High-performance flexible organic transistors

Organic -- or carbon-based -- transistors are not new and can be used to design flexible computer displays, RFID tags and sensors. However, these organic single crystals could not be mass-produced because they needed to be individually handpicked. But now, Californian researchers have developed a new method for building flexible organic transistor arrays.Even if the researchers have reached a density of 13 million crystals per square inch (or 2 million per square centimeter), there are still several issues to solve before this method can be used for commercial applications of these fast transistors.

Organic -- or carbon-based -- transistors are not new and can be used to design flexible computer displays, RFID tags and sensors. However, these organic single crystals could not be mass-produced because they needed to be individually handpicked. But now, researchers at Stanford University and the University of California-Los Angeles (UCLA) have developed a new method for building flexible organic transistor arrays. Even if the researchers have reached a density of 13 million crystals per square inch (or 2 million per square centimeter), there are still several issues to solve before this method can be used for commercial applications of these fast transistors.

This research have been led by Zhenan Bao, Associate Professor in the Department of Chemical Engineering at Stanford University, with the help of the members of her research group and other researchers at UCLA. For more information about her, you can read "From lollipops to Silly Putty, chemical engineer demonstrates that 'flexible electronics' are not child's play" (Aditi Risbud, Stanford University news release, April 17, 2006). Now, here is what she says about this new method for building flexible organic transistor arrays.

"This work demonstrates for the first time that organic single crystals can be patterned over a large area without the need to laboriously handpick and fabricate transistors one at a time," says Bao.

Below is a photo of such a flexible organic transistor array obtained with this new method (Credit: Bao Research Group, Stanford University). Here is a link to a larger version.

A flexible organic transistor array

Now here is a short description of this method which allows to print these arrays of transistors on flexible plastics or silicon wafers.

The first step is to put electrodes on these surfaces wherever a transistor is desired. Then the researchers make a stamp with the desired pattern out of a polymer called polydimethylsiloxane. After coating the stamp with a crystal growth agent called octadecyltriethoxysilane (OTS) and pressing it onto the surface, the researchers can then introduce a vapor of the organic crystal material onto the OTS-patterned surfaces. The vapor will condense and grow semiconducting organic single crystals only where the agent lies. With the crystals bridging the electrodes, transistors are formed.

And when will this method start to be used for commercial applications?

Several further advances will be necessary before the team's progress translates into commercial technologies. Among them is controlling how the crystals line up across the electrodes when the crystals form. Another key step will be ensuring better electrical contact between crystals and electrodes.

For more information, this research work has been published by Nature under the title "Patterning organic single-crystal transistor arrays" (Volume 444, Number 7121, Pages 913-917, December 14, 2006). Here are some links to the Editor's Summary, "Flexible electronics," the abstract and to a file containing many additional figures (PDF format, 19 pages, 6.09 MB).

Sources: David Orenstein, Stanford University news release, December 11, 2006; and various websites

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