Researchers at the University of California Los Angeles have demonstrated that it is possible to make intrinsically stretchable organic light-emitting diodes (OLEDs) using either carbon or more conductive silver nanotubes, potentially creating the opportunity for new LED markets and applications.
"Stretchable electronics are evolving from a conceptual curiosity into an important branch of modern electronics. Such devices can be potentially useful for a wide range of applications including wearable electronics, 'smart skins', and minimally invasive biomedical devices," wrote the UCLA researchers, Zhibin Yu, Xiaofan Niu, Zhitian Liu, and Qibing Pei.
Specifically, manufacturers are already exploring how stretchable or elastic electronics may be used to overcome challenges common with current medical devices, such as chronic wound management—which is a major health problem on a global scale—and infant respiratory monitoring, generally made more difficult because of chest and abdominal expansion normal to breathing. On a larger scale, elastic LEDs could be used to wrapper billboards, buildings, buses, or similar structures and vehicles.
The previous approach
Various researchers and manufacturers have already had some success realizing the benefits of stretchable or elastic electronics, including devices with LEDs. But the current approach is significantly different from how the UCLA researchers approached the problem. Presently, it is common to find rigid LEDs embedded or otherwise bound to a soft rubbery polymer so that the polymer provides the elasticity while the active electronics are not actually deformed.
With this approach the most difficult challenge can be the interconnections that must be able to expand and contract with the rubbery polymer. What is different about the UCLA approach is that it seeks to make the entire LED pliable, including the anode, cathode, and interconnects.
Intrinsically stretchable OLEDs
Relying in part on recent advances in organic polymer electronics, the UCLA research team has been able to demonstrate the fabrication of polymer light-emitting devices using single-walled carbon nanotube-polymer composite electrodes as both the electron- and hole-injection electrodes. The resulting hybrid polymer and semiconductor device could be made either with metal-free carbon nanotubes or with more conductive silver nanotubes and stretched linearly up to a 45 percent strain.
"The composite electrodes were prepared by in situ photopolymerization of a liquid monomer penetrating into a porous (single-walled nanotube) coating on glass and subsequent peeling off of the resulting (single-walled nanotube-polymer) composite electrode from the glass substrate," explained the researchers.
Perhaps more basically, the fabrication process is said to consist of three steps. First, conductive, previously manufactured nanotubes are applied to a glass backing. Second, a transparent, liquid monomer is added to the carbon or silver nanotubes, encasing all of the nanotubes except those directly in contact with the glass. The polymer is made solid but pliable using ultraviolet light, and, third, the resulting nanotube and polymer aggregate is peeled off of the glass backing.
"The infusion of the polymer into the carbon nanotube coatings preserved the original network and its high conductance," Professor of Material Science and Engineering, Qibing Pei, told the Massachusetts Institute of Technology's Technology Review during an interview.
Once the composite electrode is peeled away from the glass backing, those nanotubes that had been adjacent to the glass are not exposed on one surface, creating what the UCLA research team called a "conductive pathway and interconnect with the embedded" single-walled carbon or silver nanotubes.
Subsequently, the researchers also used a roll lamination process to fabricate polymer light-emitting electrochemical cells. This layer was sandwiched between two composite electrodes so that the single-walled nanotube and polymer composites are used as both the anode and cathode. The resulting intrinsically stretchable LED produced a sky-blue light at coordinates (0.22, 0.37) on the International Commission on Illumination's CIE 1931 color space chromaticity diagram.
Figure 1: The UCLA stretchable OLED shown at longitudinal strain of 0 percent, 20 percent, and 45 percent.
The UCLA researchers tested several examples of the stretchable LED. In all cases, the LED was heated to about 70°C stretched, cooled, and tested. Interestingly, conductivity actually increased as the strain on the stretchable LED increased through about 35 percent for most of the devices tested. The relative endurance of these devices still needs to be examined.
As an example, an 8V stretchable OLED was tested at 0 percent, 20 percent, and 45 percent longitudinal strain. The device demonstrated a luminance of 45 cd m-2, 30 cd m-2, and 18 cd m-2 respectively; a current density of 59.4 A m-2, 35.7 A m-2, and 20.8 A m-2 respectively; and efficiency of 0.76, 0.84, and 0.87 cd A-2 respectively.
One of the key components in the UCLA team's stretchable OLED is, of course, the single-walled carbon or silver nanotube. Nanotube's great promise is in their amazing length-to-width ratios. For example, a single-walled carbon nanotube, like those used in the stretchable OLED, is a graphene or one-atom-thick layer of carbon rolled into a cylinder, but extending for many million atoms in length.
Carbon nanotubes also have an extraordinary strength to weight ratio that makes them some of the strongest material known. And nanotubes have been shown to change in terms of conductivity depending on structure, material, and environment. For example, carbon nanotubes performed differently than silver nanotubes in the UCLA team’s experiments.
By most accounts, nanotubes will play a significant role in the development of elastic electronics, although, as with any new approach to the fabrication of electronics, there will be challenges.
The intrinsically stretchable OLEDs demonstrated at UCLA represent an important step in developing elastic electronics for medical devices, consumer electronics, or even industrial electronics. Stretchable LEDs in particular offer the promise of many new applications and may open the door to previously unimagined uses.
Even as research continues, engineers can take advantage of readily available, albeit rigid components embedded in soft and flexible polymers to begin to move to elastic electronic applications like flexible or curved displays.
- Intrinsically Stretchable Polymer Light-Emitting Devices Using Carbon Nanotube-Polymer Composite Electrodes, by Zhibin Yu, Xiaofan Niu, Zhitian Liu, and Qibing Pei.
- Silver Nanowire-Polymer Composite Electrodes for Efficient Polymer Solar Cells, by Zhibin Yu, Lu Li, Qingwu Zhand, Weili Hu, and Qibing Pei.
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