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Posted: Apr 30, 2012

Composite collaboration leads to faster plastic electronics

(Nanowerk News) 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 ("Solution-Processed Small Molecule-Polymer Blend Organic Thin-Film Transistors with Hole Mobility Greater than 5 cm2/Vs").
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.
Source: King Abdullah University of Science & Technology
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