Innovation brings flexible electronics a step closer

Last Updated: Thursday, December 14, 2006 | 3:53 PM ET

CBC News

U.S. researchers have developed a technique to make flexible electronics that could lead to high-performance devices such as electronic paper displays or computers that can be rolled up for storage.

The transistor fabrication method by scientists at Stanford and the University of California-Los Angeles, explained in a study in the science journal Nature published on Thursday, represents a leap in how flexible circuits are made.

Transistors are small semiconductor devices that can control voltage or current by using a much lower voltage or electrical current. They are used in a wide range of electronic circuits, including computers and cellular phones. Computer microchips use millions of them.

The innovation would make it possible to mass produce a speedy type of transistor known as a single-crystal organic transistor, which allows electrical current to pass through it very quickly when it is "switched on," a property known as "charge carrier mobility."

Transistors of this type have previously required an intensive manufacturing process.

As a result, researchers have tended to favour using organic thin-film transistors —an easier-to-make carbon-based version used in flat panel displays — although they have about a third of the charge carrier mobility of single-crystal organic transistors.

''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, a chemical engineering associate professor at Stanford said in a written statement.

Large arrays

In research at Stanford led by Alejandro Briseno, a master's student at UCLA who is now a doctoral candidate at the University of Washington, the scientists made large arrays of single-crystal transistors by putting an electrode into flexible plastic and stamping it with a crystal growth agent called octadecyltriethoxysilane (OTS).

The researchers then sprayed the surface with a vapour of organic crystal, which condensed and grew into organic single crystals on the OTS, forming transistors by bridging the electrodes.

Using this technique, the team was able to make circuits out of different materials.

In some cases, the researchers were able to make simple grid patterns with crystals in areas as small as 49 square microns. A micron is one-millionth of a metre.

"The transistor arrays printed on plastic continue to work well even after significant bending," they said in a written statement.

Further refinements, including greater control over crystal alignment and better electrical contact between the crystals and electrodes, are needed before the technique can be commercialized, but it proves it is possible to do what was once "unheard of and essentially impossible from previous methods,'' Briseno said in a statement.