Cadmium telluride

Cadmium telluride
Other names	Irtran-6
Identifiers
CAS number	1306-25-8 Y
Properties
Molecular formula	CdTe
Molar mass	240.01 g mol−1
Density	5.85 g/cm3
Melting point	1092 °C
Boiling point	1130 °C
Solubility in other solvents	insoluble
Band gap	1.44 eV (@300 K, direct)
Refractive index (nD)	2.67 (@10 µm)
Structure
Crystal structure	zincblende (cubic) (space group F-43m
Hazards
EU Index	048-001-00-5
EU classification	Harmful (Xn) Dangerous for the environment (N)
R-phrases	R20/21/22, R50/53
S-phrases	(S2), S60, S61
Related compounds
Other anions	Cadmium oxide Cadmium sulfide Cadmium selenide
Other cations	Zinc telluride Mercury telluride
Y (what is this?)  (verify) Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

Cadmium telluride (CdTe) is a crystalline 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.

Applications

CdTe is a highly useful material in the making of thin film solar cells. Thin-film CdTe provides a cost-effective solar cell design, but is less efficient than polysilicon.

CdTe can be alloyed 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 windows 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 An early form of CdTe for IR use was marketed under the trademarked name of Irtran-6 but this is obsolete.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 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 is used as a radiation detector for x-rays, gamma rays, beta particles and alpha particles. CdTe can operate at room temperature allowing the construction of compact detectors for a wide variety of applications in nuclear spectroscopy.^[1]

Physical properties

• Lattice constant: 0.648 nm at 300K
• Young's modulus: 52 GPa
• Poisson ratio: 0.41

Thermal properties

• Thermal conductivity: 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^[2]

Optical and electronic properties

Fluorescence spectra of colloidal CdTe quantum dots of various sizes, increasing approximately from 2 to 20 nm from left to right. The red shift of fluorescence is due to quantum confinement.

Bulk CdTe is transparent in the infrared, from close to its band gap energy (1.44 eV at 300 K,^[3] which corresponds to infrared wavelength of about 860 nm) out to wavelengths 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.

Chemical properties

This section requires expansion.

CdTe has very low solubility in water. It is etched by many acids including hydrochloric, and hydrobromic acid, forming (toxic) hydrogen telluride 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.

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.^[4]. 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^[5] 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 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 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.^[6]

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. The major concern for CdTe is inevitable presence of Cd during CdTe production and processing.^[7][8]

Availability

At the present time, the price of the raw materials cadmium 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.

See also

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
• CdTe: single crystals, grown by HPVB and HPVZM techniques; windows, substrates, electrooptical modulators. Infrared transmittance spectrum. MSDS.
• National Pollutant Inventory - Cadmium and compounds
• National Renewable Energy Laboratory - Cadmium use in Photovoltaics
• MSDS at ISP optics.com (doc)
• MDSD at espimetals.com (pdf)
• Material Safety data Sheet on isp optics web site (MS Word doc)