Exciton limits are meant to be broken: OLED surpasses 100 % exciton production efficiency
06 Jul 2018
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Researchers at Kyushu University's Center for Organic Photonics and Electronics Research (OPERA) in Japan have demonstrated a way to split energy in organic light-emitting diodes (OLEDs) and surpass the 100-per cent limit for exciton production, opening a promising new route for creating low-cost and high-intensity near-infrared light sources for sensing and communications applications.
OLEDs use layers of carbon-containing organic molecules to convert electrical charges into light. In normal OLEDs, one positive charge and one negative charge come together on a molecule to form a packet of energy called an exciton. One exciton can release its energy to create at most one beam of light, or photon.
When all charges form excitons that emit light, a maximum 100 per cent internal quantum efficiency is achieved. However, the new technology uses a process called singlet fission to split the energy from an exciton into two, making it possible to exceed the 100 per cent limit for the efficiency of converting charge pairs into excitons, also known as the exciton production efficiency
"Put simply, we incorporated molecules that act as change machines for excitons in OLEDs. Similar to a change machine that converts a $10 bill into two $5 bills, the molecules convert an expensive, high-energy exciton into two half-price, low-energy excitons," explains Hajime Nakanotani, associate professor at Kyushu University and co-author of the paper describing the new results.
Excitons come in two forms, singlets and triplets, and molecules can only receive singlets or triplets with certain energies. The researchers overcame the limit of one exciton per one pair of charges by using molecules that can accept a triplet exciton with an energy that is half the energy of the molecule's singlet exciton.
In such molecules, the singlet can transfer half of its energy to a neighboring molecule while keeping half of the energy for itself, resulting in the creation of two triplets from one singlet. This process is called singlet fission.
The triplet excitons are then transferred to a second type of molecule that uses the energy to emit near-infrared light. In the present work, the researchers were able to convert the charge pairs into 100.8% triplets, indicating that 100% is no longer the limit. This is the first report of an OLED using singlet fission, though it has previously been observed in organic solar cells.
Furthermore, the researchers could easily evaluate the singlet fission efficiency, which is often difficult to estimate, based on comparison of the near-infrared emission and trace amounts of visible emission from remaining singlets when the device is exposed to various magnetic fields.
"Near-infrared light plays a key role in biological and medical applications along with communications technologies," says Chihaya Adachi, director of OPERA. "Now that we know singlet fission can be used in an OLED, we have a new path to potentially overcome the challenge of creating an efficient near-infrared OLED, which would find immediate practical use."
Overall efficiency is still relatively low in this early work because near-infrared emission from organic emitters is traditionally inefficient, and energy efficiency will, of course, always be limited to a maximum 100 per cent.
Nonetheless, this new method offers a way to increase efficiency and intensity without changing the emitter molecule, and the researchers are also looking into improving the emitter molecules themselves.
With further improvements, the researchers hope to get the exciton production efficiency up to 125 per cent, which would be the next limit since electrical operation naturally leads to 25 per cent singlets and 75 per cent triplets. After that, they are considering ideas to convert triplets into singlets and possibly reach a quantum efficiency of 200 per cent.
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