OSA's Digital Library

Energy Express

Energy Express

  • Editor: Bernard Kippelen
  • Vol. 18, Iss. S2 — Jun. 21, 2010
  • pp: A201–A206
« Show journal navigation

Conformal phosphor coating using pulsed spray to reduce color deviation of white LEDs

Hsin-Tao Huang, Chuang-Chuang Tsai, and Yi-Pai Huang  »View Author Affiliations


Optics Express, Vol. 18, Issue S2, pp. A201-A206 (2010)
http://dx.doi.org/10.1364/OE.18.00A201


View Full Text Article

Acrobat PDF (17413 KB)





Browse Journals / Lookup Meetings

Browse by Journal and Year


   


Lookup Conference Papers

Close Browse Journals / Lookup Meetings

Article Tools

Share
Citations

Abstract

This work presents a novel “pulsed spray (PS)” process for the coating of yellow YAG:Ce3+ phosphor on blue InGaN-based light emitting diodes (LEDs). To coat a phosphor layer of high quality on an LED chip surface, the PS approach is used and studied because of the uniform color distribution, providing a wide range of color temperatures. This PS coating approach applies phosphor by exploiting mechanical principles without risk of chemical pollution. Additionally, it can be applied to wire-bonded LEDs and an array of LED chips on a substrate to fabricate a large-area, planar illumination system of high optical quality, which is easy to manufacture.

© 2010 OSA

1. Introduction

2. Experiments

Currently, most wLEDs are formed by the two-color light mixing of blue (LEDs excitation) and yellow (YAG:Ce3+ phosphor emission) wavelengths. In this study, the conventional dispensing method [Fig. 1(a)] is compared with the PS method [Fig. 1(b)] of forming a phosphor coating. The phosphor powder is blended with a silicone binder and an alkyl-based solvent to form phosphor suspension slurry for use in both described coating processes. The dispensing and PS approaches have different fluid requirements, and therefore different binder/solvent ratios. However, the phosphor powder that was used in this experiment was YAG:Ce3+, which has a broadband wavelength distribution (490~680 nm) to ensure favorable color rendering. Ninety percent of the phosphor used in this experiment had a diameter (D90) of less than 13.27 μm [Figs. 2(a)
Fig. 2 (a) The spectral properties of conformal phosphor coating using pulsed spray; (b) SEM morphology of YAG:Ce3+ phosphor.
and 2(b)]. An indium gallium nitride (InGaN)-based blue LED, which emits at a wavelength of 457 nm, was used to excite yellow phosphor.

The PS parameters are studied to optimize the coating conditions. The velocity of the spray, the amount of phosphor in the slurry (phosphor loading), the ejected air pressure and the spraying interval (pulsed spraying frequency) were controlled. The suspended slurry was prepared and disturbed mechanically to prevent the phosphor from settling before it was sprayed out. Then, the slurry was poured into the coating system to be atomized, and sprayed at a pulsing frequency of 5-10 Hz onto the surfaces of bare LED chips.

In this study, a lighting system was fabricated using a novel PS approach to yield an uniform CPC on the LEDs surfaces. As shown in Fig. 3(a)
Fig. 3 Phosphor coating using PS technique. (a) An array of blue LED chips on board (COB); (b) the magnification of individual LED.
, blue LED chips were mounted on a 4-inch aluminum alloy substrate using the COB process, and then yellow phosphor was coated on them to cover the whole perimeter of the LED with an uniform thickness layer. The advantages of CPC using the PS approach will be discussed below.

3. Results and discussion

The phosphor coating using dispensing method and PS method are both uncomplicated and involves no chemical reaction. In the following, the color distribution, the color temperature and the other optical performance parameters will be evaluated for these two methods and compared.

3.1 Color distribution

The color deviation of a lighting system, Δu′v′ [Eq. (3)], is evaluated as follows:
ui=4xi/(-2xi+12yi+3),ŠΔu'i=1-Δu'i=2,3...
(1)
vi=9yi/(-2xi+12yi+3),ŠΔv'i=1-Δv'i=2,3...
(2)
Δuv=((Δu)2+(Δv)2)1/2
(3)
where xi, yi denote the chromaticity coordinates in the CIE 1931 diagram and ui′, vi′ denote the chromaticity coordinates in the CIE 1976 diagram [Eqs. (1) and (2)]. For dispensing method, the suspended phosphor is easily delaminated by gravity before the silicone binder is cured, causing color deviation at various viewing angles [Fig. 4(a)
Fig. 4 CIE 1976 chromaticity indices versus angular distribution of LED when the light is emitted at angles from −60° to + 60°. (a) Conventional dispensing; (b) conformal coating using PS.
]. LED packages must therefore be sorted and when necessary binned before use, resulting in high production costs. The experimental Δu′v′ value reveals that the PS technique yields a much greater spatial color uniformity [Fig. 4(b), Δu′v′ of 0.07] than does the dispensing method (Δu′v′ of 0.23) when the light is emitted at angles from −60° to + 60°. Therefore, PS technique allows the accurate control of color distribution for high-quality lighting applications. Table 1

Table 1. Optical comparison between conventional dispensing and CPC by PSa

table-icon
View This Table
summarizes the comparison of optical data regarding conventional dispensing method and CPC by PS technique. When controlling Tc of experiment around 9500 K, CPC by PS reveals accurate color distribution and higher luminous efficacy (8% improvement).

3.2 Range of color temperature

Different applications have different Tc requirements. TFT-LCD TV applications require illumination at a high color temperature (Tc> 9000 K). However, general lighting has a Tc range of 3000 K (warm white) to 5000 K (cool white). This work studies the dependence of Tc for white light on phosphor loading (mg/cm2) (Fig. 5
Fig. 5 Relationship between phosphor loading (mg/cm2) and color temperature (Tc).
). The experimental results reveal that wLEDs based on the PS technique have a Tc value that is inversely proportional to the phosphor loading. Whereas a low Tc is achieved at high phosphor loading (point C), a high Tc is achieved at low phosphor loading (point A), and is associated with strongly non-uniform color distribution [7

7. N. T. Tran and F. G. Shi, “Studies of Phosphor Concentration and Thickness for Phosphor-Based White Light-Emitting-Diodes,” J. Lightwave Technol. 26(21), 3556–3559 (2008). [CrossRef]

]. Therefore, the color distribution of wLEDs at high Tc was explored. As shown in Fig. 6(a)
Fig. 6 CIE 1931 chromaticity indices versus angular distribution of LED at Tc centre around 9500 K. (a) Conventional dispensing; (b) conformal coating using PS.
, the conventional dispensing approach cannot easily ensure color uniformity with a Tc at 9533 K. The chromaticity indices display a severe variation with the viewing angle from −60° to + 60°. When the PS approach is used at a phosphor loading of 1.20 mg/cm2, the resulted Tc is 9519 K [Fig. 6(b)] with a harmonic chromaticity distribution. When the spraying interval in PS is varied to control the amount of deposited phosphor, a thin and uniform phosphor layer can be coated on the surface of the LED chips. In this case, when a higher percentage of yellow light (complementary wavelength) is mixed in, the Tc is shifted toward the yellow side (low Tc). Figures 5 and 7
Fig. 7 The CIE 1931 chromaticity diagram. Red triangle denotes the white point with different color temperature (Tc).
reveal that a Tc of 5100 K (cool white, point B), a Tc of 4174 K (warm white, point C) and a Tc of 9519 K (high color temperature, point A) can be achieved using the PS approach.

3.3 Environmentally friendly process

By EPD or self-exposure approaches, LED chips must be flip-chip mounted with a flat emitting surface [6

6. B. Hou, H. Rao, and J. Li, “Methods of Increasing Luminous Efficiency of Phosphor-Converted LED Realized by Conformal Phosphor Coating,” J. Display Technol. 5(2), 57–60 (2009). [CrossRef]

]; the top of the LED chips must be free of wire bond pads. In contrast, LED chips that are coated by PS need not have a flat emitting surface, as shown in Fig. 3(b). Although the surface of the diode is still connected using gold wires to an electrode pad, PS can be applied uniformly to the surface of LEDs. Figure 8(a)
Fig. 8 SEM micrographs of phosphor coating using PS. (a) Top view; (b) cross-sectional view; (c) illuminated lighting module with phosphor coated by PS.
presents a top view of a PS coated LED with phosphor particles in the silicone binder. Figure 8(b) presents a cross-sectional view with an uniform mean coating thickness of 45 μm and the left side of Fig. 8(c) is an illuminated demonstration of LEDs array on board coated by PS technique. The right side of Fig. 8(c) is CPC covered with another diffuser plate to get a planar illumination.

4. Conclusions

This study proposes a novel pulsed spray (PS) method of conformal phosphor coating (CPC) for LED illumination applications. The phosphor is applied directly to LED chips of the wire-bonded type. This approach is also applied to an array of blue LED chips as an inexpensive method with a high manufacturing throughput. The PS yields LEDs with superior optical characteristics including low color deviation (Δu′v′ of 0.07) when observed from various illumination angles (−60° to + 60°). Additionally, PS can yield a wide range of color temperatures (Tc) from 2500 K to 9500 K with high color accuracy, supporting a diversity of TFT-LCD backlights and general lighting applications. Meanwhile, unlike other coating techniques, such as dispensing, the settling method, spin coating, self-exposure and electrophoretic deposition (EPD), PS is a mechanical spray-based, environmentally friendly method that does not cause harmful ion pollution by chemical reactions. This study successfully demonstrates CPC by PS for future wLEDs lighting systems.

Acknowledgments

The authors would like to thank the KISmart Corporation (Hsinchu, Taiwan) for assisting in the conformal coating by pulsed spray (PS) technique and preparing the samples.

References and links

1.

F. M. Steranka, J. Bhat, D. Collins, L. Cook, M. G. Craford, R. Fletcher, N. Gardner, P. Grillot, W. Goetz, M. Keuper, R. Khare, A. Kim, M. Krames, G. Harbers, M. Ludowise, P. S. Martin, M. Misra, G. Mueller, R. Mueller-Mach, S. Rudaz, Y.-C. Shen, D. Steigerwald, S. Stockman, S. Subramanya, T. Trottier, and J. J. Wierer, “High Power LEDs—Technology Status and Market Applications,” Phys. Stat. Solidi A 194(2), 380–388 (2002). [CrossRef]

2.

M. O. Holcomb, R. Mueller-Mach, G. Mueller, D. Collins, R. M. Fletcher, D. A. Steigerwald, S. Eberle, Y. K. Lim, P. S. Martin, and M. Krames, “The LED Lightbulb: Are we there yet? Progress and Challenges for Solid State Illumination,” Lasers and Electro-Optics, 2003 CLEO’03 Conference on 1–6 June 2003.

3.

Z. Liu, S. Liu, K. Wang, and X. Luo, “Optical Analysis of Color Distribution in White LEDs with Various Packaging Methods,” IEEE Photon. Technol. Lett. 20(24), 2027–2029 (2008). [CrossRef]

4.

H. T. Huang, C. C. Tsai, and Y. P. Huang, “A Direct-View Backlight With UV Excited Trichromatic Phosphor Conversion Film,” J. Display Technol. 6(4), 128–134 (2010). [CrossRef]

5.

J. H. Yum, S. Y. Seo, S. Lee, and Y. E. Sung, “Comparison of Y3Al5O12:Ce0.05 Phosphor Coating Methods for White Light Emitting Diode on Gallium Nitride,” Proc. SPIE 4445, 60–68 (2001). [CrossRef]

6.

B. Hou, H. Rao, and J. Li, “Methods of Increasing Luminous Efficiency of Phosphor-Converted LED Realized by Conformal Phosphor Coating,” J. Display Technol. 5(2), 57–60 (2009). [CrossRef]

7.

N. T. Tran and F. G. Shi, “Studies of Phosphor Concentration and Thickness for Phosphor-Based White Light-Emitting-Diodes,” J. Lightwave Technol. 26(21), 3556–3559 (2008). [CrossRef]

OCIS Codes
(230.3670) Optical devices : Light-emitting diodes
(350.4600) Other areas of optics : Optical engineering

ToC Category:
Light-Emitting Diodes

History
Original Manuscript: May 10, 2010
Revised Manuscript: June 3, 2010
Manuscript Accepted: June 3, 2010
Published: June 7, 2010

Citation
Hsin-Tao Huang, Chuang-Chuang Tsai, and Yi-Pai Huang, "Conformal phosphor coating using pulsed spray to reduce color deviation of white LEDs," Opt. Express 18, A201-A206 (2010)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-18-S2-A201


Sort:  Author  |  Year  |  Journal  |  Reset  

References

  1. F. M. Steranka, J. Bhat, D. Collins, L. Cook, M. G. Craford, R. Fletcher, N. Gardner, P. Grillot, W. Goetz, M. Keuper, R. Khare, A. Kim, M. Krames, G. Harbers, M. Ludowise, P. S. Martin, M. Misra, G. Mueller, R. Mueller-Mach, S. Rudaz, Y.-C. Shen, D. Steigerwald, S. Stockman, S. Subramanya, T. Trottier, and J. J. Wierer, “High Power LEDs—Technology Status and Market Applications,” Phys. Stat. Solidi A 194(2), 380–388 (2002). [CrossRef]
  2. M. O. Holcomb, R. Mueller-Mach, G. Mueller, D. Collins, R. M. Fletcher, D. A. Steigerwald, S. Eberle, Y. K. Lim, P. S. Martin, and M. Krames, “The LED Lightbulb: Are we there yet? Progress and Challenges for Solid State Illumination,” Lasers and Electro-Optics, 2003 CLEO’03 Conference on 1–6 June 2003.
  3. Z. Liu, S. Liu, K. Wang, and X. Luo, “Optical Analysis of Color Distribution in White LEDs with Various Packaging Methods,” IEEE Photon. Technol. Lett. 20(24), 2027–2029 (2008). [CrossRef]
  4. H. T. Huang, C. C. Tsai, and Y. P. Huang, “A Direct-View Backlight With UV Excited Trichromatic Phosphor Conversion Film,” J. Display Technol. 6(4), 128–134 (2010). [CrossRef]
  5. J. H. Yum, S. Y. Seo, S. Lee, and Y. E. Sung, “Comparison of Y3Al5O12:Ce0.05 Phosphor Coating Methods for White Light Emitting Diode on Gallium Nitride,” Proc. SPIE 4445, 60–68 (2001). [CrossRef]
  6. B. Hou, H. Rao, and J. Li, “Methods of Increasing Luminous Efficiency of Phosphor-Converted LED Realized by Conformal Phosphor Coating,” J. Display Technol. 5(2), 57–60 (2009). [CrossRef]
  7. N. T. Tran and F. G. Shi, “Studies of Phosphor Concentration and Thickness for Phosphor-Based White Light-Emitting-Diodes,” J. Lightwave Technol. 26(21), 3556–3559 (2008). [CrossRef]

Cited By

Alert me when this paper is cited

OSA is able to provide readers links to articles that cite this paper by participating in CrossRef's Cited-By Linking service. CrossRef includes content from more than 3000 publishers and societies. In addition to listing OSA journal articles that cite this paper, citing articles from other participating publishers will also be listed.


« Previous Article  |  Next Article »

OSA is a member of CrossRef.

CrossCheck Deposited