Solid State NMR of Lead Halide Perovskites: unveiling structural complexity of 2D Ruddlesden-Popper phases

In the fast-developing research of improved and sustainable materials for optoelectronics, 2D Lead Halide Perovskites (LHP) have attracted considerable attention because they offer the possibility of tunable band gap and enhanced environmental stability with respect to the corresponding 3D perovskites. 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 LHP for its ability to study ion dynamics, compositional variations and ion incorporation, chemical interactions, and degradation mechanisms [1,3]. In this work, the 2D RP perovskites BA2MAn-1PbnI3n+1 with n=1-4 have been characterized by 207Pb, 1H, and 13C Solid-State NMR, both under Magic Angle Spinning and static conditions. The structural features of these systems have been compared with those of 3D MAPbI3 and discussed in relation to very recent literature [3]. [1] W. M. J. Franssen, A. P. M. Kentgens, Solid State Nucl. Magn. Reson., 100 (2019) 36-44 [2] L. Piveteau, V. Morad, M. V. Kovalenko, J. Am. Chem. Soc. 142 (2020) 19413-19437 [3] D. J. Kubicki, S. D. Stranks, C. P. Grey, L. Emsley Nat Rev Chem 5 (2021) 624-645 [4] J. Lee, W. Lee, K. Kang, T. Lee, S. K. Lee, Chem. Mater., 33 (2021) 370-377