Cadmium Telluride (CdTe) Thin Film Solar Cells

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Thin-film photovoltaic cells are battling for supremacy in the solar world by offering significant benefits over traditional crystalline silicon solar cells.  Cadmium Telluride (CdTe) cells offer benefits in its absorptivity, having a bandgap energy of 1.45 electron volts (eV) that makes it well suited to converting electricity into sunlight using a single junction.  These single-junction cells have a bandgap energy almost perfectly suited to the solar spectrum.

In addition to the absorptivity of CdTe, thin film photovoltaic cells are easily manufactured through low cost methods such as sublimation, chemical vapor deposition, spincoating and electrodeposition.  While the commercial production efficiency of these cells are only 10-13 percent—18.7 percent is the record efficiency for lab produced cells[1]—they require fewer materials to install and produce, potentially reducing balance of system (BoS) costs.  GE and First Solar panels use much less rack and wire equipment than other types of solar panels on the market.  Reduction of BoS costs are critical to meeting the Sunshot Initiative goal of making solar photovoltaics cost competitive with conventional forms of energy by 2020.

CdTe Thin Film cells are manufactured by continuous manufacturing processes in a semiconductor manufacturing environment.  A thin p-doped CdTe layer is grown on a thin n-doped Cadmium Sulfide (CdS) layer.  This layer is then annealed, commonly through an anaerobic cadmium chloride vapor treatment process.  A transparent conductive oxide layer, typically comprised of fluorine-doped tin oxide, is deposited on the CdS n-layer, and a back electrical contact layer is added to complete the cel.  The cells are then sandwiched between a thin composite glass sheet and a thin sheet of aluminum backing to make the panels.

According to First solar, their continuous manufacturing process uses 98% less semiconductor material compared to silicon solar cell manufacturing, and produces a finished panel in about 2.5 hours.  Most CdTe thin films also have recycling costs built into the panel cost to preserve the rare tellurium resources for future manufacturing.



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The function of CdTe thin films in the solar cell design is to reduce the cost and increase the absorptivity of solar cells over crystalline silicon solar cells.



Benefit Summary: 

CdTe thin films offer potential benefits in the energy and resource consumption during the manufacture of solar cells, increasing their life cycle sustainability. These cells are lighter, lower profile, require fewer BoS inputs and are cheaper to manufacture. While the reduction in material inputs is an added benefit for thin film CdTe modules, more R&D is being done to increase the efficiency of commercially produced CdTe thin film cells. The BoS cost reductions of CdTe thin films makes them attractive to large, utility scale systems where the marginal decrease in efficiency and better performance in high heat and adverse conditions outweighs the reductions in efficiency over silicon panels. In distributed applications, every inch of rooftop or backyard real-estate is valuable so a reduction in efficiency makes thin films less attractive to land-owners looking to install solar. Thin-film manufacturers such as First Solar and GE Energy are making considerable R&D investments in reducing the thickness of CdTe from 3 microns to 200 nm in production runs so as to increase efficiency and reduce the consumption of tellurium—a mineral with very limited known reserves. The current reserves of tellurium are enough to produce approximately 7 gigawatts (GW) of panels, or about three times what First Solar planned to produce last year. Reducing the thickness of the CdTe layer to the ideal thickness of 200nm would maximize efficiency, extend current tellurium reserves by 15 times and potentially cut the dollars-per-watt manufactured cost of CdTe in half. Once these efficiency benefits are realized, CdTe cells may be more efficient throughout their entire life cycle than other commercially available solar cell technologies. [1]


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Risk Summary: 

Cadmium toxicity raises some concerns over the use of the metal in solar panels among other consumer goods. Acute cadmium exposure, through inhalation or ingestion can cause severe respiratory and gastrointestinal effects. Chronic exposure has negative skeletal, cardiovascular, respiratory, renal and gastrointestinal effects. While these risks are real, they are very well controlled during the manufacturing process of CdTe cells. Additionally, studies have concluded that there is little risk of vaporization during fire as well as zero risk of leeching during the normal lifecycle of these cells. Well-funded and supported recycling programs also ensure that the panels will be collected and recycled upon discontinuation of use, even if the manufacturer ceases to exist. The current recycling programs and advancements in manufacturing process development also quell environmental concerns over the limited availability of tellurium. Mining concerns of cadmium are also minimal due to cadmium being a waste product of lead, zinc and copper mining. The collection of cadmium during the mining process for these other metals insures that the cadmium will not leach into the ground around the mine tailings. [1] [2]


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