The Synthesis of YAG-Based Phosphors with High Quantum Efficiency and High CRI and their Applications in the Enhancement of Luminous Efficiency and CRI of WLEDs

• Main Authors: Te-HsingLiu, 劉德星
• Other Authors: Sheng-Yuan Chu
• Format: Others
• Language: zh-TW
• Published: 2013
• Online Access: http://ndltd.ncl.edu.tw/handle/00679696663710067356

碩士 === 國立成功大學 === 電機工程學系碩博士班 === 101 === In view of the global problem of energy saving and carbon reduction nowadays, to develop efficient white light-emitting diodes (WLEDs) as the lighting source has become the concurrent research point and development direction. And the efficient phosphor with the high color rendering index (CRI) plays a key role to fabricate a WLED with the excellent performance. Considering that the yttrium aluminum garnet (YAG) phosphor has advantages such as high quantum efficiency, simple synthesis procedure, and low cost, it has always been the mainstream material in conjunction with the blue-LED chips for the fabrication of WLEDs. However, the lack of red components in its emission spectrum leads to the disadvantage of insufficient CRI for a yellow phosphor YAG:Ce-based WLED with a blue-LED chip. In order to develop YAG:Ce phosphors with high quantum efficiency and high CRI for enhancing the luminous efficiency and CRI of WLEDs, we sought to synthesize high quantum efficiency YAG:Ce and high CRI YAG:Ce on the basis of the two-step calcined solid state reaction method and composition modification, respectively. On the part of synthesizing highly efficient YAG:Ce powders, an attempt was made to reduce the required calcined temperature and expedite the formation of pure YAG phase with the addition of H3BO3 and BaF2 as fluxes. We found that the morphology of YAG:Ce particles is critical to the quantum efficiency of phosphors, and the mechanism of the particle growth was used to explain why the selection of fluxes can cause the difference in YAG:Ce morphologies. On the part of synthesizing high CRI YAG:Ce powders, an attempt was made to cause the lattice distortion and increase the covalence of YAG:Ce by replacing octahedral Al3+ and tetrahedral Al3+ with Mg2+ and Si4+ ions, respectively [Y2.95Al5-2y(Mg, Si)yO12:0.05Ce3+，YAMSG:Ce]. Such composition modification is expected to shift the emission spectrum of YAG:Ce to the longer wavelength, and therefore the increase of red components in the spectral range. To evaluate the feasibility of the said high quantum efficiency and high CRI YAG:Ce phosphors, these phosphors were combined with the 450-nm blue LEDs by using the remote coating technology. And the luminous efficiency and CRI of our fabricated WLEDs were further compared with those of the commercial YAG:Ce-based WLED under the same phosphor configuration. To validate the comparison among optical properties of these WLEDs, the thickness of the phosphor layers was kept at ~600 μm; moreover, the YAG:Ce-based and YAMSG:Ce-based WLEDs were fabricated under the correlated color temperature (CCT) of ~5500 K and ~4500 K, respectively. Under the drive current of 350 mA, the luminous efficiency of the commercial YAG:Ce-based WLED (70.87 lm/W) was enhanced by 9% compared to that of the WLED using optimized YAG:Ce phosphors (77.2 lm/W). Additionally, the CRI of the commercial YAG:Ce-based WLED was improved from 68 to 81 with the substitution of the optimized YAMSG:Ce for the commercial YAG:Ce.