Cadmium telluride photovoltaics

PV array made of cadmium telluride (CdTe) solar panels

Cadmium telluride (CdTe) photovoltaics describes a photovoltaic (PV) technology that is based on the use of cadmium telluride, a thin semiconductor layer designed to absorb and convert sunlight into electricity.[1] Cadmium telluride PV is the only thin film technology with lower costs than conventional solar cells made of crystalline silicon in multi-kilowatt systems.[1][2][3]

On a lifecycle basis, CdTe PV has the smallest carbon footprint, lowest water use and shortest energy payback time of all solar technologies.[4][5][6] CdTe's energy payback time of less than a year allows for faster carbon reductions without short-term energy deficits.

The toxicity of cadmium is an environmental concern mitigated by the recycling of CdTe modules at the end of their life time,[7] though there are still uncertainties[8][9] and the public opinion is skeptical towards this technology.[10][11] The usage of rare materials may also become a limiting factor to the industrial scalability of CdTe technology in the mid-term future. The abundance of tellurium—of which telluride is the anionic form—is comparable to that of platinum in the earth's crust and contributes significantly to the module's cost.[12]

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.[13] A prominent manufacturer of CdTe thin film technology is the company First Solar, based in Tempe, Arizona.

Background

Cross-section of a CdTe thin film solar cell.

The dominant PV technology has always been based on crystalline silicon wafers. Thin films and concentrators were early attempts to lower costs. Thin films are based on using thinner semiconductor layers to absorb and convert sunlight. Concentrators lower the number of panels by using lenses or mirrors to put more sunlight on each panel.

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.[14] The same source estimated that about 3 gigawatts were installed in 2007.

During this period cadmium telluride and copper indium diselenide or CIS-alloys remained under development. The latter is beginning to be produced in volumes of 1–30 megawatts per year due to very high, small-area cell efficiencies approaching 20% in the laboratory.[15] CdTe cell efficiency is approaching 20% in the laboratory with a record of 19.6% as of 2013.[16]

History

Research in CdTe dates back to the 1950s,[17][18][19][20][21][22] because its band gap (~1.5 eV) is almost a perfect match to the distribution of photons in the solar spectrum in terms of conversion to electricity. A simple heterojunction design evolved in which p-type CdTe was matched with n-type cadmium sulfide (CdS). The cell was completed by adding top and bottom contacts. Early leaders in CdS/CdTe cell efficiencies were GE in the 1960s, and then Kodak, Monosolar, Matsushita, and AMETEK.

By 1981, Kodak used close spaced sublimation (CSS) and made the first 10% cells and first multi-cell devices (12 cells, 8% efficiency, 30 cm2).[23] Monosolar[24] and AMETEK[25] 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.[26]

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 transparent conducting oxide (TCO), could facilitate the movement of current across the top of the cell (instead of a metal grid). One such TCO, tin oxide, was available for other uses (thermally reflective windows). Made more conductive for PV, tin oxide became and remains the norm in CdTe PV modules.

The utility-scale Waldpolenz Solar Park in Germany uses CdTe PV modules

CdTe cells achieved above 15% 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.[26]

In the early 1990s, other players experienced mixed results.[26] Golden Photon held the record for a short period for the best CdTe module measured at NREL at 7.7% using a spray deposition technique. Matsushita claimed an 11% module efficiency using CSS and then dropped the technology. A similar efficiency and fate eventually occurred at BP Solar. BP used electrodeposition (inherited from Monosolar by a circuitous route when it purchased SOHIO, Monosolar's acquirer). BP Solar dropped CdTe in the early 2000s. 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.[27]

CdTe start-ups include Calyxo[28] (formerly owned by Q-Cells), PrimeStar Solar, in Arvada, Colorado (acquired by First Solar from GE),[29] Arendi (Italy). Including Antec, their total production represents less than 70 megawatts per year.[30] Empa, the Swiss Federal Laboratories for Materials Testing and Research, focuses on the development of CdTe solar cells on flexible substrates and demonstrated cell efficiencies of 13.5% and 15.6% for flexible plastic foil and glass substrates, respectively.[31]

SCI and First Solar

The major commercial success was Solar Cells Incorporated (SCI).[32] It switched from amorphous silicon to CdTe as a better alternative to the higher-cost silicon. McMaster championed CdTe for its high-rate, high-throughput processing. SCI shifted from an adaptation of the CSS method then shifted to vapor transport.[33] In February 1999, McMaster sold the company to True North Partners, who named it First Solar.[34]

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.[35] The company manufactured in Perrysburg, Ohio and Germany.[36] In 2013, First Solar acquired GE's thin film solar panel technology in exchange for a 1.8% stake in the company.[37] Today, First Solar manufactures over 3 gigawatts with an average module efficiency of 16.4% in 2016.[38]

Technology

Cell efficiency

Solar cell efficiencies

Recently, in August 2014, First Solar announced a device with 21.1% conversion efficiency.[39] In February 2016, First Solar announced that they had reached a record 22.1% conversion efficiency in their CdTe cells. In 2014, the record module efficiency was also raised by First Solar from 16.1% up to 17.0%.[40] At this time, the company projected average production line module efficiency for its CdTe PV to be 17% by 2017, but by 2016, they predicted a module efficiency closer to ~19.5%.[41][42]

Since CdTe has the optimal band gap for single-junction devices, efficiencies close to 20% (such as already shown in CIS alloys) may be achievable in practical CdTe cells.[43]

Process optimization

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 are about $72 per 1 square metre (11 sq ft),[44] or about $90 per module.

Ambient temperature

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.[45][46][47] CdTe PV modules experience half the reduction of crystalline silicon modules, resulting in an increased annual energy output of 5-9%.[48]

Solar tracking

Almost all thin film photovoltaic module systems to-date have been non-solar tracking, because module output was too low to offset tracker capital and operating costs. But relatively inexpensive single-axis tracking systems can add 25% output per installed watt.[49] In addition, depending on the Tracker Energy Gain, the overall eco-efficiency of the PV system can be enhanced by lowering both system costs and environmental impacts.[50] This is climate-dependent. Tracking also produces a smoother output plateau around midday, better matching afternoon peaks.

Cadmium

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.[51]

Tellurium

Tellurium (Te) production and reserves estimates are subject to uncertainty and vary considerably. Tellurium is a rare, mildly toxic metalloid that is primarily used as a machining additive to steel. Te is almost exclusively obtained as a by-product of copper refining, with smaller amounts from lead and gold production. Only a small amount, estimated to be about 800 metric tons[52] per year, is available. According to USGS, global production in 2007 was 135 metric tons.[53] One gigawatt (GW) of CdTe PV modules would require about 93 metric tons (at current efficiencies and thicknesses).[54] Through improved material efficiency and increased PV recycling, the CdTe PV industry has the potential to fully rely on tellurium from recycled end-of-life modules by 2038.[55] In the last decade, new supplies have been located, e.g., in Xinju, China[56] as well as in Mexico and Sweden.[57] In 1984 astrophysicists identified tellurium as the universe's most abundant element having an atomic number over 40.[58][59] Certain undersea ridges are rich in tellurium.[59][60]

Cadmium chloride/magnesium chloride

The manufacture of a CdTe cell includes a thin coating with cadmium chloride (CdCl
2) 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 (MgCl
2) performs as well as cadmium chloride. This research may lead to cheaper and safer CdTe cells.[61][62]

Safety

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.[63] 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.[64][65] 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).[66][67]

Recycling

Due to the exponential growth of photovoltaics the number of worldwide installed PV systems has increased significantly. First Solar established the first global and comprehensive recycling program in the PV industry in 2005. Its recycling facilities operate at each of First Solar’s manufacturing plants and recover up to 95% of semiconductor material for reuse in new modules and 90% of glass for reuse in new glass products.[68][69] A life cycle assessment of CdTe module recycling by the University of Stuttgart showed a reduction in primary energy demand in End-Of-Life from 81 MJ/m^2 to -12 MJ/m^2, a reduction of around 93 MJ/m^2, and in terms of global warming potential from 6 kg CO2-equiv./m^2 to -2.5 CO2-equiv./m^2, a reduction of around -8.5 CO2-equiv./m^2. These reductions show a highly beneficial change in the overall environmental profile of CdTe photovoltaic module. The LCA also showed that the main contributors to considered environmental impact categories are due to required chemicals and energy within the processing of CdTe modules.[70]

Current Research

Grain boundaries are a topic of interest in CdTe cells . It is thought that grain boundaries allow for electrons and holes to become trapped, thus affecting conversion efficiency. Conversion efficiency affects the amount of electricity a solar cell can generate. Due to the relatively recent discovery of this problem, not much research has been done on this topic.[71] It is not currently known if the trapping of electrons and holes is a good or bad thing, current research is ongoing to answer these questions . Grain boundaries are the cracks between two grains of a material and often occur when two grains are misaligned where they meet, they are a type of defect in the cell. Defects are areas in which the crystal lattice of the atomic structure of a material is not in the correct orientation.[71] Grain boundaries are usually in the size range of a few nm in depth and length.[71] Grain interior is a term often associated with grain boundaries and stands for the interior of the grain itself.[72] The trapping of electrons in a grain boundary could potentially create localized pn junctions and make that area of the cell positively charged as holes will be attracted to the region to facilitate recombination.[71] Recombination is the recombining of electrons and holes when the electrons fall out of the conduction band recombine with a hole . On the other hand, if holes are trapped in a grain boundary, this creates a negatively charged local pn junction due to the holes in the area being repelled.[71] It is believed that these local pn junctions affect the overall pn junction of the cell, thus affecting conversion efficiency. This could potentially cause the efficiency of the cell to drop significantly, which would in turn cause the amount of electricity coming out of the cell to drop.

Market viability

The Topaz Solar Farm employs 9 million CdTe-modules. It was the world's largest PV power station in 2014.

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,[73] and capital cost per new watt of capacity is near $0.9 per watt (including land and buildings).[74]

Notable systems

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.[75] Recent installations of large First Solar CdTe PV systems were claimed to be competitive with other forms of solar energy:

See also

References and notes

  1. 1 2 "Publications, Presentations, and News Database: Cadmium Telluride". National Renewable Energy Laboratory.
  2. ↑ K. Zweibel, J. Mason, V. Fthenakis, "A Solar Grand Plan", Scientific American, Jan 2008. CdTe PV is the cheapest example of PV technologies and prices are about 16¢/kWh with US Southwest sunlight.
  3. ↑ Further mention of cost competitiveness: "Solar Power Lightens Up with Thin-Film Technology", Scientific American, April 2008.
  4. ↑ Peng et al. (2013). "Review on life cycle assessment of energy payback and greenhouse gas emission of solar photovoltaic systems". Renewable and Sustainable Energy Reviews. 19: 255–274. doi:10.1016/j.rser.2012.11.035.
  5. ↑ V. Fthenakis and H. C. Kim. (2010). "Life-cycle uses of water in U.S. electricity generation". Renewable and Sustainable Energy Reviews. 14 (7): 2039–2048. doi:10.1016/j.rser.2010.03.008.
  6. ↑ de Wild-Scholten, Mariska (2013). "Energy payback time and carbon footprint of commercial photovoltaic systems". Solar Energy Materials & Solar Cells. 119: 296–305. doi:10.1016/j.solmat.2013.08.037.
  7. ↑ Fthenakis, Vasilis M. (2004). "Life cycle impact analysis of cadmium in CdTe PV production" (PDF). Renewable and Sustainable Energy Reviews. 8 (4): 303–334. doi:10.1016/j.rser.2003.12.001. Archived (PDF) from the original on 23 September 2014.
  8. ↑ Werner, Jürgen H. (2 November 2011). "Toxic Substances In Photovoltaic Modules" (PDF). postfreemarket.net. Institute of Photovoltaics, University of Stuttgart, Germany - The 21st International Photovoltaic Science and Engineering Conference 2011 Fukuoka, Japan. p. 2. Archived from the original (PDF) on 23 September 2014. Retrieved 23 September 2014.
  9. ↑ "Water Solubility of Cadmium Telluride in a Glass-to-Glass Sealed PV Module" (PDF). Vitreous State Laboratory, and AMELIO Solar, Inc. 2011.
  10. ↑ "The Lowdown on the Safety of First Solar’s CdTe Thin Film". greentechmedia.com.
  11. ↑ Guest Column. "Cadmium: The Dark Side of Thin-Film?". gigaom.com.
  12. ↑ "NREL: Manufacturing Analysis - Supply Constraints Analysis". nrel.gov.
  13. ↑ Fraunhofer ISE Photovoltaics Report, July 28, 2014, pages 18,19
  14. ↑ Various estimates of world module production in 2007
  15. ↑ "NREL: News - Record Makes Thin-Film Solar Cell Competitive with Silicon Efficiency". nrel.gov.
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  24. ↑ B. Basol, E. Tseng, R.L. Rod, 1981, Thin film heterojunction photovoltaic cells and methods of making the same, Monosolar U.S. Patent 4,388,483. B. Basol patented numerous aspects of electrodeposition and CdTe contacting for Monosolar. Monosolar was subsequently bought by SOHIO, which was then absorbed by British Petroleum. Electrodeposition continued at BP Solar until about 2002 when it was canceled along with all thin film work at BP.
  25. ↑ Peter Meyers, originally of Ametek, provides a thread stretching from Ametek through Solar Cells Inc. to First Solar. He is on Ametek patents U.S. Patent 4,260,427, 1981; U.S. Patent 4,710,589, 1987; and SCI/First Solar patents
  26. 1 2 3 K. Zweibel (1995). "Thin Films Past Present and Future" (PDF). Nrel/tp-413-7486.R. Noufi and K. Zweibel (2006). High-Efficiency CdTe and CIGS Thin-Film Solar Cells: Highlights and Challenges. National Renewable Energy Laboratory, Golden, CO 80401, USA.
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  29. ↑ "First Solar Acquires GE’s PrimeStar Solar IP, Misses Q2 Revenue and Lowers Guidance". GrenntechMedia. 6 August 2013.
  30. ↑ "While First Solar keeps on trucking, others in CdTe thin-film PV pack keep on muddling". Fabtech.org. 2008-08-21.
  31. ↑ "Empa CdTe—Thin Films and Photovoltaics". Empa. Retrieved 5 August 2015.
  32. ↑ Harold McMaster envisioned the opportunity for 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., the precursor of First Solar; After long wait, McMaster to join hall of fame, April 29, 2008
  33. ↑ SCI CSS patent: Foote et al. Process for making photovoltaic devices and resultant product, United States Patent 5248349; and their vapor transport patent, featuring the movement of vaporized cadmium and tellurium atoms through a closed, silicon carbide distributor: Apparatus and method for depositing a semiconductor material, United States Patent 6037241. Both are now owned by First Solar.
  34. ↑ D. H. Rose; et al. (October 1999). Technology Support of High-Throughput Processing of Thin Film CdTe Panels (PDF). NREL SR-520-27149. p. Viii (preface).
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  36. ↑ Friedman, Thomas L. (5 November 2009). Hot, Flat, and Crowded: Why The World Needs A Green Revolution - and How We Can Renew Our Global Future. Penguin Books Limited. p. 388. ISBN 978-0-14-191850-1.
  37. ↑ First Solar Gives GE 1.8% Stake in Thin-Film Partnership. bloomberg.com. 2013-08-07
  38. ↑ "First Solar Annual Report" (PDF).
  39. ↑ "First Solar Builds the Highest Efficiency Thin Film PV Cell on Record". firstsolar.com.
  40. ↑ "First Solar Sets Thin-Film Module Efficiency World Record of 17.0 Percent". firstsolar.com.
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  44. ↑ This number is calculated by multiplying efficiency (12.6%) by 1000 to get output watts per square meter (126 W/m2), and then multiplying power by the stated cost of $0.57 per watt to get $72/m2
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  49. ↑ "Parabolic Trough Technology Models and Software Tools". 2008-07-25. Retrieved 2008-10-14. Like any solar price model, the Solar Advisory Model is quite sensitive to assumptions. Different sunlight, tax rates, interest rates, discount rates, loan durations, temperature coefficients, annual degradation rates, initial de-rating versus standard conditions, inverter efficiencies and O&M, and others can each have as much as a 10% impact on costs per unit power.
  50. ↑ P. Sinha and S. Dailey (November 2013). "Tracking systems boost eco-efficiency". Solar Industry.
  51. ↑ M. Raugei and V. Fthenakis (2010). "Cadmium flows and emissions from CdTe PV: future expectations". Energy Policy. 38 (9): 5223–5228. doi:10.1016/j.enpol.2010.05.007.
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  53. ↑ "Tellurium" (PDF). Mineral Commodity Summaries. United States Geological Survey. January 2008.
  54. ↑ "First Solar CdTe Photovoltaic Technology: Environmental, Health and Safety Assessment". National Renewable Energy Centre. October 2013. p. 32.
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  56. ↑ Publications of the Sichuan Xinju Mineral Resource Development Co., China
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  59. 1 2 Hein, J. (2004). "Chapter 5 from Workshop on Minerals other than Polymetallic Nodules of the International Seabed Area". Cobalt-Rich Ferromanganese Crusts: Global Distribution, Composition, Origin and Research Activities. Kingston, Jamaica: Office of Resource and Environmental Monitoring, International Seabed Authority. ISBN 976-610-647-9. 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."
  60. ↑ Hein, J.; Koschinsky, A.; Halliday, A. (2003). "Global Occurrence of tellurium-rich ferromanganese crusts and a model for enrichment of tellurium". Geochimica et Cosmochimica Acta. 67: 1117–1127. doi:10.1016/S0016-7037(00)01279-6 (inactive 2017-07-27). 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
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