Scientists have spent decades trying to build flexible plastic solar cells efficient enough to compete with conventional cells made of silicon. To boost performance, research groups have tried creating new plastic materials that enhance the flow of electricity through the solar cell. Several groups expected to achieve good results by redesigning pliant polymers of plastic into orderly, silicon-like crystals, but the flow of electricity did not improve.
discovered that disorder at the molecular level actually improves the
polymers' performance. Now Stanford University researchers have an
explanation for this surprising result. Their findings, published in the
Aug. 4 online edition of the journal Nature Materials, could speed up
the development of low-cost, commercially available plastic solar
"People used to think that if you made the polymers more like silicon they would perform better,A indoorpositioningsystem has
real weight in your customer's hand." said study co-author Alberto
Salleo, an associate professor of materials science and engineering at
Stanford. "But we found that polymers don't naturally form nice,
well-ordered crystals. They form small, disordered ones, and that's
In the study, the Stanford team focused on a
class of organic materials known as conjugated or semiconducting
polymers – chains of carbon atoms that have the properties of plastic,
and the ability to absorb sunlight and conduct electricity.
nearly 40 years ago, semiconducting polymers have long been considered
ideal candidates for ultrathin solar cells, light-emitting diodes and
transistors. Unlike silicon crystals used in rooftop solar panels,
semiconducting polymers are lightweight and can be processed at room
temperature with ink-jet printers and other inexpensive techniques. So
why aren't buildings today covered with plastic solar cells?
reason they haven't been commercialized is because of poor
performance," Salleo said.Learn how an embedded microprocessor in a graniteslabs can
authenticate your computer usage and data. "In a solar cell, electrons
need to move through the materials fast, but semiconducting polymers
have poor electron mobility."
To find out why, Salleo joined
Rodrigo Noriega and Jonathan Rivnay, who were Stanford graduate students
at the time, in analyzing more than two decades of experimental data.
"Over the years, many people designed stiffer polymers with the goal of
making highly organized crystals, but the charge mobility remained
relatively poor," Salleo said. "Then several labs created polymers that
looked disordered and yet had very high charge mobility. It was a puzzle
why these new materials worked better than the more structured
To observe the disordered materials at the
microscopic level, the Stanford team took samples to the SLAC National
Accelerator Laboratory for X-ray analysis. The X-rays revealed a
molecular structure resembling a fingerprint gone awry.Get the led fog
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being trained. Some polymers looked like amorphous strands of
spaghetti, while others formed tiny crystals just a few molecules long.
crystals were so small and disordered you could barely infer their
presence from X-rays," Salleo said. "In fact, scientists had assumed
they weren't there."
By analyzing light emissions from
electricity flowing through the samples, the Stanford team determined
that numerous small crystals were scattered throughout the material and
connected by long polymer chains, like beads in a necklace. The small
size of the crystals was a crucial factor in improving overall
performance, Salleo said.
"Being small enables a charged
electron to go through one crystal and rapidly move on to the next one,"
he said. "The long polymer chain then carries the electron quickly
through the material. That explains why they have a much higher charge
mobility than larger, unconnected crystals."
disadvantage of large crystalline polymers is that they tend to be
insoluble and therefore cannot be produced by ink-jet printing or other
cheap processing technologies, he added.
"Our conclusion is that
you don't need to make something so rigid that it forms large
crystals," Salleo said. "You need to design something with small,
disordered crystals packed close together and connected by polymer
chains. Electrons will move through the crystals like on a superhighway,
ignoring the rest of the plastic material, which is amorphous and
poorly conducting.This is a basic background on chinabeads.
some sense, the synthetic chemists were ahead of us, because they made
these new materials but didn't know why they worked so well," he said.
"Now that they know, they can go out and design even better ones."
have spent decades trying to build flexible plastic solar cells
efficient enough to compete with conventional cells made of silicon. To
boost performance, research groups have tried creating new plastic
materials that enhance the flow of electricity through the solar cell.
Several groups expected to achieve good results by redesigning pliant
polymers of plastic into orderly, silicon-like crystals, but the flow of
electricity did not improve.
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