Very efficient, pure blue organic light emitting diodes (OLEDs) developed by researchers at Kyushu University in Japan are being tested to measure their lifespan. The hyperfluorescence-based OLEDs, which use a two-molecule process to emit light, emit pure blue and exhibit significantly improved service life compared to other highly efficient devices, all without the use of expensive metal atoms. Credit: Masaki Tanaka, Kyushu University
The new approach shows promise in overcoming the bottleneck of blue emission in exhibits with organic light emitting diodes.
Using a new combination of emitter molecules, researchers in Japan have shown the promise of a new approach to finally overcoming a major challenge facing screens with organic light emitting diodes: a blue light source that matches the excellent performance of the red and green.
By dividing energy conversion and emission processes between two molecules, the researchers achieved devices that produce pure blue emission with high efficiency, retain relatively long brightness and have no expensive metal atoms – a set of properties that has been difficult so far obtained simultaneously.
Awarded for their vibrant colors and ability to form thin and even flexible devices, organic light emitting diodes, or OLEDs for short, use carbon-containing molecules to convert electricity into light.
Unlike LCD technologies that use liquid crystals to block the emission of a filtered backlight that covers many pixels, the separate red, green, and blue emission pixels of an OLED screen can be individually turned on and off, resulting in deeper blacks. liver and reduce power consumption.
However, blue OLEDs in particular have been a bottleneck in terms of efficiency and stability.
“There are a growing number of options for red and green OLEDs with excellent performance, but devices that emit blue light with a high energy output are more of a challenge, while compromises almost always occur between efficiency, color purity, cost and longevity. , “said Chin-Yiu Chan, a researcher at Kyushu University’s Center for Organic Photonics and Electronic Research (OPERA) and author of the study, which found the results in Nature Photonics.
While stable blue emitters based on a process known as fluorescence are often used in commercial exhibitions, they have a low maximum efficiency. So-called phosphorescent emitters can achieve an ideal quantum efficiency of 100%, but they usually have a shorter life and require an expensive metal such as iridium or platinum.
Alternatively, OPERA researchers are developing molecules that emit light based on the process of thermally activated retarded fluorescence, usually abbreviated as TADF, which can achieve excellent efficiency without the metal atom but often displays emissions that contain a wider range of colors.
“The range of colors that a screen can produce is directly related to the purity of the red, green and blue pixels,” explains Chihaya Adachi, Director of OPERA. “If blue emission is not pure with a narrow spectrum, filters are needed to improve the purity of the color, but it wastes energy that is radiated.”
Takuji Hatakeyama’s group at Kwansei Gakuin University recently reported a promising way to overcome the purity issue, based on a unique molecular design for an extremely efficient, pure-blue TADF emitter, but the molecule called ν-DABNA , is rapidly degraded.
In collaboration with Hatakeyama, the OPERA researchers have now found that life expectancy can be significantly improved while still achieving close emissions by combining ν-DABNA with an additional TADF molecule developed at OPERA as an intermediate , fast energy converter.
“Three-quarters of the electric charges together form energy states called triplets in OLEDs, and TADF molecules can convert these non-emitting triplets into light-emitting singles,” explains Masaki Tanaka, an OPERA researcher working closely with Chan on the study worked.
‘Ν-DABNA, however, is somewhat slow to convert the high-energy triplet, which often plays a role in deterioration. To get rid of the dangerous triplet faster, we have included an intermediate TADF molecule that can convert triplet faster into single sheets. ‘
Although the intermediate molecule rapidly converts triplets into singles, it has a wide emission spectrum that produces a sky blue emission. Nevertheless, the intermediary can transfer many of its single sheets to ν-DABNA in high energy state for fast and pure blue emission.
“Compared to most emitters, the wavelengths that ν-DABNA can absorb are very close to the color it emits. This unique feature enables it to receive much of the energy of the intermediary with large emissions and still emit a pure blue, ‘says Chan.
Using this two-molecule approach, called hyperfluorescence, the researchers achieved longer operating lifetimes with high brightness than previously reported for highly efficient OLEDs with a similar color purity.
“That this approach can extend the lifetime of pure blue emission of a molecule we previously developed is really exciting,” Hatakeyama says.
The adoption of a tandem structure that basically stacks two devices on top of each other to effectively double the emission for the same electric current has nearly doubled the lifetime at high brightness, and the researchers estimated that devices 50% of their brightness last longer than 10,000 hours could maintain. at moderate intensities.
“While still too short for practical applications, tighter control of manufacturing conditions often leads to even longer lifespans, so these initial results point to a very promising future for this approach to ultimately obtain an efficient and stable pure blue OLED,” says Adachi.
“In the near future, I hope that blue OLEDs with blue hyperfluorescent can replace current blue OLEDs for ultra-high-definition displays,” adds Chan.
Reference: “Stable, Pure Blue, High Efficiency and Narrow Emission LEDs”, by Chin-Yiu Chan, Masaki Tanaka, Yi-Ting Lee, Yiu-Wing Wong, Hajime Nakanotani, Takuji Hatakeyama, and Chihaya Adachi, January 4, 2021, Nature Photonics.
DOI: 10.1038 / s41566-020-00745-z
Funding: Program for Building Regional Innovation Ecosystems from the Ministry of Education, Culture, Sport, Science and Technology