Atmospheric Pressure Chemical Vapor Deposition of Methylammonium Bromide and Tin(II) Bromide

Abstract

With the need to rapidly decarbonize society to slow the effects of and halt climate change, new forms of renewable energy need to replace current fossil fuel consumption. Of the alternatives, solar energy is both free and universally accepted. However, today’s solar panels made of silicon are still too inefficient, too expensive, and in too short of supply to deploy as quickly as the world needs in all markets. Perovskite absorbers offer the opportunity to dramatically increase efficiency as tandem solar panels and decrease costs. This dissertation explored the use of atmospheric pressure chemical vapor deposition (APCVD) in depositing perovskite relevant films methylammonium bromide and tin(II) bromide. Precursors for making these films were selected based on metrics such as volatility, thermal stability, and ease of handling. A tin compound was used that conveyed suitable volatility and thermal stability by Thermogravimetric Analysis. The bromide source was provided by hydrogen bromide, and a process for providing hydrogen iodide was explored. Methylamine was used in its neat form as a gas. The reactor geometry used to control the gas flow was designed using computational fluid dynamics software. Two designs are discussed that aimed to minimize inlet contamination and premature deposition – two sources of film inhomogeneity. The resultant films were characterized by a suite of techniques to identify their composition and quality. For SnBr2 films, additional impurity analysis was performed to identify potential sources of the impurities. Finally, Energy Dispersive X-ray Spectroscopy was used to measure the spatial dependence of the deposition rate. These data were then used to qualitatively assess the mechanistic origins of the deposition through modeling. Results suggested an absence of gas phase reactions and substantial surface reactions of at least two kinds. These results and procedures may be used to fully develop APCVD models for the deposition of perovskite absorbers and accelerate the world towards a solar energy future.