Soon, better organic semiconductors for printable electronics

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Washington, Sept 5 : A group of researchers has taken a major step towards developing the design of practical, large-scale manufacturing techniques for a wide range of printable, flexible electronic displays - by learning how to move the top to the bottom in a new class of polymer-based semiconductors.

The finding, according to scientists at the National Institute of Standards and Technology (NIST) and Seoul National University (SNU), could help in better control of the location and alignment of the components of the blend.

Organic semiconductors are novel carbon-based molecules that have similar electrical properties to more conventional semiconducting materials like silicon and germanium.

They are a hot research topic because practical, high-performance organic semiconductors would open up whole new categories of futuristic electronic devices.

Tabloid-sized "digital paper," which one can fold up into the pocket or huge sheets of photovoltaic cells, is dirt cheap just because they're manufactured by ink-jet printing.

But, the biggest problem in this case is performance. Small organic molecules have been developed with key electrical parameters close to the benchmark set by amorphous silicon semiconductors, but they are very difficult to deposit in a stable, uniform film-a key-manufacturing requirement.

However, larger molecule polymer semiconductors make excellent thin films but have at best limited semiconductor properties.

In 2005, a patent from British researchers offered a promising compromise, which was to blend the small semiconductor molecules in with the polymer. Though this works surprisingly well, but still has a drawback.

Tests showed that actual devices, field effect transistors, made with the blend only worked well in a so-called "top-gated" structure.

The critical active part of the film was on the top, and the switching part of the device (the "gate") had to be layered on top of that, a process difficult or impossible to do on a large scale without destroying the fragile film.

However, working at NIST's Center for Neutron Research, the SNU/NIST research team used a neutron imaging technique that allowed them to observe, with nanometer resolution, how the distribution of small organic semiconductor molecules embedded in polymer films changed with depth-the films are less than 100 nanometers thick.

In the thin films originally described by the patent, the bulk of the semiconductor molecules end up at the top of the film, as suspected.

But, when the SNU/NIST research team substituted a polymer with significantly higher molecular mass, they were surprised to see that the organic semiconductor small molecules distributed themselves evenly at the top and bottom of the film.

Having an active region of the film on the bottom is key for large-scale manufacturing because it means the rest of the device-gate, source, drain-can be laid down first and the delicate film layer added last.

Also, they claim that the optimized blend of polymer and organic semiconductor actually has better performance characteristics than the organic semiconductor on its own.

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