Solid-State NMR methods for the study of Lead Halide Perovskites

In the past years, Lead Halide Perovskites (LHPs), with general formula APbX3 (A = methylammonium MA+, formamidinium FA+, Cs+; X = Cl-, Br-, I-), have received great levels of attention due to their remarkable optoelectronic properties, easy processability, abundant constituent elements, and wide compositional tunability [1]. Recent progress is mainly attributed to the exploration of various crystal engineering strategies (i.e. compositional and dimensional engineering) aimed at pushing the efficiencies higher, tuning properties, and overcoming stability issues. In 3-dimensional (3D) perovskites, the use of mixed-ion structures, obtained by introducing dopants into the perovskite structure, results in higher perovskite solar cell performance. Lowering the dimensionality of perovskites from 3D to 2D (by sandwiching organic cations, called spacers, between perovskite conductor layers) improves ambient stability, although at the expense of efficiency. Similarly, all-inorganic perovskite nanocrystals (where A is usually Cs+) show higher thermal stability and higher photoluminescence quantum yields with respect to bulk perovskites, although efficiency in solar cells is lower. Solid-State Nuclear Magnetic Resonance (SSNMR) stands out as characterization technique for LHPs for its ability to study ion dynamics, compositional variations and ion incorporation, chemical interactions, and degradation mechanisms, and can therefore be used to advance our understanding of these multifaceted materials [2]. In this study, a multiple-cation lead mixed-halide perovskite Cs0.05FA0.81MA0.14PbI2.55Br0.45[3], 2D Ruddlesden-Popper phases containing butylammonium (BA) as spacer (BA2MAn-1PbnI3n+1 with n=1-4, Figure 1), and CsPbBr3 nanocubes [4] were investigated by a range of multinuclear SSNMR techniques. 133Cs, 207Pb, 1H, and 13C spectra were recorded under Magic Angle Spinning and static conditions; variable temperature measurements of 13C and 1H spin-lattice relaxation times (T1) allowed dynamic properties of the organic cations in the series of samples to be investigated. The obtained structural and dynamic features of these systems have been compared with those of 3D pure MAPbI3 and discussed in relation to very recent literature. References: [1] J. Y. Kim, et al., Chem. Rev., 120, 7867 (2020). [2] D. J. Kubicki, et al., Nat. Rev. Chem., 5, 624-645 (2021). [3] N. Landi, et al., J. Phys. Chem. Lett.,13, 9517-9525 (2022). [4] A. Scarperi, et al., Pure Appl. Chem., 0 (2023).