Researchers at Kyushu University in Japan have devised a simple way to tweak the properties of organic light-emitting diodes (OLEDs) while increasing emission efficiency eight-fold. The development could lead to organic electronics with stronger performance, that could change their properties in response to external stimuli like pressure.

To function optimally, the active materials inside an electronic device must be tuned to a high degree of precision. Organic electronics such as OLEDs and organic solar cells hold great promise for cheap, flexible and transparent electronics, but they are also proving difficult to tweak as needed, and this drawback has stunted their development.

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A material's electrical properties partly derive from the way in which pairs of charged particles – negative electrons and positive "electron holes" – come together. In most semiconductors, these pairs exist as separate entities; but inside an organic material, they become embedded within a single organic molecule, making them much harder to tune and making organic electronics harder to work on. As an example, in the case of OLEDs, three different organic molecules must be engineered with the specific aim of emitting red, green, and blue light.

Now, researchers led by Prof. Hajime Nakanotani have found that they can significantly change the electrical properties of an OLED device without changing any of its materials, just by introducing a small spacer between two key layers of semiconductor.

The layer separates the portion of the organic semiconductor with excess negative charges (donor layer) from the portion with excess positive charges (acceptor layer) and its thickness controls how the two interact. Rather than taking hold within a single molecule, the opposing charges now remain housed in two separate molecules on either side of the spacer, forming what is known as an "exciplex."

"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," says Nakanotani.

A 5-nanometer spacer shifted the OLED's emission color from orange to yellowish-green and increased the light emission efficiency by nearly eight times.

The scientists have also suggested that they could use a 10-nanometer layer (a significant thickness on the molecular scale) to further study how excitons behave and design even better organic electronics in the future.

To Nakanotani and colleagues, this advance is also a stepping stone toward a new type of organic devices whose properties can be controlled through external forces like pressure, rather than the thickness of the spacer layer.

A study detailing these findings appeared in a recent edition of the journal Science Advances.

Source: Kyushu University

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