Solar cells are awesome, and LEDs are awesome, but what about solar cells that also act like LEDs — double awesome? It seems so, and perhaps most notably, such solar cells also have increased voltage.
The purpose of solar cells is to capture as much light as possible and produce power from it. But researchers from the University of California, Berkeley think they should do a bit of light-producing of their own. These researchers think “solar cells should be designed to be more like LEDs, able to emit light as well as absorb it,” the Optical Society notes.
Why? Because this results in greater solar cell efficiency, more electricity per solar cell.
The UC Berkeley researchers are scheduled to present their findings on their groundbreaking research on this matter at the Conference on Lasers and Electro Optics (CLEO: 2012) coming up May 6-11 in San Jose, California.
Explain this, Again — How Does Producing Light Make a Solar Cell More Efficient?
“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,” says Eli Yablonovitch, principal researcher and UC Berkeley professor of electrical engineering.
Solar cells, it is rather well known, can’t convert all the light energy they receive into electricity. This is what solar cell efficiency is all about. While some multiple-junction solar cells have hit efficiencies over 40%, the theoretical efficiency limit of typical crystalline solar cells is around 30%. Getting to that efficiency has been a challenge, though, and a bit of a mystery for about 50 years. But researchers at UC Berkeley think they’ve cracked the code. Here’s more from the Optical Society:
“Fundamentally, it’s because there’s a thermodynamic link between absorption and emission,” Miller says. Designing solar cells to emit light – so that photons do not become “lost” within a cell – has the natural effect of increasing the voltage produced by the solar cell. “If you have a solar cell that is a good emitter of light, it also makes it produce a higher voltage,” which in turn increases the amount of electrical energy that can be harvested from the cell for each unit of sunlight, Miller says.
The theory that luminescent emission and voltage go hand in hand is not new. But the idea had never been considered for the design of solar cells before now, Miller continues.
So, in the end, the researchers worked to send photons back out of the solar cell. Interesting. And certainly a bit counterintuitive.
Still confused a bit about how sending photons out of the solar cell can help increase its efficiency? Here’s a little more detail on how a solar cell works and why this is good:
“Solar cells produce electricity when photons from the Sun hit the semiconductor material within a cell. The energy from the photons knocks electrons loose from this material, allowing the electrons to flow freely. But the process of knocking electrons free can also generate new photons, in a process called luminescence. The idea behind the novel solar cell design is that these new photons – which do not come directly from the Sun – should be allowed to escape from the cell as easily as possible.”
Alta Devices Looking to Bring High-Efficiency, Light-Emitting Solar Cells to Market
Of course, at least one company has already been formed to try to pioneer such solar cells in the marketplace. Alta Devices, based in the San Francisco Bay area, “used the new concept to create a prototype solar cell made of gallium arsenide (GaAs)…. The prototype broke the [single-junction solar cell efficiency] record, jumping from 26 percent to 28.3 percent efficiency. The company achieved this milestone, in part, by designing the cell to allow light to escape as easily as possible from the cell – using techniques that include, for example, increasing the reflectivity of the rear mirror, which sends incoming photons back out through the front of the device.”
Alta Devices’ “most recent solar panel has been verified by the National Renewable Energy Laboratory (NREL) at 23.5% efficiency,” a new solar panel efficiency record, the company noted in February of this year. (Note that a solar panel has lower efficiency than a solar cell since it also contains other components.) Alta Devices is working hard to get to commercialization as fast as possible. In that process, I imagine it will continue attempting new world records and the theoretical maximum efficiency of a single-junction solar cell.
Eli Yablonovitch, principal researcher of the study “The Opto-Electronics which Broke the Efficiency Record in Solar Cells” and a UC Berkeley professor of electrical engineering, is optimistic. He hopes researchers will achieve efficiencies of close to 30% in the coming years.
Overall, this is already quite significant research for solar cell scientists and companies of all stripes, as it applies to all types of solar cells and has wide-ranging implications in the field.
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