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TYC@imperial: Origin of Dynamic Lattice Instabilities in Metal Halide Perovskites

Ruoxi Yang, Department of Materials,

Imperial College London

Time: 12pm
Venue: Lecture Room G20, Royal school of Mines, Imperial College London
Contact: Ms Hafiza Bibi

Abstract: Inorganic and hybrid halide perovskites, important members of the perovskite family, have shown exceptional photovoltaic performance with efficiencies exceeding 20%. Instability of these materials is a major issue for real world applications including both chemical breakdown and current-voltage hysteresis. Most perovskites adopt cubic crystal structures (Pm-3m type) at high temperature, and undergo various phase transitions and adopt lower symmetry space groups at lower temperature due to structural instability.[1] This phenomenon is due to a combination of octahedral tilting, molecular rotation and cation disorder within the perovskite unit cell. Depending on the specific transition path, large changes in the electronic and optical properties can take place that can affect photovoltaic performance.

Here we present a comprehensive analysis of the vibrational (phonon) structure for CH3NH3PbI3 and a series of 24 inorganic metal halides: AMX3 (A = Cs, Rb; M = Ge, Sn, Pb; X = F, Cl, Br, I). Calculations have been performed using lattice dynamics with forces based on density functional theory building on recent work [2]. We demonstrate, from quantum chemical investigations, that all inorganic halide perovskites exhibit lattice instabilities in their high temperature cubic phase. These instabilities include octahedral titling and second-order Jahn-Teller distortions depending on the chemical identity and radius of the underlying composition. We provide quantitative insights into the thermodynamic driving forces, and how the macroscopic properties will be affected. There are major implications for the applications of these materials.

 

[1] Howard, C. J.; Stokes, H. T. Acta Crystallogr. Sect. B 1998, 54, 782–789.

[2] Brivio, F.; Frost, J. M.; Skelton, J. M.; Jackson, A. J.; Weber, O. J.; Weller, M. T.; Goni, A. R.; Leguy, A. M. A.; Barnes, P. R. F.; Walsh, A. Phys. Rev. B 2015, 92, 144308.

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