A world record of 40.8 percent in solar cell efficiency with a photovoltaic device has been claimed by the Department of Energy's National Renewable Energy Laboratory (NREL) in the US.
The NREL scientists independently measured their 40.8 percent efficiency on concentrated light of 326 suns - one sun being the amount of light that typically hits Earth on a sunny day. The inverted metamorphic triple-junction solar cell is a natural candidate for the space satellite market where efficiencies are more important and for terrestrial concentrated photovoltaic arrays, which use lenses or mirrors to focus sunlight onto the solar cells.
Last year a consortium led by the University of Delaware set a world record for solar efficiency converting 42.8 percent of the suns radiation into electricity with their prototype cell which uses a novel technology that adds multiple innovations to a very high-performance crystalline silicon solar cell platform. The system's three types of solar cells are made by Emcore, the National Renewable Energy Laboratory and by University of Delaware. Their aim is to develop affordable portable solar cell battery chargers for the military.
But the NREL say their design differs in that it 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, then removing the wafer. The resulting device is extremely thin and light and 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 - their atoms don't line up. The material properties of the mismatched semiconductors allows for greater potential conversion of sunlight.
In December 2006 it was reported that the Boeing subsidiary Spectrolab Inc achieved an efficiency of 40.7 percent in terrestrial concentrator solar cell efficiency as verified by NREL.
Christiana Honsberg, associate professor of electrical and computer engineering at the University of Delaware states that the theoretical efficiency limit of solar energy conversion given completely idealized conditions and materials is 86%, but given present technology, solar cells that can potentially be made have theoretical conversion efficiency closer to 50%.
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