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Flexible Electronics: Transparent And Finally Competitive Electrodes

Tomorrow’s optoelectronics must be wearable, printable and portable: that’s how touch screens, photovoltaics, e-papers and other devices should be! However, a disruptive technology is needed to generate electrodes that are at the same time highly transparent, highly conducting, extremely flexible, cheap and producible on an industrial scale using “green methods”. Paolo Samorì and his team from the University of Strasbourg & CNRS (France) have developed a simple and inexpensive hybrid material that meets all of these criteria. This patented innovation should have a strong impact on new flexible technologies for the general public.

The rise of the market of flexible optoelectronics, such as rollable displays, electronic paper, and foldable touch screens, will require the development of new transparent electrodes. Currently, the most common technical solution relies on indium tin oxide (ITO). This material is used for its unique qualities of high electrical conductivity and transparency, but its rigidity does not make it a good candidate for flexible electronic devices. Moreover, its manufacturing process is expensive because of technical constraints and the scarcity of indium and tin.

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To address these issues, Paolo Samorì and his team have developed an original approach combining supramolecular chemistry and electronics. The resulting physical and chemical properties endow the electrodes with very good mechanical strength and very high levels of transparency, conductivity, flexibility, and resistance to aging.

The ingenuity of these groundbreaking electrodes lies in the combination of copper nanowires with a layer of graphene oxide. Copper is a very good electrical conductor, abundant, very cheap and soluble in water. However, it oxidizes rapidly in contact with air, which degrades the performance of the electrodes. To overcome this effect, a thin insulating layer of graphene oxide is deposited onto the copper nanowires. The two-component material is then chemically reduced (by sodium borohydride, NaBH4) in order to impart all its properties to the electrodes.

The reduced graphene oxide layer acts as an optimal gas barrier to hinder penetration of oxygen molecules, thereby hampering further oxidation of the copper nanowires. This manufacturing process makes it possible to produce a thin hybrid coating by simple spray deposition on large areas while being environmentally friendly. Significantly, bending tests provided unambiguous evidence that the unique electrical characteristics of the electrodes are retained when subjected to several thousand bending cycles. As a proof of the viability of these electrodes for optoelectronic applications, the researchers have fabricated electrochromic devices with a performance comparable to those obtained using ITO.

In light of its qualities and low cost of manufacture, this new generation of electrodes holds potential to replace those based on ITO and open up new fields of application in large-area flexible electronic devices (solar cells, flexible screens, interactive packaging, electroluminescent surfaces…). More generally, these electrodes could be used in sensors (pressure, humidity), batteries, energy generators…

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This study, Hybrid Copper-Nanowire–Reduced-Graphene-Oxide Coatings: A “Green Solution” Toward Highly Transparent, Highly Conductive, and Flexible Electrodes for (Opto)Electronics, was recently published in the journal Advanced Materials.

References

Aliprandi, T. Moreira, C. Anichini, M.-A. Stoeckel, M. Eredia, U. Sassi, M. Bruna, C. Pinheiro, C. A. T. Laia, S. Bonacchi, P. Samorì, “Hybrid Copper-Nanowire–Reduced-Graphene-Oxide Coatings: A “Green Solution” Toward Highly Transparent, Highly Conductive, and Flexible Electrodes for (Opto)Electronics”, Adv. Mater. 2017, in press (DOI: 10.1002/adma.201703225)

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