Copper indium gallium selenide

"Copper indium selenide" redirects here.

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 CuIn_xGa_(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 1.0 eV (for copper indium selenide) to about 1.7 eV (for copper gallium selenide). It is used as light absorber material for thin-film solar cells.

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, the structure of CIGS cells is a more complex heterojunction system. The best efficiency achieved as of December 2005 was 19.5%.^[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] 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).^[3]. As for CIS, and CGS solar cells, the world record total area efficiencies are 15.0% and 9.5%^[4] respectively.

With record CIGS efficiency wandering at just below 20% for several years, new trend of CIGS research has shifted to investigation on low cost deposition methods that could be alternatives to expensive vacuum processes. Non-vacuum solution processes progressed quickly and efficiencies of 10%-15% have been achieved by many parties, such as ISET^[5], Nanosolar and IBM^[6]and[7].

CIGS solar cells are not as efficient as crystalline silicon solar cells, for which the record efficiency lies at 24.7%,^[8], but they are expected to be substantially cheaper due to the much lower material cost and potentially lower fabrication cost. Being a direct bandgap material, CIGS has very strong light absorption and only 1-2 micron meter of CIGS is enough to absorb most part of the sun light. Yet for crystalline silicon, it would require much thicker material to do the same job. Therefore, CIGS belongs to a category of solar cells called thin film solar cell (TFSC). Other materials in this group include CdTe and amorphous Si. Their record efficiencies are slightly lower than that of CIGS for lab-scale top performance cells. Another advantage of CIGS compared to CdTe is smaller amount of toxic material cadmium are present in CIGS cells.

The active layer (CIGS) can be deposited directly onto molybdenum coated glass sheets or steel band in a polycrystalline form, saving the (energy) expensive step of growing large crystals, as necessary for solar cells made from crystalline silicon. Also, this substrates can be flexible.

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. Electroplating is another low cost alternative to apply the CIGS layer.

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: E_g,ZnO=3.2 eV and E_g,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.^[9] Additionally, susceptibility to dampness makes module encapsulation a requisite for long lifetimes.

See also

References

External links

• Copper Indium Diselenide Publications, Presentations, and News Database of the National Renewable Energy Laboratory.
• World's Largest CIGS Solar Array Operational In Arizona.
• Michael Kanellos Silicon vs. CIGS: With solar energy, the issue is material October 2, 2006 CNET News.com