Emitting Photons Key To Increased Photovoltaic Activity
Scientists in the U.S. Department of
Energy (DOE)'s Lawrence Berkeley National Laboratory (Berkeley Lab) say their
research has led to record-breaking sunlight-to-electricity conversion
efficiencies in solar cells.
"A great solar cell also needs to be
a great Light Emitting Diode," says Eli Yablonovitch, the Berkeley Lab electrical
engineer who led this research. "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."
Yablonovitch holds joint appointments
with Berkeley Lab's Materials Sciences Division and the University of
California (UC) Berkeley, where he is the James and Katherine Lau Chair in
Engineering, and also directs the NSF Centre for Energy Efficient Electronics
Science. Other scientists who contributed to
the ground breaking research are Owen Miller of Berkeley Lab, and Sarah Kurtz,
at the National Renewable Energy Laboratory.
Yablonovitch, Miller and Kurtz
describe how external fluorescence is the key to approaching the theoretical
maximum efficiency at which a solar cell can convert sunlight into electricity.
This theoretical efficiency, called the Shockley-Queisser efficiency limit (SQ
Limit), measures approximately 33.5 percent for a single p-n junction solar
cell. This means that if a solar cell collects 1,000 Watts per square metre of
solar energy, the most electricity it could produce would be about 335 Watts
per square metre.
Calculations by Miller, who is a
member of Yablonovitch's research group, showed that GaAs is capable of
reaching the SQ Limit. Based on this work, Alta Devices, a private company
co-founded by Yablonovitch, has been able to fabricate solar cells from GaAs
that have achieved a record conversion efficiency of 28.4 percent.
"Owen Miller provided an accurate
theory on how to reach the SQ Limit that for the first time included external
fluorescence efficiency," Yablonovitch says. "His calculations for gallium
arsenide showed that external fluorescence provides the voltage boost that Alta
researchers subsequently observed."
Solar or photovoltaic cells represent
one of the best possible technologies for providing an absolutely clean and
virtually inexhaustible source of electricity. However, for this dream to be
realised, solar cells must be able to efficiently and cost-competitively
convert sunlight into electricity. They must also be far less expensive to
make.
The most efficient solar cells in
commercial use today are made from monocrystalline silicon wafers and typically
reach a conversion efficiency of about 23 percent. High grade silicon is an
expensive semiconductor but is a weak collector of photons. GaAs, although even
more expensive than silicon, is more proficient at absorbing photons, which
means much less material is needed to make a solar cell.
"Gallium arsenide absorbs photons
10,000 times more strongly than silicon for a given thickness but is not 10,000
times more expensive," says Yablonovitch. "Based on performance, it is the
ideal material for making solar cells."
Past efforts to boost the conversion
efficiency of solar cells focused on increasing the number of photons that a
cell absorbs. Absorbed sunlight in a solar cell produces electrons that must be
extracted from the cell as electricity. Those electrons that are not extracted
fast enough, decay and release their energy. If that energy is released as
heat, it reduces the solar cell's power output. Miller's calculations showed
that if this released energy exits the cell as external fluorescence, it would
boost the cell's output voltage.
"This is the central
counter-intuitive result that permitted efficiency records to be broken,"
Yablonovitch says.
As Miller explains, "In the
open-circuit condition of a solar cell, electrons have no place to go so they
build up in density and, ideally, emit external fluorescence that exactly
balances the incoming sunlight. As an indicator of low internal optical losses,
efficient external fluorescence is a necessity for approaching the SQ Limit."
Using a single-crystal thin film
technology developed earlier by Yablonovitch, called "epitaxial liftoff," Alta
Devices was able to fabricate solar cells based on GaAs that not only smashed
previous solar conversion efficiency records, but can be produced at well below
the cost of any other solar cell technology. Alta Devices expects to have GaAs
solar panels on the market within a year.
"The SQ Limit is still the foundation
of solar cell technology," says Yablonovitch. "However, the physics of light
extraction and external fluorescence are clearly relevant for high performance
solar cells."
Yablonovitch believes that the
theoretical work by the group, in combination with the performance
demonstrations at Alta Devices, could dramatically change the future of solar
cells.
"We're going to be living in a world
where solar panels are very cheap and very efficient," Yablonovitch says.
This research was funded by a grant
from DOE's Light-Material Interactions in Energy Conversion Energy Frontier
Research Centre (LMI-EFRC).
Further details of this work have
been published in the paper, "Intense Internal and External Fluorescence as
Solar Cells Approach the Shockley-Queisser Efficiency Limit," by Miller et al,
published online: arXiv:1106.1603v3 [physics.optics]
