Study of LED lighting through the development of new phosphors with thermal stability

다국어 초록 (Multilingual Abstract)

The present work involves the development of new phosphor with enhanced thermal stability in order to use in light emitting diode (LED) lighting. For realizing white light, new green, greenish-yellow and red phosphors were developed by mixing various host lattices. Characteristics of these phosphors were analyzed and were verified for application in white LED. Since the developed phosphors have weak thermal properties it could not be applied in LED lighting. Studies were carried out in view of improving the thermal properties of the phosphor and a promising method to improve the thermal properties of the phosphors has been proposed. Through current work, the possibility of applying new phosphors in white LED is proven by LED device package (PKG) test. LED PKG which has various properties present via adjusting the phosphor's ratio.
New phosphors Mn-codoped Ca8Mg(SiO4)4Cl2:Eu2+, and CaY0.65Al3O7:Eu3+0.35 (CYA:Eu3+) were developed for application in white LED. Mn2+ co-doped Ca8Mg(SiO4)4Cl2:Eu2+ phosphors were synthesized by a solid–state method and their optical properties were investigated. Ca8Mg(SiO4)4Cl2:Eu2+,Mn2+ phosphors have good photoluminescent properties for phosphor-converted LED (pc-LED) because of a broad excitation band at near-UV region. The white LED prepared using InGaN chip (405 nm) with Ca8Mg(SiO4)4Cl2:Eu2+, Mn2+ phosphor emits white light. Furthermore, CaY0.65Eu0.35Al3O7 powder samples have been synthesized using a solid-state reaction method, and investigated their structural and optical properties by maximum entropy method (MEM) assisted by Rietveld refinement and optical measurements. From the MEM calculation with an X-ray source, the charge transfer band and defects resulting from antisite ordering were seen in the electron density distribution. Through an investigation of the luminescent properties of the CYA:Eu3+ phosphor, the critical transfer distance for this phosphor is calculated as 9Å.
The thermal stability of Bi-codoped Sr2SiO4:Eu2+ phosphor synthesized by a solid-state reaction method has been evaluated. Bi-codoping in the phosphor stimulated the energy transfer between Bi3+ and Eu2+, which exhibits enhanced optical properties under 450 nm excitation. Compared with the Sr2SiO4:Eu2+, the Bi-codoped Sr2SiO4:Eu2+ (x = 0.01) showed a strong thermal quenching resistance. The Bi-codoped Sr2SiO4:Eu2+, Bi3+ (x = 0.01) phosphor suggests great potential for high luminous efficacy white LEDs as well as a more stable yellow component.
Finally the manufacturing process of LED with diverse mixing ratio of color converters using the new phosphors Ca8Mg(SiO4)4Cl2:Eu2+, Mn2+ and CaYAl3O7:Eu3+ is presented. In general, color rendering index (CRI) needs to be more than 80 for being used as a lighting source and more than 90 for an application-specific. Characteristics of these phosphors were checked by synthesizing LED PKG using LED Chip which has an emission centered at 405 nm. 5050 PKG, which is commonly used in lighting, was employed in PKG process. CRI obtained were in between 60 and 89 with a corresponding correlated color temperature varying from 3370 – 6110 K depending on the mixing ratio of phosphors. Especially, CRI is proportional to CYA ratio as Red color plays an important role in CRI.
The research on phosphor for LED lighting has been receiving increasing attention. Since an existing commercialized phosphor is not able to follow this trend, the research on new phosphor to achieve high efficiency and CRI is to be progressed. Moreover it is necessary to study on diverse combination of phosphors for application in white LED. In this study, the new phosphors were developed and the application of these to LED lighting is also presented. It suggests that LED PKD studies have to be carried out together with phosphor. The new phosphors, Ca8Mg(SiO4)4Cl2:Eu2+,Mn2+ and CaYAl3O7:Eu3+ is able to synthesize as the LED PKG. However, a phosphor which has a different wavelength is mixed in order to enhance the spectral properties of LED.

목차 (Table of Contents)

• Abstract ………………………………………………………………………… i
• Contents ……………………………………………………………………………… iv
• List of Figures ………………………………………………………………………… vi
• List of Tables ………………………………………………………………………… x
• I. Introduction ……………………………………………………………………… 1
• II. Background ……………………………………………………………………… 5
• II-1. Phosphor Materials…………………………………………………………… 5
• II-1-1. Phosphor for LED ……………………………………………………… 5
• II-1-2. Thermal Qunching………………………………………………………… 12
• II-1-3. Stability by humidity…………………………………………………… 14
• II-2. LED for Solid-State Lighting……………………………………………… 16
• II-2-1. PC-LED …………………………………………………………………… 16
• II-2-2. Color Randering Index………………………………………………… 19
• III. Development for New Phosphor for LED Lighting …………………… 22
• III-1. Luminescence Properties of Mn co-doped Ca8Mg(SiO4)4Cl2:Eu2+ phorphor and its Application in white LEDs……………………………… 22
• III-1-1. Background ……………………………………………………………… 22
• III-1-2. Experimental……………………………………………………………… 23
• III-1-3. Result and Discussion………………………………………………… 24
• III-1-4. Summary…………………………………………………………………… 33
• III-1-5. Reference………………………………………………………………… 35
• III-2. Melilite-Structure CaYAlO:Eu Phosphor: Structure and Optical Characteristics for Near-UV LED-based White Light…………………… 36
• III-2-1. Background ……………………………………………………………… 36
• III-2-2. Experimental……………………………………………………………… 38
• III-2-3. Result and Discussion………………………………………………… 40
• III-2-4. Summary………………………………………………………………… 61
• III-2-5. Reference………………………………………………………………… 62
• IV. Development method for Thermal Stability of phosphor……………… 65
• IV-1. Efficiency and Thermal Stability Enhancements of Sr2SiO4:Eu2+ Phosphor via Bi3+ Co-doping for Solid-State White Lighting…………… 65
• IV-1-1. Background ……………………………………………………………… 65
• IV-1-2. Experimental……………………………………………………………… 67
• IV-1-3. Result and Discussion………………………………………………… 68
• IV-1-4. Summary…………………………………………………………………… 84
• IV-1-5. Reference………………………………………………………………… 86
• V. LED Lighting for High Color Rendering Index…………………………… 89
• V-1. Combinational Phosphor for LED Lighting…………………………… 89
• V-1-1. Background ……………………………………………………………… 89
• V-1-2. Experimental……………………………………………………………… 91
• V-1-3. Result and Discussion…………………………………………………… 92
• V-1-4. Summary………………………………………………………………… 101
• V-1-5. Reference………………………………………………………………… 102