학술논문
백색 LED용 적, 청색 발광 형광체의 합성 및 발광특성 연구 / Synthesis and luminescence of red- and blue-emitting phosphors for white LEDs
Document Type
Dissertation/ Thesis
Author
Source
Subject
Language
Korean
Abstract
현재 세계는 화석연료 사용에 따른 기후변화와 환경 문제가 세계적으로 대두됨에 따라서 CO2를 배출하지 않는 대체 연료에 대한 관심이 급증하면서 기존의 형광등, 백열등과 같은 조명기기보다 낮은 전력과 긴 수명, 그리고 훌륭한 내구성을 가진 발광 다이오드 칩과 형광체를 결합한 백색LED(white light-emitting diodes, WLEDs)의 장점에 대하여 너무 잘 알고 있다. 그래서 이것을 응용하는 LED조명에 대한 관심이 뜨겁다. 지금도 LED조명은 여러 분야에서 널리 쓰이고 있는 실정이다. 하지만 대부분의 백색LED는 청색 LED 칩과 YAG:Ce2+ 노란색 형광체를 사용하고 있다.[1, 2] 그래서 많은 연구자들이 YAG를 뛰어넘을 고성능, 고효율의 형광체에 대한 개발에 끊임없이 매달리고 있다. 현재의 형광체는 이를 보완하고자 자외선(혹은 근자외선) 또는 청색 발광 다이오드의 여기광원을 주로 사용한다. 근자외선 LED와 적, 녹, 청색 형광체를 혼합하거나, 청색 LED를 이용하여 YAG 형광체에 적색형광체를 혼합하거나, 적색과 녹색 형광체를 혼합하는 방법을 사용한다.[3, 4] 그래서 자외선 (혹은 근자외선) 영역인 350 400 nm와 청색 영역의 400 495 nm에서의 강한 흡수가 필요하고, 넓은 여기 스펙트럼이 필요하다. 이에 더불어 높은 양자 효율과 적은 열적소광과 높은 화학적 안정성이 필요하다.[1, 3, 5]그 중, 근자외선 여기 하에서 효율적인 발광 특성을 보이는 청색 형광체 모재로는 인산염(Phosphate)계3)와 규산염(Silicate)계가 있다. 대표적인 규산염계 중에서도 Eu2+ 이온이 도핑된 Sr3MgSi2O8[6–7]와 Sr2MgSi2O7[8–14]형광체는 자외선 영역에서의 넓은 여기 스펙트럼과 강한 청색 발광으로 널리 알려져 있다.그리고 이 형광체에 적용하는 도펀트로는 4f-5d 전이를 통해 근자외선 영역에서 강한 흡수를 일으키는 Eu2+ 이온이 많이 이용되고 있다. 또한 청색 발광 다이오드의 여기광원을 사용하는 nitride와 oxynitride system은 높은 화학 안정성과 낮은 열적소광으로 가장 주목받는 형광체중 하나이고, 최근 활발한 연구가 진행되고 있다.[15, 16] 대표적으로 청색 형광체로는 BaSi3Al3O4N5:Eu2+, LaAl(Si5Al)(N9O):Ce, LaSi3N5:Ce이 있고, 녹색 형광체로는 β-sialon:Eu, SrSi2O2N2:Eu, Ca-α-sialon:Yb, Ba3Si6O12N2:Ce, La3Si6N11:Ce,SrSiAl2O3N2:Eu,Y2Si3O3N4:Ce가 있으며, 마지막으로 적색 형광체는 CaAlSiN3:Eu, M2Si5N8:Eu(M=Ca,Sr,Ba), SrSiN2:Eu, BaSiN2:Eu, (Y,M)NbO4:Eu가 있다.[17] 적, 녹, 청색 형광체 중에 적색 형광체는 백색 발광다이오드의 색온도와 연색성을 구현하기 때문에 가장 중요한 형광체이다.[18] 청색 영역에서 높은 여기특성과 높은 화학적 안정성이 필요하기 때문에 Si3N4를 기반으로 한 형광체가 개발되고 있다.[3, 19]우리는 미비했던 oxynitride system의 영역에서 이전의 연구를 통해 Sr2Si(O1-xNx)4:Eu2+(SSON)이 좋은 적색 형광체의 후보 중 하나라는 것을 알아냈었고, 발광특성에 대하여 연구했었다.[20, 21] 하지만 이전의 연구에서는 SSON에 Ba2+이온을 0.4몰 치환시켰을 때 SSON보다 발광강도가 낮아졌었다. 하지만 다시 실험한 결과 Ba2+의 치환량에 따라 SSON보다 발광강도가 더 높아진다는 것을 발견하였고, 이번 연구에서는 조금 더 자세하게 SSON보다 적색 영역에서 더 좋은 효율과 더 좋은 발광강도를 갖는 Sr2-y-zBazSi(O1-xNx)4:yEu2+(SBSON)에 대하여 밝히려 한다.본 논문에서는 Sr3MgSi2O8, Sr2MgSi2O7, Sr2-y-zBazSi(O1-xNx)4:yEu2+(SBSON)형광체 분말을 고상반응법으로 합성하여 합성 분말의 결정성과 발광특성, 미세구조를 연구 하였으며, Eu2+이온에 의한 4f65d1 → 4f7 전이에 의한 청색, 적색 발광을 관찰하였다.
Phosphor-conversion white light-emitting diodes (PC-WLEDs), which are composed of LED chips and phosphors, have been extensively developed for use in solid-state lightings. There are two alternative ways of making WLEDs, which combine blue or near ultraviolet (nUV) LED chips with a phosphor blend. The latter has some advantages over the former in color rendering index, color temperature, chip performance, etc. For WLEDs using nUV chips, multi-emitting phosphor blends (red, green, and blue) are required to generate white light, while a blue-emitting phosphor is very important because of its small Stokes shift and critical contribution to white light generation. Some of the most efficient blue-emitting phosphors are Eu2+ doped halo-phosphates and silicate compounds including X3MgSi2O8, X2MgSi2O7 (X: Sr2+, Ca2+, and Ba2+). Silicon-based nitride and oxynitride phosphors have been widely investigated, because they are appropriate for use in white light emitting diodes (LEDs) as red, green, and blue phosphors. It has been stongly demanded that the phosphors for white LEDs meet some requirements: thermal quenching, chemical stability, quantum efficiency, and broad band excitation from ultraviolet (UV) to blue. Sr3MgSi2O8:Eu2+ blue-emitting phosphors were prepared by a flux method and their luminescent properties were investigated. The excitation spectra were broad (250-500 nm), showing strong excitation intensities at 370-410 nm, while the intense blue emission was observed at around 460 nm assigned to the 4f65d1-4f7transition of the Eu2+ ions incorporated into the Sr(I) sites. The strongest emission was obtained from the starting mixture with the excess Sr content. The emission intensity varied with firing temperatures, an amount of a flux, and the Eu content. These luminescent properties demonstrate that Sr3MgSi2O8:Eu2+ powders are promising blue phosphors for use in white light emitting diodes using near ultraviolet chips. Eu2+/Dy3+-doped Sr2MgSi2O7 powders were synthesized by a solid-state reaction method using flux (NH4Cl). The broad photoluminescence (PL) excitation spectra of Sr2MgSi2O7:Eu2+ were assigned to the 4f7-4f65d transition of the Eu2+ ions, showing strong intensities in the range of 375 to 425 nm. A single emission band was observed at 470nm, which resulted from two overlapping subband at 468 and 507 nm owing to Eu(I) and Eu(II) sites, respectively. The strongest emission intensity of Sr2MgSi2O7:Eu2+ was obtained at the Eu concentration of 3 mol%. This concentration quenching mechanism was attributable to a dipole-dipole interaction. The Ba2+ substitution for Sr2+ caused a blue-shift of the emission band, and this behavior was discussed by considering the difference in ionic size and covalence between Ba2+ and Sr2+. The effects of the Eu/Dy ratios on the phosphorescence of Sr2MgSi2O7:Eu2+/Dy3+ were investigated by measuring the decay time; the longest after glow was obtained for 0.01 Eu2+/0.03Dy3+. Sr2-z-yBazSi(O1-xNx):yEu2+ (SBSON) powders were prepared by a solid-state reaction method, and luminescent properties were investigated. The XRD patterns of SBSON exactly coincided with those of α'- and/or β-Sr2SiO4 (sso) phases, indicating that SBSON was a solid-solution from of SSO. The broad excitation spectra and strong red-shift of the emission spectra were ascribed to Eu-N bonds that caused a nephelauxetic effect and a strong crystal field. When Ba2+ ions were partly substituted for Sr2+ ions, the emission spectra apparently showed a blue-shift of the emission spectra.
Phosphor-conversion white light-emitting diodes (PC-WLEDs), which are composed of LED chips and phosphors, have been extensively developed for use in solid-state lightings. There are two alternative ways of making WLEDs, which combine blue or near ultraviolet (nUV) LED chips with a phosphor blend. The latter has some advantages over the former in color rendering index, color temperature, chip performance, etc. For WLEDs using nUV chips, multi-emitting phosphor blends (red, green, and blue) are required to generate white light, while a blue-emitting phosphor is very important because of its small Stokes shift and critical contribution to white light generation. Some of the most efficient blue-emitting phosphors are Eu2+ doped halo-phosphates and silicate compounds including X3MgSi2O8, X2MgSi2O7 (X: Sr2+, Ca2+, and Ba2+). Silicon-based nitride and oxynitride phosphors have been widely investigated, because they are appropriate for use in white light emitting diodes (LEDs) as red, green, and blue phosphors. It has been stongly demanded that the phosphors for white LEDs meet some requirements: thermal quenching, chemical stability, quantum efficiency, and broad band excitation from ultraviolet (UV) to blue. Sr3MgSi2O8:Eu2+ blue-emitting phosphors were prepared by a flux method and their luminescent properties were investigated. The excitation spectra were broad (250-500 nm), showing strong excitation intensities at 370-410 nm, while the intense blue emission was observed at around 460 nm assigned to the 4f65d1-4f7transition of the Eu2+ ions incorporated into the Sr(I) sites. The strongest emission was obtained from the starting mixture with the excess Sr content. The emission intensity varied with firing temperatures, an amount of a flux, and the Eu content. These luminescent properties demonstrate that Sr3MgSi2O8:Eu2+ powders are promising blue phosphors for use in white light emitting diodes using near ultraviolet chips. Eu2+/Dy3+-doped Sr2MgSi2O7 powders were synthesized by a solid-state reaction method using flux (NH4Cl). The broad photoluminescence (PL) excitation spectra of Sr2MgSi2O7:Eu2+ were assigned to the 4f7-4f65d transition of the Eu2+ ions, showing strong intensities in the range of 375 to 425 nm. A single emission band was observed at 470nm, which resulted from two overlapping subband at 468 and 507 nm owing to Eu(I) and Eu(II) sites, respectively. The strongest emission intensity of Sr2MgSi2O7:Eu2+ was obtained at the Eu concentration of 3 mol%. This concentration quenching mechanism was attributable to a dipole-dipole interaction. The Ba2+ substitution for Sr2+ caused a blue-shift of the emission band, and this behavior was discussed by considering the difference in ionic size and covalence between Ba2+ and Sr2+. The effects of the Eu/Dy ratios on the phosphorescence of Sr2MgSi2O7:Eu2+/Dy3+ were investigated by measuring the decay time; the longest after glow was obtained for 0.01 Eu2+/0.03Dy3+. Sr2-z-yBazSi(O1-xNx):yEu2+ (SBSON) powders were prepared by a solid-state reaction method, and luminescent properties were investigated. The XRD patterns of SBSON exactly coincided with those of α'- and/or β-Sr2SiO4 (sso) phases, indicating that SBSON was a solid-solution from of SSO. The broad excitation spectra and strong red-shift of the emission spectra were ascribed to Eu-N bonds that caused a nephelauxetic effect and a strong crystal field. When Ba2+ ions were partly substituted for Sr2+ ions, the emission spectra apparently showed a blue-shift of the emission spectra.