What is Cadmium telluride (CdTe) photovoltaics?
Cadmium telluride (CdTe) photovoltaics or also called Cadmium telluride solar cell is a kind of photovoltaic (PV) technology that can produce electricity from sunlight using a thin-film of compound cadmium telluride to absorb and convert sunlight into electricity. Solar photovoltaic technology that is based on cadmium is considered as a thin-film technology due to its active layers which are just a few microns thick, or about a 10 diameter of human hair. Cadmium telluride is the second most utilized solar cell material in the world and is different from a crystalline silicon photovoltaic system because CdTe uses a smaller amount of a thin-film semiconductor material.
Although solar panels based on CdTe are less efficient than crystalline silicon PV system, it is still the first and only thin-film PV technology that is cheaper to produce than crystalline silicon PV on the photovoltaic market, namely in multi-kilowatt systems. Besides, thin-film technology has a big potential to surpass silicon-based solar panels in terms of its cost per kilowatt. It may have a small share in the Solar panel market, it is still expected to grow rapidly in the future as manufacturers most focused their interest in developing an off-center manufacturing approach that can possibly unluck economies of larger-scales.
In addition, using cadmium as the main component of the thin-film solar cell is claimed to be more stable and less soluble compared to other electronics and its impact has a little risk to human health and the environment. Since the alloys are enclosed in a capsule within the modules. Though, there have been some concerns regarding cadmium leaching from the broken modules. Besides, even if it has been promoted that closed-loop recycling can possibly address any concerns over its disposal, still critics emphasized that closed-loop recycling systems are not capable enough to recover everything.
When it comes to a lifecycle basis, CdTe PV only produces little portion of carbon footprint, has the lowest water usage and even has the shortest energy payback time of unlike any current photovoltaic technology. The payback time of CdTe’s energy is only less than a year which helps in reducing the carbon easier and faster without short-term energy loss.
Materials Used in Cadmium telluride (CdTe) photovoltaics
Cadmium (Cd) is a toxic heavy metal and considered a perilous substance. It is a waste byproduct coming from smelting, mining, and refining sulfidic ores of zinc. The main reason why the PV market is not considering this a good material for photovoltaic technology. However, the CdTe PV modules provide safe and beneficial use for cadmium that can be stored for future applications and utilization. But it can be disposed of in landfills as hazardous waste. Byproducts of mining can be converted into a stable CdTe compound and safely enclosed in a capsule and deposited inside the CdTe PV solar modules for some years. Moreover, by the displacement of coal and the generation of oil power, a large growth in the CdTe PV sector has the potential to reduce global cadmium emissions.
Tellurium is considered a rare, brittle mildly toxic silvery-white metalloid that is mainly used as a machining additive to steel. Te is occasionally found native and only a small amount around 800 metric tons per year, is available. But this is more often found as the telluride of gold and is commonly combined with other metals. This element is almost obtained exclusively as a byproduct of copper refining, with smaller amounts from gold and lead production. The production of tellurium are subject to uncertainty and vary considerably. USGS stated that one gigawatt (GW) of CdTe PV technology requires about 93 metric tons at current efficiencies and thicknesses. With the aid of well-improved material efficiency and a significant increased in PV recycling, the CdTe PV industry has more potential to depend fully on tellurium from recycled end-of-life modules by the year 2038.
The production of a Cadium telluride cell includes a thin coating with cadmium chloride (CdCl) to increase the overall efficiency of the cell. It is a toxic white crystalline compound of cadmium and chlorine. It is a hygroscopic solid that is slightly soluble in alcohol but highly soluble in water which can pose a potential environmental threat during the production. It is also considered to be relatively expensive. Moreover, although it is considered as an ionic compound, it still has considerable covalent bond characteristics.
Properties of Cadmium telluride Photovoltaics
- Appearance: it is a black crystalline powder
- Odour: Odourless
- Solubility: insoluble material
- Molar Mass: 240.01 g/mol
- Melting Point: 1092 °C
- Boiling Point: 1130 °C
- Density: 6.2 g/cm3
- Vapour Pressure: 0.4 mmHg at 760 °C
- Crystal Structure: zincblende crystal structure
- Bandgap: 1.44 eV (at 300 K, direct)
- Thermal Conductivity: 6.2 W/m·K at 293 K
- Specific Heat Capacity: 210 J/kg·K at 293 K temperature
- Thermal Expansion Coefficient: 5.9×10−6/K at 293 K
- Lattice Constant: 0.648 nm at 300K
- Young’s Modulus: 52 GPa
- Poisson Ratio: 0.41
- Refractive Index: 2.67 (at 10 µm)
Development of Cadmium telluride (CdTe) Photovoltaics
A typical photovoltaic technology is always based on crystalline silicon wafers. While concentrators and thin films were early used to lower the PV costs. These thin films are based on using thinner semiconductor PV material to absorb and convert sunlight into electricity while concentrators are used to lower the number of panels by using a variety of lenses to put more sunlight on each panel.
The first thin-film technology that has been developed was the amorphous silicon where the silicon was randomly placed onto a substrate, opposing the regular crystal lattice that can be seen in the water crystals. Unfortunately, this technology had some issues. The process of placing the silicon onto the substrate is indeed time-consuming and the manufacturing is costly. Also, the cells were very less efficient. Since CdTe thin-film technology is around 11 percent more efficient compared to that amorphous silicon, it is more ideal to use. The bandgap od CdTe is around 1.4 electron volts which match the solar spectrum very well.
Aside from that, it is also more conducive when it comes to mass production, as the CdTe thin-film technology can be placed onto the substrate easier and quicker and is considered a high-throughput technology. Each cell contains different junctions such as a junction of n-doped cadmium sulphide which is known as the “window layer”, and on top of that, there’s a junction of p-doped layer of cadmium telluride which known as the “absorber.” Whereas, a transparent conductive front contact covers the cadmium sulphide and the CdTe is in contact with a conductive rear surface substrate.
Despite the numerous potentials of the CdTe PV technology, the electronics industry decided to remove the elemental cadmium from any personal electronics due to the cumulative poison that cadmium has. In fact, in some regions of Europe, the Restriction of Hazardous Substances (RoHS) legislation has been thorough in removing cadmium elements from all electronic devices due to its health effects. Health risk of the consumer is not only the reason why it has been eliminated but also its danger and harmful effect to miners when the raw materials are being extracted, for all workers who are processing the material even its health risk impact at the end of its life during its disposal.
Cadmium Telluride photovoltaics are being used in some largest photovoltaic power stations worldwide, such as the Topaz Solar Farm. This solar power farm allotted a share of 5.1 percent of photovoltaic production worldwide. Also, CdTe technology was valued in 2013 for being more than half of the thin film market in the said year. The prominent manufacturer of the CdTe thin-film Pv system is the First Solar company which is located in Tempe, Arizona.
Benefits and Drawbacks of CdTe
Benefits of Cadmium Telluride Photovoltaics
Cadmium telluride offers several advantages than crystalline silicon (Si). This substance is originally used in manufacturing photovoltaic cells. Whereas, the functional layer of the CdTe PV can be made extremely thin, on the order of a few micrometres around units of 10 to 6 metre which is far thinner than a typical crystalline silicon layer. In terms of manufacturing, CdTe solar cells only have low-cost cost manufacturing technology which also produces low-cost solar cells.
Aside from that, cadmium telluride thin film has better efficiency than crystalline silicon PV, particularly at low levels of illumination and high temperatures. Due to the high absorption rate of sunlight and bandgap energy at around 1.45 defined (eV), which is closely near the ideal solar spectrum wavelength, it has much better efficiency. Additionally, CdTe solar cells require less energy to produce than silicon-based solar cells.
Drawbacks of Cadmium Telluride Photovoltaics
On the other hand, CdTe Pv has some negative impacts on both human health and the environment. Some environmental groups have expressed their concerns regarding the existence of cadmium in CdTe solar cells. Since cadmium is a toxic heavy metal that is similar to lead and mercury it was considered dangerous in some aspects. However, the advocates of CdTe photovoltaic technology stress out that the compound cannot be dissolved in water so the Cd element will not pose any hazard effect to aquifers. Also, CdTe has no capacity to vaporize in a fire.
Nevertheless, some concerns are still existing all over the world regarding the negative impacts of CdTe solar cells. Whereas, the big supply of telluride is coming from China. But this element is existing in all seafloor, and it can be mine, so shortages of telluride will not cause a problem in the long term.
Cadmium Telluride's Future
Cadmium telluride technology will provide good offers as alternative energy sources for the next decade if traditional energy sources will continue to increase in scrutiny, regulation, and even price volatility. Currently, most kinds of photovoltaic solar cells remain considerably more expensive than the electricity produced by natural-gas-fired, coal-fired, and nuclear power plants. However, the solar industry still believes that this situation may change over time as PV technology becomes less costly and effective while conventional energy sources grow ever-more costly and risky.
Reviews about Cadmium telluride Photovoltaics (CdTe)
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Advancements in solar technology and the rapidly-expanding landscape of photovoltaic arrays are raising concerns about environmental toxicity — namely the use of Cadmium telluride (CdTe) in most photovoltaic (PV) solar cells.
The question of what happens when indictments of current energy sources are also levied towards alternative sources is an important one.
Does renewable energy pass the same scrutiny?
Biofuels have fallen under fire on numerous occasions over invasive cropsNitrous Oxide emissions, and water/land consumption that dominates food crops. The wind has also taken a slight lack over migratory bird flight paths, air currents, and noise. Geothermal is considered by some to cause earth quakes, and solar — well, it doesn’t get a free pass either.
In the case of solar, the use of CdTe — a considered toxic substance — in most solar panels is raising concerns about land pollution.
Are these concerns legit?
To answer, we need a cursory understanding of CdTe’s function.
What is Cadmium Telluride?
A crystalline compound formed between Cadmium and Tellurium, CdTe is a highly effective component of photovoltaic solar panels. Though not as effective as the previously-used polysilicon, CdTe appears to be preferred because comparably it is more cost-effective to derive and implement.
Cadmium, a byproduct of mining mostly found in Asia, is one of the six most toxic elements currently banned by the EU’s Restriction of Hazardous SubstancesDirective (RoHS) for electronic devices. Tellurium, though not as toxic as Cadmium, is a rarely-used element with extremely limited supplies.
Though its toxicity is somewhat reduced when combined with Tellurium,CdTe remains a toxic substance [pdf] when it enters the human body–whether through inhalation or ingestion.
How CdTe Functions in PV Solar:
CdTe’s use in PV technology is based on the implementation of cadmium telluride thin film, a semiconductor layer designed to absorb and convert sunlight into electricity.
The device’s structure also includes a very thin layer of Cadmiumsulfide that allows most sunlight to pass through to the CdTe layer.
What’s interesting to note is that First Solar’s panels that use CdTe are neither on the “exception” list by the RoHS, or the banned list — even though it appears that their products do not meet the rigorous RoHS standards.
Also, it is important to note is that the amounts of CdTe currently being used appear to be negligible. However, if CdTe becomes the standard compound for which all thin-film solar cell production relies, the story changes dramatically.
In Part Two, we’ll look at what the risks of CdTe are, what the industry is doing to minimize impact, and whether or not it’s enough.
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