A facile in situ surface-coating passivation strategy for improving the moisture resistance of Mn4+-activated fluoride red phosphor

Abstract Mn4+-activated fluorides have become a research hotspot in the community of inorganic red phosphors owing to their fascinating luminescent properties. However, the poor water resistance of these materials severely hinders their application in the fields of wide color gamut (CG) backlight display and warm white solid-state lighting. Herein, we propose a green, facile method, i.e., treatment with oxalic acid (H2C2O4) solution, for constructing a homogeneous Mn4+-rare surface shell layer to improve the water-resistant property of K2GeF6:Mn4+ (KGF) red phosphor. The moisture-resistant property of the treated KGF (T-KGF) phosphor was significantly enhanced; however, no significant change was observed in its luminescent properties. Furthermore, the reasons for the excellent water resistance of T-KGF were investigated in detail, and the formation mechanism of Mn4+-rare surface shell layer was proposed simultaneously. Results indicated that the Mn4+-rare surface shell layer was in situ deposited on the T-KGF core during the solid-liquid equilibrium process. Upon employing the as-treated T-KGF red phosphor, high-performance white light-emitting diode (WLED) with Ra of 86.3, correlated color temperature (CCT) of 3824 K, high luminous efficiency (LE) of 152.37 lm/W for WLED2 and an ultrahigh CG (120.1% NTSC standard and 89.7% Rec. 2020 standard) and high LE of 103.12 lm/W for WLED3 were achieved. These results demonstrate that the proposed facile strategy may open a new opportunity to promote the commercialization of Mn4+-activated fluoride phosphors.