Chemists use pump-probe lasers to track the cooling of electrons as they move through the material

Solar technology, which uses solar cells to convert sunlight into electricity or storable fuel, is gaining momentum in a world that no longer relies on fossil fuels to meet its energy needs.

The dark blue solar panels that dot rooftops and vacant lots today are often made of silicon, a well-tested semiconductor material. Silicon photovoltaic technology has its limitations, however, as it loses up to 40 percent of the energy collected from sunlight in the form of thermal waste. Researchers at Colorado State University are working on entirely new ways to improve solar power and give the industry more options to explore.

Instead of using silicon, CSU chemists propose to make solar cells using a natural material called molybdenum disulfide, which is available in large quantities. Using a creative combination of photochemical and spectroscopic techniques, the researchers conducted a series of experiments showing that extremely thin films of molybdenum disulfide exhibit unprecedented carrier properties that could one day dramatically improve solar technology.

The experiment was led by Dr. Student Rachelle Austin and postdoctoral researcher Yusef Farah. Austin works in the lab of Justin Sambur, associate professor of chemistry, and Amber Krummel, associate professor of chemistry. Farah is a former doctor. Students in Krummel's lab. Their work is in the Proceedings of the National Academy of Sciences.

The collaboration brings together Sambur's expertise in solar energy conversion using nanoscale materials, and Krummel's expertise in ultra-fast laser spectroscopy to understand how different materials are structured and how they behave. Sambur's lab has become interested in molybdenum sulfide as a possible alternative solar material, based on preliminary data on its ability to absorb light, even as little as three atoms thick, Austin explained.

That's when they turned to Krummel, whose lab has a state-of-the-art ultra-fast pump-probe transient absorption spectrometer that can measure very precisely the continuous energy state of a single electron when excited by a laser pulse. Experiments using this special instrument can provide a snapshot of how charges flow through the system. Austin used molybdenum sulfide monatomic layers to create photoelectric chemical cells, and she and Farah used a pumped-detection laser to track the cooling of electrons as they moved through the material.

What they found was a surprisingly efficient conversion of light energy. What's more, laser spectroscopy experiments enabled them to show why this efficient conversion was possible.

They found that the material is very good at converting light into energy because its crystal structure allows it to extract and harness the energy of so-called hot carriers, which are high-energy electrons that, when hit with enough visible light, briefly excite light from the ground state. Austin and Farah found that in their photochemical cells, the energy from these hot carriers was immediately converted into photocurrent, rather than being lost as heat. This phenomenon of hot carrier extraction does not exist in traditional silicon solar cells.

"This work paves the way for understanding how to design reactors containing these nanoscale materials for efficient and large-scale hydrogen production," Sambur said.

The project was done in collaboration with Prof. Andres Montoya-Castillo and Dr. Thomas Sayer of the University of Colorado Boulder, who contributed theoretical chemical and computational models to help interpret and verify the experimental data.

"This discovery requires a 'team science' approach that brings together many different types of expertise in computation, analysis and physical chemistry," Krummel said.

The results provide a way for scientists and engineers to explore new approaches to future solar technology. This work was supported by the Department of Energy's Office of Basic Energy Sciences.

Source: Laser Net