ACS Nano - Rational Design of Hybrid Graphene Films for High-Performance Transparent Electrodes
Cross-section schematic of process flow. A1-A4: Preparation of the metal grid on a transparent substrate. A1: Deposition of metal film (Metal 1) and photoresist on transparent substrate. A2: Photolithography patterning of the grid structure. A3: Wet-etching of the metal film. A4: Removal of the photoresist. B1-B4: Graphene growth using a solid carbon source (PMMA) 2. B1: Spin-coating PMMA on a copper foil (Metal 2). B2: Growing graphene film using solid carbon source. B3: Spin-coating a PMMA sacrificial layer on graphene. B4: Wetetching of the copper foil. AB1-AB2: assemble hybrid electrode. AB1: Transferring graphene on metal grid structure. AB2: Removal of PMMA sacrificial layer by dissolution in acetone.
Transparent, flexible conducting films were fabricated by using a metallic grid and graphene hybrid film. Transparent electrodes using the hybrid film and transparent substrate such as glass or polyethylene terephthalate (PET) films were assembled. The sheet resistance of the fabricated transparent electrodes was as low as 3 Ω/ with the transmittance at 80%. At 90% transmittance, the sheet resistance was 20 Ω/. Both values are among the highest for transparent electrode materials to date. The materials used for the new hybrid electrode are earth-abundant stable elements, which increase their potential usefulness for replacement of indium tin oxide (ITO) in many applications.
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 organic light-emitting diodes (OLEDs) and solar cells.
ITO works well in all of these applications, but 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 a 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, said postdoctoral researcher Yu Zhu, lead author of the new paper, 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.
The flexibility is almost a bonus, Zhu said, due to the potential savings of using carbon and aluminum instead of expensive ITO. "Right now, ITO is the only commercial electrode we have, but it's brittle," he said. "Our transparent electrode has better conductivity than ITO and it's flexible. I think flexible electronics will benefit a lot."
In tests, he found the hybrid film's conductivity decreases by 20 to 30 percent with the initial 50 bends, but after that, the material stabilizes. "There were no significant variations up to 500 bending cycles," Zhu said. More rigorous bending test will be left to commercial users, he said.
"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."
The film also proved environmentally stable. When the research paper was submitted in late 2010, test films had been exposed to the environment in the lab for six months without deterioration. After a year, they remain so.
"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.
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