The
speed with which your smart phone reacts to your touch as you swipe it
is governed by the rate at which electrical charges move through the
various display components. Scientists from Imperial College London
(ICL) have collaborated with colleagues at King Abdullah University of
Science and Technology (KAUST) to produce organic thin-film transistors
(OTFTs) that consistently achieve record-breaking carrier mobility
through careful solution-processing of a blend of two organic
semiconductors. The OTFTs and their processing methods offer a host of
future electronic applications.
Professor
Aram Amassian’s group at KAUST teamed with Dr. Thomas Anthopoulos,
Department of Physics, ICL, and colleagues Professor Iain McCulloch and
Dr. Martin Heeney, Department of Chemistry, to develop and characterize a
composite material that enhances the charge transport and enables the
fabrication of faster organic transistors. They described their novel
semiconductor blend in a joint paper published in Advanced Materials.
In
response to the challenge of expensive vacuum deposition processes,
synthetic organic chemists have been increasingly successful in
synthesizing conjugated, soluble small-molecules. “While they have a
tendency to form large crystals, reproducible formation of high quality,
continuous and uniform films remains an issue,” remarked Dr.
Anthopoulos, lead Imperial investigator. By contrast, polymer
semiconductors are often quite soluble and form high-quality continuous
films, but, until recently, could not achieve charge carrier mobilities
greater than 1 cm2/Vs.
In
this collective work, chemists from Imperial, working with device
physicists in the College’s Centre for Plastic Electronics and material
scientists at KAUST combined the advantageous properties of both polymer
and small molecules in one composite material, which offers higher
performance than do these components alone, while enhancing
device-to-device reproducibility and stability.
The
improved performance is attributed in part to the crystalline texture
of the small-molecule component of the blend and to the flatness and
smoothness achieved at the top surface of the polycrystalline film. The
latter is crucial in top-gate, bottom-contact configuration devices
whereby the top surface of the semiconductor blend forms the
semiconductor-dielectric interface when solution-coated by the polymer
dielectric.
The
smoothness and continuity of the surface and the absence of apparent
grain boundaries are uncommon for otherwise highly polycrystalline small
molecules in pure form, suggesting that the polymer binder planarizes
and may even coat the semiconductor crystals with a nanoscale thin
layer. “The performance of the polymer-molecule blend exceeds 5 cm2/Vs,
which is very close to the single-crystal mobility previously reported
for the molecule itself,” noted KAUST co-author Prof. Amassian.
The
materials scientists at KAUST addressed the phase separation,
crystallinity, and morphology of the organic semiconductor blend by
using a combination of synchrotron-based X-ray scattering at the D1 beam
line of the Cornell High Energy Synchrotron Source (CHESS),
cross-sectional energy-filtered transmission electron microscopy
(EF-TEM), and atomic force microscopy in topographic and phase modes.
“This
work is particularly exciting as it shows that by applying
complementary powerful characterization techniques on these complex
organic blends, one can learn a lot about how they work. It’s a textbook
example of a structure-property relationship study highlighting the
usefulness of such collaborations,” said Professor Alberto Salleo of
Stanford University, an expert on advanced structural characterization
of polymer semiconductors. “A mobility of 5 cm2/Vs is already a
spectacular number. The methods described chart the way for researchers
to obtain even higher mobilities.”
“In
principle, this simple blend approach could lead to the development of
organic transistors with performing characteristics well beyond the
current state-of-the-art,” added Dr. Anthopoulos.
