Researchers at The University of Texas at Austin (UT Austin) recently gained better understanding of the origin of halide perovskites' extraordinary carrier lifetimes, by showing that halide perovskites are governed by unconventional electron–phonon physics, leading to the formation of topological polarons, a class of phonon-mediated electron/hole quasiparticles.
The team's 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.
For the study, the researchers used allocations on the Lonestar6 and Frontera supercomputers awarded by the Texas Advanced Computing Center (TACC), as well as U.S. Department of Energy (DOE) supercomputers at the National Energy Research Scientific Computing Center (NERSC). Simulations on TACC's Frontera and Lonestar6 supercomputers have revealed surprising vortex structures in quasiparticles of electrons and atoms, called polarons, which contribute to generating electricity from sunlight.
The team found that electrons form localized, narrow wave packets, which are known as polarons that endow perovskites with peculiar properties. These polarons are said to display intriguing patterns. The atoms rotate around the electron and form vortices that had never been observed before. The vortex structures of polarons may help the electrons remain being in an excited state, which happens when a photon of light knocks into the compounds at the atomic level.
The team suspects that this strange vortex structure prevents the electron from going back to the unexcited energy level. So, the vortex can be seen as 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.
This research is part of a project sponsored by the Department of Energy that has been going on for several years with the support of TACC and in particular Frontera, where the team developed methodologies to study how electrons interact with the underlying atomic lattice.
To manage the complicated calculations on a supercomputer, 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. This code is currently under development by an international collaborative team.
