THE SCHOOL OF MATERIALS SCIENCE AND ENGINEERING     GEORGIA INSTITUTE OF TECHNOLOGY      Under the provisions of the regulations for the degree   DOCTOR OF PHILOSOPHY   on Friday, December 8, 2023 2:00 PM in Pettit Microelectronics Building Room 102A   will be held the   PhD DISSERTATION DEFENSE for   Andres Felipe Castro Mendez   " DEPOSITION OF METAL HALIDE PEROVSKITE THIN FILMS BY THERMAL EVAPORATION"   Committee Members:   Prof. Juan-Pablo Correa-Baena, Advisor, School of Materials Science and Engineering Prof. Mark D. Losego, School of Materials Science and Engineering Prof. Lauren Garten, School of Materials Science and Engineering Prof. Natalie Stingelin, School of Materials Science and Engineering / School of Chemical and Biomolecular Engineering Prof. Angus P. Wilkinson, School of Chemistry & Biochemistry   Abstract Hybrid organic-inorganic perovskites (HOIPs) are a class of materials characterized by the structure ABX3, where A represents an organic or inorganic cation, B is a metal cation, and X is a halide anion. Known for their remarkable optoelectronic properties, such as a tunable bandgap, high absorption coefficient, and defect tolerance, HOIPs have emerged as promising materials for photovoltaic applications. Perovskite solar cells (PSCs) have achieved impressive efficiencies, reaching a record of 25.6%, primarily, using spin coating to deposit most of the layers. However, spin coating has limitations, including low throughput, the use of toxic solvents, low quality films, and the presence of defects. Thermal evaporation presents a potential solution to these challenges, but we lack comprehensive understanding of the growth mechanisms in evaporated HOIP films, leading to lower efficiencies compared to solution processes. This dissertation aims to provide new understanding of HOIP film growth via thermal evaporation, focusing on controlling morphology, phase purity, stoichiometry, structural defects, and crystallographic orientation. These parameters are critical for optimal optoelectronic properties in PSCs, including the desired bandgap, enhanced charge carrier mobility, and reduced charge carrier recombination. This dissertation is divided into two main research thrusts, each addressing key issues associated with the thermal evaporation of HOIPs. The first thrust explores strategies to control the phase purity of methylammonium lead triiodide (MAPbI3) deposited by thermal co-evaporation of lead iodide (PbI2) and methylammonium iodide (MAI). This study reveals the crucial role of the MAI evaporation rate in film stoichiometry and formation of secondary phases. Secondary phases are identified as a major obstacle in achieving high-performance devices through vapor deposition due to the impact on charge transport and recombination. The second thrust investigates the co-evaporation of formamidinium lead triiodide (FAPbI3) by replacing MAI for formamidinium iodide (FAI). In contrast to MAI, FAI exhibits a high sticking coefficient, enabling better control over the process. This thrust provides a novel way to grow pure α-phase films without the need for substrate heating or additional post-treatments by delving into the intermolecular forces of the HOIP layer and the substrate. The substrate is functionalized with phosphonic acids to induce the growth of the α-phase as result of a change in the energetics of the system and increased sorption of the organic cation into the thin film. Overall, this dissertation sheds light on the growth mechanism of HOIPs via evaporation to control the morphology, phase purity and stoichiometry of thin films that can be used for the fabrication of PSCs.