Luminescent Behavior of the K₂SiF₆:Mn⁴⁺Red Phosphor at High Fluxes and at the Microscopic Level

Abstract

Phosphor-converted white light-emitting diodes (LEDs) are becoming increasingly popular for general lighting. The non-rare-earth phosphor K 2 SiF 6 :Mn 4+ , showing promising saturated red d-d-line emission, was investigated. To evaluate the application potential of this phosphor, the luminescence behavior was studied at high excitation intensities and on the microscopic level. The emission shows a sublinear behavior at excitation powers exceeding 40 W/cm 2 , caused by ground-state depletion due to the ms range luminescence lifetime. The thermal properties of the luminescence in K 2 SiF 6 :Mn 4+ were investigated up to 450 K, with thermal quenching only setting in above 400 K. The luminescence lifetime decreases with increasing temperature, even before thermal quenching sets in, which is favorable to counteract the sublinear response at high excitation intensity. A second, faster, decay component emerges above 295 K, which, according to crystal field calculations, is related to a fraction of the Mn 4+ ions incorporated on tetragonally deformed lattice sites. A combined investigation of structural and luminescence properties in a scanning electron microscope using energy-dispersive X-ray spectroscopy and cathodoluminescence mappings showed both phosphor degradation at high fluxes and a preferential location of the light outcoupling at irregularities in the crystal facets. The use of K 2 SiF 6 :Mn 4+ in a remote phosphor configuration is discussed. Most phosphor-converted white LEDs contain phosphors doped with rare earths such as divalent europium and trivalent cerium. These ions feature relatively broad emission bands based on the parity allowed 5d-4f transition. They are often easily excited with blue light and can show high quantum efficiency, even at elevated temperature. To improve the color rendering of white LEDs, red phosphors are added to the traditional blue LED and yellow Y 3 Al 5 O 12 :Ce (YAG:Ce) phosphor combination. These red phosphors need to be stable and have a high quantum efficiency. The emission spectrum should both be sufficiently red (>600 nm) and well within the eye sensitivity curve, to obtain a high luminous efficacy.1 Sulfide phosphors doped with Eu 2+ , such as (Ca,Sr)S:Eu 2+ are known for their efficient red emission, 2 but they lack stability in humid environments and the eye sensitivity is low for part of their broad emission band.3 Nitride phosphors doped with Eu 2+ are often chemically more stable, but their synthesis at high pressure and temperature is a drawback.4 Most europium-doped nitride phosphors show a relatively broad emission band. 5,6 Cost and supply issues of the rare-earth materials pave the way for transition-metal-doped phosphors. 7 In particular the Mn 4+ ion is a promising alternative for Eu 2+ as it shows line emission from parity and spin-forbidden d-d transitions in the red and near-infrared spectral region. Investigation of the optical properties of the Mn 4+ dopant showed that fluoride hosts are preferred for LED phosphors over oxide hosts, since only the io...

Editors' Choice—Rb2SiF6:Mn4+and Rb2TiF6:Mn4+Red-Emitting Phosphors

“…Rb 2 TiF 6 :Mn 4+ phosphors, respectively. These values fall in the range of the ever-reported Mn 4+ -activated hexafluorometallate values of ∼3−9 ms at 300 K. 2,6,13,[24][25][26][27][28][29][30][31][32][33] Temperature dependence of PL spectra.-For practical application of the red-emitting phosphors in solid-state lighting system, temperature dependence of the luminescence intensity is very important. Therefore, we measured the PL spectra for the Rb 2 SiF 6 :Mn 4+ and Rb 2 TiF 6 :Mn 4+ phosphors at temperatures between T = 20 and 500 K. Figure 6 shows the results of these measurements.…”

LaAlO3:Mn4+ as Near-Infrared Emitting Persistent Luminescence Phosphor for Medical Imaging: A Charge Compensation Study

“…Currently, manganese doped compounds, especially Mn 4+ -activated phosphors, are considered as a promising alternative to Cr 3+ -doped nanomaterials [19]. The tetravalent manganese ions (Mn 4+ ) can be doped in both fluorides and oxides [20,21,22,23]. Great effort has been made to the development of Mn 4+ -activated oxide compounds.…”

Mn4+-Doped Magnesium Titanate—A Promising Phosphor for Self-Referenced Optical Temperature Sensing

“…The highest deviations were observed for the sample with the highest doping concentration (1.00%) at the temperature of 77 ${}^{\circ }$C, where the ${\chi }_R^2$ is as high as 6.2. The presence of a faster decay component was reported in K${}_2$SiF${}_6$:Mn${}^{4+}$ and attributed to thermal activation of defect centers [35]. However, for practical applications, it is enough to characterize the intensity decay of a sample in terms of a single apparent or mean decay time.…”