Last updated on April 23, 2014 at 19:54 EDT

Composite Collaboration Leads to Faster Plastic Electronics

April 30, 2012

JEDDAH, Saudi Arabia, April 30, 2012 /PRNewswire/ –

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, http://onlinelibrary.wiley.com/doi/10.1002/adma.201200088/abstract

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 (
http://www3.imperial.ac.uk/plasticelectronics) 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.

        For more information:
        Christopher Sands, Head of University Communications
        (Dr. Aram Amassian is available for interviews)

SOURCE King Abdullah University of Science & Technology

Source: PR Newswire