Polarons, fleeting distortions in a material's atomic lattice that form around moving electrons, might hold the key to incredibly efficient solar cells made with lead hybrid perovskites.
A team of scientists at the Department of Energy's SLAC National Accelerator Laboratory and Stanford University used X-ray laser to observe the formation of polarons for the first time. They reported their findings in the scientific journal Nature Materials.
Atomic laser observations
Lead hybrid perovskites have great potential for boosting the solar cell industry. And yet, scientists aren't in agreement as to how they work.
"These materials have taken the field of solar energy research by storm because of their high efficiencies and low cost, but people still argue about why they work," said Aaron Lindenberg, an investigator with the Stanford Institute for Materials and Energy Sciences (SIMES) at SLAC and associate professor at Stanford who led the research, explained in a press statement.
Polarons, which occur over trillionths of seconds, might be the key: "the idea that polarons may be involved has been around for a number of years," he said. "But our experiments are the first to directly observe the formation of these local distortions, including their size, shape, and how they evolve."
Scientists started to incorporate perovskites into solar cells about a decade ago. They are crystalline materials named after the mineral perovskite, which has a similar atomic structure.
The materials are notoriously complex and difficult to understand, Lindenberg explained. Though they are unstable and contain poisonous lead, they have the potential to make solar cells cheaper than today's silicon cells.
For the study, Lindenberg's team used their lab's Linac Coherent Light Source (LCLS), a powerful X-ray free-electron laser with the capacity to image materials in near-atomic detail and capture movement occurring in millionths of a billionth of a second.
Through their study, they observed that the hybrid perovskite lattice structure is flexible and soft, like "a strange combination of a solid and a liquid at the same time," Lindenberg said. This, he explains, is what allows polarons to form and grow.
The observations also revealed that polaronic distortions start very small — roughly the since between atoms in a solid — and quickly expand outward in all directions to take up approximately 50 times that space.
"This distortion is actually quite large, something we had not known before," Lindenberg said. "That’s something totally unexpected."
However, as Lindenberg concludes, "while this experiment shows as directly as possible that these objects really do exist, it doesn’t show how they contribute to the efficiency of a solar cell. There’s still further work to be done to understand how these processes affect the properties of these materials."