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New OTFTs Trigger Rapid-Fire Response on Computer and Phone Displays

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April 30, 2012

New OTFTs Trigger Rapid-Fire Response on Computer and Phone Displays

By Cheryl Kaften
TMCnet Contributor

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The speed at which a smartphone reacts to your touch is governed by the rate at which electrical charges move through its display components. And in today’s world, “slow” is not an option, as the winners of the race are the swiftest, most nimble devices.


On April 30, scientists from Imperial College London (ICL) and King Abdullah University of Science and Technology (KAUST) in Jeddah, Saudi Arabia, announced that they had produced organic thin-film transistors (OTFTs) that consistently achieve record-breaking carrier mobility through careful solution-processing of a blend of two organic semiconductors.

Organic thin-film transistor (OTFT) technology involves the use of organic semiconducting compounds in electronic components, such as computer, television, and phone screens. The compounds make displays brighter, colors more vivid, and characters easier to read in most ambient lighting environments.

However, until recently, OTFTs have proven slow in terms of carrier mobility (the ease with which an atom shares electrons and holes with other atoms). Low-speed carrier mobility causes sluggish response time, which limits the ability of a display to render motion.

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 College London 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 (velocities).

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 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 (News - Alert) (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 in California, 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.




Edited by Jamie Epstein

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