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Electrophoretic Deposition of Highly Efficient Phosphors for White Solid State Lighting using near UV-Emitting LEDs /

Abstract

Electrophoretic deposition (EPD) is a method to deposit particles dispersed in a liquid onto a substrate under the force of an applied electric field, and has been applied for depositing phosphors for application in solid state lighting. The objective is to deposit phosphors in a "remote phosphor" configuration for a UV-LED-based light source for improved white light extraction efficiency. It is demonstrated that EPD can be used to deposit red-, green-, blue-, yellow- and orange-emitting phosphors to generate white light using a near UV-emitting LED by either depositing a phosphor blend or sequentially individual phosphor compositions. The phosphor coverage was excellent, demonstrating that EPD is a viable method to produce phosphor layers for the "remote phosphor" white light design. The deposition rates of the individual phosphor films were ̃1-5 [mu]m/min. The blend depositions composed of both three and four phosphor compositions emit white light located on or near the black body locus on the CIE chromaticity diagram. Phosphor films were also prepared by sequential deposition of red/orange and green/ blue compositions, to generate white light. The layered films were flipped over and illuminated in this orientation, which showed approximately the same luminescence characteristics. No change in the reabsorption ratio of green/blue emission by the red/ orange phosphor was found regardless of the deposited order of the layered films. These applications of EPD of phosphor for white solid state lighting are promising and effective due to easy tuning of emissive color by varying the phosphor blend compositions. Although nanoparticles of a variety of materials have been coated by EPD, there have been few direct comparisons of EPD of nano- and micron- sized particles of the same material. Another field of the study was to compare EPD of nano-, nano core/SiO₂ shell and micron-sized (Ba₀.₉₇Eu₀.₀₃)₂SiO₄ phosphor particles for application in a near-UV LED-based light source. EPD from an amyl alcohol bath was able to produce uniform films for all particle sizes, whereas uniform films were produced only of micron-sized particles in an isopropyl alcohol bath. A new equation was developed for predicting the deposited mass, considering the change in concentration of particles in the bath from both settling and deposition, showed good agreement with the experimental values

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