Excitons, Phonons and Symmetry Breaking in Complex Metal Halide Perovskites from First Principles

<br/>The ability to synthesize and probe a diversity of metal halide perovskites has driven the need for new intuition to interpret and predict their structural, electronic, and optical properties and how they vary with composition, dimensionality, temperature, and pressure. Here, I will discuss recent examples of first-principles calculations – based on density functional theory and ab initio many-body perturbation theory – of the structural, electronic and optical properties of complex metal halide perovskites, drawing on recent advances in non-empirically tuned hybrid functionals and our ability to compute electron-phonon and exciton-phonon interactions. I will focus on recent studies of lead halide perovskites, and of novel lead-free double and mixed-valence perovskites. I will highlight new understanding of their phase behavior, electronic structure and photoexcited states, and discuss how these properties are influenced by phonons, composition, organic cations, dimensionality, and pressure, comparing with experiments where possible and emphasizing new intuition that emerges from our calculations. Portions of this work are supported by the Theory of Materials program and Center for Computational Study of Excited-State Phenomena in Energy Materials (C2SEPEM) at the Lawrence Berkeley National Laboratory, funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division, under Contract No. DE-C02-05CH11231, and by the U.S.-Israel NSF–Binational Science Foundation under Grant No. DMR-2015991. Computational resources provided by NERSC at LBNL and XSEDE at TACC.