Squeezing rock star material could make it stable enough for solar cells

Among the materials known as perovskites, one of the most interesting is a material capable of converting sunlight into electricity as efficiently as today’s commercial silicon solar cells and has the potential to be much cheaper and easier to manufacture.

There is just one problem: of the four possible atomic configurations, or phases, that this material can take, three are effective but unstable at room temperature and in ordinary environments, and they quickly revert to the fourth phase, which is totally unnecessary. for solar applications.

Now, scientists at Stanford University and the Department of Energy’s SLAC National Accelerator Laboratory have come up with a new solution – just put the unnecessary version of the material into a diamond anvil cell and squeeze it high. temperature. This treatment pushes its atomic structure into an efficient configuration and thus maintains it, even at room temperature and in relatively humid air.

The researchers described their findings in Nature communications.

“This is the first study to use pressure to control this stability, and it really opens up a lot of possibilities,” said Yu Lin, a SLAC scientist and researcher at the Stanford Institute for Materials and Energy Sciences (SIMES).

“Now that we’ve found this optimal way to prepare the material,” she says, “it’s possible to scale it up for industrial production and use that same approach to manipulate other phases of perovskite” .

A search for stability

Perovskites get their name from a naturally occurring mineral with the same atomic structure. In this case, the scientists studied a lead halide perovskite which is a combination of iodine, lead and cesium.

One phase of this material, known as the yellow phase, does not have a true perovskite structure and cannot be used in solar cells. However, scientists discovered some time ago that if you treat it in certain ways, it transforms into a black perovskite phase that’s extremely efficient at converting sunlight into electricity. “This made it highly researched and the subject of a lot of research,” said Stanford professor and study co-author Wendy Mao.

Unfortunately, these dark phases are also structurally unstable and tend to quickly fall back into the unnecessary configuration. Plus, they only work with high efficiency at high temperatures, Mao said, and researchers will need to overcome both of these issues before they can be used in practical devices.

There had been previous attempts to stabilize dark phases with chemistry, strain, or temperature, but only in a moisture-free environment that did not reflect the real world conditions in which solar cells operate. This study combined pressure and temperature in a more realistic working environment.

Pressure and heat do the trick

Together with colleagues from Mao’s Stanford research groups and Professor Hemamala Karunadasa, Lin and postdoctoral researcher Feng Ke designed a configuration where yellow phase crystals were squeezed between the tips of diamonds in what is known as a diamond anvil cell. With the pressure still on, the crystals were heated to 450 degrees Celsius and then cooled.

Under the right combination of pressure and temperature, the crystals turned from yellow to black and remained in the black phase after the pressure was released, the scientists said. They were resistant to deterioration from humid air and remained stable and effective at room temperature for 10 to 30 days or more.

X-ray examination and other techniques confirmed the change in the crystal structure of the material, and calculations by SIMES theorists Chunjing Jia and Thomas Devereaux provided insight into how pressure changed the structure and preserved the dark phase. .

The pressure needed to turn the crystals black and keep them that way was about 1,000 to 6,000 times atmospheric pressure, Lin said – about a tenth of the pressures commonly used in the synthetic diamond industry. One of the goals of future research will therefore be to transfer what researchers have learned from their experiences with diamond anvil cells to industry and to scale up the process to bring it into the realm of manufacturing. .