An Experiment Offers a New View on How Captured Sunlight Is Converted into Electricity

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Total-Solar-Energy-diagram

 

An experiment conducted at the University of Oregon using a laser on nanoparticle photocells has created a whole new view of how captured sunlight can be converted into electricity.

 

The experiment, which could potentially inspire more efficient devices in solar energy conversion, was performed on photocells that used lead-sulfide quantum dots as pro active semiconductor material. The results were detailed in a paper published in the journal Nature Communications.

 

In the experiment, each single photon, or particle of sunlight which is absorbed potentially creates multiple packets of energy called excitons.

 

These packets can subsequently generate multiple free electrons that generate electricity in a process known as multiple exciton generation (MEG). In most solar cells, each absorbed photon creates just one potential free electron.

 

Multiple exciton generation is of great interest to researchers because it can lead to solar cells that generate more electrical current and make them more efficient. The UO work shines new light on the little understood process of MEG in nanomaterials.

 

While the potential importance of MEG in solar energy conversion is under debate by scientists, the UO spectroscopy experiment, adapted in a collaboration with scientist at Sweden’s Lund University, should be useful for studying many other processes in photovoltaic nanomaterials, said Andrew H. Marcus, professor of physical chemistry and head of the UO Department of Chemistry and Biochemistry.

 

Spectroscopic experiments previously designed by Marcus, to perform two-dimensional fluorescence spectroscopy of biological molecules, were adapted to also measure photocurrent. “Spectroscopy is all about light and molecules and what they do together,” Marcus said. “It is a really great probe that helps to tell us about the reaction pathway that connects the beginning of a chemical or physical process to its end.

 

“The approach is similar to looking at how molecules come together in DNA, but instead we looked at interactions within semiconductor materials,” said Marcus, an affiliate in UO’s Institute of Molecular Biology, Materials Science Institute and Oregon Center for Optics. “Our method made it possible to look at electronic pathways involved in creating multiple excitons. The existence of this phenomenon had only been inferred through indirect evidence. We believe we have seen the initial steps that lead to MEG-mediated photo conductivity.”

 

The controlled sequencing of laser pulses allowed the seven-member research team to see, in femtoseconds (a femtosecond is one millionth of one billionth of a second), the arrival of light, its interaction with resting electrons and the subsequent conversion into multiple excitons. The combined use of photocurrent and fluorescence two-dimensional spectroscopy, Marcus said, provided complementary information about the reaction pathway.

 

UO co-author Mark C. Lonergan likened the processes to people moving through a corn maze which has one entrance and three exits. The people entering the maze are the photons and those who exit quickly represent absorbed photons that generate unusable heat. The people leaving through the second exit represent other absorbed photons, that generate fluorescence but not usable free electrons. Finally, the people leaving the last exit signify usable electrical current.

 

“The question we are interested in is exactly what does the maze look like,” Lonergan said. “The problem is we don’t have good techniques to look inside the maze to discover the possible pathways through it.

 

The techniques that Andy has developed basically allow us to see into the maze by encoding what is coming out of the system in terms of exactly what is going in. We can visualize what is going on, whether two people coming into the maze shook hands at some point and details about the pathway that led them to come out the electricity exit.”

 

At UPS Battery Center, our goal isn’t to only sell batteries, we want to inform and teach you about the amazing world of batteries, electricity and energy. Please check back for more interesting, helpful and informative articles about batteries and electricity.

 

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