Solving mysteries conductivity in polymers

Solving mysteries of conductivity in polymers

11:15 AM, 28th July 2015
Solving mysteries of conductivity in polymers
A scanning transmission electron micrograph of the polymer material shows its division into crystalline regions (light areas of orderly dots) and the amorphous, disordered matrix, which is seen as the dark background. The original 2-D views were rendered into 3D form to create this visualization.

CAMBRIDGE, US: Materials known as conjugated polymers have been seen as promising candidates for electronics applications, including capacitors, photodiodes, sensors, organic light-emitting diodes and thermoelectric devices. But they’ve faced one major obstacle: Nobody has been able to explain just how electrical conduction worked in these materials, or to predict how they would behave when used in such devices.

Now researchers at MIT and Brookhaven National Laboratory have explained how electrical charge carriers move in these compounds, potentially opening up further research on such applications. A paper presenting the new findings are being published in the journal Advanced Materials.

“Crystals have a perfectly regular arrangement of atoms and molecules, while amorphous materials have a completely random arrangement. But conjugate polymers have some of both characteristics: regions of orderly arrangement, mixed randomly with regions of complete disorder,” said Asli Ugur, an MIT postdoc and lead author of the paper.

That’s because the trickiest part of conduction in such materials is what happens when charge carriers - in this case ions, or electrically charged atoms - reach the edge of one type of domain and have to “hop” into the next.

In this polymer, which can be very thin, there are fewer neighbouring crystalline domains to which an ion can hop. With fewer options, conduction is more efficient, Ugur said.

Previous attempts to model the electrical behaviour of such materials had focused on their chemical properties. “People didn’t take into account the crystallites,” said Karen Gleason, the Alexander and I. Michael Kasser professor of chemical engineering.

Kripa Varanasi, an associate professor of mechanical engineering, said, “We wanted to develop materials where we can independently control their thermal and electrical properties. We were inspired to develop organic-inorganic interfaces as they can give rise to many new features that are not present in the corresponding bulk materials.”

The researchers analyzed a conjugated polymer known as PEDOT, known to have a promising combination of good electrical conductivity and stability. One key question that this new research may help to answer, Gleason said, is: “What is the upper limit for conduction in this polymer?”

This information is needed to assess its potential usefulness for various applications. Initially the material had conductivity between 1 and 10 Siemens per centimetre (S/cm), Gleason said. With the new analysis by this team, conductivities of over 3,000 S/cm have been achieved. “We are able to achieve ultra-conductive, as well as highly transparent, films,” Varanasi said.

Other semiconducting materials used widely in electronics achieve even higher values, such as 8,000 S/cm in indium-tin-oxide (ITO), Gleason said. But, she pointed out, those materials are stiff and brittle, whereas conjugated polymers are flexible, opening up potential applications in curved or flexible devices.

PEDOT has a combination of three properties that give it great potential, Gleason said: electrical conductivity, transparency and flexibility. The polymer material could have applications in flexible solar cells, displays and touch screens, among others, the team said.

The research team also included MIT postdoc Ferhat Katmis and graduate student Mingda Li and Brookhaven National Laboratory research scientists Lijun Wu and Yimei Zhu. The work was supported by the MIT Institute for Soldier Nanotechnologies, under contract from the US Army Research Office and by the US Department of Energy.

© Massachusetts Institute of Technology News

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