LASER & PHOTONICSREVIEWS Laser Photonics Rev. 8, No. 1, 158–164 (2014) / DOI 10.1002/lpor.201300140

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Abstract Currently, the major commercial white light-emittingdiode (WLED) is the phosphor-converted LED made of the In-GaN blue-emitting chip and the Ce3+:Y3Al5O12 (Ce:YAG) yellowphosphor dispersed in organic epoxy resin or silicone. However,the organic binder in high-power WLED may age easily andturn yellow due to the accumulated heat emitted from the chip,which adversely affects the WLED properties such as luminousefficacy and color coordination, and therefore reduces its long-term reliability as well as lifetime. Herein, an innovative lumi-nescent material: transparent Ce:YAG phosphor-in-glass (PiG)inorganic color converter, is developed to replace the conven-tional resin/silicone-based phosphor converter for the construc-tion of high-power WLED. The PiG-based WLED exhibits notonly excellent heat-resistance and humidity-resistance charac-teristics, but also superior optical performances with a luminousefficacy of 124 lm/W, a correlated color temperature of 6674 Kand a color rendering index of 70. This easy fabrication, low-cost and long-lifetime WLED is expected to be a new-generationindoor/outdoor high-power lighting source.

A new-generation color converter for high-power whiteLED: transparent Ce3+:YAG phosphor-in-glass

Rui Zhang1, Hang Lin2, Yunlong Yu2, Daqin Chen2,∗, Ju Xu2, and Yuansheng Wang1,∗

1. Introduction

Nowadays, the white light-emitting diode, as a new type ofluminescent source, has played a crucial role in applicationsof indicator, backlight, automobile headlight and generalillumination owing to its excellent performances, such ashigh luminous efficacy (LE), energy saving, environmentfriendliness, and long lifetime [1–4]. The current leadingcommercial WLED combines an InGaN blue-emitting chipwith a Ce:YAG yellow-emitting phosphor packed on thechip surface using epoxy resin or silicone [5–8]. However,for high-power WLED the organic resin or silicone withlow thermal conductivity and poor thermal stability mayage easily and turn yellow due to the accumulated heatemitted from the chip [9, 10]. This raises several issues inWLED practical application, including degradation of LEand shift of chromaticity, and finally reduction of long-termreliability [11, 12].

1 State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, CAS, Fuzhou, Fujian 350002, P. R. China2 Key Laboratory of Design and Assembly of Functional Nanostructures, Chinese Academy of Sciences, Fuzhou, Fujian 350002, P. R. China∗Corresponding authors: e-mail: dqchen@fjirsm.ac.cn; yswang@fjirsm.ac.cn

To solve this problem, inorganic materials, such astransparent ceramics and glass ceramics, have been investi-gated recently as practical alternatives to the organic poly-mer binders. A maximum LE reaching 93 lm/W at a lowcorrelated color temperature (CCT) of 4600 K was realizedin a thin transparent Ce:YAG ceramic-based WLED [9].However, the high fabrication cost is an unavoidable chal-lenge to the mass production of the transparent Ce:YAGceramics [9, 13, 14]. Ce:YAG glass ceramic, which is akind of composite containing Ce:YAG microcrystals pre-cipitated from precursor glass via well-controlled crystal-lization, has interesting advantages of excellent heat resis-tance and easy formability, etc. [15–17]. However, the sofar reported maximum quantum yield (QY) for the Ce:YAGglass ceramic was about 30% and the optimal LE reachedmerely 20 lm/W [18]. The reasons causing poor opticalfeatures of the Ce:YAG glass ceramic are that it is difficultto partition all the Ce3+ activators into YAG host during

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YAG crystallization and to achieve high transparency insuch material.

On the other hand, phosphor-in-glass (PiG), where acertain amount of phosphor is dispersed in an inorganicglass matrix, has been considered as a promising alterna-tive for the color converter. PiG is prepared by cosinteringof a simple mixture of phosphor and glass powder in atemperature lower than 1000 ◦C [19, 20]. Importantly, var-ious commercial phosphor powders could be adopted tomix with glasses to adjust the emission colors of PiGs. Forthe PiG material, there are two key factors determining itsefficient luminescence: one is the excellent heat resistanceof phosphor particles against glass melting to keep theiroriginal properties; the other is the refractive-index match-ing between phosphor and glass matrix to reduce adverselight scattering and keep PiG transparent. In fact, increasingattention has been paid to explore the low melting tempera-ture glasses (such as borate glasses, phosphate glasses andtellurite glasses) for the dispersion of phosphors [21, 22].However, to the best of our knowledge, there have been noreports on the highly optical transparent and efficient PiGcolor converters usable in WLED to date.

In this paper, we report an innovative Ce:YAG PiGmaterial that can be used to replace the traditionalresin/silicone-based phosphor converter in WLED. Thiscomposite was fabricated by introducing Ce:YAG com-mercial phosphor into the specifically selected inorganicglass powders. The mixture was sintered at an optimaltemperature at which the glass components were meltedwhile the phosphor powders remained solid as much aspossible. By carefully designing the glass composition, ad-justing the phosphor to glass ratio, as well as controllingthe sintering temperature/time, the highly transparent PiGsample is achieved and its luminescent QY reaches as highas 92% upon 460-nm excitation. Furthermore, comparedto those of the conventional Ce:YAG phosphor-in-silicone(PiS), thermal-quenching and thermal-resistance perfor-mances of PiG are greatly improved. Impressively, the LEof the PiG-based WLED reaches as high as 124 lm/W at anoperating current of 350 mA, superior to that of the conven-tional PiS-based WLED (98 lm/W), revealing the promi-nent feasibility of the PiG material in high-power WLEDapplications.

2. Experimental section

Precursor glasses with following compositions (mol%) of10–30 Sb2O3, 10–30 B2O3, 5–30 TeO2, 10–25 ZnO, 5–20Na2O, 0–10 La2O3, and 0–10 BaO were prepared by a con-ventional melting–quenching method. The reagent gradechemicals were mixed thoroughly and melted in a platinumcrucible at 750–850 ◦C for 0.5–1.5 h in ambient atmo-sphere. The melt was poured into a cold copper mold andthen cooled to room temperature. The prepared glass wasmilled to powders using a ball grinder, and then mixedwith 1–9 wt% commercial Ce:YAG phosphors (purchasedfrom XinLi Illuminant Co. Ltd) thoroughly and sinteredin a platinum crucible at 540–690 ◦C for 10–80 min in

ambient atmosphere. The melt was poured into a 220 ◦Cpreheated copper mold and then cooled to room tempera-ture. The obtained PiG was annealed at 260 ◦C for 5 h in amuffle furnace to relinquish inner stress, polished and cutinto ϕ12 mm disks with various thicknesses (0.2–1.2 mm).

To study the thermal behaviors of the PiG samples,differential scanning calorimety (DSC) experiments werecarried out at a heating rate of 10 K/min. The refractiveindex of the sample was measured by a digital refrac-tometer (GI-RDB). The density was measured followingthe Archimedes’ principle using distilled water as medium.The hardness and tenacity were determined using a Vick-ers microindenter (DHV-1000), with a charge of 100 g.The thermal expansion coefficient and thermal conductiv-ity were measured by an electronic dilatometer (DIL402PC,Netzsch) and a laser flash apparatus (LFA457, Netzsch),respectively. Microstructures of the PiG samples werestudied using a scanning electron microscope (SEM, JSM-6700F) equipped with an energy-dispersive X-ray spec-troscopy (EDS) system. Emission, excitation spectra anddecay curves of PiGs were recorded on a spectrofluoreme-ter (FLS920, Edinburgh Instruments) equipped with bothcontinuous (450 W) and pulsed xenon lamps. All the abovemeasurements were carried out at room temperature. Thetemperature-dependent emission spectra were recorded by aspectrofluoremeter (FLS920), and the sample temperaturewas controlled by a heating stage (THMS600E, LinkamScientific Instruments).

Internal QY is defined as the ratio of the emitted photonsto the absorbed photons, and was measured by a spectroflu-oremeter (FLS920). An integrating sphere was mountedon the spectrofluoremeter with the entrance and exit portslocated in 90o geometry. The PiG sample was located inthe center of the integrating sphere. All the recorded spec-troscopic data were corrected for the spectral responsesof both the spectrofluoremeter and the integrating sphere.The responses of the detecting systems (integrating sphere,monochromators and detectors) in photon flux were de-termined using a calibrated tungsten lamp. Based on thissetup, internal QY is calculated by the following equitation[23, 24]:

η = number of photons emitted

number of photons absorbed

= Lsample

Ereference − Esample, (1)

where η represents QY, Lsample the emission intensity,Ereference and Esample the intensities of the excitation lightnot absorbed by the reference and the sample, respectively.The precursor glass was used as the standard reference. Thedifference in integrated areas between the sample and thereference represents the number of the absorbed photons.The photons emitted were determined by integrating thearea of the emission band. The error associated with theQY measurement is ±3%.

The external QY of PiG, defined as the ratio of theemitted photons to the incident photons on the sample,

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160 R. Zhang et al.: Ce:YAG phosphor-in-glass for high-power white LED

was also determined with the procedure similar to that formeasuring internal QY, except that the excitation sourcewas not a xenon lamp but a blue-emitting chip. Both theblue chip and the PiG-based WLED were located in theintegrating sphere, and their emission spectra were recordedrespectively at an operating current of 350 mA. The numberof incident photons (Ein) was evaluated by integrating theemission band of the blue chip in the wavelength rangeof 410–520 nm, while the number of the emitted photons(Eem) was calculated by integrating the emission band ofthe WLED in the range of 490–750 nm. The external QYof PiG was determined by Eem/Ein.

As a proof-of-concept experiment, the WLED devicewas constructed by encapsulating a PiG or PiS disk onthe blue chip, as schematically illustrated in Fig. S1. Thesample holder is groove-shaped with the blue chip fixed atthe bottom (purchased from Sichuan Baishi OptoelectronicTechnology Co. Ltd). The PiG or PiS color converter washorizontally fastened on the blue chip, with opaque silicagel (SD-6020, Shenzhen Saide Electronic Material Co. Ltd)coated around the edge to prevent leakage of blue light. LE,chromaticity coordinate, CCT and color rendering index(CRI) of the PiG- and PiS-based WLEDs were measuredin an integrating sphere of 50 cm diameter, which was con-nected to a CCD detector with an optical fiber (HAAS-2000,Everfine Photo-E-Info Co. Ltd). The current for exciting theblue chip was fixed at 350 mA.

Two kinds of reliability tests were carried out. One wasa thermal-resistance test, where the PiG sample and thePiG-based WLED were heat treated in an electric furnaceat 150 ◦C for 0–25 days, respectively. The other was ahumidity-resistance test, where the PiG-based WLED washeated at 85 ◦C in an environment of 85% humidity for 20days or immerged in boiling water for 24 h. In both tests,the measurements were performed after the samples andWLED devices were cooled to room temperature.

3. Results and Discussion

Photographs of the PiG samples with 1.0 mm in thickness,prepared under various experimental conditions, are shownin Figs. 1a–c. All the samples exhibit good transparency andbright yellow color. On increasing the Ce:YAG phosphorcontent, the PiG color darkens monotonously (Fig. 1a); onincreasing of the sintering temperature/time, the apparentyellow color fades gradually (Figs. 1b and c). The opti-cal transmission spectrum of the PiG containing 5 wt%Ce:YAG phosphor, as presented in Fig. 1d, shows that thetransmittance reaches 80% in the wavelength range of 550–800 nm, verifying the good transparency of PiG. The ab-sorption at ∼460 nm comes from the 4f→5d transitionof Ce3+.

The photoluminescence (PL) and PL excitation (PLE)spectra of the Ce:YAG powder and the corresponding PiGcomposite are shown in Fig. 2a. The PL spectrum of PiG ex-hibits a typical Ce3+: 5d→4f broadband emission centeredat 545 nm under 460-nm excitation, similar to the case ofthe Ce:YAG powder. The PLE spectrum of the PiG sample

Figure 1 Photographs of PiGs prepared under various experi-mental conditions: (a) containing various Ce:YAG contents (sin-tered at 570 ◦C for 20 min); (b) sintered at various temperaturesfor 20 min (containing 5 wt% Ce:YAG); (c) sintered at 570 ◦Cfor various durations (containing 5 wt% Ce:YAG). (d) Opticaltransmission spectrum of PiG with 1 mm in thickness (containing5 wt% Ce:YAG, and sintered at 570 ◦C for 20 min).

shows two excitation bands centered at 340 and 460 nmoriginating from the 4f→5d transition of Ce3+. Notably,the 340 nm excitation intensity of PiG is weaker than thatof Ce:YAG powder, ascribing to the absorption of glassmatrix in the short-wavelength range.

Figures 2b–d and Figs. S2 and S3 exhibit the phosphorcontent and sintering temperature/time dependence of thePL intensity, internal QY and decay lifetime for the PiGsample. On increasing of the phosphor content, the lumi-nescence of PiG intensifies correspondingly, while the in-ternal QY (92%) and lifetime (64 ns) are not obviouslyaffected (Fig. S2). On increasing of the sintering temper-ature/time, both the PL intensity and internal QY of PiGdecrease monotonously (Figs. 2b–d, Fig. S3), while thedecay lifetime remains unchanged since it is an intrinsicfeature of the Ce:YAG phosphor in PiG. Remarkably, thesintering temperature has a more significant impact on thePL intensity and internal QY than the sintering time.

Scanning electron microscopy (SEM) observations onthe PiG samples containing 5 wt% Ce:YAG phosphors werecarried out to investigate the microstructure variation withincreasing of the sintering temperature, as shown in Fig. 3.Evidently, the Ce:YAG particles sized 1–10 um are ho-mogeneously dispersed in the glass matrix for all the PiGsamples. However, the number of Ce:YAG particles de-creases with increasing sintering temperature, ascribed tothe serious corrosion of phosphors by the melting glass athigh temperature. This result is consistent with the fading ofapparent color, the decreasing of PL intensity and internalQY of the PiG samples on increasing sintering temperaturestated above. Figure 3d presents the SEM-EDS mappingof the PiG sample that distinguishes an individual Ce:YAGparticle from the glass matrix. The Sb-rich region representsthe glass matrix, while the Al- or Y-rich portion exhibitsthe phosphor particle.

Obviously, low sintering temperature and short sinter-ing time are beneficial to realizing highly efficient lumi-nescence of the PiG sample. However, when the sinteringtemperature is too low, or the sintering time is too short,the transparency of the prepared PiG sample is impaired, asrevealed in Fig. 1, probably owing to the existence of large

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Figure 2 (a) PLE and PL spectra of the Ce:YAG powder and the corresponding PiG sample; (b) PL intensity, (c) internal QY, and(d) decay lifetime of PiG versus sintering temperature (sintering time: 20 min). The measured internal QY and lifetime values of theCe:YAG powder are also provided in (c) and (d), respectively.

amount of tiny gas bubbles in PiG. Comprehensively evalu-ating the measured transparency, PL and QY of the samples,PiG with 5 wt% Ce:YAG phosphor sintered at 570 ◦C for20 min is regarded as the most appropriate material forthe color converter, and thus is systematically studied inthe following section. Table 1 lists some of the measuredphysical parameters of this PiG material. As a comparison,the related parameters of the silicone used for dispersingphosphor powder (QLE 1101, Shenzhen Topgun Technol-ogy Co. Ltd) and those of the Ce:YAG are also provided.The high refractive index of PiG (1.80) approaching thatof Ce:YAG (1.84) results in a low light scattering loss andtherefore high optical transparency of PiG. In addition, theexcellent mechanical properties of PiG make it applicableas the outer package for WLED. Furthermore, compared tothe silicone, PiG exhibits a much higher thermal conductiv-ity and lower thermal expansion coefficient, benefiting therapid heat-release in the high-power WLED. In the furtherexperiments, the thermal-quenching and thermal-resistancebehaviors of the PiG luminescence were investigated andcompared to those of the PiS one, as demonstrated in Fig. 4.

Table 1 Some physical parameters of PiG, silicone and Ce:YAGmaterials.

PiG Silicone Ce:YAG

Glass transition temperatureTg [ ◦C]

463 150 –

Refractive index n 1.80 1.40 1.84

Density ρ [g cm−3] 4.28 – 4.57

Hardness HV [MPa] 340 – –

Tenacity KIC [Mpa m1/2] 0.31 – –

Thermal expansion coefficienta [10−6 K−1]

16 295 –

Thermal conductivityλ [W m−1 K−1]

0.71 0.18 –

When the temperature increases from 25 to 200 ◦C, thePL intensity of PiG weakens by 5.5%, while that of PiSdecreases up to 9.8% (Fig. 4a). The improved thermal-quenching feature of PiG originates from its higher thermal

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162 R. Zhang et al.: Ce:YAG phosphor-in-glass for high-power white LED

Figure 3 SEM images of PiGs prepared at various sinteringtemperatures: (a) 570 ◦C, (b) 630 ◦C, and (c) 690 ◦C; (d) EDSmapping of an individual Ce:YAG particle embedded in glassmatrix.

Figure 4 Variations of the relative PL intensities in(a) temperature-dependent test, and (b) heat-resistance test forthe Ce:YAG PiS and Ce:YAG PiG samples.

conductivity (0.71 W m−1 K−1) than that of the silicone(0.18 W m−1 K−1), which benefits to the release of the heatemitted from the chip and subsequently reduces the proba-bility of the Ce3+ nonradiative transition. On the other hand,after heating at 150 ◦C for 20 days, only 3.9% PL degra-dation is observed in PiG, which is also much smaller thanthat in PiS (9.0%, Fig. 4b). The enhanced thermal resistanceof the inorganic PiG is due to its higher thermal stabilitythan that of PiS with organic matrix. These results revealthat the investigated PiG is superior to the conventional PiSas the color converter for the high-power WLED.

Figure 5a shows a transparent PiG color-converter-based WLED. Under an operating current of 350 mA, thelamp yields bright white light. The external QY of PiG with0.4 mm in thickness is determined to be 71% (Fig. S4), be-ing lower than the internal QY. This is reasonable sincepart of the incident light from the blue-emitting chip isnot absorbed by but passes through the Ce:YAG phosphorparticles in PiG, and thus does not contribute to the Ce3+

Table 2 Measured optical parameters of the PiG- and PiS-basedWLEDs.

PiGthickness (mm)

Chromaticitycoordinate LE (lm/W) CCT (K) CRI

0.2 (0.273, 0.249) 75 16603 69

0.4 (0.312, 0.333) 124 6674 70

0.6 (0.339, 0.391) 132 5220 64

0.8 (0.364, 0.433) 135 4728 63

1.0 (0.381, 0.448) 142 4485 61

1.2 (0.393, 0.482) 138 4261 59

PiS (0.311, 0.333) 98 6782 71

emission. The thickness-dependent PL spectra of PiGs andthe CIE color coordinates of the WLEDs are shown inFigs. 5b and c. To clearly explore the spectral variation,all the PL spectra are normalized to the blue chip emissionband (Fig. 5b). Obviously, increasing the PiG thicknessinduces monotonous intensification in the yellow lumines-cence, and the color coordinate of WLED shifts from whiteto yellow (Fig. 5c). For all the PiG–based LED devices,the yielded luminescence is very bright, as demonstratedin the inset of Fig. 5c. The measured optical parametersof WLED encapsulated with PiG of various thicknessesare listed in Table 2. On increasing the PiG thickness, LEintensifies and CCT decreases, owing to more blue light ab-sorbed and more yellow light emitted by PiG. The optimalthickness of PiG is found to be 0.4 mm, and the correspond-ing WLED has a LE of 124 lm/W, a CCT of 6674 K anda CRI of 70. As a comparison, the optical parameters ofthe PiS-based WLED are also provided in Table 2. The PiG(0.4 mm thickness)- and PiS-based WLEDs exhibit similarCCT and CRI, however, the former has much higher LE(124 lm/W) than the latter (98 lm/W). The 26.5% enhancedLE for the PiG-based WLED is attributed to the remarkablereduction in light-scattering loss in the transparent PiG andthus high light-extraction efficiency.

Furthermore, a comparison of the LE loss between PiG(0.4 mm thickness)- and PiS-based WLEDs in the heat-resistance test is performed, as shown in Fig. 6a. Notably,LE decreases monotonously with prolonging of aging at150 ◦C for both devices (Fig. 6a). However, after aging for600 h, LE loss for the PiG-base WLED (7.6%) is muchsmaller than that for the PiS-based one (16.5%). Besides,no considerable changes in CCT, CRI and chromaticity co-ordinate are detected for the PiG-based WLED, as exhibitedin Figs. 6b and c. These results clearly demonstrate that thePiG-based WLED exhibits more excellent heat-resistanceperformance than the conventional PiS-based one.

Finally, the excellent humidity-resistance feature of thePiG-based WLED was experimentally evidenced. In thehumidity-resistance test under the industry standard condi-tion (85% humidity at 85 ◦C), the LE loss for the PiG-basedWLED is not obvious (within 1%) after a testing durationof 20 days. To further characterize the humidity-resistancebehavior in a relative short time, an accelerated experiment,

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Figure 5 (a) Photographs of a WLED lamp encapsulated by a transparent PiG disk (left) and the lamp in operation (right).(b) Normalized PL spectra, and (c) CIE color coordinate of the PiG-based WLEDs with various PiG thicknesses (in mm); insetsof (c) show luminescent photographs of the PiG-based WLEDs at an operating current of 350 mA.

Figure 6 (a) LE losses of the PiG- and PiS-based WLEDs, (b) variation of CCT and CRI, and (d) CIE color coordinate of the PiG-basedWLED during heat-resistance test at 150 ◦C; inset of (c) shows chromaticity coordinate of the PiG-based WLED versus aging time.

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164 R. Zhang et al.: Ce:YAG phosphor-in-glass for high-power white LED

Table 3 Humidity-resistance test results of the PiG-based WLED.

Treating condition Chromaticity coordinate LE (lm/W) LE loss (%) CCT (K) CRI

Untreated (0.312, 0.333) 124 – 6674 70

After immerging in boiling water for 24 h (0.311, 0.333) 117 5.6 6759 71

After reheating at 150 ◦C for 1 h (0.312, 0.332) 120 3.2 6696 71

i.e. immerging the PiG sample in boiling water for 24 h, wasperformed. As exhibited in Table 3, only 5.6% LE degra-dation is observed after such test, and no marked changesare found for the chromaticity coordinate, CCT and CRI.It is worth noting that the LE recovers to 96.8% (with LEloss of 3.2%) after reheating PiG at 150 ◦C for 1 h, ascribedto partial release of the adsorbed H2O molecules acting asquenching centers.

4. Conclusion

In summary, we have developed an innovative transparentCe:YAG PiG color converter, which is proved to be an ex-cellent alternative to the conventional epoxy resin/silicone-based phosphor converter for high-power WLED. ThePiG-based WLED yields a LE of 124 lm/W, a CCT of6674 K and a CRI of 70, under an operating currentof 350 mA. Impressively, this WLED device exhibitsadmirable heat-resistance and humidity-resistance perfor-mances: only 7.6% LE loss is observed after aging at 150 ◦Cfor 600 h, much superior to that of the conventional PiS-based WLED (16.5%); and only 5.6% LE degradation isdetected after immersing PiG in the boiling water for 24 h.Benefiting from its easy fabrication, low cost, long lifetime,as well as superior optical properties, the PiG-based WLEDis expected to be a new-generation indoor/outdoor lightingsource.

Acknowledgements. This work was supported by NationalNatural Science Foundation of China (51172231, 21271170,11204301 and 51202244), the key innovation project of HaixiInstitute of CAS (SZD13001), and Natural Science Foundation ofFujian for Distinguished Young Scholars (2012J06014).

� Supporting information for this article is available free ofcharge under http://dx.doi.org/10.1002/lpor.201300140 orfrom the author.

Received: 4 September 2013, Revised: 24 October 2013,Accepted: 18 November 2013

Published online: 10 December 2013

Key words: optical materials, WLED, Ce3+:YAG, phosphor-in-glass, luminescence.

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