Better organic semiconductors

New Materials Asia, Oct, 2008

Researchers from Korea's Seoul National University (SNU) and the US National Institute of Standards and Technology (NIST) have learned how to tweak a new class of polymer-based semiconductors to better control the location and alignment of the components of the blend.

Their recent findings, namely how to move the top to the bottom, could make possible the design of practical, large-scale manufacturing techniques for a wide range of printable, flexible electronic displays and other devices, and have been published in the August edition of the Journal of the American Chemical Society.

Organic semiconductors--novel carbon-based molecules that have similar electrical properties to more conventional semiconducting materials like silicon and germanium--are a hot research topic because practical, high-performance organic semiconductors would open up whole new categories of futuristic electronic devices. Examples would be tabloid-sized digital papers that their readers could fold up into their pockets or huge sheets of photovoltaic cells that are extremely cheap because they are manufactured by basically inkjet printing.

The problem 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 requirement of the manufacturing process. Larger molecule polymer semiconductors, on the other hand, make excellent thin films, but have, at best, limited semiconductor properties. A patent from UK researchers in 2005 offered a promising compromise: blend the small semiconductor molecules in with the polymer. This works surprisingly well, but only up to a point. Tests showed that actual devices, such as 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 that is difficult or impossible to do on a large scale without destroying the fragile film.

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 nm 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. However, when the SNU/NIST research team substituted a polymer with significantly higher molecular mass, something interesting happened. 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 a key factor for large-scale manufacturing because it means the rest of the device--gate, source and drain--can be laid down first and the delicate film layer added last.

In addition, they report, the optimized blend of polymer and organic semiconductor actually has better performance characteristics than just the organic semiconductor on its own.

For further information, contact: Seoul National University, 599 Gwanak-ro, Gwanak-gu, Seoul 151-742, South Korea; tel: 2-880-4447; fax: 2-880-4449; Internet: www.snu.ac.kr; or contact: NIST, 100 Bureau Drive, Stop 1070, Gaithersburg, MD 20899-1070, USA.