Cadmium telluride

Cadmium telluride (CdTe) is a crystalline compound formed from cadmium and tellurium with a zinc blende (cubic) crystal structure (space group F43m). 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 pn junction photovoltaic solar cell.

Applications
CdTe is a useful material for solar cells (photovoltaics). It is cheaper than silicon, especially in thin-film solar cell technology, but not as efficient. 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 An early form of CdTe for IR use was marketed under the trademarked name of Irtran-6 but this is obsolete.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 me
 * Hole effective mass 0.4 me

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. It should not be ingested, nor its dust inhaled, and it should not be handled without appropriate gloves. 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.

The disposal life-cycle and long term safety of cadmium telleride could become an 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.

New Theoretical Model
Making use of the authors' experimental results and the evidence available in the literature, Prof. I.M.Dharmadasa and his co-workers at Sheffield Hallam University in the United Kingdom presented an alternative theoretical model to describe the working scientific mechanism of glass/conducting glass/CdS/CdTe/metal solar cell. This model explains the behaviour of this particular solar cell in terms of a combination of a hetero-junction and a large Schottky barrier at the CdTe/metal interface. In the article, their main experimental observations are compared with the traditionally accepted pn junction model. It is shown that the proposed model explains the experimental results more satisfactorily than the widely used pn junction model. The paper describes the guidelines to further increase the performance efficiencies based on the new model.The full article can be found here: http://www.iop.org/EJ/abstract/0268-1242/17/12/306/