This is an extraordinary piece of
work that is going to find its way out of the manufacture of solar cells into a
wide range of new layered materials capable of providing the skins for magnetic
field exclusion vehicles (MFEV)
It certainly answers the question
of how does one lay down a single atomic layer uniformly over a large area.
I become more hopeful by the day
that we will soon live to apply the present advances in physics to the
manufacture of large complex craft able to emulate the performance of UFOs.
In the meantime, these solar
cells at twenty percent are sufficiently efficient to make further gains
welcome but not deal breakers.
Additionally, it will certainly drop the raw cost of power under the $1.00
per watt level once and for all while using current built infrastructure to
retrofit. Add in energy storage and solar
becomes highly competitive.
Breakthrough Furnace Can Cut Solar Costs
October 21, 2011
The cavity inside the Solar Optical Furnace glows white hot during a
simulated firing of a solar cell.
Credit: Dennis Schroeder
Solar cells, the heart of the photovoltaic industry, must be tested for
mechanical strength, oxidized, annealed, purified, diffused, etched, and
layered.
Heat is an indispensable ingredient in each of those steps, and that's
why large furnaces dot the assembly lines of all the solar cell manufacturers.
The state of the art has been thermal or rapid-thermal-processing furnaces that
use radiant or infrared heat to quickly boost the temperature of silicon
wafers.
Now, there's something new.
A game-changing Optical Cavity Furnace developed by the U.S.
The Optical Cavity Furnace (OCF) combines the assets that photonics can
bring to the process with tightly controlled engineering to maximize efficiency
while minimizing heating and cooling costs.
NREL's OCF encloses an array of lamps within a highly reflective
chamber to achieve a level of temperature uniformity that is unprecedented.
It virtually eliminates energy loss by lining the cavity walls with
super-insulating and highly reflective ceramics, and by using a complex optimal
geometric design. The cavity design uses about half the energy of a
conventional thermal furnace because in the OCF the wafer itself absorbs
what would otherwise be energy loss. Like a microwave oven, the OCF dissipates
energy only on the target, not on the container.
Different configurations of the Optical Cavity Furnace use the benefits
of optics to screen wafers that are mechanically strong to withstand
handling and processing, remove impurities (called impurity gettering), form
junctions, lower stress, improve electronic properties, and strengthen
back-surface fields.
Bhushan Sopori, a principal engineer at National Renewable Energy
Laboratory, discusses the capabilities of the Optical Cavity Furnace with
colleagues Vishal Mehta and Peter Rupnowski.
Making 1,200 Highly Efficient Solar Cells per Hour
NREL researchers continue to improve the furnace and expect it to be
able soon to hike the efficiency by 4 percentage points, a large leap in an
industry that measures its successes a half a percentage point at a time.
"Our calculations show that some material that is at 16 percent efficiency
now is capable of reaching 20 percent if we take advantage of these photonic
effects," NREL Principal Engineer Bhushan Sopori said. "That's
huge."
Meanwhile, NREL and its private-industry partner, AOS Inc., are
building a manufacturing-size Optical Cavity Furnace capable of processing
1,200 wafers an hour.
At about a quarter to half the cost of a standard thermal furnace, the
OCF is poised to boost the solar cell manufacturing industry in the United
States by helping produce solar cells with higher quality and efficiency at a
fraction of the cost.
The furnace's process times also are significantly shorter than
conventional furnaces. The Optical Cavity Furnace takes only a few minutes to
process a solar wafer.
NREL has cooperative research and development agreements with several
of the world's largest solar-cell manufacturers, all intrigued by the OCF's
potential to boost quality and lower costs.
An NREL colleague inserts a multi-crystaline silicon solar cell sample
into the Optical Cavity Furnace while Principal Engineer Bhushan Sopori, gets
ready to read its analysis on his computer screen.
Credit: Dennis Schroeder
R&D 100 Award Winner
NREL and AOS shared a 2011 R&D 100 Award for the furnace. The
awards, from R&D Magazine, honor the most important technological
breakthroughs of the year.
Billions of solar cells are manufactured each year. A conventional
thermal furnace heats up a wafer by convection; a Rapid-Thermal-Processing
furnace uses radiative heat to boost the temperature of a silicon wafer up to
1,000 degrees Celsius within several seconds.
In contrast to RTP furnaces, the Optical Cavity Furnace processing
involves wafer heating at a relatively slower rate to take advantage of
photonic effects. Slower heating has an added advantage of significantly
lowering the power requirements and the energy loss, so it can boost efficiency
while lowering costs.
"With all solar cells, optics has a big advantage because solar
cells are designed to absorb light very efficiently," NREL Principal
Engineer Bhushan Sopori said. "You can do a lot of things. You can heat it
very fast and tailor its temperature profile so it's almost perfectly
uniform."
In fact, the OCF is so uniform, with the help of the ceramic walls,
that when the middle of the wafer reaches 1,000 degrees Celsius, every nook and
cranny of it is between 999 and 1,001 degrees.
"The amazing thing about this is that we don't use any cooling,
except some nitrogen to cool the ends of the 1-kilowatt and 2-kilowatt
lamps," Sopori said. That, of course, dramatically lowers the energy
requirements of the furnace.
The use of photons also allows junctions to be formed quicker and at
lower temperatures.
As America strives
to reach the goal of 80 percent clean energy by 2035, the White House and the U.S.
Twenty Years of Great Ideas
For more than two decades, Sopori had great ideas for making a better
furnace.
He knew that incorporating optics could produce a furnace that could
heat solar cells, purify them, ease their stress, form junctions and diffuse
just the right amount of dopants to make them more efficient.
"It's always easy on paper," Sopori said recently, recalling
the innovations that worked well on paper and in the lab, but not so well in
the real world. "There are moments … you realize that no one has ever done
something like this. Hopefully it will work, but there are always doubts."
Trouble was, he'd come up with some elegant theoretical solutions
involving optics, but wasn't able to combine them with the optimal geometry and
materials of a furnace. "We've had a whole bunch of patents (12) to do
these things, but what we were missing was an energy-efficient furnace to make
it possible," Sopori said.
And then, combining his expertise in optics with some ingenious engineering
with ceramics, he had his ah-ha moment:
NREL's Optical Cavity Furnace uses visible and infrared light to
uniformly heat crystalline silicon wafers, especially at the edges, which are
prone to cooling or heat loss, at unprecedented precision. The rays heat the
sample, but the wafer never physically contacts the lamps.
The Optical Cavity Furnace is versatile. Each step in the solar cell
manufacturing process typically requires a different furnace configuration and
temperature profile. However, with the OCF, a solar cell manufacturer simply
tells a computer (using NREL proprietary software) what temperature profile is
necessary for processing a solar cell.
So, the OCF can perform five different process steps without the
retooling and reconfiguration required by the furnaces used today, all the
while incrementally improving the sunlight-to-electricity conversion efficiency
of each solar cell.
###
Photons have special qualities that prove useful in creating solar
cells.
When light is shined on silicon atoms that are bonded electronically to
each other it changes their potential. The work by Bhushan Sopori and his
colleagues ensures that that change is for the better.
The Optical Cavity Furnace shines visible and near-infrared light to
heat the solar cell, and also shines ultraviolet light to take advantage of
photonic effects that occur deep within the atomic structure of the cell
material. This combination offers unique capabilities that lead to improved device
quality and efficiency.
Iron and other impurities can degrade the silicon quality quickly,
noted Sopori, the principal investigator for the Optical Cavity Furnace (OCF),
developed by the U.S. Department of Energy's National Renewable Energy
Laboratory and its private industry partner AOS Inc. "But shining the
right light on it can remove that impurity from the silicon," Sopori said.
"We've shown that it is possible with the photonic effects to getter these
impurities during solar cell fabrication."
Optics can make a lot of things happen at the interfaces in a cell,
where, for example, metal can reflect the light and speed the diffusion of
impurities, Sopori said.
The lamps in the furnace help fool the impurities in the silicon into
moving out of the way, by creating vacancies.
"We call it injecting vacancies," Sopori said. A vacancy is a
lack of a silicon atom. "If the atom is missing, you have a vacancy here,
an empty space." Those spaces prompt the impurities such as iron to feel
much more like moving – and they do so at a much lower temperature than would
otherwise be required. The iron moves in with the aluminum, creating an
aluminum-iron mix that, happily, is needed anyway as a contact point.
Removing impurities can change a cell's efficiency from 13 percent
to 17 percent. What that means is that 17 percent of the photons that hit the
improved cell are converted into usable electricity.
The absence of cooling water and confinement of energy in the OCF
proves to be a big advantage for lowering the energy payback time of solar
cells.
Other advantages of the photonic approach:
Silicon cells often have silver contacts in front and aluminum contacts
in back. They usually are fired simultaneously as the cell is being formed. The
OCF by selectively heating the interfaces of silicon and metal can better
control the process, and thus create stronger field surfaces and improved cell
performance.
The Optical Cavity Furnace uses photons of light to remove weak,
cracked wafers from the processing line. Photons can more easily produce a
thermal stress in a wafer and screen out bad wafers. The photon process tests
the wafers' integrity right after they are cut. The conventional method
requires physical twisting and bending of the wafers to test for weakness.
Central heating systems are easily the most cost-effective form of heating for any home.
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