Cadmium telluride

Cadmium telluride (CdTe) is a crystalline compound formed from cadmium and tellurium with a zinc blende (cubic) crystal structure (space group F-43m).

In the bulk crystalline form it is a direct bandgap semiconductor. CdTe is also a strong solar cell material. It is usually sandwiched with cadmium sulfide to form a p-n junction photovoltaic solar cell.

Applications

CdTe is a highly useful material in the making of solar cells (photovoltaics). At Tellurium and polysilicon prices as of May 2008, thin-film CdTe provides a highly cost-effective solar cell design, but at a lower efficiency than polysilicon. However, depending on the manufacturing process used, this is easily off-set by the ability of CdTe panels to handle normally problematic low-light scenarios efficiently-substantially more efficiently than traditional X-si panels, thus making the energy-production inequity a non-issue.

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 An early form of CdTe for IR use was marketed under the trademarked name of Irtran-6 but this is obsolete.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 electrooptical modulators. It has 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: 0.648 nm at 300K
• Young's modulus: 52 GPa
• Poisson ratio: 0.41

Thermal Properties

• Thermal conductivity: 6.2 W.m^-1.K^-1 at 293 K
• Specific heat capacity: 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 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 wavelengths greater than 20 µm. The refractive index is 2.649 at 10µm.

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.

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.. In addition, the cadmium telluride devices are typically dipped in a toxic compound of cadmium sulfide.

The disposal life-cycle and long term safety of cadmium telleride is becoming more of a known issue in the large scale commercialization of cadmium telluride solar panels. 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 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 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 has recently been sold for $3.55/1b. The price of tellurium peaked at about $100/1b. Tellurium's availability is 1-5 parts per billion (ppb) in the Earth's crust. Refer to Abundances of the elements (data page).

See also

• Cadmium selenide
• Cadmium telluride solar cell
• Cadmium zinc telluride
• First Solar
• Mercury telluride
• Mercury(II) cadmium(II) telluride
• Zinc telluride

References

• Properties of Narrow-Gap Cadmium-Based Compounds Ed. P. Capper (INSPEC, IEE, London, UK, 1994) ISBN 0-85296-880-9
• D W Palmer, www.semiconductors.co.uk, 2002.06.
• II-VI Infrared
• Optical properties University of Reading
• Material Safety data Sheet on isp optics web site (MS Word doc)
• CdTe page on the web-site of the Institute of Solid State Physics of the Russian Academy of Sciences (html)

External links

• 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)