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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 17, Iss. 26 — Dec. 21, 2009
  • pp: 23530–23535
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Phosphor converted LEDs with omni-directional-reflector coating

Jim-Yong Chi, Ji-Siao Chen, Chuan-Yu Liu, Cheng-wen Chu, and Kuo-Hsien Chiang  »View Author Affiliations


Optics Express, Vol. 17, Issue 26, pp. 23530-23535 (2009)
http://dx.doi.org/10.1364/OE.17.023530


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Abstract

A packaging scheme utilizing omni-directional reflective (ODR) optical coating is described to enhance the light extraction of near UV excited, phosphor-converted LEDs. A simple 1D model was developed to analyze the spectra of the extracted light measured with an integration-sphere as a function of phosphor layer concentration and thickness. Quantitative determination of the absorption coefficients at the pump and fluorescent light wavelength along with the conversion coefficient of phosphors were obtained. The reflection of the ODR film and the back reflector are also characterized. These parameters are then used for efficiency optimization of the present packaging scheme. A maximum enhancement of 40% can be expected with the materials and the configuration used in the present work.

© 2009 OSA

1. Introduction

High efficiency and high color rendering index of LEDs are the key performance indices to realize new applications from display to lighting. For the present, white light generation through color mixing of phosphor converted light is the most promising approach and efficiency surpassing the fluorescent light has been reported using blue LEDs to excite YAG:Ce phosphor [1

1. Y. Narukawa, M. Sano, M. Ichikawa, S. Minato, T. Sakamoto, T. Yamada, and T. Mukai, “Improvement of Luminous Efficiency in White Light Emitting Diodes by Reducing a Forward-bias Voltage,” Jpn. J. Appl. Phys.46, Part 2, 36–40 (2007). [CrossRef]

]. The approach based on UV excitation of red, green, and blue phosphors [2

2. J. S. Kim, P. E. Jeon, J. C. Choi, H. L. Park, S. I. Mho, and G. C. Kim, “Warm-white-light emitting diode utilizing a single-phase full-color Ba3MgSi2O8:Eu2+, Mn2+ phosphor,” Appl. Phys. Lett. 84(15), 2931 (2004). [CrossRef]

5

5. Y.-D. Huh, J.-H. Shim, Y. Kim, and Y. R. Do, “Optical properties of three-band white light emitting diodes,” J. Electrochem. Soc. 150(2), H57 (2003). [CrossRef]

], however, is hampered by the low phosphor conversion efficiency at the near UV wavelength range, despite the advantage of better color rendering index and higher power and efficiency around 400 nm [6

6. M. H. Crawford, “LEDs for Solid-State Lighting: Performance Challenges and Recent Advances,” IEEE J Sel. Top. Quantum Electron. 15, 1026 (2009).

]. We have reported a scheme to resolve this problem with omni-directional reflective (ODR) optical coating to recycle the UV light. Monte Carlo simulation was performed based on a theoretical phosphors scattering model [7

7. C. C. Chang, R. Chern, C. C. Chang, C. Chu, J. Y. Chi, J. Su, I. Chan, and J. T. Wang, “Monte Carlo Simulation of Optical Properties of Phosphor-Screened Ultraviolet Light in a White Light-Emitting Device,” Jpn. J. Appl. Phys. 44(8), 6056–6061 (2005). [CrossRef]

]. A significant enhancement in efficiency is simulated. In the present work, experiment was performed to demonstrate the improvement using the ODR film. A phenomenological one dimensional model following reference 8 is constructed to analyze the measured results [8

8. E.-Y. Kang, E. Wu, and D.-M. Wang, “Modeling white light-emitting diodes with phosphor layers,” Appl. Phys. Lett. 89(23), 231102 (2006). [CrossRef]

]. The absorption coefficients and conversion coefficient of the phosphor can be quantitatively determined using this simple approach. This model is then used to predict the optimal realistic performance using ODR with the determined coefficients. An enhancement of more than 40% is predicted by optimizing the thickness and concentration of the phosphor layer.

2. Sample preparing and experimental set up

To investigate the effect of the ODR films, a specially designed sample holder as shown in Fig. 1(c)
Fig. 1 The transmission spectra of the ODR film for the p-polarisation (a) and s-polarisation(b) at various incident angles used in the present work; (c) the schematics of the sample holds used for the present work; (d) the emission spectra for the UV excited LED with (dashed line)and without (solid) the ODR film. P1(P2) is the integrated intensity for the area shorter (longer) than 430nm.
is employed. The LED is mounted at the bottom of the holder and consists of nine GaN UV LED dies connected in series. The emission wavelength is around 395 nm with the power 24.4 mW at 20mA. The inside surface of the holder is coated with metallic reflector to reflect the back propagating light which can be blocked with a black paper mask that covers most of the surface except a small aperture around the dies. On top of the holder a glass piece coated with phosphor layers and/or ODR film can be placed. The phosphor layer is made with silicon resin (GE Plastics RTV615) mixed with green phosphors (Kasei Optonics, KX671). The thickness of the layer can be precisely controlled with a metal molding. An air gap is present between the die surface and the phosphor layer which is placed such that the phosphor layer is facing the dies. On the outer surface of the glass piece, an optical film with omni-directional reflective properties may be coated. The transmission spectra for the ODR film is such that the near UV light emitted by the LED is reflected for all angles and polarizations inside the escape cone and the visible light from the phosphor florescence can pass through with high transmission. For the present study, 40 pairs of SiO2 and Nb2O5 multi-layers are used to obtain high reflectivity (>90%) for incident angles smaller than 45 degree and for both s- and p- polarizations at the around 400nm. For the visible light, the ODR film is transparent (>90%) as shown in Figs. 1(a) and 1(b). Such a reflective property will reflect back most of the incident UV light in the escape cone regardless of the incident angle and polarization, hence the name of omni-directional reflection and will let the visible light in the escape cone pass through the film with high transmission.

To study the properties of the ODR film, phosphor layers with varying concentration and thickness are made on the glass piece coated with the ODR film on the other side. For comparison, the same phosphor layers were also made on glass pieces without the ODR film. The spectra of the output light with the current through UV die set at 20 mA are measured with an integration sphere. A typical spectrum of the output light contains two well separated peaks as shown in Fig. 1(d). The primary power (P1) and the secondary power (P2) were determined by integrating the intensity of the spectrum above or below 430nm as indicated in the figure. Light output of the phosphor layer with six thickness and six phosphor concentrations with and without ODR films are measured. The same films are then measured again by blocking the reflection from the package using the black paper mask.

3. Data analysis

To analyze the data, a phenomenological one dimensional model was constructed. This 1D model extends the assumption of ref. 8 to include the effect of the ODR films. As shown in Fig. 2(a)
Fig. 2 (a) Schematics showing the parameters used in the 1D model. (b) Transmitted UV power vs. thickness and the phosphors concentration measured with an integration sphere. The corresponding concentrations are 1,3, 5, 10, 15, and 20wt% from the top line to the bottom.
, the primary and the secondary light that are traveling in the + z and -z directions are described by the following equation,
dP1+/dz =-α1P1+
(1)
dP1-/dz =  α1P1-
(2)
dP2+/dz =-α2P2++βP1+/2+βP1-/2
(3)
dP2-/dz = α2P2 --βP1+/2-βP1-/2
(4)
where P1+ and P1- are the primary light that travel in the + and – z directions, while P2+ and P2-are that due to the secondary light.

At the bottom of the phosphor layer z = 0, the λ1 light, the light intensity generated by the LED chip reaching the phosphor layer, can be represented byP1+(0)=I, the intensity of the LED light. For the λ2 light, the back travelled light P2- will leave the phosphors layer and be reflected back by the back reflector formed by the shinny surface inside the package. In the spirit of the present 1D approximation, this extra λ2 light due to the back reflection can be represented by the boundary condition P2+(0)=r2P2-(0)  ,where r2 is the reflectivity to be determined by analyzing the measurement data.

At the top of the phosphor layer z = h, theλ1light will be reflected back to the phosphor layer if ODR is present. This is described byP1-(h)=r1P1+(h), where r 1 is the reflectivity of the ODR film at λ1. At the λ2, the reflectivity is very low and is approximated by the boundary conditionP2-(h)=0.

The above differential equation with the boundary conditions can be solved by assuming the solution of the following form.
P1+(z)=A1+exp(-α1z)
(5)
P1-(z)= A1-exp(α1z)
(6)
P2+(z)=A2+exp(-α2z)+B2+exp(-α1z+C2+exp(α1z)
(7)
P2(z)=A21exp(α2z)+B2exp(-α1z+C2exp(α1z)
(8)
Substitute Eq. (5) through (8) into Eq. (1) through (4), the coefficients A’s B’s and C’s can be readily determined by matching coefficients of the exponential terms.

4. Results and discussions

The results of the material parameters determined by the data analysis are summarized in Fig. 4(a)
Fig. 4 (a)The absorption(α1 and α2) and conversion coefficients (β) determined from the measured data; (b) reflectivity parameters (r1 and r2) due to the ODR films and the back reflectors.
for coefficients α1, α2, and β and Fig. 4(b) for r1 and r2. It can be seen that the both the absorption of the primary and the secondary light increase with the phosphor concentration and so does β, in agreement with the expected trend. It can also be seen that the coefficients tend to saturate at high concentration. This trend can be accurately fitted with a second order polynomial to allow further analysis.

The reflection coefficient due to the ODR film, r1, is close to one for most of the range except at the high concentration when most of the near UV light is absorbed in the layer to reach the surface. This high value of r1 determined from the data corresponds well with the expected effect of the ODR film. For r2, a value larger than one is obtained, indicating a significant portion of the light propagating backward into the package and reflected back by the metallic reflector on the inner surfaces. The r2 parameter also provides a quantitative characterization of the back reflection.

The parameters determined above along with the analytical solution can be utilized to find the optimized condition for the maximum efficiency. Figure 5
Fig. 5 The equal power contours based on the model and the material constants shown in Fig. 4 with the reflection coefficients are r1 = 1.0 and r2 = 2.0. The unit for the contour is mW. Part (a) corresponds to the case without ODR; Part (b) corresponds to the case with ODR. The thick red line in (b) is the contour with the maximum power value of the part (a). It represents the region of enhancement by ODR. The x’s in (a) and (b) mark the location of the maxima. The inset in (b) is a magnified view of the region shown in the dashed box.
shows the calculated efficiency contours for visible light output using the parameters shown in Fig. 4(a) for the case without the ODR (Fig. 5(a)) and with the ODR (Fig. 5(b)). The reflectivity used to obtain Fig. 5(a) is r1 = 0. For that with the ODR cover (Fig. 5(b)), r1 = 1 is used. The r2 for both structures is set to 2 to take into account that the back reflection can be further improved from the present configuration. From the calculation, the maximum occurs at 13.0 and 18.2 mW respectively for the ordinary structure without the ODR cover and the ODR covered structure. The improvement is 39.7%. The optimal concentration occurs at 5 wt% for the ODR covered structure, lower than the 7% for the ordinary structure. The region of enhancement, i.e., the region with p2 value above the maxima for the case without the ODR films, extends to the lower phosphor concentration and thinner thickness sides as indicated in the region encircled by the thick red line in Fig. 5(b). This observation indicates that the ODR covered structure can increase the visible power out put with lower concentration to lessen the deleterious effects that are normally encountered at high phosphor concentration. More phosphors can be more efficiently incorporated for color mixing which are needed for white light generation.

In summary, an enhancement scheme using optical coating to improve the efficiency of UV-excited, phosphors converted LEDs is presented. A one dimensional modeling is used to determine the absorption and conversion coefficients.

Using these parameters, it is determined that the optimal enhancement due to ODR is 40% over that without the ODR films. The optimal concentration can also be lower which is beneficial for lessening the deleterious effects at high concentration especially for the white light generation when more phosphors need to be added for color mixing.

Acknowledgement

We are grateful to Prof. Y. S. Liu for constant encouragement, to the optical coating group at ITRI to provide the ODR film for this study, to Ms. Chih-Hsuan Liu, Chiu-Ling Chen for their experimental assistance, and to Drs. Jih-Fu Wang, Ming-Te Lin, Wen-Yung Ye, Fei-Chang Hwang for fruitful discussions. This work is supported by ITRI Advanced Technology Program and National Science Foundation of Taiwan.

References and links

1.

Y. Narukawa, M. Sano, M. Ichikawa, S. Minato, T. Sakamoto, T. Yamada, and T. Mukai, “Improvement of Luminous Efficiency in White Light Emitting Diodes by Reducing a Forward-bias Voltage,” Jpn. J. Appl. Phys.46, Part 2, 36–40 (2007). [CrossRef]

2.

J. S. Kim, P. E. Jeon, J. C. Choi, H. L. Park, S. I. Mho, and G. C. Kim, “Warm-white-light emitting diode utilizing a single-phase full-color Ba3MgSi2O8:Eu2+, Mn2+ phosphor,” Appl. Phys. Lett. 84(15), 2931 (2004). [CrossRef]

3.

C.-H. Kuo, J.-K. Sheu, S.-J. Chang, Y.-K. Su, L.-W. Wu, J.-M. Tsai, C. H. Liu, and R. K. Wu, “n-UV+Blue/Green /Red White Light Emitting Diode Lamps,” Jpn. J. Appl. Phys. 42,(Part 1,No. 4B), 2284–2287 (2003). [CrossRef]

4.

J. K. Park, M. A. Lim, C. H. Kim, H. D. Park, J. T. Park, and S. Y. Choi, “White light-emitting diodes of GaN-based Sr2SiO4: Eu and the luminescent properties,” Appl. Phys. Lett. 82(5), 683 (2003). [CrossRef]

5.

Y.-D. Huh, J.-H. Shim, Y. Kim, and Y. R. Do, “Optical properties of three-band white light emitting diodes,” J. Electrochem. Soc. 150(2), H57 (2003). [CrossRef]

6.

M. H. Crawford, “LEDs for Solid-State Lighting: Performance Challenges and Recent Advances,” IEEE J Sel. Top. Quantum Electron. 15, 1026 (2009).

7.

C. C. Chang, R. Chern, C. C. Chang, C. Chu, J. Y. Chi, J. Su, I. Chan, and J. T. Wang, “Monte Carlo Simulation of Optical Properties of Phosphor-Screened Ultraviolet Light in a White Light-Emitting Device,” Jpn. J. Appl. Phys. 44(8), 6056–6061 (2005). [CrossRef]

8.

E.-Y. Kang, E. Wu, and D.-M. Wang, “Modeling white light-emitting diodes with phosphor layers,” Appl. Phys. Lett. 89(23), 231102 (2006). [CrossRef]

OCIS Codes
(230.3670) Optical devices : Light-emitting diodes
(230.5298) Optical devices : Photonic crystals

ToC Category:
Optical Devices

History
Original Manuscript: October 9, 2009
Revised Manuscript: November 16, 2009
Manuscript Accepted: November 18, 2009
Published: December 8, 2009

Citation
Jim-Yong Chi, Ji-Siao Chen, Chuan-Yu Liu, Cheng-wen Chu, and Kuo-Hsien Chiang, "Phosphor converted LEDs with omni-directional-reflector coating," Opt. Express 17, 23530-23535 (2009)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-17-26-23530


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References

  1. Y. Narukawa, M. Sano, M. Ichikawa, S. Minato, T. Sakamoto, T. Yamada, and T. Mukai, “Improvement of Luminous Efficiency in White Light Emitting Diodes by Reducing a Forward-bias Voltage,” Jpn. J. Appl. Phys. 46, Part 2, 36–40 (2007). [CrossRef]
  2. J. S. Kim, P. E. Jeon, J. C. Choi, H. L. Park, S. I. Mho, and G. C. Kim, “Warm-white-light emitting diode utilizing a single-phase full-color Ba3MgSi2O8:Eu2+, Mn2+ phosphor,” Appl. Phys. Lett. 84(15), 2931 (2004). [CrossRef]
  3. C.-H. Kuo, J.-K. Sheu, S.-J. Chang, Y.-K. Su, L.-W. Wu, J.-M. Tsai, C. H. Liu, and R. K. Wu, “n-UV+Blue/Green /Red White Light Emitting Diode Lamps,” Jpn. J. Appl. Phys. 42,(Part 1,No. 4B), 2284–2287 (2003). [CrossRef]
  4. J. K. Park, M. A. Lim, C. H. Kim, H. D. Park, J. T. Park, and S. Y. Choi, “White light-emitting diodes of GaN-based Sr2SiO4: Eu and the luminescent properties,” Appl. Phys. Lett. 82(5), 683 (2003). [CrossRef]
  5. Y.-D. Huh, J.-H. Shim, Y. Kim, and Y. R. Do, “Optical properties of three-band white light emitting diodes,” J. Electrochem. Soc. 150(2), H57 (2003). [CrossRef]
  6. M. H. Crawford, “LEDs for Solid-State Lighting: Performance Challenges and Recent Advances,” IEEE J Sel. Top. Quantum Electron. 15, 1026 (2009).
  7. C. C. Chang, R. Chern, C. C. Chang, C. Chu, J. Y. Chi, J. Su, I. Chan, and J. T. Wang, “Monte Carlo Simulation of Optical Properties of Phosphor-Screened Ultraviolet Light in a White Light-Emitting Device,” Jpn. J. Appl. Phys. 44(8), 6056–6061 (2005). [CrossRef]
  8. E.-Y. Kang, E. Wu, and D.-M. Wang, “Modeling white light-emitting diodes with phosphor layers,” Appl. Phys. Lett. 89(23), 231102 (2006). [CrossRef]

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