UC Merced

Hybrid organic metal-halide perovskites are promising materials for next generation solar cell application. Methylammonium lead iodide (CH$_3$NH$_3$PbI$_3$), sometimes called MAPI, is one of the favorable perovskites for making solar cells. Most of the research in the last decade about this material is aimed towards improving its photoconversion efficiency (PCE) and stability. In this work, I have done a detailed study on the three phases (orthorhombic, tetragonal, and cubic) of MAPI to understand how these different phases behave under stress. The total work is divided into 5 chapters. In chapter 1, I give an overview on perovskites for solar cell applications and discuss briefly about the theories that are involved in my calculations. In chapter 2, I investigate the effect of uniaxial strain on the pseudo-cubic structure and identify the most favorable vibrational modes to measure local strain using IR and Raman spectroscopy. In chapter 3, I investigate the same for low-temperature orthorhombic and room-temperature tetragonal phases. In addition to this, I explained about an improvement I made to the Quantum ESPRESSO code to enable these calculations. In chapter 4, I discussed how an analytical method we developed can help to understand hidden symmetries in the tetragonal perovskite and can be useful for any approximately symmetric structure to use symmetry for spectroscopic studies. In chapter 5, the last chapter, I studied the elastic properties of all three phases in detail and tried to determine the root cause behind the discrepancies in earlier published results. We also provide accurate reference values and an appropriate general methodology for elastic properties of metal halide perovskites. This work opens a way for a standard non-destructive bench-top characterization method to be usable for analyzing the critical role of local strain in hybrid perovskite photovoltaics. It provides an analytical method to calculate irreducible representations of vibrational modes for any approximately symmetric crystal structure which can be helpful for spectroscopic studies. Calculated detailed elastic properties of metal-halide perovskite will be useful for future reference for commercial application of perovskites for solar cell and flexible electronics.