Excitons observed in action for the first time A quasiparticle called an exciton   responsible for the transfer of energy within devices such as solar cells LEDs and semiconductor circuits   has been understood theoretically for decades. But exciton movement within materials has never been directly observed.Now scientists at MIT and the City University of New York have achieved that feat imaging excitons  motions directly. This could enable research leading to significant advances in electronics they say as well as a better understanding of natural energy-transfer processes such as photosynthesis.The research is described this week in the journal Nature Communications in a paper co-authored by MIT postdocs Gleb Akselrod and Parag Deotare professors Vladimir Bulovic and Marc Baldo and four others. This is the first direct observation of exciton diffusion processes  Bulovic says  showing that crystal structure can dramatically affect the diffusion process.  Excitons are at the heart of devices that are relevant to modern technology  Akselrod explains: The particles determine how energy moves at the nanoscale.  The efficiency of devices such as photovoltaics and LEDs depends on how well excitons move within the material  he adds.An exciton which travels through matter as though it were a particle pairs an electron which carries a negative charge with a place where an electron has been removed known as a hole. Overall it has a neutral charge but it can carry energy. For example in a solar cell an incoming photon may strike an electron kicking it to a higher energy level. That higher energy is propagated through the material as an exciton: The particles themselves don t move but the boosted energy gets passed along from one to another.While it was previously possible to determine how fast on average excitons could move between two points  we really didn t have any information about how they got there  Akselrod says. Such information is essential to understanding which aspects of a material s structure   for example the degree of molecular order or disorder   might facilitate or slow that motion. People always assumed certain behavior of the excitons  Deotare says. Now using this new technique   which combines optical microscopy with the use of particular organic compounds that make the energy of excitons visible    we can directly say what kind of behavior the excitons were moving around with.  This advance provided the researchers with the ability to observe which of two possible kinds of  hopping  motion was actually taking place. This allows us to see new things  Deotare says making it possible to demonstrate that the nanoscale structure of a material determines how quickly excitons get trapped as they move through it.For some applications such as LEDs Deotare says it is desirable to maximize this trapping so that energy is not lost to leakage; for other uses such as solar cells it is essential to minimize the trapping. The new technique should allow researchers to determine which factors are most important in increasing or decreasing this trapping. We showed how energy flow is impeded by disorder which is the defining characteristic of most materials for low-cost solar cells and LEDs  Baldo says.While these experiments were carried out using a material called tetracene   a well-studied archetype of a molecular crystal   the researchers say that the method should be applicable to almost any crystalline or thin-film material. They expect it to be widely adopted by researchers in academia and industry. It s a very simple technique once people learn about it  Akselrod says  and the equipment required is not that expensive. Exciton diffusion is also a basic mechanism underlying photosynthesis: Plants absorb energy from photons and this energy is transferred by excitons to areas where it can be stored in chemical form for later use in supporting the plant s metabolism. The new method might provide an additional tool for studying some aspects of this process the team says.David Lidzey a professor of physics and astronomy at the University of Sheffield who was not involved in this work calls the research  a really impressive demonstration of a direct measurement of the diffusion of triplet excitons and their eventual trapping.  He adds  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. The work was supported by the U.S. Department of Energy and by the National Science Foundation and used facilities of the Eni-MIT Solar Frontiers Center.