Changyeong Jeong handles the new OLED that employs an ultrathin silver film as an electrode. Image: Robert Coelius/University of Michigan Engineering, Communications & Marketing.
Changyeong Jeong handles the new OLED that employs an ultrathin silver film as an electrode. Image: Robert Coelius/University of Michigan Engineering, Communications & Marketing.

A new electrode that could free up to 20% more light from organic light-emitting diodes (OLEDs) has been developed by researchers at the University of Michigan. This new electrode could help extend the battery life of smartphones and laptops, or make next-generation televisions and displays much more energy efficient.

It works by preventing light from being trapped in the light-emitting part of an OLED, allowing OLEDs to maintain their brightness while using less power. In addition, the electrode is easy to fit into existing processes for making OLED displays and light fixtures.

"With our approach, you can do it all in the same vacuum chamber," said L. Jay Guo, professor of electrical and computer engineering at the University of Michigan and corresponding author of a paper on this work in Science Advances.

Unless engineers take action, about 80% of the light produced by an OLED gets trapped inside the device, due to an effect known as waveguiding. Essentially, the light rays that don't come out of the device at an angle close to perpendicular get reflected back and guided sideways through the device. They end up lost inside the OLED.

A good portion of this lost light is simply trapped between the two electrodes on either side of the light-emitter. One of the biggest offenders is the transparent electrode that stands between the light-emitting material and the glass, typically made of indium tin oxide (ITO). In a lab device, trapped light can be seen shooting out the sides rather than traveling through to the viewer.

"Untreated, it is the strongest waveguiding layer in the OLED," Guo said. "We want to address the root cause of the problem."

By swapping out the ITO for a transparent layer of silver just 5nm thick, deposited on a seed layer of copper, Guo's team maintained the electrode function while eliminating the waveguiding problem in the OLED layers altogether.

"Industry may be able to liberate more than 40% of the light, in part by trading the conventional indium tin oxide electrodes for our nanoscale layer of transparent silver," said Changyeong Jeong, first author and a PhD candidate in electrical and computer engineering.

This benefit is tricky to see, though, in a relatively simple lab device. Even though light is no longer guided in the OLED stack, that freed-up light can still be reflected from the glass. In industry, engineers have ways of reducing that reflection – by creating bumps on the glass surface, or adding grid patterns or particles that will scatter the light throughout the glass.

"Some researchers were able to free up about 34% of the light by using unconventional materials with special emission directions or patterning structures," Jeong said.

In order to prove that they had eliminated the waveguiding in the light-emitter, Guo's team had to stop light being trapped by the glass as well. They did this with an experimental set-up that employs a liquid with the same index of refraction as glass, so-called index-matching fluid – an oil in this case. That 'index-matching' prevents the reflection that happens at the boundary between high-index glass and low-index air.

Once the researchers had done this, they could look at their experimental set-up from the side and see whether any light was coming out sideways. They found that the edge of the light-emitting layer was almost completely dark. In turn, the light coming through the glass was about 20% brighter.

This story is adapted from material from the University of Michigan, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.