A Japanese research group led by Chihaya Adachi and Kenichi Goushi, professors at the Center for Organic Photonics and Electronics Research (OPERA) of Kyushu University, has developed a high-efficiency phosphorescent OLED device without using any rare metal.

It has an external quantum efficiency of higher than 5%, as one of the highest in the world for phosphorescent OLED devices that do not contain any rare metal.
 
Because phosphorescent materials can realize an internal quantum efficiency of 100%, they are popular in being using in high-efficiency OLED materials. But due to they contain rare metals, traditional phosphorescent materials are more expensive than normal light-emitting materials. As a result, it’s necessary to develop phosphorescent materials that do not contain rare metals.

Adachi and Goushi have been engaged in the development of a new light-emitting mechanism. In the mechanism, an excited triplet state (T1) is converted to an excited singlet state(S1). But the conversion efficiency needed to be improved.

This time, the research group developed a method to reduce energy gap (ΔEST) from 100meV to 50meV at the time of the conversion from T1 to S1 and achieved a conversion efficiency as high as 86.5%. Electron-donating molecules and electron-accepting molecules with appropriate electron structures are used, and light is emitted due to the electronic transition between the two types of molecules. As a result, the energy gap (ΔEST) was reduced by half.