Cadmium telluride (CdTe) photovoltaics is a photovoltaic (PV) technology based on the use of cadmium telluride in a thin semiconductor layer designed to absorb and convert sunlight into electricity. Cadmium telluride PV is the only thin film technology with lower costs than conventional solar cells made of crystalline silicon in multi-kilowatt systems.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: David Biello, "Solar Power Lightens Up with Thin-Film Technology", Scientific American, April 2008.

On a lifecycle basis, CdTe PV has the smallest carbon footprint, lowest water use and shortest energy payback time of any current photovoltaic technology. 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 during production and when the panels are disposed of. Some of this might be mitigated by recycling of CdTe modules at the end of their life time, as there are uncertainties regarding the recycling of CdTe modules and the public opinion is skeptical towards this technology. 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.

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.Fraunhofer ISE Photovoltaics Report, July 28, 2014, pages 18,19

Background

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.Various estimates of world module production in 2007 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. CdTe cell efficiency is approaching 20% in the laboratory with a record of 22.1% as of 2016.

History

Research in CdTe dates back to the 1950s,R. Colman, July 28, 1964 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-space sublimation (CSS) and made the first 10% efficient cells and first multi-cell devices (12 cells, 8% efficiency, 30 cm2).Y. S. Tyan, 1978, Polycrystalline thin film CdS/CdTe photovoltaic cell, Kodak, (EP0006025); Y. S. Tyan and E. A. Perez-Albuerne, 1982, Integrated array of photovoltaic cells having minimized shorting losses, Kodak, . Tyan especially published both patents and papers of significance at Kodak and helped to establish CdTe as an important thin film option. MonosolarB. Basol, E. Tseng, R.L. Rod, 1981, Thin film heterojunction photovoltaic cells and methods of making the same, Monosolar . 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. and AMETEKPeter Meyers, originally of Ametek, provides a thread stretching from Ametek through Solar Cells Inc. to First Solar. He is on Ametek patents , 1981; , 1987; and SCI/First Solar patents 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.

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.

CdTe cells achieved above 15% efficiency 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.

In the early 1990s, other players experienced mixed results. 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 November 2002. 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.

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

SCI and First Solar

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.After long wait, McMaster to join hall of fame, April 29, 2008 McMaster championed CdTe for its high-rate, high-throughput processing. In February 1999, McMaster sold the company to True North Partners, who named it First Solar.

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. The company manufactured in Perrysburg, Ohio and Germany. In 2013, First Solar acquired GE's thin film solar panel technology in exchange for a 1.8% stake in the company.First Solar Gives GE 1.8% Stake in Thin-Film Partnership. bloomberg.com. 2013-08-07 Today, First Solar manufactures over 3 gigawatts with an average module efficiency of 16.4% in 2016.

First Solar notably uses a high-rate vapor transport deposition process in lieu of CSS (closed space sublimation) for the deposition of CdTe. This is a type of physical vapor deposition where CdTe is first sublimated in an upstream region. Then, the Cd and Te2 gases flow across a cooler downstream region where they condense on a substrate to form solid CdTe.First Solar CdTe Photovoltaic Technology: Environmental, Health and Safety Assessment. (2013). National Renewable Energy Centre (CENER), Fundacion Chile. https://www.firstsolar.com/en-Emea/-/media/First-Solar/Sustainability-Documents/Sustainability-Peer-Reviews/Chile-Peer-Review---Cener_EN.ashx This process is preferred over CSS because it produces films of greater uniformity and allows for deposition on any configuration of the substrate.

Technology

Cell efficiency

In August 2014 First Solar announced a device with 21.1% conversion efficiency. 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%. 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%.

To reach these record high efficiencies of 22%, alloying is used for band gap grading. A compound incorporating selenium into CdTe is used in the solar cell to improve the quantum efficiency response for certain wavelengths of light, in addition to unalloyed CdTe. The other major contributor to this large increase in efficiency is the usage of MgZnO (MZO) within the cell. In a cell using a CdSexTe1−x/CdTe structure, MZO can be used in place of CdS. CdS is source of inefficient absorption, while MZO has a tunable band gap that can be optimized for high transparency and good alignment with CdSexTe1−x.

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 were about $72 per ,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 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. CdTe PV modules experience half the reduction of crystalline silicon modules, resulting in an increased annual energy output of 5-9%.

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