Faster Printable Circuits

A new polymer simplifies organic circuits.

By Katherine Bourzac

Organic circuits are cheap, flexible and printable. But unlike inorganic circuits, which require only silicon, high-performance organic circuits are usually made out of two different materials that must be carefully patterned. Now researchers have made a polymer that performs the function of both materials. By eliminating the need for two materials, they've made these circuits easier to fabricate. The researchers have used the new polymer to make some of the fastest organic circuits yet reported, and the process might be useful for solar cells, too.

Speedy circuit: A circuit made from a new semiconducting polymer undergoes testing in the lab. Credit: University of Washington

To perform the logic operations that run computers, cell phones and other electronics without consuming too much power, transistors need to have alternating regions that conduct negative and positive charges. But until recently, chemists had only made polymers that conducted either positive or negative charges. To make circuits from them, these polymers have to be carefully aligned with each other. "When you have two materials requiring complex patterning processes, you lose or reduce the cost advantage and simplicity" of organic electronics, says Samson Jenekhe, professor of chemistry at the University of Washington in Seattle.

"The ultimate [goal] is to have one material that can transport electrons and holes," or positive charges, says Jenekhe. He and others have been working on making such a material, called an ambipolar polymer, for a few years. "In the past, it was largely trial and error," says Jenekhe. Now he and Mark Watson, associate professor of chemistry at the University of Kentucky in Lexington, have determined what sort of structures work well in such polymers. The new material and its performance are described in the journal Advanced Materials.

The new polymer is made up of two alternating units, one that conducts electrons and another that conducts holes. It's not the first polymer to be able to do this. But electrons and holes move much faster through the new material than through those that have been made in the past. This is important, because the rate at which charges move through a semiconductor determines circuit speed.

Jenekhe's group used the polymers to make individual transistors and circuits. They put the polymer into a solution, dropped it on a substrate patterned with electrical contacts, and then spun it out into a thin film using a process called spin coating. Jenekhe says that because the polymer is water soluble, ink-jets could also be used to print out circuits. The performance of the ambipolar polymer circuits was comparable to or better than those made from two polymers.

One of the devices Jenekhe's group made is called an inverter. "Inverters are the basic building blocks of integrated circuits," says Zhenan Bao, associate professor of chemical engineering at Stanford University, who was not involved with the research. Other groups have demonstrated inverters with ambipolar polymers, but Jenekhe's polymer can operate much faster than the others, says Bao.

The new polymers may also work well in solar cells, says Jenekhe. To efficiently convert light into electricity, organic cells currently require two carefully interlaid polymers. If the structure of the two polymers in these devices isn't just right, electrons won't flow. The new polymer could eliminate that problem.

The next steps, says Jenekhe, are to make new versions of the polymer that are more conductive than the current iteration and to test them in more complex circuits.

http://www.technologyreview.com/computing/23298/?a=f