Unconventional thinking by scientists at the U.S. Department of Energy’s Lawrence Berkeley National Laboratory has led to record breaking solar cell efficiencies for single layer solar cells. While most people would think that the key to getting solar cells more efficient would be to absorb more light, the researchers found that they could make their cells better by actually having the cells emit photons in the form of fluorescent light.
What happens with conventional solar cells is that when the cells produce electrons, if they aren’t removed fast enough they decay and release their energy, most of the time as heat. This lowers the efficiency of the cells. There’s a theoretical limit of 33.5% efficiency for single junction cells (a single junction cell consists of just one material to collect light). It’s knows as the Shockley-Queisser efficiency limit (SQ Limit). However, most monocrystalline solar cells in use today get around 23 percent.
The research team is headed up by Eli Yablonovitch, who commented, “A great solar cell also needs to be a great light emitting diode. This is counter-intuitive. Why should a solar cell be emitting photons? What we demonstrated is that the better a solar cell is at emitting photons, the higher its voltage and the greater the efficiency it can produce.”
Alta Devices, Inc., the private company co-founded byYablonovitch, has produced gallium arsenide cells using this technique that convert sunlight to electricity at a 28.4% conversion rate. Although gallium arsenide is more expensive than silicon, a cell made from gallium arsenide uses much less material. An Alta cell has 1 micron thick layer of gallium arsenide compared to 150-200 microns for silicon cells and 15 microns for cadmium telluride cells. So a little goes a long way. Alta Devices expects to have their gallium arsenide solar panels on the market within a year.