Moisture-resistant and highly efficient narrow-band red-emitting fluoride phosphor K2NaGaF6:Mn4+ for warm white LED application

Abstract

Mn⁴⁺-Doped fluoride red phosphors with excellent moisture-resistant properties and high emission efficiency are highly desired for warm white LEDs and wide gamut liquid crystal displays. Herein, we synthesized a K₂NaGaF₆:Mn⁴⁺ red phosphor through a co-precipitation method at room temperature. The crystallographic structure of the K₂NaGaF₆ host was determined based on single crystal X-ray diffraction data for the first time. A high internal photoluminescence quantum yield of 82% was achieved for the 1.81 at% K₂NaGaF₆:Mn⁴⁺ sample. Through low-temperature and high-resolution spectroscopic analysis, we revealed that Mn⁴⁺ ions occupy at least four distinct sites in the K₂NaGaF₆ crystal. One of them is assigned to the Ga³⁺ site with high O_h site symmetry and the other three ones possess low non-centrosymmetric site symmetries. By employing 5.51 at% K₂NaGaF₆:Mn⁴⁺ as a red phosphor, a warm white LED with a low correlated color temperature of 3760 K, a high color rendering index of 92 (R_a) and a luminous efficacy of 156 lm W⁻¹ was packaged. Aging tests at 85 °C and 85% relative humidity on the fabricated LEDs demonstrated that K₂NaGaF₆:Mn⁴⁺ has a better moisture-resistant property than that of the popular K₂SiF₆:Mn⁴⁺ phosphor. These findings demonstrate the great potential of K₂NaGaF₆:Mn⁴⁺ as a highly efficient red phosphor with excellent moisture-resistant performance.