Investigation of Novel Luminescent Materials for Ultraviolet Light Emitting Diodes

• Main Authors: Liang-sheng Chi, 紀喨勝
• Other Authors: Ru-Shi Liu
• Format: Others
• Language: zh-TW
• Published: 2003
• Online Access: http://ndltd.ncl.edu.tw/handle/36062063912051763267

碩士 === 國立臺灣大學 === 化學研究所 === 91 === Taking LED (light emitting diode) as a light source is the tendency of modern illumination because of its advantages such as energy saving, low environmental attack and long life time. After the development of high brightness blue LED, using the white LED as light source has come true when blue GaN-LED mixing with yellow YAG (yttrium aluminate garnet) phosphor. Not only the brightness of LED but the phosphor is key factors to influence the final luminous efficiency. According to theoretical conjecture and experimental evidence, energy transformation efficiency of phosphors which can be excited under UV-LEDs (ultraviolet LEDs) is higher than excited under IR or visible light. After GaN blue LED, UV-LED is the next objective because of using many phosphors to make broaden spectrum and high color rendering index (CRI). Presently, the way to generate white light is taking UV-LED with wavelength from 360 nm to 400 nm as excitation source to excite phosphors which can produce red, green and blue lights. By blending these visible lights, white light will be obtained. In order to attain high efficiency and better quality, we should evaluate some factors such as color temperature, CRI and stability when mixing these phosphors. In consideration of the disadvantage of sulfide phosphors which is unstable in CO2, O2 and moisture environment due to the hydrolytic decomposition. We synthesized oxide based red, green, and blue tri-color phosphors and doped rare earth ions as activators. For the sake of better luminescent properties of phosphors excited under UV-LEDs with wavelength from 350 nm to 400 nm, we also doped some metal ions as sensitizers. In this investigation, we doped activators with different concentration for the purpose of realizing the change of crystal structure and luminescent properties. As regard the host lattices, different metal ions were introduced into them in order to discuss their wavelength, color, and efficiency of luminescence. We have probed into the variety of illuminant efficiency and particle size when phosphors sintering at variant temperatures. If we want to develop high brightness white LEDs, we should take the brightness enhancement of red phosphors as key point, because of its low efficiency in the present day. Consequently, homogeneous co-precipitation was used to fabricate red phosphors with high crystallinity, lattice arranging in order, and high illuminant efficiency when excited by UV-LED. In this work, we utilized X-ray diffraction (XRD), X-ray absorption near-edge spectroscopy (XANES), and extended X-ray absorption fine structure (EXAFS) to investigate the long range ordering, oxidation number of activator, and its short range ordering of these phosphors, respectively. In regard to the luminescent properties such as excitation and emission spectrum, photoluminescence spectrometer (PL) was used. And the emission spectra were transformed by the program Colortt to realize their color coordination and color purity. The reflective ultraviolet/visible spectrometer was used to investigate the mechanism of energy transfer. We also utilized scanning electron microscope (SEM) and energy dispersive X-ray spectrometer (EDX) to research their particle size, morphology, and chemical composition. Some results of this study are applying for eight patents and submitting for one publication.