Cadmium telluride

Cadmium Telluride

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Cadmium telluride

Cadmium telluride (CdTe) is a crystalline compound

Chemical compound

A chemical compound is a Chemical substance consisting of two or more different chemical element Chemical bond together in a fixed mass ratio that can be split into simpler substances....
formed from cadmium

Cadmium

Cadmium is a chemical element with the symbol Cd and atomic number 48. A relatively abundant , soft, bluish-white, transition metal, cadmium is known to cause cancer and occurs with zinc ores....
and tellurium

Tellurium

Tellurium is a chemical element that has the symbol Te and atomic number 52. A brittle silver-white metalloid which looks like tin, tellurium is chemically related to selenium and sulfur....
with a zinc blende (cubic) crystal structure (space group

Space group

The space group of a crystal or crystallographic group is a mathematical description of the symmetry inherent in the structure. The word 'group' in the name comes from the group , which is used to build the set of space groups....
F-43m).

In the bulk crystalline form it is a direct bandgap semiconductor

Semiconductor

A semiconductor is a material that has electrical conductivity between those of a Electrical conductor and an electrical insulation; it can vary over that wide range either permanently or dynamically....
. CdTe is also a strong solar cell

Solar cell

A solar cell or photovoltaic cell is a device that converts sunlight directly into electricity by the photovoltaic effect. Sometimes the term solar cell is reserved for devices intended specifically to capture energy from sunlight, while the term photovoltaic cell is used when the source is unspecified....
material. It is usually sandwiched with cadmium sulfide

Cadmium sulfide

Cadmium Sulfur is a chemical compound with the formula CdS. Cadmium sulfide is yellow in colour and is a semiconductor. It exists in nature as two different minerals, greenockite and hawleyite....
to form a p-n junction

P-n junction

A p-n junction is a junction formed by combining P-type semiconductor and N-type semiconductor semiconductors together in very close contact.The term junction refers to the region where the two regions of the semiconductor meet....
photovoltaic solar cell. Typically, CdTe cells use a n-i-p structure.

Applications
CdTe is a highly useful material in the making of thin film photovoltaic modules (photovoltaics).

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Encyclopedia

Cadmium telluride (CdTe) is a crystalline compound

Chemical compound

Cadmium

Tellurium

Space group

Semiconductor

Solar cell

Cadmium sulfide

P-n junction

Applications

CdTe is a highly useful material in the making of thin film photovoltaic modules (photovoltaics). Thin-film CdTe provides a highly cost-effective solar cell design, but at a lower efficiency than polysilicon.

CdTe can be alloyed with mercury

Mercury (element)

Mercury , also called quicksilver or hydrargyrum , is a chemical element with the symbol Hg and atomic number 80. A heavy, silvery d-block metal, mercury is one of six elements that are liquid at or near room temperature and pressure....
to make a versatile infrared detector

Infrared detector

An infrared detector is a photodetector that reacts to infrared radiation. The two main types of detectors are thermal and photonic.The thermal effects of the incident IR radiation can be followed through many temperature dependent phenomena....
material (HgCdTe). CdTe alloyed with a small amount of zinc

Zinc

Zinc is a metallic chemical element with the symbol Zn and atomic number 30. It is a first-row transition metal of the group 12 element of the periodic table....
makes an excellent solid-state x-ray

X-ray

X-radiation is a form of electromagnetic radiation. X-rays have a wavelength in the range of 10 to 0.01 nanometers, corresponding to frequency in the range 30 Hertz to 30 Hertz and energies in the range 120 Electron volt to 120 keV....
and gamma ray

Gamma ray

Gamma rays are a form of electromagnetic radiation produced by atom particle interactions, such as electron-positron annihilation or radioactive decay....
detector (CdZnTe

Cadmium zinc telluride

Cadmium zinc telluride, or CZT, is a compound of cadmium, zinc and tellurium or more strictly speaking, an alloy of cadmium telluride and zinc telluride....
).

CdTe is used as an infrared

Infrared

Infrared radiation is electromagnetic radiation whose wavelength is longer than that of visible light , but shorter than that of terahertz radiation and microwaves ....
optical material for optical window

Optical window

The meaning of this term depends on the context:* In astronomy, the optical window is the optical portion of the electromagnetic spectrum that passes through the Earth's atmosphere all the way to the ground....
s and lenses

Lens (optics)

A lens is an optics device with perfect or approximate axial symmetry which transmittance and refraction light, converging or diverging the beam....
but it has small application and is limited by its toxicity such that few optical houses will consider working with it. 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 electrooptical modulators. It has the highest electrooptical coefficient of the linear electrooptic effect among II-VI compound crystals (r₄₁=r₅₂=r₆₃=6.8*10^-12 m/V).

Physical properties

Lattice constant
Lattice constant

The Lattice Constant [or lattice parameter] refers to the constant distance between unit cells in a crystal lattice. Lattices in three dimensions generally have three lattice constants, referred to as a, b, and c....
: 0.648 nm at 300K
Young's modulus
Young's modulus

In solid mechanics, Young's modulus is a measure of the stiffness of an isotropic elastic material. It is also known as the Young modulus, modulus of elasticity, elastic modulus or tensile modulus....
: 52 GPa
Poisson ratio: 0.41

Thermal Properties

Thermal conductivity
Thermal conductivity

In physics, thermal conductivity, , is the List of materials properties of a material that indicates its ability to conduct heat. It appears primarily in Heat conduction#Fourier's law for heat conduction....
: 6.2 W.m^-1.K^-1 at 293 K
Specific heat capacity
Specific heat capacity

Specific heat capacity, also known simply as specific heat, is the measure of the energy required to increase the temperature of a of a substance by a certain Celsius#Temperatures_and_intervals....
: 210 J.kg^-1.K^-1 at 293 K
Thermal expansion coefficient: 5.9x10^-6 K^-1 at 293 K

Electronic properties

The direct band gap is 1.56 eV at 300 K.
Electron effective mass
Effective mass

In solid state physics, a particle's 'effective mass' is the mass it seems to carry in the semiclassical model of transport in a crystal. It can be shown that electrons and electron hole in a crystal respond to electric field and magnetic fields almost as if they were particles with a mass dependent upon the their direction of travel, an ...
0.11 m_e
Hole effective mass 0.4 m_e

Optical Properties

CdTe is transparent in the infrared, from close to its band gap energy (=795nm) out to wavelength

Wavelength

In physics, wavelength is the distance between repeating units of a propagating wave of a given frequency. It is commonly designated by the Greek language letter lambda ....
s greater than 20 µm. The refractive index

Refractive index

The refractive index of a medium is a measure for how much the speed of light is reduced inside the medium. For example, typical soda-lime glass has a refractive index of 1.5, which means that in glass, light travels at times the speed of light in a vacuum....
is 2.649 at 10µm.

Chemical properties

CdTe has very low solubility

Solubility

Solubility is often seen as a property of a substance; for instance the solubility of a solid substance usually refers to the concentration of the substance in a liquid that has reached equilibrium with the substance in solid phase ....
in water. It is etched by many acids including hydrochloric, and hydrobromic acid

Hydrobromic acid

Hydrobromic acid is a strong acid acid formed by dissolving the diatomic molecule hydrogen bromide in water. It has a Acid dissociation constant of −9, making it a stronger acid than hydrochloric acid, but not as strong as hydrogen iodide....
, forming (toxic) hydrogen telluride

Hydrogen telluride

Hydrogen telluride is the chemical compound with the formula hydrogen2tellurium. It is the simplest hydride of tellurium....
gas.

Cadmium telluride is commercially available as a powder, or as crystals. It can be made into nanocrystals.

Toxicity

Cadmium telluride is toxic, but only so if ingested, its dust inhaled, or if it is handled improperly (i.e. without appropriate gloves and other safety precautions). Once properly and securely captured and encapsulated, CdTe used in manufacturing processes may be rendered harmless. Please refer to materials safety data sheets for details.

The toxicity is not solely due to the cadmium content. One study found that the highly reactive surface of cadmium telluride quantum dots triggers extensive reactive oxygen damage to the cell membrane, mitochondria, and cell nucleus.. Many nanoparticle chemicals have safety issues. In addition, the cadmium telluride films are typically recrystallized in a toxic compound of cadmium chloride.

The disposal and long term safety of cadmium telluride is a known issue in the large scale commercialization of cadmium telluride solar panels. Serious efforts have been made to understand and overcome these issues. 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

First Solar, Inc. is a publicly-held U.S. energy company in the solar sector. It manufactures photovoltaic solar modules using a thin film semiconductor process based on Cadmium telluride, to produce photovoltaic modules....
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.

Health and Environmental Risks Researchers from the U.S. Department of Energy's Brookhaven National Laboratory 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 appear to be more environmentally friendly than all other current uses of Cd.

Availability

Cadmium

Tellurium

Tellurium is a chemical element that has the symbol Te and atomic number 52. A brittle silver-white metalloid which looks like tin, tellurium is chemically related to selenium and sulfur....
's availability is 1-5 parts per billion (ppb) in the Earth's crust. Refer to Abundances of the elements (data page)

Abundances of the elements (data page)

<< Chemical elements data references...
. See also Cadmium telluride photovoltaics

Cadmium telluride photovoltaics

Cadmium telluride photovoltaics describes a photovoltaic technology that is based on the use of cadmium telluride thin film, a semiconductor material layer designed to absorb and convert sunlight into electricity....
for a section devoted to this issue as it impacts the potential of CdTe to provide a significant amount of electricity.

Makers include:

AVA Solar
Calyxo
First Solar
First Solar

First Solar, Inc. is a publicly-held U.S. energy company in the solar sector. It manufactures photovoltaic solar modules using a thin film semiconductor process based on Cadmium telluride, to produce photovoltaic modules....
Matsushita Battery Industrial
PrimeStar Solar

External links

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