Investigation of the origin of polaron formation in halide perovskites

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

Recent studies have provided interesting insights into halide perovskites and their optoelectronic properties. Nevertheless, the origin of the remarkable longevity of these materials has not yet been discovered.

Researchers from the University of Texas at Austin recently conducted a study to shed new light on the origins of this extraordinary carrier longevity. Their article, published in PNASshows that halide perovskites are governed by unconventional electron-phonon physics, resulting in the formation 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.

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

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. Some researchers have specifically suggested that the key process underlying these properties could be the formation of polarons, localized quasiparticles consisting of electrons coupled to distortions of the crystal lattice.

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

“Our group had recently developed a new, powerful computational approach to study the formation of polarons, integrating the interaction between the electronic carriers and lattice vibrations, starting from first principles of quantum mechanics.”

In recent years, Lafuente, Lian, Giustino and their colleagues have attempted to facilitate the implementation of their proposed methodology using very powerful codes, which they were then able to run on some of the world’s largest supercomputers (i.e. the TACC and NERSC computers). As part of their recent research, they wanted to use these methods specifically to study the formation of polarons in halide perovskites.

“These methods allowed us to consider simulation cells ranging from a few tens to almost half a million atoms, which has never been achieved before,” Lafuente and Giustino said.

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

Lafuente’s simulations also revealed that polarons in halide perovskites can even form periodic distortions, which at sufficiently high densities manifest as charge density waves. In particular, the different types of polarons they observed in their simulations appeared to form on different time scales.

“For example, we predict that upon illumination, large polarons will first form and then transform into small polarons,” Lafuente and Giustino said.

“Our predictions agree remarkably well with available ultrafast pump-probe spectroscopy experiments. Perhaps the most surprising discovery, however, is that polarons in halide perovskites have a ‘twist’; the atomic displacements around the polarons form vortex patterns and their associated vector fields have a well-defined topology that can be described by quantized topological numbers.”

The topological structures revealed by the researchers turned out to be strikingly similar to those of skyrmions, merons and Bloch points – three types of intriguing quasiparticles previously observed in magnetic systems. The existence of non-magnetic polarons with characteristics similar to those of magnetic quasiparticles had never been reported before, so this study could open new avenues for future research, potentially leading to exciting discoveries.

“There are two main directions we would like to follow now,” said Lafuente and Giustino. “On the one hand, even if these results paint a detailed atomic-scale picture of polarons in halide perovskites, 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 transport and optical properties of these polarons in more detail.”

By developing new approaches to predict the optical properties of polarons in halide perovskites, the researchers hope to reliably predict new physical phenomena and explain their origins. 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?” Lafuente and Giustino added.

“What are the key physical ingredients required for the formation of topological polarons? Can we tune material parameters, for example via voltage, chemical composition or light, to adjust 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 could open entirely new avenues in the manipulation of quantum information through new, non-classical degrees of freedom.”

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