Title:
Structure-Property Relationships in Lead Halide Perovskites for Solar Cells
Structure-Property Relationships in Lead Halide Perovskites for Solar Cells
Author(s)
Hidalgo, Juanita
Advisor(s)
Correa-Baena, Juan-Pablo
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Abstract
Lead halide perovskites (LHPs) solar cells, particularly FA-based, have made impressive advancements in solar energy conversion, achieving high power conversion efficiencies exceeding 26% for a single junction device. However, the limited long-term stability of these devices has hindered their commercialization. The instability issues are influenced by both internal and external factors, leading to rapid degradation of the perovskite phase and overall device performance. Various strategies have been used to stabilize the perovskite phase, but it is crucial to investigate the underlying mechanisms that govern the structural characteristics of the polycrystalline thin films. This dissertation tackles the challenges associated with the instability of LHPs by investigating the complex relationship between structure, properties, and performance in perovskite solar cells. Advanced X-ray characterization techniques are employed to examine the structural properties of FA-based compositions. Understanding the mechanisms and establishing correlations between structure and properties is possible to lay the foundation of a more robust, stable, and efficient material. This dissertation presents a first step toward the design and optimization of LHPs.
The first part of this dissertation explores the crystallographic orientation in lead bromide perovskites, demonstrating that the solvent and organic cation used in the precursor solution significantly affect the preferred orientation of the deposited perovskite thin films. The solvent affects the early stages of crystallization and induces preferred orientation by preventing colloidal particle interactions. The choice of organic cation influences the degree of crystallographic orientation, with methylammonium-based perovskites showing a higher degree of orientation than formamidinium-based ones due to a lower surface energy of a specific perovskite facet. These findings identify the importance of understanding (1) the precursor solution chemistry, (2) the facet properties and their correlation with the structural properties of the polycrystalline LHP film, and (3) the effect of crystallographic orientation on charge carrier transport in perovskite solar cells.
The second part of this dissertation studies the mechanisms causing FA-based lead iodide perovskites to degrade under water and oxygen exposure. Contrary to common knowledge on humidity-induced degradation, this dissertation reveals the synergistic role of water and oxygen in accelerating phase instability of LHPs. The study uncovers a surface reaction pathway involving the dissolution of formamidinium iodide (FAI) by water followed by the oxidation of iodide, playing a crucial role in causing the subsequent and irreversible undesired phase transformations from perovskite into non-perovskite phases. The interplay of in-situ experimental techniques with theoretical calculations provides a detailed understanding of the degradation mechanisms, establishing a foundation to design more durable and efficient materials. Finally, this dissertation delves into strategies for stabilizing the perovskite phase. A hydrophobic molecule, phenethylammonium iodide (PEAI), stabilizes FA-based perovskites. Adding PEAI hinders undesired phase transformations and leads to a more stable material with improved solar cell power conversion efficiency and enhanced charge carrier mobilities and lifetimes. Further, adding Br to mixed cation lead iodide perovskites improves their phase stability at low temperatures.
Overall, understanding structure-property-performance relationships in lead halide perovskites is key for resolving the main challenge of instability in perovskite solar cells. This dissertation lays the groundwork for future research efforts to investigate the fundamentals of LHPs, improve their stability, and broaden their applications in solar cells and beyond.
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Date Issued
2023-07-14
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Dissertation