A key contributor to how halide perovskites create and transport electricity literally hinges on the way their octahedral atomic lattice twists and turns in a hinge-like fashion. Image: ORNL/Jill Hemman.
A key contributor to how halide perovskites create and transport electricity literally hinges on the way their octahedral atomic lattice twists and turns in a hinge-like fashion. Image: ORNL/Jill Hemman.

Researchers at Duke University have uncovered long-hidden molecular dynamics that confer desirable properties for solar energy and heat energy applications on an exciting class of materials called halide perovskites.

A key contributor to how these materials create and transport electricity literally hinges on the way their atomic lattice twists and turns in a hinge-like fashion. This finding, reported in a paper in Nature Materials, will help materials scientists in their quest to tailor the chemical recipes of these materials for a wide range of applications in an environmentally friendly way.

"There is a broad interest in halide perovskites for energy applications like photovoltaics, thermoelectrics, optoelectronic radiation detection and emission – the entire field is incredibly active," said Olivier Delaire, associate professor of mechanical engineering and materials science at Duke. "While we understand that the softness of these materials is important to their electronic properties, nobody really knew how the atomic motions we've uncovered underpin these features."

Perovskites are a class of materials that – with the right combination of elements – can grow into a crystalline structure that is particularly well-suited for energy applications. For example, their ability to absorb light and transfer its energy efficiently make them a common target for researchers developing new types of solar cells. They're also soft, giving them the ability to tolerate defects and avoid cracking when made into a thin film.

One size, however, does not fit all, as there is a wide range of potential recipes that can form a perovskite. Many of the simplest and most studied recipes include a halogen – such as chlorine, fluorine or bromine – giving rise to so-called halide perovskites. In the crystalline structure of halide perovskites, halides are the joints that tether adjoining octahedral crystal motifs together.

While researchers have known that these pivot points are essential to creating a perovskite's properties, nobody has been able to investigate the way they allow the structures around them to dynamically twist, turn and bend without breaking, like a Jell-O mold being vigorously shaken.

"These structural motions are notoriously difficult to pin down experimentally. The technique of choice is neutron scattering, which comes with immense instrument and data analysis effort, and very few groups have the command over the technique that Olivier and his colleagues do," said Volker Blum, professor of mechanical engineering and material science at Duke, who does theoretical modeling of perovskites but was not involved with this study. "This means that they are in a position to reveal the underpinnings of the materials properties in basic perovskites that are otherwise unreachable."

In the study, Delaire and colleagues from Argonne National Laboratory, Oak Ridge National Laboratory, the National Institute of Science and Technology and Northwestern University reveal, for the first time, important molecular dynamics of the structurally simple, commonly researched halide perovskite CsPbBr3.

The researchers started with a large, centimeter-scale, single crystal of this halide perovskite, which is notoriously difficult to grow to such sizes – a major reason why this sort of dynamic study has not been performed before now. They then barraged the crystal with neutrons at Oak Ridge National Laboratory and with X-rays at Argonne National Laboratory. By measuring how the neutrons and X-rays bounced off the crystals over many angles and at different time intervals, the researchers teased out how its constituent atoms moved over time.

After confirming their interpretation of these measurements with computer simulations, the researchers discovered just how active the crystalline network actually is. Eight-sided octahedral motifs attached to one another through bromine atoms were caught twisting collectively in plate-like domains and constantly bending back and forth in a very fluid-like manner.

"Because of the way the atoms are arranged with octahedral motifs sharing bromine atoms as joints, they're free to have these rotations and bends," said Delaire. "But we discovered that these halide perovskites in particular are much more 'floppy' than some other recipes. Rather than immediately springing back into shape, they return very slowly, almost more like Jell-O or a liquid than a conventional solid crystal."

Delaire explained that this free-spirited molecular dancing is important for understanding many of the desirable properties of halide perovskites. Their 'floppiness' stops electrons from recombining with the holes the incoming photons knocked them out of, helping halide perovskites to generate a lot of electricity from sunlight. And it likely also makes it difficult for heat energy to travel across the crystalline structure, which allows halide perovskites to create electricity from heat when one side of the material is much hotter than the other.

Because the perovskite used in this study – CsPbBr3 – has one of the simplest recipes, yet still contains the structural features common to the broad family of these compounds, Delaire believes that these findings likely apply to a large range of halide perovskites. He cites hybrid organic-inorganic perovskites (HOIPs), which have much more complicated recipes, as well as lead-free double-perovskite variants that are more environmentally friendly.

"This study shows why this perovskite framework is special even in the simplest of cases," said Delaire. "These findings very likely extend to much more complicated recipes, which many scientists throughout the world are currently researching. As they screen enormous computational databases, the dynamics we've uncovered could help decide which perovskites to pursue."

This story is adapted from material from Duke University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.