Localized dynamics of organic cations in metal halide perovskites

Doktorsavhandling, 2026

Metal halide perovskites (MHPs) are currently accumulating significant attention in the last decade because of their excellent optoelectronic properties and great potential for applications in, for example, solar cells and light-emitting diodes. However, the nature of local structure and dynamics underpinning these optoelectronic properties remain insufficiently understood. This thesis focuses on investigations of local structure and dynamics in some important MHP materials, by using quasielastic neutron scattering (QENS) and inelastic neutron scattering (INS) techniques. More specifically, the QENS studies focus on the lower-dimensional MHPs MBAMnCl3·2H2O, APbBr4 (A = 1,3-PDA; 1,4-PDA; 1,3-XDA; 1,4-XDA) and (1,3−XDA)2PbBr6. Further QENS studies have been performed on the three-dimensional (3D) lead-free MHPs FASnX3 (X = Br, I), the vacancy-ordered double perovskite variant FA2SnI6. The INS study with an in-situ illuminance environment has performed on a prototypical 3D MHP MAPbBr3.
For MBAMnCl3·2H2O, the results unraveled the nature of rotational dynamics of the MBA cations and how they evolve with temperature. Specifically, it is shown that the dynamics evolve from the functional groups -NH3, -CH3 rotational diffusion to a coupled multi-axial reorientation of the entire cation. For APbBr4 (A = 1,3-PDA;1,4-PDA;1,3-XDA;1,4-XDA) and (1,3−XDA)2PbBr6, it is shown that the nature of the organic cation dynamics can be correlated to the length and symmetry of the respective cation. Additionally, a comparison of the dynamical results with the photoluminescent spectra of the materials indicate that slower dynamics correlate with a lower thermal stability of photoluminescence due to less dynamic disorder. For FASnX3 (X= Br, I) and FA2SnI6 the QENS results unravel the reorientational dynamics of the FA cations and how they depend on halide ion and vacancy-ordering, which modify the ratio of organic cation and the volume of the sublattice cavity. In the study of MAPbBr3, for which the effect of light illumination on the local structure and vibrational dynamics was investigated, the INS results reveal a small yet distinct change of the local coordination of the MA cations upon light illumination and polaron formation.