Investigating the origin of polaron formation in halide perovskites

Halide perovskites are a class of materials with an underlying structure similar to mineral perovskites, but with the X sites occupied by halide ions, while their A and B sites are occupied by cations. These materials have various advantageous properties that make them promising candidates for the development of photovoltaics (PV), light-emitting diodes (LEDs), and other optoelectronic devices.

Recent studies have gathered interesting insights into halide perovskites and their optoelectronic properties. However, the origin of the remarkable carrier lifetimes of these materials has not yet been revealed.

Scientists at the University of Texas at Austin recently conducted a study that aimed to shed new light on the origins of these extraordinary carrier lives. Their work, published in PNASshows that halide perovskites obey unconventional electron-phonon physics, leading to the creation of a new class of quasiparticles that the authors termed “topological polarons.”

“Our motivation was experimental in nature,” Jon Lafuente, Chao Lian and Feliciano Giustino, co-authors of the paper, told Phys.org.

“Due to their exceptional optoelectronic properties, such as long carrier lifetimes and diffusion lengths, halide perovskites are extraordinary materials for applications in photovoltaics and light-emitting devices. Some of the most advanced experimental techniques have been applied to these materials to shed light on their origins.” of these unusual properties and to elucidate the origin of their extraordinary efficiency of energy conversion.’

Evidence gathered in recent experiments suggests that strong interactions between electrons and vibrations in the atomic lattice of halide perovskites could contribute to their remarkable carrier lifetimes and energy conversion efficiency. Specifically, some researchers have suggested that the key process behind these properties could be the formation of polarons, localized quasi-particles composed of electrons associated with deformations of the crystal lattice.

“The lack of suitable theoretical methodologies that encompass the full complexity of these materials and these quasiparticles has so far hindered our ability to understand polaron formation in halide perovskites at the atomic scale,” explained Lafuente and Giustino.

“Our group has recently developed a new high-throughput computational approach to study polaron formation involving the interaction between electronic carriers and lattice vibrations, starting from the first principles of quantum mechanics.”

Over the past few years, Lafuente, Lian, Giustino, and their colleagues have sought to facilitate the implementation of their proposed methodology using high-performance codes that they could then run on some of the world’s largest supercomputers (i.e., the TACC and NERSC computers). In their recent study, they specifically decided to use these methods to study polaron formation in halide perovskites.

“With these methods, we were able to consider simulation cells ranging from a few dozen to nearly half a million atoms, which has never been achieved before,” said Lafuente and Giustino.

“Our calculations led to several unexpected results. First, we found that polarons can take many different forms in halide perovskites; they can be very large, reaching a length of several nanometers, or they can be very small, localized around a single bismuth atom.” “

Simulations by Lafuente also revealed that polarons in halide perovskites can even form periodic distortions that manifest themselves at high enough densities as charge density waves. Remarkably, the different types of polarons they observed in their simulations appear to have formed on different timescales.

“For example, we predict that upon illumination, large polarons are first formed and then converted to small polarons,” said Lafuente and Giustino.

“Our predictions are in remarkable agreement with available ultrafast pump-probe spectroscopy experiments. Perhaps the most surprising discovery, however, is that the polarons in halide perovskites come with a ‘twist’; the atomic displacements surrounding the polarons create vortex patterns, and the associated field vector has a well-defined topology that can be described by quantized topological numbers.”

The topological structures uncovered by the researchers were found to be strikingly similar to the structures of skyrmions, merons and Bloch points – three types of interesting quasiparticles previously observed in magnetic systems. The existence of non-magnetic polarons with characteristics similar to those of magnetic quasiparticles has never been reported before, so this study could open new avenues for future research, which could lead to exciting discoveries.

“There are two main directions that we are eagerly pursuing now,” said Lafuente and Giustino. “On the one hand, while these results paint a detailed picture of polarons in halide perovskites at the atomic scale, they do not tell us exactly how these quasiparticles interact with light or how they propagate through the material. We would like to develop methods to predict the transport and optical properties of these polarons in more detail .”

By developing new approaches to predict the optical properties of polarons in halide perovskites, scientists hope to reliably predict new physical phenomena and explain their origin. At the same time, they plan to investigate the extent to which their findings can be generalized across different materials.

“Are topological polarons unique to halide perovskites, or can they also form in other materials?” added Lafuente and Giustino.

“What are the main physical ingredients needed to form topological polarons? Can we tune material parameters, e.g. through voltage, chemical composition or light, to tune the topological charge and helicity of polarons?

“These are some of the bigger questions we will try to answer in the future. Ultimately, the discovery of topological polarons may open up entirely new avenues in the manipulation of quantum information through new non-classical degrees of freedom.”

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