Photographs showing tilted view of sliced sample. Light-emitting images show orange background (in-plane E field), green zigzag pattern (vertical E field), and simultaneous emission of orange and green lights (in-plane + vertical E fields), respectively, after applying an AC voltage between A and B, A+B and C, and A and B+C fibers.
Photographs showing tilted view of sliced sample. Light-emitting images show orange background (in-plane E field), green zigzag pattern (vertical E field), and simultaneous emission of orange and green lights (in-plane + vertical E fields), respectively, after applying an AC voltage between A and B, A+B and C, and A and B+C fibers.

Electroluminescent (EL) devices based on light-emitting phosphors embedded in polymers could be useful in novel soft robots, self-healing systems, and wearable electronics. One of the most promising composites is zinc sulfide (ZnS) mixed with polydimethylsiloxane (PDMS), sandwiched between electrodes made from graphene, silver nanowires (Ag NWs), or indium tin oxide (ITO).

ITO is a well-established electrode material because of its transparency, but is not ideal for flexible devices because of its brittleness. To get around this problem, researchers from DGIST in Korea led by Soon Moon Jeong have designed a novel device in which durable, flexible, and electrically conductive Ag-coated nylon fibers are embedded in a PDMS + ZnS composite to serve as the electrodes. The fibers, which are aligned parallel to each other at varying distances apart, induce an electric field around them when an AC voltage is applied that drives light emission from ZnS particles in the composite.

“Previously, we tried to fabricate coplanar EL devices but the luminescent light was always insufficient because of the low transmittance of the planar electrode,” explains Jeong. “Our motivation was to remove the planar-type electrodes, which are an obstacle to internal light extraction by using an in-plane electric field instead.”

The structure not only maintains luminescence while being deformed multiple times, but also when dipped into water. The composite even demonstrates mechanoluminescence (ML) as well.

“The device employing textile-based fibers as electrodes exhibits durable electro-optical performance over 10,000 bending cycles,” points out Jeong.

The device shows higher luminescence – or brightness – than conventional planar EL devices and can be tailored to emit a range of colors by using different types of ZnS particles, which can emit green, blue, or orange light. Various other colors, including white, could be achieved by combining different phosphors in the composite.

Moreover, varied light effects can be achieved by changing the proximity and depth of the Ag-coated nylon fibers in the composite. Positioning the fibers close together, for example, produces uniform light, while arranging the fibers at different depths creates variation in the color of the emitted light.

The researchers believe that these initial results are proof-of-concept of a promising new approach to fiber-electrode-based flexible EL devices. The concept could be extended to produce light-emitting fabrics for displays, wearable electronics, and novel lighting.

“Our proposed structure could potentially be used in large-scale outdoor billboards or light-emitting banners, which don’t need high resolution, because of its high resistance to environmental factors (e.g. water and light),” points out Jeong.

The team now wants to improve resolution of the device and its stretchability, as well as find an alternative to the currently required high-power AC power source.