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 Compound may refer to: Architecture and built environments * Compound (enclosure), a cluster of buildings having a shared purpose, usually inside a fence or wall ** Compound (fortification), a version of the above fortified with defensive struct ...

formed from

cadmium Cadmium is a chemical element with the symbol Cd and atomic number 48. This soft, silvery-white metal is chemically similar to the two other stable metals in group 12, zinc and mercury. Like zinc, it demonstrates oxidation state +2 in most of ...

and

tellurium Tellurium is a chemical element with the symbol Te and atomic number 52. It is a brittle, mildly toxic, rare, silver-white metalloid. Tellurium is chemically related to selenium and sulfur, all three of which are chalcogens. It is occasionally fou ...

. It is mainly used as the

semiconducting material A semiconductor is a material which has an electrical conductivity value falling between that of a conductor, such as copper, and an insulator, such as glass. Its resistivity falls as its temperature rises; metals behave in the opposite way. ...

cadmium telluride photovoltaics Cadmium telluride (CdTe) photovoltaics is a photovoltaic (PV) technology based on the use of cadmium telluride in a thin semiconductor layer designed to absorb and convert sunlight into electricity. Cadmium telluride PV is the only thin film ...

and an

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 around ...

optical window The optical window is a range of wavelengths that are not 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 to approxi ...

. It is usually sandwiched with

cadmium sulfide Cadmium sulfide is the inorganic compound with the formula CdS. Cadmium sulfide is a yellow solid.Egon Wiberg, Arnold Frederick Holleman (2001''Inorganic Chemistry'' Elsevier It occurs in nature with two different crystal structures as the rare m ...

to form a

p–n junction A p–n junction is a boundary or interface between two types of semiconductor materials, p-type and n-type, inside a single crystal of semiconductor. The "p" (positive) side contains an excess of holes, while the "n" (negative) side contains ...

solar PV cell.

Applications

CdTe is used to make

thin film solar cell A thin-film solar cell is a second generation solar cell that is made by depositing one or more thin layers, or thin film (TF) of photovoltaic material on a substrate, such as glass, plastic or metal. Thin-film solar cells are commercially use ...

s, 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 First Solar, Inc. is an American manufacturer of solar panels, and a provider of utility-scale PV power plants and supporting services that include finance, construction, maintenance and end-of-life panel recycling. First Solar uses rigid thi ...

. In 2011, around 2 GW_p of CdTe solar cells were produced; For more details and discussion see

. CdTe can be

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, ductility, ...

ed with

mercury Mercury commonly refers to: * Mercury (planet), the nearest planet to the Sun * Mercury (element), a metallic chemical element with the symbol Hg * Mercury (mythology), a Roman god Mercury or The Mercury may also refer to: Companies * Merc ...

to make a versatile

infrared detector An infrared detector is a detector that reacts to infrared (IR) radiation. The two main types of detectors are thermal and photonic (photodetectors). The thermal effects of the incident IR radiation can be followed through many temperature depen ...

material (

HgCdTe Hg1−xCdxTe or mercury cadmium telluride (also cadmium mercury telluride, MCT, MerCad Telluride, MerCadTel, MerCaT or CMT) is a chemical compound of cadmium telluride (CdTe) and mercury telluride (HgTe) with a tunable bandgap spanning the shortwave ...

). CdTe alloyed with a small amount of

zinc Zinc is a chemical element with the symbol Zn and atomic number 30. Zinc 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 periodi ...

makes an excellent solid-state

X-ray An X-ray, or, much less commonly, X-radiation, is a penetrating form of high-energy electromagnetic radiation. Most X-rays have a wavelength ranging from 10 picometers to 10 nanometers, corresponding to frequencies in the range 30&nb ...

and

gamma ray A gamma ray, also known as gamma radiation (symbol γ or \gamma), is a penetrating form of electromagnetic radiation arising from the radioactive decay of atomic nuclei. It consists of the shortest wavelength electromagnetic waves, typically ...

detector ( CdZnTe). CdTe is used as an

optical material for

s and

lenses A lens is a transmissive optical device which focuses or disperses a light beam by means of refraction. A simple lens consists of a single piece of transparent material, while a compound lens consists of several simple lenses (''elements''), ...

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 modulator An electro-optic modulator (EOM) is an optical device in which a signal-controlled element exhibiting an electro-optic effect is used to modulate a beam of light. The modulation may be imposed on the phase, frequency, amplitude, or polarization ...

s. It has the greatest electro-optic coefficient of the linear

electro-optic effect Electro–optics is a branch of electrical engineering, electronic engineering, materials science, and material physics involving components, electronic devices such as lasers, laser diodes, LEDs, waveguides, etc. which operate by the propaga ...

among II-VI compound crystals (r₄₁=r₅₂=r₆₃=6.8×10⁻¹² m/V). CdTe doped with

chlorine Chlorine is a chemical element with the 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 betwee ...

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 during the process of beta decay. There are two forms of beta decay, β� ...

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 Thermal expansion is the tendency of matter to change its shape, area, volume, and density in response to a change in temperature, usually not including phase transitions. Temperature is a monotonic function of the average molecular kinetic ...

: 5.9×10⁻⁶/K at 293 K *

Young's modulus Young's modulus E, the Young modulus, or the modulus of elasticity in tension or compression (i.e., negative tension), is a mechanical property that measures the tensile or compressive stiffness of a solid material when the force is applied leng ...

: 52 GPa *

Poisson ratio In materials science and solid mechanics, Poisson's ratio \nu ( nu) is a measure of the Poisson effect, the deformation (expansion or contraction) of a material in directions perpendicular to the specific direction of loading. The value of Poi ...

: 0.41

Optical and electronic properties

Bulk CdTe is

transparent Transparency, transparence or transparent most often refer to: * Transparency (optics), the physical property of allowing the transmission of light through a material They may also refer to: Literal uses * Transparency (photography), a still, ...

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, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats. It is the distance between consecutive corresponding points of the same phase on the wave, such as two adjacent crests, tro ...

s greater than 20 µm; correspondingly, CdTe is

fluorescent Fluorescence is the emission of light by a substance that has absorbed light or other electromagnetic radiation. It is a form of luminescence. In most cases, the emitted light has a longer wavelength, and therefore a lower photon energy, tha ...

at 790 nm. As the size of CdTe crystals are reduced to a few nanometers or less, thus making them CdTe

quantum dot Quantum dots (QDs) are semiconductor particles a few nanometres in size, having light, optical and electronics, electronic properties that differ from those of larger particles as a result of quantum mechanics. They are a central topic in nanote ...

s, the fluorescence peak shifts through the visible range into the ultraviolet.

Chemical properties

CdTe is

insoluble In chemistry, solubility is the ability of a substance, the solute, to form a solution with another substance, the solvent. Insolubility is the opposite property, the inability of the solute to form such a solution. The extent of the solubil ...

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 1041 °C with evaporation starting at 1050 °C. 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 The European Chemicals Agency (ECHA; ) is an agency of the European Union which manages the technical and administrative aspects of the implementation of the European Union regulation called Registration, Evaluation, Authorisation and Restricti ...

(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 that:

Brookhaven National Laboratory (BNL) and the U.S. Department of Energy (DOE) are nominating Cadmium Telluride (CdTe) for inclusion in the National Toxicology Program (NTP). This nomination is strongly supported by the National Renewable Energy Laboratory (NREL) and
First Solar First Solar, Inc. is an American manufacturer of solar panels, and a provider of utility-scale PV power plants and supporting services that include finance, construction, maintenance and end-of-life panel recycling. First Solar uses rigid thi ...
Inc. The material has the potential for widespread applications in photovoltaic energy generation that will involve extensive human interfaces. Hence, we consider that a definitive toxicological study of the effects of long-term exposure to CdTe is a necessity.

Researchers from the U.S. Department of Energy's

Brookhaven National Laboratory Brookhaven National Laboratory (BNL) is a United States Department of Energy national laboratory located in Upton, Long Island, and was formally established in 1947 at the site of Camp Upton, a former U.S. Army base and Japanese internment c ...

have found that large-scale use of CdTe PV modules does not present any risks to health and the environment, and recycling the modules at the end of their useful life completely resolves any environmental concerns. During their operation, these modules do not produce any pollutants, and, furthermore, by displacing fossil fuels, they offer great environmental benefits. CdTe PV modules that use cadmium as a raw material appear to be more environmentally friendly than all other current uses of Cd. CdTe PV provides a sustainable purpose for the potential oversupply of cadmium in the near future. Cadmium is generated as a waste by-product of

zinc refining Zinc refining is the process of purifying zinc to ''special high grade'' (SHG) zinc, which is at least 99.995% pure. This process is not usually required when smelting of zinc is done through electrolysis processes, but is needed when zinc is produc ...

and is generated in substantial amounts regardless of its use in PV, due to the demand for steel products. 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

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 Cadmium telluride (CdTe) photovoltaics is a photovoltaic (PV) technology based on the use of cadmium telluride in a thin semiconductor layer designed to absorb and convert sunlight into electricity. Cadmium telluride PV is the only thin film ...

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