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Preparation and properties of the flexible remote phosphor film for blue chip-based white LED
Materials and Design 102 (2016) 8–13
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Materials and Design journal homepage: www.elsevier.com/locate/matdes
Preparation and properties of the flexible remote phosphor film for blue chip-based white LED Jing Jia a,b, Aiqin Zhang a,c,⁎, Dongxin Li a,b, Xuguang Liu a,d, Bingshe Xu a,e, Husheng Jia a,b,⁎⁎ a
Key laboratory of Interface Science and Engineering in Advanced Materials, Ministry of Education, Taiyuan University of Technology, Taiyuan 030024, PR China College of Materials Science and Engineering, Taiyuan University of Technology, Taiyuan 030024, PR China College of Textile Engineering, Taiyuan University of Technology, Taiyuan 030600, PR China d College of Chemistry and Chemical Engineering, Taiyuan University of Technology, Taiyuan 030024, PR China e Shanxi Research Center of Advanced Materials Science and Technology, Taiyuan 030024, PR China b c
a r t i c l e
i n f o
Article history: Received 27 January 2016 Received in revised form 5 April 2016 Accepted 7 April 2016 Available online 11 April 2016 Keywords: WLED Yttrium aluminum garnet (YAG) Remote phosphor film Luminescence Flexible
a b s t r a c t A simple and effective method for larger area flexible and relatively uniform phosphor film layer for blue chipbased white LED is presented in this paper. Besides having a certain ultraviolet resistance and thermal stability, the proposed yttrium aluminum garnet (YAG) phosphor film also achieved applicable color temperature (Tc of 5480 K and 4900 K), color rendering index (CRI of 68.4 and 70) and luminous efficiency (121.7 and 77 lm/W) when simply assembled with single blue chip and high-power COB separately. The preparation process and its test results suggested flexible phosphor film consists of YAG phosphor particles and common silicone without additional converter devices, which has positive practical significance for the packaging of remote phosphorconverted white LEDs. © 2016 Elsevier Ltd. All rights reserved.
1. Introduction White light-emitting diodes (WLEDs), which are regarded as potential replacements in future display and solid-state lighting, have been widely applied in solid-state lighting, outdoor information display, liquid crystal display (LCD) backlighting, landscape lighting and even the automotive field, owing to their superior advantages such as energy saving, long lifetime, free pollution and low weight, etc. [1,2]. Currently, WLEDs can be mainly achieved through light converting techniques which are low-cost, simple and easy to realize industrialization, while two other means, known as multi-chip and quantum well technology, have not reached industrial scale because of high cost, instability and immature technologies [3–5]. One of conventional light converting systems for WLEDs is comprised of blue LED chip and yttrium aluminum garnet (YAG) yellow phosphor excited by blue light, in which process phosphor powders mixed with transparent epoxy resin or silicone are dropped directly onto the surface of blue nitride-based LED chip [see Fig. 1a]. To improve the differences among LEDs such as color deviation caused by uneven
* Corresponding author. ** Corresponding author at: Key Laboratory of Interface Science and Engineering in Advanced Materials, Taiyuan University of Technology, Ministry of Education, No. 79 Yingze Street, Taiyuan 030024, PR China. E-mail addresses: [email protected] (A. Zhang), [email protected] (H. Jia).
http://dx.doi.org/10.1016/j.matdes.2016.04.022 0264-1275/© 2016 Elsevier Ltd. All rights reserved.
distribution of YAG phosphor, surface mounted devices (SMD) LEDs have been developed, in which a caky mixture of phosphors and epoxy resin or phosphor sprayed onto the surface of some kind of substrate is stick on blue chip to generate white light. For instance, Joongyeon Cho et al. reported the preparation by direct spin-on glass (SOG) printing of nanopatterned yttrium aluminum garnet phosphorincorporated film to improve the light output power of white LEDs [6]. However, almost 60% of the blue light would be backscattered by the phosphor and lost in the chip because the phosphor is tightly close to the LED die in both of those WLED packages [7,8]. To increase luminance efficiency, some efforts have been made, showing that placing phosphor away from the chip die [Fig. 1b] and using a diffuse reflector cup significantly reduce the absorption of backscattered blue light for LED chip and thus improve the luminance efficiency [9–12]. Moreover, remote phosphor structure can reduce the decline of conversion efficiency and high operating temperature for phosphors resulted from chip heat. Currently, the studies on remote phosphor configuration for phosphor-converted white LEDs used in lighting applications have become popular. Huang et al. proposed a planar lighting system using array of blue LEDs to excite a YAG:Ce3+ yellow phosphor remote film fabricated through depositing the slurry of phosphor onto a polyethylene terephthalate (PET) film [13]. Tsai et al. reported a glass phosphor layer with ultra-high thermal stability appropriate for phosphorconverted white light-emitting diodes (PC-WLEDs) [14]. Wang et al. presented a new method for fabricating fluorescent film by the rareearth phosphor powder filled polycarbonate (PC) resin was prepared
J. Jia et al. / Materials and Design 102 (2016) 8–13
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silicone, and a little n-C6H14 as dispersant was dripped so as to form a uniform phosphor suspension. Subsequently, the uniform mixture was filled into PTFE mold and the small air bubbles inside the phosphor suspension were removed through vacuum pumping for 2 min. Finally, phosphor film was baked and cured at 100 °C for 1 h. For ultraviolet aging with UV wavelength of 272 nm and thermal aging tests, prepared film samples were aged at room temperature and 200 °C for one week, respectively. 2.3. Apparatus Fig. 1. Schematic diagram of LED packaging. (a) Package of dispensing phosphor, (b) Package of remote phosphor. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
by melt-extrusion and rolled into a fluorescent PC film by open mill [15]. Ying et al. concluded that the convex remote phosphor structure had higher luminous efficiency with higher color uniformity than conventional flat remote phosphor structure [16]. Silicone is commonly used for packaging materials with high transparency, good stability, flexibility and good gas-permeability, whereas studies of the remote phosphor layer prepared directly from slurry of phosphor powder and silicone are less. In this work, we demonstrated a simple and efficient process to fabricate large-area and uniform flexible yellow fluorescent film that is applicable for the PC-WLED package such as Flip chip and Chip on Board (COB). Moreover, the optical properties, thermal aging and ultraviolet aging tests of the flexible phosphor film used in LED packaging samples were described in detail.
The morphologies of raw YAG phosphor and remote phosphor film were characterized using field emission scanning electron microscope (FE-SEM, JSM-6700F). The surface topography of the phosphor film was measured by atomic force microscope (AFM, SPA-300 HV). UV– vis absorption spectrum was taken using Hitachi-3900 spectrometer. Fluorescence spectra were measured by Horiba Fluoromax4 Spectrometer (the slit width was 1 nm). Electroluminescence (EL) spectra were measured using a computer controlled PMS-50 plus UV–vis-near IR spectrometer with an integrating sphere. The thermal aging and ultraviolet aging tests of remote phosphor film were carried out in heat oven and ultraviolet irradiation lightbox, respectively. The tensile testing was conducted at an electronic universal testing machine with the dumbbell-shaped specimen that had a gauge length of 10 mm, a gauge width of 10 mm and a thickness of 1 mm. All measurements were made at room temperature unless otherwise stated. 3. Results and discussion
2. Experiments
3.1. Micro-morphology of the YAG phosphor film
2.1. Materials
Fig. 3a shows the appearance of the YAG phosphor film prepared by the process proposed in this work. YAG phosphor particles gradually subside into one side of PDMS during the solidification because of gravity and immiscibility. Fig. 3b exhibits the surface topography of the sediment side of phosphor film, which is relatively flat with a surface roughness RMS of 1.35 nm. As can be seen from Fig. 3c and d, phosphor particles are in orderly lateral rows in PDMS and separated with obvious spacing among each other. Compared with the agglomerated raw YAG particles with D50 of 17 μm, the YAG phosphors in prepared phosphor film is just 10 μm or so in average size, confirming that n-hexane acts effectively in dispersing YAG phosphor particles. However, the particles accumulate compactly with the layer thickness of about 70 μm in the longitudinal direction. This structure can allow sufficient blue light pass through the prepared phosphor film to produce white light by mixing with converted yellow light since PDMS has good transmittance. On the other hand, it can also scatter the incident blue light efficiently.
A Ce-doped yttrium aluminum garnet (YAG-04, Intematix) with excitation wavelength ranging from 430 nm to 490 nm was used in this experiment. As shown in Fig. 2a, the YAG phosphor powders are approximate micro-spherical particles, the mean size (D50) is about 17 μm [Fig. 2b]. Furthermore, adopting polydimethylsiloxane (PDMS, OE-6550, DOW CORNING), tetrahydrofuran (THF, AR), n-hexane (nC6H14, AR) and teflon (PTFE) coagulating mold with a size of 28 × 28 × 1 mm. 2.2. Processing procedure First, the silicone PDMS glue A with glue B were blended at the ratio of 1:1 in weight and trace THF acting as solvent was injected. Then YAG: Ce phosphor was mixed, which accounted for 12.5% of total quality of
Fig. 2. SEM images of YAG:Ce phosphor.
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Fig. 3. Morphologies of YAG phosphor film (a) appearance, (b) AFM micrograph, (c) and (d) are the SEM images.
3.2. Optical spectra The UV–vis-near IR absorption spectra of prepared YAG phosphor film and raw YAG phosphor are displayed in Fig. 4. It is quite clear that the curve shape and peak wavelength of phosphor film are basically in accordance with those of raw phosphor particles, although the absorbance for film decreases a little when compared with that for raw particles. The characteristic absorption peak appears at 458 nm (459 nm for original phosphor), demonstrating that the prepared film can be excited by blue light as original YAG phosphor. The solid-state excitation and emission spectra of prepared YAG phosphor film and raw YAG phosphor recorded at room temperature are presented in Fig. 5a and b. Their peak shapes are very similar for either photoluminescence excitation (PLE) spectra or photoluminescence (PL) spectra. But there is a big difference in relative fluorescent intensity because of fluorescence quenching between YAG phosphor and PDMS matrix, which is often divided into two categories: static quenching and dynamic quenching [17]. The black curve in Fig. 5b displays two optimal excitation wavelengths at 340 nm and 457 nm when monitored at 539 nm, which cover the whole area of UV-blue light and thus ensure that the prepared YAG phosphor film can match blue chips with λp of 457 nm or so. Moreover, it is speculated that the red-shift of excitation peak for YAG phosphor in the film (445 nm → 457 nm) is caused by dynamic quenching, which only affects the excited state of YAG phosphor but not its absorption spectrum. In contrast, static quenching typically generates nonluminous complex, leading to changes in absorption spectrum for fluorescent molecules. The red curve in Fig. 5b shows typical emission band at 539 nm when excited at 457 nm. Correspondingly, the yellow-green
fluorescence of prepared phosphor layer can be blended with unconverted blue light to get white light, and the correlated 1931 Commission Internationale de L'Eclairage (CIE) coordinates for the tested phosphor film (0.382, 0.586) is closer to the green light region than that for raw YAG phosphor powders (0.404, 0.569), as marked in Fig. 5c. As can be seen the excitation spectra of both raw YAG:Ce phosphor and the prepared phosphor film have two peaks. Besides the highest peaks in the visible range are attributed to the 2F7/2 → 2D5/2 transitions of Ce3+, there are sub-highest peaks at 340 nm around related to the transitions from 2F5/2 → 2D5/2 of Ce3+. Correspondingly, the symmetrical emission spectra are compounded of emission bands formed by 2D3/ 2 2 2 3+ [18]. 2 → F7/2 and D3/2 → F5/2 transitions of Ce 3.3. Display performance and aging The remote phosphor-conversion method reportedly has a higher luminance efficiency than conventional phosphor dispensing method when applied to individual LED packages [19,20]. Hence to start with, the EL spectrum of regularly assembled WLED lamp package composed of the same blue chip and differently treated YAG phosphor film, and their visible emission colors around white light zone are shown in Table 1 and Fig. 6. The test phosphor films before and after thermal aging or ultraviolet aging are encapsulated into 1 W lamp beads, inside which slices of different samples are pressed on the blue chips with particle face down. Except for thermally aged samples, which are measured at 2.8 V because of signal overflow in blue emission bands, other samples are measured at 3.0 V. When the blue light with wavelength of 455 nm is radiated from LED chip to excite the phosphor film, some of incident blue light is converted
Table 1 Optical characteristics of assembled WLED using different YAG phosphor film. Samples
Mean Tc (K)
Mean CRI (Ra)
Mean luminance efficiency (lm/W)
CIE chromaticity coordinates
Light decay (%)
Untreated Thermal aging for 170 h at 200 °C UV aging for 170 h at 272 nm
5480 10,796 5122
68.4 81 66.9
121.7 65.4 70.2
(0.333, 0.401) (0.275, 0.285) (0.347, 0.428)
null 46.26 42.32
J. Jia et al. / Materials and Design 102 (2016) 8–13
Fig. 4. UV–vis-near IR absorption spectra of prepared YAG phosphor film and raw YAG phosphor.
into yellow light, the rest is diffused, and finally these rays are mixed to produce white light, as presented in Fig. 6a. The CIE chromaticity coordinates corresponding to the electroluminescence of fabricated WLED with untreated film (0.333, 0.401) is close to pure white light (0.330, 0.330) according to the 1931 CIE coordinate diagram [21–23]. Furthermore, the mean Tc of untreated test sample is 5480 K, mean CRI (Ra) is 68.4, and the mean luminance efficiency is 121.7 lm/W. The emission intensity of YAG phosphor film after ultraviolet aging for 170 h, by contrast, decreases by half owing to light loss caused by ultraviolet irradiation. For UV aged samples, the Tc of 5122 K and CRI of 66.9 show a little changes and the average power efficiency is reduced to 70.2 lm/W. Actually, the emission of UV aged WLED is warmer white light since the relative contribution to white light of emitted yellow light increased, as shown in the inset photograph of Fig. 6c and d.
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The emission intensity for thermally aged sample driven at 2.8 V is significantly reduced, and the corresponding assembled WLED exhibits blue-white emission with higher Tc of 10,796 K and CRI of 81, the average luminance efficiency drops to 65.4 lm/W. This means that the light decay is up to 46.26% after aged under 200 °C, though the junction temperature of LED is generally less than 150 °C. Accordingly, the CIE chromaticity coordinates (0.275, 0.285) is closer to the blue light region as a result of lower conversion efficiency for thermally aged YAG phosphor [Fig. 6b and d]. These results demonstrate that the fabricated WLED with untreated YAG phosphor film shows moderate Tc value, qualified luminance efficiency and lower CRI value, which are similar to those for UV aged sample because organic silicon has a certain resistance against UV except for decreased lumen efficiency. At the same time, thermal aging can lead to greatly increased proportion of blue emission radiated from LED and higher Tc value and CRI value, and significantly reduced luminous efficiency. In other words, the proposed YAG phosphor film has good ultraviolet resistance but poor thermostability. The luminous flux emitted from finished products could be minimum while the distance between phosphor and chip surface is zero, so a very simple remote phosphor LED module without reflector cup is built with the air gap of 10 mm around and composed of 80 W high-power blue COB and proposed yellow phosphor film. Blue light derived from COB LED driven at 20 V coupled with yellow emission of remote YAG phosphor layer yields milder warm white light, which shows average Tc of 4900 K, CRI of 70 and the mean luminance efficiency of 77 lm/W (for naked blue COB is only 15–18.75 lm/W). Obviously, the luminous efficiency of a remote phosphor configuration is strongly influenced by the value of the air gap between remote phosphor layer and blue chip, that is, the luminance efficiency will reach a maximum only when the air gap is appropriate. In regard to CRI, change in air gap would not make a difference since the phosphor conversion efficiency
Fig. 5. PLE and PL spectra of raw YAG phosphor (a), YAG phosphor film(b) and the CIE chromaticity coordinates (c) for raw YAG phosphor (1) and YAG phosphor film(2). (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
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J. Jia et al. / Materials and Design 102 (2016) 8–13
Fig. 6. Electroluminescence (EL) spectrum of assembled WLED with different YAG phosphor film (a) untreated sample, (b) sample after thermal aging at 200 °C for 170 h, (c) sample after UV aging for 170 h and (d) shows the corresponding CIE chromaticity coordinates. The inset photographs are the WLED lamp package. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
depends on the energy of excitation wavelength not on the light intensity. 3.4. Tensile testing Further investigation was carried out to understand the mechanical properties of prepared flexible YAG phosphor film. Tensile testing result (Fig. 7) reveals that the loaded force increases gradually along with time up to around 6.5 N—the fracture strength of the phosphor film, and then it declines until the test sample is completely separated into two pieces. Obviously, the fracture of prepared YAG phosphor film is brittle fracture at low stress since there are no yield point and plastic deformation occurring throughout the fracturing process. The reason for poor plasticity is clear that the stress concentration under force is created by each YAG phosphor particle with average diameter of about 10 μm dispersed in PDMS, and the generation and expansion of cracks are intensified so that the brittleness of phosphor film increases greatly, even though the PDMS is a transparent elastomer.
CRI were 4900 K and 70, respectively. N-hexane had been shown to disperse the YAG phosphor particles efficiently. Additionally, the proposed method was also applicable to preparation for other inorganic particle layer. However, the luminous efficiency of the prepared plane remote phosphor configuration for high-power COB, the toughness of the proposed YAG phosphor film were not ideal, which need to be improved by means of adjusting the distance between YAG phosphor film and
Conclusions The YAG phosphor film for blue chip-based white LED was produced by a simple and pollution-free method. The proposed phosphor film not only retains the optical properties of raw YAG phosphor, but also presents good UV resistance. Furthermore, the corresponding assembled WLED yielded moderate Tc value of 5480 K, CRI value of 68.4 and luminance efficiency of 121.7 lm/W when yellow phosphor film was in direct contact with blue chip. While proposed film was combined with 80 W high-power blue COB remotely with a 10 mm air gap, the luminous efficiency measured was only 77 lm/W, and the value of Tc and
Fig. 7. Tensile curve of YAG remote phosphor film.
J. Jia et al. / Materials and Design 102 (2016) 8–13
blue chips, and altering the proportion of YAG phosphor in silicone respectively in future work. Acknowledgements This work was funded by the Program for International Science &Technology Cooperation Program of China (2011DFA52290, 2012DFR50460), National Natural Science Funds (21471111), Shanxi Provincial Key Innovative Research Team in Science and Technology (2015013002-10), Natural Science Foundation of Shanxi Province (2013011013-2), and Program for Science and Technology Development of Shanxi (20140321012-01), project of Excellent Scientist Fund in Inner Mongolia. References [1] E. Fred Schubert, Light-Emitting Diodes, second ed. Cambridge University Press, Cambridge, 2006. [2] R.V. Steele, The story of a new light source, Nat. Photonics 1 (2006) 25–26. [3] X. Guo, G.D. Shen, B.L. Guan, et al., Cascade single-chip phosphor-free white lightemitting diodes, Appl. Phys. Lett. 92 (2008) 013507. [4] L.W. Ji, Y.K. Su, S.J. Chang, et al., InGaN/GaN multi-quantum dot light-emitting diodes, J. Cryst. Growth 263 (2004) 114–118. [5] R.Z. Liang, M. Wei, et al., Quantum dots-based flexible films and their application as the phosphor in white light-emitting diodes, Chem. Mater. 26 (2014) 2595–2600. [6] Joong-yeon Cho, Sang-Jun Park, et al., Nanopatterned yttrium aluminum garnet phosphor incorporated film for high-brightness GaN-based white light emitting diodes, Thin Solid Films 570 (2014) 326–329. [7] K. Yamada, Y. Imai, K. Ishii, Optical simulation of light source devices composed of blue LEDs and YAG phosphor, J. Light. Vis. Environ. 27 (2003) 70–74. [8] N. Narendran, Y. Gu, J.P. Freyssinier-Nova, et al., Extracting phosphor-scattered photons to improve white LED efficiency, Phys. Status Solidi A 202 (2005) 60–62.
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[9] Nadarajah Narendran, L.E.D. Improved Performance White, Fifth International Conference on Solid State Lighting, Proceedings of SPIE 5941 (2005) 45–50. [10] Steven C. Allen, Andrew J. Steckl, ELiXIR—Solid-State luminaire with enhanced light extraction by internal reflection, J. Disp. Technol. 3 (2007) 155–159. [11] Jong Kyu Kim, Hong Luo, Eric Fred Schubert, et al., Strongly enhanced phosphor efficiency in GaInN white light-emitting diodes using remote phosphor configuration and diffuse reflector cup, Jpn. J. Appl. Phys. 44 (2005) 649–651. [12] Zongyuan Liu, Sheng Liu, Kai Wang, et al., Optical analysis of phosphor's location for high-power light-emitting diodes, IEEE T. Device Mat. Re. 9 (2009) 65–73. [13] Hsin-Tao Huang, Yi-Pai Huang, Chuang-Chuang Tsai, Planar lighting system using array of blue LEDs to excite yellow remote phosphor film, J. Disp. Technol. 7 (2011) 44–51. [14] Chun-Chin Tsai, Wei-Chih Cheng, Jin-Kai Chang, et al., Ultra-high thermal-stable glass phosphor layer for phosphor-converted white light-emitting diodes, J. Disp. Technol. 9 (2013) 427–431. [15] Zezhong Wang, Kefeng Wang, et al., Study on processing feasibility and fluorescence properties of rare-earth fluorescent/polycarbonate, J. S. China Norm. Univ. 47 (2015) 32–37. [16] Y. Shang-Ping, F. Han-Kuei, T. Huang-Zong, Curved remote phosphor structure for phosphor-converted white LEDs, Appl. Opt. 53 (2014) 160. [17] Guozhen Chen, et al., Fluorescence Analytical Method, Science Press, Beijing, 1990. [18] R.R. Jacobs, W.F. Krupke, M.J. Weber, Measurement of excited-state-absorption loss for Ce3+ in Y3Al5O12 and implications for tunable 5d → 4f rare-earth lasers, Appl. Phys. Lett. 33 (1978) 410–412. [19] Y. Zhu, N. Narendran, Optimizing the performance of remote phosphor LEDs, J. Light. Vis. Environ. 32 (2008) 115–119. [20] H. Luo, J.K. Kim, E.F. Schubert, et al., Analysis of high-power packages for phosphorbased white-light-emitting diodes, Appl. Phys. Lett. 86 (2005) 243505. [21] J. Guild, The colorimetric properties of the spectrum, Philos. Trans. R. Soc. Lond. 230 (1932) 149–187. [22] T. Forster, Transfer mechanisms of electronic excitation, Discuss. Faraday Soc. 27 (1959) 7–17. [23] T.F. Guo, T.C. Wen, Y.S. Huang, et al., White-emissive tandem-type hybrid organic/ polymer diodes with (0.33,0.33) chromaticity coordinates, Opt. Express 17 (2009) 21205–21215.
Contents lists available at ScienceDirect
Materials and Design journal homepage: www.elsevier.com/locate/matdes
Preparation and properties of the flexible remote phosphor film for blue chip-based white LED Jing Jia a,b, Aiqin Zhang a,c,⁎, Dongxin Li a,b, Xuguang Liu a,d, Bingshe Xu a,e, Husheng Jia a,b,⁎⁎ a
Key laboratory of Interface Science and Engineering in Advanced Materials, Ministry of Education, Taiyuan University of Technology, Taiyuan 030024, PR China College of Materials Science and Engineering, Taiyuan University of Technology, Taiyuan 030024, PR China College of Textile Engineering, Taiyuan University of Technology, Taiyuan 030600, PR China d College of Chemistry and Chemical Engineering, Taiyuan University of Technology, Taiyuan 030024, PR China e Shanxi Research Center of Advanced Materials Science and Technology, Taiyuan 030024, PR China b c
a r t i c l e
i n f o
Article history: Received 27 January 2016 Received in revised form 5 April 2016 Accepted 7 April 2016 Available online 11 April 2016 Keywords: WLED Yttrium aluminum garnet (YAG) Remote phosphor film Luminescence Flexible
a b s t r a c t A simple and effective method for larger area flexible and relatively uniform phosphor film layer for blue chipbased white LED is presented in this paper. Besides having a certain ultraviolet resistance and thermal stability, the proposed yttrium aluminum garnet (YAG) phosphor film also achieved applicable color temperature (Tc of 5480 K and 4900 K), color rendering index (CRI of 68.4 and 70) and luminous efficiency (121.7 and 77 lm/W) when simply assembled with single blue chip and high-power COB separately. The preparation process and its test results suggested flexible phosphor film consists of YAG phosphor particles and common silicone without additional converter devices, which has positive practical significance for the packaging of remote phosphorconverted white LEDs. © 2016 Elsevier Ltd. All rights reserved.
1. Introduction White light-emitting diodes (WLEDs), which are regarded as potential replacements in future display and solid-state lighting, have been widely applied in solid-state lighting, outdoor information display, liquid crystal display (LCD) backlighting, landscape lighting and even the automotive field, owing to their superior advantages such as energy saving, long lifetime, free pollution and low weight, etc. [1,2]. Currently, WLEDs can be mainly achieved through light converting techniques which are low-cost, simple and easy to realize industrialization, while two other means, known as multi-chip and quantum well technology, have not reached industrial scale because of high cost, instability and immature technologies [3–5]. One of conventional light converting systems for WLEDs is comprised of blue LED chip and yttrium aluminum garnet (YAG) yellow phosphor excited by blue light, in which process phosphor powders mixed with transparent epoxy resin or silicone are dropped directly onto the surface of blue nitride-based LED chip [see Fig. 1a]. To improve the differences among LEDs such as color deviation caused by uneven
* Corresponding author. ** Corresponding author at: Key Laboratory of Interface Science and Engineering in Advanced Materials, Taiyuan University of Technology, Ministry of Education, No. 79 Yingze Street, Taiyuan 030024, PR China. E-mail addresses: [email protected] (A. Zhang), [email protected] (H. Jia).
http://dx.doi.org/10.1016/j.matdes.2016.04.022 0264-1275/© 2016 Elsevier Ltd. All rights reserved.
distribution of YAG phosphor, surface mounted devices (SMD) LEDs have been developed, in which a caky mixture of phosphors and epoxy resin or phosphor sprayed onto the surface of some kind of substrate is stick on blue chip to generate white light. For instance, Joongyeon Cho et al. reported the preparation by direct spin-on glass (SOG) printing of nanopatterned yttrium aluminum garnet phosphorincorporated film to improve the light output power of white LEDs [6]. However, almost 60% of the blue light would be backscattered by the phosphor and lost in the chip because the phosphor is tightly close to the LED die in both of those WLED packages [7,8]. To increase luminance efficiency, some efforts have been made, showing that placing phosphor away from the chip die [Fig. 1b] and using a diffuse reflector cup significantly reduce the absorption of backscattered blue light for LED chip and thus improve the luminance efficiency [9–12]. Moreover, remote phosphor structure can reduce the decline of conversion efficiency and high operating temperature for phosphors resulted from chip heat. Currently, the studies on remote phosphor configuration for phosphor-converted white LEDs used in lighting applications have become popular. Huang et al. proposed a planar lighting system using array of blue LEDs to excite a YAG:Ce3+ yellow phosphor remote film fabricated through depositing the slurry of phosphor onto a polyethylene terephthalate (PET) film [13]. Tsai et al. reported a glass phosphor layer with ultra-high thermal stability appropriate for phosphorconverted white light-emitting diodes (PC-WLEDs) [14]. Wang et al. presented a new method for fabricating fluorescent film by the rareearth phosphor powder filled polycarbonate (PC) resin was prepared
J. Jia et al. / Materials and Design 102 (2016) 8–13
9
silicone, and a little n-C6H14 as dispersant was dripped so as to form a uniform phosphor suspension. Subsequently, the uniform mixture was filled into PTFE mold and the small air bubbles inside the phosphor suspension were removed through vacuum pumping for 2 min. Finally, phosphor film was baked and cured at 100 °C for 1 h. For ultraviolet aging with UV wavelength of 272 nm and thermal aging tests, prepared film samples were aged at room temperature and 200 °C for one week, respectively. 2.3. Apparatus Fig. 1. Schematic diagram of LED packaging. (a) Package of dispensing phosphor, (b) Package of remote phosphor. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
by melt-extrusion and rolled into a fluorescent PC film by open mill [15]. Ying et al. concluded that the convex remote phosphor structure had higher luminous efficiency with higher color uniformity than conventional flat remote phosphor structure [16]. Silicone is commonly used for packaging materials with high transparency, good stability, flexibility and good gas-permeability, whereas studies of the remote phosphor layer prepared directly from slurry of phosphor powder and silicone are less. In this work, we demonstrated a simple and efficient process to fabricate large-area and uniform flexible yellow fluorescent film that is applicable for the PC-WLED package such as Flip chip and Chip on Board (COB). Moreover, the optical properties, thermal aging and ultraviolet aging tests of the flexible phosphor film used in LED packaging samples were described in detail.
The morphologies of raw YAG phosphor and remote phosphor film were characterized using field emission scanning electron microscope (FE-SEM, JSM-6700F). The surface topography of the phosphor film was measured by atomic force microscope (AFM, SPA-300 HV). UV– vis absorption spectrum was taken using Hitachi-3900 spectrometer. Fluorescence spectra were measured by Horiba Fluoromax4 Spectrometer (the slit width was 1 nm). Electroluminescence (EL) spectra were measured using a computer controlled PMS-50 plus UV–vis-near IR spectrometer with an integrating sphere. The thermal aging and ultraviolet aging tests of remote phosphor film were carried out in heat oven and ultraviolet irradiation lightbox, respectively. The tensile testing was conducted at an electronic universal testing machine with the dumbbell-shaped specimen that had a gauge length of 10 mm, a gauge width of 10 mm and a thickness of 1 mm. All measurements were made at room temperature unless otherwise stated. 3. Results and discussion
2. Experiments
3.1. Micro-morphology of the YAG phosphor film
2.1. Materials
Fig. 3a shows the appearance of the YAG phosphor film prepared by the process proposed in this work. YAG phosphor particles gradually subside into one side of PDMS during the solidification because of gravity and immiscibility. Fig. 3b exhibits the surface topography of the sediment side of phosphor film, which is relatively flat with a surface roughness RMS of 1.35 nm. As can be seen from Fig. 3c and d, phosphor particles are in orderly lateral rows in PDMS and separated with obvious spacing among each other. Compared with the agglomerated raw YAG particles with D50 of 17 μm, the YAG phosphors in prepared phosphor film is just 10 μm or so in average size, confirming that n-hexane acts effectively in dispersing YAG phosphor particles. However, the particles accumulate compactly with the layer thickness of about 70 μm in the longitudinal direction. This structure can allow sufficient blue light pass through the prepared phosphor film to produce white light by mixing with converted yellow light since PDMS has good transmittance. On the other hand, it can also scatter the incident blue light efficiently.
A Ce-doped yttrium aluminum garnet (YAG-04, Intematix) with excitation wavelength ranging from 430 nm to 490 nm was used in this experiment. As shown in Fig. 2a, the YAG phosphor powders are approximate micro-spherical particles, the mean size (D50) is about 17 μm [Fig. 2b]. Furthermore, adopting polydimethylsiloxane (PDMS, OE-6550, DOW CORNING), tetrahydrofuran (THF, AR), n-hexane (nC6H14, AR) and teflon (PTFE) coagulating mold with a size of 28 × 28 × 1 mm. 2.2. Processing procedure First, the silicone PDMS glue A with glue B were blended at the ratio of 1:1 in weight and trace THF acting as solvent was injected. Then YAG: Ce phosphor was mixed, which accounted for 12.5% of total quality of
Fig. 2. SEM images of YAG:Ce phosphor.
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J. Jia et al. / Materials and Design 102 (2016) 8–13
Fig. 3. Morphologies of YAG phosphor film (a) appearance, (b) AFM micrograph, (c) and (d) are the SEM images.
3.2. Optical spectra The UV–vis-near IR absorption spectra of prepared YAG phosphor film and raw YAG phosphor are displayed in Fig. 4. It is quite clear that the curve shape and peak wavelength of phosphor film are basically in accordance with those of raw phosphor particles, although the absorbance for film decreases a little when compared with that for raw particles. The characteristic absorption peak appears at 458 nm (459 nm for original phosphor), demonstrating that the prepared film can be excited by blue light as original YAG phosphor. The solid-state excitation and emission spectra of prepared YAG phosphor film and raw YAG phosphor recorded at room temperature are presented in Fig. 5a and b. Their peak shapes are very similar for either photoluminescence excitation (PLE) spectra or photoluminescence (PL) spectra. But there is a big difference in relative fluorescent intensity because of fluorescence quenching between YAG phosphor and PDMS matrix, which is often divided into two categories: static quenching and dynamic quenching [17]. The black curve in Fig. 5b displays two optimal excitation wavelengths at 340 nm and 457 nm when monitored at 539 nm, which cover the whole area of UV-blue light and thus ensure that the prepared YAG phosphor film can match blue chips with λp of 457 nm or so. Moreover, it is speculated that the red-shift of excitation peak for YAG phosphor in the film (445 nm → 457 nm) is caused by dynamic quenching, which only affects the excited state of YAG phosphor but not its absorption spectrum. In contrast, static quenching typically generates nonluminous complex, leading to changes in absorption spectrum for fluorescent molecules. The red curve in Fig. 5b shows typical emission band at 539 nm when excited at 457 nm. Correspondingly, the yellow-green
fluorescence of prepared phosphor layer can be blended with unconverted blue light to get white light, and the correlated 1931 Commission Internationale de L'Eclairage (CIE) coordinates for the tested phosphor film (0.382, 0.586) is closer to the green light region than that for raw YAG phosphor powders (0.404, 0.569), as marked in Fig. 5c. As can be seen the excitation spectra of both raw YAG:Ce phosphor and the prepared phosphor film have two peaks. Besides the highest peaks in the visible range are attributed to the 2F7/2 → 2D5/2 transitions of Ce3+, there are sub-highest peaks at 340 nm around related to the transitions from 2F5/2 → 2D5/2 of Ce3+. Correspondingly, the symmetrical emission spectra are compounded of emission bands formed by 2D3/ 2 2 2 3+ [18]. 2 → F7/2 and D3/2 → F5/2 transitions of Ce 3.3. Display performance and aging The remote phosphor-conversion method reportedly has a higher luminance efficiency than conventional phosphor dispensing method when applied to individual LED packages [19,20]. Hence to start with, the EL spectrum of regularly assembled WLED lamp package composed of the same blue chip and differently treated YAG phosphor film, and their visible emission colors around white light zone are shown in Table 1 and Fig. 6. The test phosphor films before and after thermal aging or ultraviolet aging are encapsulated into 1 W lamp beads, inside which slices of different samples are pressed on the blue chips with particle face down. Except for thermally aged samples, which are measured at 2.8 V because of signal overflow in blue emission bands, other samples are measured at 3.0 V. When the blue light with wavelength of 455 nm is radiated from LED chip to excite the phosphor film, some of incident blue light is converted
Table 1 Optical characteristics of assembled WLED using different YAG phosphor film. Samples
Mean Tc (K)
Mean CRI (Ra)
Mean luminance efficiency (lm/W)
CIE chromaticity coordinates
Light decay (%)
Untreated Thermal aging for 170 h at 200 °C UV aging for 170 h at 272 nm
5480 10,796 5122
68.4 81 66.9
121.7 65.4 70.2
(0.333, 0.401) (0.275, 0.285) (0.347, 0.428)
null 46.26 42.32
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Fig. 4. UV–vis-near IR absorption spectra of prepared YAG phosphor film and raw YAG phosphor.
into yellow light, the rest is diffused, and finally these rays are mixed to produce white light, as presented in Fig. 6a. The CIE chromaticity coordinates corresponding to the electroluminescence of fabricated WLED with untreated film (0.333, 0.401) is close to pure white light (0.330, 0.330) according to the 1931 CIE coordinate diagram [21–23]. Furthermore, the mean Tc of untreated test sample is 5480 K, mean CRI (Ra) is 68.4, and the mean luminance efficiency is 121.7 lm/W. The emission intensity of YAG phosphor film after ultraviolet aging for 170 h, by contrast, decreases by half owing to light loss caused by ultraviolet irradiation. For UV aged samples, the Tc of 5122 K and CRI of 66.9 show a little changes and the average power efficiency is reduced to 70.2 lm/W. Actually, the emission of UV aged WLED is warmer white light since the relative contribution to white light of emitted yellow light increased, as shown in the inset photograph of Fig. 6c and d.
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The emission intensity for thermally aged sample driven at 2.8 V is significantly reduced, and the corresponding assembled WLED exhibits blue-white emission with higher Tc of 10,796 K and CRI of 81, the average luminance efficiency drops to 65.4 lm/W. This means that the light decay is up to 46.26% after aged under 200 °C, though the junction temperature of LED is generally less than 150 °C. Accordingly, the CIE chromaticity coordinates (0.275, 0.285) is closer to the blue light region as a result of lower conversion efficiency for thermally aged YAG phosphor [Fig. 6b and d]. These results demonstrate that the fabricated WLED with untreated YAG phosphor film shows moderate Tc value, qualified luminance efficiency and lower CRI value, which are similar to those for UV aged sample because organic silicon has a certain resistance against UV except for decreased lumen efficiency. At the same time, thermal aging can lead to greatly increased proportion of blue emission radiated from LED and higher Tc value and CRI value, and significantly reduced luminous efficiency. In other words, the proposed YAG phosphor film has good ultraviolet resistance but poor thermostability. The luminous flux emitted from finished products could be minimum while the distance between phosphor and chip surface is zero, so a very simple remote phosphor LED module without reflector cup is built with the air gap of 10 mm around and composed of 80 W high-power blue COB and proposed yellow phosphor film. Blue light derived from COB LED driven at 20 V coupled with yellow emission of remote YAG phosphor layer yields milder warm white light, which shows average Tc of 4900 K, CRI of 70 and the mean luminance efficiency of 77 lm/W (for naked blue COB is only 15–18.75 lm/W). Obviously, the luminous efficiency of a remote phosphor configuration is strongly influenced by the value of the air gap between remote phosphor layer and blue chip, that is, the luminance efficiency will reach a maximum only when the air gap is appropriate. In regard to CRI, change in air gap would not make a difference since the phosphor conversion efficiency
Fig. 5. PLE and PL spectra of raw YAG phosphor (a), YAG phosphor film(b) and the CIE chromaticity coordinates (c) for raw YAG phosphor (1) and YAG phosphor film(2). (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
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Fig. 6. Electroluminescence (EL) spectrum of assembled WLED with different YAG phosphor film (a) untreated sample, (b) sample after thermal aging at 200 °C for 170 h, (c) sample after UV aging for 170 h and (d) shows the corresponding CIE chromaticity coordinates. The inset photographs are the WLED lamp package. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
depends on the energy of excitation wavelength not on the light intensity. 3.4. Tensile testing Further investigation was carried out to understand the mechanical properties of prepared flexible YAG phosphor film. Tensile testing result (Fig. 7) reveals that the loaded force increases gradually along with time up to around 6.5 N—the fracture strength of the phosphor film, and then it declines until the test sample is completely separated into two pieces. Obviously, the fracture of prepared YAG phosphor film is brittle fracture at low stress since there are no yield point and plastic deformation occurring throughout the fracturing process. The reason for poor plasticity is clear that the stress concentration under force is created by each YAG phosphor particle with average diameter of about 10 μm dispersed in PDMS, and the generation and expansion of cracks are intensified so that the brittleness of phosphor film increases greatly, even though the PDMS is a transparent elastomer.
CRI were 4900 K and 70, respectively. N-hexane had been shown to disperse the YAG phosphor particles efficiently. Additionally, the proposed method was also applicable to preparation for other inorganic particle layer. However, the luminous efficiency of the prepared plane remote phosphor configuration for high-power COB, the toughness of the proposed YAG phosphor film were not ideal, which need to be improved by means of adjusting the distance between YAG phosphor film and
Conclusions The YAG phosphor film for blue chip-based white LED was produced by a simple and pollution-free method. The proposed phosphor film not only retains the optical properties of raw YAG phosphor, but also presents good UV resistance. Furthermore, the corresponding assembled WLED yielded moderate Tc value of 5480 K, CRI value of 68.4 and luminance efficiency of 121.7 lm/W when yellow phosphor film was in direct contact with blue chip. While proposed film was combined with 80 W high-power blue COB remotely with a 10 mm air gap, the luminous efficiency measured was only 77 lm/W, and the value of Tc and
Fig. 7. Tensile curve of YAG remote phosphor film.
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blue chips, and altering the proportion of YAG phosphor in silicone respectively in future work. Acknowledgements This work was funded by the Program for International Science &Technology Cooperation Program of China (2011DFA52290, 2012DFR50460), National Natural Science Funds (21471111), Shanxi Provincial Key Innovative Research Team in Science and Technology (2015013002-10), Natural Science Foundation of Shanxi Province (2013011013-2), and Program for Science and Technology Development of Shanxi (20140321012-01), project of Excellent Scientist Fund in Inner Mongolia. References [1] E. Fred Schubert, Light-Emitting Diodes, second ed. Cambridge University Press, Cambridge, 2006. [2] R.V. Steele, The story of a new light source, Nat. Photonics 1 (2006) 25–26. [3] X. Guo, G.D. Shen, B.L. Guan, et al., Cascade single-chip phosphor-free white lightemitting diodes, Appl. Phys. Lett. 92 (2008) 013507. [4] L.W. Ji, Y.K. Su, S.J. Chang, et al., InGaN/GaN multi-quantum dot light-emitting diodes, J. Cryst. Growth 263 (2004) 114–118. [5] R.Z. Liang, M. Wei, et al., Quantum dots-based flexible films and their application as the phosphor in white light-emitting diodes, Chem. Mater. 26 (2014) 2595–2600. [6] Joong-yeon Cho, Sang-Jun Park, et al., Nanopatterned yttrium aluminum garnet phosphor incorporated film for high-brightness GaN-based white light emitting diodes, Thin Solid Films 570 (2014) 326–329. [7] K. Yamada, Y. Imai, K. Ishii, Optical simulation of light source devices composed of blue LEDs and YAG phosphor, J. Light. Vis. Environ. 27 (2003) 70–74. [8] N. Narendran, Y. Gu, J.P. Freyssinier-Nova, et al., Extracting phosphor-scattered photons to improve white LED efficiency, Phys. Status Solidi A 202 (2005) 60–62.
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