From Stanford: “Disorder can improve the performance of plastic solar cells, Stanford scientists say”

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Stanford University

Instead of mimicking rigid solar cells made of silicon crystals, scientists should embrace the inherently disordered nature of plastic polymers, a Stanford study has found

August 4, 2013
Mark Shwartz

“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.

Recently, scientists 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 cells.

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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.

‘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.”

X-ray analysis

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. Some polymers looked 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.

Other authors of the study are postdoctoral scholar Koen Vandewal of Stanford; Felix Koch and Paul Smith of ETH Zurich; Natalie Stingelin of Imperial College London; and Michael Toney of the SLAC Stanford Synchrotron Radiation Lightsource.”

See the full article here.

Leland and Jane Stanford founded the University to “promote the public welfare by exercising an influence on behalf of humanity and civilization.” Stanford opened its doors in 1891, and more than a century later, it remains dedicated to finding solutions to the great challenges of the day and to preparing our students for leadership in today’s complex world. Stanford, is an American private research university located in Stanford, California on an 8,180-acre (3,310 ha) campus near Palo Alto. Since 1952, more than 54 Stanford faculty, staff, and alumni have won the Nobel Prize, including 19 current faculty members

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