Moving society toward a greener future hinges on efficient technologies. Silicon solar cells, currently the leading commercial technology in the solar cell arena, reach energy conversion efficiencies of 18% to 22%, according to the U.S. Department of Energy. That means roughly four-fifths of the energy from the sunlight isn’t being harnessed by current commercial photovoltaic technology.

Frank Osterloh, a chemistry professor in the College of Letters and Science at UC Davis, wants to access that unused energy. He’s dedicated his lab to the foundational science necessary to optimize solar cell technology and produce clean fuels. In a new paper appearing in the Journal of the American Chemical Society, Osterloh and his colleagues unveiled a new type of solar cell that might be used in tandem with current commercial solar cell technologies to improve solar conversion efficiency and produce clean hydrogen fuel.

“The solar cell that we published is very different from the current technologies,” Osterloh said. “It utilizes a metal oxide called bismuth vanadate, whereas the other more established technologies use main group element semiconductors like silicon.”

While the new photovoltaic cell currently only reaches 0.2% solar energy conversion efficiency, Osterloh hopes the research will encourage scientists to explore bismuth vanadate and other metal oxides as viable solar energy conversion technologies.

“If you were to take a metal oxide solar cell that absorbs high-energy photons near the ultraviolet emission and combine that with traditional, more established semiconductor materials that absorb light in the visible spectrum, you’d end up with a device that overall has higher efficiency because we’re dividing the work of absorbing low-energy photons and high-energy photons between two different materials,” said Osterloh.

Harvesting happenstance

For the past 10 years, Osterloh and his lab colleagues have explored using bismuth vanadate for direct solar energy to fuel conversion due to its photocatalytic properties.

“When bismuth vanadate is exposed to water and then light hits it, it doesn’t create electricity,” Osterloh said. “It oxidizes the water to oxygen, harvesting electrons from the water and those electrons can be used to turn water into green hydrogen fuel. This is known as overall water splitting.”

Sahar Daemi, a postdoctoral researcher in the Osterloh Group and first author of the study, has been performing fundamental studies on bismuth vanadate with support from the Department of Energy. Thanks to her fundamental research, the Osterloh Group learned that bismuth vanadate not only oxidizes water but also iodine ion, a known conductor of electric charge.       

“From there, the idea came up that if this works so well with this iodine solution, why don’t we try to make a solar cell out of that?” Osterloh said. 

An electric sandwich

The resulting bismuth vanadate solar cell can be thought of like a sandwich. A bismuth vanadate oxide film is placed on an electrode. A sodium iodide (the electron-rich form of iodine) solution is then inserted between that and a platinum-based counter electrode. Following its fabrication, the solar cell performed at 0.2% solar energy conversion efficiency.  

“We’re a factor of 100 below what you can get with commercial technology,” said Osterloh, referring to the roughly 20% efficiency rates seen in current solar technology. “So there’s a need to improve this efficiency and in this paper, we analyze why the efficiency is so low.”

One potential avenue for improvement lies in replacing the iodine solution.

“If we change the iodine to something with higher voltage storing ability, we might be able to increase the efficiency by maybe a few more percentage points,” Osterloh said. 

For Osterloh, parsing the innerworkings of solar cells to increase efficiency is an iterative process necessary for a green revolution. His lab is also exploring tin sulfide and copper oxide as alternative thin film-based solar cells.

“Copper is very abundant and not expensive,” Osterloh said. “Essentially, we’re building a solar cell using a recipe that most people in the world will be able to replicate without the need for expensive facilities.” 

“This is what we need in order to move our society to a more sustainable future,” he added. “We need to tap into the energy sources that we’re provided with naturally instead of getting energy by digging fossil fuels out of the ground.”  

Other contributors to the study include Samhita Kaushik, Soumik Das and Thomas W. Hamann, all from Michigan State University.