K2SiF6:Mn4+ red-luminescent crystals with high external quantum efficiency (EQEmax of 78%) and high thermal quenching temperature (T1/2 > 500 K) enabling high brightness warm white LEDs†

Abstract

Mn⁴⁺-activated fluoride phosphors are some of the crucial red-emitting phosphors for white lighting systems driven by light-emitting diodes (LEDs). In this study, we successfully synthesized high-quality K₂Si_1−xF₆:xMn⁴⁺ (nominal x = 0.03–0.70, measured x of 0.22%–33.3%) crystals, measuring approximately 1 mm in size, through a new cooling-induced crystallization method. These crystals exhibit narrow line-type photoluminescence emission bands and exceptional optical properties, including a high absorption efficiency of up to 92.9% and an external quantum efficiency of up to 78.5%, which are among the record values for Mn⁴⁺-doped fluoride phosphors. Furthermore, they exhibit a notably elevated luminescence thermal quenching temperature (T_1/2) exceeding 500 K. A comprehensive investigation into the phase transformation, zero-phonon line tuning, and fluorescence decay behavior as a function of increasing Mn concentrations was also conducted. Practical applications were demonstrated through the fabrication of warm white LEDs, integrating a blue LED with a garnet yellow phosphor and the as-synthesized K₂Si_1−xF₆:xMn⁴⁺ (measured x of 2.94%) crystals as the red component. The white LEDs exhibit an impressive luminous efficacy of 192.1 lm W⁻¹, a color rendering index of R_a = 87.5 and R₉ = 62 under a 20 mA driving current. Our study highlights the outstanding properties of Mn⁴⁺-doped fluoride crystals with bulk size rather than conventional powdery form and the potential of the cooling-induced crystallization method for cultivating the crystals.