Synthesis of New Green-Emitting Bredigite-Structure Orthosilicate Phosphor and Application to White LEDs

이경화 전남대학교 대학원 2013 국내석사

In the last few years, white light emitting diodes (WLEDs) have been the increasing subject due to their advantages of low energy consumption, long life, lack of pollutants such as Hg and their potential applications in backlights, automobile headlights and general illuminations. Generally white light can be generated by a combination of blue LED chips coated with the yellow-emitting phosphor. This method, based on phosphor-down conversion, has high luminous efficiency (> 30 lm/W), but a poor color-rendering index (< 65) due to weak red emission is one of its main drawbacks. In this regard, a new class of WLEDs has been developed to guarantee excellent color-rendering properties compared to the conventional one e.g. near-ultraviolet LEDs (near-UV LEDs) combined with multi-phase phosphors. In Chap. 2, it is reported a green-emitting Ca14-xEuxMg2[SiO4]8(CMS:Eu2+) phosphor with Bredigite structure. Their structural and optical properties have investigated. WLEDs based on a combination of an InGaN LED chip (λmax = 395 nm) with the CMS:Eu2+and red phosphors have been fabricated and are discussed. In Chap. 3, a single-phase green-red emitting phosphor, Ca13.7Eu0.3Mg2-xMnx[SiO4]8 (CMS:Eu2+,Mn2+), was prepared by a solid-state reaction, and its energy transfer from Eu2+ to Mn2+ was investigated as a function of Mn2+ concentration. In the crystal structure of CMS, there are eight different Ca sites that can be substituted by Eu2+ as an activator and two different Mg sites that can be substituted by Mn2+. The CMS:Eu2+phosphor, which has good luminescence properties under near-UV excitation, can be enhanced with incorporation of Mn2+ions in the red spectral range. The mechanism of energy transfer in a CMS phosphor with critical concentrations of Eu2+ and Mn2+ and Dexter’s theory for energy transfer have been discussed and confirmed. When CMS:Eu2+,Mn2+ was incorporated with an encapsulant in ultraviolet (λmax = 400 nm) light emitting diodes (LEDs), white light with a color rendering index of 67 under a forward bias current of 20 mA was obtained. The results of this work indicate that CMS:Eu2+,Mn2+ could be a promising single-phase phosphor for white LEDs under a near-ultraviolet source. In Chap. 4, we have synthesized of a partially nitridated Ca13.7Eu0.3Mg2Si8O32 (CMSN:Eu2+) using Si3N4 with a conventional solid-state reaction and successfully determined the structure parameters by a combined Rietveld refinement method. The partial nitridation of Ca13.7Eu0.3Mg2Si8O32 (CMS:Eu2+) led to a large enhancement in the luminescence intensity, as much as 148 %. From the Rietveld refinement results, the anisotropic changes of the lattice parameters were observed for the partially nitridated samples. By incorporating this phosphor + red phosphor with an encapsulant on an InGaN light-emitting diode (λmax = 395 nm), white light with a color rendering index of 92 and a color temperature of 5320 K under 20 mA was obtained.

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

박승혁 Graduate School, Korea University 2013 국내박사

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.

Photoluminescence characterization of (Y,Gd)(V,P)O4:Eu3+ phosphor and the effects of Sc3+ and Bi3+ addition

경현애 고려대학교 대학원 2009 국내석사

(Y,Gd)(V,P)O₄:Eu^(3+) and Sc- or Bi-added (Y,Gd)(V,P)O₄:Eu^(3+), which could be potential red-emitting components for cold cathode fluorescent or light emitting diodes, were prepared by a solid-state reaction. The crystalline phases of the prepared phosphor powders were identified using an X-ray diffraction system. The morphology and the particle size of the phosphors were characterized by SEM. The photoluminescence properties of the phosphors under the excitation of ultraviolet ray were investigated. In both cases, the concentration of Eu activator exhibiting maximum emission intensity was 10 mol%. The photoluminescence results showed that the optimum chemical compositions of the phosphors are (Y_(0.5)Gd_(0.4)Eu_(0.1))(V_(0.8)P_(0.2))O₄, (Sc_(0.05)Y_(0.45)Gd_(0.4)Eu_(0.1))(V_(0.8)P_(0.2))O₄ and (Bi_(0.05)Y_(0.45)Gd_(0.4)Eu_(0.1))(V_(0.8)P_(0.2))O₄. The XRD results indicated that these phosphors have a single phase of tetragonal structure with space group I4₁/amd, which is identical to the crystal structure of YVO4. The (Y_(0.5)Gd_(0.4)Eu_(0.1))(V_(0.8)P_(0.2))O₄ phosphor showed higher emission and better chromaticity characteristics compared to commercial Y₂O₃:Eu^(3+) red phosphor. It was shown that the addition of Sc^(3+) improves the color purity of the (Y_(0.5)Gd_(0.4)Eu_(0.1))(V_(0.8)P_(0.2))O₄ phosphor. It was further shown that the band edge of the excitation spectra with increasing Bi^(3+) is shifted toward the longer wavelength region. The temperature dependence of the emission intensity of the (Y_(0.5)Gd_(0.4)Eu_(0.1))(V_(0.8)P_(0.2))O₄ phosphor was also evaluated in the range of room temperature to 150℃.

반도체 광원의 응용에 있어 열적 안정성이 우수하고 희토류 이온이 첨가된 NaCaPO4 형광체의 합성 및 발광특성 : Synthesis and Luminescent Properties of Rare Earth Activated Thermally Stable NaCaPO4 Phosphors for Solid State Lighting Applications : 보난 벤카타

보난 벤카타 라트남 창원대학교 2016 국내박사

Nowadays, solid state lighting (SSL) is a pivotal engineering technology that promises to fundamentally alter and improve lighting systems in future. Recently, one of the rapidly developing fields of research for new and efficient phosphors is the SSL technology especially white light emitting diodes (w-LEDs), which can compete with traditional incandescent and fluorescent lamps. This technology has the potential to attract significant scientific attention for the development of fourth generation light sources in recent years due to their excellent characteristics in comparison to conventional light sources including long lifetime, high efficiency, energy saving, low power consumption, low cost, reliability, small size and the environmental friendliness. The widespread use of solid state based lighting is of great importance to significantly reduce the global electricity consumption and the use of fossil fuels. Moreover, the features of long lifetime and mercury-free would contribute in solving environmental problems. Nowadays, the most popular approach for fabricating w-LED is by coating a yellow emitting phosphor on a blue emitting InGaN LED. However, the current generation white LEDs have many disadvantages. To overcome the deficiencies of the commercially available phosphor converted white LEDs (pc-white LEDs) as mentioned above, thermally stable orthophosphate host is selected for the current study to produce white light emission with the strategy of combining near ultraviolet (near-UV) chip with blue and yellow color (or) tricolor [red, green and blue (or) blue, cyan and orange] emitting phosphor. In addition, the near-UV based pc-LED is expected to have great potential in the field of solid-state lighting, when comparing with blue LED based pc-LED. Therefore, optimization of the red/green/blue/orange emission with the high absorption of n-UV light is needed for rare earth doped phosphors to develop highly efficient white light emitting diodes. Motivated by the above facts, white light, green, orange, orange-red color emitting trivalent rare earth activated NaCaPO4 phosphors were synthesized by using a conventional solid state reaction (SSR) method. Moreover, Eu3+ doped NaCaPO4 has also been synthesized by two other synthesis techniques such as molten salt synthesis (MSS) and sol-gel combustion (SGC) method. The synthesis procedure, sintering temperature have been optimized based on the results of TGA-DTA and emission properties. The phase and the structure of the as-prepared powders were characterized by using X-ray diffraction data and revealed the formation of pure NaCaPO4 with orthorhombic structure using Reitveld refinement analysis. In addition, thermal, morphological and structural properties have been investigated by employing TGA-DTA, FE-SEM, FT-IR and Raman techniques and the obtained results are presented in this thesis. Research has been further extended to study the luminescent and colorimetric properties of rare earth doped phosphors. The excitation and emission spectra were measured to characterize the luminescent properties of all rare earth doped NaCaPO4 (NCP) phosphors and the results are as follows: For NaCaPO4:Dy3+ phosphors, sharp emission peaks were observed at 482 nm (blue) and 575 nm (yellow) upon 367 nm excitation, which are attributed to the characteristic 4F9/2 →6HJ (J =15/2 and 13/2) transitions of trivalent Dy3+ ions, respectively. The suitable control of the blue and yellow intensity ratio is expected to realize a white luminescent system. The lifetime of the 4F9/2 level was measured by exciting Dy3+ ions at 355 nm excitation. The Commission Internationale de l’Eclarage (CIE) color coordinates fall in the white light region (x = 0.32, y = 0.37) under different UV and NUV excitations. These results indicate that NaCaPO4:Dy3+ phosphor could be a potential candidate for near-UV based white light emitting diodes (w-LEDs) (Section: 3.1) For NaCaPO4:Tb3+ phosphors, the excitation spectrum exhibited one broad band located in the UV -region at 274 nm and is assigned to 4f8→ 4f75d1 transition and some other excitation bands in the longer wavelength region are attributed to f-f transitions within the Tb3+ (4f8) configuration. Among all the excitation peaks, the strong 4f-4f transition at around 370 nm has a higher intensity. The emission spectra were measured upon 370 nm excitation and the most intense peak is observed in the green region at 547 nm, corresponding to the 5D4 → 7F5 transition. Analysis of the emission spectra with different Tb3+ concentrations revealed that the optimum dopant concentration for these NCP phosphors is about 5 mol% of Tb3+. The emitting color of Tb3+ doped NaCaPO4 phosphor was discussed based on the chromaticity coordinates and are indicated in CIE diagram. The excellent luminescent properties of Tb3+ doped NaCaPO4 green phosphor makes it as a potential candidate to use in near-UV based w-LEDs (Section: 3.2). For NaCaPO4:Sm3+ phosphors, the excitation spectra indicate that this phosphor can be effectively excited by NUV and blue light. The emission spectra indicated that the strong emission peak at wavelength of 599 nm originated from the transition of 4G5/2 →6H7/2. The optimum concentration of Sm3+ is determined as 1.0 mol% based on the concentration dependent emission spectra. These results suggest that the NaCaPO4:Sm3+ phosphor is a promising orange emitting phosphor under 404 nm excitation with CIE coordinates of x= 0.545, y= 0.410, which might be used in the development of materials for LEDs and other optical devices in the visible region (Section: 3.3). For NaCaPO4:Eu3+ phosphors, excitation spectra indicates the strong absorption in n-UV and blue region due to intraconfigrational f-f transitions of Eu3+ ions. The emission spectra exhibit strong orange-red emission at 595 nm corresponds to 5D0 → 7F1 transition under n-UV (λex=392 nm) excitation. The SGC route synthesized phosphor indicates intense emission than that of the SSR and MSS method. Therefore, series of Eu3+ ion doped NaCaPO4 phosphor has been prepared by SGC method. The effect of concentration on the emission intensity and concentration quenching mechanism has been discussed in detail. The Commission International de I’Eclairage (CIE) chromaticity coordinates (0.621, 0.377), (0.620, 0.378) and (0.622, 0.376) are nearly equal for the phosphors synthesized by SSR, MSS and SGC method, respectively (Section: 3.4). Thus, this thesis is devoted to synthesize and development of efficient rare earth doped NaCaPO4 phosphors with enhanced luminescent properties to generate whtie light and multicolor emission and further to assess the feasiblity to use these phosphors in near-UV based white LEDs.

Fabrication and Luminescence Characterization of Rare Earth Based Phosphors for X-ray Imaging : X-선 영상을 위한 희토류계 형광체의 제조와 발광 특성

오명진 경북대학교 대학원 2017 국내박사

이 연구는 디지털 방사선 영상 및 X-선 영상판 및 증감지 등의 의료 방사선 영상에 사용되는 새로운 형광체를 개발하는 것이 주목적이다. 형광체는 이 밖에도 다양한 산업 분야 즉, 램프나 플라즈마 디스플레이, 레이저 물질, 음극관 튜브 및 비파괴 검사 분야에서 다양하게 이용된다. 이 실험에서Eu 또는Sm 이 도핑된 LLBO (Li6Lu(BO3)3) 와 Ln3+ 이 도핑된 Gd2GeO5, Eu 또는 Ce 이 도핑된 GGAG (Gd3Ga2Al3O12) 그리고 Gd를 도핑한 LYBO (Li6Y(BO3)3) 을 각각 제조하였다. 이 원료 물질들은 희토류계 원소로 다른 물질과 비교하여 높은 유효 원자번호를 가지기 때문에 의료 방사선 영상 분야에서 사용되는 X-선과 감마선에 대한 흡수효율이 매우 우수하다. 높은 순도의 주 물질과 활성제로 이루어진 대부분의 형광체는 고체 반응법을 이용하여 전기로에서 높은 온도로 소성한다. 그런 다음 제조된 형광체는UV, X-선 그리고, 양성자선을 이용하여 발광 스펙트럼 특성을 조사하였으며, 형광 붕괴 시간, 상대적 발광량, 엑스선회절시험 (X-ray diffraction, XRD), 전계방사형주사전자현미경(field emission scanning electron microscope, FE-SEM) 등을 측정하였다. 제조된 형광체의 각각의 발광 스펙트럼은 대략 500~700 nm로 장파장 영역에서 발생하였으며 이것은 광전자증배관(PMT)과 같은 광 센서보다 전하결합소자 (charge coupled device, CCD), 상보성금속산화 (complementary metal-oxide semiconductor; CMOS) 소자, 광다이오드 (photodiode, PD) 혹은 박막트랜지스터(thin film transistor, TFT) 와 같은 광센서의 양자효율 특성에 잘 매칭되므로 이들 수광 소자를 이용한 X=선 영상 장비에 유용하다. 상대적 발광량은 형광체의 특성을 결정하는 가장 중요한 요소 중 하나이다. 실험 결과 GGAG (Gd3Ga2Al3O12) 와 LLBO (Li6Lu(BO3)3) 형광체는 상용화된 가독스(Gd2O2S:Tb) 형광체와 비교하여 높은 효율의 빛을 방출하였다. The purpose of this study is to develop the novel phosphor for medical radiographic imaging, such as digital radiography (DR), X-ray imaging plate (IP) and intensifiers etc. Besides, the phosphor can be used various industrial fields, such as various lamps, plasma display, laser materials, cathode-ray tubes and non-destructive inspection and etc. In this experiment, the novel phosphors such as Eu3+ or Sm3+ doped Lithium Lutetium Borate; LLBO (Li6Lu(BO3)3), Ln3+ doped Gd2GeO5, Eu3+ or Ce3+ doped Gadolinium Gallium Aluminum Garnet; GGAG (Gd3Ga2Al3O12) and Gd3+ doped Lithium Yttrium Borate; LYBO (Li6Y(BO3)3) are fabricated. These raw materials made up of rare earth elements have a high effective Z-number (Zeff) in comparison to other phosphors and the high effective Z-number led to the high absorption efficiency for X-ray or gamma rays in medical radiation imaging fields. Most of phosphor are synthesized by solid state reaction with high purity of the host and activator materials at high temperature by using electric furnace. Then, the luminescent properties of the phosphor were characterized bys photoluminescence, X-ray induced luminescence, proton induced luminescence, decay time, relative light yield, X-ray diffraction (XRD) and field emission scanning electron microscope (FE-SEM) measurements. The emission spectra of fabricated phosphors show the long wavelength at 500~700 nm roughly. The long wavelength is well matched to silicon based light sensor such as photo diode (PD), charge coupled devices (CCDs), thin film transistor (TFT) and complementary metal oxide semiconductor (CMOS) sensor because of their high quantum efficiency compared to the photomultiplier tube (PMT) and another light detectors. Therefore fabricated phosphors are suitable for X-ray imaging application using CCD, CMOS, PD or TFT which is the high quantum efficiency in the wavelength range of 500~700 nm. The relative light yield is one of the most important parameter to determine the characteristic of phosphor. As a result, GGAG (Gd3Ga2Al3O12) and LLBO (Li6Lu(BO3)3) phosphor shows the high effective light compared to reference phosphor such as Gadox (Gd2O2S:Tb).

Structurally engineered phosphors using two-dimensional photonic crystal and resonant cavity platforms

이태윤 서울대학교 대학원 2023 국내박사

Phosphor, a color conversion material, is an indispensable and important component in the modern display industry. Improvement in color conversion efficiency of phosphor has been required for more advanced displays. Research to improve phosphor efficiency has been developed by focusing on the material itself so far. However, our research group recently proposed a new research paradigm shift—structurally engineered approach. The concept is to improve the efficiency of phosphor by controlling the field profile of light using photonic structures. It has been demonstrated that it is possible to dramatically improve color conversion efficiency by using a photonic crystal structure. This thesis contains the further developments of this photonic structure-based phosphor research. In previous studies, one-dimensional photonic crystal was adopted as a photonic structure. However, since it is one-dimensional, it has an unavoidable directionality problem. Therefore, in this study, efforts were made to maximize the improvement in color conversion efficiency by using two waveguide modes, TE and TM, while solving the problem of directionality by extending the photonic crystal to two dimensions. As a result, four times higher efficiency compared to the previous one-dimensional structure has been demonstrated. As a next step, a more enhanced two-dimensional photonic crystal phosphor was proposed by changing the photonic crystal backbone material from Si3N4 to TiO2 and planarizing the surface by using squeeze method. This enhances the resonance of light. By manufacturing the real device, it was shown that the attempt was successful. Structurally engineered phosphors using two-dimensional photonic crystal structure exhibited higher color conversion efficiency than conventional phosphors. But the absolute amount of light absorption is small unless they are stacked. To solve this problem, a resonant cavity structure was proposed. The RC phosphor proposed here differs from the PhC phosphor in that it utilizes a tightly localized vertical cavity mode whereas the PhC phosphor relies on a periodically extended lateral band-edge mode. In this structure most of excitation beam was absorbed. The resonant cavity phosphor exhibits up to 30 times better absorption compared to reference phosphors. This high color conversion efficiency will be useful for advanced displays. 색변환 물질인 형광체는 현대 디스플레이 산업에서 없어서는 안 될 중요한 구성요소이다. 보다 발전된 디스플레이를 위해 형광체의 색변환 효율 향상이 요구되어 왔다. 그동안의 형광체 효율 향상을 위한 연구는 물질자체에 집중하여 발전해 왔다. 그러나 최근 본 연구 그룹에서 구조에 기반한 새로운 연구 패러다임을 제시하였다. 광학적 구조를 이용해 빛의 거동을 제어하여 형광체의 효율 향상을 꾀한다는 컨셉이다. 이를 직접 소자로 제작해 획기적인 색변환 효율 향상이 가능하다는 것을 시연한 바 있다. 본 논문은 이런 광학적 구조 기반 형광체 연구를 이어받아 더욱 발전시킨 내용을 담고 있다. 앞선 연구에서는 광학적 구조로 1차원 광자결정을 채택하였다. 그런데 이는 1차원이라서 어쩔수 없이 갖게 되는 방향성 문제점를 갖고 있다. 그래서 본 연구에서는 광자결정을 2차원으로 확장하는 것으로 문제를 해결하는 동시에 TE, TM 2가지 모드를 이용함으로써 색변환 효율 향상을 극대화시키는 노력을 하였다. 그 결과 기존 연구의 4배에 달하는 효율 향상을 이룩할 수 있었다. 그리고 그 후속 연구로 광학적 구조를 구성하는 물질의 변경과 표면형상의 제어로 더욱 개선된 2차원 광자결정 형광체를 제안하였다. 이는 빛의 공진을 더욱 강하게 하기 위함이다. 이 직접 소자를 만들어 시도가 성공적이었음을 보였다. 비록 2차원 광자결정을 이용한 형광체가 기존 형광체 대비 높은 색변환 효율을 보였어도 절대적인 빛의 흡수량이 적어서 적층을 하지 않으면 실용성이 떨어진다. 이 문제를 해결하기 위해 공진 공동 구조를 도입하였고 빛의 흡수량이 부족하던 문제를 해결할 수 있었다. 광자결정 형광체는 주기적으로 펼쳐진 측면 방향의 밴드 에지 모드를 이용하는 반면 공진 공동 구조 형광체는 강하게 국지화된 수직 공동 모드를 이용한다. 이렇게 만든 공진 공동 형광체는 기존 형광체의 30배에 달하는 흡수를 보였다. 이는 보다 진보한 디스플레이 응용에 큰 도움이 될것으로 기대 된다.

Study of the luminescent properties of Ca-α-SiAlON phosphors synthesized by spark plasma sintering

최성우 서울대학교 대학원 2012 국내박사

White light emitting diodes (LEDs) as eco-friendly light sources have been extensively investigated due to their potential applications in backlight units for liquid crystal displays (LCD), indoor/outdoor lamps, traffic signals, and general illuminations. For enerating white LEDs, several approaches have been explored to produce the white light including multi chip LEDs and a combination of color converting phosphors and LEDs (so called pc-LEDs). Generally, phosphors converted-LEDs (pc-LEDs) have been commercially adopted due to good performances such as efficiency, single life time, and CRI. Phosphors in this system play an important role in determining the color temperature, chromaticity coordinates, color rendering index, luminous efficiency and life time of phosphor-converted white LEDs. At present, pc-LED with using combination of blue LED chip and yellow phosphor Y3Al5O12:Ce3+ has been the most popular on the market. A deficiency for pc-LED that use only YAG:Ce phosphor is that they are limited to high CCT and lower CRI due to a lack of a red spectral component. Therefore, phosphors used in white LEDs have been considerably developed in order to replace YAG:Ce yellow phosphor. In general, the requirements for phosphors include that the excitation wavelength of phosphors should match well with the emission wavelength of the LED chips (Blue chips: 450~460 nm); the phosphors must have high efficiency at the emission wavelength of the LED chips; and the phosphors have to be stable against high temperature (at least > 150 ˚C), humidity, and strong irradiation of LED chip. Commonly used phosphors for white LEDs have been mainly based on oxides and sulfides such as (Sr,Ca)S:Eu2+, SrGa2S4:Eu2+, and YAG:Ce3+. These sulfide phosphors have low thermal/chemical stability, resulting in a strong temperature dependence of chromaticity and the degradation in efficiency of white LED. While, oxide-based phosphors except YAG:Ce3+ are barely suitable for absorbing visible light efficiently, which make them impossible to combine with UV or blue LEDs to generate white light. Therefore, novel materials with requirements for white LEDs are in great demand. Very recently, nitride and oxynitride based phosphors such as a green (β-SiAlON:Eu2+, and Ba3Si6O12N2:Eu2+), yellow (Ca-α-SiAlON:Eu2+), and red phosphors (M2Si5N8:Eu2+ ; M=Ca, Sr, and Ba, CaAlSiN3:Eu2+) have been studied as a new hot topic for phosphors, which exhibit unusual, interesting luminescence properties when activated by rare-earth ions. Especially, these phosphors importantly emit visible light efficiently under near UV or blue light irradiation and have superior thermal and chemical stability to their oxide and sulfide counterparts, allowing them to be used as down conversion luminescent materials for white light emitting diodes (LEDs). Their luminescence properties are attributed to the strong nephelauxetic effect and large crystal field splitting effect. Among these various phosphors, Ca-α-SiAlON:Eu2+ phosphor has advantage to replace YAG:Ce3+ yellow phosphor as well as makes up for deficiency of red region as a single phosphor converted LEDs. Up to now, Ca-α-SiAlON phosphors have been synthesized by gas pressure sintering (GPS), gas reduction-nitridation (GRN), and carbothermal reduction-nitridation (CRN) methods. In this work, all α-SiAlON powder samples with composition of Cam/2-xSi12-(m+n)Alm+nOnN16-n:Eux were synthesized by using the spark plasma sintering (SPS) method for the first time. Spark plasma sintering is possible to synthesized nitride phosphors relatively at lower temperature for a short time without reducing gas. α-Si3N4, AlN, CaO, and Eu2O3 (CeO2, Yb2O3) were used as starting materials and the synthetic conditions were optimized, varying the sintering temperature, time, amount of dopants, and compositions. The photoluminescent properties of SiAlON doped with Eu2+ were investigated with a relation to temperature, composition, and concentration. Also, to investigate the effect of dopants on luminescent properties and thermal stability within the same crystal structure, the Eu2+, Yb2+ and Ce3+-doped Ca-α-SiAlON were prepared. Especially, thermal quenching is one of the important parameters for phosphors used in white LED. The temperature-dependent luminescent properties were measured, and the results of different behaviors were compared, relatively. In addition, the phosphors synthesized by spark plasma sintering were post-treated by annealing in a reduction atmosphere and washing with acidic solution for improvement of luminescent properties. The post-annealing conditions for all samples were optimized, varying the annealing temperature, time and gas flow. It was confirmed that the post-annealing led to improve crystallinity, increase the amount of dopant with reduced ion state and reduce glass phase in grain boundary, resulting in the enhanced yellow-orange emission without any change in the emission spectrum. Also, it is thought that washing of phosphors with using an acidic solution affected to selectively dissolve non-participated elements in glass phase on luminescent properties. It was confirmed that the removal or crystallization of glass phase in grain boundary bring respectively the improvement of luminescent property.

Micro/nano-structured phosphor materials for white light-emitting diodes and biomedical applications

엘루리 파비트라 경희대학교 일반대학원 2015 국내박사

The fascinating scientific and technological developments, especially, the rapidly growing technology in the solid-state lighting market creates a need for the development of novel phosphors with high conversion efficiencies, excellent thermal quenching behavior and the proficient emissions in the visible spectral range. Particularly, oxide-based phosphors activated with trivalent rare-earth ions are recognized as promising materials for next-generation display devices and white light-emitting diodes due to their environmental friendly nature with stable physical and chemical properties. Currently, a big challenge is to develop novel phosphor materials with high efficiency, thermal stability and reliability for indoor and outdoor illuminations. Besides, efforts have been focused on the development of nontoxic multifunctional mesoporous materials for biomedical applications. Several biocompatible materials with different morphologies and compositions, such as metals, metal oxides, and polymers, have been employed as multifunctional biomaterials to target cancer and other diseased cells. Due to the lack of permeable capability, the majority of these materials concentrate in the cytoplasm. Therefore, research efforts have been focused on targeting the cell nucleus by improving the penetration capability of the particles. In this thesis, various trivalent rare-earth ions (Eu3+, Tb3+, Sm3+, Dy3+, Er3+, Tm3+, and Yb3+) activated SrY2O4 (SY) nanocrystalline phosphors were synthesized using a modified sol-gel technique. The X-ray diffraction (XRD) patterns confirmed their orthorhombic structure and scanning electron microscope (SEM) image showed the closely packed particles. SY: Eu3+ phosphor showed the reddish-orange emission and SY: Tb3+ phosphor exhibited the bright green color. For the first time, in the case of Eu3+/Tb3+ ions co-doped SY phosphors, concentration independent white light emission was achieved by controlling the energy transfer between Tb3+ and Eu3+ ions and excitation induced emissions were observed from the Sm3+ co-doped SY: Tb3+ phosphor. The Er3+ ions doped phosphors displayed intense green emission and Dy3+/Er3+ ions co-doped phosphors showed yellowish green emission. While, the Er3+/Dy3+/Sm3+ ions triple-doped SY phosphors exhibited a fair white light emission due to the energy transfer from both Dy3+ and Er3+ ions to Sm3+ ions. The upconversion emission properties were also studied for Er3+/Tm3+/Yb3+ ions tri-doped SY host lattice at different pump powers. The achieved results suggest that these SY nanocrystalline phosphors were efficient materials for solid-state lighting applications. It is well known that, the phosphors with spherical morphology have many advantages like higher packing density, lower scattering of light, and brighter luminescence performance. Also, the suitable morphology and excitation wavelengths are very important for mixing with Y3Al5O12 (YAG):Ce3+ to generate perfect white light emission. In this context, we established the synthesis of Eu3+ ions activated Y2Ti2O7 spheres with two efficient excitations in the excitation and emission regions of YAG:Ce3+. Likewise, Eu3+ ions doped Gd2O3 nanoflowers were synthesized by facile large-scale route. The structural and morphological properties were carried out by XRD and SEM investigations. The emission intensity and asymmetric ratio between red and orange (R/O) are higher for our Gd2O3:Eu3+ when compared with the commercial Y2O3:Eu3+. Nowadays, there is increasing demand for novel yellow phosphor materials with excitation in near-ultra violet (UV) region. So, a novel self-activated yellow Ca5Zn3.92In0.08(V0.99Ta0.01O4)6 (CZIVT) phosphor which efficiently convert violet excitation light into yellow luminescence was synthesized. The crystal structure and lattice parameters of these CZIVT phosphors are elucidated by Rietveld refinement. By doping the In3+ and Ta5+ ions, the emission intensity is enhanced in the red region and the Stokes shift is controlled to acquire good color rendition. When a near-UV LED chip is coated with the combination of CZIVT and commercial blue phosphors, a pleasant WLED is achieved with high CRI of 82.51 and low CCT of 5231 K which are essential for indoor illuminations. In this present study, we reported the PEGylated -Gd2(MoO4)3 (GMO) ternary complex compound mesoporous flowers using two-step synthesis such as solvothermal for amorphous precursor and hydrothermal for crystalline with PEGylated precursor. The growth mechanism of flower-like morphology has been explained by taking SEM images of the intermediate products. PEGylation was verified by the XRD patterns and FTIR spectra and the nitrogen adsorptiondesorption isotherms of PEGylated -GMO particles established their mesoporous nature. When exciting with UV or visible wavelengths these mesoporous particles displayed gorgeous red emission. These mesoporous particles are proved to be promising biomaterials with hydrophilic nature and have the capacity to penetrate cells, translocate to the nucleus, and trigger high-quality signals from the cellular compartment than earlier nanoparticles. Likewise, size tunable mesoporous Gd:Eu3+@mSiO2@FA core-shell nanoparticles were synthesized and these core-shell nanoparticles exhibited high quality red emission in the near-UV and visible regions. The preliminary results confirmed that, after conjugated with folic acid, increased number of Gd:Eu3+@mSiO2@FA core-shell nanoparticles were entered in to the both nuclear and cytoplasm in U2OS cell lines due to the increased hydrophilic nature of the particles.

형광체의 표면 코팅에 따른 평면형광램프의 방전특성

우창민 경북대학교 대학원 2008 국내석사

Flat Fluorescent Lamp(FFL) has been actively studied for application to the back light unit(BLU) of thin film transist liquid crystal display(TFT-LCD). FFL is actually a much simple version of plasma display panel(PDP) since the two devices use same phosphor materials and emit light by same mechanism. In PDP fine patterning of red, green, and blue phosphor layer is required corresponding to each pixels to display fine full color images, however, FFL is fabricated with the R, G, B mixture of phosphors since FFL is used as BLU or luminance device with just white light. In this work R, G, B phosphors were treated with TEOS solution to make thin film of SiO2 on the surface of phosphor particles by the Sol-Gel method. Surface treated R, G, B phosphors were characterized by SEM, TEM, EDS and XRD to check its effect on surface of phosphor particle, size of particle, and luminance properties of phosphors. The luminance of R, G, B phosphors were decreased to a minal degree compared to the original R, G, B phosphors as verified by photoluminance spectra utilizing vacuum UV sources. The surfaced treated R, G, B phosphors were used to make phosphor slurry. The slurry was made by vigorous stirring of phosphor powder in the vehicle composed of nitro cellulose / ethyl cellulose mixture(1:2 wt ratio) as binder polymer, BCA as solvent and dispersant as additives. The rheological properties of various formulation of phosphor slurry were evaluated from the viewpoint of applying coating with sprayer and slurry dispensing equipment. After optimizing phosphor slurry formulation, the FFL device was fabricated by coating the R, G, B phosphor mixture slurry on the bottom panel of FFL devices. The luminance property of FFL fabricated with single phosphor slurry and R, G, B phosphor mixture slurry were tested by applying voltage. The white color coordinates of the FFL with R, G, B phosphor mixture was almost same either original R, G, B phosphor or surface treated phosphor was used to make FFL devices. The whit luminance of FFL was decreased to a very small degree when surface treated R, G, B phosphors were used compared to the original phosphors. However, it was expected the long term stability of life time of FFL may be increased in the case of FFL made with surface treated phosphor since the thin SiO2 layer on the surface of phosphor will protect the active center of luminating phosphor material from the high energy VUV radiation.

UV-curable planar silicate/halide phosphor structures for highly efficient white light emitting diodes

장진우 Graduate School, Yonsei University 2020 국내박사

Phosphor converted white light-emitting diodes (LEDs) have received much attention due to their relatively low power consumption, high efficiency, long lifetime, and environmental friendliness. In general, a white LED is fabricated by applying a yellow phosphor that is emitted by a blue LED on a blue LED chip. Increasing the input current of a blue LED chip, the general method for increasing the light emission intensity of a white LED, degrades the phosphor and yellows the polymer used as an encapsulant from the high temperature of the blue chip. This may reduce reliability due to decreased luminous efficacy and color change. Various LED structures have been proposed to solve this problem. Among them is a remote phosphor structure that can minimize the reduction in performance from heat generated from the chip by separating the distance between the blue LED chip and the phosphor layer. Phosphor materials for converted white LEDs play an important role in luminous efficacy, color gamut, color rendering index (CRI), and correlated color temperature (CCT). For this reason, studies of phosphor materials for white LEDs have recently been conducted. In the case of the basic blue LED chip and yellow phosphor combination, there are problems of low CRI and low color gamut. Those problems render the combination unsuitable for application as a white LED backlight. Therefore, we considered introducing red phosphor into a yellow phosphor to improve CRI and CCT as well as the luminous efficacy of the white LED. In addition, it was necessary to use red and green phosphors instead of yellow phosphors to prevent large overlaps of green and red-light emission spectra when passing through the RGB color filters used in liquid crystal displays (LCDs). For this reason, we needed a phosphor with a narrow emission wavelength region. Further research on a suitable quantum dot and halide perovskite nanocrystals is also necessary. One of the key scientific challenges for the phosphor-converted white LED industry is to adjust the technology for versatile device structures, including planar and/or flexible systems requiring a low-temperature process, to allow easy transfer at a commercial scale. Screen printing processes are widely used industrially because they yield well-defined layers and are easily controllable in thickness and size, as well as simple and inexpensive. Ultraviolet (UV)-curable polymers can be used to easily make remote phosphor layers through a simple process of UV irradiation at room temperature. Especially for temperature-sensitive phosphors, such as halide perovskite, UV-curable polymer can be considered as an important matrix for room temperature processes. Silicone resins and glass frits, however, are not suitable matrices because they require high temperatures and long time to cure. Combining a commercially simple and inexpensive screen-printing process with UV-curable polymers will enable mass production of phosphor layers at very low cost. This thesis focuses primarily on the development of various phosphor thick films embedded in the UV-curable polymer. First, we developed a simple way of forming a nonconventional remote phosphor layer for white light-emitting diodes. Silicate phosphor ((Ba,Sr,Ca)2SiO4:Eu2+) and an UV-curable polymer can be applied to form solid planar films on a common soda lime silicate glass substrate using UV radiation. The stability of the phosphor and polymer with UV irradiation time were evaluated. The relative content of the phosphor was adjusted to achieve the best dispersion of the phosphor particles in the polymer matrix along with increased emission and luminescence performance. As a result, the 70 wt% phosphor-embedded film had a luminous efficacy of ~70.1 lm∕W at 200 mA. In the second phase, we investigated different types of remote phosphors to enhance light-emitting performance with less time-dependent degradation of emission properties. Two remote phosphor approaches were introduced to optimize the effect of red phosphor in the yellow phosphor-driven white LEDs. Both approaches combine the printing technology with a UV-curing process and are designed for flexible planar white LEDs with a thin layer thickness of a few tens of a micrometer. A screen-printing process was utilized to obtain the mixed or stacked phosphor layers based on commercial yellow and red phosphors. The resulting luminescence characteristics depended on the type of remote phosphor and the relative content of the red phosphor. A luminous efficacy of ~101 lm/W and a CRI of ~83.1 were obtained for the mixed sample with 5 wt% red phosphor. This corresponded to a decrease of ~11.2% and an increase of ~9.3% compared to the reference sample, respectively. In the final phase, UV-cured polymer-based phosphor thick films were prepared using green CsPbBr3 and red CsPbBr0.75I2.25 halide perovskite nanocrystals as a phosphor. Green and red halide perovskite layers were prepared with different concentrations and thicknesses, and phosphor layers were prepared by varying the stacking order to green/red and red/green. The samples stacked in green/red order emitted white light and samples stacked in red/green order emitted predominantly red light. By optimizing the concentration of halide perovskite nanocrystals and the thickness of phosphor films, a luminous efficacy of ~45.31 lm/W and CCT of 6,841 K suitable for display backlights were obtained. In addition, studies were conducted to improve luminous efficacy. The total internal reflection (TIR) was improved by introducing a prism pattern in the red phosphor thick films. The red layer with the prism pattern was prepared by casting a red halide perovskite nanocrystal-embedded UV-curable polymer paste onto the prism pattern and then removing the cured red thick films from the prism pattern. The red layer with a prismatic pattern of ~9 m in pitch length enhanced the luminous efficacy of the white LED by ~20% compared with the reference sample due to decrease in total internal reflection (TIR) at the surface of red layer. Red and green halide perovskite nanocrystals phosphor layers showed much higher long-term stability in humid air by applying silica-coating on the halide perovskite nanocrystals.