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electricity prices are high and solar cells are most e cient, the
power produced by photovoltaics is still far too expensive.
Dozens of startups have formed in recent years to pursue tech-
nologies that their founders hope will be more cost e ective. To
Swanson's mind, however, the attention given to these e orts
is misplaced. The cost of electricity from silicon photovoltaics
is decreasing by 5 to 8 percent a year as the industry grows at a
rapid pace, he says; within five years, as the existing technology
improves and manufacturers realize economies of scale, it will be
competitive without federal incentives.
"We don't need a breakthrough," Swanson says. "Waiting for
the next big breakthrough [in photovoltaics] will do nothing but
cause you to grow moss underneath your feet." He adds, "We have
a road map where we can very clearly see how to halve the cost
from where we are today. And that is su cient to fuel explosive
TURNING THE CORNER
The solar industry might not need a breakthrough to continue
healthy growth rates. But many scientists say that without dra-
matic advances, solar power will never supply the vast amount of
power needed to eventually displace fossil fuels.
Of the 46 new energy research centers announced by the sec-
retary of energy in late April, 24 are doing work related to solar
power, and each is receiving $2 million to $5 million annually over
the next five years. Likewise, two of the eight new DOE innova-
tion hubs will focus on solar technologies: one on electricity and
the other on techniques for storing the energy from sunlight in
the form of fuels. And the proposed 2010 DOE budget, which
(coming just a few months after the stimulus bill) contained rela-
tively modest increases for most new energy technologies, nearly
doubled the research budget for solar power.
Much of the research focuses on overcoming the fundamental
dilemma of photovoltaic technology: the trade-o between cost
and e ciency. Conventional solar cells are e cient because the
silicon from which they're made is grown as a single crystal, yield-
ing a perfectly ordered molecular structure; when the semicon-
ductor absorbs sunlight, the light's energy excites electrons that
can travel through this crystal structure unimpeded, escaping
to create an electrical current. But making devices out of single-
crystal silicon is relatively di cult and expensive. Newer photo-
voltaic technologies use materials that have a less ordered structure
and can be deposited as thin films; they are potentially easier and
cheaper to make, but they're also less e cient.
"With photovoltaics you have either high e ciencies or low cost,
but what we urgently need are [photovoltaics] with both attributes,"
says Harry Atwater, a professor of physics and materials science
at Caltech. "One of the challenges of solar power is how to get
hundreds of gigawatts to a terawatt of power in a way that is cost
e ective." Achieving that, he says, may take technology "very dif-
ferent than what we use today."
Atwater will head a DOE-funded energy research center at
Caltech, where scientists will work on developing materials that
could enable thin-film photovoltaics to absorb sunlight more e -
ciently. These materials, whose microstructure is designed to inter-
act with light in new ways, could be made using di erent types of
semiconductors. Light that strikes solar cells made from them,
Atwater says, can be forced to "turn a corner" and travel parallel to
the surface of the thin film. As a result, the cell has a chance to absorb
much more light than it would if the light passed through perpen-
dicular to the surface.
Researchers elsewhere are hoping to overcome the challenges
inherent in using disordered materials for photovoltaic cells. When
light strikes the jumble of molecules in such materials, the excited
electrons and the electron "holes" left behind when they're knocked
free form particle-like pairs called excitons. Excitons play a role in
the process that plants use to capture energy through photosynthe-
sis, says Marc Baldo, a professor of electrical engineering at MIT; in
addition, organic light-emitting diodes use them to generate light.
And, he says, it might be possible to manipulate these excitons on
the nanoscale to improve the photovoltaic properties of disordered
materials. Baldo heads a DOE-funded energy research center for
excitonics, which includes researchers from MIT, Harvard Uni-
versity, and Brookhaven National Laboratory.
Ultimately, however, using sunlight to produce electricity will
never supply enough of the energy we need: existing solar tech-
nologies, after all, produce power only during the day, and elec-
tricity can't easily be stored. Instead, we must find a way to use
sunlight to make fuels such as hydrogen, which can readily and
cheaply be stored until they're needed.
Learning how to e ciently make such fuels directly from the
sun---a process called artificial photosynthesis, because the aim is to
essentially mimic the natural process used by green plants---is "still
20 to 30 years down the road," says Harry Gray, a chemist at Caltech
and director of a solar-research collaboration that includes scien-
tists from a number of universities. Although researchers, includ-
ing some in his group, are getting "nice results" on certain aspects
of artificial photosynthesis, lots of di cult problems remain to be
solved. "It's going to take a long time to get it together," he says.
Silicon photovoltaics will be the dominant solar technology "for
quite a while," says Gray. "If all goes well, we will move into cheaper
solar cells that are not single-crystal silicon, such as organic photo-
voltaics. But the transition [to cheaper photovoltaics] is not going
to come all that fast."
Will the stimulus bill facilitate that much-needed transition
to more e cient technologies? Severin Borenstein, for one, is
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