To produce the maximum amount of energy, solar cells are designed to absorb as much light from the Sun as possible.
Now researchers from the University of California, Berkeley, have suggested – and demonstrated – a counterintuitive concept: solar cells should be designed to be more like LEDs, able to emit light as well as absorb it. The Berkeley team will present its findings at the Conference on Lasers and Electro Optics (CLEO: 2012), to be held May 6-11 in San Jose, Calif.
"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.
Since 1961, scientists have known that, under ideal conditions, there is a limit to the amount of electrical energy that can be harvested from sunlight hitting a typical solar cell. This absolute limit is, theoretically, about 33.5 percent. That means that at most 33.5 percent of the energy from incoming photons will be absorbed and converted into useful electrical energy.
Yet for five decades, researchers were unable to come close to achieving this efficiency: as of 2010, the highest anyone had come was just more than 26 percent. (This is for flat-plate, "single junction" solar cells, which absorb light waves above a specific frequency. "Multi-junction" cells, which have multiple layers and absorb multiple frequencies, are able to achieve higher efficiencies.)
More recently, Yablonovitch and his colleagues were trying to understand why there has been such a large gap between the theoretical limit and the limit that researchers have been able to achieve. As they worked, a "coherent picture emerged," says Owen Miller, a graduate student at UC Berkeley and a member of Yablonovitch's group. They came across a relatively simple, if perhaps counterintuitive, solution based on a mathematical connection between absorption and emission of light.
"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.
This past year, a Bay area-based company called Alta Devices, co-founded by Yablonovitch, used the new concept to create a prototype solar cell made of gallium arsenide (GaAs), a material often used to make solar cells in satellites. The prototype broke the 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.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.
The work is "a good, useful way" of determining how scientists can improve the performance of solar cells, as well as of finding creative new ways to test and study solar cells, says Leo Schowalter of Crystal IS, Inc. and visiting professor at Rensselaer Polytechnic Institute, who is chairman of the CLEO committee on LEDs, photovoltaics, and energy-efficient photonics.
Yablonovitch says he hopes researchers will be able to use this technique to achieve efficiencies close to 30 percent in the coming years. And since the work applies to all types of solar cells, the findings have implications throughout the field.
The CLEO: 2012 presentation CF2J.1, "The Opto-Electronics which Broke the Efficiency Record in Solar Cells," by Eli Yablonovitch and Owen D. Miller, is at 10:30 a.m. Friday May 11 in the San Jose Convention Center.
EDITOR'S NOTE: High-resolution images are available to members of the media upon request. Contact Angela Stark, firstname.lastname@example.org.
A Press Room for credentialed press and analysts will be located on-site at CLEO: 2012 in the San Jose Convention Center, May 6 – May 11. Media interested in attending the conference should register on the CLEO website or contact Angela Stark at 202.416.1443, email@example.com.About CLEO
Angela Stark | EurekAlert!
Researchers use light to remotely control curvature of plastics
23.03.2017 | North Carolina State University
TU Graz researchers show that enzyme function inhibits battery ageing
21.03.2017 | Technische Universität Graz
Astronomers from Bonn and Tautenburg in Thuringia (Germany) used the 100-m radio telescope at Effelsberg to observe several galaxy clusters. At the edges of these large accumulations of dark matter, stellar systems (galaxies), hot gas, and charged particles, they found magnetic fields that are exceptionally ordered over distances of many million light years. This makes them the most extended magnetic fields in the universe known so far.
The results will be published on March 22 in the journal „Astronomy & Astrophysics“.
Galaxy clusters are the largest gravitationally bound structures in the universe. With a typical extent of about 10 million light years, i.e. 100 times the...
Researchers at the Goethe University Frankfurt, together with partners from the University of Tübingen in Germany and Queen Mary University as well as Francis Crick Institute from London (UK) have developed a novel technology to decipher the secret ubiquitin code.
Ubiquitin is a small protein that can be linked to other cellular proteins, thereby controlling and modulating their functions. The attachment occurs in many...
In the eternal search for next generation high-efficiency solar cells and LEDs, scientists at Los Alamos National Laboratory and their partners are creating...
Silicon nanosheets are thin, two-dimensional layers with exceptional optoelectronic properties very similar to those of graphene. Albeit, the nanosheets are less stable. Now researchers at the Technical University of Munich (TUM) have, for the first time ever, produced a composite material combining silicon nanosheets and a polymer that is both UV-resistant and easy to process. This brings the scientists a significant step closer to industrial applications like flexible displays and photosensors.
Silicon nanosheets are thin, two-dimensional layers with exceptional optoelectronic properties very similar to those of graphene. Albeit, the nanosheets are...
Enzymes behave differently in a test tube compared with the molecular scrum of a living cell. Chemists from the University of Basel have now been able to simulate these confined natural conditions in artificial vesicles for the first time. As reported in the academic journal Small, the results are offering better insight into the development of nanoreactors and artificial organelles.
Enzymes behave differently in a test tube compared with the molecular scrum of a living cell. Chemists from the University of Basel have now been able to...
20.03.2017 | Event News
14.03.2017 | Event News
07.03.2017 | Event News
24.03.2017 | Materials Sciences
24.03.2017 | Physics and Astronomy
24.03.2017 | Physics and Astronomy