Nurcihan AYDEMİR Kürşad UYANIK

 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.  Cadmium telluride (CdTe) is a photovoltaic (PV) technology based on the use of a thin film of CdTe to absorb and convert sunlight into electricity.

 Best cell efficiency has plateaued at 16.5% since 2001 (a record held by NREL). The opportunity to increase current has been almost fully exploited, but more difficult challenges associated with junction quality, with properties of CdTe and with contacting have not been as successful.  Improved doping of CdTe and increased understanding of key processing steps (e.g., cadmium chloride recrystallization and contacting) are key to improving cell efficiency. Since CdTe has the optimal band gap for single-junction devices, it may be expected that efficiencies close to exceeding 20% (such as already shown in CIS alloys) should be achievable in mass produced CdTe cells.  In 2009, EMPA, the Swiss Federal Laboratories for Materials Testing and Research, demonstrated a 12.4% efficient solar cell on flexible plastic substrate.

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

Fig. 1. Crystallographic orientation and (a 1 ) Chemical etching of the pure CdTe substrate.

 Available as a powder or as a crystal.

 CdTe photovoltaics is a technology that uses the compound Cadmium Telluride (CdTe) as the semiconductor material in photovoltaics cell that convert sunlight into DC (direct current) electricity.  Photovoltaic (PV) cells are an integral part of solar electric-energy systems which are becoming increasingly important as alternative sources of utility power.

Fig.2. Structure of CdTe

 CdTe offers some advantages over crystalline silicon (Si), the substance originally used in the manufacture of PV cells.  A functional layer of CdTe can be made extremely thin, on the order of a few micrometers (units of meter), far thinner than a typical Si layer.  CdTe offers better efficiency than Si at high temperatures and low levels of illumination.

Fig.3. Flexible CdTe Thin Film

 In addition, CdTe PV cells require less energy to produce than Si PV cells do.  Cadmium (Cd) and Tellurium (Te) are both products of zinc, copper and gold mining.

Fig.4. Solar panels equipped with CdTe

 Otherwise, some concern over the existance of Cd in CdTe PV cells. Cadmium is a toxic heay metal. However, the proponents of CdTe technology point out that the compound does not dissolve in water, so the elemental Cd should not pose any hazard to aquifers.

 Cadmium is one of the top 6 deadliest and toxic materials known. However, CdTe appears to be less toxic than elemental cadmium, at least in terms of acute exposure.  This is not to say it is harmless. Cadmium telluride is toxic if ingested, if it’s dust is inhaled, or if it is handled improperly (i.e. without appropriate gloves and other safety precautions).

 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 compound of cadmium chloride.

 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 resolves any environmental concerns.

 Ease of manufacturing: The necessary electric field, which makes turning solar energy into electricity possible, stems from properties of two types of cadmium molecules, cadmium sulfide and cadmium telluride. This means a simple mixture of molecules achieves the required properties, simplifying manufacturing compared to the multi-step process of joining two different types of doped silicon in a silicon solar panel.

 Good match with sunlight: Cadmium telluride absorbs sunlight at close to the ideal wavelength, capturing energy at shorter wavelengths than is possible with silicon panels

 Cadmium is abundant: Cadmium is abundant, produced as a by-product of other important industrial metals such as zinc, consequently it has not had the wider price swings that have happened in the past two years with silicon prices.