Organic LEDs, carbon nanotubes may light up future fabrics

With the emergence of wearable electronics that monitor fitness and health, there is a growing need for more flexible light-emitting devices. One option that researchers have been interested in is developing fabrics with integrated light-emitting devices. Unfortunately, fabrics themselves are not a suitable surface for current light-emitting materials. However, a team of scientists have found a way around this issue by integrating the light-emitting devices directly into fabrics using a new technology: light-emitting device fibers.

These research team, based in China, worked with polymer light-emitting electrochemical cells (PLECs). Like many other light-emitting devices, PLECs have a structure that is composed of two metal electrodes connected to a thin organic layer that acts as a semiconductor. Because PLECs have mobile ions incorporated into the semiconductor, they have many benefits compared to other light-emitting diodes (LEDs): low operating voltage, high efficiency in converting electrons to photons, and high power efficiency. PLECs are also a good option because they do not require the use of metals that are sensitive to air and they can be used on rougher surfaces; these characteristics make them suitable for large-scale manufacturing.

These fiber-shaped PLECs have a coaxial structure with four layers. Using a solution-based processing, a steel wire, which acts as the base of this fiber, is dip-coated with a thin layer of ZnO nanoparticles. This layer has two key functions: protecting the light-emitting layer that's applied next; and decreasing the leakage of the current, thus enhancing current efficiency.

Next, the electroluminescent polymer layer is deposited onto the wire using dip-coating. Finally, a sheet of aligned carbon nanotubes is wrapped around the bundle using a dry-drawn form of spinnable carbon nanotubes. Because the carbon nanotubes were highly aligned, they provided the fiber with high electrical conductivities. Imaging revealed that the fibers had a uniform diameter and a smooth outer surface.

The scientists who created these fibers determined the lifetime of the devices. They found that the fibers gradually light up over a 21-minute period and gradually dim over a four-hour period; in these studies, the light emitted by the fibers was blue. The fiber lit up when a voltage of 5.6V was applied and reached a peak intensity at 13V. When the fiber is pre-charged, it displays a rapid turn-on response that is similar to conventional LEDs.

The brightness of the light emitted by the fibers was almost entirely independent of viewing angle. When the fibers were bent, they maintained their brightness above 90 percent and no obvious damage was observed. Though only blue light was explored in these studies, the team believes other colors could be displayed as well.

Together, these studies demonstrate that these light-emitting fibers can be woven into fabrics without compromising their luminescence, which makes them a promising candidate for the development of wearable electronics and other fabric-based electronics in the future.

Nature Photonics, 2015. DOI: 10.1038/NPHOTON.2015.37  (About DOIs).

Listing image by Huisheng Peng