Kyushu University: Small Molecules, Giant (Surface) Potential

In a molecular feat akin to getting pedestrians in a crosswalk to spontaneously start stepping, Kyushu University researchers have created a set of molecules that tend to point in one direction when they evaporate on the surface to create a “giant surface potential.” to form

The researchers hope to use this approach to generate controlled electric fields that will help improve the efficiency of organic light-emitting diodes used in displays and lighting, and open new avenues for realizing devices that convert vibrations into electricity with organic materials. open

Based on carbon’s extraordinary chemical versatility, which enables life, organic electronics are currently driving a wave of vibrant and even flexible smartphone and television displays, with applications in solar cells, lasers, and It guides the orbits on the horizon.

This flexibility is due in part to the disordered nature of the thin layers of materials used in the devices. Unlike conventional inorganic electronics based on silicon atoms bound together in rigid, ordered crystals, organic materials typically form “amorphous” layers that are not nearly as well organized.

Despite the seemingly random organization of molecules, researchers have found that some of them actually tend to align in similar directions, which profoundly affects a device’s properties and opens up new possibilities for controlling device performance.

Masaki Tanaka, an assistant professor at Tokyo University of Agriculture and Technology (TUAT) who initiated the current work, said: “There has been considerable work done on molecules that are aligned in such a way that the light they emit can escape the device more easily. slow at Kyushu University’s Photonics and Organic Electronics Research Center (OPERA) and continued to further study molecular alignment in amorphous films after his transfer to TUAT.

However, other molecules align in such a way that they place more electrons on one side of the layer, resulting in a so-called surface potential with an electric field. The field can help move charges into or out of a device to make it more efficient or unlock new electrical properties, but finding ways to control the formation of the field has been a challenge.

The layers used in organic electronics are typically only tens of nanometers thick—a fraction of the thickness of a human hair—and are often made gradually by heating an organic powder in a vacuum to convert it directly from a solid to a gas. A process known as sublimation. When the sublimated powder molecules reach the cool surface, they stick together to form a layer.

“In the gas phase, the molecules are randomly spinning and bumping into each other, so they’re likely to be deposited on a film in a random direction,” explains Morgan Afrey, who synthesized the molecules. However, we found that certain molecular units with fluorine atoms essentially move away from the surface of the deposit. By incorporating these units into a molecule, we can align the deposited molecules roughly with the fluorinated units facing outwards.

The researchers then attached pieces that move negatively charged electrons toward or away from the fluorinated unit. This imbalance of charges across aligned molecules on a surface leads to the so-called surface potential and resulting electric field.

“Because the deposited molecules and their associated electric fields are aligned in the same direction, the small individual fields add up to produce a much larger overall field,” says Tanaka. “Not only can we have a relatively larger field, but we can direct it to the surface, something that has rarely been reported before.”

These layers produce a gigantic surface potential of nearly 10 V, which is especially impressive when we consider that it was spontaneously produced by a 100 nm thick film.

Such a large voltage in such a small thickness produces a high electric field that can help introduce positive and negative charges into different layers of devices such as OLEDs, thus improving the overall power conversion efficiency.

Moreover, these controlled and internal electrical structures can help in the construction of new devices. Researchers have already shown that these layers could be used in a new type of device that converts vibrations into electricity, but more work remains to make such devices practical.

“Science is showing us new ways to control electrical processes at smaller and smaller scales by arranging atoms in organic molecules,” says Chihaya Adachi, director of OPERA. “This research adds to our toolkit, which will enable new devices as we continue to grow.

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