Methylammonium Cation Dynamics in Methylammonium Lead Halide Perovskites: A Solid-State NMR Perspective

Abstract

In light of the intense recent interest in the methylammonium lead halides, CHNHPbX (X = Cl, Br, and I) as sensitizers for photovoltaic cells, the dynamics of the methylammonium (MA) cation in these perovskite salts has been reinvestigated as a function of temperature via H,N, and Pb NMR spectroscopy. In the cubic phase of all three salts, the MA cation undergoes pseudoisotropic tumbling (picosecond time scale). For example, the correlation time, τ, for the C-N axis of the iodide salt is 0.85 ± 0.30 ps at 330 K. The dynamics of the MA cation are essentially continuous across the cubic ↔ tetragonal phase transition; however, H andN NMR line shapes indicate that subtle ordering of the MA cation occurs in the tetragonal phase. The temperature dependence of the cation ordering is rationalized using a six-site model, with two equivalent sites along the c-axis and four equivalent sites either perpendicular or approximately perpendicular to this axis. As the cubic ↔ tetragonal phase transition temperature is approached, the six sites are nearly equally populated. Below the tetragonal ↔ orthorhombic phase transition, H NMR line shapes indicate that the C-N axis is essentially frozen.

Short-range ion dynamics in methylammonium lead iodide by multinuclear solid state NMR and127I NQR

“…Nevertheless, they are fully in agreement with the ones extracted from other 14 N NMR measurements. 56 The observed decrease of the activation barrier across the phase transition implies a less hindered rotation of MA in the cubic phase, where the 14 N signal displays a Lorentzian line shape with FWHM of only 710 Hz at 333 K, decreasing slowly as the temperature increases to 453 K (Fig. 4c).…”

“…These values indicate a rapid reorientation of the C-N axis, and are in full agreement with previous reports. 19,21,56 A much higher correlation time of 108 ps has also been reported, 28 however this value was obtained by a drastically different method that does not allow for a direct comparison. The T 1 temperature dependences (in Arrhenius coordinates, Fig.…”

Hamiltonians and order parameters for crystals of orientable molecules

“…While the idealized perovskite host crystal has cubic space-group symmetry (P m3m) at elevated temperature [41], the methylammonium ions have C 3v trigonal point symmetry. This, coupled with the fact that the methylammonium ions dynamically reorient within the crystal at elevated temperature [23,24,26], make it a fascinating and technologically important case study. There is one CH 3 NH 3 + ion per CH 3 NH 3 PbX 3 unit cell, which we treat as a rigid rotor and imbue with an orientational degree of freedom, represented by a unit quaternion variable.…”

“…A unique feature of hybrid perovskites is the presence of molecular interstitial cations, such as CH 3 NH + 3 , that can rotate within the cuboctohedral interstitial sites of a halide based perovskite host [19][20][21][22]. The molecular cations of hybrid perovskites are observed to undergo orientational order-disorder transformations that couple with the structural transitions of the host [23,24]. In the low temperature orthorhombic phase of CH 3 NH 3 PbI 3 , for example, molecular orderings favor an orientationally locked head-to-tail arrangement with rotational fluctuations occurring only along the high symmetry axis of the CH 3 NH 3 + ion.…”

Advanced partial nucleation for single-phase FA0.92MA0.08PbI3-based high-efficiency perovskite solar cells

“…However, increasing the FA content in the mixed system has resulted in a change in the quality of the perovskite film. Han et al [13][14][15][16] proposed a novel vertical recrystallization technique to prepare high FA content of FA x MA 1−x PbI 3 perovskite. This method achieved an FA content of as high as 0.95, but the highest efficiency reported was an FA ratio of 0.86.…”