Metamorphic junctions push solar cell efficiency
The US Department of Energy National Renewable Energy Laboratory (NREL) has set a new world record for photovoltaic cell efficiency by replacing the germanium wafer on the device bottom junction with a composition of gallium indium phosphide (GaInP) and gallium indium phosphide arsenide (GaInAs) to split the solar spectrum into three equal part that are absorbed by each of the cell's three junctions for higher potential efficiency. The PV device now converts 40.8% of the light that hits it into electricity and is the highest confirmed efficiency of any photovoltaic device to date.
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 space satellite markets and for terrestrial concentrated PV arrays, which use lenses or mirrors to focus sunlight onto the solar cells.
The new solar cell differs significantly from the previous record holder – also based on a NREL design which used a germanium wafer as the bottom junction of the device. In the new design, using InP and InAs is accomplished by growing the solar cell on a gallium arsenide (GaAs) 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 optimised the junction energies by making the middle junction metamorphic as well as the bottom junction. Metamorphic junctions are lattice mismatched –so their atoms don't line up. The material properties of the mismatched semiconductors allows for greater potential conversion of sunlight.
But if even an inverted cell can't get over 50%, what will it actually take to make the high 80's?
Source:http://www.nrel.gov/