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DOE supports innovative research focused on overcoming the current technological and commercial barriers for copper indium gallium diselenide [Cu(In_xGa_1-x)Se₂], or CIGS, solar cells. A list of current projects, summary of the benefits, and discussion on the production and manufacturing of this solar technology are below.
Since its initial development, copper indium diselenide (CuInSe₂) thin-film technology has been considered promising for solar cells because of its favorable electronic and optical properties. It was later found that by substituting gallium (Ga) for indium (In), the bandgap can be increased from about 1.04 electron-volts (eV) for copper indium diselenide (CIS) films to about 1.68 eV for copper gallium diselenide (CGS) films. Optimal devices have been fabricated with only a partial substitution of Ga for In, leading to a substantial increase in overall efficiency and more optimal bandgap. These solar cells are commonly known as a copper indium gallium diselenide [Cu(In_xGa_1-x)Se₂], or CIGS, cells.
Although laboratory-scale cell efficiencies have exceeded 20%, commercial CIGS modules typically have efficiencies between 12% and 14%.
Learn more about the DOE Solar Energy Technologies Office awardees and the projects involving CIGS below.
The benefits of CIGS solar cells include:
Two of the low-cost deposition methods that produce the highest device and module efficiencies were developed in the 1980s. These methods are:
After the CIGS deposition, the junction is formed by chemical-bath deposition of the n-type CdS layer. To finish the solar cell, a high-resistance zinc oxide (ZnO) layer and a high-conductivity n+-type ZnO layer are deposited by either sputtering or chemical-vapor deposition. Laser-scribing processes at different steps in the production process create the individual solar cells connected in series.
Alternative manufacturing techniques have been explored, such as reactive sputtering, magnetron sputtering (Cu, In, and Ga are sputtered while Se is evaporated), and electrodeposition. However, co-evaporation and precursor reaction processes still remain the most popular.
A major increase in device performance was achieved when the ceramic or borosilicate glass substrate was replaced by soda-lime glass. Although soda-lime glass was chosen because it has closer thermal expansion properties to CIGS, it was ultimately determined that the primary advantage of using soda-lime glass results from the diffusion of sodium (Na) ions from the glass into the CIGS absorber layer. Work is currently being done to identify the role of Na in improving CIGS performance and what tolerances CIGS has to the inclusion of Na. Current manufacturing techniques incorporate Na either from soda-lime glass or a separate Na source. Soda-lime glass has an added advantage of being less expensive than previous glass substrates.
All high‐efficiency CIS and CIGS devices use molybdenum (Mo) as the back contact primarily because of its work function and the high reflectivity of the Mo film. These films are typically deposited through direct-current (DC) sputtering. The sputtering deposition process requires precise pressure to control the stress in the film. Because of some inherent problems with the Mo back-contact, such as the possibility of a hole-blocking Schottky diode effect at the interface, other metals have been investigated to replace Mo, but have had limited success.
For more information on CIGS solar cells, visit the Energy Basics website.
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