Scientists at the US Department of Energy’s National Renewable Energy Laboratory (NREL) claim to have set a world record in solar cell efficiency with a photovoltaic device that converts 40.8 per cent of the light that hits it into electricity.
The inverted metamorphic triple-junction solar cell was designed, fabricated and independently measured at NREL. The 40.8 per cent 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. NREL says 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.
The new solar cell differs significantly from the previous record holder. Instead of using a germanium wafer as the bottom junction of the device, the new design 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, rotating it, then removing the wafer. The resulting device is thin and light and is said to represent a new class of solar cells with advantages in performance, design, operation and cost.
NREL’s Mark Wanlass invented the original inverted cell, which was then 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, as their atoms do not line up. The material properties of the mismatched semiconductors allow for greater potential conversion of sunlight.