Organic-inorganic lead halide perovskites have unusual, temperature-dependent emission characteristics.

An interatomic model potential for molecular dynamics is derived from first-principles and used to study the molecular rotations and relaxation times in methylammonium lead halide, here considered the prototypical example of a hybrid crystal with a strong reorientational dynamics. Within the limits of a simple ionic scheme, the potential is able to catch the main qualitative features of the material at zero and finite temperature and opens the way to the development of classical potentials for hybrid perovskites. In agreement with experiments and previous theoretical findings, the molecule trajectories exhibit a transition from a dynamics dominated by high symmetry directions at low temperature to a fast dynamics at room temperature in which the molecule can reorient quasi-randomly. By computing the angular time correlation function we discuss the reorientational time as a function of temperature in comparison with existing literature, providing a simple model and a clear attribution of the relaxation times in terms of their temperature dependence. This work clarifies the temperature dependence of the relaxation times and the interpretation of the experimental data in terms of the different mechanisms contributing to the molecule dynamics.

No abstract

Homologous classes of Polycyclic Aromatic Hydrocarbons (PAHs) in their crystalline state are among the most promising materials for organic opto-electronics. Following previous works on oligoacenes we present a systematic comparative study of the electronic, optical, and transport properties of oligoacenes, phenacenes, circumacenes, and oligorylenes. Using density functional theory (DFT) and timedependent DFT we computed: (i) electron affinities and first ionization energies; (ii) quasiparticle correction to the highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) gap; (iii) molecular reorganization energies; (iv) electronic absorption spectra of neutral and ±1 charged systems. The excitonic effects are estimated by comparing the optical gap and the quasiparticle corrected HOMO-LUMO energy gap. For each molecular property computed, general trends as a function of molecular size and charge state are discussed. Overall, we find that circumacenes have the best transport properties, displaying a steeper decrease of the molecular reorganization energy at increasing sizes, while oligorylenes are much more efficient in absorbing low-energy photons in comparison to the other classes.

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.

Halide perovskites are emerging as revolutionary materials for optoelectronics. Their ionic nature and the presence of mobile ionic defects within the crystal structure have a dramatic influence on the operation of thin‐film devices such as solar cells, light‐emitting diodes, and transistors. Thin films are often polycrystalline and it is still under debate how grain boundaries affect the migration of ions and corresponding ionic defects. Laser excitation during photoluminescence (PL) microscopy experiments leads to formation and subsequent migration of ionic defects, which affects the dynamics of charge carrier recombination. From the microscopic observation of lateral PL distribution, the change in the distribution of ionic defects over time can be inferred. Resolving the PL dynamics in time and space of single crystals and thin films with different grain sizes thus, provides crucial information about the influence of grain boundaries on the ionic defect movement. In conjunction with experimental observations, atomistic simulations show that defects are trapped at the grain boundaries, thus inhibiting their diffusion. Hence, with this study, a comprehensive picture highlighting a fundamental property of the material is provided while also setting a theoretical framework in which the interaction between grain boundaries and ionic defect migration can be understood.

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.

We report on the thermal conductivities of two-dimensional metal halide perovskite films measured by time domain thermoreflectance. Depending on the molecular substructure of ammonium cations and owing to the weaker interactions in the layered structures, the thermal conductivities of our two-dimensional hybrid perovskites range from 0.10 to 0.19 W m–1 K–1, which is drastically lower than that of their three-dimensional counterparts. We use molecular dynamics simulations to show that the organic component induces a reduction of the stiffness and sound velocities along with giving rise to vibrational modes in the 5–15 THz range that are absent in the three-dimensional counterparts. By systematically studying eight different two-dimensional hybrid perovskites, we show that the thermal conductivities of our hybrid films do not depend on the thicknesses of the organic layers and instead are highly dependent on the relative orientation of the organic chains sandwiched between the inorganic constituents.