Clearing a path for electrons in polymers: closing in on the speed limits
A
new class of low-cost polymer materials, which can carry electric
charge with almost no losses despite their seemingly random
structure, could lead to flexible electronics and displays which are
faster and more efficient.
Researchers
from the University of Cambridge have identified a class of low-cost,
easily-processed semiconducting polymers which, despite their
seemingly disorganised internal structure, can transport electrons as
efficiently as expensive crystalline inorganic semiconductors.
In
this new polymer, about 70% of the electrons are free to travel,
whereas in conventional polymers that number can be less than 50%.
The materials approach intrinsic disorder-free limits, which would
enable faster, more efficient flexible electronics and displays. The
results are published today (5 November) in the journal Nature.
For
years, researchers have been searching for semiconducting polymers
that can be solution processed and printed – which makes them much
cheaper – but also retain well-defined electronic properties. These
materials are used in printed electronic circuits, large-area solar
cells and flexible LED displays.
However,
a major problem with these materials – especially after they go
through a messy wet coating, fast-drying printing process – is that
they have an internal structure more like a bowl of spaghetti than
the beautifully ordered crystal lattice found in most electronic or
optoelectronic devices.
These
nooks and crannies normally lead to poorer performance, as they make
ideal places for the electrons which carry charge throughout the
structure to become trapped and slowed down.
Polymer
molecules consist of at least one long backbone chain, with shorter
chains at the sides. It is these side chains which make conjugated
polymers easy to process, but they also increase the amount of
disorder, leading to more trapped electrons and poorer performance.
Now,
the Cambridge researchers have discovered a class of conjugated
polymers that are extremely tolerant to any form of disorder that is
introduced by the side chains. “What is most surprising about these
materials is that they appear amorphous, that is very disordered, at
the microstructural level, while at the electronic level they allow
electrons to move nearly as freely as in crystalline inorganic
semiconductors,” said Mark Nikolka, a PhD student at the
University’s Cavendish Laboratory and one of the lead authors of
the study .
Using
a combination of electrical and optical measurements combined with
molecular simulations, the team of researchers led by Professor
Henning Sirringhaus were able to measure that, electronically, the
materials are approaching disorder-free limits and that every
molecular unit along the polymer chain is able to participate in the
transport of charges.
“These
materials resemble tiny ribbons of graphene in which the electrons
can zoom fast along the length of the polymer backbone, although not
yet as fast as in graphene,” said Dr Deepak Venkateshvaran, the
paper’s other lead author. “What makes them better than graphene,
however, is they are much easier to process, and therefore much
cheaper.”
The
researchers plan to use these results to provide molecular design
guidelines for a wider class of disorder-free conjugated polymers,
which could open up a new range of flexible electronic applications.
For example, these materials might be suitable for the electronics
that will be needed to make the colour and video displays that are
used in smartphones and tablets more lightweight, flexible and
robust.
This
research was funded by the Engineering and Physical Sciences Research
Council (EPSRC) and Innovate UK.
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