Forum for Science, Industry and Business

Sponsored by:     3M 
Search our Site:

 

Disorder can improve the performance of plastic solar cells, Stanford scientists say

05.08.2013
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.


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.

Credit: Jonathan Rivnay (Stanford) and Michael Toney (SSRL/SLAC)

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.

"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.

Speedy electrons

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.

Discovered 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?

"One reason they haven't been commercialized is because of poor performance," Salleo said. "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 crystalline ones."

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.

"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."

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.

The study was supported by a Stanford Center for Advanced Molecular Photovoltaics award from the King Abdullah University of Science and Technology; and by the European Research Council.

This article was written by Mark Shwartz of the Precourt Institute for Energy at Stanford University.

Comment:

Alberto Salleo, Department of Materials Science and Engineering: (650) 725-1025, asalleo@stanford.edu

Mark Shwartz | EurekAlert!
Further information:
http://www.stanford.edu

More articles from Materials Sciences:

nachricht Move over, Superman! NIST method sees through concrete to detect early-stage corrosion
27.04.2017 | National Institute of Standards and Technology (NIST)

nachricht Control of molecular motion by metal-plated 3-D printed plastic pieces
27.04.2017 | Ecole Polytechnique Fédérale de Lausanne

All articles from Materials Sciences >>>

The most recent press releases about innovation >>>

Die letzten 5 Focus-News des innovations-reports im Überblick:

Im Focus: Making lightweight construction suitable for series production

More and more automobile companies are focusing on body parts made of carbon fiber reinforced plastics (CFRP). However, manufacturing and repair costs must be further reduced in order to make CFRP more economical in use. Together with the Volkswagen AG and five other partners in the project HolQueSt 3D, the Laser Zentrum Hannover e.V. (LZH) has developed laser processes for the automatic trimming, drilling and repair of three-dimensional components.

Automated manufacturing processes are the basis for ultimately establishing the series production of CFRP components. In the project HolQueSt 3D, the LZH has...

Im Focus: Wonder material? Novel nanotube structure strengthens thin films for flexible electronics

Reflecting the structure of composites found in nature and the ancient world, researchers at the University of Illinois at Urbana-Champaign have synthesized thin carbon nanotube (CNT) textiles that exhibit both high electrical conductivity and a level of toughness that is about fifty times higher than copper films, currently used in electronics.

"The structural robustness of thin metal films has significant importance for the reliable operation of smart skin and flexible electronics including...

Im Focus: Deep inside Galaxy M87

The nearby, giant radio galaxy M87 hosts a supermassive black hole (BH) and is well-known for its bright jet dominating the spectrum over ten orders of magnitude in frequency. Due to its proximity, jet prominence, and the large black hole mass, M87 is the best laboratory for investigating the formation, acceleration, and collimation of relativistic jets. A research team led by Silke Britzen from the Max Planck Institute for Radio Astronomy in Bonn, Germany, has found strong indication for turbulent processes connecting the accretion disk and the jet of that galaxy providing insights into the longstanding problem of the origin of astrophysical jets.

Supermassive black holes form some of the most enigmatic phenomena in astrophysics. Their enormous energy output is supposed to be generated by the...

Im Focus: A Quantum Low Pass for Photons

Physicists in Garching observe novel quantum effect that limits the number of emitted photons.

The probability to find a certain number of photons inside a laser pulse usually corresponds to a classical distribution of independent events, the so-called...

Im Focus: Microprocessors based on a layer of just three atoms

Microprocessors based on atomically thin materials hold the promise of the evolution of traditional processors as well as new applications in the field of flexible electronics. Now, a TU Wien research team led by Thomas Müller has made a breakthrough in this field as part of an ongoing research project.

Two-dimensional materials, or 2D materials for short, are extremely versatile, although – or often more precisely because – they are made up of just one or a...

All Focus news of the innovation-report >>>

Anzeige

Anzeige

Event News

Fighting drug resistant tuberculosis – InfectoGnostics meets MYCO-NET² partners in Peru

28.04.2017 | Event News

Expert meeting “Health Business Connect” will connect international medical technology companies

20.04.2017 | Event News

Wenn der Computer das Gehirn austrickst

18.04.2017 | Event News

 
Latest News

Wireless power can drive tiny electronic devices in the GI tract

28.04.2017 | Medical Engineering

Ice cave in Transylvania yields window into region's past

28.04.2017 | Earth Sciences

Nose2Brain – Better Therapy for Multiple Sclerosis

28.04.2017 | Life Sciences

VideoLinks
B2B-VideoLinks
More VideoLinks >>>