Manipulation of Exciplexes Paves Way for Novel Electronics with Tunable Properties

Researchers at Kyushu University have formulated a new approach that could be the foundation of a whole new range of electronic devices possessing exceptionally tunable properties. Using this approach, the team was able to extensively vary the efficiency and emission color of organic light-emitting diodes based on exciplexes merely by modifying the distance by a few nanometers between key molecules in the devices.

This schematic shows the basic structure of an exciplex-based OLED with emission color and efficiency that can be controlled simply by changing the spacer thickness. An exciplex forms when a hole in the highest occupied molecular orbital (HOMO) of a donor molecule is attracted to an electron in the lowest unoccupied molecular orbital (LUMO) of an acceptor molecule. Light is emitted when the electron releases energy as light and transfers across the spacer layer to the donor molecule, thus replacing the missing electron represented by the hole. The thickness of the spacer can be used to modify the attraction between the hole and electron and tune the exciplex energy. Without changing any of the molecules, emission color could be varied from orange to yellowish green and the efficiency enhanced eight fold by increasing the spacer thickness to 5nm. This device was first reported in the paper titled 'Long-range coupling of electron-hole pairs in spatially separated organic donor-acceptor layers' written by H. Nakanotani at Kyushu University's Center for Organic Photonics and Electronics Research (OPERA) in Fukuoka, Japan, and colleagues and published online Feb. 26, 2016 in the journal Science Advances. (Photo credit: Hajime Nakanotani and William John Potscavage Jr.)

This novel electrical property-controlling method could pave the way for a unique range of organic electronic devices having switching actions or light emission, which responds to external factors.

Organic electronic devices such as organic solar cells and OLEDs utilize thin films of organic molecules for materials that are electrically active, thus making the devices as versatile and economical as possible. The behavior of electrical energy packets known as excitons is a main factor that determines the properties of organic devices.

An exciton comprises a negative electron that has affinity towards a positive hole, which can be considered as a missing electron. The energy in these excitons is emitted as light in OLEDs when the electron loses energy and fills the empty hole. For instance, altering the exciton energy will alter the emission color.

Conversely, excitons are normally localized on a single organic molecule and firmly bound with binding energies of approximately 0.5eV. Therefore, completely new molecules will have to be typically designed and synthesized to acquire varied properties from these Frenkel-type excitons, such as green, blue or red emission for displays.

Scientists at Kyushu University's Center for Organic Photonics and Electronics Research (OPERA) as an alternative studied a different type of exciton known as an exciplex, which is formed by an electron and hole situated on two diverse molecules rather than the same molecule.

The team was able to alter the properties of the poorly bound excitons, by exploiting the molecular distance between the electron-donating molecule termed as donor and the electron-accepting molecule termed as acceptor, which carry the hole and electron of the exciplex, respectively,

What we did is similar to placing sheets of paper between a magnet and a refrigerator. The results of the research were published online on 26th February, 2016, in the Science Advances journal.

Hajime Nakanotani, Associate Professor - Kyushu University

By increasing the thickness of an extremely thin layer of organic molecules inserted as a spacer between the donor and acceptor, we could reduce the attraction between the hole and electron in the exciplex and thereby greatly influence the exciplex's energy, lifetime, and emission color and efficiency.

Hajime Nakanotani, Associate Professor - Kyushu University

Definitely, the changes will be huge. The emission color changed from orange to yellowish green and the efficiency of light emission increased 700% when a spacer layer of just 5nm thickness was inserted between an acceptor layer and a donor layer in an OLED. To ensure that this function appropriately, the organic molecule utilized for the spacer layer has to possess excitation energy that is higher than those of the acceptor and the donor. These types of materials are however easily available.

At the moment, the thickness of the vacuum-deposited spacer layer determines the molecular distance, but researchers want to find new methods to control the distance.

"This gives us a powerful way to greatly vary device properties without redesigning or changing any of the materials," said Professor Chihaya Adachi, director of OPERA. "In the future, we envision new types of exciton-based devices that respond to external forces like pressure to control the distance and electrical behavior."

Furthermore, the team discovered that the exciplexes could still be formed when the spacer measured 10nm in thickness, which is considered long on a molecular scale.

"This is some of the first evidence that electrons and holes could still interact like this across such a long distance," commented Professor Adachi, "so this structure may also be a useful tool for studying and understanding the physics of excitons to design better OLEDs and organic solar cells in the future."

"From both scientific and applications standpoints, we are excited to see where this new path for exciton engineering takes us and hope to establish a new category of exciton-based electronics."

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