Researchers in the University of Toronto's Department of Materials Science & Engineering have developed what they say are the world’s most efficient organic light-emitting diodes (OLEDs) on plastic. This result enables a flexible form factor as well as a less expensive alternative to traditional OLED manufacturing.

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The flexible OLEDs on plastic as built by the researchers in the Department of Materials Science & Engineering at the University of Toronto.
Current state-of-the-art OLEDs are produced using heavymetal doped glass in order to achieve high efficiency and brightness, which makes them expensive to manufacture, heavy, rigid, and fragile. Using plastic can substantially reduce the cost of production, while providing designers with a more durable and flexible material to use in their products.

Wang and Helander were able to reconstruct the high-refractive index property previously limited to heavy metal-doped glass by using a 50-100-nanometer-thick layer of Ta2O5, an advanced optical thinfilm coating material. This coating technique, when applied on flexible plastic, allowed the team to build the highest-efficiency OLED device reported with a glass-free design.
“For years, the biggest excitement behind OLED technologies has been the potential to effectively produce them on flexible plastic,” says Materials Science & Engineering Professor Zheng-Hong Lu, who supervised the research. The research was led by Ph.D. candidates Zhibin Wang and Michael G. Helander.

Typical high-efficiency OLEDs require exotic high-refractive- index (n ≥ 1.8) substrates to enhance the outcoupling of trapped light in the device. Flexible plastic substrates unfortunately have a low refractive index (n ≤ 1.6). To unlock the full potential of OLEDs on flexible plastic, the researchers have reported high-efficiency phosphorescent OLEDs using a thinfilm outcoupling enhancement method that does not depend on high-index substrates.

In these devices, multifunctional anode stacks — consisting of a high-index tantalum (V) oxide (Ta2O5) optical coupling layer, electrically conductive gold layer, and hole-injection molybdenum trioxide (MoO3) layer — are collectively optimized to achieve high efficiency. The maximum external quantum efficiency reaches 63% for green, which remains as high as 60% at > 10,000 cd/m-2.

The performance of their device is comparable with the best glass-based OLEDs, while providing the benefits offered by using plastic.

“This discovery unlocks the full potential of OLEDs, leading the way to energy-efficient, flexible, and impact-resistant displays,” says Lu.

For more information, visit www.utoronto.ca.