_NATURE ENVIRONMENT Photovoltaics

_Solar Energy Breakthrough

Researchers have identified a compound that could make solar cells less expensive, easier to manufacture and more efficient.

_Jonathan Spanier

Spanier is a professor in the College of Engineering’s Department of Materials Science and Engineering.

The best solar panels wring every drop of energy from as many photons as possible. This goal has sent chemistry, materials science and physics researchers on a quest to boost the energy-absorption efficiency of photovoltaic devices, but existing techniques are running up against limits set by the laws of physics. 

Now, researchers from Drexel and the University of Pennsylvania have identified a new material for solar cell construction that may push these boundaries.

Their project — published in Nature — was led by Jonathan E. Spanier, a professor in the College of Engineering at Drexel; Andrew M. Rappe and research specialist Ilya Grinberg of the Chemistry Department in Penn’s School of Arts and Sciences; and Peter K. Davies, chair of the Department of Materials Science and Engineering in the School of Engineering and Applied Science at Penn.

Their goal was to find a robust oxide that exhibits the bulk photovoltaic effect (in which electrons travel in one direction without having to cross from one material to another) for visible light such as that which comes from the sun.

The “bulk” phenomenon has been known since the 1970s, but manufacturers haven’t built solar cells that way because the effect has only been demonstrated with ultraviolet light, and most of the energy from the sun is in the visible and infrared spectrum.

As no known materials exhibited a strong bulk photovoltaic effect for the most important part of the solar spectrum, the research team relied on chemistry and materials science to devise how a new one might be fashioned and its properties measured.

For more than five years of research, they searched for the desired properties in a type of crystal material called perovskites. Perovskite crystals can have the same cubic lattice of metal atoms, but inside each cube is an octahedron of oxygen atoms, and inside each octahedron is another kind of metal atom. The relationship between these two metallic elements can cause them to move off center, making the structure polar and giving directionality to the flow of electrons.

Perovskite KBNN01

BRIGHT_IDEAS

New perovskite oxide materials could be used to create more efficient solar panels.

After several failed attempts to physically produce the specific perovskite crystals they had theorized, the researchers had success with a combination of potassium niobate, the parent, polar material and barium nickel niobate, which contributes to the final product’s bandgap (the range of light frequencies it captures).

“The parent’s bandgap is in the UV range,” Spanier says, “but adding just 10 percent of the barium nickel niobate moves the bandgap into the visible range and close to the desired value for efficient solar energy conversion.

“This family of materials is all the more remarkable because it is comprised of inexpensive, non-toxic and earth-abundant elements,” Spanier says.