Thin Silver-Films for Flexible Screens

The researchers used colloidal lithography to create a silver nanopattern that conducts electricity while letting light through the holes. The new transparent electrode films could be useful for solar cells as well as flexible displays and touch screens. (Source: J. Linnet, Univ. of Southern Denmark)

Researchers have demons­trated large-scale fabrication of a new type of trans­parent conduc­tive electrode film based on nano­patterned silver. Smartphone touch screens and flat panel tele­visions use trans­parent electrodes to detect touch and to quickly switch the color of each pixel. Because silver is less brittle and more chemically resistant than materials cur­rently used to make these electrodes, the new films could offer a high-perfor­mance and long-lasting option for use with flexible screens and electronics. The silver-based films could also enable flexible solar cells for instal­lation on windows, roofs and even personal devices.

The researchers fabricated a trans­parent conduc­ting thin-film on glass discs 10 centi­meters in diameter. Based on theo­retical esti­mations that matched closely with experi­mental measure­ments, they calculate that the thin-film electrodes could perform signi­ficantly better than those used for existing flexible displays and touch screens. “The approach we used for fabrication is highly repro­ducible and creates a chemically stable confi­guration with a tunable tradeoff between transparency and conductive properties,” said Jes Linnet from the Univer­sity of Southern Denmark. “This means that if a device needs higher trans­parency but less conduc­tivity, the film can be made to accommo­date by changing the thick­ness of the film.”

Most of today’s transparent elec­trodes are made of indium tin oxide (ITO), which can exhibit up to 92 percent trans­parency – comparable to glass. Although highly trans­parent, ITO thin films must be processed care­fully to achieve repro­ducible perfor­mance and are too brittle to use with flexible electronics or displays. Researchers are seeking alter­natives to ITO because of these drawbacks. The anti-corrosive nature of noble metals such as gold, silver and platinum makes them promising ITO alter­natives for creating long-lasting, chemically resistant elec­trodes that could be used with flexible substrates. However, until now, noble metal trans­parent conductive films have suffered from high surface roughness, which can degrade perfor­mance because the inter­face between the film and other layers isn’t flat. Trans­parent conduc­tive films can also be made using carbon nano­tubes, but these films don’t currently exhibit high enough conduc­tance for all applications and tend to also suffer from surface roughness due to the nano­tubes stacking on top of each other.

Now, the researchers used colloidal litho­graphy to create trans­parent conductive silver thin films. They first created a masking layer, or template, by coating a 10-centi­meter wafer with a single layer of evenly sized, close-packed plastic nano­particles. They placed these coated wafers into a plasma oven to shrink the size of all the particles evenly. When they deposited a thin film of silver onto the masking layer, the silver entered the spaces between the particles. They then dissolved the particles, leaving a precise pattern of honey­comb-like holes that allow light to pass through, producing an electri­cally conduc­tive and opti­cally trans­parent film.

The researchers demonstrated that their large-scale fabri­cation method can be used to create 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 carbon-nanotube-based films with the equi­valent trans­parency. The lower the elec­trical resistance, the better the electrodes are at conduc­ting an electrical charge.

“The most novel aspect of our work is that we accounted for both the trans­mission properties and the conduc­tance properties of this thin film using theo­retical analysis that corre­lated well with measured results,” said Linnet. “Fabri­cation problems typi­cally make it hard to get the best theo­retical perfor­mance from a new material. We decided to report what we encoun­tered experi­mentally and postu­late remedies so that this infor­mation could be used in the future to avoid or minimize problems that may affect perfor­mance.” The researchers say that their findings show that colloidal litho­graphy can be used to fabri­cate trans­parent con­ductive thin films that are chemi­cally stable and could be useful for a variety of appli­cations. (Source: OSA)

Reference: J. Linnet et al.: Transparent and conductive electrodes by large-scale nano-structuring of noble metal thin-films, Opt. Mat. Exp. 8, 1733 (2018); DOI: 10.1364/OME.8.001733

Link: Center for Nano Optics, Mads Clausen Institute, University of Southern Denmark, Odense M, Denmark

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