Synthesis And Optical Performances Of Rare-earth Doped YAG Phosphor Films

YAG:Ce³⁺ phosphors converted white light emitting diodes（Pc-WLEDs） are particularly desirable in the LED white lighting market and are recognized as a new lighting source for the next generation due to their excellent optical properties, such as high luminous efficacy, low consumption of energy, long lifetime, high reliability and good chemical stability. However, commercial YAG:Ce³⁺ phosphors based Pc-WLEDs exhibit limited conversion efficiency due to their high scattering and reflective loss of the emission from irregular micro-size phosphors/epoxy resin layers. In addition, the organic binder with low thermal conductivity in commercial pc-WLEDs may age easily and turn yellow due to the accumulating heat emitted from the bluelight chip, which adversely affects the Pc-WLEDs properties such as luminous efficacy and colour coordination, and therefore reduces the long-term life-span and reliability.In compared to the traditional YAG:Ce³⁺ phosphor powder, transparent YAG:Ce³⁺ phosphor films with nanometer particle size, superior thermal conductivity, better adhesion, and especially lower scattering and reflective effect are recognized as good candidates in pc-WLEDs applications and havd been attracted much attention. The key purpose of the work is to prepare Res-doped YAG phosphor films with considerable optical perferfances for the application of WLED, and achievements can besummarized as the following:1. YAG:3.3%Ce³⁺ phosphor films were deposited on quartz substrate by RF magnetron sputtering using powder target, and the effects of sputtering power, O2 concentration, annealing process on the deposition rate, crystallinity and optical performances were also studied. Owing to the lower surface binding energy, it holds a higher deposition rate of about 1.2 μm/h. Transparent film a thickness of about 2.92 μm via sputtering 2.5 h under an optimized preparation process, which showed good crystallinity of YAG phase and a quantum yield of 23% after a annealing process at 1100℃under H2 /Ar for 10 h.2. YAG:3.3%Ce³⁺phosphor films with micron thickness were prepared by a modified sol-gel technology with a higher sol concentration using pure ethylene glycolas solvent due to its high solubility. Via 40-layers deposition, transparentand compact YAG:Ce³⁺ film of thickness about 4.3 μm and homogeneous grain size about 80 nm was fabricated on quartz substrate. The as-prepared YAG:3.3%Ce³⁺ thick film shows broad absorption at blue-light range and excellent photoluminescence performance with a quantum yield of 47% at 457 nm excitation, and can be used in pc-WLED applications.3. YAG:3.3%Ce³⁺thick films were synthesized via a novel sol-gel method which introduced the combustion pyrolysis process into the conventional sol-gel method using yttrium, cerium, aluminium nitrates as precursors and glycine and urea as fuel. Compact YAG:Ce³⁺ phosphor film with a thickness of 5.1 μm and homogeneous grain size of 60 nm was achieved on a quartz substrate via 40-layers depositions. The film shows a quantum yield of 56% at 457 nm excitation. YAG:3.3%Ce³⁺thick film incorporated WLED displays an appreciable luminous efficacy（LE） of 82 lm/W, while maintaining a warm correlated colour temperature（CCT） of 4536 K as well as a high colour rendering index（Ra） of 75.8.4. The effects the crystal structure on Ce³⁺ photoluminescence performances were studied. The substituting Y³⁺ with larger Lanthanide ions(e.g., Tb³⁺, Gd³⁺) lead to a stronger crystal field and lattice distortion, which results in a broader and red-shifted emission of Ce³⁺. The FWHM and emission peak of the as-prepared TAG:3.3%Ce³⁺ and（Y1-xGdx）3Al5O12:3.3%Ce³⁺ PFs were enlarged and red-shifted in compared to YAG:Ce³⁺ PFs. The as-prepared YSAG:Ce³⁺ phosphor fims diplayed an enlarged FWHM but without red-shifted emission. An appropriate enlarging of the unit cell would improve the emission intensities of Ce³⁺ owing to reduce of the probability of luminescence quenching. The WLED fabricated with TAG:3.3%Ce³⁺ films showed a LE of 72 lm/W, CCT of 4886 K and Ra of 79.2. The WLED fabricated with（Y0.7Gd0.3）3Al5O12:Ce³⁺ films displayed a LE of 88 lm/W,CCT of 4356 K and Ra of 78.2,while WLED based on（Y0.25Gd0.7）3Al5O12:3.3%Ce³⁺ phosphor film a much higher Ra of 81.2.5. The energy transfer processes of Ce³⁺→Eu³⁺, Ce³⁺→Pr³⁺ and optical performances of Ce³⁺/Eu³⁺ and Ce³⁺/Pr³⁺ codoped YAG phosphor films were studied to improve the red-light emission of the as-prepared PFs. Results showed that, viatransfer processes of Ce³⁺→Eu³⁺, Ce³⁺→Pr³⁺, the emission peaks displayed much improved sharp peaks located respectively at 610 nm and 611 nm for the YAG:Ce³⁺/Eu³⁺ and YAG:Ce³⁺/Pr³⁺ phosphor films. The YAG:Ce³⁺/Eu³⁺ displayed a serious deterioration of the whole intensity of the emission peak owing to that, the introduce of the co-doping Eu³⁺ cause luminescence quenching of the Ce³⁺, while YAG:3.3%Ce³⁺/0.5%Pr³⁺ phosphor film also keep a over 95% intensity with a low concentration of 0.5% Pr³⁺. Triple Ce³⁺/Pr³⁺/Gd³⁺ doped, which showed much broader and red-shifted emission as well as and improved sharp peaks located 611 nm. The as-fabricated WLED displayed improved performances including a much higher Ra of 83.4, a warmer CCT of 4327 K as well as a considerable LE of 84 lm/W.6. An alternative approach is presented to improve the absorption ability of PF via a co-doping an energy transfer sensitizer. Owing to the allowed 4f→5d transitions of Tb³⁺ with a reasonable doping concentration of 20% in the YAG matrix, the as-prepared YTb AG:5%Eu³⁺ and YTb AG:3.3%Ce³⁺ films displayed strong abilities of absorption in the UV-light range owing to the allowed 4f→5d transitions of Tb³⁺ at a reasonable doping concentration of 20% in the YAG matrix. Owing to the improvement of the absorption abilities and energy transfer process of Tb³⁺→Ce³⁺ and Tb³⁺→Eu³⁺, the as-prepared YTb AG:5%Eu³⁺ and YTb AG:3.3%Ce³⁺ displayed much higher luminescence intensites at 275 nm excitation. The YTb AG:3.3%Ce³⁺ films displayed right green-yellow emission and YTb AG:x%Eu³⁺ films displayed bright tunable color（green→ green-yellow→ orange→ orange-red→ red） emissions with variations in Eu³⁺ doped concentration.