Power/Performance Bits: June 26

Organic pigment for optoelectronics; transparent, conductive silver film.


Organic pigment for optoelectronics
Researchers at Oregon State University are investigating xylindein, an organic pigment produced by fungi, to find low-cost, sustainable alternatives to silicon in electronic or optoelectronic applications where the high-performance capabilities of silicon aren’t required.

Xylindien is secreted by two wood-eating fungi in the Chlorociboria genus. Any wood that’s infected by the fungi is stained a blue-green color, and has long been sought after by artists. The pigment is so stable that decorative products made half a millennium ago still exhibit its distinctive hue. It holds up against prolonged exposure to heat, ultraviolet light and electrical stress.

“If we can learn the secret for why those fungi-produced pigments are so stable, we could solve a problem that exists with organic electronics,” said Oksana Ostroverkhova, a professor in the department of physics at OSU. “Also, many organic electronic materials are too expensive to produce, so we’re looking to do something inexpensively in an ecologically friendly way that’s good for the economy.”

Wood afflicted by xylindien-producing Chlorociboria fungi. (Source: Oregon State University)

However, with current fabrication techniques, xylindein tends to form non-uniform films with a porous, irregular, “rocky” structure, leading to a lot of performance variation and preventing large-scale application.

Seeking to improve the quality of xylindein film, the team blended it with a transparent, non-conductive polymer, poly(methyl methacrylate), (PMMA). They drop-cast solutions both of pristine xylindein and a xlyindein-PMMA blend onto electrodes on a glass substrate for testing.

They found the non-conducting polymer greatly improved the film structure without a detrimental effect on xylindein’s electrical properties. Additionally, the blended films showed better photosensitivity.

“Exactly why that happened, and its potential value in solar cells, is something we’ll be investigating in future research,” Ostroverkhova said. “We’ll also look into replacing the polymer with a natural product – something sustainable made from cellulose. We could grow the pigment from the cellulose and be able to make a device that’s all ready to go.”

“Xylindein will never beat silicon, but for many applications, it doesn’t need to beat silicon,” she added. “It could work well for depositing onto large, flexible substrates, like for making wearable electronics.”

Transparent, conductive silver film
Researchers from the University of Southern Denmark propose a new method for large-scale fabrication of a transparent conductive electrode film based on nanopatterned silver.

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.

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 team used colloidal lithography to create the transparent conductive silver thin films. They first created a masking layer by coating a 10-centimeter wafer with a single layer of evenly sized, close-packed plastic nanoparticles. The researchers placed these coated wafers into a plasma oven to shrink the size of all the particles evenly. After depositing a thin film of silver on the masking layer, they dissolved the plastic particles to leave a pattern of holes that allows light to pass through, producing an electrically conductive and optically transparent film.

Colloidal lithography was used to create a transparent and conductive thin film. (a) Schematic illustration of the fabrication process. (b) A single nanohole after the silver was deposited deposition and dissolving of the plastic particle. Scale bar: 200 nm. (c) Low magnification micrograph of deposited silver thin-film on homogenous particle monolayer, demonstrating large-scale feasibility. Scale bar: 50 microns. (d) Particle monolayer on substrate after spin-coating and a short (60 s) time in the plasma oven: Scale bar: 2 microns. (e) Particle monolayer after a long (3 min) time in the plasma oven, demonstrating that original particle positions are preserved even after significant size reduction. Scale bar: 10 microns. (Source: Jes Linnet, University of Southern Denmark)

“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, a PhD student at 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.”

The 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.

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