A test OLED incorporating thick layers of hybrid perovskite emits green light. Photo: William J. Potscavage Jr., Kyushu University.
A test OLED incorporating thick layers of hybrid perovskite emits green light. Photo: William J. Potscavage Jr., Kyushu University.

By combining thin organic layers with thick layers of hybrid perovskite, researchers at Kyushu University in Japan have developed micrometer-thick organic light-emitting diodes (OLEDs) that could improve the affordability and viewing angles of high-performance displays and televisions in the near future.

OLEDs use layers of organic molecules to efficiently convert electricity into light. Though great emitters, the molecules are generally poor electrical conductors, so the name of the game has been thin – as in 100nm. Only by using such thin layers can electricity easily reach where emission occurs in the middle of the devices.

While extremely thin layers benefit from needing only a small amount of material, the use of such thin films complicates the reliable fabrication of millions of pixels, since extremely small defects can cause device failure. Furthermore, light reflecting between the front and back of the thin layers often results in interactions known as cavity effects that slightly distort the emission color at large viewing angles.

Thus, the challenge has been to make the devices thicker while avoiding the drawbacks of organics. To do this, researchers at Kyushu University turned to an alternative class of materials called perovskites, which are defined by their distinct crystal structure.

"Although perovskites have recently attracted a huge amount of attention as light-absorbing layers in solar cells, some perovskites are actually transparent while also being highly conductive," says Toshinori Matsushima, associate professor of the International Institute for Carbon-Neutral Energy Research at Kyushu University and lead author of a paper in Nature on this work.

"In addition, perovskites based on a blend of organic and inorganic components can be processed from low-cost starting materials using the same fabrication processes as for organics, making perovskites and organics a perfect match."

In their devices, the researchers sandwiched an emitting layer of organic molecules typically used in OLEDs between perovskite layers with a total thickness of 2000nm. The resulting devices have active layers that are 10-times thicker than typical OLEDs – though still a fraction of the width of a human hair.

The thick devices exhibited efficiencies that were similar to those in reference thin OLEDs, while also displaying the same color from every viewing angle. In contrast, OLEDs with thick organic layers did not emit any light at similar operating voltages.

"These results overturn 30 years of thinking that OLEDs are limited to thin films and open new paths for low-cost, reliable and uniform fabrication of OLED-based displays and lighting," says Chihaya Adachi, director of Kyushu University's Center for Organic Photonics and Electronics Research.

While researchers have also been attempting to use perovskites directly as light emitters, the lifetimes of the devices have so far been short. By keeping the emission process in the organic materials and using perovskites just for transporting electricity, the Kyushu team achieved similar lifetimes for both thick devices and reference OLEDs.

"Based on this work, perovskites will be seen in a new light as versatile, high-performance materials for supporting roles in not only OLEDs but also other organic electronic devices, such as lasers, memory devices and sensors," predicts Adachi.

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