A high-efficiency Alta Devices solar cell. Credit: Joe Foster, Alta Devices. |
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%. That means that at most 33.5% 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%. (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.
Eli Yablonovitch and Owen Miller, who worked out the theory for the new solar cell efficiency. The monitor in the picture illustrates the new physics concept where increased light emission yields higher efficiency. Photo courtesy Eli Yablonovitch. |
“The
first reaction is usually, why does it help [to let these photons
escape]?” Miller says. “Don’t you want to keep [the photons] in, where
maybe they could create more electrons?” However, mathematically,
allowing the new photons to escape increases the voltage that the cell
is able to produce.
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% 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.
Source: Optical Society of America
