Improving CRI and Luminous Efficiency of Phosphor-Converted Full-Spectrum WLEDs by Powder Sedimentation Packaging
Nan Zhang, Zhijun Wang, Jinxin Zhao, Dawei Wang, Xueqian Liu, Zhiping Yang, Panlai Li
Abstract
The reabsorption between the powders of phosphor-converted full-spectrum white-light-emitting diodes (WLEDs) was improved by using the powder sedimentation packaging method. The phosphors of red-emitting CaAlSiN3:Eu2+ with large particles and blue-cyan-emitting (Sr,Ca,Ba)5(PO4)3Cl:Eu2+ with small particles were synthesized by the solid-state method. Large-particle Ca0.99AlSiN3:0.01Eu2+ was obtained with an intense red emission at 650 nm by sintering three times, and the luminescence intensity increased by 63% compared with the primary sintering. Meanwhile, the crystallization performance greatly improved, with the D50 of particle sizes reaching 20.37 μm. A high-luminance blue-emitting Sr4.95(PO4)3Cl:0.05Eu2+ phosphor with emission at 455 nm was obtained in a weak reduction (NH3) environment. By adding CaCl2 flux to regulate the morphology, small spherical particles with a D50 of approximately 7.60 μm were obtained. The crystal field environment of Eu2+ was changed by adding Ca2+ and Ba2+ to Sr4.95(PO4)3Cl:0.05Eu2+, and the luminescence colors could be changed from blue to cyan. By combining large-particle red phosphor Ca0.99AlSiN3:0.01Eu2+ and small-particle blue phosphor Sr4.95(PO4)3Cl:0.05Eu2+ with commercial green-emitting phosphor Y3(Al,Ga)5O12:Ce3+, low correlated color temperatures (CCT) full-spectrum WLEDs were fabricated using a 400 nm LED chip as the excitation source. The particles of Ca0.99AlSiN3:0.01Eu2+ will settle during the packaging process because it is relatively large, whereas small-particle Sr4.95(PO4)3Cl:0.05Eu2+ will not, and the packaging method avoids the reabsorption between red and blue phosphors. The color rendering indices (CRI) and luminous efficiency were greatly improved after sedimentation packaging. Full-spectrum WLEDs with tunable CCTs (i.e., 2700, 3000, 4000, 5000, 6000, and 6500 K) were obtained by changing the proportion of different color phosphors. Also, the CRI of each device exceeded 97, with a maximum of 99.3 (5000 K), which is close to the CRI of sunlight (Ra = 100).