Unlike conventional high quantum efficiency materials, the newly developed one, called “Hyperfluorescence” does not use rare metals, as announced by Nature magazine last week.
According to the project leader and professor at Kyushu, the material eliminates the need for phosphorescent materials. Being a fluorescent material it emits light only when excitors go through a spin, or singlet, state. In comparison, phosphorescent materials emit light in singlet as well as in the so-called triplet state. Because of this, it is known that the maximum quantum efficiency of the former is 25% and therefore not suitable to use for lightening, while the efficiency of the latter is 100%.
This is the main reason why phosphorescent materials are widely used in high quality OLED displays and lightening apparatuses.
The use of rare metals in these materials, however, makes them very expensive. In addition, they are patented and therefore permission and fee are required prior to their use. And last but not least, blue light-emitting phosphorescent materials have a very short lifespan, making them unusable for practical purposes.
Adachi Lab of OPERA, however, made use of fluorescent materials that were previously found to have internal quantum efficiency higher than 25%. They established light emission principle called thermally activated delayed fluorescence (TDAF) and worked on designing a material to increase its luminous efficiency.
TADF emits light only at singlet state of the excitors, however the emitted heat excites the excitors in the triplet state, making them partly responsible for emitting light. Using this principle, the team of scientists managed to develop materials with more than 90% internal quantum efficiencies, consisting of five to nine benzene rings.
The team also designed a prototype of an OLED device and displays that use these low-molecular materials, with equal efficiency to any phosphorescent OLED device. The green light-emitting material performs best, however Adachi is certain they will soon be able to achieve maximum efficiency for any colour.