University of Oregon (UO) researchers have a new method to follow light into nanoparticle photocells in a spectroscopy experiment on photocells that used lead-sulfide quantum dots as photoactive semiconductor material.

Its research that should prove to be of enormous consequence. The efficiency of conventional solar cells could be significantly increased from the current limit of 30 percent to more than 60 percent with research on semiconductor nanocrystals, or quantum dots.

Andrew H. Marcus, professor of physical chemistry and head of the UO Department of Chemistry and Biochemistry explains, “The approach is similar to looking at how molecules come together in DNA, but instead we looked at interactions within semiconductor materials. 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.”

Marcus and Lonergan with Spectroscopy Imager.  Click image for the largest view.

Marcus and Lonergan with Spectroscopy Imager. Click image for the largest view.

The multiple exciton concept has been the basis for years of theory and research. Multiple exciton generation (MEG) is of interest 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.

Here’s why it matters, in the process studied, each single photon, or particle of sunlight, that 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. In most solar cells, each absorbed photon creates just one potential free electron. That’s where the doubling of efficiency theory comes from.

The work potentially should inspire devices with improved efficiency in solar energy conversion. The team’s research was performed on photocells that used lead-sulfide quantum dots as photoactive semiconductor material. The research is detailed in a paper published online by the journal Nature Communications.

For now the potential importance of MEG in solar energy conversion is under debate by scientists. Meanwhile the UO spectroscopy experiment – adapted in a collaboration with scientists at Sweden’s Lund University – should be useful for studying many other processes in photovoltaic nanomaterials.

Using four pulses of laser light on nanoparticle photocells and controlling 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.

Marcus had previously designed spectroscopic experiments to perform two-dimensional fluorescence spectroscopy of biological molecules. The team adapted the experiments 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.

To understand what UO team’s work offers co-author Mark C. Lonergan, professor of physical and materials chemistry, who studies electrical and electrochemical phenomena in solid-state systems, likened the processes being observed to people moving through a corn maze that has one entrance and three exits.

“People entering the maze are photons. Those who exit quickly represent absorbed photons that generate unusable heat. People leaving the second exit represent other absorbed photons that generate fluorescence but not usable free electrons. People leaving the final exit signify usable electrical current.”

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

Credit the Swedes for asking the right question and kicking the research off. The project began when Tonu Pullerits, who studies ultrafast photochemistry in semiconductor molecular materials at Lund University, approached Marcus about adopting his spectroscopic system to look at solar materials. Khadga J. Karki, a postdoctoral researcher in Pullerits’ lab, then visited the UO and teamed with the Marcus and Lonergan groups to reconfigure the equipment.

Note that UO doctoral student Julia R. Widom was a co-leading author on the paper with Joachim Seibt of Lund University and UO graduate student Ian Moody collaborating.

Quantum dot photo cells are still quite a ways off and still up for debate as even possible. But this research gets the field a means to look and measure that should lead to a stimulation of more research and faster paths to working solutions.


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