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Hope for `sun proofing' solar cells
To improve the efficiency
and reliability of solar cells, researchers are investigating
mixed-phase solar cell materials — a mixture of clusters of
nanocrystalline silicon embedded in an amorphous matrix.
SCIENTISTS AT the U.S. Department of Energy's Ames Laboratory and
Iowa State University's Microelectronics Research Centre (MRC) may
have solved a mystery that has plagued the research community for
more than 20 years: Why do solar cells degrade in sunlight? Finding
the answer to that question is essential to the advancement of solar
cell research and the ability to produce lower-cost electricity from
sunlight.
``The problem is that when you put solar cells in
sunlight, the efficiency starts to decrease by as much as 15-20 per
cent over a period of several days,'' said Rana Biswas, a physicist
at Ames Laboratory and the MRC.
Solar cells made from hydrogenated amorphous
silicon, a noncrystalline form of silicon, absorb light far more
effectively than traditional crystalline silicon solar cells.
``Instead of a thick, 20-micron crystalline silicon film, you can
just deal with a very thin, half-micron amorphous silicon film,''
said Biswas. ``These cells are more cost-effective as they involve
much less material and processing time — driving forces for
industry. But, although amorphous silicon absorbs light efficiently,
it suffers from degradation effect.''
Biswas and his co-workers have been studying the
troublesome degradation effect, also known as the Staebler-Wronski
effect, for the past few years. The effort includes investigations
into the atomic origins of the S-W effect and the subsequent
exploration of possible new solar cell materials through computer
molecular dynamics simulations.
Biswas explained that exposure to light can cause
changes in hydrogenated amorphous silicon, resulting in defects
known as metastable dangling bonds — bonds that can go away only
when heated to a high temperature. Dangling bonds are missing a
neighbour to which they can bond. To remedy the situation, they will
`capture' electrons, reducing the electricity that light can produce
and decreasing solar cell efficiencies. ``The question,'' Biswas
said, ``is how does light create dangling bonds?''
The answer has long been a mystery, but now Biswas
and his co-workers, Bicai Pan and Yiying Ye, are helping resolve
many puzzling aspects of the problem with their three-step atomistic
rebonding model. The model is based on rearrangements of silicon and
hydrogen atoms in the hydrogenated amorphous silicon material, says
from Ames Laboratory. In the first step, sunlight creates excited
electrons and holes (vacant electron energy states) in the material.
When the electrons recombine, they pair up with holes on the weak
silicon bonds. The recombination energy causes the weak silicon
bonds to break, creating silicon dangling bond-floating bond pairs.
During the second step, the floating bonds break away from the
dangling bonds and move freely throughout the material. This occurs
when the extra floating bond from one silicon atom moves to a
neighbouring silicon atom.
The third step reveals that the short-lived
floating bonds disappear. Some recombine with the silicon dangling
bonds, which results in no material defects. Others `hop' away from
the dangling bonds and are annihilated when hydrogen atoms in the
network move into the floating bond sites.
Biswas' three-step rebonding model shows that
defect creation in hydrogenated amorphous silicon solar cells is
first driven by breaking of weak silicon bonds followed by rebonding
of silicon and hydrogen sites in the material.
The research represents a significant achievement
in understanding atomic origins of light-induced degradation effect
in hydrogenated amorphous silicon and so provides a vantage point
for eliminating this effect in development of new cell materials.
To improve the efficiency and reliability of solar
cells, Biswas and his co-workers are investigating mixed-phase solar
cell materials — a mixture of clusters of nanocrystalline silicon
embedded in an amorphous matrix. "One of the most promising
developments has been the success of hydrogen-diluted materials
grown at the edge of crystallinity — the phase boundary between
microcrystalline and amorphous film growth," said Biswas. "These
materials and solar cells made from them have a much greater
stability to light-induced degradation than traditional amorphous
material."
By developing molecular dynamics computer
simulations, Biswas hopes to learn more about this mixed-phase
material at the atomic level and discover what aspect of the mixture
is responsible for the improved material properties.
His research efforts may even extend to
manipulating the nanoscale structure of the material, allowing the
design and creation of improved materials.
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