical resistivity (also called specific electrical resistance or volume resistivity) is a fundamental property of a material that measures how strongly it resists electric current. A low resistivity indicates a material that readily allows ...

of ~10⁻⁴ Ω·cm, and a thin film can have an optical transmittance of greater than 80%. These properties are utilized to great advantage in touch-screen applications such as

mobile phone A mobile phone, cellular phone, cell phone, cellphone, handphone, hand phone or pocket phone, sometimes shortened to simply mobile, cell, or just phone, is a portable telephone that can make and receive telephone call, calls over a radio freq ...

Common uses

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. ITO green tapes are utilized for the production of lamps that are electroluminescent, functional, and fully flexible. 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. ITO is often used to make transparent conductive coating for displays such as

liquid crystal display A liquid-crystal display (LCD) is a flat-panel display or other electronically modulated optical device that uses the light-modulating properties of liquid crystals combined with polarizers. Liquid crystals do not emit light directly but ...

OLED An organic light-emitting diode (OLED or organic LED), also known as organic electroluminescent (organic EL) diode, is a light-emitting diode (LED) in which the emissive electroluminescent layer is a film of organic compound that emits light ...

displays,

plasma display A plasma display panel (PDP) is a type of flat panel display that uses small cells containing Plasma (physics), plasma: ionized gas that responds to electric fields. Plasma televisions were the first large (over 32 inches diagonal) flat panel displ ...

s, touch panels, and electronic ink applications. Thin films of ITO are also used in organic light-emitting diodes,

solar cell A solar cell, or photovoltaic cell, is an electronic device that converts the energy of light directly into electricity by the photovoltaic effect, which is a physical and chemical phenomenon.antistatic coatings and EMI shieldings. In organic light-emitting diodes, ITO is used as the

anode An anode is an electrode of a polarized electrical device through which conventional current enters the device. This contrasts with a cathode, an electrode of the device through which conventional current leaves the device. A common mnemoni ...

(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 In physics, electromagnetic radiation (EMR) consists of waves of the electromagnetic (EM) field, which propagate through space and carry momentum and electromagnetic radiant energy. It includes radio waves, microwaves, infrared, (visible ...

. The

F-22 Raptor The Lockheed Martin F-22 Raptor is an American single-seat, twin-engine, all-weather stealth tactical fighter aircraft developed for the United States Air Force (USAF). As the result of the USAF's Advanced Tactical Fighter (ATF) program, t ...

's canopy has an ITO coating that reflects

radar Radar is a detection system that uses radio waves to determine the distance ('' ranging''), angle, and radial velocity of objects relative to the site. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, w ...

waves, enhancing its

stealth Stealth may refer to: Military *Stealth technology, technology used to conceal ships, aircraft, and missiles **Stealth aircraft, aircraft which use stealth technology **Stealth ground vehicle, ground vehicles which use stealth technology ** Stea ...

capabilities and giving it a distinctive gold tint. ITO is also used for various optical coatings, most notably

infrared Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from aroun ...

-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 The vertical-cavity surface-emitting laser, or VCSEL , is a type of semiconductor laser diode with laser beam emission perpendicular from the top surface, contrary to conventional edge-emitting semiconductor lasers (also ''in-plane'' lasers) wh ...

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 thin film strain gauges can operate at temperatures up to 1400 °C and can be used in harsh environments, such as

gas turbine A gas turbine, also called a combustion turbine, is a type of continuous flow internal combustion engine. The main parts common to all gas turbine engines form the power-producing part (known as the gas generator or core) and are, in the directio ...

jet engine A jet engine is a type of reaction engine discharging a fast-moving jet (fluid), jet of heated gas (usually air) that generates thrust by jet propulsion. While this broad definition can include Rocket engine, rocket, Pump-jet, water jet, and ...

s, and

rocket engine A rocket engine uses stored rocket propellants as the reaction mass for forming a high-speed propulsive Jet (fluid), jet of fluid, usually high-temperature gas. Rocket engines are reaction engines, producing thrust by ejecting mass rearward, i ...

Alternative synthesis methods 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, alternative methods of preparing ITO and alternative materials are being investigated. Promising alternatives based on zinc oxide doped with various elements.

Doped compounds

Several transition metal dopants in indium oxide, particularly molybdenum, give much higher electron mobility and conductivity than obtained with tin. Doped binary compounds such as aluminum-doped

zinc oxide Zinc oxide is an inorganic compound with the Chemical formula, formula . It is a white powder that is insoluble in water. ZnO is used as an additive in numerous materials and products including cosmetics, food supplements, rubbers, plastics, ceram ...

(AZO) and indium-doped cadmium oxide have been proposed as alternative materials. Other inorganic alternatives include

aluminum Aluminium (aluminum in American and Canadian English) is a chemical element with the symbol Al and atomic number 13. Aluminium has a density lower than those of other common metals, at approximately one third that of steel. It h ...

gallium Gallium is a chemical element with the symbol Ga and atomic number 31. Discovered by French chemist Paul-Émile Lecoq de Boisbaudran in 1875, Gallium is in group 13 of the periodic table and is similar to the other metals of the group ( alum ...

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

Carbon nanotubes

Carbon nanotube conductive coatings are a prospective replacement.

Graphene

As another carbon-based alternative, films of

graphene Graphene () is an allotrope of carbon consisting of a Single-layer materials, single layer of atoms arranged in a hexagonal lattice nanostructure.

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 Silver is a chemical element with the Symbol (chemistry), symbol Ag (from the Latin ', derived from the Proto-Indo-European wikt:Reconstruction:Proto-Indo-European/h₂erǵ-, ''h₂erǵ'': "shiny" or "white") and atomic number 47. A soft, whi ...

nanowires and covered with

graphene Graphene () is an allotrope of carbon consisting of a Single-layer materials, single layer of atoms arranged in a hexagonal lattice nanostructure.

. The advantages to such materials include maintaining transparency while simultaneously being electrically conductive and flexible.

Conductive polymers

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 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 crystallization is disrupted by the difference in the ratio of oxygen to metal atoms between In₂O₃ and ZnO. Indium-zinc-oxide has some comparable 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.

Silver nanoparticle–ITO hybrid

ITO has been popularly used as a high-quality flexible substrate to produce flexible electronics. However, this substrate's flexibility decreases as its conductivity improves. Previous research have indicated that the mechanical properties of ITO can be improved through increasing the degree of crystallinity. Doping with silver (Ag) can improve this property, but results in a loss of transparency. An improved method that embeds Ag

nanoparticles A nanoparticle or ultrafine particle is usually defined as a particle of matter that is between 1 and 100 nanometres (nm) in diameter. The term is sometimes used for larger particles, up to 500 nm, or fibers and tubes that are less than 10 ...

(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.

Alternative synthesis methods

Tape casting process

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. 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. Benzyl phthalate plasticizer and 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 The electrical resistance of an object is a measure of its opposition to the flow of electric current. Its reciprocal quantity is , measuring the ease with which an electric current passes. Electrical resistance shares some conceptual parallel ...

of 2 Ω·cm.

Laser sintering

Using ITO

imposes a limit on the choice of substrate, owing to the high temperature required for sintering. As an alternative starting material, In-Sn alloy

allow for a more diverse range of possible substrates. A continuous conductive In-Sn alloy film is formed firstly, followed by oxidation to bring transparency. This two step process involves thermal annealing, which requires special atmosphere control and increased processing time. Because metal

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.

Ambient gas conditions

For example, using conventional methods but varying the ambient gas conditions to improve the optoelectronic properties as, for example,

plays a major role in the properties of ITO.

Chemical shaving for very thin films

There has been numerical modeling of plasmonic metallic nanostructures have shown great potential as a method of light management in thin-film nanodisc-patterned

hydrogenated amorphous silicon Amorphous silicon (a-Si) is the non-crystalline form of silicon used for solar cells and thin-film transistors in LCDs. Used as semiconductor material for a-Si solar cells, or thin-film silicon solar cells, it is deposited in thin films onto ...

(a-Si:H) solar 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.

Constraints and trade-offs

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

aluminium zinc oxide Aluminium (aluminum in American and Canadian English) is a chemical element with the symbol Al and atomic number 13. Aluminium has a density lower than those of other common metals, at approximately one third that of steel. It has ...

(AZO). AZO is a common choice of transparent conducting oxide (TCO) because of its lower cost and relatively good optical transmission performance in the solar spectrum. However, ITO is superior to AZO in many other important performance categories including chemical resistance to moisture. ITO is not affected by moisture, and is stable as part of 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

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. 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.

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

Health and safety

Inhalation of indium tin oxide may cause mild irritation to the

respiratory tracts The respiratory tract is the subdivision of the respiratory system involved with the process of respiration in mammals. The respiratory tract is lined with respiratory epithelium as respiratory mucosa. Air is breathed in through the nose to t ...

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. 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 and face the possibility of pulmonary alveolar proteinosis, pulmonary fibrosis,

emphysema Emphysema, or pulmonary emphysema, is a lower respiratory tract disease, characterised by air-filled spaces ( pneumatoses) in the lungs, that can vary in size and may be very large. The spaces are caused by the breakdown of the walls of the a ...

, and granulomas. Workers in the US, China, and Japan have been diagnosed with cholesterol clefts under indium exposure. Silver

existed in improved ITOs have been found

in vitro ''In vitro'' (meaning in glass, or ''in the glass'') studies are performed with microorganisms, cells, or biological molecules outside their normal biological context. Colloquially called "test-tube experiments", these studies in biology and ...

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. A new occupational problem called indium lung disease was developed through contact with indium-containing dusts. The first patient is a worker associated with wet surface grinding of ITO who suffered from interstitial pneumonia: his lung was filled with ITO related particles. These particles can also induce

cytokine Cytokines are a broad and loose category of small proteins (~5–25 kDa) important in cell signaling. Cytokines are peptides and cannot cross the lipid bilayer of cells to enter the cytoplasm. Cytokines have been shown to be involved in a ...

production and macrophage dysfunction. Sintered ITOs particles alone can cause phagocytic dysfunction but not

release in macrophage cells; however, they can intrigue a pro-inflammatory

response in pulmonary epithelial cells. Unlike uITO, they can also bring 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. Because of these issues, alternatives to ITO have been found.

Recycling

The

etching Etching is traditionally the process of using strong acid or mordant to cut into the unprotected parts of a metal surface to create a design in intaglio (incised) in the metal. In modern manufacturing, other chemicals may be used on other type ...

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.Yokel, R. A. (2014) pp. 116–119 in ''Encyclopedia of the Neurological Sciences'', ed. M. J. Aminoff and R. B. Daroff, Academic Press, Oxford, 2nd ed..

References

{{reflist

External links

Spectroscopic studies of conducting metal oxides
with many slides about ITO Articles containing unverified chemical infoboxes Oxides Indium compounds Tin Display technology Transparent electrodes