A close examination of the structure and dynamics of HC(NH₂)₂PbI₃ by MD simulations and group theory

Abstract

The formamidinium lead iodide hybrid perovskite is studied using first principles molecular dynamics simulations and further analyzed using group theory. The simulations are performed on large supercells containing 768 atoms under isothermal and fully anisotropic isobaric conditions. Two trajectories, one at 300 K and another at 450 K, were extended for over 50 ps in order to perform a detailed assessment of the rotational dynamics of organic cations. The characteristic rotations of the cation are analyzed by defining two rotation axes. It is found that the formamidinium molecules rotate preferentially around the direction parallel to the line connecting the two nitrogen atoms. The rotational dynamics shows some characteristics already observed in methylammonium lead iodide, like the heterogeneous dynamics at room temperature that disappears at 450 K. The orientational probability of the molecules is explored in terms of an expansion in cubic harmonics up to the 12th order. It reveals a strong directionality at room temperature that relaxes when increasing the temperature. These findings are further rationalized using Landau and group theories suggesting a mixed displacive/order-disorder structural instability at lower temperatures.

Universal Dynamics of Molecular Reorientation in Hybrid Lead Iodide Perovskites

“…Rotations about an axis parallel to the N···N line (φ 3 axis) are the preferred mode in all phases, in agreement with a recent report of heterogeneous dynamics at 300 K from ab initio molecular dynamics. 36 As shown in Table 1, the activation barriers for φ 3 rotations are in remarkable agreement with experiment for the α-and β-phases, and a slight underestimate of the experimental value for the disordered γ-phase. Though a few meV higher in energy than the φ 1 = 0…”

“…For each rotation mode about the principal axes of the molecule, the other two rotation angles are fixed (for φ 1 rotations, φ 2 = φ 3 = 0; for φ 2 rotations, φ 1 = (0, 90 29 Reported reorientation times from GHz spectroscopy, quasi-elastic neutron scattering (QENS), ab initio molecular dynamics (MD), and 14 N NMR are presented for comparison. 15,16,23,24,36,42,73 14 reorientation. In particular, it is intriguing to consider if the differing barriers to molecular dipole rotation and differing dipole moments lead to important differences between FAPbI 3 and MAPbI 3 on timescales relevant to exciton dissociation, interaction of charge carriers with polar optical phonons or polarons, or momentum separation of excited electrons and holes due to possible dynamic Rashba-Dresselhaus effects.…”

“…This represents a synthesis of the results of this work with the findings of complementary prior reports. 14,16,19,23,29,36,41 As FAPbI 3 is cooled, molecular motion slows and the inorganic octahedra tilt to improve coordination of the A-site cation and enhance electrostatic binding, but the molecular geometry appears to be incompatible with the preferred ground state octahedral tilting pattern. This leads to substantially more complex behavior in FAPbI 3 than in MAPbI 3 , with a disordered pseudo-cubic γ-phase with persistent dipole motion, 19 followed by a fragile glassy freezing into a disordered ground state.…”

“…Since the first preparation of the hybrid lead halide perovskites, 12 the behavior of the molecular cations has been studied by calorimetry, 13,14 infrared spectroscopy, 13,15,16 dielectric spectroscopy, 14,[17][18][19][20] Raman spectroscopy, 21 quasi-elastic and inelastic neutron scattering, [22][23][24][25][26] nuclear magnetic resonance (NMR) spectroscopy, [27][28][29][30][31][32] isotopic substitution, 33 and ab initio methods, [34][35][36][37][38][39] and these findings have been correlated to the structure evolution from crystallographic techniques. 19,[40][41][42][43][44][45][46] Most of these efforts have focused on MAPbX 3…”

Effect of Halogen Substitution on the Absorption and Emission Profile of Organometallic Perovskites

“…The effect of non-stoichiometric precursors on MAPbI3 PSCs had also been investigated, showing an improvement on the photovoltaic performance [11] as well as the fill factor [12] in step with the increasing PbI2 amount. Instead of real-life experiment, several studies are done using computer simulation, mostly using Density Functional Theory (DFT), MD simulation, Group theory and GW approximation, to unravel the underlying working mechanism and physics of the ABX3 perovskite material [13][14][15][16].…”