Investigation of Environmental Stability of Methylammonium Lead Iodide Perovskite and Its Interaction with Water
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Organic-inorganic halide perovskite materials (e.g., CH3NH3PbI3) have recently received significant attention due to their excellent performance in photovoltaic and optoelectronic applications along with low cost. Since the first report of long-term, durable solid-state perovskite solar cell in 2012, the performance of perovskite solar cells (PSCs) experienced an unprecedented rise over the past few years. The highest certified power conversion efficiency (PCE) has reached 22.1% which is close to that of single crystal silicon solar cells. Other than photovoltaic applications, various kinds of perovskite-based optoelectronic devices (e.g., photodetectors, light-emitting diodes) have also been demonstrated to show outstanding performance. Nevertheless, there are still many issues which hindered its large scale commercial application and the environmental stability of perovskite materials is the most critical issue. The degradation of perovskite by water was observed in the first perovskite solar cell and the long-term stability of PSCs in the ambient environment has been the Achilles' heel of this emerging technology. Despite extensive research efforts since then, a clear microscopic understanding of perovskite interaction with water molecules is still missing. In this thesis, we systematically investigate the interaction between CH3NH3PbI3 perovskite and water molecules, and its stability in the presence of moisture. We first synthesize high-quality CH3NH3PbI3 thin-film and single-crystal samples. Using X-ray diffraction, we examine their chemical composition and crystal structure during moisture treatment. We identify that CH3NH3PbI3 thin-film does not turn into hydrates or lead iodide after a 1-hour moisture exposure. The film morphology change is studied by using a scanning electron microscope at the nanoscale. We monitor the moisture uptake in the CH3NH3PbI3 thin-film by using a quartz crystal microbalance and uncover the fact that the film absorbs a negligible amount of moisture until nearing a saturated humidity level. Using Fourier transform infrared spectroscopy, we investigate the interaction between water molecules and CH3NH3PbI3: from physical water absorption to phase transition. Ultraviolet-visible absorption, photoluminescence, and photocurrent measurements are performed to study the change in optical and electrical properties upon exposure to moisture. Finally, we discuss the reasons for high stability of CH3NH3PbI3 in our study and provide new insights into alternative degradation mechanisms and highly stable perovskite solar cells.