Magnetic Resonance and Hyperpolarization Methods for Perovskite Photovoltaics

Solid-state NMR can provide information about the atomic-level microstructure and dynamics in materials as it directly probes the local nuclear environment. In the last decade, halide perovskites have drawn intense interest because of their intriguing optoelectronic properties making them suitable for solar cells, LEDs, and photodetectors. Meanwhile, perovskite solar cells already achieve power conversion efficiencies over 25%, but they are also unstable and degrade under operating conditions. The community has developed various strategies to minimize these detrimental effects, however, it is challenging to determine the atomic level structure using conventional techniques. In contrast, solid-state NMR can provide information about structure at the atomic level. NMR applications are often limited by the inherently low sensitivity arising from low concentration, low gyromagnetic ratio, and/or low natural abundance of the NMR active nuclei. In particular, the low mass of thin films limits the usage of NMR for technologically relevant samples. Dynamic nuclear polarization (DNP) is a method to acquire NMR spectra with higher sensitivity, however, it has had limited success for halide perovskites. The objective of this thesis is to develop and apply solid-state NMR and DNP methods to investigate perovskite materials. This thesis contains three sections: In the first section, a few examples of the application of solid-state NMR to halide perovskite systems are presented. Firstly, an NMR crystallography-based approach was developed to determine the supramolecular structure of layered hybrid perovskites with a mixture of two spacer cations. The observed nano-scale phase segregation was proposed to be responsible for providing high efficiency and operational stability. Secondly, using conventional solid-state NMR, the incorporation of the dimethylammonium ion was examined under various conditions. Unusually, solution processed samples were found to have a different metastable structure than mechanosynthesised samples. Finally, the NMR-derived atomic-level structure in current state-of-the-art hybrid and inorganic perovskites is presented. Notably, structural hypotheses were thoroughly investigated using mechanosynthesised perovskites and solution-processed thin films. These structural insights have guided the studies to achieve unprecedented solar cell performance. In the second section, a protocol is developed to investigate cation dynamics in single and multi-cation perovskite systems. This protocol employs quadrupolar relaxometry at high field under magic angle spinning, combined with a rotational diffusion model, to provide the rate of rotation about each principal axis of the cation. The dynamics were found to depend upon the symmetry of the inorganic lattice, but were insensitive to cation alloying. In the third section, a DNP based method is developed enabling the structure to be determined for the surface layer of a single perovskite thin-film. Fast relaxation was found to be the major impediment in DNP performance, but can be partially mitigated by optimal deuteration, resulting in enhancement factors approaching 100. Applying this DNP strategy at 21 T with a 0.7 mm outer diameter rotor, the surface layer structure of a single thin-film was identified. The work presented in this thesis will further open doors to develop and apply NMR methods to this family of optoelectronic materials.