Exciton Movement Observed Directly

For decades, scientists’ understanding of excitons has been theoretical at best. However, a recent breakthrough has changed the game.

A team from MIT and the City College of New York has developed a new technique for imaging exciton movement directly. They say better understanding of the quasiparticle could lead to significant advances in electronics, as well as deeper knowledge about natural energy-transfer processes such as photosynthesis.

A diagram of an exciton within a tetracene crystal, used in these experiments, shows the line across which data was collected. That data, plotted as a function of both position (horizontal axis) and time (vertical axis) provides the most detailed information ever obtained on how excitons move through the material. Courtesy of MIT.

“People always assumed certain behavior of the excitons,” said Parag Deotare, a postdoctoral researcher at MIT. “[Now] we can directly say what kind of behavior the excitons were moving around with.”

The new technique uses optical microscopy and organic compounds to make the energy of excitons visible, allowing the researchers to observe their movements directly. The researchers also used tetracene in their experiments, but agreed that the new technique could be applied to any crystalline or thin-film material.

Until recently, scientists could only determine an exciton’s average speed.

“We really didn't have any information about how they got there,” said Gleb Akselrod, a postdoctoral researcher at MIT, noting that such information is essential to understanding which aspects of a material's structure might facilitate or slow exciton motion.

“The efficiency of devices such as photovoltaics and LEDs depends on how well excitons move within the material,” he said.

In their study, the researchers demonstrated that the nanoscale structure of a material determines how quickly excitons get trapped as they move through it. Maximizing trapping can assist in applications such as LEDs, as it prevents energy leakage. Minimizing it can be helpful in other applications, including solar cells.

“We showed how energy flow is impeded by disorder, which is the defining characteristic of most materials for low-cost solar cells and LEDs,” said MIT professor Marc Baldo.

The work was supported by the U.S. Department of Energy and the National Science Foundation. The research is published in Nature Communications. (doi: 10.1038/ncomms4646)

For more information, visit: www.mit.edu