Polarons first look – Transient distortions – Forming next-generation energy into a promising material

An illustration shows polarites – elusive distortions in the atomic lattice of a material – in a high-energy energy material of the next generation, lead hybrid perovskite. Credit: Greg Stewart / SLAC National Accelerator Laboratory

These elusive interruptions first observed in hybrid lead perovskites may be helpful in explaining why these materials are an extraordinary skill in converting sunlight into electrical current in solar cells.

Polarons are elusive distortions in the atomic lattice of a material that form within a few trillion seconds around a moving electron and then disappear rapidly. As they are ephemeral, they affect the behavior of the material, and solar cells made with hybrid lead perovskites can also be a reason for their high efficiency in the laboratory.

Now scientists at the SLAC National Accelerator Laboratory in the Department of Energy and Stanford University have used the lab’s X-ray laser to directly observe and measure polar formation. The findings were found in Materials of nature January 4, 2021.

“These materials have taken the field of solar energy research by storm because of their high efficiency and low cost, but people are still debating why they work,” said Aaron Lindenberg, a Stanford Institute for Materials and Energy Sciences (SIMES) researcher. Associate Professor at SLAC and Stanford who led the research.

“The idea that Polarons can participate has been around for a few years,” he said. “But our experiments are the first to directly observe the emergence of these local distortions, including their size, shape, and evolution.”

How Polarons propagate through a next-generation energy material

One illustration shows polarites, elusive distortions in the atomic lattice of the material, a material of promising energy for the next generation, a hybrid lead perovskite. Scientists at SLAC and Stanford first saw how these distortion “bubbles” are created around the transport charges – electrons and holes released by light pulses – that appear here as bright points. This process can help explain why electrons travel so efficiently in these materials, leading to high solar cell performance. Credit: Greg Stewart / SLAC National Accelerator Laboratory

Exciting, complex and difficult to understand

Perovskites are crystalline materials called mineral perovskites because of their similar atomic structure. Scientists began penetrating solar cells about a decade ago, and the efficiency of converting sunlight into these cells has steadily increased, even though there are many flaws in the components of their perovskite that should inhibit current flow.

These materials are very complex and difficult to understand, Lindenberg said. Scientists find them exciting because they are both efficient and easy to make, increasing the chances of solar cells being cheaper than today’s silicon cells, highly unstable, breaking when exposed to air, and having lead to maintain. outside the environment.

Previous research at SLAC has delved into the nature of perovskites with “electron chambers” or X-ray beams. Among other things, they revealed that light surrounds atoms in perovskites, and also measured the life of acoustic phonons (sound waves) that carry heat through materials.

For this research, Lindenberg’s team used the laboratory’s Linac Coherent Light Source (LCLS), a powerful X-ray free electron laser that can represent materials in almost atomic detail and capture atomic motions that occur in billions of seconds. Associate Professor Hemamala Karunadasa studied the unique crystals of the material synthesized by the Stanford team.

They struck a small sample of the material with the light of an optical laser and then used an X-ray laser to observe how the material responded over tens of billions of seconds.

Polarons spread quickly

As this animation shows, the polaronic distortions start very small and spread rapidly outward in all directions, up to a diameter of about 5 billion meters, which is 50 times the size. This pushes about 10 layers of atoms into an approximately spherical field, in tens of picoseconds or trillion seconds. These distortions were measured in hybrid lead perovskites with an X-ray laserless electron at the National SLAC Accelerator Laboratory. Credit: Greg Stewart / SLAC National Accelerator Laboratory

Expanding distortion bubbles

“As in a solar cell, when you charge a material when you hit it with light, the electrons are released and those free electrons start moving around the material,” said Burak Guzelturk, a scientist at the Argonne National Laboratory in the DOE. at the time of the experiments he was a postdoctoral researcher at Stanford.

“It is surrounded and surrounded by a kind of bubble of local distortion that soon travels with them,” he said. “Some people have argued that this ‘bubble’ protects electrons from scattering material defects and helps them move so efficiently that they come out of contact with solar cells as electricity.”

The hybrid structure of the perovskite lattice is flexible and soft – like “a strange combination of solid and liquid at the same time,” as Lindenberg puts it – and that’s what allows polarons to form and grow.

Their observations revealed that the polar polarity distortions are very small — on the scale of some angstroms, about the distance between the atoms of a solid — and that they spread rapidly in all directions, up to about 5 billion meters in diameter, an improved 50-foot rise. This pushes about 10 layers of atoms into an approximately spherical field, in tens of picoseconds or trillion seconds.

“That distortion is pretty big, something we didn’t know before,” Lindenberg said. “That’s something completely unexpected.”

“Although this experiment shows as accurately as possible that these objects actually exist, it does not show how they contribute to the efficiency of a solar cell. There is still more work to be done to understand how these processes affect the properties of these materials.”

Reference: Head Guzelturk, Thomas Winkler, Tim WJ Van de Goor, Matthew D. Smith, Sean A. Bourelle, Sascha Feldmann, Mariano Trigo, Samuel W. Teitelbaum, Hans -Georg Steinrück, Gilberto A. de la Pena, Roberto Alonso- Mori, Diling Zhu, Takahiro Sato, Hemamala I. Karunadasa, Michael F. Toney, Felix Deschler and Aaron M. Lindenberg, January 4, 2021, Materials of nature.
DOI: 10.1038 / s41563-020-00865-5

LCLS is a DOE Science of Office user installation. Lindenberg is also a researcher at the Stanford PULSE Institute, as a joint SLAC and Stanford institute like SIMES. Scientists at the University of Cambridge in the United Kingdom; Aarhus University Denmark; and the German University of Paderborn and the Technical University of Munich also contributed to this research. The major funding came from the DOE Science Office.

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