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Researchers find new twist in bendy electronics

A new technique for manufacturing transistors may make mass producing flexible electronics much more feasible.

University researchers have shown that flexible electronics made with organic, carbon-based transistors are much more feasible than previously thought, potentially leading to devices like electronic paper.

Researchers at Stanford University and the University of California, Los Angeles, published a study Wednesday illustrating that single-crystal organic transistors can be mass-produced with a new technique. Typically, high-performance organic transistors are set by hand, making them rare in electronics manufacturing.

"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," Zhenan Bao, associate professor of chemical engineering, said in a statement. The study was published in the December 14 issue of the journal Nature.

Photo of flexible transistors
Credit: Alejandro Briseno
A flexible array of organic single-
crystal transistors is wrapped around
a glass vial.

The researchers devised a method of printing patterns of transistors on surfaces like silicon wafers and flexible plastic. The method begins with placing electrodes on the surfaces of these materials wherever they want a transistor, then producing a stamp (with the desired pattern) out of a polymer called polydimethylsiloxane, a common silicon-based polymer. Following that step, researchers coat the stamp with a crystal growth agent called octadecyltriethoxysilane, and a vapor of the organic crystal material causes the single crystals to grow. A transistor is formed when those crystals bridge with the electrodes.

In some experiments using various materials, the researchers made grid patterns with crystals in areas as small as 8 hundred-millionths of a square inch. Although not nearly as packed as modern silicon processors or memory chips, the team's patterns could still produce richly functioning circuits and high-resolution displays, according to Bao.

Because they demonstrated that these organic transistors still worked well while being bent, the researchers say they believe the technique could lead to the manufacturing of bendable electronics, such as low-cost sensors for product packaging or electronic paper displays. Still, the team must iron out some kinks before they can commercialize the techniques. Among the issues are controlling how the crystals line up across the electrodes when they form, and ensuring better electrical contact between crystals and electrodes.

Engineers say single-crystal organic transistors have a high "charge carrier mobility," meaning they can move an electrical current through crystal very quickly. Despite this, manufacturers favor organic "thin-film" transistors because they can be mass-produced for products like flat-panel computer monitors, yet they have only a third the charge mobility of single-crystal organic transistors. That means that the best mass-produced organic transistors have only so-so performance, and the products using them have yet to come to market, according to Stanford.

"Until now, the possibility of fabricating hundreds of (organic single-crystal) devices on a single platform (had) been unheard of and essentially impossible from previous methods," Alex Briseno, the study's lead author and a doctoral student at the University of Washington, said in a statement.

Funding sponsors of the research include the Bell Labs Graduate Research Fellowship, the Air Force Office of Scientific Research, the National Science Foundation's Center on Polymeric Interfaces and Macromolecular Assemblies, and the Stanford School of Engineering.