Jul 30 2019
Scientists at Kyushu University in Japan have successfully created micrometer-thick organic light-emitting diodes by integrating thick layers of hybrid perovskite with thin organic layers.
A test organic light-emitting diode (OLED) incorporating thick layers of hybrid perovskite emits green light. (Image credit: William J. Potscavage Jr., Kyushu University)
These innovative devices can possibly enhance the viewing angles and affordability of high-performance televisions and displays in the coming days.
Layers of organic molecules are used by organic light-emitting diodes, or OLEDs, to efficiently change electricity into light. Although the molecules are excellent emitters, they are usually poor conductors of electric current.
Hence, thin—as in 100 nm, or approximately 1/500 the thickness of a single strand of human hair—became the name of the game. The use of such thin layers is essential to allow electricity to easily reach where emission takes place in the center of the devices.
Although very thin layers will require just a tiny amount of material, the application of such thin films is known to complicate the consistent development of an unlimited number of pixels. This is because very minute defects can lead to device failure.
In addition, the light reflected between the back and front of the thin layers usually leads to interactions, known as cavity effects. The emission color at huge viewing angles is slightly distorted by these interactions.
As a result, it has been difficult to develop thicker devices and simultaneously prevent the disadvantages of organics. In order to accomplish this feat, Kyushu University researchers used an alternative group of materials known as perovskites, which are characterized by their evident 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.
Toshinori Matsushima, Study Lead Researcher and Associate Professor, International Institute for Carbon-Neutral Energy Research, Kyushu University
The study results have been reported in Nature.
Matsushima continued, “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.”
The scientists closely packed an emitting layer of molecules within their devices. These molecules are often utilized in OLEDs between perovskite layers that have an overall thickness of 2,000 nm. The devices, thus obtained, possess active layers that are as much as 10-fold thicker than standard OLED devices—although still a fraction of the thickness of a human hair.
The efficiencies of these thick devices were analogous to those found in reference thin OLEDs. In addition, these devices have the same color from each viewing angle. By contrast, OLEDs predicated on thick organic layers did not produce any light at analogous 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,” stated Professor Chihaya Adachi, director of the Center for Organic Photonics and Electronics Research at Kyushu University.
Scientists have also been trying to apply perovskites directly as light emitters, but to date, the lifespans of these devices have been very short.
By maintaining the emission process in the organic materials and utilizing perovskites simply for transmitting electricity, the Kyushu University research team accomplished comparable lifetimes for both reference OLEDs and thick devices.
“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,” predicted Adachi.