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Date: 06-08-13

Scientists at Stanford exploit disorders in plastic solar cell to improve performance

Scientists at Stanford University have noticed plastic material used for solar photovoltaic cells have structural differences with silicon solar PV cell. They cannot be used or made like the way silicon solar PV cells made. The disorders at the molecule level are exploited by scientists to build plastic solar PV cells of higher efficiency. Most of the solar researchers are following similar process what they are followed for silicon to make plastic solar cells, which is not working.

"People used to think that if you made the polymers more like silicon they would perform better," 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 perfectly fine."

Instead of trying to mimic the rigid structure of silicon, Salleo and his colleagues recommend that scientists learn to cope with the inherently disordered nature of plastics. Standard team have used organic material known as conjugated semiconducting polymer which is conductive and optoelectronic in nature. This organic semiconductor material is also a good material for light emitting diodes. Since it is plastic/polymer, manufacturing solar devices out of this material can be done at room temperature with printing technologies and other such less-expensive processes. The challenge engineers and scientists facing now in using polymer material is, they are less efficient in converting light energy into electrical energy and also poor in performance in terms of electron mobility.

Stanford team did the x-ray analysis of sample at SLAC National Accelerator Laboratory and noticed molecular structure resembling a fingerprint gone awry with some looking like amorphous strands of spaghetti, while others formed tiny crystals just a few molecules long.

"The 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."

Another 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."

"In 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."

And Salleo offered a final piece of advice. "Try to design a material that can live with as much disorder as possible," he said. "Take the disorder for granted. Personally, I really like disorder. Just look at my office."

plastic solar

These X-ray images reveal the microscopic structure of two semiconducting plastic polymers. The bottom image, with several big crystals stacked in a row, is from a highly ordered polymer sample. The top image shows a disordered polymer with numerous tiny crystals that are barely discernible.

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