Going red to make good white, or red phosphors for white light emitting diodes

The red-light emitting phosphors get more and more attention in the recent years due to various applications, one of the most important of which is related to the solid state lighting and white light emitting diodes (LEDs). The currently used white LEDs suffer from the lack of the red component, which results in the high color correlated temperature (CCT) and low color rendering index (CRI). The phosphors based on the red emission of the Mn4+ ions are excellent candidates for improvement of the white LEDs characteristics. The advantage of these phosphors over those based on the Eu3+ and Sm3+ ions is strong absorption of 450 nm blue LED emission via the 4A2g→4T2g optical transition. For practical applications, an additional advantage of the Mn4+ ion as a red emitter is that the wavelength of its 2Eg→4A2g emission transition can be tuned in a wide spectral range from 620 nm (K2SiF6) to 723 nm (SrTiO3) [1]. Systematic analysis of the existing experimental spectroscopic data for a large number of phosphors activated by the Mn4+ ions [1-4] has led to the conclusion that such variation is caused by the nephelauxetic effect (or by varying degree of covalency/ionicity of chemical bonds between the Mn4+ ions and their nearest neighbors). Examples of how to analyze the experimental data for the hosts with several available sites for the Mn4+ ions will be discussed [5, 6]. The factors that greatly influence the energy of the Mn4+ 2Eg→4A2g emission transition will be highlighted. Several trends across the considered compounds (related to the structure of the host materials, local symmetry of the dopant sites, and chemical bonds properties) will be indentified; in addition, some practical advices on how to get red emission at a desired wavelength or enhance the zero-phonon line intensity in the emission spectra [7] will be suggested.

References

1. M.G. Brik, A.M. Srivastava, J. Lumin. 133 (2013) 69.
2. M.G. Brik, S.J. Camardello, A.M. Srivastava, ECS J. Solid State Sci. & Technol. 4 (2015) 4 R39.
3. M.G. Brik, S.J. Camardello, A.M. Srivastava, N.M. Avram, A. Suchocki, ECS J. Solid State Sci. & Technol. 5 (2016) R3067.
4. M.G. Brik, A.M. Srivastava, ECS J. Solid State Science & Technol. 7 (2018) R3079.
5. M.Y. Peng, X.W. Yin, P.A. Tanner, M.G. Brik, P.F. Li, Chem. Mater. 27 (2015) 2938.
6. M.G. Brik, A.M. Srivastava, Opt. Mater. 54 (2016) 245.
7. D.-X. Liu, C.-G. Ma, P.-W. Hu, Z. Li, Y. Tian, P. Su, M.G. Brik, A.M. Srivastava, S. Tanabe, J. Am. Ceram. Soc. 101 (2018) 2368.