A nanomaterial flex — MXene electrodes help OLED display technology shine, while bending and stretching

(Press-News.org) The organic light-emitting diode (OLED) technology behind flexible cell phones, curved monitors, and televisions could one day be used to make on-skin sensors that show changes in temperature, blood flow, and pressure in real time. An international collaboration, led by researchers from Seoul National University in the Republic of Korea and Drexel University, has developed a flexible and stretchable OLED that could put the technology on track for this use and a range of new applications.

Recently reported in Nature, their work improves on existing technology by integrating a flexible, phosphorescent polymer layer and transparent electrodes made from MXene nanomaterial. The result is an OLED that can be stretched to 1.6 times its original size, while maintaining most of its luminescence. 

“This study addresses a longstanding challenge in flexible OLED technology, namely, the durability of its luminescence after repeated mechanical flexion,” said Yury Gogotsi, PhD, Distinguished University and Bach professor in Drexel’s College of Engineering. “While the advances creating flexible light-emitting diodes have been substantial, progress has leveled off in the last decade due to limitations introduced by the transparent conductor layer, limiting their stretchability.”

Limits of Flexibility  OLEDs produce light through a process called electroluminescence. When a current is applied to the device, the positive and negative charges shuttling between electrodes — through an organic polymer layer — emit light when they unite, forming a particle called an exciton, and settle into an electrical bias stable state. The color of the light can be controlled by varying the chemical composition of the organic layer.

Flexible OLEDs, which are produced by depositing the layers on a flexible plastic substrate, can operate while being folded, bent or rolled. First developed in the 1990s, by the 2010s, Samsung had integrated the technology in its shatterproof and flexible devices and curved-edge phones. But it became apparent that the OLED’s pixel brightness and flexibility waned over time, due to the gradual degradation of materials used in its electrodes and organic layer.

“Imparting conducting materials with flexibility usually involves incorporating an insulating but stretchable polymer that hinders charge transport and, as a result, reduces light emission,” said Danzhen Zhang, PhD, a co-author and postdoctoral researcher at Northeastern University, who performed pioneering work on the development of transparent conductive MXene films with tunable properties as a PhD student in Gogotsi’s lab at Drexel. “In addition, the material most commonly used in electrodes can become brittle and more likely to break the longer the OLED is flexed and stretched. This issue was addressed by using MXene-contact stretchable electrodes, which feature high mechanical robustness and tunable work function, ensuring efficient hole or electron injection.”

A Material Solution To address this design problem, the researchers used a special type of organic layer that chemically coaxes more charge unions, exciton creation, and light production.

The material, which the researchers call an exciplex-assisted phosphorescent (ExciPh) layer, is intrinsically stretchable and its chemical composition can alter the energy level of the charges to enable more of them to form excitons and produce light — like slowing the spin of a merry-go-round to make it easier to jump onto it.

The ExciPh material allows more than 57% of excitons to be used to produce light; by comparison, emissive materials adding the polymer layer currently used in OLEDs have only a 12-22% conversation efficiency rate.

The team also added a thermoplastic polyurethane elastomer matrix material to improve the stretchability of the layer. And to further enhance the performance of the OLEDs, the researchers developed high-quality, highly conductive, transparent, stretchable electrodes that improve the disbursal of charges into the ExciPh layer.

Combining MXene, a type of highly conductive, two-dimensional nanomaterial created by Drexel researchers in 2011, with silver nanowires, the electrodes provide a percolation network for the charges that helps to ensure more of them reach the light-producing polymer layer before they combine to form excitons. By doing this, the stretchable MXene-based electrodes optimize charge injection and help the OLED maintain its luminescence while it is being flexed.

“Owing to their exceptional conductivity and layered form, MXenes provide an exceptional electrode material for flexible OLEDs,” Gogotsi said. “We have demonstrated the performance of flexible, transparent MXene electrodes in multiple applications; thus, including them in efforts to improve OLED technology is a natural step for our research.”

Flexing Combining these enhancements, the team created a set of flexible green OLED displays, one in the shape of a heart and another displaying a set of numbers. They measured the charge-to-exciton conversion rate — a measure of the OLEDs’ ability to efficiently produce light — as well as their performance under strain and repeated use.

To further demonstrate the wide applicability of their fully stretchable OLED, researchers at Seoul National University produced a full-color, fully stretchable display that incorporated four dopant substances in the stretchable ExciPh layer. They also created a set of fully stretchable passive-matrix OLEDs to demonstrate a simple, low-power interface that could be readily manufactured and used in wearable electronics.

The OLEDs performed better than those reported in previous research, both in light production and energy efficiency. Stretching tests also showed that the device’s performance dropped by only 10.6% at 60% of its maximum strain. And it retained 83% of its light production efficiency after 100 cycles of 2% strain, demonstrating improved durability over current OLEDs.

“We anticipate the success of this approach to designing flexible, high-efficiency optoelectronic devices will enable the next generation of wearable and deformable displays,” said Teng Zhang, PhD, a co-author who was a post-doctoral researcher in Gogotsi’s lab. “This technology will play an important role in real-time health care monitoring and wearable communications technology.

Future research could entail testing different flexible substrate materials, tailoring organic layers for producing different colors and light intensity and streamlining the OLED production process.

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