In a study published in the Dec. 14 issue of the journal Nature,
researchers at Stanford and UCLA describe mass manufacturing flexible
electronics made with organic, or carbon-based, transistors that could enable
technologies such as low-cost sensors on product packaging and "electronic
paper" displays as thin and floppy as a placemat.
Single-crystal organic transistors are fast. 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 patterns of transistors on surfaces such as silicon wafers and
flexible plastic. 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.
In the experiments reported in the paper, the researchers
were able to make simple grid patterns with crystals in areas as small as 8
hundred-millionths of a square inch (49 square microns). Although not nearly as
packed as modern silicon processors or memory chips, with up to 13 million
crystals per square inch, the research team believes that the patterns could
still yield functioning circuits and displays. The researchers also 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 the study's lead
author Alejandro Briseno. "All of this can now be accomplished on an area
the size of a human fingernail."