New 'Fluorescent' Material Has Almost 100% Internal Quantum Efficiency
The Center for Organic Photonics and Electronics Research (OPERA) of Kyushu University announced that it has developed a new OLED light-emitting material that is a fluorescent material and has an internal quantum efficiency of almost 100%.
Thus far, phosphorescent materials using rare metals have been the only materials with high internal quantum efficiencies. But the new material does not use a rare metal.
OPERA named the material "Hyperfluorescence" and announced its details on Nature magazine.
"It eliminates the need for phosphorescent materials," said Chihaya Adachi, professor at the university and project leader of OPERA.
OLED light-emitting materials can be classified into fluorescent and phosphorescent materials by the difference of light emission principle. Fluorescent materials are recombined (emit light) only when excitors go through a spin state called "singlet state." On the other hand, phosphorescent materials emit light not only when they are in the singlet state but also when they are in a spin state called "triplet state."
Because the ratio of the occurrences of the singlet and triplet states is 1:3, it has been considered that the maximum internal quantum efficiency of fluorescent materials is 25% and that of phosphorescent materials is 100%. The energy of fluorescent material's excitors in the triplet state is not used for light emission, and most of it is lost as heat.
This leads to the difference in the luminous efficiency of OLED device. Therefore, phosphorescent materials are more and more employed in the development of OLED displays and OLED lighting apparatuses. OLED devices whose luminous efficiencies are higher than 50lm/W are realized with phosphorescent materials expect for blue light-emitting materials.
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However, there are some problems with phosphorescent materials. First, they are expensive because they contain rare metals. Second, Universal Display Corp (UDC) owns the basic patents related to phosphorescent materials, and it is necessary to negotiate with UDC before using them. Third, the life of blue light-emitting phosphorescent materials is so short that most of them cannot be used for practical purposes.
On the other hand, some fluorescent materials having an internal quantum efficiency higher than 25% have been found. Adachi Lab of OPERA took notice of this trend, called one of their light emission principles "thermally activated delayed fluorescence (TADF)" and started a research on the design of a material that increases the luminous efficiency of TADF.
TADF emits light only when excitors go through the singlet state. In this sense, it is a fluorescent material. But the excitors in the triplet state are "excited" to the singlet state by heat. Therefore, every excitor has a possibility of contributing to light emission.
This time, Adachi Lab developed materials whose internal quantum efficiencies are 90% or higher by using the TADF principle. They are low-molecular materials consisting of five to nine benzene rings and do not require any rare metal or rare earth material.
In addition, the lab prototyped an OLED device and displays by using the low-molecular materials. The external quantum efficiency of the device is 19% or higher, which is equivalent to the external quantum efficiency of a device using a phosphorescent material. At this point, a green light-emitting material has the highest efficiency, but Adachi said, "We will be able to emit lights of most colors including deep blue."