Copper indium gallium selenide

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Copper indium gallium (di)selenide (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, and a bandgap varying continuously with x from about 1.0eV (for copper indium selenide) to about 1.7eV (for copper gallium selenide). It is used as light absorber material for thin-film solar cells.


[edit] CIGS photovoltaic cells

CIGS is mainly used in photovoltaic cells (CIGS cells), in the form of polycrystalline thin films. Unlike the silicon cells based on a homojunction p-n junction, the structure of CIGS cells is a more complex heterojunction system. The best efficiency achieved as of December 2005 was 19.5% reported by Contreras et al [1]. A team at the National Renewable Energy Laboratory achieved 19.9% new world record efficiency by modifying the CIGS surface and making it look like CIS[2]. This idea was first introduced in the IEEE conference in 2005[3]. The 19.9% efficiency is by far the highest compared with those achieved by other thin film technologies such as Cadmium Telluride (CdTe) or amorphous silicon (a-Si). [4]. As for CIS, and CGS solar cells, the world record total area efficiencies are 15.0% and 10.2% respectively.

CIGS solar cells are not as efficient as crystalline silicon solar cells, for which the record efficiency lies at 24.7%[5], but they are expected to be substantially cheaper. CIGS can be deposited directly onto molybdenum coated glass sheets in a polycrystalline form, saving the (energy) expensive step of growing large crystals, as necessary for solar cells made from crystalline silicon. The latter are made of slices of solid silicon and require therefore more expensive semiconductor material.

CIGS films can be manufactured by several different methods. The most common vacuum-based process co-evaporates or co-sputters copper, gallium, and indium, then anneals the resulting film with a selenide vapor to form the final CIGS structure. An alternative is to directly co-evaporate copper, gallium, indium and selenium onto a heated substrate. A non-vacuum-based alternative process deposits nanoparticles of the precursor materials on the substrate and then sinters them in situ.

[edit] Structure of a CIGS thin-film solar cell

Cross-section of Cu(In,Ga)Se2 solar cell

The semiconductors used as absorber layer in thin-film photovoltaics exhibit direct bandgaps allowing the cells to be a few micrometers thin; hence, the term thin-film solar cells is used. The basic structure of a Cu(In,Ga)Se2 thin-film solar cell is depicted in the image to the right. The most common substrate is soda-lime glass of 1-3 mm thickness. This is coated on one side with molybdenum (Mo) that serves as metal back contact. The heterojunction is formed between the semiconductors CIGS and ZnO, buffered by a thin layer of CdS and a layer of intrinsic ZnO. The CIGS is doped p-type from intrinsic defects, while the ZnO is doped n-type to a much larger extent through the incorporation of aluminum (Al). This asymmetric doping causes the space-charge region to extend much further into the CIGS than into the ZnO. Matched to this are the layer thicknesses and the bandgaps of the materials: the wide CIGS layer serves as absorber with a bandgap between 1.02 eV (CuInSe2) and 1.65 eV (CuGaSe2). Absorption is minimized in the upper layers, called window, by the choice of larger bandgaps: Eg,ZnO = 3.2 eV and Eg,CdS = 2.4 eV. The doped ZnO also serves as front contact for current collection. Laboratory scale devices, typically 0.5 cm2 large, are provided with a Ni/Al-grid deposited onto the front side to contact the ZnO. For the production of modules, individual cells are divided and monolithically interconnected by a series of scribing steps between the layer depositions [6]. Additionally, susceptibility to dampness makes module encapsulation a requisite for long lifetimes.

[edit] Companies

Some companies working with CIGS/CIS cells include:

[edit] Sales

Global Solar Energy, Inc. [15] sells portable, flexible solar panels made with thin film CIGS cells[16].

Sunload GmbH [17] offers mobile solar solutions based on CIGS technology / thin film technology too.

Würth Solar was the first German company to launch a full-scale CIGS production facility in 2006 and currently produces 30MW/a[18].

Honda Soltec Co., Ltd., located in Kikuchi-gun, Kumamoto is going to begin sales throughout Japan of CIGS cells for public and industrial use on October 24, 2008[19] .

[edit] See also

[edit] References

  1. ^ Miguel Contreras, Kannan Ramanathan, Jehad AbuShama, Falah Hasoon, David Young, Brain Egaas and Rommel Noufi “Diode Characteristics of State-of-the art ZnO/CdS/Cu(In1-xGax)Se2 Solar Cells,” Progress in Photovoltaics: Research and Applications 13, 209 (2005 )
  2. ^ Ingrid Repins, Miguel A. Contreras, Brian Egaas, Clay DeHart, John Scharf, Craig L. Perkins, Bobby To, Rommel Noufi,19.9%-efficient ZnO/CdS/CuInGaSe2 solar cell with 81.2% fill factor, Progress in Photovoltaics: Research and Applications 16, 235 (2008)
  3. ^ Jehad A. AbuShama, R. Noufi, S. Johnston, S. Ward, and X. Wu, “Improved Performance in CuInSe2 and Surface-Modified CuGaSe2 (CGS) Thin Film Solar Cells,” Proceedings of 31st IEEE Photovoltaic Specialists Conference, Lake Buena Vista, Florida, January 2005.
  4. ^ Noufi, Rommel; Ken Zweibel. "HIGH-EFFICIENCY CDTE AND CIGS THIN-FILM SOLAR CELLS: HIGHLIGHTS AND CHALLENGES". National Renewable Energy Laboratory. 
  5. ^ M.A. Green, Jianhua Zhao, A. Wang and S.R.Wenham, "Very high efficiency silicon solar cells-science and technology", IEEE Transactions on Electron Devices 46(10), 1940, October 1999
  6. ^ W. N. Shafarman and L. Stolt. Handbook of Photovoltaic Science and Engineering, chapter Cu(InGa)Se2 Solar Cells, pages 567–616. Wiley, 2003
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