Conductive thin film clears the way for improved solar cells

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A new thin sheet combining high transparency and high conductivity could have future applications in touchscreens, solar cells and flexible displays(Credit: Korea University)

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Researchers at the University of Korea and the University of Illinois at Chicago have developed a new thin film material that packs a unique combination of features: it's highly electrically conductive, bendable, stretchable, and almost entirely transparent. The film could help build more efficient solar panels, self-heating smart windows, flexible displays, and high-performance cooling surfaces.

The most remarkable property of the film is the unique (and useful) combination of transparency and conductivity. In most materials, one comes at the expense of the other because the particles that carry electric charge also tend to interact with light, turning the material opaque. But researchers Alexander Yarin, Sam Yoon, and their team got around this limitation by building a web of highly conductive copper fibers with holes between them large enough for light to still come through.

Yarin, Yoon, and colleagues created the film by first coating polyacrylonitrile (PAN), a flexible polymer with very thin fibers, with small concentrations of metal to make it conductive. They then spun the polymer into a deeply intertwined web and coated the fibers again, this time with a thin layer of copper.

The result is a massive tangle in which the copper fibers, fused at the junctions, can conduct electricity with little resistance while the large gaps between the fibers still allow the film to be 92 percent transparent.

Such a material could be a strong choice for the transparent conducting electrodes (TCEs) that feature in solar panels and smart windows. The industry-standard material for building TCEs is indium-doped tin oxide (ITO). Indium, however, is brittle, sacrifices conductivity for transparency, and according to some reports (PDF), could run out sometime soon (though others think not).

The copper-plated nanofibers, on the other hand, establish a "world-record combination of high transparency and low electrical resistance" and also sport excellent mechanical properties. According to the researchers, the film can in fact be stretched by 580 percent and still retain about a fifth of its conductivity, or it can be bent at a 5 mm (0.2 in) radius for a thousand times and still show no noticeable change in performance.

The scientists involved say processes like electrospinning (to create the thin, tangled fibers) and electroplating (to coat the fibers with copper) are commonplace, fast and high-throughput. Combined with the low cost and abundance of the prime materials, this could make the new film a cheap and highly effective electronic component.

Aside from future applications in touchscreens, solar cells and flexible displays, the researchers say they could transfer the fibers to any surface, creating, for instance, nano-textured materials that can dissipate heat with high efficiency. Another area of interest, Yoon tells Gizmag, will be to exploit the resistive heating effect and use the film as a heater for a smart window.

The study appears in this week's issue of the journal Advanced Materials.

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