Graphene promises to revolutionize electronics but we're still waiting for graphene-based technologies to hit the market. Rice University researchers have now created transparent, graphene-based electrodes that they say could be the "killer app" that finally puts graphene into the commercial spotlight. The graphene-based electrodes could be used to replace the increasingly expensive indium tin oxide (ITO) in touch-screen displays, photovoltaic solar cells and LED lighting.
ITO is used as a transparent, conductive coating in virtually all flat-panel displays, including touch screens on smartphones and tablet computers, and is used in organic light-emitting diodes (OLEDs) and solar cells. With the increase in popularity of these products the element indium has become increasingly rare and therefore more expensive. It is also brittle, which rules it out for use in flexible displays and heightens the risk of the screen of your smartphone cracking when the device is dropped.
For these reasons, researchers have been looking for an electrically conductive ITO replacement that can be put on a flexible substrate such as plastic. Now the lab of Rice chemist James Tour has created thin films that could combine with other flexible, transparent electronic components and lead to computers that wrap around the wrist and solar cells that wrap around just about anything.
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.
"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," says Tour.
Conversely, said postdoctoral researcher Yu Zhu, lead author of the new paper detailing the team's work, 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, Zhu said. 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 aluminum 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 said 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, he said; commercial labs have already developed a roll-to-roll graphene production technique.
"This material is ready to scale right now," he said.
Although tests showed the hybrid film's conductivity decreases by 20 to 30 percent with the initial 50 bends, the material then stabilizes and no significant variations were observed at up to 500 bending cycles. Tour said more rigorous bending testing will be left up 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," Tour said.
The film is also environmentally stable with test films exposed to the environment in the lab showing no signs of deterioration after a period of a year. Additionally, the use aluminum and carbon offers significant savings compared to the increasingly expensive ITO.
"Now that we know it works fine on flexible substrates, this brings the efficacy of graphene a step up to its potential utility," Tour said.
The Rice University team's paper, co-authored by graduate students Zhengzong Sun and Zheng Yan and former postdoctoral researcher Zhong Jin, appears in the online edition of ACS Nano.