Most of today’s display screens, including the one on your phone, are likely coated with transparent electrodes made of indium tin oxide (ITO). Although highly transparent, ITO thin films are brittle and must be processed carefully.
PhD student Jes Linnet from the University of Southern Denmark hopes that his silver-based, transparent conductive electrode film offers a longer-lasting alternative for flexible screens and electronics.
In the journal Optical Materials Express, Linnet and the Denmark team reported the fabrication of their nano-patterned silver material on 10-cm-diameter glass discs.
Based on theoretical estimations that matched closely with their experimental measurements, Linnet says that the silver thin-film electrodes are poised to perform significantly better than the materials currently used in existing flexible displays and touch screens.
The Importance of Flexibility
Screen flexibility is a highly sought-after characteristic, especially by manufacturers working on the next-generation displays appearing in cars, phones, and curved televisions. The common, but less flexible materials like indium tin-oxide are stiff and often crack or flex after multiple taps – or a drop on the ground.
Noble metals like gold and silver feature anti-corrosive, “tap-proof” properties, but their surfaces are traditionally rough, which can degrade performance. Another drawback: the metals reflect too much.
“In terms of the silver-based electrode film, the problem with using noble metals for transparency is that they are highly reflective in the visible [range],” said Linnet. “This can be minimized by reducing the thickness of the thin-film, as the skin-depth of silver will enable more transmission through thinner films than thicker films.”
To let more of the light in and to increase transparency, Linnet wanted to make the thin film even thinner – and poke some holes.
The Fabrication Method
As part of an approach called colloidal lithography, the researchers gave a honeycomb-like structure to the transparent conductive silver thin film.
Here’s how the process worked:
Linnet and his team first coated the 10-centimeter wafer with a single layer of evenly sized, close-packed plastic nanoparticles. The researchers then placed these coated wafers into a plasma oven to shrink all the particles.
When a thin film of silver was deposited onto the primary “masking” layer, the silver reached the spaces between the particles. After dissolving the particles, the screen-testers ended up with a precise pattern of honeycomb-like holes left behind.
The resulting arrangement allowed light to pass through, producing an optically transparent film that was also electrically conductive.
Linnet and his colleagues demonstrated that their large-scale fabrication method created silver transparent electrodes with as much as 80 percent transmittance, while keeping electrical sheet resistance below 10 ohms per square – about a tenth of what has been reported for the carbon-nanotube-based films used in devices like automotive GPS units.
The material additionally offers a kind of tunability: If a device needs higher transparency but less conductivity, the film can be made to accommodate by changing the thickness of the film.
“Reducing the thickness of the film makes it more transparent; however it also reduces the cross-sectional area for a flow of current in the film,” Linnet told Tech Briefs. “That is the tradeoff: more transparency at the cost of more resistance.”
Linnet hopes that the colloidal lithography can be used to fabricate transparent conductive thin films that are chemically stable and could be useful for a variety of applications, including solar cells.
“The solution we used only has surface roughness defined by the deposited thickness, which can be as low as 10-20 nanometers,” said Linnet, who hopes to someday commercialize the material. “This very flat, functional film may be preferable in solar cell technologies which are primarily fabricated using layers on layers of thin-films.”
What do you think? Will these thin conductive films support flexible screens? Solar cells? Share your thoughts and questions below.