Indium-free transparent flexible electrodes

02 August 2011

A hybrid material that combines a fine aluminum mesh with a single-atom-thick layer of graphene outperforms materials common to current touch screens and solar cells. (Credit: Yu Zhu/Rice University)

Flexible, transparent electronics are closer to reality with the creation of graphene-based electrodes at Rice University.

The lab of chemist James Tour has created thin films that could revolutionise touch-screen displays, solar panels and LED lighting.

Flexible, see-through video screens may be the ‘killer app’ that finally puts graphene into the commercial spotlight once and for all.

Combined with other flexible, transparent electronic components being developed at Rice and elsewhere, the breakthrough could lead to computers that wrap around the wrist and solar cells that wrap around just about anything.

The lab's hybrid graphene film is a strong candidate to replace indium tin oxide (ITO), a commercial product widely used as a transparent, conductive coating. It's the essential element in virtually all flat-panel displays, including touch screens on smart phones and iPads, and is part of OLEDs and solar cells.

ITO works well in all of these applications, but it has several disadvantages. The element indium is increasingly rare and expensive.

It's also brittle, which heightens the risk of a screen cracking when a smart phone is dropped and further rules ITO out as the basis for flexible displays.

The Tour Lab's thin film combines a single-layer sheet of highly conductive graphene with a fine grid of metal nanowire. The researchers claim the material easily outperforms ITO and other competing materials, with better transparency and lower resistance to electric current.

"Many people are working on ITO replacements, especially as it relates to flexible substrates," said Tour, Rice's T.T. and W.F. Chao Chair in Chemistry as well as Professor of Mechanical Engineering and Materials Science and of Computer Science. "Other labs have looked at using pure graphene. It might work theoretically, but when you put it on a substrate, it doesn't have high enough conductivity at a high enough transparency. It has to be assisted in some way."

Conversely, says postdoctoral researcher Yu Zhu, fine metal meshes show good conductivity, but gaps in the nanowires to keep them transparent make them unsuitable as stand-alone components in conductive electrodes.

But combining the materials works superbly. The metal grid strengthens the graphene, and the graphene fills all the empty spaces between the grid. The researchers found a grid of five-micron nanowires made of inexpensive, lightweight aluminium did not detract from the material's transparency.

"Five-micron grid lines are about a 10th the size of a human hair, and a human hair is hard to see," Tour said.

Tour commented that metal grids could be easily produced on a flexible substrate via standard techniques, including roll-to-roll and ink-jet printing. Techniques for making large sheets of graphene are also improving rapidly, and commercial labs have already developed a roll-to-roll graphene production technique. "This material is ready to scale right now," he added.

The flexibility is almost a bonus due to the potential savings of using carbon and aluminium instead of ITO.

"Right now, ITO is the only commercial electrode we have, but it's brittle," said Zhu. "Our transparent electrode has better conductivity than ITO and it's flexible. I think flexible electronics will benefit a lot."

In tests, he found that the hybrid film's conductivity decreases by 20% to 30% with the initial 50 bends, but after that, the material stabilises.

"There were no significant variations up to 500 bending cycles," Zhu said, adding that more rigorous bending test will be left to commercial users.

"I don't know how many times a person would roll up a computer," Tour added. "Maybe 1,000 times? Ten thousand times? It's hard to see how it would wear out in the lifetime you would normally keep a device. Now that we know it works fine on flexible substrates, this brings the efficacy of graphene a step up to its potential utility," he said.

Rice graduate students Zhengzong Sun and Zheng Yan and former postdoctoral researcher Zhong Jin are co-authors of the paper.

The Office of Naval Research Graphene MURI programme, the Air Force Research Laboratory through the University Technology Corporation, the Air Force Office of Scientific Research and the Lockheed Martin Corp./LANCER IV programme supported the research.

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