Self-Assembled Organic Circuits

Molecules that form an ordered layer could lead to low-cost, bendable plastic electronics.

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.

Self-made circuit: This logic circuit features more than 300 organic transistors made using a new self-assembly trick. The semiconducting layer in each device is formed from molecules that arrange themselves on the circuit surface. Credit: Philips Research Laboratories

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.

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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.