Moisture‐Resistant Mn⁴⁺‐Doped Core–Shell‐Structured Fluoride Red Phosphor Exhibiting High Luminous Efficacy for Warm White Light‐Emitting Diodes

Abstract

K2TiF6:Mn4+ is a highly efficient narrow‐band emission red phosphor with promising applications in white light‐emitting diodes (LEDs) and wide‐gamut displays. Nevertheless, the poor moisture‐resistant properties of this material hinder commercialization. A convenient reverse cation‐exchange strategy is introduced for constructing a core–shell‐structured K2TiF6:Mn4+@K2TiF6 phosphor. The outer K2TiF6 shell acts as a shield for preventing moisture in the air from hydrolyzing the internal MnF62− group, while effectively cutting off the path of energy migration to surface defects, thereby increasing the emission efficiency (especially for the phosphors doped with high concentrations of Mn4+). Employed as a red phosphor, the packaged white LED exhibits an extraordinarily high luminous efficacy of 162 lm W−1, a correlated color temperature (CCT) of 3510 K, and a color rendering index of 93 (Ra). Aging tests performed on this device at 85 °C and 85 % humidity for 480 h retain up to 89 % luminous efficacy. The findings could facilitate commercial application of K2TiF6:Mn4+@K2TiF6 phosphor.

Single‐Crystal Red Phosphors and Their Core–Shell Structure for Improved Water‐Resistance for Laser Diodes Applications

“…Many efforts have been devoted to improving the surface coating technique for moisture‐resistant phosphors, for example, by using isostructural fluoride as a shell to protect Mn 4+ from moisture [16] . Typically, a core–shell structured KTF:Mn 4+ @KTF powdery phosphor prepared with a reverse cation exchange strategy to improve moisture‐resistant properties has been reported [17] . However, the outcomes of changes in surface structure during the moisture‐resistant operation are still unclear.…”

“…Furthermore, we evaluated the moisture resistance of CTFM single crystal after aging under high humidity (HH, 85 %) and high temperature (HT, 85 °C) conditions as an accepted method [17] . The results showed that the yellowish color of CTFM polycrystalline powder quickly turned into brown after 6 h, whereas the CTFM single‐crystal color remained yellow even after 48 h (Supporting Information, Figure S13a).…”

“…It has been reported that the poor stability of Mn 4+ ‐activated fluoride phosphors in a moisture environment is mainly due to the hydrolysis of the MnF 6 2− group into Mn oxides and hydroxides [15] . In such studies, the K 2 TiF 6 or K 2 SiF 6 shell was successfully coated on its related Mn 4+ ‐doped fluoride powders to improve moisture resistance [16, 17] . Here, to further improve the waterproof stability of CTFM, the CTF shell was coated with an epitaxial growth method on the CTFM single crystal (Supporting Information, Figure S15a,b), denoted by CTFM@CTF.…”

Surface Passivation toward Highly Stable Mn4+‐Activated Red‐Emitting Fluoride Phosphors and Enhanced Photostability for White LEDs

“…Besides, Murphy et al developed a washing process to reduce surface active manganese from KSF using K 2 SiF 6 and hydrofluoric acid (HF) saturated solutions or H 2 SiF 6 solutions . And core–shell‐structured KTF was successfully prepared by Huang et al Wang and co‐workers further synthesized K 2 SiF 6 :Mn 4+ @K 2 SiF 6 composites without the consumption of toxic HF solution, and prepared new reductive dl‐mandelic acid loaded K 2 GeF 6 :Mn 4+ (KGF) phosphor . These strategies can evidently improve the moisture resistance of fluoride red phosphors, while most of the surface modification procedures are relatively complex, and it is a big challenge to enhance the stability with the surface protective layer.…”

A new langbeinite-type phosphate K2GdHf(PO4)3: synthesis, crystal structure, band structure and luminescence properties

“…An efficient method for acquiring an illuminating lamp with a high CRI is the use of a near-UV chip and red, green and blue (RGB) phosphors. Meeting this requirement has inspired research interest in the development of new phosphors and many phosphors in various oxometallate systems have been reported in recent years (Ho et al, 2007;Huang et al, 2019;Park et al, 2014).…”