Organic Semiconductor Breakthrough Could Speed Flexible Circuits

21 January 2009—Researchers at Polyera Corp., of Skokie, Ill., say they’ve invented a new polymer that closes an important gap in the field of printing electronics on plastic. The polymer is an organic semiconductor that, unlike other such materials, conducts electrons, they report today in the online version of the journal Nature . Together with existing polymer semiconductors, the new material was used to print complementary metal-oxide semiconductor (CMOS) circuits, the type that make up today’s silicon logic.

Although there are already polymer semiconductors that allow the printing of simple electronic circuits, for efficient flexible display screens or complex radio-frequency identification (RFID) tags, manufacturers need to be able to print semiconductors that are p -type—conducting positive charge carriers, or holes—and n -type, which use negative charge carriers, or electrons. The combination of the two generally makes for more power-efficient digital circuits because current should flow through them only when their bits are flipping.

Several p -channel semiconductors exist, but ”polymeric n -channel semiconductors—practical ones—were unknown until our work,” says Antonio Facchetti, Polyera’s chief technology officer and an adjunct chemistry professor at Northwestern University. ”If you want to enable high-performance CMOS electronics, you need both p -channel and n -channel semiconductors.”

The material that Facchetti’s team developed showed electron mobility—the speed with which an electron traverses the semiconductor and a key determinant of how quickly a transistor can switch—between 0.45 and 0.85 centimeter squared per volt second. That’s short of the 1 cm²/Vs they would like to achieve but well ahead of other polymers. Facchetti says the maximum he has heard of in a lab is about 0.5, and more typically the measurement is in the range of 0.01.

The challenge was that the material needed not only high mobility but also other properties, such as solubility, that allow it to be constituted as an ink for printing. This can be difficult because molecular characteristics that make a material more soluble can also lower the electron mobility.

The material the team came up with is based on naphthalene-bis(dicarboximide). Facchetti says the molecule has a regular structure that looks like a series of tightly clustered rods, which allows the electrons to move easily from one rod to the next. Also contributing to the material’s high electron mobility is that its conduction band energy—the energy an electron must have to escape the bounds of an atom and flow as current—is among the lowest ever reported for a polymer. At the same time, the material is highly soluble (60 grams per liter).

To further improve the material, which Polyera has dubbed ActivInk N2200, and perhaps reach mobilities of 1 or slightly higher, Facchetti plans to try chemical variations on the naphthalene-based molecule. He’d also like to experiment with different dielectrics and contacts in the printed thin-film transistors. The team built transistors that used gold contacts, but applications such as RFID tags would require cheaper materials.

Hagen Klauk, head of the organic electronics group at the Max Planck Institute for Solid State Research, in Stuttgart, Germany, says he considers Facchetti’s material the first n -channel polymer that really works outside the lab. ”It’s not just that the performance of the transistors is really good but that they were made using methods that I could actually see in a production environment,” Klauk says.

Indeed, the group tested the material in several printing methods, including spin-coating, inkjet, flexographic, and gravure printing, and found it worked with all of them. Gravure printing, in which the pattern to be printed is inscribed as tiny holes on a drum, is the process with the highest throughput, but being able to choose the printing process based on the specific product is a key advantage, says Polyera CEO Philippe Inagaki. He predicts flexible displays based on ActivInk within three years. ”Things are ready to be on the market now, but to tap into the real potential, there needs to be more development,” he says.

Polyera is, of course, not alone in the quest for efficient plastic electronics. Researchers at the University of Illinois at Urbana-Champaign, led by materials science and chemistry professor John A. Rogers, reported last year that they’d made CMOS circuits by stamping slivers of n-type and p-type silicon onto plastic sheets.