A single crystal of the new organic semiconductor material shown in polarized light. It is approximately twice as fast as the parent organic material from which it was derived. The white scale bar at the bottom center of the photo represents 10 microns (10 millionths of a meter). Anatoliy Sokolov |
Organic
semiconductors hold immense promise for use in thin film and flexible
displays?picture an iPad you can roll up?but they haven’t yet reached
the speeds needed to drive high definition displays. Inorganic materials
such as silicon are fast and durable, but don’t bend, so the search for
a fast, durable organic semiconductor continues.
Now
a team led by researchers at Stanford and Harvard universities has
developed a new organic semiconductor material that is among the
speediest yet. The scientists also accelerated the development process
by using a predictive approach that lopped many months?and could lop
years?off the typical timeline.
For
the most part, developing a new organic electronic material has been a
time-intensive, somewhat hit-or-miss process, requiring researchers to
synthesize large numbers of candidate materials and then test them.
The
Stanford and Harvard-led group decided to try a computational
predictive approach to substantially narrow the field of candidates
before expending the time and energy to make any of them.
“Synthesizing
some of these compounds can take years,” said Anatoliy Sokolov, a
postdoctoral researcher in chemical engineering at Stanford, who worked
on synthesizing the material the team eventually settled on. “It is not
a simple thing to do.”
Sokolov
works in the laboratory of Zhenan Bao, an associate professor of
chemical engineering at Stanford. They are among the authors of a paper
describing the work, published in the Aug. 16 issue of Nature Communications.
Alán Aspuru-Guzik, an associate professor of chemistry and chemical
biology at Harvard, led the research group there and directed the theory
and computation efforts.
The
researchers used a material known as DNTT, which had already been shown
to be a good organic semiconductor, as their starting point, then
considered various compounds possessing chemical and electrical
properties that seemed likely to enhance the parent material’s
performance if they were attached.
They came up with seven promising candidates.
Semiconductors
are all about moving an electrical charge from one place to another as
fast as possible. How well a material performs that task is determined
by how easy is it for a charge to hop onto the material and how easily
that charge can move from one molecule to another within the material.
Using
the expected chemical and structural properties of the modified
materials, the Harvard team predicted that two of the seven candidates
would most readily accept a charge. They calculated that one of those
two was markedly faster in passing that charge from molecule to
molecule, so that became their choice. From their analysis, they
expected the new material to be about twice as fast as its parent.
Sokolov,
the Stanford researcher, said it took about a year and a half to
perfect the synthesis of the new compound and make enough of it to test.
“Our final yield from what we produced was something like 3 percent
usable material and then we still had to purify it.”
When
the team members tested the final product, their predictions were borne
out. The modified material doubled the speed of the parent material.
For comparison, the new material is more than 30 times faster than the
amorphous silicon currently used for liquid crystal displays in products
such as flat panel televisions and computer monitors.
“It
would have taken several years to both synthesize and characterize all
the seven candidate compounds. With this approach, we were able to focus
on the most promising candidate with the best performance, as predicted
by theory,” Bao said. “This is a rare example of truly ‘rational’
design of new high performance materials.”
The
researchers hope their predictive approach can serve as a blueprint for
other research groups working to find a better material for organic
semiconductors.
And they’re eager to apply their method to the development of new, high-efficiency material for organic solar cells.
“In
the case of renewable energy, we have no time for synthesizing all the
possible candidates, we need theory to complement synthetic approaches
to accelerate materials discovery,” said Aspuru-Guzik.
Other
Stanford researchers contributing to the research include Rajib Mondal
and Hylke Akkerman, postdoctoral fellows in the department of chemical
engineering when the research was done; Stefan Mannsfeld, a staff
scientist at the Stanford Synchrotron Radiation Lightsource; and Arjan
Zoombelt, a postdoctoral fellow in chemical engineering.
The
research was supported financially by the Stanford Global Climate and
Energy Project, Netherlands Organization for Scientific Research,
National Science Foundation, King Abdullah University of Science and
Technology, Air Force Office of Scientific Research, Harvard Materials
Research Science and Engineering Center, the Camille & Henry Dreyfus
Foundation and the Sloan Foundation.
