Plastic Chips: New materials boost organic electronics

Over the past decade, research groups in academia and industry have been racing to fabricate electronic devices–integrated circuits, displays for handheld computers, and solar cells–not from silicon but from semiconducting polymers (SN: 5/17/03, p. 312: Available to subscribers at Plastic Electric). Components made from such organic materials could be flexible, as well as cheaper and easier to manufacture than their silicon counterparts.

Now researchers at Northwestern University in Evanston, Ill., and Lucent Technologies in Murray Hill, N.J., have devised a new class of organic semiconductor materials that could hasten the arrival of what could be the electronics revolution’s next big wave.

Until recently, the fabrication of plastic electronics has been limited by the number of molecular building blocks suitable for making semiconducting polymers. Transistors–which are the switches in an integrated circuit–require two types of semiconductor materials: n-type and p-type. In n-type materials, charge flows through the material via electrons. P-type materials transport charge through “holes,” places where electrons are missing.

“Yet, most of the organic materials examined so far have all been p-type,” says lead investigator Tobin Marks at Northwestern. Existing n-type organics are rare and unstable. “So there’s a real need for n-type materials,” he says.

His team’s new class of molecules assembles into semiconductors of both p- and n-type. A rod-shaped organic molecule made of six thiophene units forms the basis for each type of material. Each thiophene, in turn, is a ring of five carbons and one sulfur. When the researchers replaced the rod’s two end thiophenes with a perfluoroarene group (a ring of six carbons decorated with fluorines), the organic molecule behaved like an n-type semiconductor. When the researchers instead replaced the next two thiophenes from the ends, the molecule behaved as a p-type semiconductor.

The researchers describe their molecular constructions in the Aug. 25 Angewandte Chemie International Edition.

“It turns out, the way we move the perfluoroarenes around also allows us to control the packing between the molecules,” says Marks. The closer the molecules are to each other, the faster a charge can hop from one molecule to another in either type of semiconductor.

So far, the team has fabricated prototype transistors from the materials, which performed just as well as existing organic semiconductors do, as measured by the mobility of the electrons and holes. But Marks says his lab expects to increase the n-type material’s electron mobility by at least a factor of 5, an advance that would boost the switching speed of the material.

The pantry of organic materials for making n-type semiconductors has been particularly sparse, says Ananth Dodabalapur of the University of Texas at Austin. “This will be very useful for people like myself who make organic circuits.” One of the biggest appeals of plastic electronics is that manufacturers could spray liquid polymer circuits onto a surface using ink-jet printers, instead of resorting to the multibillion-dollar fabrication equipment used to etch circuitry on silicon wafers.

Marks predicts that low-cost, even disposable plastic electronic devices, such as smart cards, electronic tags for tracking inventory, and chemical sensors, will emerge in the next couple of years.

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