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Working at the limits of space and time

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Toward building a better solar cell, two Penn State scientists are using light to probe photovoltaic materials on incredible scales.

14 September 2018

Student in the Asbury lab. Credit: Nate Follmer, Penn State.
Student in the Asbury lab. Credit: Nate Follmer, Penn State.
What do lasers, magnets, and solar cells have in common?

As it turns out, two Penn State scientists—John Asbury and Ken Knappenberger—who are working to develop more-efficient light-harvesting, or photovoltaic, materials.

Best known as the stuff of solar cells and panels, photovoltaic materials work by converting light into electricity at the atomic level: When photons are absorbed by a photovoltaic material, electrons are simultaneously released and then captured by a circuit—creating an electric current that can be used to charge batteries and power other devices. This process happens in less than a nanometer’s space and in less than a millionth of a second’s time, and observing it requires highly sophisticated instrumentation—like the one-of-a-kind laser-based microscopy and spectroscopy systems Asbury and Knappenberger have built in their labs.

The photovoltaic process can be thought of as transpiring in two parts—essentially, an initial triggering event where light is absorbed by the material and an ensuing reaction where the charge is generated. Knappenberger studies the trigger— an event of between an attosecond and several femtoseconds’ length, or between a quintillionth and a few quadrillionths of a second—and Asbury studies the reaction that follows in the next few femtoseconds to microsecond, or between a few quadrillionths and a millionth of a second. As Asbury explains, “Ken focuses on how light interacts with a material’s structure to create energy ow, and my focus is on the chemical or diffusive process that results.”

Despite their different focuses, the two scientists have a shared interest. “We’re both interested in understanding how the structure of a material inffluences or determines how energy passes through it, or how energy gets used by that material,” Knappenberger says. “We’re interested in making that correlation—how you could control the structure to manipulate how that triggering event ultimately leads to different chemistry.”

Asbury works with several types of emerging inorganic and polymer-based organic photovoltaic materials—among them an exciting new class of materials called singlet fission sensitizers, which he explains are targeted as a thin-film add-on to silicon solar cells and could double photocurrent from the blue or green part of the solar spectrum, potentially increasing solar cells’ efficiency by as much as 40 percent. The structure of these materials, “particularly the mixture of crystalline and amorphous components and the interactions between molecules, has a significant impact on their performance,” he says. “So we’re trying to understand how this affects the singlet fission process.”

Using ultrafast laser spectroscopy—where extremely rapid laser pulses energize and then pass through a material for spectral analysis—Asbury is able to characterize the singlet fission process and collect correlative data on the material’s molecular structure. “The grand challenge,” he says, “is to first understand how singlet fission is affected by the structure of the material, and then figure out how to turn that into design rules that we can take to synthetic chemists to say ‘This is what you should make.’”

These materials show potential for use in next-generation photovoltaics, but there are others that may also be able to increase solar cell efficiency. Knappenberger studies so-called plasmonic materials, which he explains act essentially like an optical antenna, gathering light and intensifying its transmission to an underlying catalytic material. “The details of how they do this,” he says, “are intimately related to structure and on a very small, sub-nanometer, scale.”

“Typically, the best amplification occurs when the nanoparticles are arranged in a certain way so they funnel the light into a region and really concentrate it,” he explains. “So you have to understand how energy flows in time and how energy flows spatially, and then you need to be able to correlate that flow to some kind of structure.”

Scientists in the Asbury lab. Credit: Nate Follmer, Penn State.
Scientists in the Asbury lab. Credit: Nate Follmer, Penn State.

Electronics are too slow to be used to measure the flow of energy on the timescales Knappenberger is interested in. But with a laser-based microscope he is able to make those measurements, which he then correlates to structural images from an electron microscope, “so we get feedback about the performance and then we find out what the design is that gave us that performance,” he says.

To make a solar cell, the plasmonic material needs to be matched to a catalyst—perhaps a two- dimensional material—to produce current; but in this coupling, the materials’ electronic properties are changed and the result is, Knappenberger says, “a synergy of the two—no longer like the individual components isolated.” By applying magnetic elds to the coupled material and employing ultrafast laser spectroscopy, he is able to determine its electronic structure and how it prefers to interact with different states of light—information that can then be used “to design the perfect antenna to make the perfect trigger.”

Together, Knappenberger and Asbury are developing new instrumentation that will allow them to delve deeper into exploring the potential of two-dimensional materials for use in photovoltaics—with a particular focus on materials made by researchers in Penn State’s 2D Crystal Consortium, Center for 2D and Layered Materials, and Materials Characterization Laboratory. Housed in an ultrahigh vacuum system, this new instrumentation will enable them to examine newly synthesized 2D materials under pristine conditions, so they will be able to directly correlate synthesis and processing data with materials’ electronic properties—essential to informing future applications, such as in solar cell manufacturing.

“In the end,” Knappenberger says, “we’re driven by understanding how materials’ properties are interrelated and determined by structure and building the tools to see that on the right scales—I’ve always called it ‘spectroscopy at the limits of space and time.’”

By Seth Palmer

John Asbury is an associate professor of chemistry at Penn State. Ken Knappenberger is a professor of chemistry at Penn State.