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Semiconductor Design Strategies for Implementation in Solar Fuel Applications

Solar power technologies, which employ solar cells to turn sunlight into electricity or storable fuels, are becoming popular in a world seeking alternatives to fossil fuels for energy.

Semiconductor Design Strategies for Implementation in Solar Fuel Applications
CSU researchers Yusef Farah and Rachelle Austin with the ultrafast pump-probe transient absorption spectrometer they used to measure charge carrier properties of molybdenum disulfide thin films. Image Credit: John Eislele.

The dark bluish solar panels that dot today’s rooftops and open fields primarily comprise silicon, a tested semiconductor material. However, silicon photovoltaic technology has limits, losing up to 40% of the energy it absorbs from sunlight as heat waste. Colorado State University researchers are investigating radical new ways to boost solar power and provide more options for the industry to explore.

CSU chemists propose making solar cells out of a naturally occurring substance called molybdenum disulfide rather than silicon.

The investigators conducted experiments using a novel combination of photoelectrochemical and spectroscopic techniques to demonstrate that extremely thin molybdenum disulfide films exhibit unparalleled charge carrier properties that could someday greatly enhance solar technologies.

Rachelle Austin, a Ph.D. Student in chemistry, and Yusef Farah, a Postdoctoral Researcher, performed the research. Austin works jointly in the labs of Justin Sambur, Associate Professor in the Department of Chemistry, and Amber Krummel, Associate Professor in the same department. Farah worked at Krummel’s lab as a PhD Student.

The findings have been published in the Proceedings of the National Academy of Sciences.

Sambur’s expertise in solar energy conversion using nanoscale materials was combined with Krummel’s knowledge in ultrafast laser spectroscopy to better understand how different materials are structured and function.

According to Austin, Sambur’s lab became interested in molybdenum sulfide as a prospective alternative solar material based on preliminary research on its light absorption capabilities even when only three atoms thick.

They turned to Krummel, whose lab houses a cutting-edge ultrafast pump-probe transient absorption spectrometer capable of measuring the successive energy states of individual electrons as a laser pulse stimulates them.

Experiments performed with this specialized device can show how charges flow in a system. Austin built a photoelectrochemical cell from a single atomic layer of molybdenum sulfide. She and Farah tracked the cooling of electrons as they traveled through the material using the pump-probe laser.

They discovered astonishingly effective light-to-energy conversion. More crucially, the laser spectroscopy studies allowed them to demonstrate why such efficient conversion was possible.

They discovered that the material’s crystal structure allows it to harvest and exploit the energy of so-called hot carriers, which are highly energetic electrons quickly stimulated from their ground states when exposed to enough visible light.

Austin and Farah discovered that the energy from these heated carriers was directly transformed into photocurrent rather than being lost as heat in their photoelectrochemical cell. This hot carrier extraction phenomenon does not exist in standard silicon solar cells.

This work paves the way for knowing how to design reactors that contain these nanoscale materials for efficient and large-scale hydrogen production.

Justin Sambur, Associate Professor, Department of Chemistry, Colorado State University

The venture was a collaborative effort with Professor Andrés Montoya-Castillo and Dr. Thomas Sayer of the University of Colorado Boulder, who contributed theoretical chemistry and computational modeling to better clarify and ascertain the experimental data.

The discovery required a ‘team science’ approach that brought together many different types of expertise, in computational, analytical and physical chemistry.

Amber Krummel, Associate Professor, Colorado State University

The findings provide scientists and engineers with a fresh line of investigation to explore new approaches to tomorrow’s solar energy technology. The United States Department of Energy’s Office of Basic Energy Sciences funded the research.

Journal Reference

Austin, R., et al. (2023). Hot carrier extraction from 2D semiconductor photoelectrodes. Proceedings of the National Academy of Sciences.


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