Exploring the origin of polaron formation in halide perovskites

Halide perovskites are a class of materials whose underlying structure resembles that of mineral perovskites, but with X sites occupied by halide ions, while their A and B sites are occupied by cations. These materials possess 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 yielded interesting insights into halide perovskites and their optoelectronic properties. However, the origin of the remarkable lifespan of the supports of these materials has not yet been discovered.

Researchers at the University of Texas at Austin recently conducted a study aimed at shedding new light on the origin of these extraordinary carrier lifespans. 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 have dubbed “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 extraordinary materials for photovoltaic applications and light-emitting devices due to their exceptional optoelectronic properties, such as long carrier lifetimes and scattering lengths. Some of the most advanced experimental techniques have been applied to these materials to illuminate the origin of these unusual properties and to clarify the origin of their extraordinary energy conversion efficiency.

Evidence from recent experiments suggests that strong interactions between electrons and vibrations in the atomic lattice of halide perovskites may contribute to the remarkable lifetime of their carriers and their power conversion efficiency. Specifically, some researchers have suggested that the key process underlying these properties may be the formation of polarons, localized quasiparticles made of electrons coupled to distortions in the crystal lattice.

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

“Our group recently developed a new high-performance computing approach to study polaron formation integrating the interaction between electronic media and lattice vibrations, starting from first principles of quantum mechanics.”

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

“Using these methods, we were able to consider simulation cells ranging from several dozen 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, spanning several nanometers in length, or they can be very small, located around a single bismuth atom.

Simulations performed by Lafuente also revealed that polarons in halide perovskites can even form periodic distortions, manifesting at high enough densities as charge density waves. Notably, the different types of polarons observed in their simulations appeared to form on different time scales.

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

“Our predictions agree remarkably well with available ultrafast pump-probe spectroscopy experiments. Perhaps the most surprising finding, however, is that polarons in halide perovskites exhibit a ‘twist’; the atomic shifts surrounding the polarons form vortex models, and the 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 quasi-particles previously observed in magnetic systems. The existence of non-magnetic polarons with characteristics resembling those of magnetic quasiparticles had never been reported before. This study could therefore open new avenues for future research, potentially leading to exciting discoveries.

“We now look forward to pursuing two main directions,” Lafuente and Giustino said. “On the one hand, although 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 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, researchers hope to reliably predict new physical phenomena and explain their origin. At the same time, they plan to explore the extent to which their findings can be generalized to different materials.

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

“What are the main physical ingredients necessary for the formation of topological polarons? Can we adjust material parameters, for example via deformation, chemical composition or light, to change the topological charge and helicity of polarons?

“These are some of the biggest questions we will attempt to answer in the future. Ultimately, the discovery of topological polarons could open up entirely new avenues in manipulating quantum information via new degrees of freedom non-classical.”

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