MIT, CUNY Researchers Observe Energy-Carrying Quasiparticles For The First Time

April 17, 2014

While excitons have been understood in theory for decades, the authors of a new Nature Communications study have directly observed these quasiparticles responsible for energy transfer in solar cells, LEDs and semiconductor circuits for the first time.

According to the MIT and City University of New York (CUNY) scientists behind the research, being able to image the motions of the excitons directly could result in significant advances in electronics. Furthermore, their work could lead to a better understanding of natural energy-transfer processes like photosynthesis.

“This is the first direct observation of exciton diffusion processes, showing that crystal structure can dramatically affect the diffusion process,” MIT professor of emerging technology Vladimir Bulovic explained in a statement on Wednesday.

His colleague and MIT postdoctoral researcher Gleb Akselrod noted that excitons “are at the heart of devices that are relevant to modern technology,” and that these particles determine how energy moves at nanoscale levels. “The efficiency of devices such as photovoltaics and LEDs depends on how well excitons move within the material.”

As it travels through matter, an exciton pairs a negatively-charged electron with a place where an electron had been removed, otherwise known as a hole. While it has a net neutral charge, it is still capable of carrying energy – for example, an incoming photon could strike an electron in a solar cell, moving it to a higher energy level.

The higher energy level is propagated through the material as an exciton. The particles themselves are unmoved, but the elevated energy levels are passed along from one to another. While scientists could previously determine the average speed traveled by moving excitons, Akselrod said that they had no information about how they arrived – information essential to understanding how a material’s structure could speed up or slow down that process.

“People always assumed certain behavior of the excitons,” said Parag Deotare, also an MIT postdoc. However, by using a new technique which combines optical microscopy with the use of organic compounds that can make exciton energy visible, the study authors can determine the behavior of the moving quasiparticles.

Using this technique, the MIT and CUNY investigators gained the ability to see which one of the two possible types of “hopping motion” was actually occurring. Deotare added that the method made it possible to demonstrate that a material’s nanoscale structure determined how quickly excitons became trapped while moving through it.

In some applications – LEDs, for example – the authors report that it is better to maximize the trapping capabilities to make sure that energy is not lost due to leakage.

However, for solar cells and other uses, it is important to minimize the trapping. The technique is expected to make it possible for researchers to figure out which individual factors are the most important in either increasing or decreasing exciton trapping capability.

The experiments conducted as part of the study utilized a molecular crystal archetype known as tetracene, but the authors note that the results should be applicable to any crystalline or thin-film material. They believe that the technique will become widely adopted by scientists and industrial researchers, namely because of the technique’s simplicity and the fact that only relatively inexpensive equipment is required for the method.

University of Sheffield physics and astronomy professor David Lidzey, who was not involved in the US Department of Energy/National Science Foundation-funded study, called the research “a really impressive demonstration of a direct measurement of the diffusion of triplet excitons and their eventual trapping.

“Exciton diffusion and transport are important processes in solar-cell devices, so understanding what limits these may well help the design of better materials, or the development of better ways to process materials so that energy losses during exciton migration are limited,” he added.

Originally posted by