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Materials

Aligning Nanowires For Transparent Electrodes

Materials Science: Printing method lines up silver nanowires for more transparent, flexible electrodes

by Katherine Bourzac
December 7, 2015

UNCROSSED WIRES
20151207lnp4-oldalignedrandom.jpg
Credit: Nano Lett.
Scanning electron micrographs show aligned silver nanowires (left) printed using the new technique and randomly distributed nanowires (right) which produce a less transparent display.

A new method prints highly transparent, silver nanowire electrodes in one step. These electrodes could be particularly useful for flexible electronics, such as display touchscreens (Nano Lett. 2015, DOI:10.1021/acs.nanolett.5b03019).

Transparent, flexible electrodes will be part of fully flexible devices of the future, such as cell phones that can be rolled up, or displays that can be applied like cling film to any surface. Even today, such electrodes would have advantages in portable electronics, replacing the standard transparent electrode material—brittle indium tin oxide on glass—with electrodes on plastic and other materials that won’t shatter when you drop your phone. Silver nanowires offer transparency and can be readily printed on flexible substrates.

With silver nanowire electrodes, there’s a tradeoff between transparency, which ensures a good-looking display, and conductivity, which ensures a well-functioning touchscreen. Many researchers have developed various tricks for improving the conductivity of sparse nanowire arrays—for example, by scattering the nanowires using simple printing methods, then fusing them together with heat or pressure. Hyunhyub Ko, a chemical engineer at the Ulsan National Institute of Science & Technology, wanted to get good transparency and conductivity in one step.

Ko developed a simple way to align the nanowires. First the researchers coat a surface with a suspension of the 20 to 30 μm-long wires in ethanol. Then they drag a rubbery polydimethylsiloxane stamp over the surface. The stamp is patterned with channels that align the nanowires like a hairbrush smoothing messy tresses. As the solvent evaporates from the printed nanowires, capillary forces further align them. The alignment encourages the flow of electrons between wires, boosting overall conductivity while allowing the nanowires to be relatively sparsely distributed. This sparseness improves transparency: An organic light-emitting diode made with the new material is 30% brighter than one made using a randomly aligned silver nanowire electrode. But the randomly aligned electrodes still have slightly higher conductivity than the aligned ones.

L. Jay Guo, an electrical engineer at the University of Michigan, says he would like to see the group eliminate the tradeoff in conductivity. Ko is working on scaling up the printer system from the current 2 cm-long print head to a much wider one, which will increase the throughput.

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