Research conducted at the Lawrence Berkeley National Laboratory (LBL) has created single-junction solar cells that operate at up to 28.4 percent conversion efficiencies. That’s the closest yet to the Shockley-Queisser efficiency limit, which states that the maximum possible efficiency for a single-junction photovoltaic device is at 33.5 percent. And the key to doing it is emitting light.
“A great solar cell also needs to be a great light emitting diode,” said Eli Yablonovitch, James & Katherine Lau chair in engineering director of the NSF Center for Energy Efficient Electronics Science. “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.”
The trick was emitting infrared light and not absorbing it.
Yablonovich worked with Owen Miller of LBL and Sarah Kurtz of the National Renewable Energy Laboratory on the research and published “Intense Internal and External Fluorescence as Solar Cells Approach the Shockley-Queisser Efficiency Limit” earlier this year.
Miller was able to calculate that gallium arsenide (GaAs) is capable of reaching the limit. The material is more expensive than silicon or semiconductors used in thin-film photovoltaics, but it’s efficiency makes up for that, according to Yablonovitch.
“Gallium arsenide absorbs photons 10,000 times more strongly than silicon for a given thickness but is not 10,000 times more expensive,” he said. “Based on performance, it is the ideal material for making solar cells.”
So, despite the added cost, the increased efficiency should more than make up for it.
”The GaAs panels suffer very little efficiency loss when they are hot, or on cloudy days. They will be by far the most efficient solar panels, thereby saving installation cost,” Yablonovitch said. “I expect that the thin-film GaAs cells produced by epitaxial liftoff will be less expensive than even the least expensive thin-film cell, while retaining their efficiency advantage.”
Resulting devices would produce more power per square meter than other photovoltaics on the market.
Yablonovitch helped cofound Alta Devices, a company that will make photovoltaics, including flexible photovoltaics. It’s expected to bring the modules to market within a year, according to LBL.
While they have produced PV cells that are 28.4 percent efficient, the device that comes to market will likely be less efficient.
“There is always a difference between the champion cell and mass production at the panel level,” Yablonovitch said. “In the case of c-Si cells, the efficiency difference between the champion cell versus the mass-produced panel is about 3 percent. This would imply GaAs panels at 25 percent efficiency.”
That would outpace the most efficient single-junction cells on the market, SunPower’s Maxeon cells, which have reached 22.4 percent efficiency.
Image courtesy of Lawrence Berkeley National Laboratory—Yablonovitch is on the left; Miller is on the right.