We study the diffusion of point defects in crystalline methylammonium lead halide (MAPI) at finite temperatures by using all-atoms molecular dynamics. We find that, for what concerns intrinsic defects, iodine diffusion is by far the dominant mechanism of ionic transport in MAPI, with diffusivities as high as 7.4 × 10(-7) and 4.3 × 10(-6) cm(2) s(-1) at 300 K and single activation energies of 0.24 and 0.10 eV, for interstitials and vacancies, respectively. The comparison with common covalent and oxide crystals reveals the ultrahigh mobility of defects in MAPI. Though at room temperature the vacancies are about 1 order of magnitude more diffusive, the anisotropic interstitial dynamics increases more rapidly with temperature, and it can be dominant at high temperatures. Present results are fully consistent with the involvement of iodide ions in hysteresis and have implications for improvement of the material quality by better control of defect diffusion.

The temperature evolution of vibrations of CH3NH3PbI3 (MAPI) is studied by combining first principles and classical molecular dynamics and compared to available experimental data. The work has a fundamental character showing that it is possible to reproduce the key features of the vibrational spectrum by the simple physical quantities included in the classical model, namely the ionic-dispersive hybrid interactions and the mass difference between organic and inorganic components. The dynamics reveals a sizable temperature evolution of the MAPI spectrum along with the orthorhombic-to-tetragonal-to-cubic transformation and a strong dependence on molecular confinement and order. The thermally induced weakening of the H-I interactions and the anharmonic mixing of modes give two vibrational peaks at 200-250 cm(-1) that are not present at zero temperature and are expected to have detectable infrared activity. The infrared inactive vibrational peak at ∼140 cm(-1) due to molecular spinning disappears abruptly at the orthorhombic-to-tetragonal transition and forms a broad molecular band red-shifting progressively with temperature. This trend is correlated to the reduced confinement of the rotating cations due to thermal expansion of the lattice.

The topical review describes the recent progress in the modeling of hybrid perovskites by molecular dynamics simulations. Hybrid perovskites and in particular methylammonium lead halide (MAPI) have a tremendous technological relevance representing the fastest-advancing solar material to date. They also represent the paradigm of an organic-inorganic crystalline material with some conceptual peculiarities: an inorganic semiconductor for what concerns the electronic and absorption properties with a hybrid and solution processable organic-inorganic body. After briefly explaining the basic concepts of ab initio and classical molecular dynamics, the model potential recently developed for hybrid perovskites is described together with its physical motivation as a simple ionic model able to reproduce the main dynamical properties of the material. Advantages and limits of the two strategies (either ab initio or classical) are discussed in comparison with the time and length scales (from pico to microsecond scale) necessary to comprehensively study the relevant properties of hybrid perovskites from molecular reorientations to electrocaloric effects. The state-of-the-art of the molecular dynamics modeling of hybrid perovskites is reviewed by focusing on a selection of showcase applications of methylammonium lead halide: molecular cations disorder; temperature evolution of vibrations; thermally activated defects diffusion; thermal transport. We finally discuss the perspectives in the modeling of hybrid perovskites by molecular dynamics.

The origin of the dissolution of methylammonium lead trihalide (MAPI) crystals in liquid water is clarified by finite-temperature molecular dynamics by developing a MYP-based force field (MYP1) for water-MAPI systems. A thermally activated process is found with an energy barrier of 0.36 eV consisting of a layer-by-layer degradation with generation of inorganic PbI films and solvation of MA and I ions. We rationalize the effect of water on MAPI by identifying a transition from a reversible absorption and diffusion in the presence of vapor to the irreversible destruction of the crystal lattice in liquid due to a cooperative action of water molecules. A strong water-MAPI interaction is found with a binding energy of 0.41 eV/HO and wetting energy of 0.23 N/m. The water vapor absorption is energetically favored (0.29 eV/HO), and the infiltrated molecules can migrate within the crystal with a diffusion coefficient D = 1.7 × 10 cm/s and activation energy of 0.28 eV.

The fundamental properties of lead halide perovskites, rivaling those of conventional semiconductors, make these systems attractive not just for solar cells but also for a broader playground of energy and nanotechnology applications. The recently measured ultralow thermal conductivity of the perovskites suggests the possibility of high thermoelectric efficiency and the possible use of the perovskites for solar-thermoelectric generation capable to capture both above-gap and below-gap sun illumination. Here we explore this possibility presenting a theoretical analysis of the thermoelectric behavior of CH 3 NH 3 PbI 3 for a wide range of temperatures and carrier concentrations. For electron doping, we find optimal carrier density n ∼ 10 19 cm −3 , at which this material displays room-T power factor σS 2 ∼ 0.8 × 10 −3 W/mK 2 , derived by moderate electrical conductivity σ and robust thermopower, with Seebeck coefficient S of approximately hundreds of μV/K, typical of polar insulating perovskites. In combination with a measured thermal conductivity ∼0.3−0.5 W/ mK, this delivers figure of merits Z ∼ 1−3 × 10 −3 K −1 , thus in the league of the best performing thermoelectric tellurides and skutterudites. For hole doping, on the other hand, the figure of merit is sensitively reduced by a factor 2 to 3, due to the isotropic nature of the valence band edge. These results can be a stimulus and a guideline to the search of strategies for chemical doping, which has been scarcely investigated so far, for these materials.