A Reverse Strategy to Restore the Moisture-deteriorated Luminescence Properties and Improve the Humidity Resistance of Mn4+ -doped Fluoride Phosphors.

Fluoride phosphors as red components for warm white LEDs have attracted a tremendous amount of research attention. But these phosphors are extremely sensitive to moisture, which seriously limits their practical industrial applications. To tackle this problem, unlike all the straightforward preventive strategies, a reverse strategy "Good comes from bad" was successfully developed to treat the degraded K2 SiF6 : Mn4+ (D-KSFM) phosphor in the present study, which not only completely restores the luminescence properties, but also significantly enhances the moisture resistance at the same time. After treatment with an oxalic acid solution as restoration modifier, the emission intensity of the D-KSFM phosphor can be restored to 103.7% of the original K2 SiF6 : Mn4+ red phosphor (O-KSFM), and the moisture resistance is remarkably improved. The restored K2 SiF6 : Mn4+ (R-KSFM) maintains approximately 62.3% of its initial relative emission intensity after immersing in deionized water for 300 min, while the reference commercial K2 SiF6 : Mn4+ with a protective coating (C-KSFM) is only 33.2%. As a proof of general applicability, this strategy was also conducted to K2 TiF6 : Mn4+ phosphor, which is less moisture-stable than K2 SiF6 : Mn4+ . The luminescence intensity of the degraded K2 TiF6 : Mn4+ (D-KTFM) phosphor can be restored to 162.6% of original level of the K2 TiF6 : Mn4+ synthesized through a cation exchange approach without any treatment (O-KTFM). The emission intensity of the restored K2 TiF6 : Mn4+ (R-KTFM) phosphor retains 62.8% of its initial emission intensity after soaking in deionized water for 300 min. Finally, the R-KSFM phosphors were packaged into white light-emitting diodes with blue InGaN chips and Y3 Al5 O12 : Ce3+ yellow phosphors. The WLEDs display excellent color rendition with higher color rendering index, lower color temperature (WLED-II: Ra =83.6, R9 =57.3, 3743 K, ηl =199.68 lm/W; WLED-III: Ra =90.4, R9 =94.2, 2892 K, ηl =183.3 1 m/W). The above results show that the reverse strategy can be applied in those phosphor materials with poor moisture resistance to restore luminescence properties and improve moisture resistance without excessively care about the deterioration during the production, storage and transportation.

A novel Mn4+ doped oxyfluoride red phosphor for rapid-response backlights display.

An oxyfluoride red phosphor Cs2MoO2F4:Mn4+ was synthesized via a facile co-precipitation route with a certain molecular ratio of CsF and MoO3. X-ray diffraction analysis and its Rietveld refinement reveal that Cs2MoO2F4:Mn4+ crystallized in an orthorhombic structure with the Amam (63) space group. Upon blue light excitation, Cs2MoO2F4:Mn4+ exhibits a series of sharp red emission lines around ∼634 nm and the zero-phonon line (ZPL) is visible at 619 nm. The optimal doping amount of Mn4+ in Cs2MoO2F4 is 1.12%, and the decay curves show a good fit with the single exponential decay model. The fluorescence lifetime of the synthesized phosphors is relatively short and calculated as 3.18 to 2.46 ms, the Mn4+ ions in Cs2MoO2F4 experience a strong crystal field strength with a Dq/B of ∼4.87, and the distinct nephelauxetic ratio ß1 is determined to be ∼1.0226. The thermal quenching mechanism of Mn4+ was also studied. Furthermore, by using the as-synthesized Cs2MoO2F4:Mn4+ phosphor as a red component and ß-SiALON as a green light component, a WLED was fabricated with a high luminous efficacy of 114.70 lm·W-1 and wide color gamut of 109.1% of the National Television Standard Committee (NTSC) value. Hence, the Cs2MoO2F4:Mn4+ phosphor with a short fluorescence lifetime could potentially be an efficient red compensator for application in rapid-response backlight displays.

Cs2MnF6 Red Phosphor with Ultrahigh Absorption Efficiency.

To improve absorption efficiency (AE) and subsequently improve external quantum efficiency (EQE) remains one of the significant challenges for Mn4+-doped red-emitting fluoride phosphors. In this study, we propose to use Mn4+ as a part of matrix to enhance the AE of fluoride phosphors. Red-emission phosphors Cs2MnF6, Cs2MnF6:Sc3+, and Cs2MnF6:Si4+ were synthesized successfully by a coprecipitation method. The Rietveld refinement of X-ray diffraction reveals that this red phosphor exhibits a cubic structure in Fm3̅m space group. Owing to Mn4+ being a part of matrix, this kind of red phosphor possesses an extremely high AE, which can be promoted to 88%. The doping of Sc3+ and Si4+ ions into Cs2MnF6 can effectively increase the luminescence intensity to 253 and 232%, respectively, relative to that of Cs2MnF6. The relative emission intensity of Cs2MnF6:5%Si4+ red phosphor preserves about 115% when temperature rises to 175 °C. By employing Cs2MnF6:5%Si4+ as a red-emitting component, high-performance LED-1 with Ra = 86.2, R9 = 82.1 and CCT = 3297 K, and LED-2 with an ultrawide color gamut (NTSC value of 122.3% and rec. 2020 value of 91.3%) are obtained. This work may provide a new idea to explore a new type of fluoride phosphor with high EQE for high-performance white-light-emitting diodes.