Semiconductor design strategies implemented by researchers in solar fuel applications

Solar technology, which uses solar cells to convert sunlight into electricity or storable fuel, is becoming increasingly popular in a world looking for alternatives to fossil fuel energy.

The dark blue solar panels that dot today's rooftops and vacant lots are made mostly of silicon, a tested semiconductor material. However, silicon photovoltaic technology has its limitations, as much as 40 percent of the energy it absorbs from sunlight is lost as heat. Researchers at Colorado State University are working on entirely new ways to boost solar power generation and give the industry more options to explore.

CSU chemists suggest using a natural substance called molybdenum disulfide instead of silicon to make solar cells.

The researchers conducted experiments using a novel combination of photochemical and spectroscopic techniques to demonstrate that extremely thin films of molybdenum disulfide exhibit unmatched charge carrier properties that could one day greatly improve solar technology.

Rachel Austin, PhD chemistry student and postdoctoral researcher Yusef Farah conducted the study. Austin works in the lab of Justin Sambur, associate professor of chemistry, and Amber Krummel, associate professor of chemistry. Farah worked in Krummel's lab as a doctoral student.

The findings are in the Proceedings of the National Academy of Sciences.

Sambur's expertise in solar energy conversion using nanoscale materials is combined with Krummel's knowledge of ultrafine laser spectroscopy to better understand the structure and function of different materials.

According to Austin, Sambur's lab became interested in molybdenum sulfide as a potential alternative solar energy material based on preliminary studies of its ability to absorb light, even as little as three atoms thick.

They turned to Krummel, whose lab is equipped with a cutting-edge ultra-fast pump-probe transient absorption spectrometer capable of measuring the continuous energy state of individual electrons when stimulated by laser pulses.

Experiments using this specialized device can show how charges flow through the system. Austin built a photoelectric chemical cell using a single atomic layer of molybdenum sulfide. She and Farah used a pump-detection laser to track the cooling of electrons as they passed through the material.

What they found was a surprisingly efficient conversion of light energy. More importantly, the laser spectroscopy study allowed them to demonstrate why such an efficient conversion could be achieved.

They found that the material's crystalline structure allows it to collect and harness the energy of so-called hot carriers, which are energetic electrons that are rapidly excited from the ground state when exposed to enough visible light.

Austin and Farah found that the energy from these heated carriers is converted directly into photocurrent, rather than being lost as heat in their photochemical cells. This phenomenon of hot carrier extraction does not exist in standard silicon solar cells.

The project is a collaboration with Prof. Andres Montoya-Castillo and Dr. Thomas Sayer of the University of Colorado Boulder, who contributed theoretical chemical and computational models to better clarify and determine experimental data.

The findings provide scientists and engineers with a new line of investigation to explore new approaches to future solar technology. The U.S. Department of Energy's Office of Basic Energy Sciences funded the study.

Source: Laser Net