2D RUDDLESDEN-POPPER PEROVSKITES BA2MAn-1PbnI3n+1 AS STUDIED BY SOLID-STATE NMR.

Lead Halide Perovskites are interesting semiconductors used in different optoelectronic devices (e.g. sensitizers for solar cells, photodetectors, LEDs). Recently, 2D analogues of Hybrid Lead Halide Perovskites (HLHP) have attracted considerable attention because they offer the possibility of tunable band gap and enhanced environmental stability with respect to the corresponding 3D systems. 2D Ruddlesden-Popper (RP) perovskites can be prepared by adding a large organic mono ammonium cation L+ in the precursor solution. In this way the 3D structure of corner-sharing octahedra (ABX3) is disrupted and a structure with a bilayer of spacer cations between metal halide sheets is formed (L2An-1BnX3n+1). For example, butylammonium (BA) is a suitable organic cation to force the archetypical perovskite MAPbI3 into 2D RP perovskites BA2MAn-1PbnI3n+1 (Figure 1), which are the object of the present study. The layer thickness of metal halide sheets is specified by n and can be adjusted by tuning precursor stoichiometry. Solid-State NMR stands out as characterization technique for HLHP for its ability to study ion dynamics, compositional variations and ion incorporation, chemical interactions and degradation mechanisms [1,2]. In this work, the 2D RP perovskites BA2MAn-1PbnI3n+1 with n=1, 2, 3 have been characterized by Solid-State NMR and compared with 3D MAPbI3 as a reference compound. The structural features of these systems have been investigated by 207Pb, 1H, and 13C spectra recorded under Magic Angle Spinning and static conditions; the obtained results have been discussed also by comparison with very recent literature [3]. In addition, the variable temperature measurement of 13C and 1H spin-lattice relaxation times (T1) allowed dynamic properties of the organic cations in the series of samples to be investigated. Figure 1. Schematic structure of 2D RP perovskites BA2MAn-1PbnI3n+1 for n=1, 2, 3, 4, and of the corresponding 3D perovskites MAPbI3. References: [1] W. M. J. Franssen, A. P. M. Kentgens, Solid State Nucl. Magn. Reson. 2019, 100, 36-44. [2] L. Piveteau, V. Morad, M. V. Kovalenko, J. Am. Chem. Soc.. 2020, 142 (46), 19413-19437 [3] J. Lee, W. Lee, K. Kang, T. Lee, S. K. Lee, Chem. Mater. 2021, 33, 370-377.