High-Efficiency White LED Packaging With Reduced ... - IEEE Xplore

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phosphor concentration can be reduced to meet the requested correlated color temperature. Index Terms—Light emitting diodes, phosphors, photon recycling.
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IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 25, NO. 7, APRIL 1, 2013

High-Efficiency White LED Packaging with Reduced Phosphor Concentration Ching-Yi Chen, Tsung-Hsun Yang, Chien-Hung Hsu, and Ching-Cherng Sun

Silicone Lens

Abstract— In this letter, important factors for white light emitting diodes (LED) packaging, including lens encapsulation, reflectivity of cavity reflectors, and phosphor concentration are studied. Simulation as well as the corresponding experiment shows that reflectivity of the reflectors in packaging cavity is more effective than lens encapsulation to reach high efficiency. Besides, a white LED packaging without lens encapsulation but with high reflectivity of cavity reflectors may increase interaction possibility between blue photons and the phosphor so that the phosphor concentration can be reduced to meet the requested correlated color temperature. Index Terms— Light emitting diodes, phosphors, photon recycling.

I. I NTRODUCTION

L

IGHT emitting diode (LED), as a solid-state light source, has been regarded as the most powerful light source in next-generation lighting, owing to its advantages such as high efficiency, saturated color, fast response, compact size, long life and potential environmental benefit. As a light source potentially applied to general lighting, LED should perform high-quality white light with high luminous efficacy and vivid color. Optics and color performance of a white LED are determined mainly by packaging, where conventionally a blue die is covered with yellow phosphor. One of the important issues in packaging is to build up a recipe related to phosphor including concentration and geometry to reach possible highest luminous efficacy as well as correct chromatic performance [1]–[4]. In past years, several studies reported possible key factors in white LED packaging, where Luo et al. studied the lens effect in light extraction of the packaging cavity with a reflective cup [5], Kim et al. proposed to roughen the cup surface to enhance scattering of blue and yellow rays and then enhances the output flux [6], Tran et al. presented a study of optimized output flux as a function of phosphor volume and correlated color temperature [7], Allen et al. proposed to insert

Manuscript received November 23, 2012; revised January 22, 2013; accepted February 11, 2013. Date of publication March 7, 2013; date of current version March 15, 2013. This work was supported in part by the National Central University’s Plan to Develop First-class Universities and Toplevel Research Centers under Grant 995939 and Grant 100G-903-2, and in part by the National Science Council of the Republic of China under Contract 99-2623-E-008-002-ET and Contract NSC100-3113-E-008-001. The authors are with the Institute of Lighting and Display Science and the Department of Optics and Photonics, National Central University, Jhongli 32001, Taiwan (e-mail: [email protected]; [email protected]; [email protected]; [email protected]. edu.tw). Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LPT.2013.2248141

Phosphor Metal Cup Blue Die Fig. 1.

Schematic diagram of the studied packaging structure.

an air gap between the blue die and the phosphor to enhance the light output [8], Oh et al. proposed to use a color filter to reflect blue ray back to the packaging cavity to enhance light conversion [9], and Moreno et al. discussed light extraction by an encapsulated lens [10]. Although various studies partially address the issue of light extraction or color presentation, an effective way to find a rule for a specific packaging geometry is desired. In this Letter, we present a study of the effects by phosphor concentration and photon recycle ability for both yellow and blue rays in a packaging cavity to find a way to extract most lights in the cavity and also meet the requested correlated color temperature (CCT). II. S PATIAL D ISTRIBUTION OF O PTICAL F LUX IN PACKAGING C AVITY The objective of the study is to figure out the effect associated with phosphor concentration, surface reflectivity and packaging geometry in the packaging cavity to reach high luminous efficacy as well as the desired CCT. Fig. 1 shows a common packaging geometry, where a blue LED die is attached on the bottom of a metal cup. The yellow phosphor, e.g., YAG, is dispensed in the cup to transfer part of blue photons to yellow photons. Besides, owing to light extraction enhancement, a hemi-sphere lens could be attached on the top surface of phosphor [10], [11]. Before the experimental verification with real packaging, we applied a precise phosphor model to simulate the blue and yellow rays in the packaging cavity. The model was developed with several important procedures, including to precisely simulate light scattering in the phosphor, to obtain absorption coefficient and conversion coefficient of the phosphor from the experimental measurement of output yellow and blue rays from phosphor plates with different thickness and concentration. Then Monte Carlo ray tracing with Mie

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CHEN et al.: WHITE LED PACKAGING WITH REDUCED PHOSPHOR CONCENTRATION

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(b) Fig. 2. Simulation of the blue and yellow rays hitting the bottom and side reflectors in the case of (a) without lens, and (b) with lens encapsulation.

scattering was applied to simulate the spatial distribution for both blue and yellow rays in the designed structure [12]. For a white LED, both blue and yellow rays encounter Mie scattering when they are propagated in the phosphor volume. The spatial distribution between yellow and blue rays, however, is inherently different since yellow rays are emitted by phosphor through spontaneous emission. Therefore, there are more backward yellow rays observed. The effect caused by such an inherent difference is an important subject in whiteLED packaging. Fig. 2 shows a simulation of incidence flux of the blue and yellow rays on the bottom and side reflectors for different phosphor concentrations and surface reflectivity. In the simulation, the emitting optical power of the blue die was set 1 W, the thickness of the blue die was set 4 µm and the absorption coefficient was 500 mm−1 [11], [13]. When a lens is encapsulated on the top surface of the phosphor, less backward blue and yellow rays hit the side surface or the bottom surface owing to better light extraction of the lens. Besides, Fig. 2 also shows that yellow rays hit the side surface and bottom reflector more intensively when the concentration of phosphor increases. It means that recycling of the yellow rays in the packaging cavity is important. Fig. 3 shows a simulation of output blue and yellow rays as a function of the reflectivity of the cavity reflectors (simplified RCR) and phosphor concentration, where the response by blue rays is different from that by yellow rays. For the output of blue rays, both phosphor concentration and surface reflectivity play important roles, but phosphor concentration is a more important factor for blue rays. In contrast, the output of yellow rays depends on surface reflectivity more

Fig. 3. Simulation or output flux for (a) blue and (b) yellow rays as functions of phosphor concentration and surface reflectivity.

than phosphor concentration. Fig. 3 can be used to clarify the role of phosphor concentration and the RCR in white LED packaging. From the viewpoint of blue rays, as the phosphor concentration increases, the decrease of blue rays is not caused by more backward scattering, but by more absorption by the phosphor. This is why increase of the RCR makes less impact as phosphor concentration. In contrast, increase of phosphor concentration means that more yellow rays are generated. If there is no effective way to extract the yellow rays from the packaging cavity, the yellow rays will be trapped and finally absorbed. Therefore, to increase the RCR becomes important to yellow rays, especially in the case of higher phosphor concentration. This is why we can find more than 400% enhancement in yellow ray output when the surface reflectivity is increased from 0% to 90%. III. H IGH O PTICAL E FFICIENCY W ITH A R EQUESTED CCT After clarifying the factors affecting the output of blue and yellow rays, we turn to figure out the key factors for obtain a correct CCT as well as high optical efficiency. We know the increase of phosphor concentration will cause decrease of the CCT owing to less blue rays but with more yellow rays in output flux. In the viewpoint of light extraction, both lens encapsulation (as shown in Fig. 2) and high RCR (as shown in Fig. 3) are effective ways in increasing optical efficiency [11], [13], and [14]. However, these two schemes have different characteristics. For lens encapsulation, more blue and yellow rays are extracted and few rays are propagated backward. Therefore, such an approach reduces the possibility for blue rays to hit the phosphor. In contrast, if there is no lens encapsulation, more blue and yellow rays will be scattered or

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more important role to extract yellow rays. Both experiment and simulation show that increasing the RCR may reach high light extraction even without lens encapsulation. Besides, since such an approach will result in more backward lights and then induces a lower CCT. Both the experiment and simulation show that less phosphor can perform a requested CCT without sacrificing the total output power. When the RCR increases from 50% to 100%, the phosphor concentration can be reduced to 65% in the case without lens encapsulation.

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Fig. 4. Simulation and the corresponding experimental measurement of light output and needed phosphor concentration as a function of surface reflectivity for CCT around 6500K.

emitted backward owing to less light extraction to the air by the lens. The interesting point is that the loss of light extraction could be compensated if the RCR in the packaging cavity is increased to enhance photon recycling. That is why the RCR plays an important role in such a situation. Besides, since more blue rays are propagated backward, phosphor can be hit by more blue rays and results in generation of more yellow rays and then reduce the CCT. In other words, such a way may enable to use less phosphor in performing a requested CCT because the phosphor is hit by blue rays more frequently. Fig. 4 shows simulation and corresponding experimental measurement for white LEDs of CCT around 6500K. It shows that as the RCR increases to as high as 90%, lens encapsulation makes less impact to light extraction. More important, the experiment and simulation confirm that less phosphor can perform a requested CCT without sacrificing the total output power. When the surface reflectivity increases from 50% to 100%, the phosphor concentration can be reduced to 65% in the case without lens encapsulation. IV. C ONCLUSION In summary, we have presented a study of packaging factors to obtain a correct CCT with high optical efficiency. The physics insight is originated from the inherent difference in light emission between blue and yellow rays. The output of blue rays mainly depends on phosphor concentration, but the output of yellow rays mainly depend on degree of photon recycling in packaging cavity, which includes the factors of the RCR and lens encapsulation. In regarding to such two important approaches, the study shows that the RCR plays a

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