Color Conversion Of Molybdate Phosphor Via Hypersensitive Transition Of Trivalent Europium For Warm White Light Emitting Diodes

Light emitting diodes (LEDs) develop fastly for their merits such as energy consumption, environment. To generate white light, color mix with blue InGaN/GaN chip and blue excitable Ce³⁺-activated yttrium aluminum garnet phosphor has now been a mainstream technology. However, the white light generated by this mode is usually with a high correlated color temperature (CCT) and low color rendering index (Ra). In this M. S. thesis, we have synthesized a molybdate phosphor LiY_0.5Eu_0.5(MoO₄)₂ and tried to use this red phosphor in the white LEDs as a remote phosphor. the main achievements made in this work include:A. Molybdate phosphor LiY_0.5Eu_0.5(MoO₄)₂ with tetragonal scheelite structure has been synthesized. By changing the temperature, we can control the morphology of the phosphor. Monitering the main emission of 616 nm, we find the most intensive excitation of this phosphor located at 538 nm ascribing to the ⁷F₁→⁵D₁ transition of Eu³⁺ ions, followed by the ⁷F₀→⁵D₂, ⁷F₀→⁵L₆ transitions at 467 nm and 396 nm respectively. Based on this finding, we can conclude that this phosphor can be used as a near-ultraviolet/blue/yellowish-green excitable red phosphor.B. Eu³⁺ activated phosphors are generally understood as near-ultraviolet or blue excitable red phosphors. However, the hypersensitive transition ⁷F₁→⁵D₁ of Eu³⁺ ions at 538 nm, which is highly depended on the site environment of Eu³⁺ ions, is usually ignored in the past investigations. Here for this highest hypersensitive transition of Eu³⁺ ions, this phosphor is really suitable to be used as a yellowish-green excitable red phosphor.C. By using this LiY_0.5Eu_0.5(MoO₄)₂ phosphor as a relay phosphor, we can excite this molybdate by the emission of YAG:Ce. By the spectrum of the LED, we can find the energy transfer from Ce³⁺ to Eu³⁺ is realized by reabsorption between the YAG:Ce and LiY_0.5Eu_0.5(MoO₄)₂ layers. By incorporating the LiY_0.5Eu_0.5(MoO₄)₂ phosphor, the CCT of the device could be lowered to a desired value.By the synthesis of the LiY_0.5Eu_0.5(MoO₄)₂ phosphor, we find that, except being blue-excitable, the most intensive excitation of this phosphor is located at yellow-green band and substantially ascribed to the hypersensitive transition of Eu³⁺ ions to give red emission, the results we obtained provide a route to construct warm white light by adding a relay phosphor in LEDs.