Evolution inspires more efficient solar cell design
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| McCormick researchers have designed a geometrically-patterned light scattering layer that could make solar cells more efficient and less expensive. |
The sun's energy is virtually limitless,
but harnessing its electricity with today's single-crystal silicon solar cells
is extremely expensive -- 10 times pricier than coal, according to some
estimates. Organic solar cells -- polymer solar cells that use organic
materials to absorb light and convert it into electricity -- could be a
solution, but current designs suffer because polymers have less-than-optimal
electrical properties. Researchers at Northwestern University have now
developed a new design for organic solar cells that could lead to more
efficient, less expensive solar power. Instead of attempting to increase
efficiency by altering the thickness of the solar cell's polymer layer -- a
tactic that has preciously garnered mixed results -- the researchers sought to
design the geometric pattern of the scattering layer to maximize the amount of
time light remained trapped within the cell.
Using a
mathematical search algorithm based on natural evolution, the researchers
pinpointed a specific geometrical pattern that is optimal for capturing and
holding light in thin-cell organic solar cells.
The
resulting design exhibited a three-fold increase over the Yablonovitch Limit, a
thermodynamic limit developed in the 1980s that statistically describes how
long a photon can be trapped in a semiconductor.
In the
newly designed organic solar cell, light first enters a 100-nanometer-thick
"scattering layer," a geometrically-patterned dielectric layer
designed to maximize the amount of light transmitted into the cell. The light
is then transmitted to the active layer, where it is converted into
electricity.
"We
wanted to determine the geometry for the scattering layer that would give us
optimal performance," said Cheng Sun, assistant professor of mechanical
engineering in Northwestern's McCormick School of Engineering and Applied
Science and co-author of the paper. "But with so many possibilities, it's
difficult to know where to start, so we looked to laws of natural selection to
guide us."
The
researchers employed a genetic algorithm, a search process that mimics the
process of natural evolution, explained Wei Chen, Wilson-Cook Professor in
Engineering Design and professor of mechanical engineering at McCormick and
co-investigator of the research.
"Due
to the highly nonlinear and irregular behavior of the system, you must use an
intelligent approach to find the optimal solution," Chen said. "Our
approach is based on the biologically evolutionary process of survival of the
fittest."
The
researchers began with dozens of random design elements, then "mated"
and analyzed their offspring to determine their particular light-trapping
performance. This process was carried out over more than 20 generations and
also accounted for evolutionary principles of crossover and genetic mutation.
The
resulting pattern will be fabricated with partners at Argonne National
Laboratory.
Also
co-authoring the paper were co-lead authors Chen Wang and Shuangcheng Yu,
graduate students in McCormick's Department of Mechanical Engineering.
Source: Northwestern University
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Posted by Unknown
on Saturday, January 26, 2013.
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