Controllable morphology, excellent stability and non-equivalent doping-induced optical properties of Mn4+-activated K2NaScF6 red phosphor for high-quality warm WLED

Abstract

Currently, Mn4+ activated fluoride red phosphor has been widely studied to improve the optical performance of white light-emitting diode (WLED) in the field of high-efficiency lighting and wide color gamut backlight display. In this paper, a series of red-emitting K2NaScF6:Mn4+ phosphors are synthesized by a simple co-precipitation method with a non-equivalent substitution of Mn4+ for Sc3+ at room temperature. The crystal structure, morphology and elemental composition of K2NaScF6:Mn4+ phosphors prepared using different reaction conditions are systematically investigated. Interestingly, the K2NaScF6:Mn4+ phosphors exhibit narrowband red emission with low correlated color temperature (CCT), high color purity and short fluorescence lifetime. Compared to equivalent doping, the mechanism of short fluorescence lifetime caused by non-equivalent doping has also been systematically discussed. The optimal doping concentration is determined by setting a series of Mn4+ doping concentrations and the concentration quenching mechanism is analyzed. The crystal field strength (Dq), Racah parameters (B and C) and nephelauxetic ratio (β1) are calculated in detail to evaluate the crystal field environment of Mn4+ in K2NaScF6. The good water resistance of the as-prepared sample K2NaScF6:Mn4+ is verified by comparing with the commercially available phosphor K2SiF6:Mn4+. Moreover, the thermal stability is also analyzed by calculating the activation energy (Ea), chromaticity shift (ΔE) and chromaticity coordinate variation (CCV). Importantly, the as-packaged warm WLED (InGaN chip + YAG:Ce3+ + K2NaScF6:Mn4+) has excellent optical properties (CCT = 4082 K, color rendering index (CRI, Ra = 91.2) and luminous efficiency (LE) = 78.86 lm/W), and it exhibits good output stability at high drive currents. In summary, our work demonstrates that K2NaScF6:Mn4+ red phosphors possess a great potential application in warm WLED for the indoor lighting.