Cadmium telluride (CdTe) is a stable

crystal A crystal or crystalline solid is a solid material whose constituents (such as atoms, molecules, or ions) are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions. In addition, macros ...

line compound formed from

cadmium Cadmium is a chemical element; it has chemical symbol, symbol Cd and atomic number 48. This soft, silvery-white metal is chemically similar to the two other stable metals in group 12 element, group 12, zinc and mercury (element), mercury. Like z ...

and tellurium. It is mainly used as the semiconducting material in cadmium telluride photovoltaics and an

infrared Infrared (IR; sometimes called infrared light) is electromagnetic radiation (EMR) with wavelengths longer than that of visible light but shorter than microwaves. The infrared spectral band begins with the waves that are just longer than those ...

optical window The optical window is the portion of the optical spectrum that is blocked by the Earth's atmosphere. The window runs from around 300 nanometers ( ultraviolet-B) up into the range the human eye can detect, roughly 400–700 nm and continues up ...

. It is usually sandwiched with cadmium sulfide to form a p–n junction solar PV cell.

Applications

CdTe is used to make thin film solar cells, accounting for about 8% of all solar cells installed in 2011. They are among the lowest-cost types of solar cell, although a comparison of total installed cost depends on installation size and many other factors, and has changed rapidly from year to year. The CdTe solar cell market is dominated by First Solar. In 2011, around 2 GW_p of CdTe solar cells were produced; For more details and discussion see cadmium telluride photovoltaics. CdTe can be

alloy An alloy is a mixture of chemical elements of which in most cases at least one is a metal, metallic element, although it is also sometimes used for mixtures of elements; herein only metallic alloys are described. Metallic alloys often have prop ...

ed with mercury to make a versatile infrared detector material ( HgCdTe). CdTe alloyed with a small amount of

zinc Zinc is a chemical element; it has symbol Zn and atomic number 30. It is a slightly brittle metal at room temperature and has a shiny-greyish appearance when oxidation is removed. It is the first element in group 12 (IIB) of the periodic tabl ...

makes an excellent solid-state

X-ray An X-ray (also known in many languages as Röntgen radiation) is a form of high-energy electromagnetic radiation with a wavelength shorter than those of ultraviolet rays and longer than those of gamma rays. Roughly, X-rays have a wavelength ran ...

and

gamma ray A gamma ray, also known as gamma radiation (symbol ), is a penetrating form of electromagnetic radiation arising from high energy interactions like the radioactive decay of atomic nuclei or astronomical events like solar flares. It consists o ...

detector ( CdZnTe). CdTe is used as an

optical material for

s and lenses and is proven to provide a good performance across a wide range of temperatures. An early form of CdTe for IR use was marketed under the trademarked name of ''Irtran-6'', but this is obsolete. CdTe is also applied for electro-optic modulators. It has the greatest electro-optic coefficient of the linear electro-optic effect among II-VI compound crystals (r₄₁=r₅₂=r₆₃=6.8×10⁻¹² m/V). CdTe doped with

chlorine Chlorine is a chemical element; it has Symbol (chemistry), symbol Cl and atomic number 17. The second-lightest of the halogens, it appears between fluorine and bromine in the periodic table and its properties are mostly intermediate between ...

is used as a radiation detector for x-rays, gamma rays,

beta particle A beta particle, also called beta ray or beta radiation (symbol β), is a high-energy, high-speed electron or positron emitted by the radioactive decay of an atomic nucleus, known as beta decay. There are two forms of beta decay, β− decay and � ...

s and

alpha particle Alpha particles, also called alpha rays or alpha radiation, consist of two protons and two neutrons bound together into a particle identical to a helium-4 nucleus. They are generally produced in the process of alpha decay but may also be produce ...

s. CdTe can operate at room temperature allowing the construction of compact detectors for a wide variety of applications in nuclear spectroscopy. The properties that make CdTe superior for the realization of high performance gamma- and x-ray detectors are high atomic number, large bandgap and high electron mobility ~1100 cm²/V·s, which result in high intrinsic μτ (mobility-lifetime) product and therefore high degree of charge collection and excellent spectral resolution. Due to the poor charge transport properties of holes, ~100 cm²/V·s, single-carrier-sensing detector geometries are used to produce high resolution spectroscopy; these include coplanar grids, Frisch-collar detectors and small pixel detectors.

Physical properties

* Thermal expansion coefficient: 5.9×10⁻⁶/K at 293 K *

Young's modulus Young's modulus (or the Young modulus) is a mechanical property of solid materials that measures the tensile or compressive stiffness when the force is applied lengthwise. It is the modulus of elasticity for tension or axial compression. Youn ...

: 52 GPa * Poisson ratio: 0.41

Optical and electronic properties

Bulk CdTe is transparent in the

, from close to its band gap energy (1.5 eV at 300 K, which corresponds to infrared wavelength of about 830 nm) out to

wavelength In physics and mathematics, wavelength or spatial period of a wave or periodic function is the distance over which the wave's shape repeats. In other words, it is the distance between consecutive corresponding points of the same ''phase (waves ...

s greater than 20 μm; correspondingly, CdTe is fluorescent at 790 nm. As the size of CdTe crystals are reduced to a few nanometers or less, thus making them CdTe quantum dots, the fluorescence peak shifts through the visible range into the ultraviolet.

Chemical properties

CdTe is insoluble in water.Solubility is below 0.1mg/L which equals a classification as insoluble- reference, "ECHA Substance Registratio

CdTe has a high melting point of with evaporation starting at . CdTe has a vapor pressure of zero at ambient temperatures. CdTe is more stable than its parent compounds cadmium and tellurium and most other Cd compounds, due to its high melting point and insolubility. Cadmium telluride is commercially available as a powder, or as crystals. It can be made into nanocrystals.

Toxicology assessment

The compound CdTe has different qualities than the two elements, cadmium and tellurium, taken separately. CdTe has low acute inhalation, oral, and aquatic toxicity, and is negative in the Ames mutagenicity test. Based on notification of these results to the European Chemicals Agency (ECHA), CdTe is no longer classified as harmful if ingested nor harmful in contact with skin, and the toxicity classification to aquatic life has been reduced. Once properly and securely captured and encapsulated, CdTe used in manufacturing processes may be rendered harmless. Current CdTe modules pass the U.S. EPA's Toxicity Characteristic Leaching Procedure (TCLP) test, designed to assess the potential for long-term leaching of products disposed in landfills. A document hosted by the U.S. National Institutes of Health dated 2003 discloses the following: According to the classification provided by companies to the European Chemicals Agency (ECHA) in REACH registrations, it is still harmful to aquatic life with long lasting effects. Additionally, the classification provided by companies to ECHA notifications classifies it as very toxic to aquatic life with long lasting effects, very toxic to aquatic life, harmful if inhaled or swallowed and is harmful in contact with skin.

Availability

At the present time, the prices of the raw materials

and tellurium are a negligible proportion of the cost of CdTe solar cells and other CdTe devices. However, tellurium is a relatively rare element (1–5 parts per billion in the Earth's crust; see Abundances of the elements (data page)). Through improved material efficiency and increased PV recycling systems, the CdTe PV industry has the potential to fully rely on tellurium from recycled end-of-life modules by 2038. See Cadmium telluride photovoltaics for more information. Another study shows that CdTe PV recycling will add a significant secondary resource of Te which, in conjunction with improved material utilization, will enable a cumulative capacity of about 2 TW by 2050 and 10 TW by the end of the century.

References

External links

CdTe page on the web-site of the Institute of Solid State Physics of the Russian Academy of Sciences (html)Optical properties
University of Reading, Infrared Multilayer Laboratory

* ttps://web.archive.org/web/20061210213049/http://www.npi.gov.au/database/substance-info/profiles/17.html National Pollutant Inventory – Cadmium and compoundsbr>MSDS at ISP optics.com
(doc)
MDSD at espimetals.com
(pdf)
Material Safety data Sheet
on isp optics web site (MS Word doc) {{DEFAULTSORT:Cadmium Telluride Cadmium compounds Tellurides Photovoltaics II-VI semiconductors Zincblende crystal structure