New Technique Brings Flexible Electronics Closer to Reality

Researchers have devised a method for printing patterns of high-performance organic single-crystal transistors on surfaces such as silicon wafers and flexible plastic, which could enable fast, bendable electronics such as low-cost sensors on product packaging and "electronic paper" displays.

"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," said Stanford University chemical engineering associate professor Zhenan Bao.

The study by researchers at Stanford and the University of California-Los Angeles is reported in the Dec. 14 issue of the journal Nature. Lead author is Alejandro Briseno, who was a master's student at UCLA performing part of this research at Stanford. He is now a doctoral student at the University of Washington. The study's other authors are Stefan C. B. Mannsfeld, Mang M. Ling, Shuhong Liu, Colin Reese, Mark E. Roberts and Bao at Stanford, and Ricky J. Tseng, Yang Yang and Fred Wudl at UCLA.

Single-crystal organic transistors are fast -- engineers say they have a high charge carrier mobility, meaning that when they are "switched on," electrical current can move through the crystal very quickly. Organic thin-film transistors, carbon-based versions of the kind of transistor commonly found in flat-panel computer monitors, have only about a third the charge mobility. Researchers have nevertheless favored the thin-film transistors because they could be manufactured en masse, while single-crystal devices always had to be made by manual selection and placing of individual crystals.

The trick to being able to manufacture -- rather than handcraft -- large arrays of single-crystal transistors was to devise a method for printing transistor patterns on silicon wafers, flexible plastic and other surfaces. 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.

A flexible rubrene device is wrapped around a glass vial. (Photo: Alejandro Briseno)

In the experiments reported in the paper, the team made arrays out of several different crystal materials including rubrene (it makes the fastest transistors) and even ''buckyballs,'' soccer balls made out of 60 carbon atoms each. In some cases, the researchers were able to make simple grid patterns with crystals in areas as small as 8 hundred-millionths of a square inch (49 sq µm). Although not nearly as packed as modern silicon processors or memory chips, with up to 13 million crystals per square inch, the team's patterns could still yield richly functioning circuits and high-resolution displays, Bao said.

In other experiments reported in the paper, the researchers showed that the transistor arrays printed on plastic continue to work well even after significant bending, a key finding for anything that will be used in flexible electronics.

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.

Still, the results show that organic single-crystal transistors are now feasible for making a variety of useful devices. "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," said Briseno. "All of this can now be accomplished on an area the size of a human fingernail."

The research was funded by a Bell Labs Graduate Research Fellowship, the Air Force Office of Scientific Research, a German research foundation fellowship, the National Science Foundation's Center on Polymeric Interfaces and Macromolecular Assemblies and the Stanford School of Engineering.

For more information, visit: www.stanford.edu