Tuesday, November 10, 2009
Electronic devices can't work well unless all of the transistors,
or switches, within them allow electrical current to flow easily
when they are turned on. A team of engineers has determined why
some transistors made of organic crystals don't perform well,
yielding ideas about how to make them work better.
Providing insight into a frustrating inconsistency in the
performance of electronics made with organic materials, Stanford
researchers have shown that the way boundaries between individual
crystals in a film are aligned can make a 70-fold difference in how
easily current, or electrical charges, can move through
transistors.
The research, which could help engineers design better digital
displays and other devices, was published online Nov. 8 in the
journal Nature Materials.
Organic semiconductors have a lot to offer in electronics. They
are cheap and flexible, and the production process is much simpler
than for traditional silicon chips. Applications such as computer
display screens, digital signs or magazines made of "electronic
paper" have been possibilities for more than a decade, but their
full potential seems always just around the corner. A persistent
problem is that performance from transistor to transistor varies
much more than can be allowed in commercially viable devices.
"You can make a single device that has high 'charge mobility,'
but you really need to make thousands of them," said Alberto
Salleo, an assistant professor of materials science and engineering
at Stanford and a senior co-author
of the paper. "Most research groups report a high variation in that
mobility. What we did here is try to understand what causes the
variation."
Systematic study
Salleo's group led a multidisciplinary team of researchers in
making a systematic study of a likely culprit of the inconsistent
transistor performance in polycrystalline devices: the "grain"
boundaries between crystals. It turns out that the differences in
boundary alignment can make the path that electric charges must
follow through a transistor look more like a disjointed slog
through airport security than a sprinter's dash.
To examine the role that boundary alignment plays, the paper's
lead author, graduate student Jonathan Rivnay, grew crystals of an
organic semiconductor called PDI8-CN2, synthesized at Northwestern
University and Polyera Corp., an organic electronics company, using
a process that ensures consistent alignment from crystal to crystal
in a particular direction.
He then made transistors in which charges could flow through
molecules that were well aligned with each other, and others where
the molecules were misaligned across the grain boundaries. The
first kind of transistors performed far better. He went further to
link the properties of these boundaries to the molecular packing in
the crystals.
In addition to the team's direct electrical measurements, the
researchers employed information from extensive theoretical
calculations, made by co-author John E. Northrup at Xerox Palo Alto
Research Center, and with X-ray analysis headed by co-author
Michael Toney at the Stanford Synchrotron Radiation
Lightsource.
Could influence future production
Rivnay said the team's work could strongly influence how organic
crystal electronics are made in the future.
"The problem of understanding defects in organic electronic
materials including grain boundaries is very important for any
device application," Rivnay said. "By better understanding what
goes on at these boundaries, and how detrimental they are,
improvements can be made at the chemistry end as well as at the
design and fabrication end of the process. This way devices can be
more reproducible and better performing."
Other authors were Stanford graduate students Leslie Jimison in
Materials Science and Engineering and Rodrigo Noriega in Applied
Physics; Northwestern University chemist Tobin Marks; Polyera Corp.
researcher Shaofeng Lu; and Northwestern faculty member and Polyera
Chief Technology Officer Antonio Facchetti. Funding came from
multiple U.S. federal institutions, including the departments of
Defense and Energy and the National Science Foundation, as well as
the King Abdullah University of Science and Technology in Saudi
Arabia.
SOURCE