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."