Huisheng Peng
With the rise of wearable electronics that monitor fitness and health, there is a growing need for more flexible light-emitting devices. One option researchers are interested in is developing fabrics with integrated light-emitting devices. Fabrics themselves are unfortunately not a suitable substrate for the current luminous materials. However, a team of scientists has found a way around this problem by integrating the light-emitting devices directly into fabrics using a new technology: light-emitting device fibers.
This research team, based in China, worked with polymeric light-emitting electrochemical cells (PLECs). Like many other light-emitting devices, PLECs have a structure consisting of two metal electrodes connected to a thin organic layer that acts as a semiconductor. Because PLECs have mobile ions built into the semiconductor, they have many advantages over other light-emitting diodes (LEDs): low operating voltage, high efficiency in converting electrons to photons, and high energy efficiency. PLECs are also a good option because they don’t require metals that are sensitive to air and can be used on rougher surfaces; these properties make them suitable for large-scale production.
These fibrous PLECs have a four-layer coaxial structure. Using a solution-based processing, a steel wire, which acts as the basis of this fiber, is dipped with a thin layer of ZnO nanoparticles. This layer has two important functions: to protect the light-emitting layer that is subsequently applied; and reducing current leakage, improving current efficiency.
Subsequently, the electroluminescent polymer layer is applied to the wire by dip coating. Finally, a sheet of aligned carbon nanotubes is wrapped around the bundle using a draw-dry form of spinnable carbon nanotubes. Because the carbon nanotubes were highly aligned, they provided the fiber with high electrical conductivity. Imaging revealed that the fibers had a uniform diameter and a smooth outer surface.
The scientists who created these fibers determined the lifespan of the devices. They found that the fibers gradually brightened over a 21-minute period and gradually dimmed 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 peaked at 13V. When the fiber optic is pre-charged, it exhibits a fast turn-on response similar to conventional LEDs.
The brightness of the light emitted by the fibers was almost entirely independent of the viewing angle. When the fibers were bent, they maintained their clarity above 90 percent and no obvious damage was observed. While only blue light has been examined in these studies, the team believes that other colors may be represented as well.
Together, these studies show that these light-emitting fibers can be woven into fabrics without compromising their luminescence, making them a promising candidate for the development of wearable electronics and other fabric-based electronics in the future.
Nature photonics2015. DOI: 10.1038/NPHOTON.2015.37 (About DOIs).
Frame image by Huisheng Peng