New Rare Metal-free Phosphorescent OLED Device Has 5% External Quantum Efficiency

Mar 14, 2012
Naoki Tanaka, Tech-On!
The light-emitting mechanism that converts an excited triplet state (T1) to an excited singlet state(S1)
The light-emitting mechanism that converts an excited triplet state (T1) to an excited singlet state(S1)
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The structures of the two kinds of molecules used for the new OLED device (m-MTDATA and t-Bu-PBD)
The structures of the two kinds of molecules used for the new OLED device (m-MTDATA and t-Bu-PBD)
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The energy levels and emission transitions of the two kinds of molecules
The energy levels and emission transitions of the two kinds of molecules
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Emission spectrum
Emission spectrum
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Japanese researchers developed a high-efficiency phosphorescent OLED device without using any rare metal.

The device was developed by a research group led by Chihaya Adachi and Kenichi Goushi, professors at the Center for Organic Photonics and Electronics Research (OPERA) of Kyushu University.

It has an external quantum efficiency of higher than 5%, which is one of the highest in the world for phosphorescent OLED devices that do not contain any rare metal, according to the university. The latest research results were published in the online edition of Nature Photonics magazine.

Phosphorescent materials are drawing attention as high-efficiency OLED materials because they can realize an internal quantum efficiency of 100%. However, traditional phosphorescent materials are more expensive than normal light-emitting materials because they contain rare metals such as iridium (Ir). Therefore, there are needs for phosphorescent materials that do not contain rare metals.

To realize such materials, 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, the group said.