Fabricating and Tailoring Halide Perovskites for Photovoltaic Applications

Abstract

Green energy is a contemporary global concern, and research of materials for solar energy harvesting is the heart of potential solutions for the energy crisis. Halide perovskites are leading candidates to replace silicon in next generation solar cells. This thesis focuses on halide perovskite materials, aiming to understand their structure, electronic and ionic properties and photo-activity; and to re-direct their fabrication techniques to address global market needs and requirements. In this work we developed alternative, vapor-based fabrication techniques, based on manufacturing-compatible, safe, rapid and scalable processes, that have the potential to improve material stability and efficiency. Vapor Transport Deposition (VTD) is investigated as a promising fabrication method for thin film halide perovskites and beyond. We explored the deposition parameter space and elucidated relationships and trends regarding composition, structure and deposition rate. We examined the morphology, crystal phase formation, optical and electrical properties, and finally the performance of the deposited films when incorporated into solar cells. We begin by exemplifying the viability of vapor transport co-deposition in fabricating active perovskite films, utilizing methylammonium lead iodide (MAPbI3) as a simplified model system. We then design an improved version of the vapor transport deposition system and transition to the more technologically attractive perovskite composition formamidinium lead iodide (FAPbI3). Learning from previous attempts to fabricate this material, we developed a novel technique that we call Hybrid two-step vapor-solution deposition in which we use VTD to deposit the inorganic 4 precursor, not readily dissolved in industry acceptable solvents, and then react it with a solution of the organic precursors dissolved in a benign solvent. This technique allowed us to fabricate functioning FAPbI3 based solar cell devices, in a safe, fast-paced, scalable and manufacturing compatible fashion. The deposition rate is significantly influenced by chamber pressure and source temperature, and by controlling all deposition parameters, we systematically reached rates of up to 1200 nm/min, that is orders of magnitude faster than current comparable techniques. We found the technique to be reproducible, yielding 13% efficient devices, with champion efficiencies of up to 15.3%. Based on the proposed novel fabrication process, we believe it offers an avenue for further improvement in solar cell stability and efficiency. CsPbBr3, a fully inorganic halide perovskite, also shows great promise as a photo and gamma ray detector and like the other halide perovskites is known to support halide ion conductivity that contributes to device instability and reduced sensitivity to irradiation. We choose this as a model system to apply concepts from defect chemistry and demonstrate the ability to measure and manipulate the ionic conductivity in the material by stoichiometry control and doping.