Polaritons offer the best of two very different worlds. These hybrid particles combine light and molecules of organic material, making them ideal energy transfer vessels in organic semiconductors. Both are compatible with modern electronics, but also move fast, thanks to their photonic origin.

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

A project led by Andrew Musser, an assistant professor of chemistry and chemical biology at Cornell University’s College of Arts and Sciences, has found a way to adjust the speed of this energy flow. This “choke” can move polaritons from near-stagnation to something that approaches the speed of light and increase their range – an approach that could eventually lead to more efficient solar cells, sensors and LEDs.

The team’s paper, “Adjusting the Coherent Distribution of Organic Polariton Excitons by Delocation of the Dark State,” was published April 27, 2022 in the journal Advanced science. The lead author is Raj Pandya of the University of Cambridge.

For the past few years, Musser and colleagues at the University of Sheffield have been researching a method of creating polaritons using small sandwich structures of mirrors called microcavities that capture light and force it to interact with excitons, moving energy bundles made up of a connected electron pair. -hole.

They previously shown how microcavities can save organic semiconductors from “dark states” in which they do not emit light, with consequences for improved organic LEDs.

For the new project, the team uses a series of laser pulses that function as an ultrafast video camera to measure in real time how energy moves in the structures of the microcavity. But the team came at its own speed. Polaritons are so complex that even interpreting such measurements can be a difficult process.

“What we found was completely unexpected. We sat on the data for a good two years and thought about what it all meant, “said Musser, a senior contributor to the paper.

Eventually, the researchers realized that by turning on more mirrors and increasing the reflectivity in the microcavity resonator, they were able to practically turbocharge the polaritons.

“The way we changed the speed of these particles is still unprecedented in the literature,” he said. “But now we have not only confirmed that placing materials in these structures can make countries move much faster and much further, but we have a lever that actually controls how fast they move. This gives us a very clear roadmap now on how to try to improve them. “

In typical organic materials, elementary excitations range from 10 nanometers per nanosecond, which is roughly equivalent to the speed of world champion sprinter Usain Bolt, according to Musser.

This may be fast for humans, he noted, but it is actually a rather slow process on a nanoscale.

The microcavity approach, in contrast, fires polaritons a hundred thousand times faster – speeds on the order of 1% of the speed of light. While transport is short – instead of less than a nanosecond, it is less than a picosecond or about 1000 times shorter – polaritons move 50 times farther.

“Absolute speed is not necessarily important,” Muser said. “Distance is more useful. So if hundreds of nanometers can travel when you miniature the device – say, with terminals that are 10 nanometers away – that means they will go from A to B with zero loss. And that’s really what this is about. “

This brings physicists, chemists and materials scientists closer to their goal of creating new, efficient structures for next-generation devices and electronics that are not blocked by overheating.

“Many technologies that use excitons instead of electrons only work at cryogenic temperatures,” Musser said. “But with organic semiconductors, you can start to achieve very interesting, exciting functionality at room temperature. So these same phenomena can be powered even by new types of lasers, quantum simulators or computers. There are many applications for these polariton particles, if we can understand them better. “

Reference: “Tuning the Coherent Distribution of Organic Polariton Excitons by Delocalization of the Dark State” by Raj Pandya, Arjun Ashoka, Kyriakos Georgiou, Juyung Sung, Rahul Jayaprakash, Scott Renken, Liji Gai, Zhen Shen, Akshai Rao and Andrzej Rao. April 27, 2022, Advanced science.
DOI: 10.1002 / advs.202105569

Co-authors include Scott Renken, MS ’21 from Musser Group; and researchers from the University of Cambridge, the University of Sheffield and the University of Nanjing.

The study was supported by the Research Council for Engineering and Physical Sciences in the United Kingdom, the University of Cambridge and the US Department of Energy.

Light-Infused Hybrid Particles Speed Energy Transfer in Organic Semiconductors

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