Solar Cell Sets World Efficiency Record at Over 40%

Chemical Engineering Progress, Sep 2008 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.