Polaronic Distortions Could be Leveraged to Make Low-Cost Solar Cells

For the first time, investigators from the SLAC National Accelerator Laboratory at the Department of Energy (DOE) and Stanford University have applied the laboratory’s X-ray laser to observe and directly quantify the development of polarons. The researchers have recently reported their results in the Nature Materials journal.

Polarons are essentially transient distortions that occur in the atomic lattice of a material. These distortions form around a moving electron in just a few trillionths of a second, and then vanish rapidly.

Although polarons are fleeting, they tend to affect the behavior of a material and may also explain why solar cells created with lead hybrid perovskites are able to achieve incredibly high efficiencies in laboratory settings.

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.

Aaron Lindenberg, Study Lead and Associate Professor, Stanford University

Lindenberg is an investigator with the Stanford Institute for Materials and Energy Sciences (SIMES) at the SLAC National Accelerator Laboratory.

“The idea that polarons may be involved has been around for a number of years,” Lindenberg added. “But our experiments are the first to directly observe the formation of these local distortions, including their size, shape and how they evolve.”

Exciting, Complex, and Hard to Understand

Perovskites, in essence, are crystalline materials dubbed after the mineral perovskite, which exhibits an analogous atomic structure. About 10 years ago, investigators began to incorporate these perovskites into solar cells, and the efficiency of these cells at changing sunlight to energy has progressively increased, even though their perovskite constituents have plenty of defects that must prevent current flow.

According to Lindenberg, such materials are eminently complex and difficult to figure out. Investigators find these materials fascinating because they are not only efficient but can also be made easily, providing a possibility to make solar cells more affordable than existing silicon cells. However, these materials break down upon exposure to air, are extremely unstable, and contain lead that should be protected from the environment.

Earlier analyses made at SLAC have explored perovskites with X-ray beams or with an “electron camera.” Among other things, the studies showed that light normally whirls the atoms around in perovskite materials, and they allowed scientists to quantify the lifetimes of acoustic phonons—sound waves—that carry heat via the materials.

For the new study, Lindenberg’s group used the laboratory’s Linac Coherent Light Source (LCLS). LCLS is a strong X-ray free-electron laser that is capable of capturing atomic motions that occur in millionths of a billionth of a second, and imaging materials in near-atomic detail.

The researchers initially studied the single crystals of the material produced by Associate Professor Hemamala Karunadasa’s team from Stanford University. Then, they bombarded a small material sample using light from an optical laser, and finally used the X-ray laser to visualize the way the material reacted across the course of tens of trillionths of a second.

Expanding Bubbles of Distortion

When you put a charge into a material by hitting it with light, like what happens in a solar cell, electrons are liberated, and those free electrons start to move around the material.

Burak Guzelturk, Scientist, Argonne National Laboratory

Guzelturk was a postdoctoral researcher at Stanford University during the experiments.

Guzelturk continued, “Soon they are surrounded and engulfed by a sort of bubble of local distortion—the polaron—that travels along with them. Some people have argued that this ‘bubble’ protects electrons from scattering off defects in the material, and helps explain why they travel so efficiently to the solar cell’s contact to flow out as electricity.”

The structure of the hybrid perovskite lattice is soft and flexible, similar to “a strange combination of a solid and a liquid at the same time,” as described by Lindenberg—and this is what enables polarons to both form and grow.

The team’s observations demonstrated that polaronic distortions begin in a small way—on the scale of just a few angstroms, about the spacing between atoms present in a solid—and quickly expand outward in all the directions to a diameter of around five billionths of a meter, which is roughly an increase of 50 times.

This nudges around 10 layers of atoms somewhat outward inside a coarsely spherical region over the course of trillionths of a second, or tens of picoseconds.

“This distortion is actually quite large, something we had not known before,” added Lindenberg. “That’s something totally unexpected.”

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.

Aaron Lindenberg, Study Lead and Associate Professor, Stanford University

LCLS is a DOE Office of Science user facility. Associate Professor Lindenberg is also an investigator with the Stanford PULSE Institute, which, similar to SIMES, is a joint institute of SLAC and Stanford University.

Researchers from the University of Cambridge in the United Kingdom, Aarhus University in Denmark, and Paderborn University and the Technical University of Munich in Germany were also involved in the research work. The DOE Office of Science provided major funding.

Journal Reference:

Guzelturk, B., et al. (2021) Visualization of dynamic polaronic strain fields in hybrid lead halide perovskites. Nature Materials. doi.org/10.1038/s41563-020-00865-5.