Rare-Earth-Free High-Efficiency Narrow-Band Red-Emitting Mg3Ga2GeO8:Mn4+ Phosphor Excited by Near-UV Light for White-Light-Emitting Diodes

A series of novel red emission Mg3Ga2GeO8 (MGG):Mn4+ phosphors under near-UV (NUV) excitation are synthesized successfully by traditional high-temperature solid-state reaction. The structure of Mg3Ga2GeO8 is investigated by high-resolution transmission electron microscopy, scanning electron microscopy, and powder X-ray diffraction (XRD) Rietveld refinement. It has one octahedral site and one tetrahedral site in the crystal structure. According to XRD and photoluminescence (PL) property analysis, Mn4+ can occupy an octahedral (Mg2+/Ga3+) site. The PL properties are investigated by diffuse-reflectance, emission, excitation, and temperature-dependent spectroscopy and decay curves. It can emit red light peaking at 659 nm under NUV excitation. The critical quenching concentration of Mn4+ was about 0.5 mol %. The concentration quenching mechanism could be a d-d interaction for the Mn4+ center. The CIE chromaticity coordinates and full-width at half-maximum are (0.295, 0.677) and 24 nm, respectively. It demonstrated that MGG:Mn4+ has high color purity. The PL intensity of MGG:0.5% Mn4+ drops to 72% when the temperature is raised up to 150 degrees C. Furthermore, MGG:0.5% Mn4+ exhibits outstanding quantum efficiency (64.7%). By tuning of the weight ratio of blue, green, and red phosphors, the fabricated white-light-emitting diodes using a 405 nm GaN NUV chip combined with a blend of blue phosphor BAM:Eu2+, green phosphor Sr2SiO4:Eu2+, and red-emitting phosphor MGG:Mn4+ driven by 40 mA current can get white light with chromaticity coordinates (0.316, 0.375) and CCT = 3340 K. This demonstrates that MGG:Mn4+ is a potential red phosphor matching NUV LED chips to get white light.