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The inverted metamorphic triple-junction solar cell was designed, fabricated and independently measured at NREL. The 40.8% efficiency was measured under concentrated light of 326 suns. (One sun is about the amount of light that typically hits Earth on a sunny day.) The new cell is a natural candidate for the space satellite market and for terrestrial concentrated photovoltaic arrays, which use lenses or mirrors to focus sunlight onto the solar cells.
The new solar cell differs significantly from the previous record holder, which was also based on a NREL design. Instead of using a germanium wafer as the bottom junction of the device, the new design uses compositions of gallium indium phosphide and gallium indium arsenide to split the solar spectrum into three equal parts that are absorbed by each of the cell’s three junctions for higher potential efficiencies. This is accomplished by growing the solar cell on a gallium arsenide wafer, flipping it over, and then removing the wafer. The resulting device is extremely thin and light, and it represents a new class of solar cells with advantages in performance, design, operation and cost.
NREL’s Mark Wanlass invented the original inverted cell, which recently won a R&D 100 award. His design was modified by a team led by John Geisz that further optimized the junction energies by making the middle junction metamorphic as well as the bottom junction. Metamorphic junctions are lattice mismatched (i.e., their atoms don’t line up). The material properties of the mismatched semiconductors allow for greater potential conversion of sunlight.
NREL is the DOE’s primary national laboratory for renewable energy and energy efficiency research and development. NREL is operated for the DOE by Midwest Research Institute and Battelle. For more information, call (303) 275-4090 or visit www.nrel.gov.