A reverse strategy to restore the moisture-deteriorated luminescence properties and improve humidity resistance of Mn4+-doped fluoride phosphors

Fluoride phosphors as red components for warm WLEDs have attracted a tremendous amount of research attention. But these phosphors are extremely sensitive to moisture, which seriously limits its practical industrial applications. To tackle with this problem, unlike all the straight-forward preventive strategies, a novel strategy "Good comes from bad" was successfully developed to treat the degraded K2SiF6 :Mn 4+ (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. With an oxalic acid solution as restoration modifier, the emission intensity of D-KSFM phosphor can be restored to 103.68% of the original K 2 SiF 6 :Mn 4+ red phosphor (O-KSFM), and the moisture resistance is remarkably improved. The restored K 2 SiF 6 :Mn 4+ (R-KSFM) maintains 62.3% of its initial relative emission intensity after 300 min water immersion, while the reference commercial K 2 SiF 6 :Mn 4+ with a protective coating (C-KSFM) is only 33.2%. Similarly, the luminescence intensity of the degraded K 2 TiF 6 :Mn 4+ (D-KTFM) phosphor can be restored to 162.59% of original level of the K 2 TiF 6 :Mn 4+ without any treatment (O-KTFM). The emission intensity of the restored K 2 TiF 6 :Mn 4+ (R-KTFM) phosphor retains 62.81% of its initial emission intensity after 300 min water immersion. The R-KSFM phosphors were packaged into WLEDs with InGaN chips and Y 3 Al 5 O 12 :Ce 3+ phosphors. The WLEDs display excellent color rendition with higher color rendering index, lower color temperature (WLED-Ⅱ: Ra = 83.6, R 9 = 57.3, 3743 K, η l = 199.68 lm/W; WLED-Ⅲ: Ra = 90.4, R 9 = 94.2, 2892 K, η l = 183.3 1m/W). The above results show that the strategy can be applied in those phosphor materials with poor moisture resistance to restore luminescence properties and improve moisture resistance without excessively care about deterioration.

Keywords: K2SiF6:Mn4+; K2TiF6:Mn4+; moisture resistance; red phosphor; white LEDs.