CONTROLLING LIGHT EMISSION IN HYBRID METAL HALIDES THROUGH CRYSTAL ENGINEERING FOR OPTICAL AND ELECTRONIC APPLICATIONS

Abstract

The development of photoluminescent materials continues to attract the interest of both industry and academic researchers due to their widespread applications in solid-state lasers, scintillators, sensing, anticounterfeiting, bio-imaging, lighting and display technologies, to name just a few. According to a recent estimation by the US Department of Energy, solid-state lighting technologies are currently used in 30% of all lighting applications in the US. This figure is expected to rise to 84% by 2035. Separately, there has been a growing recent interest on the development of advanced materials, often referred to as smart luminescent materials, due to their ability to alter optical properties (e.g., emission wavelength and intensity) in response to different chemical and/or physical stimuli. Therefore, smart luminescent materials could be employed in a wide range of applications in fields such as sensing, advanced security, data storage, anticounterfeiting, and more. While academic research is ongoing and some progress has been made, the practical applications of such materials are lagging due to a variety of materials related issues. Among highly luminescent materials, Cu(I) halides have gained significant attention over the past decade due to their low cost, non-toxic, abundance of elemental components, ability to be processed at low temperature in solution, tunable structural dimensionality, and outstanding light emission properties. My graduate research has largely been focused on syntheses and characterizations of new families of luminescent A – Cu(I) – X halides containing organic A cations. This research follows some exciting findings from our group on outstanding light emission properties of all-inorganic Cu(I) halides. On the other hand, all-inorganic Cu(I) halides have some major disadvantages such as poor environmental stability and weak tunability of their optical properties. Our new A – Cu(I) – X halides preparation work followed a hypothesis that the inclusion of structurally diverse organic cations may influence the coordination environment and geometry of Cu(I) centers, thereby providing a handle for influencing the resultant crystal and electronic structures, and photophysical properties; as an added bonus, the inclusion of hydrophobic organics may also lead to the increased stability of the resultant hybrid organic-inorganic Cu(I) halides. This dissertation has 6 chapters including Introduction (Chapter 1), which describes the background of my graduate work and materials design perspectives that have been utilized in this study. Chapter 2 reports our findings that the inclusion of a bulky organic cation can improve the stability of hybrid Cu(I) halides. However, these compounds are found to exhibit weak light emission properties due to their unique crystal and electronic (Type-II) structures. Utilizing the outcome of the study described in Chapter 2, Chapters 3 and 4 demonstrate that adaptable and flexible organic cations (as opposed to the rigid aromatic cations used in Chapter 2) can lead to structurally diverse light emitting hybrid Cu(I) halides. Hybrid Cu(I) halides presented in these chapters exhibit pure blue, bright greenish-white and orange-red emissions with remarkable photoluminescence quantum yield (PLQY) values >90%. Optical spectroscopy measurements and computational results reveal that photoemission in these hybrid Cu(I) halides originate from self-trapped excitons due to the excited state distortions in the inorganic units. The ultrabright luminescence coupled with their sensitivity to external radiation, chemical and/or thermal stimuli, allows their consideration for practical applications. The potential of these stimuli-responsive materials for various applications is showcased, along with proof-of-concept demonstrations of their use in solid-state lighting. Analogues hybrid Ag halides have been synthesized and studied in chapters 2 – 4, to investigate the suitability of metal substitution in luminescent Cu(I) halides. Hybrid Ag halides are found to exhibit weak light emission. We find that light emission in hybrid metal halides is often unpredictable, although some rationalization based on the pattern of the metal halide units of the observed properties can be made for metal halides in which emission is from the inorganic structural units. A more predictable way of designing light emitters is reported in Chapter 5, which demonstrates that the incorporation of group 12 metal (Zn, Cd) halide frameworks can enhance the efficiency of the organic emitters without changing their emission behavior, thereby ensuring predictable organic photoemission. Overall, fundamental understanding of the structure-property relationships presented in this work will eventually help to design and develop future luminescent hybrid materials with better tunability of the optical properties for desired optoelectronic applications.