Lehigh University physicists Ivan Biaggio, professor of physics, and Pavel Irkhin, a Ph.D. candidate, have developed an imaging technique that makes it possible to directly observe light-emitting excitons as they diffuse in a new material that is being explored for its extraordinary electronic properties.

Biaggio with Exciton Viewing Apparatus. Click image for more info.

The problem for low cost plastic based solar cells the absorption of light creates excitons in the plastic instead of directly inducing a current, as it does in the most commonly used glass covered silicon systems.

Excitons, which are created by the incoming light, play a central role in the harvesting of solar energy using plastic solar cells.  Biaggio explains with an example, “One way to understand the mechanics of excitons is to pour a cup of milk on the floor. The milk spreads out in all directions from the point of impact. How far it goes depends on the type of surface on which it lands. Now imagine that the milk has been replaced with particle-like bundles of energy and the floor with an ordered arrangement of organic molecules.”  An understanding of the exciton diffusion is critical for plastic solar cell technology.

Rubrene is one of a new generation of single-crystal plastic organic semiconductors.  The Lehigh team is using an advanced imaging technique to witness the long-range diffusion of energy-carrying excitons in the rubrene.

The team uses a focused laser beam to create the particles – the excitons – in a crystal made of rubrene. They tracked the movements of the excitons over distances smaller than the size of a human hair by directly taking pictures of the light coming from the laser. Unlike the spilled milk, the excitons spread only in a direction corresponding to a particular arrangement of the molecules.

After the excitons are created in plastic solar cells, they diffuse toward specially designed interfaces where they drive electrons into an external circuit, creating the flow of electrons we know as electric current.

Exciton Diffusion. Click image for more info.

“This is the first time that excitons have been directly viewed in a molecular material at room temperature,” said Biaggio. “We believe the technique we have demonstrated will be exploited by other researchers to develop a better understanding of exciton diffusion and the bottleneck it forms in plastic solar cells.”

The team’s paper entitled “Direct Imaging of Anisotropic Exciton Diffusion and Triplet Diffusion Length in Rubrene Single Crystals” was published July 1 by the journal Physical Review Letters.

The target of research is to improve exciton diffusion lengths until they become as large as the light absorption – that’s the point when sunlight is most efficiently collected and converted into energy.

Irkhin and Biaggio were able to obtain precise measurement of their diffusion length by directly imaging the diffusing excitons.  This length was found to be very large in a particular direction, reaching a value several hundreds of times larger than in the plastic solar cells that are presently in use. The payoff is this is the first time that excitons have been directly viewed in a molecular material at room temperature, and it is believed that the widespread adoption of the technique developed by Irkhin and Biaggio will lead to significantly more progress in the field.

Biaggio sums up with, “It is important that physicists explore the most fundamental phenomena underlying the mechanisms that enable solar energy harvesting with cheap organic materials. Organics have lots of unexplored potential and the very efficient exciton diffusion that we have observed in rubrene may build the basis for new ideas and technologies towards the development of ever more efficient and plastic solar cells.”

For those with a need to build low cost and lightweight solar cells, the Lehigh team has a tool to see what the material candidates will do.  Trial and error can be replaced with visual inspection. This is just the sort of research that speeds things up.  Silicon cells under glass at any appreciable size are expensive, vulnerable to wind and hail and just plain heavy.  Plastics could offer a much larger range and more useful applications for solar collection.  Good work, gentleman.


1 Comment so far

  1. Nina Christopher on May 16, 2012 1:07 PM

    Sounds like my physics/chemisty class in highschool.Im not really good at it though.This article is perfect for those science geniuses out there.

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