Acting like a waffle iron, silicon nanodomes, each about 300 nanometers in diameter and 200 nanometers tall, imprint a honeycomb pattern of nanoscale dimples into a layer of metal within the solar cell. Image courtesy of Michael McGehee |
Researchers
in solar energy speak of a day when millions of otherwise fallow square
meters of sun-drenched roofs, windows, deserts and even clothing will
be integrated with inexpensive solar cells that are many times thinner
and lighter than the bulky rooftop panels familiar today.
So,
when your iPod is on the nod, you might plug it into your shirt to
recharge. Lost in the Serengeti with a sapped cell phone? No problem;
rolled in your backpack is a lightweight solar pad. Sailing the seven
seas and your GPS needs some juice? Hoist a solar sail and be one with
the gods of geosynchronous orbit.
It
is not hard to envision a time when such technologies will be
ubiquitous in our increasingly energy-hungry lives. That day may come a
bit sooner thanks to a multidisciplinary team of Stanford engineers led
by Mike McGehee, Yi Cui and Mark Brongersma, and joined by Michael
Graetzel at the École Polytechnique Fédérale de Lausanne (EPFL).
Waves of energy
In
an article published in Advance Energy Materials, the Stanford/EPFL
team announced a new type of thin solar cell that could offer a new
direction for the field. They succeeded in harnessing plasmonics – an
emerging branch of science and technology – to more effectively trap
light within thin solar cells to improve performance and push them one
step closer to daily reality.
“Plasmonics
makes it much easier to improve the efficiency of solar cells,” said
McGehee, an associate professor of materials science and engineering at
Stanford.
McGehee
is the director of CAMP – the Center for Advanced Molecular
Photovoltaics – a multidisciplinary, multi-university team tackling the
challenges of thin-film solar cells.
“Using
plasmonics we can absorb the light in thinner films than ever before,”
McGehee said. “The thinner the film, the closer the charged particles
are to the electrodes. In essence, more electrons can make it to the
electrode to become electricity.”
Plasmonics
is the study of the interaction of light and metal. Under precise
circumstances, these interactions create a flow of high-frequency, dense
electrical waves rather than electron particles. The electronic pulse
travels in extremely fast waves of greater and lesser density, like
sound through the air.
A perfect solar waffle
The
lightbulb moment for the team came when they imprinted a honeycomb
pattern of nanoscale dimples into a layer of metal within the solar
cell. Think of it as a nanoscale waffle, only the bumps on the waffle
iron are domes rather than cubes – nanodomes to be exact, each only a
few billionths of a meter across.
To
fashion their waffle, McGehee and team members spread a thin layer of
batter on a transparent, electrically conductive base. This batter is
mostly titania, a semi-porous metal that is also transparent to light.
Next, they use their nano waffle iron to imprint the dimples into the
batter. Next, they layer on some butter – a light-sensitive dye – which
oozes into the dimples and pores of the waffle. Lastly, the engineers
add some syrup – a layer of silver, which hardens almost immediately.
When all those nanodimples fill up, the result is a pattern of nanodomes on the light-ward side of the silver.
This
bumpy layer of silver has two primary benefits. First, it acts as a
mirror, scattering unabsorbed light back into the dye for another shot
at collection. Second, the light interacts with the silver nanodomes to
produce plasmonic effects. Those domes of silver are crucial. Reflectors
without them will not produce the desired effect. And any old nanodomes
won’t do either; they must be just the right diameter and height, and
spaced just so, to fully optimize the plasmonics.
Titania within the solar cell is imprinted by the silicon nanodomes like a waffle imprinted by the iron. Image courtesy of Michael McGehee |
If
you imagine your nanoself observing one of these solar cells in slow
motion, you would see photons enter and pass through the transparent
base and the titania (the waffle), at which point some photons would be
absorbed by the light-sensitive dye (the butter), creating an electric
current. Most of the remaining photons would hit the silver back
reflector (the hardened syrup) and bounce back into the solar cell. A
certain portion of the photons that reach the silver, however, will
strike the nanodomes and cause plasmonic waves to course outward. And
there you have it – the first-ever plasmonic dye-sensitized solar cell.
Trapping the light fantastic
It
is easy to see why researchers are focused on thin-film solar
technology. In recent years, much hope has been directed toward these
lightweight, flexible cells that use photosensitive dyes to generate
electricity. These cells have many advantages: They are less energy
intensive and less costly to produce, flowing like newsprint off huge
roll presses. They are thinner even than other “thin” solar cells. They
are also printable on flexible bases that can be rolled up and taken
virtually anywhere. Many use non-toxic, abundantly available materials,
as well – a huge plus in the push for sustainability.
Dye-sensitized
solar cells are not without challenges, however. First off, the very
best convert only a small percentage of light into electricity – about 8
percent. The bulkier commercial technologies available today have
reached 25 percent efficiency, and certain advanced applications have
topped 40 percent. And then there is durability. The latest thin solar
cell will last about seven years under continuous exposure to the
elements. Not bad until you consider that 20 to 30 years is the
commercial standard.
Both
efficiency and reliability will have to improve. Nonetheless, engineers
like McGehee believe that if they can convert just 15 percent of the
light into electricity – a figure that is not out of reach – and tease
the lifespan to a decade, we might soon find ourselves in the age of
personal solar cells. An advance like plasmonics just might provide the
spark necessary to take the field down a new and exciting path.
A matter of economics
Cheaper
and cleaner will be the keys. Coal-based power is plentiful and cheap,
but also comes at a steep environmental cost in gouged landscapes and
polluted skies. At today’s commercial rates, however, even the best
solar alternatives cost five times more per kilowatt-hour than coal. It
is clear that economics, and not technology, is what stands between us
and our solar future.
But McGehee and others are confident they can make thin solar cells more attractive.
Andrew Myers is the associate communications director for the School of Engineering.
