Copper indium gallium selenide solar cells

A copper indium gallium selenide solar cell (or CIGS cell, sometimes CI(G)S or CIS cell) is a thin-film solar cell used to convert sunlight into electric power. It is manufactured by depositing a thin layer of copper, indium, gallium and selenium on glass or plastic backing, along with electrodes on the front and back to collect current. Because the material has a high absorption coefficient and strongly absorbs sunlight, a much thinner film is required than of other semiconductor materials. A copper indium gallium selenide solar cell (or CIGS cell, sometimes CI(G)S or CIS cell) is a thin-film solar cell used to convert sunlight into electric power. It is manufactured by depositing a thin layer of copper, indium, gallium and selenium on glass or plastic backing, along with electrodes on the front and back to collect current. Because the material has a high absorption coefficient and strongly absorbs sunlight, a much thinner film is required than of other semiconductor materials. CIGS is one of three mainstream thin-film PV technologies, the other two being cadmium telluride and amorphous silicon. Like these materials, CIGS layers are thin enough to be flexible, allowing them to be deposited on flexible substrates. However, as all of these technologies normally use high-temperature deposition techniques, the best performance normally comes from cells deposited on glass, even though advances in low-temperature deposition of CIGS cells have erased much of this performance difference. CIGS outperforms polysilicon at the cell level, however its module efficiency is still lower, due to a less mature upscaling. Thin-film market share is stagnated at around 15 percent, leaving the rest of the PV market to conventional solar cells made of crystalline silicon. In 2013, the market share of CIGS alone was about 2 percent and all thin-film technologies combined fell below 10 percent. CIGS cells continue being developed, as they promise to reach silicon-like efficiencies, while maintaining their low costs, as is typical for thin-film technology. Prominent manufacturers of CIGS photovoltaics were the now-bankrupt companies Nanosolar and Solyndra. Current market leader is the Japanese company Solar Frontier, with Global Solar and GSHK Solar also producing solar modules free of any heavy metals such as cadmium and/or lead. CIGS is a I-III-VI2 compound semiconductor material composed of copper, indium, gallium, and selenium. The material is a solid solution of copper indium selenide (often abbreviated 'CIS') and copper gallium selenide, with a chemical formula of CuInxGa(1-x)Se2, where the value of x can vary from 1 (pure copper indium selenide) to 0 (pure copper gallium selenide). It is a tetrahedrally bonded semiconductor, with the chalcopyrite crystal structure. The bandgap varies continuously with x from about 1.0 eV (for copper indium selenide) to about 1.7 eV (for copper gallium selenide). CIGS has an exceptionally high absorption coefficient of more than 105/cm for 1.5 eV and higher energy photons. CIGS solar cells with efficiencies around 20% have been claimed by the National Renewable Energy Laboratory (NREL), the Swiss Federal Laboratories for Materials Science and Technology (Empa), and the German Zentrum für Sonnenenergie und Wasserstoff Forschung (ZSW) (translated: Center for Solar Energy and Hydrogen Research), which is the record to date for any thin film solar cell. The most common device structure for CIGS solar cells is shown in the diagram (see Figure 1: Structure of a CIGS device). Soda-lime glass of about of 1–3 milimetres thickness is commonly used as a substrate, because the glass sheets contains sodium, which has been shown to yield a substantial open-circuit voltage increase, notably through surface and grain boundary defects passivation. However, many companies are also looking at lighter and more flexible substrates such as polyimide or metal foils. A molybdenum (Mo) metal layer is deposited (commonly by sputtering) which serves as the back contact and reflects most unabsorbed light back into the CIGS absorber. Following molybdenum deposition a p-type CIGS absorber layer is grown by one of several unique methods. A thin n-type buffer layer is added on top of the absorber. The buffer is typically cadmium sulfide (CdS) deposited via chemical bath deposition. The buffer is overlaid with a thin, intrinsic zinc oxide layer (i-ZnO) which is capped by a thicker, aluminum (Al) doped ZnO layer. The i-ZnO layer is used to protect the CdS and the absorber layer from sputtering damage while depositing the ZnO:Al window layer, since the latter is usually deposited by DC sputtering, known as a damaging process. The Al doped ZnO serves as a transparent conducting oxide to collect and move electrons out of the cell while absorbing as little light as possible. The CuInSe2-based materials that are of interest for photovoltaic applications include several elements from groups I, III and VI in the periodic table. These semiconductors are especially attractive for solar applications because of their high optical absorption coefficients and versatile optical and electrical characteristics, which can in principle be manipulated and tuned for a specific need in a given device. CIGS is mainly used in the form of polycrystalline thin films. The best efficiency achieved as of September 2014 was 21.7%. A team at the National Renewable Energy Laboratory achieved 19.9%, a record at the time, by modifying the CIGS surface and making it look like CIS. These examples were deposited on glass, which meant the products were not mechanically flexible. In 2013, scientists at the Swiss Federal Laboratories for Materials Science and Technology developed CIGS cells on flexible polymer foils with a new record efficiency of 20.4%. These display both the highest efficiency and greatest flexibility. The U.S. National Renewable Energy Laboratory confirmed 13.8% module efficiency of a large-area (meter-square) production panel, and 13% total-area (and 14.2% aperture-area) efficiency with some production modules. In September 2012 the German Manz AG presented a CIGS solar module with an efficiency of 14.6% on total module surface and 15.9% on aperture, which was produced on a mass production facility. MiaSolé obtained a certified 15.7% aperture-area efficiency on a 1m2 production module, and Solar Frontier claimed a 17.8% efficiency on a 900 cm2 module.

[ "Thin film", "Solar cell", "layer", "Copper indium gallium selenide", "Copper gallium diselenide" ]
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