Materials known as conjugated polymers have been
seen as very 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.
The conjugated polymers fall somewhere between crystalline and amorphous
materials and that's caused some of the difficulty in explaining how they work.
Crystals have a perfectly regular
arrangement of atoms and molecules, while amorphous materials have a completely
random arrangement. But the conjugate polymers have some of both
characteristics: regions of orderly arrangement, mixed randomly with regions of
complete disorder.
A scanning tunneling electron micrograph (STEM) 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 STEM views were rendered into 3-D form to create this visualization.
"Some models have tried to explain how these materials behave, but
there's been no direct evidence," Ugur says, for which model matches the
reality. "Here, we've shown that the effect of crystallite size" —the
sizes of the ordered domains within the material—plays a crucial role.
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 bulk materials, those ions can go in any direction. But in this
polymer, which can be very thin, there are fewer neighboring crystalline
domains to which an ion can hop. With fewer options, conduction is more
efficient, Ugur says, adding that, "As you get thinner, the conditions
[for conduction] improve, even though the material didn't change."
Diagram shows the possible orientations of PEDOT polymer chains relative to a substrate surface (white plane at bottom), in experiments carried out by the MIT team.
Previous attempts to model the electrical behavior of such materials had
focused on their chemical properties. "People didn't take into account the
crystallites," says Karen Gleason, the Alexander and I. Michael Kasser
Professor of Chemical Engineering. As a result, understanding of the electrical
properties of such materials "remains incomplete even after decades of
investigation," the team writes in their paper.
Other semiconducting materials used widely in electronics achieve even higher values, such as 8,000 S/cm in indium-tin-oxide, or ITO, Gleason says. But, she points out, those materials are stiff and brittle, whereas conjugated polymers are flexible, opening up potential applications in curved or flexible devices.
Kripa Varanasi, an associate professor of mechanical engineering, says,
"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".
Varanasi explains that most of the time, electrical and thermal
conductivity of materials go together, but achieving independent tuning of
thermal and charge transport can lead to broad applications for thermal
management, flexible electronics and photonics, thermoelectrics, and thermal
and electrical cloaking.
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 says, is:
"What is the upper limit for conduction in this polymer?".
That's information needed to assess its potential usefulness for various
applications. When the material was first developed it had conductivity of
between 1 and 10 Siemens per centimeter, or S/cm, Gleason says; over time, it
was improved to a level of "close to 100." Now, with the new analysis
carried out by this team, conductivities of over 3,000 S/cm have been achieved.
By creating ultrathin layers that amplify the hopping mechanism, "We are
able to achieve ultraconductive, as well as highly transparent, films,"
Varanasi says.
Other semiconducting materials used widely in electronics achieve even higher values, such as 8,000 S/cm in indium-tin-oxide, or ITO, Gleason says. But, she points out, those materials are stiff and brittle, whereas conjugated polymers are flexible, opening up potential applications in curved or flexible devices.
Although the research was conducted with PEDOT, Ugur says, the findings
"should be generalizable to all conjugated polymers." (Polymers'
structures consist of long chains; conjugated polymers are those that have at
least one "backbone" consisting of alternating double and single
chemical bonds, making them conductive).
PEDOT has a combination of three properties that give it great
potential, Gleason says: electrical conductivity,
transparency, and flexibility. "Anywhere that ITO is used, you could think
of using this"—with added flexibility. The polymer material could have
applications in flexible solar cells, displays, and touch screens, the team
says, among other possibilities.
"This work is a significant step in the development and
understanding of conductive polymer films," says Ruud Schropp, a professor
of thin-film photovoltaics at the Eindhoven University of Technology in the
Netherlands, who was not involved in this work. He adds that the finding
"explains the counterintuitive effect that ungrafted, amorphous PEDOT
films have higher conductivity than grafted films. This insight could provide
an avenue for bringing the conductivity of polymer films close to that of their
transparent oxide counterparts, such as ITO."
by: http://phys.org/news/2015-07-mysteries-polymers.html
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