In a new approach to improving OLED efficiency, researchers from the Center for Organic Photonics and Electronics Research (OPERA) at Kyushu University in Japan used single-state fission to split the energy of an exciton into two parts. This method has the potential to make OLED exciton production efficiency exceed the 100% limit.
An exciton is an energy packet in an OLED that is formed by the connection of a positive charge and a negative charge on a molecule. An exciton can produce a photon by releasing energy. There are two forms of excitons, singlet and triplet.
Researchers at the Center for Organic Photonics and Electronics Research at Kyushu University in Japan use molecules that accept triplet excitons, where the energy of triplet excitons is half that of singlet excitons, overcoming each pair of charges can only form a radical Child's limit. Singlet excitons can transfer half of the energy to adjacent molecules while retaining half of the energy. This single-state fission process can cause a singlet exciton to produce two triplet excitons and then transfer the triplet excitons to a second type of molecule that uses energy to emit near-infrared (NIR) light.
Professor Hajime Nakanotani said: "In short, we use molecules as a transforming machine for excitons in OLEDs. It is similar to a conversion machine that converts $10 bills into two $5 bills that convert expensive high-energy excitons. Into two half-price low-energy excitons."
The researchers evaluated the efficiency of the single-state fission process by comparing the NIR emission with the trace visible light emission of the remaining singlet excitons when the device was exposed to various magnetic fields.
Through experiments, the researchers confirmed that the triplet state produced by singlet fission emits NIR electroluminescence after the exciton energy is transferred from the dark triplet state to the emissive state, and the total exciton generation efficiency is 100.8%. Researchers believe their research is the first to use single-state fission to improve OLED efficiency, although single-state fission has previously been used in organic solar cells.
The team said that the overall efficiency of using single-state fission remains relatively low, as the near-infrared emission of organic emitters is traditionally inefficient. Nonetheless, this new approach can provide a way to increase OLED efficiency and strength without changing the emitter molecules. To further increase efficiency, researchers are investigating ways to improve the emitter molecules themselves.
With further improvements, the research team hopes to increase exciton generation efficiency to 125%, which is the next limitation for researchers, because electrical operation naturally produces 25% of singlet excitons and 75% of triplet states. child. Once the team achieved this goal, the research team began to study how to convert triplet excitons into singlet excitons to achieve 200% quantum efficiency.
OPERA Director Chihaya Adachi said: "Near-infrared light plays a key role in biology, medical applications, and communication technologies. Now we understand that single-state fission can be used in OLEDs, and there is a new way to overcome the potential for efficient near-infrared OLEDs. The problem, and will soon be practical."
Experiments show that even under electrical excitation, triplet excitons generated by single-state fission are also likely to be electroluminescence, thereby increasing the quantum efficiency of OLEDs. Electroluminescence with single-state fission can provide a way to develop high-intensity NIR sources, which are of particular importance in sensing, optical communication, and medical applications.
The research progress was published in Advanced Materials.
