Faster Flexible Electronics

A new way to build microwire transistors could double flexible electronics speeds.For years, engineers have been trying to make flexible electronics faster and less expensive to manufacture, but this has proved difficult. For instance, organic microwires, which can be used to make flexible electronics, are hard to align as circuits. Now researchers at Stanford University and Samsung have developed a technique that allows them to precisely position organic microwires on a substrate and build complex circuits with relative ease. The new technique involves putting microwires in a liquid solution and filtering them through paper to form the circuit's transistors.

Zhenan Bao, a professor of chemical engineering at Stanford, who led the work, says that engineers can maximize the number of microwires in a circuit using the new technique. "That allows us to significantly increase the output current from these devices," she says. 

Bao explains that the organic microwire transistors made using the new technique operate two and a half times faster than previous ones. This means that a flexible display made in this way could refresh about twice as quickly as those made using existing methods.

Most electronic devices, such as cell phones and computers, use microchips that are made from silicon. Transistors made from organic microwires may not be as fast as silicon ones, but they are better suited to making flexible electronics. They can be created cheaply, without using high temperatures that would melt plastic, and can also coat large areas, potentially creating huge flexible displays.

"Our goal is to make electronic devices that are lighter in weight and can be coated over a large area," says Bao. "This includes displays that are put onto a plastic substrate and can be folded, low-cost sensors that are disposable, and electronic tags put on merchandise."

Organic microwires can already be added to a solution and printed onto a substrate. But they tend to clump together and lie at odd angles, making it difficult to connect the electrodes needed to form a transistor. "Previously, many groups have shown that they can make transistors out of nanowires and microwires," says Bao. But they have not been able to align all these wires affectively. "All these wires are sitting on top of each other randomly," Bao says. "It's difficult to pack a dense layer of wires into the same area."

The new approach begins with depositing patterned metal onto a substrate made of silicon dioxide. The metal will form the electronic terminals of the microwire transistors. Next is the most important step: aligning the microwires across these terminals. The researchers do this by making a mask with a design that corresponds to the alignment of transistors on top of the terminals, and placing it onto a piece of filter paper. The microwires, which are contained in a liquid solution, are then poured onto the mask, and vacuum suction pulls them to the open areas of the mask. When the mask is removed, the wires are aligned on the filter paper correspondingly. Next, the filter paper with the patterned wires is put in contact with the electrodes on the substrate, and the wires are transferred when the whole thing is dipped into water.

John Rogers, a professor of materials science and engineering at the University of Illinois at Urbana-Champaign, says that the Stanford and Samsung approach could be used to quickly make large-scale microwire circuits that are as good as those constructed using more complex methods.

Bao and her colleagues are currently testing the idea that their alignment technique could work with other materials, including inorganic microwires, which have electrical and structural properties that are different from organic microwires. In addition, she says, her team plans to increase the complexity of patterns on a paper mask in order to create even more complex circuitry.