Cadmium telluride photovoltaics
Cadmium telluride (CdTe) photovoltaics describes a photovoltaic (PV) technology that is based on the use of cadmium telluride thin film, a semiconductor layer designed to absorb and convert sunlight into electricity. Cadmium telluride PV is the only thin film photovoltaic technology to surpass crystalline silicon PV in cheapness for a significant portion of the PV market, namely in multi-kilowatt systems.
Since inception, the dominant solar cell technology in the marketplace has been based on wafers of crystalline silicon. During the same period, the idea of developing alternative, lower cost PV technologies led to the consideration of thin films and concentrators. Thin films are based on using thinner semiconductor layers to absorb and convert sunlight; concentrators, on the idea of replacing expensive semiconductors with lenses or mirrors. Both reduce cost, in theory, by reducing the use of semiconductor material. However, both faced critical challenges.
The first thin film technology to be extensively developed and manufactured was amorphous silicon. However, this technology suffers from low efficiencies and slow deposition rates (leading to high capital costs) and has not become a market leader. Instead, the PV market has grown to almost 4 gigawatts with wafer-based crystalline silicon comprising almost 90% of sales. Installation trails production by a slight time lag, and the same source estimates about 3 gigawatts were installed in 2007.
During this period, two other thin films continued in development (cadmium telluride, and copper indium diselenide or CIS-alloys). The latter is beginning to be produced in start-up volumes of 1–30 megawatts per year by individual companies and remains an unproven, but promising market competitor due to very high, small-area cell efficiencies approaching 20% in the laboratory.
Research in CdTe dates back to the 1950s, because it was quickly identified as having a band gap (about 1.5 eV) almost perfectly matched to the distribution of photons in the solar spectrum in terms of optimal 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). Monosolar and AMETEK 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 being made by the early 1980s at Kodak, Matsushita, Monosolar, and Ametek.
An important step forward occurred when cells were being scaled-up in size to make larger area products called modules. These products require 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 already being applied to glass for other uses (thermally reflective windows). Made more conductive for PV, tin oxide became and remains the norm in CdTe PV modules.
Professor Ting L. Chu of Southern Methodist University and subsequently of University of South Florida, Tampa, made significant contributions to moving the efficiency of CdTe cells to above 15% in 1992, a critical level of success in terms of potential commercial competitiveness. This was done when he added an intervening or buffer layer to the TCO/CdS/CdTe stack and then thinned the CdS to allow more light through. 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. By removing this loss while maintaining the other properties of the device, Chu reached 15% efficiency in 1991, the first thin film to do so, as verified at the National Renewable Energy Laboratory(NREL). Chu used CSS for depositing the CdTe. For his achievements in taking CdTe from its status as “also-ran” to a primary candidate for commercialization, some think of Ting L. Chu as the key technologist in the history of CdTe development.
In the early 1990s, another set of entrants were active in CdTe commercial development, but with mixed results. A short-lived company, Golden Photon replaced Photon Energy, when it was bought by the Coors Company in 1992. Golden Photon, led by Scot Albright and John Jordan, actually held the record for a short period for the best CdTe module measured at NREL at 7.7% using a spray deposition technique. Meanwhile Matsushita, BP Solar, and Solar Cells Inc. were active. Matsushita claimed an 11% module efficiency using CSS and then dropped out of the technology, perhaps due to internal corporate pressures over cadmium. A similar efficiency and fate eventually occurred at BP Solar. BP used electrodeposition inherited from Monosolar by a circuitous route when it purchased SOHIO. SOHIO had previously bought Monosolar. BP Solar however never made a complete commitment to their CdTe technology despite its achievements and dropped it in the early 2000s. Another ineffective corporate evolution occurred at a European entrant, Antec. Founded by CdTe pioneer Dieter Bonnet (who made cells in the 1960s), Antec was able to make about 7%-efficient modules, but went bankrupt when it started producing commercially during a short, sharp downturn in the market in 2002. Purchased from bankruptcy, it never regained the technical traction needed to make further progress. However, as of 2008 Antec does make and sell CdTe PV modules.
There are a number of start-ups in CdTe today: Q-Cells' Calyxo (Germany), GE’s PrimeStar Solar (Arvada, Colorado), Arendi (Italy), and Abound Solar (Fort Collins, Colorado). Including Antec, their total production represents less than 70 megawatts per year. In February 2009, Roth & Rau announced to develop turnkey CdTe production lines and launch the business before end of 2009.
There has been a lot of research on fabricating CdTe cells on flexible substrate since 1999. In 2009, EMPA, the Swiss Federal Laboratories for Materials Testing and Research, demonstrated a 12.4% efficient solar cell on flexible polyimide substrate.
SCI and First Solar 
The major commercial success to emerge from the turmoil of the 1990s was Solar Cells Incorporated (SCI). Founded in 1999 as an outgrowth of a prior company, Glasstech Solar (founded 1984), led by inventor/entrepreneur Harold McMaster, it switched from amorphous silicon to CdTe as a better solution to the higher-cost crystalline silicon PV. McMaster championed CdTe for its high-rate, high-throughput processing. Technical leadership came from a team that included Jim Nolan, Rick Powell, Jim Foote, and Peter Meyers, with consulting help from Ting Chu and Al Compaan (U. Toledo). SCI started with an adaptation of the CSS method then shifted to a vapor transport approach, inspired by Powell. In February 1999, McMaster sold the company to True North Partners, an investment arm of the Walton family, owners of Wal-Mart. John T. Walton joined the Board of the new company, and Mike Ahearn of True North became the CEO of the newly minted First Solar.
In its early years First Solar suffered setbacks, and initial module efficiencies were modest, about 7%. Commercial product became available in 2002. But production did not reach 25 megawatts until 2005. The company built an additional line in Perrysburg, Ohio, then four lines in Germany, supported by the then substantial German production incentives (about 50% of capital costs). In 2006 First Solar reached 75 MW of annual production and announced a further 16 lines in Malaysia. The more recently announced lines have been operational ahead of schedule. As of 2008, First Solar is producing at nearly half a gigawatt annual rate, and in 2006 and 2007 was among the largest PV module manufacturers in the world.
GE announced plans to spend $600 million on a new plant and enter this market 
Cell efficiency 
Best cell efficiency has plateaued at 16.5% since 2001. 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. However, until recently the number of active scientists in CdTe PV was small. Improved doping of CdTe and increased understanding of key processing steps (e.g., cadmium chloride recrystallization and contacting) are key to progress. 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 practical CdTe cells. Modules of 15% would then be possible.
Process optimization 
Process optimization allows greater throughput at smaller cost. Typical improvements are broader substrates (since capital costs scale sublinearly, and installation costs can be reduced), thinner layers (to save material, electricity, and throughput time), and better material utilization (to save material and cleaning costs). Making components rather than buying them is also a traditional way for great manufacturers to save costs. Today’s CdTe module costs are about $110/m2 (normalized to a square meter). Costs are expected to reduce to $75/m2.
Thus a practical, long-term (10–20 year) goal for CdTe modules resulting from combining cost and efficiency goals would be $75 per 150 watts, or about $0.5 per watt. With commodity-like margins and combined with balance-of-system (BOS) costs, installed systems near $1.5/W seem achievable. With Southern California sunlight, this would be in the 6 to 8 US cents per kWh range (e.g., based on economic and other assumptions used in algorithms such as in the United States Department of Energy and NREL's Solar Advisory Model).
Tellurium supply 
Perhaps the most subtle and least understood problem with CdTe PV is the supply of tellurium. Tellurium (Te) is an element not currently used for many applications. Only a small amount, estimated to be about 800 metric tons  per year, is available. According to USGS, global tellurium production in 2007 was 135 metric tons. Most of it comes as a by-product of copper, with smaller byproduct amounts from lead and gold. One gigawatt (GW) of CdTe PV modules would require about 93 metric tons (at current efficiencies and thicknesses), so this seems like a limiting factor. However, because tellurium has had so few uses, it has not been the focus of geologic exploration. In the last decade, new supplies of tellurium-rich ores have been located, e.g., in Xinju, China. Since CdTe is now regarded as an important technology in terms of PV’s future impact on global energy and environment, the issue of tellurium availability is significant. Recently, researchers have added an unusual twist – astrophysicists identify tellurium as the most abundant element in the universe with an atomic number over 40. This surpasses, e.g., heavier materials like tin, bismuth, and lead, which are common. Researchers have shown that well-known undersea ridges (which are now being evaluated for their economic recoverability) are rich in tellurium and by themselves could supply more tellurium than we could ever use for all of our global energy. It is not yet known whether this undersea tellurium is recoverable, nor whether there is much more tellurium elsewhere that can be recovered.
Other issues 
Another issue frequently mentioned, is the use and recycling of the extremely toxic metal cadmium, one of the six most toxic materials banned by European Union's RoHS regulation. According to First Solar's annual report, the CdTe solar panel is not in RoHS compliance, not listed in the exemption product list, but not currently listed in the restricted product list either. So the product's future RoHS compliance status is uncertain. First Solar has a self-imposed recycling regimen that provides a deposited amount (<$0.05 a watt) that covers the costs of transport and recycling of the module at the end of its useful life. Recycling has been fully demonstrated on scrap modules although recent work has questioned the economic viability of recycling CdTe PV modules and called for extended producer responsibility policy to ensure all CdTe PV is recycled. In a validating test, Vasilis Fthenakis of the Brookhaven National Laboratory showed that the glass plates surrounding CdTe material sandwiched between them (as they are in all commercial modules) seal during a fire and do not allow any cadmium release. 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).
Price vulnerability 
A subtle issue with CdTe and with all thin films in relation to greater efficiency PV module technologies is the potential impact of commodity inflation. Lower efficiency modules incur a higher balance of system commodity cost per unit output, as a result such inflation can have a greater percentage impact on system cost. This is another reason that continued efficiency improvements are important.
Solar tracking 
Almost all thin film photovoltaic module systems to-date have been non-solar tracking, because the output of modules has been too low to offset tracker capital and operating costs. But relatively inexpensive single-axis tracking systems can add 25% output per installed watt. This is climate-dependent. Tracking also produces a smoother output plateau around midday, allowing afternoon peaks to be met.
Market viability 
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 $0.76 per watt in 2010, and capital cost per new watt of capacity is near $0.9 per watt (including land and buildings). However, module cost alone is not enough to assure the lowest installed system price. Thin films, including CdTe, are less efficient than most wafer silicon modules. Typical wafer silicon modules are 13% to 20% efficient, while the best CdTe modules were about 10.7% efficient; recent modules produced at First Solar and measured by NREL have shown CdTe modules with efficiencies at 12.5% or greater. Many components of an installed PV system (e.g., support structures, installation labor, land) scale with system area; and less-efficient modules require more area to produce the same output (all other things being equal). The impact of area-related costs on CdTe systems is about $0.5 per watt of extra cost.
Notable systems 
Recent installations of large CdTe PV systems by First Solar confirm the competitiveness of CdTe PV with other forms of solar energy and how close it is to being competitive with conventional natural gas peakers:
- A 40MW system being installed by juwi group in Waldpolenz Solar Park, Germany: at the time of its announcement, it was both the largest planned and lowest cost PV system in the world. The price of 3.25 euros translated then (when the euro was equal to US$1.3) to $4.2/watt, much lower than any other known system.
- A 7.5-megawatt system to be installed in Blythe, CA, where the California Public Utilities Commission has accepted a 12 US cent per kWh power purchase agreement with First Solar (after the application of all incentives). Defined in California as the "Market Referent Price," this is the price the PUC will pay for any daytime peaking power source, e.g., natural gas. Although PV systems are intermittent and not dispatchable the way natural gas is, natural gas generators have an ongoing fuel price risk that PV does not have.
- A contract for two megawatts of rooftop installations with Southern California Edison, where the SCE program is designed to install 250 megawatts at a total cost of $875M (averaging $3.5/watt), after incentives.
See also 
References and notes 
- "Publications, Presentations, and News Database: Cadmium Telluride". National Renewable Energy Laboratory.
- 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.
- Further mention of cost competitiveness: "Solar Power Lightens Up with Thin-Film Technology", Scientific American, April 2008.
- Various estimates of world module production in 2007 are about 4 gigawatts (e.g., http://worldwatch.org/node/5449#notes).
- 19.9% CIGS cell made at NREL:
- D. A. Jenny and R. H. Bube (1954). "Semiconducting CdTe". Phys. Rev. 96 (5): 1190. doi:10.1103/PhysRev.96.1190.
- R. H. Bube (1955). "Photoconductivity of the Sulfide, Selenide, and Telluride of Zinc or Cadmium". Proceedings of the IRE 43 (12): 1836–1850. doi:10.1109/JRPROC.1955.278046. ISSN 0096-8390.
- D. A. Cusano (1963). "CdTe Solar Cells and PV Heterojunctions in II-VI Compounds". Solid State Electronics 6 (3): 217. doi:10.1016/0038-1101(63)90078-9.
- B. Goldstein (1958). "Properties of PV Films of CdTe". Phys. Rev 109 (2): 601. doi:10.1103/PhysRev.109.601.2.
- Y. A. Vodakov, G. A. Lomakina, G. P. Naumov, Y. P. Maslakovets (1960). "A P-N Junction photocell made of CdTe". Soviet Physics, Solid State 2 (1): 1.
- R. Colman, July 28, 1964 U.S. Patent 3,142,586
- D. A. Cusano led a group at GE in the 1960s.
- Y. S. Tyan, 1978, Polycrystalline thin film CdS/CdTe photovoltaic cell, Kodak, U.S. Patent 4,207,119 (EP0006025); Y. S. Tyan and E. A. Perez-Albuerne, 1982, Integrated array of photovoltaic cells having minimized shorting losses, Kodak, U.S. Patent 4,315,096. Tyan especially published both patents and papers of significance at Kodak and helped to establish CdTe as an important thin film option.
- 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.
- 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
- K. Zweibel (1995). "Thin Films Past Present and Future". 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.
- "While First Solar keeps on trucking, others in CdTe thin-film PV pack keep on muddling". Fabtech.org. 2008-08-21. \
- "Roth & Rau AG plans entry into thin film technology based on cadmium telluride". roth-rau.de. 2009-02-23.
- Flexible CdTe thin-film solar cells reach efficiency of 12.4% http://www.worldofrenewables.com/renewables_news/photovoltaics/thin_film_pv/flexible_cdte_thin-film_solar_cells_reach_efficiency_of_12_4.html
- 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/804290323, April 29, 2008
- 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.
- D. H. Rose et al. (October 1999). Technology Support of High-Throughput Processing of Thin Film CdTe Panels. NREL SR-520-27149. p. Viii (preface).
- "First Solar annual manufacturing levels". FirstSolar.com. 2008.
- Friedman, Thomas. “Hot, Flat, and Crowded” Farrar, Straus and Giroux, New York; 2008. Page 388
- "First Solar Goes Supernova". Forbes.com. 2007-11-08.
- Jonathan G. Dorn (December 27, 2007). "Solar Cell Production Jumps 50 Percent in 2007".
- X. Wu et al. (October 2001). High Efficiency CTO/ZTO/CdS/CdTe Polycrystalline Thin Film Solar Cells. NREL/CP-520-31025.. Since NREL’s X. Wu produced a cell with 16.5% efficiency using advanced front TCO material that allowed more light while being more conductive than prior cells.
- Most US R&D activities, which indeed means most world activities in CdTe, were organized and partially funded through NREL’s Department of Energy funded Thin Film PV Partnership (http://www.nrel.gov/pv/thin_film/ and http://www.nrel.gov/pv/thin_film/about.html), which included a CdTe national R&D team of about 50 members. The Partnership was ended with the advent of the recent DOE Solar America Initiative which de-emphasized technology-specific research.
- This number is calculated by multiplying efficiency (10.7%) by 1000 to get output watts per square meter (107 W/m2), and then multiplying power by the stated cost of $1.04 per watt to get $111/m2 , or $80 per module.
- This number is calculated by dividing the cost per unit (e.g, $75/m2) by output per the same unit (15% produces 150 watts per square meter): $75/150 W = $0.5/W.
- "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.
- "Assessment of critical thin film resources" (.doc file).
- "Tellurium". Mineral Commodity Summaries. United States Geological Survey. January 2008.
- Density of CdTe is 5.85 g/cm³ with Te being 53% of the mass, there is about 3.1 g/cm³ Te in CdTe. One micrometre over a square meter area is 1 cubic centimetre, or 3.1 grams of Te. Typical CdTe layer thicknesses are about 3 micrometres, so there are 9.3 g/m2. First Solar CdTe solar panels are 8 square feet (0.74 m2) each and provides average of 73.75 watts, or roughly 100 watts per square meter. So one gigawatt is 10 km2, or 93 metric tons.
- Publications of the Sichuan Xinju Mineral Resource Development Co., China
- B. L. Cohen (1984). "Anomalous behavior of tellurium abundances". Geochim. Cosmochim. Acta 38: 279–300.
- J. Hein (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 (http://www.webelements.com), yet it is one of the least abundant elements in the Earth’s crust and in ocean water.""
- J. Hein, A. Koschinsky, and A. Halliday (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. "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"
- "First Solar 2007 Annual Report". First Solar, Inc.
- "Cadmium Telluride Casts Shadow on First Solar".
- "Recycle First Solar Modules". First Solar, Inc.
- "Publications, Presentations, and News Database: Environment, Safety, and Health".
- McDonald N. C., Pearce J. M. (2010). "Producer Responsibility and Recycling Solar Photovoltaic Modules". Energy Policy 38 (11): 7041–7047. doi:10.1016/j.enpol.2010.07.023.
- V. Fthenakis, M. Fuhrmann, J. Heiser, W. Wang (June 7–11, 2004). Experimental Investigation of Emissions and Redistribution of Elements in CdTe PV Modules during Fires. Paris, France: 19th European PV Solar Energy Conference. p. 5BV.1.32.
- V. Fthenakis, H. C. Kim (May 29-June 2, 2006). CdTe Photovoltaics: Life Cycle Environmental Profile and Comparisons. Nice, France: European Material Research Society Meeting, Symposium O. for environmental issues; and D. H. Rose et al. (October 1999). Technology Support of High-Throughput Processing of Thin Film CdTe Panels. NREL SR-520-27149. for manufacturing approaches
- Pacific Crest Presentation, August 3–5, 2008
- PDF (401 KB)
- "First Solar announces two solar projects with Southern California Edison". Semiconductor-Today.com. 2008-07-17.
- "California Utility to Install 250MW of Roof-Top Solar". SustainableBusiness.com. 2008-03-27.
Further reading 
- V. Fthenakis, H. C. Kim, 2007, "CdTe photovoltaics: Life cycle environmental profile and comparisons," Thin Solid Films, Volume 515, Issue 15, 31 May 2007, Pages 5961-5963, doi:10.1016/j.tsf.2006.12.138; article at http://www.clca.columbia.edu/papers/CdTe_Photovoltaics_Life_Cycle_Environmental_Profile.pdf
- A. H. Hill, "Progress in Photovoltaic Energy Conversion," NASA, Washington, DC, http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19660006117_1966006117.pdf
- Richard Stevenson, August 2008, "First Solar: Quest for the $1 Watt," IEEE Spectrum Online, http://spectrum.ieee.org/aug08/6464