Expanding the Crystal Chemistry of Rare-Earth Substituted Oxide and Mixed Anionic Inorganic Phosphors

Date

2023-04-05

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Abstract

Phosphor-converted light-emitting diode (LED) technology has replaced fluorescent and plasma display lighting due to its higher efficiency, longer operational lifetimes, and environmentally benign components. These devices produce white light by down-converting the emission from an LED chip using inorganic phosphors. Improving this technology further while enabling new application spaces will require the development of new phosphors. Phosphor discovery has historically been time-intensive and expensive. Fortunately, literature has suggested design rules that will increase the likelihood of identifying novel phosphors with specific optical properties, for example a high photoluminescence efficiency. This dissertation first validates the use of a material’s Debye temperature, ΘD,DFT, as a proxy for quantum yield using an experimental and computational analysis of unsubstituted and substituted phosphor hosts. The ΘD,DFT was then utilized to identify new phosphors: Cs2ZnSi5O12:Eu2+ and BaScO2F:Eu2+. BaScO2F:Eu2+ was highly efficient, as predicted, but optical spectroscopy revealed a surprisingly broad emission stemming from a local structure distortion upon rare-earth substitution. Incrementally substituting Sr2+ in BaScO2F:Eu2+ subsequently caused a decrease in the phosphor’s efficiency and thermal stability, which was attributed to a loss in the local rare-earth rigidity using ab initio molecular dynamics simulations. These studies support the critical need to analyze the local rare-earth environment during phosphor development.

Unfortunately, the ubiquity of LED-driven technology is not problem-free. Exposure to residual blue LED light has been found to suppress human melatonin production, leading to disturbances in sleep patterns resulting in insomnia and mood disorders. “Human-centric” lighting aims to minimize the harmful effects of this “blue light hazard” by transitioning to a violet LED and trichromatic phosphor mixture. The challenge is finding a phosphor with an appropriately small Stokes shift that can efficiently convert violet light to blue light. This research demonstrates that two novel phosphors discovered here, Na2MgPO4F:Eu2+ and K3AlP3O9N:Eu2+, combined with green- and red-emitting phosphors and a violet LED produces a human-centric white light for general illumination and display backlighting. Developing phosphors with humans’ health in mind will undoubtedly pave the way for next generation human-centric LED lighting.

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Keywords

Inorganic phosphors, LED lighting, Human-centric lighting, Debye temperature, Density functional theory

Citation

Portions of this document appear in: Hariyani, S.; Brgoch, J., 4.09 - Luminescence in the Solid State. In Comprehensive Inorganic Chemistry III (Third Edition), Reedijk, J.; Poeppelmeier, K. R., Eds. Elsevier: Oxford, 2023; pp 262-307; and in: Hariyani, S.; Brgoch, J. Spectral Design of Phosphor-Converted LED Lighting Guided by Color Theory. Inorganic Chemistry 2022, 61 (10), 4205-4218; and in: Hariyani, S.; Duke, A. C.; Krauskopf, T.; Zeier, W. G.; Brgoch, J. The Effect of Rare-Earth Substitution on the Debye Temperature of Inorganic Phosphors. Appl. Phys. Lett. 2020, 116 (5), 051901; and in: Hariyani, S.; Armijo, E.; Brgoch, J. Broad Green Emission in the Leucite-Like Cs2ZnSi5O12:Eu2+ Phosphor. ECS J. Solid State Sci. Technol. 2020, 9 (1), 016015; and in: Hariyani, S.; Brgoch, J. Local Structure Distortion Induced Broad Band Emission in the All-Inorganic BaScO2F:Eu2+ Perovskite. Chem. Mater. 2020, 32 (15), 6640-6649; and in: Hariyani, S.; Amachraa, M.; Khan, M.; Ong, S. P.; Brgoch, J. Local Environment Rigidity and the Evolution of Optical Properties in the Green-Emitting Phosphor Ba1−xSrxScO2F:Eu2+. J. Mater. Chem. C 2022, 10 (8), 2955-2964; and in: Hariyani, S.; Brgoch, J. Advancing Human-Centric LED Lighting Using Na2MgPO4F:Eu2+. ACS Appl. Mater. Interfaces. 2021, 13 (14), 16669-16676; and in: Hariyani, S.; Xing, X.; Amachraa, M.; Bao, J.;, Ong, S. P.; Brgoch, J. Realizing Wide-Gamut Human-Centric Display Lighting with K3AlP3O9N:Eu2+. Adv. Opt. Mater. 2023; and in: Hariyani, S.; Brgoch, J.; Garcia-Santamaria, F.; Sista, S. P.; Murphy, J. E.; Setlur, A. A. From Lab to Lamp: Understanding Downconverter Degradation in LED Packages. J. Appl. Phys. 2022, 132 (19), 190901.