New discovery paves the way for more efficient perovskite solar cells

Researchers from University of Texas have used computational methods to study the formation of polarons in halide perovskites. The findings revealed topological vortices in polaron quasiparticles.

August 6, 2024 Patrick Jowett

Skyrmion-like large hole polaron in the halide double perovskite Cs2AgBiBr6.

Image: J. Lafuente-Bartolome

From pv magazine Global

Researchers at the University of Texas in Austin have explored the formation of polarons by examining the properties of halide perovskites.

Halide perovskites are used in applications such as photovoltaics due to their optoelectric properties. In the research paper “Topological polarons in halide pervoskites,” published in the Proceedings of the National Academy of Sciences, the scientists used supercomputers Lonestar6 and Frontera from the Texas Advanced Computing Center (TACC) to analyze these properties at the individual atom level.

The team developed EPW, an open-source Fortran and message-passing interface code that calculates properties related to electron-phonon interaction. The EPW code specializes in studying how electrons interact with vibrations in the lattice of a solid, which causes the formation of polarons.

“We found that electrons form localized, narrow wave packets, which are known as polarons,” Feliciano Giustino, one of the paper’s lead authors, told TACC. “These ‘lumps of charge’ – the quasiparticle polarons – endow perovskites with peculiar properties.”

“These polarons show very intriguing patterns. The atoms rotate around the electron and form vortices that had never been observed before. We suspect that this strange vortex structure prevents the electron from going back to the unexcited energy level,” Giustino explained. “This vortex is a protected topological structure in the halide perovskite lattice material that remains in place for a long time and allows the electrons to flow without losing energy.”

In the research paper, the scientists add that polarons take many different forms in halide perovskites, including small polarons, large polarons and charge density waves.

“We find that these emergent quasiparticles support topologically nontrivial phonon fields with quantized topological charge, making them nonmagnetic analog of the helical Bloch points found in magnetic skyrmion lattices,” the researchers said. “Our findings suggest that halide perovskites may be regarded as a class of quantum materials where electron-phonon couplings replace the traditional electron–electron interactions of correlated electron systems.”

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