September 28, 2008

Solar Cell Sets World Efficiency Record at Over 40%

By Shelley, Suzanne

ENERGY Scientists at the U.S. Dept. of Energy's National Renewable Energy Laboratory (NREL) have set a world record in solar cell efficiency with a photovoltaic device that converts 40.8% of the light that hits it into electricity. This is the highest confirmed efficiency of any photovoltaic device to date.

The so-called inverted metamorphic triple-junction solar cell was designed, fabricated and independently tested at NREL. The 40.8% efficiency was measured under concentrated light with an intensity of 326 suns. (One sun is about the amount of light that typically hits Earth on a sunny day.)

The new solar cell differs significantly from the previous record holder (based on an older NREL design). Instead of using a germanium wafer as the bottom junction of the device, it uses gallium indium phosphide and two compositions of gallium indium arsenide. 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 performance, design, operation and cost advantages.

Traditional triple-junction solar cells use lattice-matched junctions (i.e., the atoms are aligned), which constrains the band gap energies of the junctions. The NREL team optimized the junction band-gap energies for high efficiency by growing metamorphic middle and bottom junctions. The metamorphic junctions consist of a single- crystal semiconductor layer grown on top of another single-crystal layer with different atomic lattice spacing. This arrangement splits the solar spectrum into three equal parts, each of which is absorbed by one of the cell's series-connected junctions for the highest possible conversion efficiency.

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.

Copyright American Institute of Chemical Engineers Sep 2008

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