Researchers beat 100% exciton production efficiency for NIR OLEDs

July 12, 2018 // By Julien Happich
In a paper titled "Exploiting singlet fission in organic light-emitting diodes" published in the journal of Advanced Materials, researchers from Kyushu University demonstrate how they can boost near-infrared OLED efficiency.

Harvesting of both triplets and singlets yields electroluminescence quantum efficiencies of nearly 100% in OLEDs, but the production efficiency of excitons that can undergo radiative decay is theoretically limited to 100% of the electron–hole pairs, the paper explains.

The researchers broke this limit by exploiting singlet fission to produce triplets excitons in a rubrene host matrix, emitted as near‐infrared (NIR) electroluminescence by erbium(III) tris(8‐hydroxyquinoline) (ErQ3) after excitonic energy transfer from the “dark” triplet state of rubrene to an “emissive” state of ErQ3.

This singlets to triplets excitons conversion lead to NIR electroluminescence with an overall exciton production efficiency of 100.8%, reports the paper, which indicates new strategies to develop high‐intensity NIR light sources


The singlet fission process used to boost the number of excitons in an OLED breaks the 100 percent limit for exciton production efficiency. The emitting layer consists of a mixture of rubrene molecules, which are responsible for singlet fission, and ErQ3 molecules, which produce the emission. A singlet exciton, which is created when a positive charge and a negative charge combine on a rubrene molecule, can transfer half of its energy to a second rubrene molecule through the process of singlet fission, resulting in two triplet excitons. The triplet excitons then transfer to ErQ3 molecules, and the exciton energy is released as near-infrared emission by ErQ3.
Credit: William J. Potscavage Jr.

Although overall efficiency is still relatively low, the 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%, which would be the next limit since electrical operation naturally leads to 25% singlets and 75% triplets. After that, they are considering ideas to convert triplets into singlets and possibly reach a quantum efficiency of 200%.

Kyushu University - www.kyushu-u.ac.jp


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