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%.. 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. 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).. As for CIS, and CGS solar cells, the world record total area efficiencies are 15.0% and 9.5% 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, Nanosolar and IBMand.
CIGS solar cells are not as efficient as crystalline silicon solar cells, for which the record efficiency lies at 24.7%,, 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 micrometers of CIGS is enough to absorb most of the sunlight. A much greater thickness of crystalline silicon is required for the same absorption. CIGS therefore belongs in the category of thin film solar cells (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 in a polycrystalline form directly onto molybdenum coated glass sheets or steel bands. This uses less energy than growing large crystals, which is a necessary step in the manufacture of crystalline silicon solar cells. Also unlike crystalline silicon, these 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 13 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. Additionally, susceptibility to dampness makes module encapsulation a requisite for long lifetimes.
Cadmium telluride photovoltaics
List of CIGS companies
List of photovoltaics companies
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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
Categories: Semiconductor materials | Copper compounds | Indium compounds | Gallium compounds | Selenides | Thin films | Thin-film cells | CIGS solar cells | Printed electronics