BERKELEY, CA — A new generation of solar cells
that combines nanotechnology with plastic electronics has been launched
with the development of a semiconductor-polymer photovoltaic device by
researchers with the U.S. Department of Energy's Lawrence Berkeley National
Laboratory (Berkeley Lab) and the University of California at Berkeley
(UCB). Such hybrid solar cells will be cheaper and easier to make than
their semiconductor counterparts, and could be made in the same nearly
infinite variety of shapes as pure polymers.
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Janke Dittmer, Wendy Huynh, and team
leader Paul
Alivisatos created tiny solar cell assemblies (shown held by
a tweezer, upper right) using nanoscale semiconducting rods
of polymer and cadmium selenide |
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Paul Alivisatos, a chemist who holds a joint appointment with Berkeley
Lab's Materials Science Division (MSD) and UCB's Chemistry Department,
led the research team, who reported their hybrid solar cell development
in the March 29, 2002 issue of the journal Science. Other members
of the team were Wendy Huynh, a graduate student with UCB's Chemistry
Department, and Janke Dittmer, an MSD staff scientist.
"We have demonstrated that semiconductor nanorods can be used to
fabricate readily processed and energy-efficient hybrid solar cells together
with polymers," says Alivisatos, a leading authority on the production
of nanosized semiconductor crystals and director of the Molecular Foundry,
a center for nanoscience now being established at Berkeley Lab.
The use of solar, or photovoltaic, cells devices that can absorb and
convert light into electrical power has been limited to date because
production costs are so high. Even the fabrication of the simplest semiconductor
cell is a complex process that has to take place under exactly controlled
conditions, such as high vacuum and temperatures between 400 and 1,400
degrees Celsius.
Ever since the discovery in 1977 of conducting plastics (polymers which
feature conjugated double chemical bonds, that enable electrons to move
through them), there has been interest in using these materials in the
fabrication of solar cells. Plastic solar cells can be made in bulk quantities
for a few cents each; however, the efficiency with which they convert
light into electricity has been quite poor compared to the power-conversion
efficiencies of semiconductor cells.
"The advantage of hybrid materials consisting of inorganic semiconductors
and organic polymers is that potentially you get the best of both worlds,"
says Dittmer. "Inorganic semiconductors offer excellent, well established
electronic properties, and they are very well suited as solar cell materials.
Polymers offer the advantage of solution processing at room temperature,
which is cheaper and allows for using fully flexible substrates, such
as plastics."
At the heart of all photovoltaic devices are two separate layers of materials,
one with an abundance of electrons that functions as a "negative
pole," and one with an abundance of electron holes (vacant, positively-charged
energy spaces) that functions as a "positive pole." When photons
from the sun or some other light source are absorbed, their energy is
transferred to the extra electrons in the negative pole, causing them
to flow to the positive pole and creating new holes that start flowing
to the negative pole. This electrical current can then be used to power
other devices such as a pocket calculator.
In a typical semiconductor solar cell, the two poles are made from n-type
and p-type semiconductors. In a plastic solar cell, they're made from
hole-acceptor and electron-acceptor polymers. In their new hybrid solar
cell, Alivisatos, Huynh, and Dittmer used the semicrystalline polymer
known as poly(3-hexylthiophene), or P3HT, for the hole acceptor or negative
pole, and nanometer-sized cadmium selenide (CdSe) rods as the positive
pole.
"We chose P3HT because it can be processed in solution and has been
used by many research groups around the world who are working on plastic
transistors," says Huynh. "Also, it is the conjugated polymer
with the highest hole mobility found so far. Higher hole (and electron)
mobility means that charges are transported more quickly, which reduces
current losses."
The cadmium selenide rods measured 7 nanometers in diameter and 60 nanometers
in length (a nanometer is one billionth of a meter, less than one-hundred
millionth of an inch). Alivisatos led an earlier study in which the technique
for growing semiconductor nanocrystals into two-dimensional rods was first
developed. Prior to that work, nanocrystals had always been grown as one-dimensional
spheres. Using rod-shaped nanocrystals rather than spheres provided a
directed path for electron transport to help improve solar cell performance.
"With CdSe rods measuring 7 nanometers by 60 nanometers, our hybrid
solar cells achieved a monochromatic power conversion efficiency of 6.9
percent, one of the highest ever reported for a plastic photovoltaic device,"
says Alivisatos. Monochromatic power efficiency measures the ability to
convert room light (which is mostly visible light) into electricity.
The Berkeley researchers prepared their solar cells by codissolving the
nanorods with the P3HT and spin-casting the hybrid solution onto a glass
substrate. They found that by keeping the length of the rods constant
while modifying the diameter enabled them to tune the absorption spectrum
of the cells so that it overlapped with the solar emission spectrum. This
not only enables their hybrid cells to collect more light than typical
plastic solar cells, but it also opens the door for high-efficiency devices
in the future, such as tandem solar cells.
Although the efficiency of the Berkeley hybrid cells for converting sunlight
into electricity was only 1.7 percent at A.M. 1.5 (when the sun is at
a 41.8-degree angle to the horizon), which is far off the mark of the
best semiconductor solar cells, Dittmer says there is ample opportunity
for improvement.
"The most important step is to increase the amount of sunlight absorbed
in the red part of the spectrum, which we can do by going to other semiconductor
materials such as cadmium telluride. Also, our published hybrid solar
cells have a very simple structure, in order to investigate the science
behind them. In the future, many engineering tricks can be applied to
make the cells more efficient." The Berkeley researchers have already
been approached by companies that are interested in commercializing this
technology.
Berkeley Lab is a U.S. Department of Energy national laboratory located
in Berkeley, California. It conducts unclassified scientific research
and is managed by the University of California.
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