Laser tests show pathway from sunlight to electricity

Multiple exciton generation is of interest because it can lead to efficient solar cells that generate more electrical current. (Credit: iStockphoto)

Researchers fired lasers at photocells to learn more about how solar energy is converted into electricity.

The photocells in the experiment used lead-sulfide quantum dots as photoactive semiconductor material.

During the process, 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 (MEG). In most solar cells, each absorbed photon creates just one potential free electron.

Multiple exciton generation is of interest because it can lead to efficient solar cells that generate more electrical current.

The work by University of Oregon researchers shed light on the little understood process of MEG in nanomaterials.

While the potential importance of MEG in solar energy conversion is under debate, the experiment should be useful for studying many other processes in photovoltaic nanomaterials, says Andrew H. Marcus, professor of physical chemistry.

A new use for spectroscopy

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 says. “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,” explains Marcus. “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.”

Like people in a corn maze

The controlled sequencing of laser pulses allowed scientists 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.

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The combined use of photocurrent and fluorescence two-dimensional spectroscopy, Marcus says, provided complementary information about the reaction pathway.

Study co-author Mark C. Lonergan, professor of physical and materials chemistry, 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,” Lonergan says. “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.”

Scientists at Sweden’s Lund University collaborated on the work, which was described in Nature Communications.

The National Science Foundation and US Department of Energy supported the project via grants to Marcus and Lonergan. The Wenner-Gren Foundation, Knut and Alice Wallenberg Foundation, Swedish Foundation for International Cooperation in Research and Higher Education, Swedish Energy Agency, and Swedish Research Council funded the Lund University researchers.

Source: University of Oregon