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High efficiency phosphor-converted white light-emitting diodes with superior color uniformity have been investigated. It is proposed that the cymbal-shaped phosphor structure can improve the uniformity of the angle-dependent correlated color temperature (CCT) and also increase the luminous intensity compared to the conventional dispensing phosphor structure. In this experiment, we form the cymbal-shaped structure, which features a bump upon the central surface of the bottom layer, by employing an injection process after the dispensing coating. The upper bump phosphor layer not only enhances the extraction efficiency of lights, but also compensates the difference of the excitation optical path in the dispensing bottom layer between the normal-concentrated forward-scattered blue rays and those emitted with larger angles. This considerably eliminates the “blue center” phenomenon. The CCT deviation have been reduced from 315 K to 120 K using the cymbal-shaped phosphor coating method, and the light extraction efficiency (LEE) is enhanced by 8.5% compared with conventional dispensing phosphor-converted white LEDs. This new cymbal-shaped design was verified both experimentally and theoretically.
© 2015 Optical Society of America
1. Introduction
Highly efficient white light-emitting diodes (LEDs) have been well developed for solid-state lighting (SSL) sources [1]. In current mass production, the white LEDs are fabricated commonly by combining the blue LED-chips and yellow phosphor, in which the phosphor-converted yellow rays and the transmitted chip-emitted blue rays mix together to generate the white light emission. Hence, they are known as phosphor-converted white LEDs (pc-WLEDs).
In the generic pc-WLED configuration, the yellow-emitting phosphor powders are firstly mixed with transparent encapsulating resin (such as silicone) to form the phosphor slurry. The slurry is dispensed into a cup reflector with a mounted LED chip, and then baked in a chamber to harden the encapsulants. This direct phosphor dispensing method can offer high conversion efficiency and facilitates the manufacture, and thus has been widely used in mass productions.
However, due to the surface tension at the slurry-air interface, the encapsulant surface, i.e. the outer surface of phosphor conversion layer, is generally convex during the dispensing. This slightly curved surface causes the phosphor thickness inhomogeneous with respect to the surfaces of the LED chip, which increases the excitation optical path in larger emission angles. As a result, more down-converted yellow rays will be generated in the perimeter of the package. This is known as the “blue center” or “yellow ring” phenomenon and is readily observable in the commercial pc-WLEDs products [2,3].
In order to remove this inhomogeneous angle-dependent CCT which exists in the conventional dispensing structure, the conformal phosphor coating was developed by Lumileds for wirebond-free flip-chip LED configuration, in which phosphor layer with a uniform thickness was employed on the top surface of the chip as well as the sidewalls. The angular CCT variation can be reduced by a factor of almost 10 when using the conformal coating compared to using the conventional coating [2,4].
Kuo et al. [3] demonstrated that a patterned remote phosphor structure with a non-phosphor coating ring window in the perimeter can enhance the extraction of blue rays with large angles. Using this patterned remote phosphor method, the angle-dependent CCT was eliminated by mixing those blue rays with yellow rays, and the CCT deviation was reduced from 1320 K to 266 K. It is suggested that the “yellow ring” phenomenon can be solved via the patterned remote phosphor structure that is fabricated using the pulsed spray coating approach.
Hu et al. [5] proposed a design of freeform lens for LED uniform illumination, where a conformal phosphor coating could be realized between the chip surface and the inner surface of lens. The simulation showed that the angular color uniformity (ACU) could be enhanced by 282.3% compared with conventional freeform lens when the illumination uniformity was kept equivalent. The simulation results also by Hu et al. [6] showed that the conformal coating phosphor with optimized thickness of 50-70 μm could realize the better optical performance when the light extraction efficiency (LEE), CCT and ACU were taken into consideration.
Sun et al. [7] demonstrated that a hemisphere phosphor dome with extended vertical thickness performed an extremely small angular CCT deviation (ACCTD) of 105 K in simulation and 182 K in experiment for a white pc-LED with the CCT near 6500 K.
Zheng et al. [8] demonstrated that a thin and convex phosphor layer on the top surface of LED chip by dip-transfer coating performed a higher ACU compared with the conformal coating for the vertical LED chip configuration. The extreme small CCT deviation of 63 K was gained for a white LED at the average CCT of 3918 K.
An intensity self-adaptive phosphor structure using self-exposure slurry coating was developed in our previous work [9]. In the self-exposure process, the LED chip was used as a self-aligned exposure source. The exposed area was insolubilized and remained on the LED surface in the development process. As a result, a light intensity-adaptive phosphor conversion layer for pc-WLEDs was formed. The shape and size, i.e. the thickness profile, of this self-adaptive phosphor layer are proportional to the intensity profile of the LED-emitting blue light, which helps to keep the intensity ratio of yellow to blue rays with good consistency over a wide emitting angle and thus it improves the uniformity of the angle-dependent CCT compared to the conventional conformal coating.
In this study, we propose a cymbal-shaped phosphor structure that is based on a concept of intensity-proportional thickness distribution to improve the CCT uniformity of the dispensing phosphor pc-WLEDs. Comparing with the conventional dispensing phosphor structure, the cymbal-shaped phosphor structure possesses a phosphor bump upon the central surface of the dispensing bottom layer (i.e. the dispensing base), which visually resembles the bell part of the cymbal. This extra bump phosphor layer can increase the intensity ratio of yellow to blue rays because it increases the excitation optical paths in small angles and thus eliminates the “blue center” phenomenon. It also increases the extraction efficiency of lights at the central area compared to the case in the conventional structures.
2. Experiment
In this investigation, the conventional and the bottom layer of the cymbal-shaped phosphor were prepared using the common dispensing method. The method is convenient and suitable for mass productions. In the first step, the YAG: Ce3+ phosphor powders were sufficiently blended with silicone binder in the ratio 1: 8 to form the phosphor suspension slurry. The phosphor slurry was dispensed into a cup reflector with a mounted LED chip and then baked in a chamber to cure the encapsulant. For the cymbal-shaped structure, the dispensing bottom layer was half-cured in order to settle the shape without fully harden the encapsulant. In the second step, a bell-like structure of the cymbal was formed by injecting the phosphor slurry with a syringe right underneath the central surface of the half-cured bottom layer. With the increase of the injection volume, the injected area swelled into a bell-like bump upon the dispensing bottom layer that is mentioned in the first step. The size of this bump layer is designed to be slightly larger than that of LED chip by controlling the injected volume of the slurry. Important parameters that affect light propagation within the package and therefore reduce the angular CCT variation of the pc-WLEDs with the cymbal-shaped phosphor are the dimension (height h and angle α with z axis) of the bump layer, as shown in Fig. 1.
Fig. 1 Schematic diagrams of two phosphor distribution models, (a) the conventional dispensing phosphor structure, having a slightly curved convex surface over the opening of cup reflector; (b) the cymbal-shaped phosphor structure, a bump layer protruding out of the central surface of the dispensing bottom layer. Parameters that affect the angle-dependent CCT are the surface curvature of two structures, the height h and the angle α with z axis in the cymbal-shaped structure.
The chip size is 35 mil square chips. The blue LED chips were bonded with gold wire to bond pad. The lead-frame size is 8 mm in diameter. The radiant fluxes of all packages with bare blue LED chips were measured and selected to be 395 ± 2 mW at 350 mA to ensure the same initial condition for all phosphor structures.
The photometric and colorimetric parameters of the pc-WLED samples such as the CCT, luminous intensity, luminous flux and emission spectra were measured using a goniophotometer (Everfine, model GO-SPEX500) or a spectroradiometer with an integrating sphere of 1 meter in diameter (Everfine, model PMS-80). To better compare these two structures, the difference in effective CCT of packages was strictly controlled at ± 50 K because the luminous flux and luminous efficacy of the pc-WLEDs are affected by the color temperature.
3. Measurement and analysis
The phosphor distribution of the two kind of samples discussed in this work was observed and compared under a light microscope. Figure 2(a) shows the conventional dispensing phosphor structure, having a slightly curved convex surface over the opening of the cup reflector. It is demonstrated that the angular CCT variation in the radiation pattern of the conventional dispensing structure is caused by the inhomogeneous thicknesses of the phosphor layer with respect to the surfaces of the LED chip [2, 3].
Fig. 2 The overlook (left) and side view (right) of two phosphor distribution samples: (a) the conventional dispensing phosphor structure, which shows a convex surface over the opening of cup reflector; (b) the cymbal-shaped phosphor structure, where a bump layer protruded out of the central surface of the dispensing bottom layer.
The cymbal-shaped phosphor structure, shown in Fig. 2(b), is proposed to reduce the angle-dependent CCT variation and to increase the extraction efficiency of the small angle photons. In addition to the dispensing bottom layer, we develop a second upper bump layer to increase phosphor thickness in central area of the package to compensate the angle-dependent optical path difference of excitation blue rays caused by inhomogeneous phosphor thicknesses in the dispensing bottom layer. The upper bump layer causes more down-converted yellow rays by the extended color conversion process and considerably alleviates the blue center phenomenon. At the same time, the bump layer improves the light extraction efficiency (LEE) with refraction of its curved surface.
With a bell-like upper layer upon the middle of the wider bottom layer, this cymbal-shaped phosphor structure reflects to some extent the intensity pattern of LED-emitting blue light that is mostly concentrated in the normal direction. The realistic cymbal-shaped phosphor distribution on top of surface is shown in Fig. 2(b), where the phosphor is thicker in the center than in the surrounding areas.
If we consider the angle-dependent CCT from the normal direction to 90 degree, the CCT deviations of the conventional dispensing phosphor and the cymbal-shaped phosphor structures were 315 K, 120 K, respectively, as shown in Fig. 3. It is obvious that the angle-dependent CCT of the cymbal-shaped phosphor structure is more uniform in larger angle distribution in our measurement. When travelling within the phosphor-containing encapsulant, the blue rays are forwardly scattered and concentrated in normal direction according to Mie’s theory [3,10], while the converted yellow rays are isotropically emitted from the phosphor particles. Due to shorter excitation optical paths around the normal direction in the slightly curved dispensing structure, most of the blue rays tend to pass through in the normal direction without converting to yellow rays. As the blue photons take higher percentages around the center of the package, a higher CCT light will appear, which is known as the “blue center” phenomenon. This is also the case with the dispensing bottom layer of the cymbal-shaped structure. In our cymbal-shaped design, large angle of light exits the package through the outer perimeter of the dispensing bottom layer, which was not covered by the upper bump layer, as shown in the right of Fig. 2(b). Those blue rays that are concentrated in the normal angle travel from the dispensing bottom layer into the upper bump layer. The excitation optical paths in upper bump layer cause more down-conversion yellow rays around the center area of the package, which decrease the intensity ratio of yellow to blue rays and mitigate the angular CCT variation from the center to the outer perimeter of the package. Consequently, the “blue center” phenomenon can be solved via this cymbal-shaped phosphor structure.
Table 1 shows the comparison of the angle-dependent CCT variation of these two structures with the different effective CCT. The cymbal-shaped phosphor structure shows a better uniformity in angle-dependent CCT as compared to the conventional dispensing phosphor structure.
Table 1. Angle-dependent CCT deviation (ΔCCT) of two phosphor structures
The spectral power distribution of the radiation (i.e. the radiant power spectra) of two phosphor structures at the effective CCT of about 4800 K are shown in the Fig. 4. The external quantum efficiency (EQE) of the cymbal-shaped phosphor and the conventional dispensing phosphor structures are respectively 34.5% and 31.8% at the driving current of 350 mA. The EQE of the cymbal-shaped phosphor increases by 8.5% compared to the conventional dispensing phosphor structure. Because the initial conditions (such as radiant fluxes of the bare blue LED chips, the phosphor concentration) of all packages were selected to be same for two phosphor structures, we can attribute the higher EQE of the cymbal-shaped phosphor to the enhancement of LEE in the upper bump layer.
Fig. 4 The spectral power distribution of the radiation of the pc-WLEDs with two phosphor structures.
In order to further understand the optical characteristics for both phosphor structures, we measured the angle-dependent luminous intensity, current-dependent luminous flux and calculated the luminous efficacy of two phosphor structures. The luminous efficacy is defined as the ratio of luminous flux to power, which is obtained by dividing the luminous flux by the input power (i.e. the injection current multiplied by the driving voltage). As observed in Fig. 5, The luminous flux and luminous efficacy of the cymbal-shaped phosphor are 97.1 lm and 88.1 lm/W, while those of the conventional dispensing phosphor are 91.7 lm and 81.9 lm/W with a driving current of 350 mA. It is clear that the luminous flux and luminous efficacy of the cymbal-shaped phosphor increase by 5.9% and 7.5% compared to the conventional dispensing phosphor structure.
Fig. 5 The current-dependent luminous flux and luminous efficacy of the pc-WLEDs with the conventional dispensing phosphor and the cymbal-shaped phosphor structures.
As shown in Fig. 6, the cymbal-shaped structure shows a higher value of luminous intensity in all directions as compared to the conventional dispensing structure. The higher brightness of pc-WLEDs with the cymbal-shaped phosphor can be reasoned as follows: in order to achieve the same color temperature to compare with both examples, the phosphor of the cymbal-shaped structure is thicker in the center and thinner in the perimeter than the conventional dispensing phosphor layer because the upper bump layer did not cover all over the surface of the dispensing bottom layer in our design, as observed in Fig. 2. When the blue rays pass through the thinner phosphor layer in a large emission angle, there are less down-conversion yellow rays trapped and re-absorbed in the cymbal-shaped structure as compared to the conventional dispensing structure, which causes a slightly higher intensity in the perimeter area of the package. Meanwhile, the curved surface of the upper bump layer in the cymbal-shaped phosphor structure greatly enhanced the extraction efficiency of lights, and causes a distinct increase in the luminous intensity in the center of the package.
Fig. 6 The luminous intensity distribution of the pc-WLEDs with the conventional dispensing phosphor and the cymbal-shaped phosphor structures.
4. Simulation
We use a self-developed ray tracing program [11] based on the Monte-Carlo method to simulate both the conventional dispensing and the cymbal-shaped phosphor structures. In the simulation, we assume the following parameters: nphosphor = 1.82, nsilicone = 1.54, nfree space = 1.0, silver reflectance RAg reflector = 95%, nblue chip = 2.4, LED chip thickness of 200 μm, area of 35 mil square, depth of the cup reflector of 500 μm. Parameters that can be varied are the surface curvature of two structures, the height h and the angle α with respect to the z axis in the cymbal-shaped structure, as shown in the Fig. 1. Varying those three parameters gives rise to white light emission at a different color temperature. Taking one optimized set of parameters (i. e. height h = 0.20 mm, angle α = 40°), we calculated the angle-dependent intensity ratio of yellow to blue rays (Iyellow/Iblue) to compare these two structures at a CCT of 4800 K, as shown in Fig. 7. The cymbal-shaped phosphor structure keeps high ratio consistency from zero to 70 degrees. However, the conventional dispensing phosphor structure shows equivalent ratio stability only up to 50 degrees, and has serious ratio deviation after that. For the cymbal-shaped structure, the thicker phosphor coating in center bumpy area causes more down-conversion yellow rays and increases the intensity ratio, while the thinner phosphor coating in the outer perimeter brings the intensity ratio down as compared to the conventional dispensing structure, as shown in Fig. 7. Consequently, the cymbal-shaped phosphor structure keeps the intensity ratio of yellow to blue rays with good consistency over a wider emission angle range. These results correspond well to what we observed in the experiment. Therefore, the cymbal-shaped structure has the better illumination quality in terms of angle-dependent CCT and intensity compared to the conventional dispensing phosphor structure.
Fig. 7 The ray tracing simulation results: the angle-dependent intensity ratio of yellow to blue rays at a CCT of 4800 K. The conventional dispensing phosphor structure has serious ratio deviation in angles more than 50 degrees. However the cymbal-shaped phosphor structure keeps high consistency even at angles up to 70 degrees.
5. Conclusion
In this study, the angle-dependent CCT variation of pc-WLEDs was reduced effectively with the cymbal-shaped phosphor structure. We believe that the angular CCT deviation related “blue center” phenomenon can be modified by a thickened phosphor in the corresponding area of the package and thus the increased ratio of down-conversion yellow rays, and a bump phosphor layer upon the center of the dispensing bottom layer can serve this important function, which takes a cymbal-like shape. Furthermore, the curved surface of the bump layer can effectively enhance the light extraction efficiency and thus improve the luminous intensity in the center area of the package. Consequently, through the implementation of this novel design, we believe that mass productions of high quality lighting sources can be realized using the common dispensing method.
References and links
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