Indium tin oxide

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. Thin-film ITO deposited for use in the infrared can act as a dichroic mirror. Indium tin oxide is one of the most widely used transparent conducting oxides, not just for its electrical conductivity and 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. 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.

Material and properties

ITO is a mixed oxide of indium 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 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 phones.

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 displays, OLED 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 a voltage across the film. ITO is also used to reflect or absorb electromagnetic radiation. The F-22 Raptor's canopy has an ITO coating that absorbs radar waves and reflects infrared waves, enhancing its stealth capabilities and giving it a distinctive gold tint. 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 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. ITO is used as a transparent conductive layer in solar cells, including thin-film, perovskite, and tandem cells. Its transparency allows sunlight to reach the active layers, while its conductivity enables efficient charge collection, improving overall efficiency.

Silver doping ITO has been popularly used as a high-quality flexible substrate to produce flexible electronics. However, this substrate's flexibility decreases as electrical 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 doping method that uses silver nanoparticles (AgNPs) 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 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. Because of the cost and energy of physical vapor deposition, with the required vacuum processing, alternative methods of preparing ITO are being investigated.

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. 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 of 2 Ω·cm.

… excerpt ends here. Continue reading the full article.