Light-saturated particles travel a distance in organic semiconductors

the particle

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Polaritons offers the best of two very different worlds. These hybrid particles combine light and molecules of organic matter, making them ideal receptacles for energy transfer in organic semiconductors. It is compatible with modern electronics but is also fast moving thanks to its optical origins.

However, they are difficult to control, and much of their behavior is a mystery.

A project led by Andrew Moser, Assistant Professor of Chemistry and chemical biology In the College of Arts and Sciences, a method for adjusting the speed of this energy flow. This “throttle” could move polaritons from a near standstill to something approaching the speed of light and increasing its range – an approach that could eventually lead to more efficient solar cells, sensors and LEDs.

The team’s paper, “Tuneing the Coherent Reproduction of Organic Excitons by Delocalization of the Dark State,” was published April 27 in advanced science. The lead author is Raj Pandya from the University of Cambridge.

Over the past several years, Moser and colleagues at the University of Sheffield have discovered a way to form polaritons via tiny sandwich structures of mirrors, called microgaps, that trap light and force it to interact with excitons — moving energy packets made up of a bound electron-hole pair.

They previously demonstrated how small gaps can rescue organic semiconductors from “dark states” in which they do not emit light, with implications for improved organic LEDs.

For the new project, the team used a series of laser pulses, which act like an ultra-fast video camera, to measure how energy moves in real time within the micro-cavity structures. But the team hit a fast pump on its own. Polaritons are so complex that interpreting such measurements can be a daunting process.

“What we found was completely unexpected,” said Moser, the paper’s senior author. “We’ve sat on the data for a good two years thinking about what it all means.”

Ultimately, the researchers realized that by incorporating more mirrors and increasing the reflectivity into the small-cavity resonator, they were, in effect, able to charge the polaritons.

“The way we’ve been changing the speed of movement of these particles is still basically unprecedented in the literature,” he said. “But now, not only have we confirmed that putting materials into these structures can make countries move much faster and farther, but we have a lever to actually control how fast they can go. And that gives us a very clear roadmap now for how to try to improve it.”

In typical organics, the initial excitations move on the order of 10 nanometers per nanosecond, which is roughly equivalent to the speed of world champion sprinter Usain Bolt, according to Moser.

He noted that this may be fast for humans, but is actually a very slow process at the nanoscale.

By contrast, the micro-cavitation approach releases polaritons a hundred thousand times faster—a speed on the order of 1% of light’s speed. While the transfer is short-lived—instead of taking less than a nanosecond, it’s less than a picosecond, or about 1,000 times shorter—the polaritons move 50 again.

“Absolute speed isn’t necessarily important,” Moser said. “What’s more useful is the distance. So if they can travel hundreds of nanometers, when the device is miniaturized — say, with terminals 10 nanometers apart — that means it will go from A to B without losses. That’s really what it’s about.”

This brings together physicists, chemists, and Materials Scientists It gets closer to their goal of creating new, efficient device structures and next-generation electronics that are not hampered by overheating.

“A lot of technologies that use excitons instead of electrons only work at freezing temperatures,” Moser said. “But with organic semiconductors, you can start to realize a lot of interesting and interesting functions at room temperature. So these same phenomena can fuel new kinds of lasers, quantum simulators, or even computers, and there are a lot of applications for these Polariton particles if we can understand them better.”

Strong coupling of light in organic crystals

more information:
Raj Pandya et al., Tuning the coherent diffusion of organic excitons‐polaritons through dark‐state delocalization, advanced science (2022). DOI: 10.1002 / advs.202105569

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