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Abstract:
Rice University scientists have developed a new method for cost-effectively
producing four-armed quantum dots that have previously been shown to be
particularly effective at converting sunlight into electrical energy. The
discovery, which appears this week in the journal Small, could clear the way
for better, cheaper solar energy panels.

Quantum dot recipe may lead to cheaper solar panels

Houston, TX | Posted on May 2nd, 2007

Rice scientists clear hurdle on path to nanotech-based photovoltaics

Rice University scientists today revealed a breakthrough method for producing
molecular specks of semiconductors called quantum dots, a discovery that could
clear the way for better, cheaper solar energy panels.

The research, by scientists at Rice's Center for Biological and
Environmental Nanotechnology (CBEN), appears this week in the journal Small.
It describes a new chemical method for making four-legged cadmium selenide
quantum dots, which previous research has shown to be particularly effective
at converting sunlight into electrical energy.

"Our work knocks down a big barrier in developing quantum-dot-based
photovoltaics as an alternative to the conventional, more expensive
silicon-based solar cells," said paper co-author and principal investigator
Michael Wong, assistant professor of chemical and biomolecular engineering.

Quantum dots are "megamolecules" of semiconducting materials that are
smaller than living cells. They interact with light in unique ways, to give
off different-colored light or to create electrons and holes, due partly to
their tiny size, partly to their shape and partly to the material they're
made of. Scientists have studied quantum dots for more than a decade, with
an eye toward using them in medical tests, chemical sensors and other
devices.

One way towards cheaper solar cells is to make them out of quantum dots.
Prior research by others has shown that four-legged quantum dots, which are
called tetrapods, are many times more efficient at converting sunlight into
electricity than regular quantum dots. But, Wong said the problem is that
there is still no good way of producing tetrapods. Current methods lead to a
lot of particles with uneven-length arms, crooked arms, and even missing
arms. Even in the best recipe, 30 percent of the prepared particles are not
tetrapods, he said.

CBEN's formula, which was developed by Wong and his graduate student
Subashini Asokan with CBEN Director Vicki Colvin and graduate student Karl
Krueger, produces same-sized particles, in which more than 90 percent are
tetrapods. Significantly, these tetrapods are made of cadmium selenide,
which have been very difficult to make, until now. The essence of the new
recipe is to use cetyltrimethylammonium bromide instead of the standard
alkylphosphonic acid compounds. Cetyltrimethylammonium bromide happens to be
safer - it's used in some shampoos, for example - and it's much cheaper than
alkylphosphonic acids. For producers looking to eventually ramp up tetrapod
production, this means cheaper raw materials and less purification steps,
Wong said.

"One of the major bottlenecks in developing tetrapod-based solar cell
devices has been removed, namely the unavailability of high-quality
tetrapods of the cadmium selenide kind," Wong said. "We might be able to
make high-quality nanoshapes of other compositions also, using this new
synthesis chemistry."

The research was funded by the National Science Foundation, 3M Corp.,
Advanced Aromatics LP, the Air Force Office of Scientific Research and Rice
University.

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About Rice University
Rice University is consistently ranked one of America’s best teaching and
research universities. It is distinguished by its: size—2,850 undergraduates
and 1,950 graduate students; selectivity—10 applicants for each place in the
freshman class; resources—an undergraduate student-to-faculty ratio of
6-to-1, and the fifth largest endowment per student among American
universities; residential college system, which builds communities that are
both close-knit and diverse; and collaborative culture, which crosses
disciplines, integrates teaching and research, and intermingles
undergraduate and graduate work. Rice’s wooded campus is located in the
nation’s fourth largest city and on America’s South Coast.

About CBEN
The Center for Biological and Environmental Nanotechnology is a National
Science Foundation Nanoscale Science and Engineering Center dedicated to
developing sustainable nanotechnologies that improve human health and the
environment. Located at Rice University in Houston, CBEN is a leader in
ensuring that nanotechnology develops responsibly and with strong public
support. For more information visit http://cben.rice.edu .