Nanowires for Transparent LEDs

Flexible transparent LED screens based on their new silver nanowire transparent conductive circuits. (Source: L. Yang, Zhejiang U.)

Connecting LEDs with transparent conductive circuits has made it possible to turn glass windows, walls and building exteriors into see-through displays that inform or entertain viewers with videos and images. A new approach to making these circuits could help lower the costs of transparent LED screens and allow the tech­nology to be used on substrates that are flexible or curved. Now, researchers from Zhejiang Univer­sity in China and the KTH Royal Institute of Tech­nology in Sweden report fabrication of meter-scale transparent conductive circuits based on silver nanowire networks, and, for the first time, demons­trate these circuits in both rigid and flexible transparent LED screens.

“Transparent LED screens act much like traditional LCDs or LED tele­visions but the fact that they don’t block light enables creative applications not possible with conventional display technology,” said Liu Yang, who led the Zhejiang University research team. “The circuits we fabricated are highly transparent, conductive and flexible, and thus are very promising as a replacement for trans­parent circuits used today.” Transparent LED displays typically incor­porate transparent conductive circuits made from fluorine-doped tin oxide (FTO) or indium tin oxide (ITO). Scientists have been looking for alter­natives to these materials because of indium’s high cost and the complex and expensive manu­facturing process necessary to create circuits for large-format transparent displays. In addition, circuits made from ITO and FTO tend to be too brittle for flexible appli­cations.

Silver nanowire networks are a promising replacement because silver nanowires can be easily synthesized and distri­buted over a large area, have excellent optical trans­parency, are highly conductive and can bend without breaking or compro­mising performance. Although silver nanowire networks have been used to create transparent conductive films, using them to make long circuits has proved challenging. In the new work, the researchers developed a straightforward fabrication process for using silver nanowires to make the ultralong trans­parent circuits necessary for meter-scale transparent LED screens. Using a spray coating method and sacri­ficial masks, they created a 1.2-meter silver nanowire transparent conductive circuit.

The new transparent conductive circuits consist of randomly distributed silver nanowires that are applied in a pattern to a substrate such as plastic or glass. The nanowire network must be dense enough for the electrical current to travel well but not so dense as to compromise trans­parency. “Because of our very simple and low-cost fabri­cation method and the inherent flexibility of silver nanowires, these new transparent conductive circuits could lower the cost and expand the applications of the trans­parent LED screens to flexible and large-angle curved areas,” said Yang.

Analysis of the silver nanowire circuits showed that they were highly trans­parent while exhibiting higher conduc­tivity than ITO. The researchers also performed tests in which they bent silver nanowire conductive strips and ITO transparent strips deposited on a plastic film to a radius as small as 2 millimeters. The silver nanowire strip showed mechanical flexi­bility and stable perfor­mance during the bending tests while ITO didn’t maintain performance. Next, the researchers incor­porated the sliver nanowire transparent conductive circuits into prototype LED displays with glass or plastic substrates. These included a plastic display that performed well even when wrapped around a small bottle or dynami­cally bent to a radius as small as about 15 milli­meters.

The researchers point out that just a few simple steps could turn their proto­type screens into practical transparent screens. For example, the transparent conductive circuits could easily be designed to allow pro­gramming of connected LEDs for displaying videos. Also, the circuits would need to be protected with a coating to avoid chemical reactions with the environ­ment and to enhance their adhesion to the substrates. The screens could also be inte­grated with sandwiched glass or transparent plastic films for further protection and easy main­tenance. (Source: OSA)

Reference: L. Yang et al.: Meter-scale transparent conductive circuits based on silver nanowire networks for rigid and flexible transparent light-emitting diode screens, Opt. Mater. Exp. 9, 4483 (2019); DOI: 10.1364/OME.9.004483

Link: Centre for Optical and Electromagnetic Research, National Engineering Research Center of Optical Instrumentation, Zhejiang University, Hangzhou, China

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