Probing cation dynamics and phase transition in hybrid organic-inorganic perovskites by 13C solid-state NMR spectroscopy at very high resolution

Abstract

The spatial dynamics of cations have a significant impact on the photodynamic behavior of excited states in high-performance hybrid organic-inorganic perovskites. Multinuclear (¹H, ²H, and ¹⁴N) solid-state NMR (SSNMR) spectroscopy has traditionally been utilized to study the motion of methylammonium (MA) cations in methylammonium lead (II) halides MAPbX₃ (X = I, Br, Cl). NMR methods based on spin-lattice relaxation or quadrupolar line shape analysis over a limited temperature range demonstrate rapid MA reorientation, but the cation dynamics in a wider temperature range covering phase transition of all major crystallographic phases is lacking. Due to its low sensitivity, ¹³C NMR is rarely used to assess MA dynamics in these perovskites. Herein, we adopte variable-temperature (VT) ¹³C MAS NMR at very high resolution and dipolar-coupled transverse relaxation analysis as a new tool for dynamical characterization without isotopic enrichment, and systematically investigated MA dynamics in MAPbX₃ across phase transitions. This new approach enables retrieval of activation energy of MA reorientation and assessment of motion regimes. We propose a generalized “Camel model” that describes the common trend of cation dynamics for MAPbX₃, suggesting possible complicated reorientation modes. Furthermore, we discover the evolution of multiple MA sites in orthorhombic MAPbCl₃, consistent with X-ray crystallography, demonstrating its unique advantage in resolving and characterizing multi-cation dynamics. The VT ¹³C SSNMR effectively probes organic ion motions and phase transitions in hybrid perovskites, helpful for further elucidating the structure-property relationship in photovoltaic conversion mechanisms.