The first sight of polarons forming in next-gen promising energy material

Poles move out fields in the atomic surface of a material that creates around a moving electron in a few trillion seconds, and then disappear abruptly. Because of their unusualness, they affect material behavior, which may be why solar cells made of lead hybrid perovskites achieve extremely high laboratory efficiency.

Scientists at the Department of Energy’s SLAC National Acceleration Laboratory and Stanford University have now used the lab’s X-ray laser to observe and measure polarons formation for the first time. They described their findings Natural Materials today.

“These products have taken the field of solar energy research by storm because of their high efficiency and low cost, but people are still arguing about why they work,” said Aaron Lindenberg, a researcher with the Stanford Institute for Materials and Energy Sciences (SIMES) at SLAC and an associate professor at Stanford led the research.

“The idea that polarons may be involved has been around for several years,” he said. “But our experiments are the first to look directly at the formation of these local areas, including their size, shape and how they grow.”

Inspiring, complex and difficult to understand

Perovskites are crystalline materials named after the mineral perovskite, which has a similar atomic structure. Scientists began introducing them into solar cells about a decade ago, and the efficiency of these cells in converting sunlight into energy has gradually increased, despite the fact that there are many defects in the cells. their perovskite components that should impede the flow of current.

These well-known products are complex and difficult to understand, Lindenberg said. While scientists find them inspiring because they are both effective and easy to make, raising the possibility that they could make solar cells cheaper than today’s silicon cells, they are also very unstable, they break down when exposed to air and contain lead that needs to be kept out of the environment.

Previous studies at SLAC have explored the nature of perovskites with “electron camera” or X-ray behavior. Among other things, they revealed that lights surround atoms in perovskites, and they also measured the life of acoustic phones – sound waves – that carry heat through the materials.

For this study, the Lindenberg team used Linac Coherent Light Source (LCLS) at the laboratory, a powerful free-electron X-ray laser capable of imaging near near-atomic detail and atomic motions. a capture that appears in millions of billions of a second. They looked at single crystals of the material co-authored by Associate Professor Hemamala Karunadasa in Stanford.

They hit a small sample of the material with light from an optical laser and then used the X-ray laser to see how the material reacted over tens of trillions of seconds.

“When you charge a material by striking it with light, as in a solar cell, electricity is released, and those free electrons begin to move around the material,” said Burak Guzelturk, a scientist at DOE ‘s Argonne National Laboratory. who was a graduate researcher at Stanford at the time of the experiments.

“Soon they are surrounded and surrounded by a kind of bubble of local distortion – the polaron – that travels with them,” he said. “Some people have argued that the These ‘bubbles’ protect electricity from scattering defects in the material, and help explain why they travel as efficiently as solar cell communications to flow out as electricity. “

The knee structure of a hybrid perovskite is flexible and soft – as “a strange mixture of solid and liquid at the same time,” as Lindenberg puts it – and this is what allows polarities to form and grow.

Their observations showed that polaronic separations begin very small – on the scale of a few angstroms, about the distance between atoms in a solid – and expand rapidly in all directions to a diameter of about 5 billion meters. , which is about a 50-fold increase. This moves about 10 rows of atoms slightly out within a relatively spherical area over tens of picoseconds, or trillionths of a second.

“This move is huge, something we didn’t know before,” Lindenberg said. “That’s something completely unexpected.”

He said, “While this test shows as directly as possible the existence of these factors, it does not show how they contribute to the efficiency of a solar cell. There is still more work to be done to understand find out how these processes affect the properties of these products. “

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LCLS is a DOE Science Office user resource. Lindenberg is also a researcher with the Stanford PULSE Institute, which like SIMES is a joint institute of SLAC and Stanford. Scientists from the University of Cambridge in the UK; Aarhus University in Denmark; and the University of Paderborn and the Technical University of Munich in Germany also contributed to this study. Significant funding came from the DOE Science Office.

Citation: Burak Guzelturk et al., Natural Materials, 4 January 2021 (10.1038 / s41563-020-00865-5)

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