(Invited) Synthesis and Development of High-Performance Phosphors to Brighten the Future

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Phosphors for the ever growing LED-lighting sector are an highly important research field in the current research of solid state chemistry.1 Within this class of materials, europium-activated phases have emerged as a promising substance class for inorganic phosphors with some of them, such as ß-SiAlON, RE 2Si5N8:Eu2+ (RE = Ca, Sr, Ba), and CaAlSiN3:Eu2+, becoming standard phosphors in the industry. Recent developments, such as the discovery of SLA, also demonstrated that there is still much potential within this substance class and the number of Eu2+ based phosphors is still increasing. The key properties of these phosphors (spectral position, emission profile, thermal quenching, etc.) emerge from their respective structures and must be optimized to match the requirements of any given application.In this context, the search for new classes of compounds led us to the question, if alkali metal silicates or lithosilicates could be suitable for modern phosphor materials. To answer this question, several alkali metal oxides such as Na[Li3SiO4] and K[Li3SiO4], originally synthesized by Hoppe in the 80s and 90s, have been revisited. Due to the fact that this substance-class only hosts monovalent cations, it was widely believed that their host structures would be unsuitable for doping with Eu2+. In a first step, we discovered three lithosilicate phosphors Na[Li3SiO4]:Eu2+, K[Li3SiO4]:Eu2+, and NaK7[Li3SiO4]8:Eu2+ (new structure type), which represent the first examples of this substance class.2 Surprisingly, these novel phosphors exhibit extremely diverse luminescence properties, despite their structural similarities, while maintaining a relatively high quantum efficiency (QE) and low thermal quenching (TQ). Their emissions range from narrow blue to a broadband warm white, which we explain in terms of structure-property relationships.3 The search for new narrow band emitters in the red spectral region led us to the discovery of SrAl2Li2O2N2:Eu2+(SALON).4,5 Upon excitation with UV to blue light, SALON exhibits a narrow band red emission with a maximum at 614 nm and a remarkably narrow half width of only 48 nm (1286 cm-1). It comes close to the desired spectral position for a red LED phosphor as defined in the 2016 Research & Development-plan6 of the U.S. Department of Energy. Beside the interesting spectral position and band width, SALON also exhibits high quantum efficiency, strong absorption of blue light, and almost no temperature quenching. The reason for the excellent luminescence performance is based on a very special atom arrangement. In SrAl2Li2O2N2:Eu2+, the oxygen and nitrogen atoms occupy distinct positions leading to a uniform, highly symmetric coordination of the Eu2+ activator ions. Using this new phosphor, it was possible to construct a prototype pc-LED which demonstrated an increase of 16% in luminous efficacy compared to currently available commercial high CRI pc-LEDs, while retaining excellent high color rendition (CRI = 91, R9 > 40, CCT = 2700 K, CIEx = 0.463 CIEy = 0.415).Starting from SALON, a tunable oxonitride-phosphor can be derived by introducing disorder into the structure.7 To achieve this, the oxygen content of the reaction mixture is increased thereby prohibiting the oxygen/nitrogen ordering observed in SALON. Lin, C. C. & Liu, R.-S. Advances in Phosphors for Light-emitting Diodes. J. Phys. Chem. Lett. 2, 1268–1277 (2011). Dutzler, D., Seibald, M., Baumann, D., Huppertz, H. Alkali Lithosilicates: Renaissance of a Reputable Substance Class with Surprising Luminescence Properties. Angew. Chem. Int. Ed. Engl. 57, 13676–13680 (2018). Ruegenberg, García-Fuente, A., Seibald, M., Baumann, D., Peschke, S., Urland, W., Meijerink, A., Huppertz, H., Suta, M. Chasing Down the Eu2+ Ions: The Delicate Structure−Property Relationships in the Ultra-Narrow Band Phosphor K1.6Na2.1Li0.3[Li3SiO4]4:Eu2+. Adv. Opt. Mater. 9, 2101643 (2021). Hoerder, G. J., Huppertz, H. et al. Sr[Li2Al2O2N2]:Eu2+ —A high performance red phosphor to brighten the future. Nat. Commun. 10, 1824 (2019). Ruegenberg, F., García-Fuente, A., Seibald, M., Baumann, D., Hoerder, G. J., Fiedler, T., Urland, W., Huppertz, H., Meijerink, A., Suta, M. Mixed Microscopic Eu2+ Occupancies in the Next-Generation Red LED Phosphor Sr[Li2Al2O2N2]:Eu2+ (SALON:Eu2+). Adv. Opt. Mater. 11, 2202732 (2023). Energy, U. D. O. Solid state lighting research and development plan. U.S. Department of Energy (2016). Available at: https://www.energy.gov/sites/prod/files/2016/06/f32/ssl_rd-plan_%20jun2016_2.pdf. (Accessed: 9th January 2024)Hoerder, G. J., Peschke, S., Wurst, K., Seibald, M., Baumann, D., Stoll, I., Huppertz, H., SrAl2–xLi2+xO2+2xN2–2x:Eu2+ (0.12 ≤ x ≤ 0.66)—Tunable Luminescence in an Oxonitride Phosphor. Inorg. Chem. 58, 12146-12151 (2019).