Self-Assembled Organic Circuits

Researchers have found a simple way to make high-performance electronic circuits from organic semiconductors. The advance, reported in this week’s Nature, brings us one step closer to low-cost, bendable plastic electronics.

A research team led by Dago de Leeuw at the Philips Research Laboratories, in Eindhoven, the Netherlands, developed semiconductor molecules that automatically arrange themselves on a surface in a layer just a few nanometers thick. These “self-assembling” molecules could make it much easier to fabricate organic transistors, the essential building blocks of plastic electronics. In experiments, the researchers used the technique to make hundreds of transistors and arranged them into complex circuits.

In the past, others have used similar self-assembly tricks to make organic transistors, but the new method is much simpler. Moreover, researchers have been unable to accurately and reliably replicate self-assembled devices until now. “You need every transistor to be working in order for the circuit to work,” says John Kymissis, an electrical-engineering professor at Columbia University. “Here, there are hundreds of transistors, all of which work. The yield is extremely good for complicated circuits.”

Organic semiconductors are cheaper and more flexible than silicon. Today’s flat-panel displays use transistors made from rigid amorphous silicon to switch pixels on and off. Conversely, transistors made of plastics could lead to large, cheap, bendable displays and a range of other inexpensive devices, such as RFID tags. However, the cost and practicality of fabricating organic electronic circuits is a challenge.

Many researchers believe that self-assembly–a technique that relies on molecules arranging themselves into complex structures–could be the most practical way to produce cheap plastic electronics. The methods currently used to fabricate organic circuits include lithographic etching and ink-jet printing. Self-assembly eliminates the need to progressively pattern the semiconductor layer and does not waste the semiconductor by etching it away.

The ultimate goal for self-assembled circuits “is to be able to throw molecules in a beaker and let them organize into desired structures,” says Edsger Smits, a researcher at the Philips Research Laboratories, who was involved in the work. Pulling circuits out of a beaker is still some ways away, but the present work is a step toward that goal. The researchers deposited gold source and drain electrodes with a silicon-dioxide insulator in between using traditional lithography and surface etching. Then they dipped this transistor circuit in a solution containing the organic semiconductor.

The semiconductor molecules arranged themselves in a densely packed single layer about three nanometers thick on top of the silicon-dioxide insulator between the source and drain. “The molecules attach as long as there is an open space, so there’s truly a mono-molecule layer,” says paper coauthor Stephan Kirchmeyer, who is vice president of H.C. Starck, the chemical company based in Leverkusen, Germany, that designed and produced the molecules.

In previous work by other groups, circuits were first dipped in an anchor chemical and then coated with a semiconductor solution, causing the semiconductor to attach to the anchor molecules. In the new molecules, the anchor and semiconductor are already strung together. “This becomes a one-step manufacturing process,” says Yang Yang, a materials-science and engineering professor at the University of California, Los Angeles. “[It’s] a smart approach.”

Producing a well-ordered semiconductor layer creates a high-performance device. It improves a transistor’s electron mobility, which in turn determines how much current it can carry and how fast it can switch on and off. “The performances of the devices [are] comparable to bulk transistors based on similar materials,” Smits says.

The researchers finally combined their transistors into functioning circuits. In their Nature paper, they demonstrate several important logic components such as inverters and ring oscillators. They also demonstrate a complicated circuit called a code generator, using 300 transistors.

Kymissis admits that faster, better circuits have previously been made using self-assembly. But he says that the simplicity of the one-step assembly method and the ability of these transistors to function in such complex circuits “is a terrific advance.”