A Complete Picture of Cation Dynamics in Hybrid Perovskite Materials from Solid-State NMR Spectroscopy

Abstract

The organic cations in hybrid organic−inorganic perovskites rotate rapidly inside the cuboctahedral cavities formed by the inorganic lattice, influencing optoelectronic properties. Here, we provide a complete quantitative picture of cation dynamics for formamidinium-based perovskites and mixed-cation compositions, which are the most widely used and promising absorber layers for perovskite solar cells today. We use 2 H and 14 N quadrupolar solid-state NMR relaxometry under magic-angle spinning to determine the activation energy (E a ) and correlation time (τ c ) at room temperature for rotation about each principal axis of a series of organic cations. Specifically, we investigate methylammonium (MA + ), formamidinium (FA + ), and guanidinium (GUA + ) cations in current state-of-the-art single-and multication perovskite compositions. We find that MA + , FA + , and GUA + all have at least one component of rotation that occurs on the picosecond timescale at room temperature, with MA + and GUA + also exhibiting faster and slower components, respectively. The cation dynamics depend on the symmetry of the inorganic lattice but are found to be insensitive to the degree of cation substitution. In particular, the FA + rotation is invariant across all compositions studied here, when sufficiently above the phase transition temperature. We further identify an unusual relaxation mechanism for the 2 H of MA + in mechanosynthesized FA x MA 1−x PbI 3 , which was found to result from physical diffusion to paramagnetic defects. This precise picture of cation dynamics will enable better understanding of the relationship between the organic cations and the optoelectronic properties of perovskites, guiding the design principles for more efficient perovskite solar cells in the future.

Rotational Dynamics of Organic Cations in Formamidinium Lead Iodide Perovskites

“…This indicates that there are additional relaxational dynamics at 200 K. The data can be approximately described by a jump diffusion model consisting of a 4-fold rotation around the C–H axis and jumps around the N···N axis, suggesting that the main axes of rotation in the tetragonal β-phase are the N···N axis and the C–H axis. This is also in agreement with the recent NMR study that shows that the relaxation time is indeed the shortest for rotation around these specific axes. The data at T = 150 K are merely a shift to higher elastic scattering from the T = 200 K data, with no significant change in the q dependence, which suggests that the geometry of the dynamics is the same at T = 200 and 150 K. The higher elastic intensity at T = 150 K may be explained by, at this temperature, some of the FA cations being immobile on the probed time scale.…”

“…In addition, the ratio of the two line widths of the two fitted Lorentzians is about 3.5 at 300 K, which is in good agreement with what is expected (3) from the isotropic rotational jump diffusion model . Isotropic rotations in the cubic phase of FAPbI 3 are also in agreement with a previous neutron diffraction study, which indicated no preferred orientations of the FA cations, and a recent NMR study, which shows that the relaxation times for all FA cation rotations are less than 2 ps.…”

“…In addition, the ratio of the two line widths of the two fitted Lorentzians is about 3.5 at 300 K, which is in good agreement with what is expected (3) from the isotropic rotational jump diffusion model. 48 Isotropic rotations in the cubic phase of FAPbI 3 are also in agreement with a previous neutron diffraction study, 41 which indicated no preferred orientations of the FA cations, and a recent NMR study, 34 which shows that the relaxation times for all FA cation rotations are less than 2 ps. In the tetragonal β-phase, at T = 200 and 150 K, the minimum in the EISF is shifted to higher q, to about 2.5 Å −1 at 200 K. This indicates that the effective jump distance is smaller than in the cubic α-phase, which could occur if there are some strong preferred orientations of the FA cations and/or rotations occur only around some specific axes.…”

“…In contrast to the MA-based HOIPs, the dynamical nature of FA cations in HOIPs has been studied much less and to the best of our knowledge no QENS studies have been reported on FAPbI 3 , but recently, QENS studies were performed on the related material FAPbBr 3 . 31 , 32 In particular, the study by Sharma et al 31 indicated that the FA cation dynamics are isotropic in nature, with a continuously increasing number of FA cations participating in the dynamics with increasing temperature from 100 to 350 K. However, other studies on FAPbBr 3 and FAPbI 3 , based on temperature-dependent photoconductivity, 32 nuclear magnetic resonance (NMR), 11 , 13 , 32 − 34 optical spectroscopy, 35 and molecular dynamics simulations, 36 have indicated that the FA cation dynamics are anisotropic and feature preferred reorientations around the N···N axis. 13 Consequently, a consensus regarding FA cation dynamics in HOIPs is missing.…”

Temperature and pressure induced structural transitions of lead iodide perovskites

“…18,77 Different studies have pointed out that the I⋯H hydrogen bonds play an important role in the MA + dynamics, thereby in the phase transition of MAPbI 3 . 78,79 Therefore, the order-disorder dynamics of the MA + cation has been widely studied using a range of techniques such as solid-state NMR spectroscopy, 10,80 dielectric spectroscopy, 81 and quasi-elastic neutron scattering measurements. 82–84 The β- to γ-phase transition is a consequence of two simultaneous in-phase and out-of-phase tilts (Glazer's tilt notation a − b + a − ) with respect to the α-phase.…”