![]() ![]() Using concentrating optics requires the use of dual-axis sun-tracking, which must be factored into the cost of the system. The concentrating optics increase the amount of light incident on the solar cell, thus leading to more power production. For terrestrial applications, the high costs of these semiconductor substrates (compared to silicon, for example) may be offset by using concentrating optics, with current systems primarily using Fresnel lenses. In the past, multijunction devices have primarily been used in space, where there is a premium placed on lightweight power generation, which allows for the use of this relatively high-cost solar technology. This architecture can also be transferred to other solar cell technologies, and multijunction cells made from CIGS, CdSe, silicon, organic molecules, and other materials are being investigated. Three-junction devices using III-V semiconductors have reached efficiencies of greater than 45% using concentrated sunlight. Early research into multijunction devices leveraged the properties of semiconductors comprised from elements in the III and V columns of the Periodic table, such as gallium indium phosphate (GaInP), gallium indium arsenide (GaInAs), and gallium arsenide (GaAs). Typical multijunction cells use two or more absorbing junctions, and the theoretical maximum efficiency increases with the number of junctions. A material with a slightly lower bandgap is then placed below the high-bandgap junction to absorb photons with slightly less energy (longer wavelengths). Multijunction devices use a high-bandgap top cell to absorb high-energy photons while allowing the lower-energy photons to pass through. Photons that have energies greater than the bandgap are absorbed, but the energy greater than the bandgap is lost as heat. This limiting efficiency, known as the Shockley-Queisser limit, arises from the fact that the open-circuit voltage (Voc) of a solar cell is limited by the bandgap of the absorbing material and that photons with energies below the bandgap are not absorbed. The maximum theoretical efficiency that a single-bandgap solar cell can achieve with non-concentrated sunlight is about 33.5%, primarily because of the broad distribution of solar emitted photons. ![]() High-efficiency multijunction devices use multiple bandgaps, or junctions, that are tuned to absorb a specific region of the solar spectrum to create solar cells having record efficiencies over 45%. Below is a list of the projects, summary of the benefits, and discussion on the production and manufacturing of this solar technology. DOE invests in multijunction III-V solar cell research to drive down the costs of the materials, manufacturing, tracking techniques, and concentration methods used with this technology. ![]()
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