NREL Spurred the Success of Mul�junc�on Solar Ce - PDF document

NREL Spurred the Success of Mul�junc�on Solar Cells Many scien�sts once believed that high-quality gallium indium phosphide (GaInP) al loys could not be grown for use as semiconductors because the alloys would separate. However, researchers at the Na�onal Renewable Energy Laboratory (NREL) thought di�erently, and they employed GaInP in a material combina�on that allowed the mul�junc�on cell to �ourish. The mul�junc�on cell is now the workhorse that powers satellites and the catalyst for renewed interest in concentrator photovoltaic products. A mul�junc�on cell is like a semi-transparent layer cake: the top cell produces elec tricity from the higher-energy por�on of the solar spectrum, and the lower-energy sunlight passes through to the lower cells to be converted into electricity, resul�ng in a highly e�cient produc�on of power. To make it work, the stacked cells need to ab sorb complementary wavelengths of sunlight, and those absorp�on wavelengths are certain energy bands that the material’s electrons can occupy. While many researchers were focused on materials with ideal combina�ons of band gaps, NREL researchers thought the focus should change to �nding materials with chemical and structural compa�bility—materials that had a bandgap combina�on that would give a high, but not necessarily the highest, theore�cal e�ciency. With that in mind, NREL researchers focused their e�orts on GaInP and gallium arsenide (GaAs), which are well-matched chemically and have the same la�ce constant, an indicator of how their crystals �t together at the atomic and molecular levels. NREL applied for a patent for this “dual-junc�on” solar cell, and although the �rst cells were less than 10% e�cient, NREL researchers thought the e�ciency could be and a bo�om cell of GaAs could capture and convert photons into electricity more e�ciently than other materials. Subsequently, all major research en��es working on similar mul�junc�on solar cells adopted some version of the GaInP/GaAs cell. True to the predic�ons of NREL researchers, as knowledge and processes advanced, e�ciency surged. A�er a decade of research, NREL’s dual-junc�on solar cell surpassed 30% e�ciency, and a�erward, the NREL team and scien�sts worldwide started adding more semiconductor junc �ons, and the record steadily increased. More than a dozen companies are now capable of producing mul�junc�on solar cells. This archive photo shows NREL scien�sts Jerry Olson and Sarah Kurtz tes�ng the voltage of their award-winning gallium indium phosphide/gallium arsenide solar conver�ng more than 30% of incident sunlight into electricity. Photo by Warren Gretz, NREL 02002 Quick Facts Although a number of research en��es were once inves�ga�ng dual-junc�on solar cells, NREL researchers found the path to the �rst prac�cal version. Their innova�ve choice for the top ac�ve layer of the solar cell remains the choice for all mul�junc�on solar cells produced today. Mul�junc�on solar cells are the choice for most solar-powered space applica�ons, such as satellites. In 2010, Boeing Spectro lab, a major supplier of space-based solar cells, announced that it had provided three million solar cells for 225 spacecra�. Mul�junc�on solar cells also powered the nity, and their durability helped to extend the rovers’ life�mes beyond predic�ons. R&D Magazine has selected a number of mul�junc�on solar cells for its R&D 100 Award over the years, including the dual-junc�on solar cell in 1991 and a va riety of triple-junc�on solar cells in 2001, 2007, 2008, and 2012—all inven�ons to which NREL contributed. The award pro claims the inven�ons to be among the top 100 technological innova�ons of the year. In October 2012, Solar Junc�on, of San José, California—a company that works closely with NREL—achieved a new solar cell conversion e�ciency record with hi�ng it into electricity. The record was achieved under sunlight concentrated to 947 �mes its normal strength, condi�ons the cell could experience when used in a concentra�ng photovoltaic system. INNOVATION IMPACTSolar National Renewable Energy Laboratory 15013 Denver West Parkway, Golden, CO 80401 303-275-3000  www.nrel.gov NREL is a na�onal laboratory of the U.S. Department of Energy, O�ce of Energy E�ciency and Renewable Energy, operated by the Alliance for Sustainable Energy, LLC. NREL/FS-6A42-59142  August 2013 NREL prints on paper that contains recycled content. For more NREL innova�ons, visit www.nrel.gov/innova�on/