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Fraunhofer beats European record with 39.7-percent cell
October, 2008: Researchers at the Freiburg, Germany-based Fraunhofer Institute for Solar Energy Systems (ISE) in late September reported a new European record for the sunlight conversion efficiency of a solar cell at 39.7 percent.
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© Fraunhofer-Institut für Solare Energiesysteme ISE
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Best in Europe: Fraunhofer researchers increased the efficiency of their concentrator cell by enhancing the grid structure.
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The measurement of the multi-junction device – a so-called metamorphic triple-junction solar cell – was recorded using a concentration of 293 suns. Such cells are used in high-concentration PV (HCPV) systems being developed by Fraunhofer-spin-off Concentrix Solar GmbH, SolFocus Inc., Isofoton and others.
In terms of efficiency, Fraunhofer ISE’s new achievement beat its previous European record cell of 37.6 percent, which was accomplished in July using a similar multi-junction device under a concentration of 1,600 suns. According to Andreas Bett, head of the solar cells and technology department at Fraunhofer and a supervisory board member at Concentrix, Fraunhofer was able to improve the efficiency of its concentrator cell by adapting the contact grid structure. »However, our earlier record was important because we could show the excellent performance at high concentration,« he notes.
Fraunhofer is the research partner of Azur Space Solar Power GmbH, which supplies multi-junction cells for space applications as well as to terrestrial concentrator start-ups such as Concentrix and others. Bett expects Azur to offer the cell commercially »within the next year.«
The new champion European cell brings Fraunhofer closer to the current world record cell efficiency of 40.8 percent under 326 suns, which was achieved by researchers at the US-based National Renewable Energy Laboratory (NREL) in August. California, US-based Spectrolab Inc. was the previous record holder with a confirmed multi-junction cell efficiency of 40.7 percent.
Each of these multi-junction devices use various cell architectures based on dozens of layers of semiconductor materials from the third and fifth columns of the periodic table, such as gallium, indium, phosphide and arsenide. In both Fraunhofer’s and Azur’s case, these are deposited onto a germanium wafer substrate using metal organic vapor-phase epitaxy (MOVPE) machines supplied by Aixtron AG located in Aachen, Germany. Such concentrator cells enable much greater absorption of the solar spectrum than silicon cells, hence the higher efficiency.
Current commercial cells supplied by Azur to Concentrix have an efficiency of approximately 36 percent. Packaged in Concentrix’s Fresnel lens modules, these result in a module level efficiency of 26 percent – due to optical losses – and a system AC efficiency of 23 percent (see PI 7/2008, p. 15). This September, Concentrix moved into its first fully automated 25 MW production facility in Freiburg.
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