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Organic Transistors That Assemble Themselves

A simple way to pattern organic semiconductor material could mean cheap, large, bendable electronics.
March 5, 2008

Researchers have found a quick and simple way to make arrays of high-performance electronic devices from organic semiconductor material. The development, led by researchers at the National Institute of Standards and Technology (NIST), in Gaithersburg, MD, could lead to a simple, low-cost method to manufacture large, flexible electronic circuits that use organic semiconductors.

Self-made transistors: Organic semiconductor molecules line up when deposited on top of metal electrodes that have been chemically treated (top). The aligned molecules, which can conduct electricity, bridge the two electrodes, forming the essential structure of a transistor. Untreated electrodes (bottom) result in randomly arranged semiconductor molecules, which form an insulating layer.

The researchers coax organic semiconductor molecules to self-assemble around chemically pretreated electrodes to form field-effect transistors, which are often used to switch pixels on and off in displays. The technique results in an array of transistors that have good electrical properties and are insulated from one another. The researchers demonstrated the process using flexible substrates, which could be useful for commercial applications, says David Gundlach, the NIST researcher who led the work.*

Current flat-panel displays, such as liquid crystal displays, are rigid because they use amorphous silicon to make the transistors that control the pixels. Organic electronic circuits could pave the way for roll-up displays: foldable electronic readers, large screens that can be rolled up and tucked into cell phones, and smart bandages that monitor wounds and sense the need for drugs. However, a practical method to cheaply produce high-performance organic electronic circuits has proved elusive.

The new technique, presented in Nature Materials, could be faster, and hence cheaper, than current methods to make flexible circuits. There are several existing ways to make organic circuits over large areas. One is a lithographic technique similar to those used to make conventional silicon chips; this involves coating the entire circuit’s surface with the organic semiconductor and then etching it away wherever it is not needed. A more efficient method is inkjet printing, in which nozzles put down liquid droplets of plastic semiconductors in a desired pattern. In fact, two companies that have announced plans to commercially manufacture plastic electronics use these two different methods. (See “Plastic Electronics Head for Market.”)

The new method eliminates the need to pattern the semiconductor layer. Once the researchers have patterned the source and drain electrodes using lithography, they dip the circuit in a special chemical to treat the electrode surface. Then they coat the circuit with a thin layer of an organic semiconductor solution.

* The original version of this article contained an error. It incorrectly stated that the researchers had not demonstrated the technique on a flexible substrate.

Near the electrodes, the semiconductor crystals assemble themselves in an ordered way so that they carry current well. Away from the electrodes, however, crystals are randomly oriented so that the material acts as an insulator. “Now we can make circuits without patterning [the semiconductor] at all,” says Thomas Jackson, an electrical engineering professor at Pennsylvania State University, who was involved in the work. “We simply spin it on and we’re done. We don’t have to go through the step of removing the material where we don’t want it.”

Getting rid of this step makes the manufacturing process significantly simpler, says John Kymissis, an electrical engineering professor at Columbia University. Patterning the semiconductor layer is one of the most delicate steps in making an organic electronic circuit, he says. If any semiconductor material accidentally spans the electrodes of two adjacent transistors, that could allow current to flow between transistors, making the circuit dysfunctional. In a display, for instance, two pixels might go on instead of one. “Even if you print the electrodes, if you don’t have to pattern the organic semiconductor, [the process] is going to be faster,” says Kymissis. “It is a huge advantage.”

Compared with current techniques, the simplicity of the new method should make it more practical to manufacture organic electronic circuits on a large scale, says Natalie Stingelin-Stutzmann, a materials-science researcher at Queen Mary, University of London. “Inkjet is simple, but if you can cover large areas with a simple coating technique, it will be cheaper,” she says. “At the end, it is the cost which will determine if organic electronics makes it or not.”

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