Cadmium telluride
Encyclopedia
Cadmium telluride is a crystal
line compound
formed from cadmium
and tellurium. It is used as an infrared
optical window
and a solar cell
material. It is usually sandwiched with cadmium sulfide
to form a p-n junction
photovoltaic solar cell. Typically, CdTe cells use a n-i-p structure.
s, accounting for about 6% of all solar cells installed in 2010. 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 2010, around 1.5 GWp
of CdTe solar cells were produced; if this figure were massively increased, there might eventually be a shortage of tellurium, as Te is among the rarest elements in the Earth's crust (see below). Specifically, production could be expanded by a factor of 1000 to 10000 (estimates vary) before running out of Te. Another problem for the technology is the toxicity of cadmium
, see below. For more details and discussion see cadmium telluride photovoltaics
.
CdTe can be alloy
ed with mercury
to make a versatile infrared detector
material (HgCdTe
). CdTe alloyed with a small amount of zinc
makes an excellent solid-state X-ray
and gamma ray
detector (CdZnTe
).
CdTe is used as an infrared
optical material for optical window
s and lenses
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 An early form of CdTe for IR use was marketed under the trademarked name of Irtran-6 but this is obsolete.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 electro-optic modulator
s. 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
is used as a radiation detector for x-rays, gamma rays, beta particle
s and alpha particle
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.
, from close to its band gap energy (1.44 eV at 300 K, which corresponds to infrared wavelength of about 860 nm) out to wavelength
s greater than 20 µm; correspondingly, CdTe is fluorescent
at 790 nm. When the size of CdTe crystal is being reduced to a few nanometers and below, thus making a CdTe quantum dot
, the fluorescence peak shifts towards through the visible range to the ultraviolet.
in water. It is etched
by many acid
s including hydrochloric
, and hydrobromic acid
, forming (toxic) hydrogen telluride
gas and toxic cadmium salts. It is a reducing agent and is unstable in air at high temperatures.
Cadmium telluride is commercially available as a powder, or as crystals. It can be made into nanocrystals.
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
. In addition, the cadmium telluride films are typically recrystallized in a toxic solution 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:
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.
The approach to CdTe safety in the European Union and China is much more cautious: cadmium and cadmium compounds are considered as toxic carcinogens in EU whereas China regulations allow Cd products for export only.
and tellurium are a negligible proportion of the cost of CdTe solar cells and other CdTe devices. However, tellurium is an extremely rare element (1-5 parts per billion in the Earth's crust; see Abundances of the elements (data page)
), and if CdTe were to be used in sufficiently large quantities (for example, to make enough solar cells to provide a significant proportion of worldwide energy consumption), tellurium availability could be a serious problem. See Cadmium telluride photovoltaics
for more information.
Crystal
A crystal or crystalline solid is a solid material whose constituent atoms, molecules, or ions are arranged in an orderly repeating pattern extending in all three spatial dimensions. The scientific study of crystals and crystal formation is known as crystallography...
line compound
Chemical compound
A chemical compound is a pure chemical substance consisting of two or more different chemical elements that can be separated into simpler substances by chemical reactions. Chemical compounds have a unique and defined chemical structure; they consist of a fixed ratio of atoms that are held together...
formed from cadmium
Cadmium
Cadmium is a chemical element with the symbol Cd and atomic number 48. This soft, bluish-white metal is chemically similar to the two other stable metals in group 12, zinc and mercury. Similar to zinc, it prefers oxidation state +2 in most of its compounds and similar to mercury it shows a low...
and tellurium. It is used as an infrared
Infrared
Infrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...
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 atmosphere all the way to the ground...
and a solar cell
Solar cell
A solar cell is a solid state electrical device that converts the energy of light directly into electricity by the photovoltaic effect....
material. It is usually sandwiched with cadmium sulfide
Cadmium sulfide
Cadmium sulfide is the inorganic compound with the formula CdS. Cadmium sulfide is a yellow solid. It occurs in nature with two different crystal structures as the rare minerals greenockite and hawleyite, but is more prevalent as an impurity substituent in the similarly structured zinc ores...
to form a p-n junction
P-n junction
A p–n junction is formed at the boundary between a P-type and N-type semiconductor created in a single crystal of semiconductor by doping, for example by ion implantation, diffusion of dopants, or by epitaxy .If two separate pieces of material were used, this would...
photovoltaic solar cell. Typically, CdTe cells use a n-i-p structure.
Applications
CdTe is a used to make thin film solar cellThin film solar cell
A thin-film solar cell , also called a thin-film photovoltaic cell , is a solar cell that is made by depositing one or more thin layers of photovoltaic material on a substrate...
s, accounting for about 6% of all solar cells installed in 2010. 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
First Solar, Inc. is an American manufacturer of thin film photovoltaic modules, or solar panels, and a provider to PV power plants of supporting services that include finance, construction, maintenance and end-of-life panel recycling...
. In 2010, around 1.5 GWp
Watt-peak
Watt-peak is a measure of the nominal power of a photovoltaic solar energy device under laboratory illumination conditions. Related units such as kilowatt-peak or kilowatts-peak and megawatt-peak are also used, and in the context of domestic installations kWp is the most common unit encountered...
of CdTe solar cells were produced; if this figure were massively increased, there might eventually be a shortage of tellurium, as Te is among the rarest elements in the Earth's crust (see below). Specifically, production could be expanded by a factor of 1000 to 10000 (estimates vary) before running out of Te. Another problem for the technology is the toxicity of cadmium
Cadmium
Cadmium is a chemical element with the symbol Cd and atomic number 48. This soft, bluish-white metal is chemically similar to the two other stable metals in group 12, zinc and mercury. Similar to zinc, it prefers oxidation state +2 in most of its compounds and similar to mercury it shows a low...
, see below. For more details and discussion see 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 layer designed to absorb and convert sunlight into electricity...
.
CdTe can be alloy
Alloy
An alloy is a mixture or metallic solid solution composed of two or more elements. Complete solid solution alloys give single solid phase microstructure, while partial solutions give two or more phases that may or may not be homogeneous in distribution, depending on thermal history...
ed with mercury
Mercury (element)
Mercury is a chemical element with the symbol Hg and atomic number 80. It is also known as quicksilver or hydrargyrum...
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
Mercury(II) cadmium(II) telluride
HgCdTe or mercury cadmium telluride is an alloy of CdTe and HgTe and is sometimes claimed to be the third semiconductor of technological importance after silicon and gallium arsenide...
). CdTe alloyed with a small amount of zinc
Zinc
Zinc , or spelter , is a metallic chemical element; it has the symbol Zn and atomic number 30. It is the first element in group 12 of the periodic table. Zinc is, in some respects, chemically similar to magnesium, because its ion is of similar size and its only common oxidation state is +2...
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 0.01 to 10 nanometers, corresponding to frequencies in the range 30 petahertz to 30 exahertz and energies in the range 120 eV to 120 keV. They are shorter in wavelength than UV rays and longer than gamma...
and gamma ray
Gamma ray
Gamma radiation, also known as gamma rays or hyphenated as gamma-rays and denoted as γ, is electromagnetic radiation of high frequency . Gamma rays are usually naturally produced on Earth by decay of high energy states in atomic nuclei...
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. A direct bandgap semiconductor, it is used in a variety of applications, including radiation detectors, photorefractive gratings,...
).
CdTe is used as an infrared
Infrared
Infrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...
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 atmosphere all the way to the ground...
s and lenses
Lens (optics)
A lens is an optical device with perfect or approximate axial symmetry which transmits and refracts light, converging or diverging the beam. A simple lens consists of a single optical element...
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 electro-optic modulator
Electro-optic modulator
Electro-optic modulator is an optical device in which a signal-controlled element displaying electro-optic effect is used to modulate a beam of light. The modulation may be imposed on the phase, frequency, amplitude, or polarization of the modulated beam...
s. It has the greatest electro-optic coefficient of the linear electro-optic effect
Electro-optic effect
An electro-optic effect is a change in the optical properties of a material in response to an electric field that varies slowly compared with the frequency of light...
among II-VI compound crystals (r41=r52=r63=6.8×10−12 m/V).
CdTe doped with chlorine
Chlorine
Chlorine is the chemical element with atomic number 17 and symbol Cl. It is the second lightest halogen, found in the periodic table in group 17. The element forms diatomic molecules under standard conditions, called dichlorine...
is used as a radiation detector for x-rays, gamma rays, beta particle
Beta particle
Beta particles are high-energy, high-speed electrons or positrons emitted by certain types of radioactive nuclei such as potassium-40. The beta particles emitted are a form of ionizing radiation also known as beta rays. The production of beta particles is termed beta decay...
s and alpha particle
Alpha particle
Alpha particles consist of two protons and two neutrons bound together into a particle identical to a helium nucleus, which is classically produced in the process of alpha decay, but may be produced also in other ways and given the same name...
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.
Physical properties
- Lattice constantLattice constantThe 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. However, in the special case of cubic crystal structures, all of the constants are...
: 0.648 nm at 300K - Young's modulusYoung's modulusYoung's modulus is a measure of the stiffness of an elastic material and is a quantity used to characterize materials. It is defined as the ratio of the uniaxial stress over the uniaxial strain in the range of stress in which Hooke's Law holds. In solid mechanics, the slope of the stress-strain...
: 52 GPa - Poisson ratio: 0.41
Thermal properties
- Thermal conductivityThermal conductivityIn physics, thermal conductivity, k, is the property of a material's ability to conduct heat. It appears primarily in Fourier's Law for heat conduction....
: 6.2 W·m/m2·K at 293 K - Specific heat capacity: 210 J/kg·K at 293 K
- Thermal expansion coefficient: 5.9×10−6/K at 293 K
Optical and electronic properties
Bulk CdTe is transparent in the infraredInfrared
Infrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...
, from close to its band gap energy (1.44 eV at 300 K, which corresponds to infrared wavelength of about 860 nm) out to wavelength
Wavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...
s greater than 20 µm; correspondingly, CdTe is fluorescent
Fluorescence
Fluorescence is the emission of light by a substance that has absorbed light or other electromagnetic radiation of a different wavelength. It is a form of luminescence. In most cases, emitted light has a longer wavelength, and therefore lower energy, than the absorbed radiation...
at 790 nm. When the size of CdTe crystal is being reduced to a few nanometers and below, thus making a CdTe quantum dot
Quantum dot
A quantum dot is a portion of matter whose excitons are confined in all three spatial dimensions. Consequently, such materials have electronic properties intermediate between those of bulk semiconductors and those of discrete molecules. They were discovered at the beginning of the 1980s by Alexei...
, the fluorescence peak shifts towards through the visible range to the ultraviolet.
Chemical properties
CdTe has very low solubilitySolubility
Solubility is the property of a solid, liquid, or gaseous chemical substance called solute to dissolve in a solid, liquid, or gaseous solvent to form a homogeneous solution of the solute in the solvent. The solubility of a substance fundamentally depends on the used solvent as well as on...
in water. It is etched
Etching
Etching is the process of using strong acid or mordant to cut into the unprotected parts of a metal surface to create a design in intaglio in the metal...
by many acid
Acid
An acid is a substance which reacts with a base. Commonly, acids can be identified as tasting sour, reacting with metals such as calcium, and bases like sodium carbonate. Aqueous acids have a pH of less than 7, where an acid of lower pH is typically stronger, and turn blue litmus paper red...
s including hydrochloric
Hydrochloric acid
Hydrochloric acid is a solution of hydrogen chloride in water, that is a highly corrosive, strong mineral acid with many industrial uses. It is found naturally in gastric acid....
, and hydrobromic acid
Hydrobromic acid
Hydrobromic acid is a strong acid formed by dissolving the diatomic molecule hydrogen bromide in water. "Constant boiling" hydrobromic acid is an aqueous solution that distills at 124.3 °C and contains 47.6% HBr by weight, which is 8.89 mol/L. Hydrobromic acid has a pKa of −9, making it a...
, forming (toxic) hydrogen telluride
Hydrogen telluride
Hydrogen telluride is the inorganic compound with the formula H2Te. The simplest hydride of tellurium, it is rarely encountered because of it decomposes rapidly to the constituent elements. Most compounds with Te-H bonds are unstable with respect to loss of H2. H2Te is chemically and structurally...
gas and toxic cadmium salts. It is a reducing agent and is unstable in air at high temperatures.
Cadmium telluride is commercially available as a powder, or as crystals. It can be made into nanocrystals.
Toxicity
Cadmium telluride is toxic if ingested, if its dust is 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. CdTe appears to be less toxic than elemental cadmium, at least in terms of acute exposure.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
Cell membrane
The cell membrane or plasma membrane is a biological membrane that separates the interior of all cells from the outside environment. The cell membrane is selectively permeable to ions and organic molecules and controls the movement of substances in and out of cells. It basically protects the cell...
, mitochondria, and cell nucleus
Cell nucleus
In cell biology, the nucleus is a membrane-enclosed organelle found in eukaryotic cells. It contains most of the cell's genetic material, organized as multiple long linear DNA molecules in complex with a large variety of proteins, such as histones, to form chromosomes. The genes within these...
. In addition, the cadmium telluride films are typically recrystallized in a toxic solution of cadmium chloride
Cadmium chloride
Cadmium chloride is a white crystalline compound of cadmium and chlorine, with the formula CdCl2. It is a hygroscopic solid that is highly soluble in water and slightly soluble in alcohol. Although it is considered to be ionic, it has considerable covalent character to its bonding...
.
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 SolarFirst SolarFirst Solar, Inc. is an American manufacturer of thin film photovoltaic modules, or solar panels, and a provider to PV power plants of supporting services that include finance, construction, maintenance and end-of-life panel recycling...
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
Brookhaven National Laboratory , is a United States national laboratory located in Upton, New York on Long Island, and was formally established in 1947 at the site of Camp Upton, a former U.S. Army base...
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.
The approach to CdTe safety in the European Union and China is much more cautious: cadmium and cadmium compounds are considered as toxic carcinogens in EU whereas China regulations allow Cd products for export only.
Availability
At the present time, the price of the raw materials cadmiumCadmium
Cadmium is a chemical element with the symbol Cd and atomic number 48. This soft, bluish-white metal is chemically similar to the two other stable metals in group 12, zinc and mercury. Similar to zinc, it prefers oxidation state +2 in most of its compounds and similar to mercury it shows a low...
and tellurium are a negligible proportion of the cost of CdTe solar cells and other CdTe devices. However, tellurium is an extremely rare element (1-5 parts per billion in the Earth's crust; see Abundances of the elements (data page)
Abundances of the elements (data page)
-Earth bulk continental crust and upper continental crust:*C1 — Crust: CRC Handbook*C2 — Crust: Kaye & Laby*C3 — Crust: Greenwood*C4 — Crust: Ahrens *C5 — Crust: Ahrens...
), and if CdTe were to be used in sufficiently large quantities (for example, to make enough solar cells to provide a significant proportion of worldwide energy consumption), tellurium availability could be a serious problem. See 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 layer designed to absorb and convert sunlight into electricity...
for more information.
See also
- Cadmium selenideCadmium selenideCadmium selenide is a solid, binary compound of cadmium and selenium. Common names for this compound are cadmium selenide, cadmium selenide, and cadmoselite ....
- Cadmium telluride photovoltaicsCadmium telluride photovoltaicsCadmium telluride photovoltaics describes a photovoltaic technology that is based on the use of cadmium telluride thin film, a semiconductor layer designed to absorb and convert sunlight into electricity...
- Cadmium zinc tellurideCadmium zinc tellurideCadmium zinc telluride, or CZT, is a compound of cadmium, zinc and tellurium or, more strictly speaking, an alloy of cadmium telluride and zinc telluride. A direct bandgap semiconductor, it is used in a variety of applications, including radiation detectors, photorefractive gratings,...
- First SolarFirst SolarFirst Solar, Inc. is an American manufacturer of thin film photovoltaic modules, or solar panels, and a provider to PV power plants of supporting services that include finance, construction, maintenance and end-of-life panel recycling...
- Mercury telluride
- Mercury(II) cadmium(II) tellurideMercury(II) cadmium(II) tellurideHgCdTe or mercury cadmium telluride is an alloy of CdTe and HgTe and is sometimes claimed to be the third semiconductor of technological importance after silicon and gallium arsenide...
- Zinc tellurideZinc tellurideZinc telluride is a binary chemical compound with the formula ZnTe. This solid is a semiconductor material with band gap of 2.23–2.25 eV. It is usually a P-type semiconductor. Its crystal structure is cubic, like that for sphalerite and diamond....
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
- CdTe: single crystals, grown by HPVB and HPVZM techniques; windows, substrates, electrooptical modulators. Infrared transmittance spectrum. MSDS.
- National Pollutant Inventory - Cadmium and compounds
- MSDS at ISP optics.com (doc)
- MDSD at espimetals.com (pdf)
- Material Safety data Sheet on isp optics web site (MS Word doc)