Cadmium telluride photovoltaics

Cadmium telluride (CdTe) photovoltaics describes a photovoltaic (PV) technology that is based on the use of cadmium telluride in 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 any current photo voltaic technology.[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 regarding the recycling of CdTe modules[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.

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 22.1% as of 2016.[16]

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.[citation needed]

By 1981, Kodak used close-space sublimation (CSS) and made the first 10%[clarification needed] 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%[clarification needed] 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%[clarification needed] 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 November 2002.[27] 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.[28]

CdTe start-ups include Calyxo[29] (formerly owned by Q-Cells), PrimeStar Solar, in Arvada, Colorado (acquired by First Solar from GE),[30] (Italy).[citation needed] Including Antec, their total production represents less than 70 megawatts per year.[31] 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.[32]

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.[33] McMaster championed CdTe for its high-rate, high-throughput processing. SCI shifted from an adaptation of the CSS method then shifted to vapor transport.[34] 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]

In August 2014 First Solar announced a device with 21.1% conversion efficiency.[40] 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%.[41] 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%.[42][43]

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.[44]

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),[45] 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.[46][47][48] CdTe PV modules experience half the reduction of crystalline silicon modules, resulting in an increased annual energy output of 5-9%.[49]

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. 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.[51] This is climate-dependent. Tracking also produces a smoother output plateau around midday, better matching afternoon peaks.

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.[52]

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[53] per year, is available. According to USGS, global production in 2007 was 135 metric tons.[54] One gigawatt (GW) of CdTe PV modules would require about 93 metric tons (at current efficiencies and thicknesses).[55] 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.[56] In the last decade[when?], new supplies have been located, e.g., in Xinju, China[57] as well as in Mexico and Sweden.[58] In 1984 astrophysicists identified tellurium as the universe's most abundant element having an atomic number over 40.[59][60] Certain undersea ridges are rich in tellurium.[60][61]

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.[62][63]

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.[64] 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.[65][66] 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).[67][68]

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.[69] 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.

Photovoltaic modules can last anywhere from 25 – 30 years. Improper disposal of PV modules can release toxic materials into the environment.[70] 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.[70]

Materials that can be recovered in the recycling process include metals, mounts, glass, and, in high value cases, the whole PV module.[71]

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.[70] 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%.[72] The Double Green Process consists of almost entirely mechanical processes.[73]

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.[74][75] 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/m2 to -12 MJ/m2, a reduction of around 93 MJ/m2, and in terms of global warming potential from 6 kg CO2-equiv./m2 to -2.5 CO2-equiv./m2, a reduction of around -8.5 CO2-equiv./m2. 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.[76]

a small PV array used as part of Energy Systems Integration research ongoing at the National Wind Technology Center (NWTC)

Photovoltaics can assist in reducing toxic emissions and pollution caused by fossil fuels.[70] Emissions from fossil fuels that impact global climates such as Nitrogen Oxide (NOx), Carbon Dioxide (CO2) and Sulfur Dioxide (SO2) are not emitted from PV. A single gigawatt-hour of electricity produced from PV would decrease the emissions of SO2 by 10 tons, NOx by 4 tons and CO2 by 1000 tons when compared to coal.[77]

CdTe cells are considered to be both toxic and carcinogenic when inhaled or ingested as Cd is a toxic compound by the US Occupational Safety and Health Administration. Workers in processing facilities may be exposed to, and inhale, fine particles or fumes of Cd.[77]

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.[77]

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 lit aflame or ground into a fine dust. When exposed to temperatures of approximately 1100 °C in lab tests around 0.4% to 0.6% of the Cd content was released.[71]

The overall Cd air emission estimates can range from 0.02 to 0.5 grams per gigawatt-hour.[71]

Early CdTe modules failed elution tests, however more recent models can pass some elution tests. Despite the small amounts of Cd that may leach out, CdTe modules have low overall leachability as the hazardous materials within them are encased within two layers of glass. Despite their low leachability, CdTe modules have very poor biodegradability.[71]

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,[78] and capital cost per new watt of capacity is near $0.9 per watt (including land and buildings).[79]

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