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High-performance, single-crystal plastic transistors reveal hidden behavior
James E. Kloeppel, Physical Sciences Editor
217-244-1073; kloeppel@illinois.edu
3/11/04
CHAMPAIGN, Ill. —
Printing circuits on sheets of plastic may offer a low-cost technique
for manufacturing thin-film transistors for flexible displays, but maximizing
the performance of such devices will require a detailed, fundamental
understanding of how charge flows through organic semiconductors.
Now, an unusual way of fabricating single-crystal organic transistors
has allowed scientists to probe charge transport within the crystals
and to observe a strong anisotropy of the charge transport mobility
within the crystal plane never before seen.
“We construct transistors simply by laminating a piece of silicone
rubber that supports electrodes and dielectric layers for the transistor
– an element that we refer to as a transistor stamp – against
the surface of a single crystal,” said John A. Rogers, a professor
of materials science and
engineering at the University of Illinois at Urbana-Champaign and
corresponding author of a paper to appear in the March 12 issue of the
journal Science.
“This method separates the synthesis of the crystal from the fabrication
of the other elements needed for the transistors,” Rogers said.
“It thereby eliminates exposure of the fragile surface of the
organic crystals to the hazards of conventional processing.”
The fabrication technique – developed by researchers from Illinois,
Rutgers University and Bell Laboratories, Lucent Technologies –
not only provides a way to study the physics at the heart of charge
transport in these unusual materials, it also has resulted in the highest
mobility recorded in an organic semiconductor.
The use of transistor stamps promises to open up the field of basic
study of organic semiconductors by allowing devices to be fabricated
from pristine organic crystal samples that remain untouched by conventional
chemical or mechanical processing.
To build their high-performance organic transistors, the researchers
start with a simple rubber substrate, upon which they deposit gold films
and thin rubber layers to create the gate dielectric and the source,
drain and gate electrodes. A high-quality rubrene crystal – grown
by the Rutgers group – is then bonded to the substrate to complete
assembly. The bonding is performed by a lamination process carried out
in ambient conditions without pressure or adhesives.
“While this assembly process could be performed commercially to
produce complex circuits, we really designed it to get at the physics,”
Rogers said. “Understanding the fundamental behavior of charge
transport in these transistors will help us make better devices for
the wide range of electronic applications that are now emerging for
these classes of materials.”
As charges flow through conventional thin-film polycrystalline materials,
they encounter boundaries between the crystals that disrupt their movement.
By studying single crystals, Rogers and his colleagues can eliminate
the effects of these grain boundaries and examine the intrinsic transport
properties of the crystalline material itself.
“The mobility we measured in these single-crystal devices was
about 50 to 100 times larger than in thin-film plastic transistors,”
Rogers said. “This result suggests that scattering at grain boundaries
is significantly reducing the performance of normal transistors, and
points us toward a way of improving these devices.”
Because the bond between stamp and crystal is not permanent, the researchers
also can remove the crystal, rotate it, and reattach it to the substrate.
Repositioning the crystal allows the scientists to explore the dependence
of the mobility on the orientation of the transistor channel relative
to the crystal axes.
“We found a huge dependence upon transport direction in the currents
that we measured,” Rogers said. “This anisotropy was unexpected,
and indicates that transistor performance depends strongly on how the
electrodes are oriented relative to the packing of molecules in the
crystal.”
The researchers’ findings have clear device implications. In addition
to removing grain boundaries, Rogers said, “if you could preferentially
order the crystals in these thin films, that would benefit device performance
as well.”
Collaborators included Vitaly Podzorov and Michael E. Gershenson at
Rutgers, Vikram C. Sundar, Jana Zaumseil, Robert L. Willett and Takao
Someya at Bell Labs, and Etienne Menard at Illinois.
The National Science Foundation and the U.S. Department of Energy funded
the work.
