Real-Time Observation of Organic Cation Reorientation in Methylammonium Lead Iodide Perovskites

Abstract

Abstract:The introduction of a mobile and polarised organic moiety as a cation in three-dimensional lead-iodide perovskites brings fascinating optoelectronic properties to these materials. The extent and the timescales of the orientational mobility of the organic cation and the molecular mechanism behind its motion remain unclear, with different experimental and computational approaches providing very different qualitative and quantitative description of the molecular dynamics. Here we use ultrafast two-dimensional vibrational spectroscopy of methylammonium (MA) lead iodide, to directly resolve the rotation of the organic cations within the MAPbI3 lattice. Our results reveal two characteristic time constants of motion. Using ab-initio molecular dynamics simulations, we identify these as a fast (~300 fs) 'wobbling-ina-cone' motion around the crystal axis, and a relatively slow (~3 ps) jump-like reorientation of the molecular dipole with respect to the iodide lattice. The observed dynamics are essential for understanding the electronic properties of perovskite materials. TOC figure:

Giant Rashba Splitting inCH3NH3PbBr3Organic-Inorganic Perovskite

“…Yet, the individual bromide atoms reside 0.04 a from the high-symmetry sites in a disordered arrangement [36,50,51]. In CH 3 NH 3 PbI 3 , the methylamine ion wobbles and flips on 0.3 ps and 3 ps time scales [52], and similar motions may be expected for CH 3 NH 3 PbBr 3 . The resulting spatially and temporally fluctuating fields give rise to a dynamical Rashba effect.…”

Universal Dynamics of Molecular Reorientation in Hybrid Lead Iodide Perovskites

“…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…”

Energy Landscape of Molecular Motion in Cubic Methylammonium Lead Iodide from First-Principles

“…At elevated temperature, the PbI 6 octahedra of the inorganic host undergo large tilt-mode oscillations relative to their average positions in the ideal cubic structure, 10 while the molecular cations rotate rapidly. 11,12 Distortions of the inorganic lattice by halide substitution and octahedral tilting have been shown to influence the band gap and absorption properties. 7,[13][14][15] Although the A-cation does not contribute to electronic states near the band gap, [16][17][18] it has been shown to affect the nature of the band gap through interaction with the inorganic Pb−I sublattice.…”

“…11,[20][21][22][23] CH 3 NH 3 PbI 3 undergoes a transition from the high-temperature pseudo-cubic aristotype P m3m structure to the tetragonal I4/mcm phase (a 0 a 0 c − tilt system in Glazer notation) 24 at 330 K and then to the orthorhombic P nma phase (a + b − c − tilt system) below 160 K. [25][26][27][28] The transitions associated with A-cation ordering have been the subject of some debate. [11][12][13]20,[29][30][31] Nevertheless, recent quasi-elastic neutron scattering experiments indicate that the CH 3 NH 3 + molecules in the cubic phase dynamically disorder and undergo both fast reorientations of the C−N bond axis (≈5 ps at 300 K) and faster on-axis rotations about the C−N bond axis (≈1 ps at 300 K). 11 Some degrees of freedom freeze out upon cooling and only on-axis rotations are observed in the low temperature orthorhombic phase (≈4 ns at 70 K) accompanied by a dramatic loss in dielectric permittivity.…”