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Hybrid electrode paves way for next-gen electronics

Posted: 03 Jun 2013 ?? ?Print Version ?Bookmark and Share

Keywords:inorganic LED? transparent electrode? indium tin oxide? graphene synthesis?

A new breed of transparent and stretchable electrodes designed by Ulsan National Institute of Science & Technology (UNIST) researchers may make way for more flexible electronics, displays, and solar cells.

The research team demonstrated a live rabbit wearing contact lenses fitted with organic light-emitting diode (LED) with no side effects. The device was made of graphene with silver nanowires that forms a thin, transparent, and stretchable electrode that overcome the weakness of each individual material. The electrode hybrid used in the demonstration could be used in possible future applications such as scanning and taking pictures using soft contact lenses.

Transparent electrodes have been widely used in things like touch screens, flat-screen TVs, solar cells and light-emitting devices. Commonly made from indium tin oxide (ITO), it is brittle and cracks thus losing functionality if flexed. It also degrades over time, and is expensive due to the limited quantities of indium metal.

As an alternative, the networks of randomly distributed mNWs have been considered as promising candidates for next-generation transparent electrodes, due to their low-cost, high-speed fabrication of transparent electrodes. However, the number of disadvantages of the mNW networks limited their integration into commercial devices. They have low breakdown voltage, typically high NW-NW junction resistance, high-contact resistance between network and active materials, material instability and poor adhesion to plastic substrates.

Graphene is also well known as good a candidate for transparent electrode because of their unique electrical properties and high mechanical flexibility. However, scalable graphene synthesis methods for commercialisation produces lower quality graphene with individual segments called grains that increase the electrical resistance at boundaries between these grains.

Silver nanowires, on the other hand, have high resistance because they are randomly oriented like a jumble of toothpicks facing in different directions. In this random orientation, there are many contacts between nanowires, resulting in high resistance due to large junction resistance of nanowires. Due to these drawbacks, neither is good for conducting electricity, but a hybrid structure, combined from two materials, is.

LED fitted soft contact lens

LED fitted soft eye contact lens. This was worn by a live rabbit during the UNIST demonstration.

The hybrid material presents a high electrical and optical performance with mechanical flexibility and stretchability for flexible electronics. The hybrid transparent electrode has a low "sheet resistance" and high transmittance. There's almost no change in its resistance when bent and folded. Where the ITO is bent, its resistance increases significantly. Additionally the hybrid material preserves its electrical and optical properties against thermal oxidation condition

In the demonstration conducted by the research team using a live rabbit, the contact lens was worn by the rabbit for five hours and no abnormal behaviour, such as bloodshot eye or the rubbing of eye areas were observed in the live rabbit. Using eye contact lenses to take pictures and scan images may not be science fiction anymore.

The research was led by Jang-Ung Park, professor of the School of Nano-Bioscience and Chemical Engineering at UNIST. "We believe the hybridisation between two-dimensional and one-dimensional nanomaterials presents a promising strategy towards flexible, wearable electronics and implantable biosensor devices, and indicate the substantial promise of future electronics," said Park.

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