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Technical Insight

Partners push for 40% efficient solar cell (Opto News)

NASA Glen Research Center has initiated a three-year project to develop a 40% efficient solar cell. Tecstar (City of Industry, CA) will act as principal contract holder for the $1.5 million project, and will work with the California Institute of Technology (Caltech) and the National Renewable Energy Laboratory (NREL). The aim is to deliver lightweight prototype solar cells using concentrator arrays. "NASA is looking for a high efficiency solar cell design with a low dollars/watt ratio," said Tecstar s VP of R&D, Frank Ho. "Under concentration, these prototype cells will enable more than 1000 W/kg of specific power, and will employ techniques to significantly reduce mass and cost." Efficiency stacks up Tecstar will employ a four-junction design that combines its mature GaInP2/GaAs dual junction cell with a further dual junction cell based on InGaAsP and InGaAs. The top and bottom cells are joined with a grid providing electrical connections to both cells, while at the same time allowing long wavelength light passing through the top cell to penetrate to the bottom InGaAsP/InGaAs collector (see ). The InGaAsP/InGaAs sub-cell is an optimized structure, which is grown by NREL on an InP-compliant layer on a silicon substrate. This compliant substrate, supplied by Caltech, is fabricated by growing a 24 m layer of InP (and intermediate layers) on an InP substrate. This structure is wafer-bonded to a silicon substrate and the InP wafer is removed, leaving a thin compliant InP layer on Si. This procedure is key in reducing costs, since the InP wafer can be re-used. The top GaInP2/GaAs sub-cell is designed to provide 30.3% at a concentration of 15 suns (AM0), and the bottom collector will target 9.5% for the same conditions (see ). The structures are chosen from materials that convert solar energy into electricity in the 1.90.72 eV range. "Monolithic quadruple cell designs with bottom cell junctions based on GaInNAs suffer from fundamental material problems associated with the incorporation of nitrogen," says Ho. "While a GaInNAs-based quadruple structure has been demonstrated, the current-voltage properties of the bottom cell severely limit the operation of the top cell. The approach we are taking can be considered as a medium risk one because we are employing well-established technology for both top and bottom cells, and using bonding to achieve the best combination to get us toward a 40% efficient device."
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