The OLED technology found in flexible smartphones, curved computer monitors, and modern televisions may eventually be used in wearable devices that sit directly on the skin. These future systems could display real-time information such as changes in temperature, blood flow, or pressure. An international research team led by scientists from Seoul National University in the Republic of Korea and Drexel University has now developed a flexible and stretchable OLED that could move this idea closer to real-world use and unlock new applications.
The research, recently published in Nature, introduces a redesigned OLED that combines a flexible phosphorescent polymer layer with transparent electrodes made from MXene nanomaterial. This approach allows the display to stretch up to 1.6 times its original length while retaining most of its brightness.
"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."
Why OLEDs Lose Performance When Bent
OLEDs generate light through a process known as electroluminescence. When electricity flows through the device, positive and negative charges move between electrodes and pass through an organic polymer layer. When these charges meet, they release light and form a particle called an exciton before settling into a stable electrical state. Adjusting the chemical composition of the organic layer determines the color of the emitted light.
Flexible OLEDs are made by depositing these layers onto bendable plastic substrates, allowing them to function while folded, bent, or rolled. The technology was first developed in the 1990s and became widely visible in the 2010s when Samsung incorporated flexible displays into shatter-resistant devices and curved-edge phones. Over time, however, it became clear that repeated bending caused OLED brightness and flexibility to decline due to gradual damage in the electrodes and organic materials.
"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 conducted early work on transparent conductive MXene films 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 New Light-Emitting Layer
To overcome these challenges, the researchers redesigned the light-emitting portion of the OLED. Their solution uses a specialized organic layer that increases how often electrical charges combine to form excitons, leading to stronger light output.
This material, called an exciplex-assisted phosphorescent (ExciPh) layer, is naturally stretchable and engineered to adjust the energy levels of moving charges. By making it easier for charges to meet and form excitons, the layer boosts light production, similar to slowing a spinning ride so more people can step on safely.
More than 57% of excitons created in the ExciPh layer are converted into light. In comparison, the polymer-based emissive layers commonly used in today's OLEDs achieve only a 12-22% conversation efficiency rate.
To further improve flexibility, the team incorporated a thermoplastic polyurethane elastomer matrix into the ExciPh layer. They also focused on improving how electrical charges move through the device by redesigning the electrodes.
MXene Electrodes Boost Durability and Brightness
The new electrodes combine MXene, a highly conductive two-dimensional nanomaterial developed by Drexel researchers in 2011, with silver nanowires. Together, these materials form a conductive network that helps electrical charges reach the light-emitting polymer layer more efficiently before forming excitons.
This structure improves charge injection and allows the OLED to maintain its brightness even while being bent and stretched.
"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."
Testing OLEDs Under Repeated Strain
Using these combined improvements, the researchers produced flexible green OLED displays, including one shaped like a heart and another showing numerical digits. They measured the charge-to-exciton conversion rate -- a measure of the OLEDs' ability to efficiently produce light -- along with performance during repeated stretching.
To demonstrate broader potential, researchers at Seoul National University also built a full-color, fully stretchable OLED display using four dopant materials within the ExciPh layer. In addition, they created fully stretchable passive-matrix OLEDs that showcase a simple, low-power design suitable for wearable electronics.
Compared with previous designs, the new OLEDs showed higher brightness and better energy efficiency. When stretched to 60% of their maximum strain, performance dropped by only 10.6%. After 100 cycles of repeated stretching at 2% strain, the displays retained 83% of their light output, indicating significantly improved durability.
Toward Wearable and Deformable Displays
"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 and former 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 work may involve testing alternative flexible substrates, fine-tuning organic layers to produce different colors and brightness levels, and simplifying the manufacturing process to support large-scale production of stretchable OLED devices.
