An ultra-high yield of spherical K2NaScF6:Mn4+ red phosphor and its application in ultra-wide color gamut liquid crystal displays

Abstract

Mn⁴⁺-Activated narrow-band red-emitting fluoride phosphors are an efficient red compensator able to widen the color gamut and improve the color quality of pc-WLEDs for backlight display applications. However, the low synthetic yields and extremely non-uniform, irregular morphologies of these fluorides seriously restrict their practical industrial applications. Herein, a novel Mn⁴⁺-doped narrow-band red-emitting phosphor K₂NaScF₆:Mn⁴⁺ was prepared through a simple co-precipitation route. High-resolution synchrotron XRD and its Rietveld refinement reveal that K₂NaScF₆:Mn⁴⁺ crystallized in the space group Fmm with a cubic structure and that the octahedral Sc³⁺ sites were occupied by Mn⁴⁺, resulting in a very small amount of alkali metal cation vacancies for charge compensation. The electronic band gap of K₂NaScF₆ was calculated to be 6.46 eV by DFT calculations, which ensured the location of Mn⁴⁺ energy levels in the host band gap and was verified by DRS results. Unlike previously reported Mn⁴⁺-activated fluorides, K₂NaScF₆:Mn⁴⁺ has an ultra-high synthetic yield (∼100%) and a uniform spherical morphology with extremely narrow size distribution (∼1.17 μm). Excess KHF₂ and the extremely low solubility of K₂NaScF₆ were shown to be responsible for the high synthetic yield of K₂NaScF₆:Mn⁴⁺. Under blue light illumination, K₂NaScF₆:Mn⁴⁺ exhibited an intense sharp-line red fluorescence peak at 630 nm. Meanwhile, the ZPL intensity could be tailored through the control of Mn⁴⁺ amount in the K₂NaScF₆ matrix. By employing the efficient and stable K₂NaScF₆:Mn⁴⁺ as an efficient red emitter, two pc-WLEDs with ultra-wide color gamut (Rec. 2020 value of 95.1% and NTSC value of 127.3% for LED-1, Rec. 2020 value of 96.7% and NTSC value of 129.5% for LED-2) were successfully obtained. These results suggest the great potential of K₂NaScF₆:Mn⁴⁺ as a red-emitting phosphor for applications in high-quality backlight displays, and this work may open a perspective for researchers to design new luminescence materials with high synthetic yield and uniform spherical morphology.