Unveiling the structure and dynamics of treated methyl-ammonium lead iodide: insights from scattering and magnetic resonance techniques

This doctoral thesis examines the structure and dynamics of methyl-ammonium lead iodide (MAPI), a prominent hybrid perovskite material, through advanced scattering and magnetic resonance techniques. With its high power conversion efficiencies and cost-effective fabrication, MAPI is a promising candidate for photovoltaic technology.

However, its stability under environmental stressors, such as moisture and heat, poses significant challenges to commercial deployment. This research investigates the influence of various post-synthetic treatments, including vacuum and iodine annealing, on MAPI's structural integrity and phase transitions. Through techniques such as X-ray diffraction (XRD), quasi-elastic neutron scattering

(QENS), and nuclear magnetic resonance (NMR), this study provides insights into the flexible nature of the perovskite lattice and the dynamic behavior of the methylammonium cation, highlighting critical aspects of cation mobility and phase stability. Additionally, the thesis addresses the formation of impurity phases, notably NH4PbI3, during post-synthetic treatments and explores the potential mechanisms driving their formation. This work also advances the application of nuclear quadrupole resonance (NQR) for halide perovskites, demonstrating the possibility to use numerical simulations

to optimise NQR experiments for the observation of 127I nuclei. By refining annealing parameters, this research demonstrates a method of post synthetically altering the structure and dynamics in MAPI, contributing valuable insights to the development of perovskite-based solar cells. The findings offer promising pathways to enhance the stability and efficiency of these materials, reinforcing their

potential for next-generation photovoltaic applications.