Self-Powered Flexible Electronics
Touch-screen computing is all the rage, appearing in countless smart phones, laptops, and tablet computers.
Now researchers at Samsung and Sungkyunkwan University in Korea have come up with a way to capture power when a touch screen flexes under a user’s touch. The researchers have integrated flexible, transparent electrodes with an energy-scavenging material to make a film that could provide supplementary power for portable electronics. The film can be printed over large areas using roll-to-roll processes, but are at least five years from the market.
The screens take advantage of the piezoelectric effect–the tendency of some materials to generate an electrical potential when they’re mechanically stressed. Materials scientists are developing devices that use nanoscale piezoelectronics to scavenge mechanical energy, such as the vibrations caused by footsteps. But the field is young, and some major challenges remain. The power output of a single piezoelectric nanowire is quite small (around a picowatt), so harvesting significant power requires integrating many wires into a large array; materials scientists are still experimenting with how to engineer these screens to make larger devices.
Samsung’s experimental device sandwiches piezoelectric nanorods between highly conductive graphene electrodes on top of flexible plastic sheets. The group’s aim is to replace the rigid and power-consuming electrodes and sensors used on the front of today’s touch-screen displays with a flexible touch-sensor system that powers itself. Ultimately, this setup might generate enough power to help run the display and other parts of the device functions. Rolling up such a screen, for instance, could help recharge its batteries.
“The flexibility and rollability of the nano-generators gives us unique application areas such as wireless power sources for future foldable, stretchable, and wearable electronics systems,” says Sang-Woo Kim, professor of materials science and engineering at Sungkyunkwan University. Kim led the research with Jae-Young Choi, a researcher at Samsung Advanced Institute of Technology.
The same group previously put nano-generators on indium tin oxide electrodes. This transparent, conductive material is used to make the electrodes on today’s displays, but it is inflexible.
To make the new nano-generators, the researchers start by growing graphene–a single-atom-thick carbon material that’s highly conductive, transparent, and stretchy–on top of a silicon substrate, using chemical vapor deposition. Next, through an etching process developed by the group last year, the graphene is released from the silicon; and the graphene is removed by rolling a sheet of plastic over the surface. The graphene-plastic substrate is then submerged in a chemical bath containing a zinc reactant and heated, causing a dense lawn of zinc-oxide nanorods to grow on its surface. Finally, the device is topped off with another sheet of graphene on plastic.
In a paper published this month in the journal Advanced Materials, the Samsung researchers describe several small prototype devices made this way. Pressing the screen induces a local change in electrical potential across the nanowires that can be used to sense the location of, for example, a finger, as in a conventional touch screen. The material can generate about 20 nanowatts per square centimeter. Kim says the group has subsequently made more powerful devices about 200 centimeters squared. These produce about a microwatt per square centimeter. Kim says this is enough for a self-powered touch sensor and “indicates we can realize self-powered flexible portable devices without any help of additional power sources such as batteries in the near future.”
“It’s pretty impressive to integrate all these things in a foldable, macroscale device,” says Michael McAlpine, professor of mechanical engineering at Princeton University. He notes that the potential of zinc oxide nanowires as a piezoelectric sensing material and nanoscale power source was previously demonstrated by Georgia Tech materials scientist Zhong Lin Wang. But integrating these materials over a large area with a flexible, transparent electrode opens up new applications, says McAlpine.
The methods used to make the nano-generators are compatible with large-scale manufacturing, according to Kim. His group is working to boost the power output of the films–the main obstacle is the quality of the electrodes. One possible solution is to improve the connection between the nanowires and the electrodes by eliminating flaws in the structure of the graphene. The Korean group is also experimenting with adding small amounts of impurities to the material, a process called doping, to improve its conductivity.
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