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Japanese researchers develop new methods to improve OLED efficiency and intensity

Oct 29, 2018

Excitons are energy packets in OLED, formed by a positive charge and a negative charge on the molecule. An exciton can produce a photon by releasing energy. There are two forms of exciton: single state and triplet state.

Researchers at the Center for Organic Photonics and Electronics at Kyushu University in Japan use molecules capable of accepting triplet excitons, of which the energy of triplet excitons is half that of single excitons. The limitation that only one exciton can be formed per pair of charge is overcome. The single state excitons can transfer half of the energy to adjacent molecules while retaining half of the energy. This process of mono-state fission can lead to the production of two triplet excitons by a single state exciton, and then transfer the triplet exciton to a second kind of molecule, which can emit near-infrared (NIR) light using energy.

"in short, we use molecules as excitons in OLED," says Professor Hajime Nakanotani. "similar to a conversion machine that converts $10 bills into two $5 bills, these molecules can convert expensive high-energy excitons into two half-price low-energy excitons." The authors evaluate the efficiency of the single state fission process by comparing the NIR emission with the trace visible emission of the remaining exciton when the device is exposed to various magnetic fields.

Through experiments, the researchers have confirmed that the triplet produced by single state fission emits NIR electroluminescence after the exciton energy is transferred from dark triplet to emission state, and the total exciton production efficiency is 100.8. The researchers believe their work is the first to use mono-state fission to improve OLED efficiency, even though monomorphic fission has previously been used in organic solar cells.

The team said the overall efficiency of using monomorphic fission remains relatively low because of the traditional inefficiency of near-infrared emissions from organic transmitters. Nevertheless, the new method can provide a way to improve the efficiency and strength of OLED without changing the emitter molecule. To further improve efficiency, researchers are looking at ways to improve the emitters themselves.

With further improvement, the team hopes to increase the exciton production efficiency to 125, which is the next limit for the researchers, as electrical operations naturally produce 25 percent of monomorphic excitons and 75 percent of triplet excitons. Once the team has achieved this goal, the team has begun to study how to convert triplet excitons to single-state excitons to achieve a quantum efficiency of 200%.

"Near-infrared light plays a key role in biological, medical applications and communications technologies," said Chihaya Adachi, director of OPERA. Now we have learned that monomorphic fission can be used in OLED, so that there is a new way to overcome the problem of producing high efficiency near infrared OLED, and it can be used in practice soon. "

It is shown that even under electrical excitation, triplet excitons produced by single state fission can be used as electroluminescence, thus improving the quantum efficiency of OLED. The use of single-state fission electroluminescence can provide a way to develop high-intensity NIR light sources, which is of great significance in the fields of sensing, optical communication and medical applications.

The study was published in Advanced Materials.

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