1. Introduction

White light-emitting diodes (WLEDs) have been promising light sources in recent years due to their high efficiency, low power consumption, applicability in a wide range of sizes, and environmental friendliness. These remarkable features are helping WLEDs rapidly replace conventional light sources in the area of full-color displays backlight, automobile headlight, and general lightings [1,2]. Lighting is a key element in assuring user’s satisfaction. Many researchers have indicated that correlated color temperatures (CCT) and color rendering index (CRI) in addition to luminous of light sources affect humans both psychology and physiology [3,4]. A continuous broad-band spectrum is required to realize high CRI WLEDs. However, a blue InGaN LED excites yellow phosphors in resin matrix, the most common and cost-effective technique in WLEDs, usually exhibits CRI only around 70. To address this problem, multiple phosphors have been introduced into resin matrix, leading to the development of high-quality WLEDs that achieve CRI above 85 [5,6]. It is worth noting that high-power WLEDs, which show increasing demand in lighting applications, have a thermal stability issue caused by the resin matrix. The degradation of silicone resins due to the heat emitted from LED chips adversely affects the optical properties and chromaticity characteristics of WLEDs. Therefore, alternative matrix materials with high thermal stability are essential for the lighting industry. Ceramics-based [7,8] and glass-based phosphors [9,10] with high thermal durability and acceptable optical efficiency have been reported. However, high processing temperature (>1000°C) of recrystallizing technique used in the ceramics-based and glass-based phosphors raise the fabricating cost of the thermal-stable phosphors. In our previous work, we have successfully demonstrated thermally stable glass-based phosphors with high efficiency [11]. By the strategy of directly dispersing phosphor crystals into glass matrix, the processing temperature of the glass phosphors can be effectively lowered to 680°C [12]. Recently, the efficiency as well as color rendering capability of the glass phosphors has been greatly improved by restraining the inter-diffusion between glass and phosphor crystals [13]. The optical model of the novel glass phosphors has also been constructed with promising accuracy [14]. These results reveal the potential of glass-based phosphors in the applications of high-power WLEDs, especially in the region of indoor lightings.

In this paper, we propose a cost-effective method to realize a wide range of CCT emission by dispersing two kinds of phosphor crystals with a golden proportion into sodium-based glass matrix. The high transparency and excellent thermal stability of the sodium-based glass make the resulting glass phosphors exhibit satisfactory optical properties and thermal durability [15,16]. The golden proportion of the two phosphor crystals realizes high CRI WLEDs with different CCTs though the thickness of the glass phosphors. The CCT of the WLEDs utilizing the glass phosphors in different thickness ranges between 4200 and 7740K, including most of the CCT range used in our daily life. The CRI of these WLEDs ranges between 83 and 90, which is required for the widespread use of solid state lighting especially in indoors. In this contribution, the glass phosphors with excellent color and optical performance show their highly potential feasibility to the applications for solid-state indoor lightings.

2. Experimental details

Sodium mother glass with composition of 60 mol% SiO₂, 25 mol% Na₂CO₃, 9 mol% Al₂O₃, and 6 mol% CaO were uniformly mixed and then heated at temperature of 1300°C to melt, followed by gradual cooling to room temperature. The resulting cullet glass was ground into glass powders and then screened to under ca. 125μm in size. Phosphor crystals, Lu₃Al₅O₁₂:Ce³⁺(LuAG:Ce) and CaAlSiN₃:Eu²⁺(CASN:Eu) with the size of ca. 10μm, chosen as the green and red color conversion elements, respectively, were uniformly blended into the glass powders to form glass phosphor precursor. The concentration of LuAG:Ce and CASN:Eu in the precursor was 8wt% and 2wt%, respectively. The precursor was sintered at 680°C for 30min and then annealed at 350°C for 3h, followed by cooling to room temperature. The resulting glass phosphor was then cut into disks with a diameter of 15mm and thickness of 0.8, 0.85, 0.95, 1.0, 1.1, and 1.2mm. The crystallographic phase of the glass phosphors was determined by X-ray diffraction (XRD) with a Bruker D8 diffractometer. Absorption spectra of these samples were recorded by an UV-vis spectrophotometer (U4100, HITACHI). Photoluminescence excitation (PLE) and photoluminescence (PL) spectra were both acquired by a fluorescence spectrophotometer (F-4500, HITACHI). Internal quantum yield (QY_int) was obtained by analyzing the PL and absorption spectra of the glass phosphors excited by a bare GaN LED chip with the aid of an integrating sphere. Luminescent spectra of WLED modules utilizing glass phosphors were also measured using the configuration of a charge-coupled device detector equipped on an integrating sphere calibrated with a Photo Research, Inc. PR-650 SpectraScan Colorimeter. The above measurements were all carried out at room temperature. Color rendering index, Commission Internationale de l’Eclairage (CIE) chromaticity coordinates, correlated color temperature, and luminous efficiency of the WLED modules were analyzed by the emitting spectra and the driving current of the WLED modules.

3. Results and discussion

Figure 1 shows X-ray diffraction patterns of LuAG:Ce, CASN:Eu, and the glass phosphor. The pattern confirms the presence of LuAG:Ce and CASN:Eu in the glass phosphors, indicating that the phosphor crystals are chemically stable under the sintering process. Figure 2 shows absorption, PLE and PL spectra of the glass phosphor in 0.95mm thick. It can be observed that a strong and broad absorption ranging from 350 to 550nm, contributed by both LuAG:Ce and CASN:Eu. There are two major bands, centered at 346nm and 451nm in the PLE spectrum with the detection wavelength of 510nm. The 346-nm and 451-nm PLE bands originate from 4f to 5d transition of Ce³⁺ ions. The 451-nm PLE band covers the emission spectrum of general blue LEDs, so the glass phosphors can be efficiently excited by blue LEDs. The PL spectrum obtained by exciting the wavelength of 451nm shows a peak wavelength of 510nm. Such wide PL emission covers most of visible spectral region which makes the glass phosphors as a perfect complement to blue LED and largely enriches the color rendering capability. The inset of Fig. 2 is the photograph of the glass phosphors, which clearly shows that the glass phosphor is translucent.

 

Fig. 1 XRD patterns of LuAG:Ce, CASN:Eu, and glass phosphor.

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Fig. 2 Absorption, excitation and emission spectra of glass phosphor in 0.95mm thick. Inset is the photograph of the glass phosphor.

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Figure 3 shows the emission spectra of the WLEDs utilizing 0.8-, 0.85-, 0.95-, 1.0-, 1.1- and 1.2-mm-thick glass phosphors. To clearly explore the spectral variation, the driving current of the blue LED were fixed at 350mA. The band centered at 448nm corresponds to the excited blue LED, and the broad emission band between 480nm and 700nm is LuAG:Ce and CASN:Eu luminescence, respectively. The inset of Fig. 3 shows the normalized emission spectra, it shows that the emission from the glass phosphors is enhanced distinctly with the increased thickness of the glass phosphor. It is worthy to note that the emission spectra of the glass phosphors obtained in the WLED modules are somewhat different from it obtained in the PL measurement. The mismatch between the emission spectra can be attributed to the absorption nature of CASN:Eu. The PL spectrum was obtained by exciting the glass phosphors with a monochromatic light at the wavelength of 451nm, while the WLED modules were excited by a blue LED chip with peak wavelength at 448nm and a FWHM of 23nm. CASN:Eu possesses a strong and broad absorption ranging from 300nm to 500nm, which entirely covers the emission of the blue LED and also overlaps the emission of LuAG:Ce, so a stronger emission of CASN:Eu in the WLED modules can be observed.

 

Fig. 3 Emission spectra of the glass-phosphor-based WLEDs with various glass phosphor thickness at an operating current of 350mA. Inset is the normalized emission spectra.

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Table 1 shows the chromaticity coordinate, CCT, CRI, and luminous efficiency of the WLEDs utilizing glass phosphors with thicknesses of 0.8, 0.85, 0.95, 1.0, 1.1, and 1.2mm. CCT’s of 6711, 6211, 5366, 5021, 4513, and 4246 were estimated for 0.8, 0.85, 0.95, 1.0, 1.1, and 1.2mm-thick glass phosphors, respectively. Increasing the thickness of glass phosphors, CCT decreases, owing to blue light is re-absorbed and scattered by the glass phosphors, and this trend is also depicted in Fig. 3. The CRI of the WLEDs with 0.8-mm-thick glass phosphors is up to 90, and all the WLEDs in this work exhibit CRI beyond 80, due to significant red component in the luminescence spectrum contributed by CASN:Eu phosphors. The excellent CRI performance of the WLEDs makes them adequate for indoor illumination applications. The luminous efficiency of the WLEDs is in the range of 88.8 to 99.6lm/W and decreases with the increase of the thickness of glass phosphors. The luminous efficiency is lower than that of WLEDs with Ce:YAG phosphors (~124lm/W) [17]. The relatively low luminous efficiency of the WLED can be attributed to the lower fluorescent quantum yield of LuAG:Ce and CASN:Eu compared to it of Ce:YAG. However, it should be noted that the luminous efficiency of the WLEDs in this work is much higher than other CASN:Eu-based WLEDs (~32lm/W) [18,19] because of the much lower fabricating temperature we used (680°C). The slight decrease of CRI and luminous efficiency with increase of the thickness of glass phosphors can be ascribed to the decrease of blue and green light emission caused by re-absorption by LuAG:Ce and CASN:Eu, respectively.

Table 1. Performances of WLEDs Utilizing Glass Phosphors with various thicknesses

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Figure 4 shows the photograph and CIE chromaticity diagram of WLEDs utilizing the glass phosphors. As shown in Fig. 4, the chromaticity of the WLEDs was effectively tuned by varying the thickness of glass phosphors and it followed the Planckian locus well. The CCT of the WLEDs locates in the 4000K, 4500K, 5000K, 5700K, and 6500K specifications of ANSI’s eight nominal CCT quadrangles. Thus, precise control of WLED chromaticity, from cool white (CCT = 4000K) to daylight (CCT = 6500K), can be easily achieved by simply adjusting the thickness of the glass phosphors.

 

Fig. 4 CIE chromaticity diagram with ANSI’s eight nominal CCT quadrangles of WLEDs utilizing 0.8, 0.85, 0.95, 1.0, 1.1, and 1.2 mm-thick glass phosphors.

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4. Conclusion

In summary, we developed glass phosphors with LuAG:Ce and CASN:Eu in a weight ratio of 4:1, while readily varying the CCT of the resulting WLEDs along the Planckian locus by simply altering their thickness. The WLEDs exhibit CCT ranging from ~4000K to ~7000K with high CRI ranging for 83 to 90 and yield luminous efficiency up to 99.6lm/W. Benefiting from its easy fabrication, excellent thermal stability, as well as wide CCT tailorability, the glass phosphors in this work can be a promising candidate phosphor materials for high-power WLEDs, especially in the applications for indoor lightings.