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 An early form of CdTe for IR use was marketed under the trademarked name of Irtran-6 but this is obsolete.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] 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 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

CdTe has very low solubility in water. It is etched by many acids 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.


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.[4]
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.[5]. 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[6] 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.[7]
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.[8][9]

References

P. Capper (1994). Properties of Narrow-Gap Cadmium-Based Compounds. London, UK: INSPEC, IEE. ISBN 0-85296-880-9.
Palmer, D W (March 2008). "Properties of II-VI Compound Semiconductors". Semiconductors-Information.
Bube, R. H. (1955). "Temperature dependence of the width of the band gap in several photoconductors". Physical Review 98: 431–3.
(PDF) Acute Oral and Inhalation Toxicities in Rats With Cadmium Telluride. International Journal of Toxicology. 2009-08.
"Unmodified Cadmium Telluride Quantum Dots Prove Toxic". Nano News (National Cancer Institute). 2005-12-12.
(PDF) Nomination of Cadmium Telluride to the National Toxicology Program. United States Department of Health and Human Services. 2003-04-11.
Fthenakis, V M (2004). "Life Cycle Impact Analysis of Cadmium in CdTe PV Production". Renewable & Sustainable Energy Reviews 8: 303–334. doi:10.1016/j.rser.2003.12.001.
Sinha, Parikhit; Kriegner, Christopher J.; Schew, William A.; Kaczmar, Swiatoslav W.; Traister, Matthew; Wilson, David J. (2008). "Regulatory policy governing cadmium-telluride photovoltaics: A case study contrasting life cycle management with the precautionary principle". Energy Policy 36: 381. doi:10.1016/j.enpol.2007.09.017.
Cadmium Telluride Casts Shadow of Death on First Solar