Interest in thermally activated delayed fluorescence (TADF) and hyper fluorescence, which are referred to as the next generation of luminescence materials for an organic light-emitting diode, is increasing.
Recently the display industry has been expanding the market to include large-sized OLEDs in earnest, following small- and medium-sized OLEDs. This is why liquid-crystal display(LCD)-related companies and customers are constantly comparatively evaluating the differences between LDCs and OLEDs.
One of the biggest problems and most contentious issues is Burn-in or pixel deterioration (Image Sticking). These are a phenomenon in which a mark remains on part of the screen, when still frames or images are left on at the same position for a long time.
OLEDs emit light with organic materials. Therefore, as time passes by, the deterioration of organic materials decreases the lifespan of the screen and causes the burn-in phenomenon. In particular, it mainly appears in images using blue(B) color with the lowest efficiency out of the three primary colors, red (R), green (G), and blue (B).
The reason why blue has the lowest lifespan and efficiency is because it uses fluorescent materials. Fluorescent materials have an internal efficiency of 25%, which is inevitably lower in efficiency and lifespan compared to phosphorescent materials with an internal efficiency of 100% used for red and green. Several companies have invested a lot of time and money to develop blue phosphorescent materials, but they haven’t come up with any notable results.
TADF is the technology that Samsung Display and LG Display have kept their eyes on. It is a concept of the singlet energy level of the exciton inversely transitions in the triplet and emits the light. This technology enables the internal efficiency of fluorescent materials to rise from 25% to 100%. Since TADF can theoretically acquire 100% internal efficiency, the same as phosphorescent materials by using its fluorescent materials, it’s considered the best alternative to improve the lifespan and efficiency of fluorescent materials. TADF drew great interest when it was first applied to OLED by Professor Adachi’s team of Japan in 2012.
TADF has mainly been developed as a fluorescent blue material with the least lifetime and efficiency. Development is on-going while the high efficiency is prioritized, compared to conventional fluorescent blue. However, the problem is that it is difficult to increase the color purity because the full width at half maximum (FWHM) is large compared to phosphorescent material.
Recently, the display flow has been following BT2020 color coordinates, and dark blue is essential for implementing them. If the FWHM is large, it’s difficult to implement it. To resolve this issue, a new acceptor such as a boron type has been introduced and developed to reduce FWHM of the TADF’s blue. In addition, Hyper fluorescent materials are being developed that add a fluorescent dopant to TADF.
The hyper fluorescence has a benefit that improves the stability of materials, efficiency and color purity by the structure in which the exciton is transmitted from the TADF to the fluorescent dopant to emit light.
Cynora of Germany and Kyulux of Japan have been actively developing these next generation of luminescence material technology.
Cynora was founded in Germany in 2008 and has focused on developing a blue TADF. From 2015, they started revealing the development results to the public in earnest. In 2017, it received an investment of about 15 million Euros (about 20 billion KRW) from LG Display and about 10 million Euros (about 13 billion KRW) from Samsung Venture Investment.
Kyulux started in 2012 with a team selected by Professor Adachi from Kyushu University in Japan. It was spun off from Kyushu University in 2015 and was established as a specialized OLED material company. TADF and hyper fluorescent materials are being developed at the same time. In 2016, Kyulux received an investment of 1.5 billion Yen (about 16 billion KRW) from LG Display and Samsung Venture Investment. In 2018, they signed a joint development agreement (JDA) with LG Display and Samsung Display.
At the ‘2018 International Meeting on Information Display’ recently hosted in Busan, Cynora was found to achieve the efficiency(EQE) of TADF deep-blue color materials of the TADF color coordinate CIEy 0.13 at 20%(@1000nit) and the 15 hours(LT97@700nit) of its lifespan.
It is hard to make an exact comparison because the two companies seperately have the development results and measurement conditions of the fluorescent blue material applied to mass-production. However, compared to the generally known factor that the efficiency of fluorescent blue material at about 7~10%, blue TADF and fluorescent blue material contains an efficiency more than twice as high as that of the conventional fluorescent blue.
The most important part is to ensure the luminescent efficiency and the lifespan as well. Currently, the lifespan required for a blue luminescent material is known to be about LT97 100 hours or more. Materials that have yet to be announced do not meet these requirements and need further improvement.
However, development of TADF materials is accelerating due to investment and joint development of various OLED-related companies including Samsung Display and LG Display. It is expected that blue TADF and blue hyper fluorescence will become the next generation OLED luminescent materials.