OLED Surpasses 100 Percent Exciton Production Efficiency

In a new approach to improving OLED efficiency, researchers are using singlet fission to split the energy of one exciton in two. This approach has made it possible for the team to exceed the 100 percent limit for exciton production efficiency in OLEDs.

An exciton is a packet of energy in the OLED that is formed from the joining of one positive charge and one negative charge on a molecule. One exciton can release its energy to create one photon. Excitons come in two forms: singlets and triplets.

Hajime Nakanotani (left), Ryo Nagata (center), and Chihaya Adachi (right) at Kyushu University's Center for Organic Photonics and Electronics Research (OPERA) reported an NIR OLED that uses singlet fission to boost the fraction of excitons created per pair of electrical charges to over 100 percent. Using the pictured electromagnet, the researchers evaluated the efficiency of singlet fission based on changes in the OLED emission with different applied magnetic fields. Courtesy of Ko Inada.

Researchers at Kyushu University's Center for Organic Photonics and Electronics Research (OPERA) 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 one-half of the energy of the molecule’s singlet exciton. The singlet can transfer half of its energy to a neighboring molecule while keeping half of the energy for itself. This process of singlet fission results in the creation of two triplets from one singlet. The triplet excitons are then transferred to a second type of molecule that uses the energy to emit NIR light.

“Put simply, we incorporated molecules that act as change machines for excitons in OLEDs,” said professor Hajime Nakanotani. “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.”

Researchers evaluated the efficiency of the singlet fission process by comparing the NIR emission with trace amounts of visible emission from remaining singlets when the device was exposed to various magnetic fields.

Illustration of the singlet fission process used to boost the number of excitons in an OLED and break 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. Courtesy of William J. Potscavage Jr.

In experiments researchers confirmed that triplets produced by singlet fission were emitted as NIR electroluminescence after excitonic energy transfer from the dark triplet state to an emissive state, leading to NIR electroluminescence with an overall exciton production efficiency of 100.8 percent. Researchers believe their work to be the first report of improving OLED efficiency using singlet fission, although singlet fission has previously been used in organic solar cells.

Overall efficiency using singlet fission is still relatively low, because NIR emission from organic emitters is traditionally inefficient, says the team. Nonetheless, this new approach could offer a way to increase OLED efficiency and intensity without changing the emitter molecule. To further boost efficiency, researchers are investigating ways to improve the emitter molecules themselves.

With further improvements, the team hopes to boost exciton production efficiency to 125 percent — the next limit facing the researchers, since electrical operation naturally leads to 25 percent singlets and 75 percent triplets. Once the team has achieved this goal, it plans to investigate how to convert triplets into singlets to achieve a potential quantum efficiency of 200 percent.

“Near-infrared light plays a key role in biological and medical applications along with communications technologies,” said 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.

The experiments show that the harvesting of triplets produced by singlet fission as electroluminescence is possible even under electrical excitation, leading to an enhancement of the quantum efficiency of the OLEDs. Electroluminescence employing singlet fission could provide a route toward developing high-intensity NIR light sources, which are of particular interest for sensing, optical communications, and medical applications.

The research was published in Advanced Materials (doi:10.1002/adma.201801484).