On a lifecycle basis, CdTe PV has the smallest carbon footprint, lowest water use and shortest energy payback time of any current photovoltaic technology.[4][5][6][7] CdTe's energy payback time of less than a year allows for faster carbon reductions without short-term energy deficits.
The toxicity of
CdTe photovoltaics are used in some of the world's largest photovoltaic power stations, such as the Topaz Solar Farm. With a share of 5.1% of worldwide PV production, CdTe technology accounted for more than half of the thin film market in 2013.[14] A prominent manufacturer of CdTe thin film technology is the company First Solar, based in Tempe, Arizona.
The dominant PV technology has always been based on
The first thin film technology to be extensively developed was amorphous silicon. However, this technology suffers from low efficiencies and slow deposition rates (leading to high capital costs). Instead, the PV market reached some 4 gigawatts in 2007 with crystalline silicon comprising almost 90% of sales.[15] The same source estimated that about 3 gigawatts were installed in 2007.
During this period cadmium telluride and
Research in CdTe dates back to the 1950s,Matsushita, and AMETEK.[citation needed]
By 1981, Kodak used close-space sublimation (CSS) and made the first 10% efficient cells and first multi-cell devices (12 cells, 8% efficiency, 30 cm2).[24] Monosolar[25] and AMETEK[26] used electrodeposition, a popular early method. Matsushita started with screen printing but shifted in the 1990s to CSS. Cells of about 10% sunlight-to-electricity efficiency were produced by the early 1980s at Kodak, Matsushita, Monosolar and AMETEK.[27]
An important step forward occurred when cells were scaled-up in size to make larger area products called modules. These products required higher currents than small cells and it was found that an additional layer, called a
CdTe cells achieved above 15% efficiency in 1992 by adding a buffer layer to the TCO/CdS/CdTe stack and then thinned the CdS to admit more light. Chu used resistive tin oxide as the buffer layer and then thinned the CdS from several micrometres to under half a micrometre in thickness. Thick CdS, as it was used in prior devices, blocked about 5 mA/cm2 of light, or about 20% of the light usable by a CdTe device. The additional layer did not compromise the device's other properties.[27]
In the early 1990s, other players experienced mixed results.SOHIO, Monosolar's acquirer). BP Solar dropped CdTe in November 2002.[28] Antec was able to make about 7%-efficient modules, but went bankrupt when it started producing commercially during a short, sharp market downturn in 2002. However, as of 2014 Antec still made CdTe PV modules.[29]
CdTe start-ups include Toledo Solar Inc (100 megawatts per year),
The major commercial success was by Solar Cells Incorporated (SCI). Its founder, Harold McMaster, envisioned low-cost thin films made on a large scale. After trying amorphous silicon, he shifted to CdTe at the urging of Jim Nolan and founded Solar Cells Inc., which later became First Solar.[34] McMaster championed CdTe for its high-rate, high-throughput processing. In February 1999, McMaster sold the company to True North Partners, who named it First Solar.[35]
In its early years First Solar suffered setbacks, and initial module efficiencies were modest, about 7%. Commercial product became available in 2002. Production reached 25 megawatts in 2005.[36] The company manufactured in Perrysburg, Ohio and Germany.[37] In 2013, First Solar acquired GE's thin film solar panel technology in exchange for a 1.8% stake in the company.[38] Today, First Solar manufactures over 3 gigawatts with an average module efficiency of 16.4% in 2016.[39]
First Solar notably uses a high-rate vapor transport deposition process in lieu of CSS (closed space sublimation) for the deposition of CdTe. This is a type of physical vapor deposition where CdTe is first sublimated in an upstream region. Then, the Cd and Te2 gases flow across a cooler downstream region where they condense on a substrate to form solid CdTe.[40] This process is preferred over CSS because it produces films of greater uniformity and allows for deposition on any configuration of the substrate.[41]
In August 2014 First Solar announced a device with 21.1%
To reach these record high efficiencies of 22%, alloying is used for band gap grading. A compound incorporating selenium into CdTe is used in the solar cell to improve the quantum efficiency response for certain wavelengths of light, in addition to unalloyed CdTe.[46] The other major contributor to this large increase in efficiency is the usage of MgZnO (MZO) within the cell. In a cell using a CdSexTe1−x/CdTe structure, MZO can be used in place of CdS. CdS is source of inefficient absorption, while MZO has a tunable band gap that can be optimized for high transparency and good alignment with CdSexTe1−x.[47]
Process optimization improved throughput and lowered costs. Improvements included broader substrates (since capital costs scale sublinearly and installation costs can be reduced), thinner layers (to save material, electricity, and processing time), and better material utilization (to save material and cleaning costs). 2014 CdTe module costs were about $72 per 1 square metre (11 sq ft),[48] or about $90 per module.[citation needed]
Module efficiencies are measured in laboratories at standard testing temperatures of 25 °C, however in the field modules are often exposed to much higher temperatures. CdTe's relatively low temperature coefficient protects performance at higher temperatures.[49][50][51] CdTe PV modules experience half the reduction of crystalline silicon modules, resulting in an increased annual energy output of 5-9%.[52]
Almost all thin film
Cadmium (Cd), a toxic heavy metal considered a hazardous substance, is a waste byproduct of mining, smelting and refining sulfidic ores of zinc during zinc refining, and therefore its production does not depend on PV market demand. CdTe PV modules provide a beneficial and safe use for cadmium that would otherwise be stored for future use or disposed of in landfills as hazardous waste. Mining byproducts can be converted into a stable CdTe compound and safely encapsulated inside CdTe PV solar modules for years. A large growth in the CdTe PV sector has the potential to reduce global cadmium emissions by displacing coal and oil power generation.[55]
The manufacture of a CdTe cell includes a thin coating with cadmium chloride (CdCl2) to increase the cell's overall efficiency. Cadmium chloride is toxic, relatively expensive and highly soluble in water, posing a potential environmental threat during manufacture. In 2014 research discovered that abundant and harmless magnesium chloride (MgCl2) performs as well as cadmium chloride. This research may lead to cheaper and safer CdTe cells.[65][66]
By themselves, cadmium and tellurium are toxic and carcinogenic, but CdTe forms a crystalline lattice that is highly stable, and is several orders of magnitude less toxic than cadmium.[67] The glass plates surrounding CdTe material sandwiched between them (as in all commercial modules) seal during a fire and do not allow any cadmium release unless the glass is broken.[68][69] All other uses and exposures related to cadmium are minor and similar in kind and magnitude to exposures from other materials in the broader PV value chain, e.g., to toxic gases, lead solder, or solvents (most of which are not used in CdTe manufacturing).[70][71]
The grain boundary is the interface between two grains of a crystalline material and occur when two grains meet. They are a type of crystalline defect. It is often assumed that the open-circuit voltage gap seen in CdTe, in comparison to both single-crystal GaAs and the theoretical limit, may be in some way attributable to the grain boundaries within the material. There have however been a number of studies which have suggested not only that GBs are not deleterious to performance but may in fact be beneficial as sources of enhanced carrier collection. So, the exact role of the grain boundaries in limitation of performance of CdTe-based solar cells remains unclear and the research is ongoing to address this question. However, in as-grown CdTe, grain boundaries are detrimental to performance. Subsequent processing may change this, but those effects should be studied on a case-by-case basis.[72]
The size of the grains, and thus, the number of grain boundaries in a CdTe film are dependent on substrate temperature during film deposition. The higher the substrate temperature is, the larger the grain size, and the fewer the number of grain boundaries in the film. If a low substrate temperature is used during deposition, grain size is commonly increased by depositing CdCl2 on the film and subsequently annealing. This is a crucial processing step, as cells deposited at low temperatures that lack this step are unable to reach conversion efficiencies above 10%.[47]
Photovoltaic modules can last anywhere from 25 – 30 years. Improper disposal of PV modules can release toxic materials into the environment.[73] Only three methods of high-value recycling are industrially available for thin-film PV modules, as of 2013. SENSE (Sustainability EvaluatioN of Solar Energy systems) and RESOLVED (REcovery of SOLar Valuable materials, Enrichment, and Decontamination) are European funded procedures. SENSE relies on mechanical, chemical and thermal treatments. RESOLVED relies on mainly mechanical treatments. The final method, First Solar, relies on mechanical and chemical processes. Mechanical methods of recycling are more environmentally friendly as they do not rely on the use of chemicals.[73]
Materials that can be recovered in the recycling process include metals, mounts, glass, and, in high value cases, the whole PV module.[74]
As of 2013 the recycling costs for CdTe modules are higher than the re-sale of recycled materials. However, possible future recycling methods may decrease in cost through reduction of expensive and environmentally unfriendly processes.[73] Promising future recycling methods include vulcanization-vacuum distillation and the Double Green Process. Vulcanization-vacuum distillation has been suggested as a possible recycling process to obtain Te and can recover Te with purities up to 99.92%.[75] The Double Green Process consists of almost entirely mechanical processes.[76]
Due to the exponential
Photovoltaics can assist in reducing toxic emissions and pollution caused by
Cadmium telluride photovoltaic cells have negative impacts on both workers and the ecosystem.[81] When inhaled or ingested the materials of CdTe cells are considered to be both toxic and carcinogenic by the US Occupational Safety and Health Administration. Workers in processing facilities may be exposed to, and inhale, fine particles or fumes of Cd.[80]
CdTe production facilities may cause environmental issues when there are accidents in high-efficiency production or from by-product exhaust in less efficient production methods.[80]
During a module's lifespan it will not release any particles or vapors if used as intended. The only way for a completed module to release dust or vapor is by being ignited, or ground into a fine dust. When exposed to temperatures of approximately 1100 °C in laboratory tests, 0.4% to 0.6% of the Cd content was released.[74]
The overall Cd air emission estimates can range from 0.02 to 0.5 grams per gigawatt-hour.[74]
Early CdTe modules failed
Success of cadmium telluride PV has been due to the low cost achievable with the CdTe technology, made possible by combining adequate efficiency with lower module area costs. Direct manufacturing cost for CdTe PV modules reached $0.57 per watt in 2013,[82] and capital cost per new watt of capacity was about $0.9 per watt (including land and buildings) in 2008.[83]
Utility-scale CdTe PV solutions were claimed to be able to compete with peaking fossil fuel generation sources depending on irradiance levels, interest rates and other factors such as development costs.[84] Recent installations of large First Solar CdTe PV systems were claimed to be competitive with other forms of solar energy:
2011/02 Solar Fields LLC takes over Q-Cells shares
It has been suggested that Te is unique in the universe in that its cosmic abundance is as great or greater than any of other element with an atomic number higher than 40, yet it is one of the least abundant elements in the Earth's crust and in ocean water."
The ridges occur at 400-4000 m depths where currents have kept the rocks swept clean of sediments for millions of years. Crusts…forming pavements up to 250 mm thick
{{cite journal}}