Inside Plastic Transistors: Crystal-clear window opens on hidden flows

Plastic semiconductors are spawning a new breed of electronic devices that are cheap to make, lightweight, and flexible. The microscopic details of how electric charges move through transistors and other devices made of such materials have remained obscure, however.

Now, by creating a new type of transistor from such materials, known as organic semiconductors (SN: 8/30/03, p. 133: Plastic Chips: New materials boost organic electronics), Vitaly Podzorov of Rutgers University in Piscataway, N.J., and his colleagues there and at the University of Illinois at Urbana-Champaign have identified crucial details regarding electric flow through those substances.

Charges move more slowly through plastic transistors than they do through transistors based on inorganic semiconductors such as silicon, the stuff of conventional electronics. The new findings indicate that this sluggish rate stems from a ball-and-chain effect: Traveling charges distort the organic materials’ malleable crystal lattices and then have to drag around those distortions.

Such understanding of the fundamental behavior of organic semiconductors is vital to the future of the technology, comments Allen Goldman of the University of Minnesota, Twin Cities.

Some flat-screen computer displays already exploit organic semiconductors as light-emitting pixels. However, the range of future uses is expected to mushroom to include such products as electronic newspapers (SN: 1/31/04, p. 67: Flexible E-Paper: Plastic circuits drive paperlike displays) and digital gadgetry sewn into clothing (SN: 11/20/99, p. 330: https://www.sciencenews.org/pages/sn_arc99/11_20_99/bob1.htm).

Researchers have had a tough time getting a clear picture of how charges move in organic semiconductors. That’s because structural defects invariably riddle the thin crystalline films required for making transistors or other devices. Those defects dominate any moving charges’ behavior, thereby blinding researchers to the crystal’s intrinsic contribution to electronic movement.

The Rutgers-Illinois team reports the first organic transistor structure sufficiently free of crystal flaws for the intrinsic behavior of the organic material to stand out. In a yearlong progression of eliminating ever more defects, the researchers have boosted by as much as 200-fold the speed at which charges traverse their transistors.

“That’s definitely an amazing leap ahead,” says Alberto F. Morpurgo of the Delft University of Technology in the Netherlands.

Made of a thick and uniform crystal of the organic chemical rubrene, the structure also has an insulating gap of air instead of a layer of electrically insulating material, which would initiate defects in the crystal. The team, led by Michael E. Gershenson of Rutgers and John A. Rogers of Illinois, describes its work in an upcoming Physical Review Letters.

In their tests, the researchers observed changes in charge speed that theoretical studies and other experimental work have linked to lattice distortions, they say.

The new findings are “technically impressive,” comments Morpurgo. “Two years ago, [attaining] these results would have been considered science fiction,” he says.

Although organic semiconductors will probably never pose a speed challenge to silicon, traits such as their flexibility offer important advantages, Podzorov says. To match those advantages with maximum performance, he notes, researchers must figure out how to eliminate crystal defects in the thin-film components actually used in products.