Light-saturated hybrid particles transfer energy in organic semiconductors

Polaritons offer the best of two very different worlds. These hybrid particles combine light and molecules of organic material, making them ideal vessels for energy transfer in organic semiconductors. They are both compatible with modern electronics, but also move quickly due to photonic origins.

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

The project, led by Andrew Musser, an associate 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 “throttle” can move polaritons from near-dead point to something approaching the speed of light, and increase their range – an approach that could eventually lead to more efficient solar cells, sensors and LEDs.

The document of the team “Setting up the coherent propagation of organic exciton-polaritons through the delocalization of the dark state” was published on April 27, 2022 in the journal Advanced science. The main author is Raj Panda from Cambridge University.

Over the past few years, Musser and colleagues at the University of Sheffield have studied the method of creating polaritons using tiny sandwich structures made of mirrors called microrelaves that capture light and cause it to interact with excitons, moving energy beams made of electron-coupled pairs.

They are previously shown how microenvelopes can save 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 that functioned as a high-speed video camera to measure in real time how energy moves inside the structures of the microcavity. But the team got the result. 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, thinking about what it all means, ”said Musser, a senior contributor to the newspaper.

Eventually, the researchers realized that by turning on more mirrors and increasing the reflectivity in a resonator with a microcavity, they were able to essentially 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 the introduction of materials into these structures can make states move much faster and much further, but we have the leverage to really control how fast they go. It gives us a very clear road map of how to try to improve them. ”

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

It may be fast for humans, he noted, but it’s actually a rather slow process on a nanoscale.

The microcavity approach, on the other hand, triggers polaritons a hundred thousand times faster – speeds of the order of 1% of the speed of light. Although transport is short-lived – instead of taking less than a nanosecond, it is less than a picosecond, or about 1,000 times shorter – polaritons move 50 times farther.

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

This brings physicists, chemists and materials scientists closer to their goal of creating new, efficient next-generation device structures and electronics that do not prevent overheating.

“Many technologies that use excitons, not electrons, only work at cryogenic temperatures,” Musser said. “But with organic semiconductors you can start to achieve a lot of interesting, exciting functionality at room temperature. Thus, the same phenomena can be introduced into new types of lasers, quantum simulators or computers. There are many applications for these polariton particles if we can better understand them. ”

Reference: “Tuning the Coherent Distribution of Organic Exiton Polaritons through Darklocation Delocalization” Raj Panda, Arjun Ashoka, Kyriakos Georgiou, Joon Son, Rahul Jayaprakash, Scott Ranken, Lizhi Gai, Zhen Sheng, Endru Rajo. April 27, 2022 Advanced science.
DOI: 10.1002 / advs.202105569

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

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

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