Creation of molecules that possess a giant surface potential

Scientists at Kyushu University have constructed a succession of molecules that prefer to head in the same direction when they evaporate off a surface.

This figure shows two molecules spontaneously aligning on a surface to generate controlled electric fields, with red indicating areas of more negative charge and blue indicating areas of less negative charge. Although the common unit in the middle is usually aligned away from the surface, the different surrounding units result in positive or even negative fields at the surface. Image credit: Kyushu University.

Scientists want to use the method to generate controlled electric fields that help the effectiveness of organic light-emitting diodes used in displays and lighting, as well as open new avenues for creating devices that convert vibrations into electricity using organic materials.

Based on the incredible chemical adaptability of carbon that allows living things to exist, organic electronics is already driving a wave of vibrant and potentially flexible smartphone and television screens, with applications such as solar cells, lasers and electronic circuitry. the horizon.

The uneven character of the thin layers of the materials used in the devices contributes to this flexibility. Organic compounds often create “amorphous” shells that are not as well-ordered as standard inorganic electronic components, which rely on silicon atoms securely bound together in hard, well-organized crystals.

Despite the apparent random structure of the molecules, studies have identified that some of them tend to connect in similar directions, dramatically influencing device attributes and opening new avenues for managing device performance.

Significant work has already been done on molecules that line up so that the light they emit can more easily escape from a device..

Masaki Tanaka, Assistant Professor, Tokyo University of Agriculture and Technology

Tanaka began his work at Kyushu University’s Center for Organic Electronics and Photonics Research (OPERA) and resumed his research on molecular alignment in amorphous films after moving on to TUAT.

However, other molecules were known to line up in a way that put more of their electrons on one side of the shell, generating a so-called surface potential accompanied by an electric field. This field can help get 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.Tanaka says.

The films used in organic electronics are typically tens of nanometers thick, a fraction of the thickness of a human hair, and are often slowly built up by first heating an organic powder in a vacuum so that it converts directly from a solid to a gas. process called sublimation. When the sublimated powder molecules meet a cold surface, they stick together to create a layer.

In the gas phase, molecules randomly spin and collide with each other, so they are likely to be deposited in a random direction on a film.”, clarifies Morgan Auffray, who synthesized the molecules.

However, we find that certain molecular units with fluorine atoms will move away from the deposition surface. By including these units in a molecule, we can make the deposited molecules line up roughly, with the fluorinated units facing out.”, says Auffray.

After that, the researchers added components that push and pull negatively charged electrons to and from the fluorinated unit. This voltage difference between paired molecules on a surface produces the surface potential and the accompanying electric field.

Since the deposited molecules and their associated electric fields point in a similar direction, the tiny individual fields add up to produce a much larger overall field. Not only can we get a relatively larger field, but we can make it point towards the surface, which has rarely been reported so far..

Masaki Tanaka, Assistant Professor, Tokyo University of Agriculture and Technology

These layers provide a massive surface potential of about 10 V, which is especially amazing given that it was naturally produced by a film only 100 nm thick.

One of those high voltages within such a thin layer generates a strong electric field, which can help get positive and negative charges across the different layers of electronic components like OLEDs, improving the overall efficiency of power conversion.

Furthermore, these embedded and regulated electrical structures can help in the development of new devices. The layers have already been shown to be useful in a new type of device that converts vibrations into electricity, but more work is needed to make these devices viable.

Science continues to show us new ways to control electrical processes on an ever smaller scale by arranging atoms in organic molecules. This research adds to our tool bag, which will enable new devices as it continues to grow.”, says Chihaya Adachi, director of OPERA.

magazine reference

Tanaka, M. et al. (2022) Spontaneous formation of metastable orientation with well-organized permanent dipole moment in organic glassy films. materials from nature.


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