Bismuth-activated,narrow-band,cyan garnet phosphor Ca3Y2Ge3O12:Bi3+ for near-ultraviolet-pumped white LED application

Herein, a novel Bi³⁺-activated Ca₃Y₂Ge₃O₁₂ (CYGO) narrow-band cyan-emitting phosphor was synthesized. It can be excited from 320–420 nm, and the strongest excitation peak is located at 370 nm, which is suitable for current near-ultraviolet (NUV) chips perfectly. The full width at half maximum is at 52 nm. By analyzing the crystal structure of the sample, we infer that the Bi³⁺ ions replace the Y³⁺ site to form a highly symmetrical BiO₆ octahedron. The time-resolved photoluminescence (TRPL) spectra of CYGO: Bi³⁺ reveal that the only a single emission center exists in the host lattice. A warm white light–emitting diode (WLED) device with a low correlated color temperature (3148 K) and a high color rendering index (90.2) was fabricated by using the as-prepared sample, and the significant thermal stability of CYGO: Bi³⁺ guarantees its potential application in WLEDs. It is verified that the structure with only one crystallographic Y site for Bi³⁺ dopant occupation and highly symmetrical and dense structure is conducive to realize narrow-band emission, which will provide experience for researchers to explore more Bi³⁺-activated phosphors used for high-end lighting.     

Organic wavelength‐converting‐film‐based hybrid planar white light‐emitting diodes

Abstract— In this study, organic wavelength‐converting films (WCFs) applied to InGaN blue LED‐based hybrid planar WLED has been fabricated. The organic dye layer in the WCF was formed between the upper and bottom polymer sheets by using a simple roll‐laminating technique. Subsequently, the hybrid planar WLEDs have been fabricated based upon these films. The luminous efficiency of green WCF‐based hybrid planar WLEDs with a single blue LED chip was 34.6 lm/W and that of red‐WCF‐assisted green WCF‐based hybrid planar WLEDs was 27.3 lm/W under 20 mA. The use of WCF to fabricate hybrid planar WLEDs showed better stability than that of directly coating organic color‐convergence materials (CCMs) on the LED chips. It only decreased to about 10% of the initial wavelength‐converting intensity after 1 hour of continual operation at 20 mA.     

Chromaticity-tunable remote LuYAG: Ce phosphor-in-glass film on regular textured glass substrate for white light emitting diodes

All inorganic remote phosphor-in-glass film exhibits excellent properties in high power white light-emitting-diodes (WLEDs) thanks to their easy fabrication and thermal stability. Herein, fabrication of (Lu, Y)₃Al₅O₁₂: Ce³⁺ (LuYAG: Ce)phosphors embedded in borosilicate glass film by the conventional solid state reaction and spin coating technology has been reported. The introduction of Y³⁺ ions reduces the difference of relative growth rate along some directions in growth of LuYAG microparticles, yielding a finer grain with smooth edges. By adjusting the molar concentration of Y³⁺ ions in LuAG phosphor, a series of tunable broadband emission from green to yellow region is observed and maintains excellent thermal stability. Meanwhile, the decay curves of samples with different Y³⁺ are almost same. SEM images show that phosphor particles are homogenously distributed within the glass matrix and keep their original morphology, suggesting the phosphor-in-glass films were synthesized as expected. Finally, a simple WLEDs based on the films was constructed using the commercial blue chip. The correlated color temperature ranging from 4853K to 4627K and high color rendering index from 81.4–79.7 were obtained. Upon the different driving current, the chromaticity coordinates of as-fabricated film exhibit good light color stability. These results bring an inspiring insight to tune the luminescent performance for remote WLEDs.     

Near-ultraviolet light–induced dazzling red emission in CaGd2(MoO4)4:2xSm3+ compounds for phosphor-converted WLEDs

The Sm³⁺-activated CaGd₂(MoO₄)₄ phosphors were prepared through a sol-gel reaction route. From the results of excitation spectrum, three-dimensional emission spectra and contour lines, it was confirmed that the near-ultraviolet (NUV) light was the proper excitation light source for the synthesized phosphors. Under 405 nm irradiation, the luminescent behaviors of the studied samples were revealed to be dependent on the Sm³⁺ ion concentration and its optimal value of 0.03 mol was obtained. Through theoretical analysis, it is evident that the dipole-dipole interaction can be responsible for the involved concentration quenching mechanism in the final products and the critical distance was 39.7 Å. Moreover, the temperature-dependent emission spectra demonstrated that the studied samples had admirable thermal stability and the activation energy was decided to be 0.21 eV. Furthermore, the internal quantum efficiency of the Sm³⁺-activated CaGd₂(MoO₄)₄ phosphors was found to be 21.6%. Finally, to explore the practical applications of obtained compounds for indoor illumination, a white light-emitting diode (WLED) device which contained a NUV chip, prepared phosphors, and commercial blue-emitting and green-emitting phosphors was packaged. The packaged WLEDs device can emit dazzling white light with satisfied color coordinate of (0.305, 0.318), proper color rendering index (82.6), and correlated color temperature (7069 K).     

In Situ Embedding Synthesis of Highly Stable CsPbBr3/CsPb2Br5@PbBr(OH) Nano/Microspheres through Water Assisted Strategy

The disappointing stability of perovskites, especially in water, remains a key issue hindering their further commercialization. Here, CsPbBr₃/CsPb₂Br₅@PbBr(OH) (PQDs@PbBr(OH)) nano/microspheres with superior stability and outstanding photoluminescence quantum yield (PLQY, ≈98%) are fabricated through a water-assisted process. The nano/microspheres can maintain excellent photoluminescence (PL) intensity and high PLQY (≈90%) when immersed in water for more than 18 months. By changing the water content in the reaction mixture, the phase, particle size, and PL peaks of the nano/microspheres will change. Compared with CsPbBr₃/Cs₄PbBr₆ nanocrystals synthesized without water, PQDs@PbBr(OH) nano/microspheres exhibit better thermal stability, photostability, and superior stability in water. Based on the first-principles calculations, the enhanced stability results from PbBr(OH) with high decomposition enthalpy in water, which can effectively prevent water from contacting PQDs embedded in it. Moreover, white light-emitting diodes are fabricated by mixing green-emitting PQDs@PbBr(OH) powder and K₂SiF₆:Mn⁴⁺ (KSF) red phosphor on a 460 nm blue chip and the device shows a high luminous efficacy of 101.27 lm W⁻¹ at 10 mA. This work not only provides a reliable method for the facile preparation of ultrastable perovskites, but also has great potentials for future practical applications.     

High luminescent thermal stability and water resistance of K2SiF6:Mn4+@CaF2 red emitting phosphor

K₂SiF₆:Mn⁴⁺ (KSF:Mn⁴⁺), as an efficient red-emitting phosphor, has a promising application in WLEDs (white light-emitting diodes). However, poor moisture resistance performance still hinders its deeper commercialization. Here, KSF:Mn⁴⁺@ CaF₂ with high water resistance and luminescent thermal stability has been prepared though H₂O₂-free hydrothermal method and surface coating process. Both KSF:Mn⁴⁺ and KSF:Mn⁴⁺@CaF₂ all have high luminescent thermal stability, due to negative thermal quenching (NTQ) effect. Mechanism of the NTQ has been discussed and suggested as thermal-light energy conversion mechanism. Compared with KSF:Mn⁴⁺, water resistance of KSF:Mn⁴⁺@CaF₂ is greatly improved by coating of CaF₂, because the outer shell of CaF₂ can effectively prevent the [MnF₆]^2- group on the surface of the phosphor from being hydrolyzed into MnO₂. The results of water resistance test shows that after immersing in water for 360 min (6 h), luminescent intensity of the uncoated product drops to 41.68% of the initial one, while that of the coated product remains to have 88.24% of its initial one. Warm white light with good luminescent performances (CCT = 3956 K and Ra = 89.3) is got from prototype WLEDs assembled by using the optimal coated sample. The results suggest that the optimal coated sample has potential application in blue-based warm WLEDs.     

白光LED用(CaO-CaCl_2-SiO_2):Eu~(2+)的合成及光谱特性研究

采用高温固相法合成(CaO-CaCl2-SiO2):Eu2+荧光材料。利用X射线衍射、荧光激发和发射光谱对材料的结构和光谱特性进行了研究。在近紫外光(350～420nm)激发下,900℃下合成的(CaO-CaCl2-SiO2):Eu2+可有效发射出峰值波长位于510nm的绿色荧光;1100℃下合成的(CaO-CaCl2-SiO2):Eu2+则发射出峰值位于582nm的黄色荧光。利用VanUit-ert公式讨论了1100℃下合成的(CaO-CaCl2-SiO2):Eu2+中Eu2+的晶格环境和发光特性,推断该体系中存在绿色和黄色两种发光中心。探讨了Eu2+在(CaO-CaCl2-SiO2)基质中的浓度猝灭效应,其机理为激活剂邻近离子间的相互作用。     

Design of a novel WLED structure based on the non-rare-earth Ca2Y(Nb,Sb)O6:Mn4+ materials

Herein, the novel white light-emitting diode (WLED) structures based on the Ca₂Y(Nb,Sb)O₆:Mn⁴⁺ materials were presented. The crystal structure, morphology, elemental composition, luminescent behavior, thermal stability, quantum yield, decay curve, and color purity of Ca₂YSbO₆:Mn⁴⁺ (CYS:Mn⁴⁺) and Ca₂YNbO₆:Mn⁴⁺ (CYN:Mn⁴⁺) phosphors were investigated and compared in detail. For these phosphors, the concentration quenching (CQ) mechanism is different. For CYS:Mn⁴⁺ phosphors, the dipole-dipole interaction dominates the CQ while the dipole-quadrupole interaction is dominant in CYN:Mn⁴⁺ phosphors. The emission intensity of the optimal CYN:Mn⁴⁺ phosphors was stronger than that of the CYS:Mn⁴⁺ phosphors. Meanwhile, the color purities of the optimal CYS:0.003Mn⁴⁺ and CYN:0.003Mn⁴⁺ phosphors were calculated to be as high as 96.40% and 96.91%, respectively. Eventually, it is interesting that the packaged novel WLED structure would improve the color rendering index and correlated color temperature values. From the above results, non-rare-earth Ca₂Y(Nb,Sb)O₆:Mn⁴⁺ materials with high color purities could be proposed for WLED devices.     

Unusual concentration induced antithermal quenching of the Eu2+ emission at 490 nm in Sr4Al14O25:Eu2+ for near ultraviolet excited white LEDs

Thermal quenching of phosphor is an important challenge for its practical application in phosphor-converted white light-emitting diodes (pc-WLEDs) and it usually becomes aggravated with the increase of activator concentration. Conversely, this work finds the thermal quenching of Eu²⁺ emission at 490 nm in Sr₄Al₁₄O₂₅:Eu²⁺ does not follow this in the temperature range of 300 to 480 K, and the rate of it is even slowed down as the concentration of Eu²⁺ increases. However, at the same time, the experiment on three heating-cooling cycles of Sr₄Al₁₄O₂₅:Eu²⁺ reveals that the thermal degradation of Eu²⁺ emission becomes improved. Once Eu²⁺ ions are doped into Sr₄Al₁₄O₂₅, they will prefer substituting for the 10- and 7-coordinated strontium sites Sr1 and Sr2, respectively. The emission centers Eu1 and Eu2, therefore, appear. The abnormal phenomenon is perhaps partly due to the enhanced energy transfer from the emission center Eu1 at 407 nm to the one Eu2 at 490 nm. It is also found interesting that the introduction of AlN can enhance the emission of Sr₄Al₁₄O₂₅:Eu²⁺ without leading to the deterioration of thermal degradation. In the end, a prototype of pc-WLED was fabricated with Sr₄Al₁₄O₂₅:Eu²⁺ to demonstrate the application of white lighting. This work is not only beneficial to the understanding of the relationship between concentration and thermal quenching, but also conducive to the design of the heavily doped phosphor for WLEDs with better resistance to thermal quenching.