Demonstrating a strategy that could form the basis for a new class of electronic devices with uniquely tunable properties, researchers at Kyushu University were able to widely vary the emission color and efficiency of organic light-emitting diodes based on exciplexes(*1) simply by changing the distance between key molecules in the devices by a few nanometers. This new way to control electrical properties by slightly changing the device thickness instead of the materials could lead to new kinds of organic electronic devices with switching behavior or light emission that reacts to external factors. Furthermore, this is the first experimental evidence that such exciplexes can form between two molecules separated by up to 10 nm.
The paper reporting these results was published online February 26, 2016, in the journal Science Advances. This research is ongoing under the Adachi Molecular Exciton Engineering Project funded by the Exploratory Research for Advanced Technology (ERATO) program of Japan Science and Technology Agency (JST).
Background
Organic electronic devices such as organic light-emitting diodes (OLEDs) and organic solar cells use thin films of organic molecules for the electrically active materials, making flexible and low-cost devices possible. A key factor determining the properties of organic devices is the behavior of packets of electrical energy called excitons(*2). An exciton consists of a negative electron attracted to a positive hole, which can be thought of as a missing electron. In OLEDs, the energy in these excitons is released as light when the electron loses energy and fills the vacancy of the hole. Varying the exciton energy, for example, will change the emission color. However, excitons are commonly localized on a single organic molecule and tightly bound with binding energies of about 0.5 eV. Thus, entirely new molecules must usually be designed and synthesized to obtain different properties from these Frenkel-type excitons(*3), such as red, green, or blue emission for displays.
Results
Researchers at Kyushu University’s Center for Organic Photonics and Electronics Research (OPERA) instead focused on a different type of exciton called an exciplex, which is formed by a hole and electron located on two different molecules instead of the same molecule. By manipulating the molecular distance between the electron-donating molecule (donor) and the electron-accepting molecule (acceptor) that carry the exciplex’s hole and electron, respectively, the researchers could modify the properties of these weakly bound excitons.
“What we did is similar to placing sheets of paper between a magnet and a refrigerator,” said Associate Professor Hajime Nakanotani, lead author of the paper reporting these results published online February 26, 2016, in the journal Science Advances. “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.”
Indeed, the changes can be large: by inserting a spacer layer with a thickness of only 5 nm between a donor layer and an acceptor layer in an OLED, the emission color shifted from orange to yellowish green and the light emission efficiency increased 700%. For this to work, the organic molecule used for the spacer layer must have an excitation energy higher than those of the donor and acceptor, but such materials are already widely available. While the molecular distance is currently determined by the thickness of the vacuum-deposited(*4) spacer layer, the researchers are now looking into other ways 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.”
In addition, the researchers found that the exciplexes were still formed when the spacer was 10 nm thick, which is 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.”
Future outlook
Researchers at the Adachi and Nakanotani laboratory are continuing to study the underlying physics of the processes and to develop new materials with the goal of creating new types of exciton-based organic electronics that operate in ways not possible with conventional devices.
Definitions
- 1) Exciplex:
- The excited state formed between an electron on an acceptor molecule and a hole on a separate donor molecule. Light emission occurs when the exciplex returns to the ground state through a process involving the electron releasing its energy and transferring to the donor molecule to fill the vacancy represented by the hole.
- 2) Exciton:
- The energetic state formed between a bound electron and hole pair located on organic molecules. Excitons directly affect the electrical and optical properties of devices using organic semiconductors. For example, light emission from organic molecules is the process when an exciton returns to the ground state by releasing its energy as light.
- 3) Frenkel-type exciton:
- A type of exciton with the hole and electron confined to the same molecule because of a strong Coulomb attraction.
- 4) Vacuum deposition:
- A deposition technique in which source material is placed in a chamber under vacuum and heated so that it sublimes and slowly deposits on a surface or substrate located opposite to the source.
Figures
- A: Chemical structures and energy levels of the organic semiconductor materials used in this study along with the basic device structure used to control the exciplex energy.
- B: Dependence of the emission spectrum of the fabricated OLEDs on the spacer layer thickness. The emission color shifts from orange to yellowish green as the thickness of the spacer layer is increased.
- C: External electroluminescence efficiency characteristics of OLEDs with several different spacer layer thicknesses showing a dramatic increase in efficiency as thickness is increased to 5 nm.
- D: Excitation lifetime as a function of spacer layer thickness.
Contact
[Research-related inquiries]
Chihaya Adachi, Center Director and Hajime Nakanotani, Associate Professor
Center for Organic Photonics and Electronics Research (OPERA)
Tel: +81-92-802-6920 Fax: +81-92-802-6921
E-mail: 
,
[JST-related inquiries]
Hisao Mizuta
Department of Research Project, JST
Tel: +81-3-3512-3528 Fax: +81-3-3222-2068
E-mail: 