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Essential Factor for Determining Optical Output of Phosphor-Converted LEDs Volume 6, Number 2, April 2014 Tsung-Hsun Yang Cheng-Chien Chen Ching-Yi Chen Yu-Yu Chang Ching-Cherng Sun, Member, IEEE

DOI: 10.1109/JPHOT.2014.2308630 1943-0655 Ó 2014 IEEE

IEEE Photonics Journal

Factor for Determining Optical Output of LED

Essential Factor for Determining Optical Output of Phosphor-Converted LEDs Tsung-Hsun Yang, Cheng-Chien Chen, Ching-Yi Chen, Yu-Yu Chang, and Ching-Cherng Sun, Member, IEEE Department of Optics and Photonics National Central University, Chungli 32001, Taiwan DOI: 10.1109/JPHOT.2014.2308630 1943-0655 Ó 2014 IEEE. Translations and content mining are permitted for academic research only. Personal use is also permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.

Manuscript received February 16, 2014; accepted February 23, 2014. Date of publication February 26, 2014; date of current version March 6, 2014. This work was supported in part by the National Central University’s Plan to Develop First-Class Universities and Top-level Research Centers under Grants 995939 and 100G-903-2 and in part by the National Science Council of Taiwan under Contracts 97-2221-E-008-025-MY3, 99-2623-E-008-002-ET, NSC 101-2911-I-008-501, 102-3113-E-008-001, and 102-2221-E-008-066. Corresponding author: C.-C. Sun (e-mail: [email protected]).

Abstract: An essential factor of the particle number is exploited for the phosphor excitation in phosphor-converted white LEDs. The particle number can clearly reveal the dependence of the light output flux and the correlated color temperature upon the conventional parameters, thickness, and concentration of the phosphors in a simpler way. In addition, we also find that there might exist an optimal particle number for the maximal luminous light output. An empirical function is then proposed for successfully modeling the relation between the output light and the particle number. Index Terms: Light-emitting diodes, fluorescent and luminescent materials, wavelength conversion devices.

1. Introduction High-brightness white LEDs have been utilized as the most promising light source in the general lighting due to its small size, long life, fast response, high reliability, energy saving and environmental benefit [1], [2]. There are three major ways for LEDs to generate white light in practical application. The first way is color mixing with RGB LEDs [3], which requires a complicated electric driver and optical elements to achieve accurate white light. Thus, the chromatic uniformity and thermal effects will be hard to deal with. Another way is to use UV LEDs to pump RGB phosphors to produce the white light [4], but the efficiency of the UV LEDs and UV light leakage are two important issues to solve. The other way is to mix the blue light emitted by GaN LEDs with yellow lights excited from the yellow phosphor [5], [6]. The last scheme becomes as the simplest, the most convenient, and an efficient way to generate white light, so that it has been commonly adapted in white LEDs. Recently, many efforts in improving internal quantum efficiency, light extraction efficiency, phosphor conversion efficiency and packaging efficiency have dramatically increased the luminous efficacy of yellow phosphor-converted white LEDs. The available commercialized high-power white LEDs achieve more than 200 lm/W in luminous efficacy [7], which is larger than most light sources in general lighting. Up today, how to packaging the high-brightness white LEDs with the blue dies and appropriate phosphors has become an essential, urgent, but nontrivial subject both in the academic research and in the practical industry.

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Factor for Determining Optical Output of LED

Fig. 1. (a) Configuration and (b) picture of the remote package type LED without the photon recycling.

As of the most popular scheme, the phosphors do play a very important role in the phosphorconverted LEDs to the luminous efficacy. Most of the crucial factors include not only the geometry shape, the particle size, the thickness, and the location, but also the concentration of the phosphor in the resin gels. As well-known, the phosphor concentration induces some obvious effects on the performance of white LEDs [8]–[10]. Among them, the effect of the phosphor placement and the phosphor shapes have been extensively discussed in recent years, such as package types of the conformal coating [1], the dispensing [11], and the remote phosphors [12]–[16]. Those reports show that the package type of remote phosphors have more light output and higher efficacy as compared with the other package types. As the matter of fact, the light output of white LEDs depends on the thickness and the concentration of phosphor both. In addition, the particle size of phosphor is another important factor [17]. Previously, Narendran et al. [13] and Yamada et al. [18] have studied the excitation properties of the phosphors under various concentrations. Later, the role of the thickness playing in the phosphor excitation has been taken into consideration by Tran et al. [19]–[21]. They found that the cases of lower concentration and larger phosphor thickness have higher luminous efficacy. The concentration and the thickness of the phosphors are treated as two independent parameters in the phosphor excitation for quit a long time. It makes the composition recipe of the phosphors much complicated for their targeted performance both in chromaticity and correlated color temperature (CCT). However, the thickness and the concentration might not be two independent parameters for the phosphor excitation. They are much more relevant to the same hidden parameter, particle number, and that is the objective of this study. In this paper, the proposed hidden parameter of the particle number has been explored for its relationship among the phosphor concentration, the phosphor thickness, and the final performance of the white LEDs with the remote and the dispensing packaging of phosphors. The final performance of the white LEDs includes the light output, the chromaticity, and CCT of white LEDs. Consequently, the composition recipe of the phosphors for their goal performance now becomes much simpler with only considering the particle number rather than the thickness and the concentration, which are regarded as two parameters.

2. Measurement on Phosphor Excitation To figure out the effects of the thickness and the concentration of the phosphor layers to the phosphor excitation, the various measurements including the optical and the chromatic properties have been executed on the remote packaging LED with aid of a 12-inch integrating sphere. In the measurement, the remote packaging configuration as shown in Fig. 1 is consisted of a MCPCB board, an EZ-700 blue LED chip from Cree [22], a black acrylic plate, and the phosphor plate which is a mixture of yellow YAG phosphor (Cool White YAG 000902 from Hung Ta Co. [23]) and silicone (KER-2600 (A/B) from ShinEtsu Chemical Co. [24]). The MCPCB board provides with good heat dissipation from blue-LED chip to stabilize the light output. The black acrylic plate is applied for confining the remote phosphor plate. In addition, we also change the reflectivity of the cavity surfaces in the black acrylic plate by different color coatings to control the degree of the photon recycling on the backward reflection of lights from the phosphor plate.

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Factor for Determining Optical Output of LED

Fig. 2. Parts of the phosphor plates prepared in the experiment. The phosphor plates are of various thickness (0.2 mm, 0.4 mm, 0.6 mm, 0.8 mm, 1.0 mm, and 1.5 mm) and of different concentration (5%, 10%, 15%, 20%, 25%, and 30% by weight).

Fig. 3. Total mixed light output power of the blue LED and the phosphor plates with various thicknesses (0.2 mm, 0.4 mm, 0.6 mm, 0.8 mm, 1.0 mm, 1.2 mm, and 1.5 mm) and concentrations (5%, 6%, 8%, 10%, 15%, 20%, 25%, and 30% by weight).

After the completing the packaging, the blue LED is driven with a constant electric current of 350 mA. After reaching thermal equivalence, the LED die emitted blue lights with the peak wavelength of 452 nm. To explore the excitation of the phosphors, the yellow YAG phosphor plates are prepared with the various thicknesses of 0.2 mm, 0.4 mm, 0.6 mm, 0.8 mm, 1.0 mm, 1.2 mm, and 1.5 mm, and various weight concentrations of 5%, 6%, 8%, 10%, 15%, 20%, 25%, and 30%, as shown in Fig. 2. In every measurement, we put each different phosphor plate on the acrylic plate in turns to generate white light, which is of color mixing between the blue light and the yellow light. Accordingly, the final white light output features with the different chromaticity, correlated color temperature, and luminous flux, respectively. Fig. 3 shows the output light power with different phosphor plates with various thicknesses, concentrations and cavity reflectivity. The blue LED chip under a constant driving condition is applied as the common light source for all phosphor plates in the measurement. We observed that the output power changed in different combinations. For the cases of lower concentration, the optical power increased as the thickness of the phosphor plate increased. In contrast, for the cases of higher concentration, the light power decreased as the thickness increased.

3. Particle Number of Phosphors In order to explore the influence of the thickness and the concentration of the phosphor layers, an essential factor for the phosphor excitation is proposed to simplify the complicated process in tuning the phosphor recipes. Based on a precise phosphor optic model [25]–[27], the interaction between the blue LED light and the phosphor particles are taken further consideration all over. Here all the

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Fig. 4. Particle size distribution of phosphors.

phosphor particles are assumed to be spherical and are uniformly distributed across the volume of the phosphor plates. Thus, the effective average particle number of the phosphors illuminated by the blue light from LED can be well estimated according to the particle size distribution of the phosphors, the thickness of the phosphor plates, and the concentration of the phosphors. In a general LED packaging, the phosphor is dispersed within the silicone resin. Under the same volume of phosphor, the concentrations are changed to obtain an expected CCT of the output light. In fact, different concentration of phosphor resin causes a change in average density of the phosphor plate, which is an average of the density of phosphor (4.3 g/cm3 [23]) and silicone resin (1.03 g/cm3 [24]). Therefore, the average density of the phosphor plate can be expressed   1 Wpho þ Wsil Wpho þ Wsil pho pho 1 ¼ W ¼ 1þ 1 (1) plt ¼ Wsil pho Vpho þ Vsil wpho sil wpho ho þ sil where plt is the density of the phosphor plate individually; Wsil and sil are the weight and the density of the silicone resin individually, respectively; Wpho and pho are the weight and the density of the phosphors, respectively; wpho ( Wpho =ðWpho þ Wsil Þ) denotes the concentration of the phosphors by weight. In order to precisely describe the effect by the phosphors, the distribution of the size of phosphor particles is taken into consideration. Fig. 4 shows the measured particle size distribution. As assuming the phosphor particles are all spherical, the total volume of the phosphor particles is Vpho

  k k X X 4 dj 3  3  d Nd ¼ Ndj ¼ 3 6 j j 2 j¼1 j¼1

(2)

where dj ’s are the diameters of the phosphor particles. Let Npho be the total particle number that is the sum of the numbers of the phosphor particles Ndj ’s with diameters dj ’s, respectively, and it can be expressed Npho ¼

k X

Ndj

(3)

j¼1

where Ndj ’s are further related to the total particle number Npho and their percentage pðdj Þ which is written Ndj ¼ Npho  pðdj Þ;

j ¼ 1; 2; . . . ; k :

(4)

In our experiment, the phosphor plates are made in a flat cylindrical shape, and the volume of the phosphor plate is A  h, where A denotes the base area and h indicates the thickness. Based on the average density of the phosphor plates and the concentrations of the phosphor by weighting in Eq. (1), the volume of the phosphor particle in the phosphor plate is Vpho ¼

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plt  Vplt  wpho Ah  i ¼h pho 1 pho 1 1 þ sil wpho

(5)

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Factor for Determining Optical Output of LED

Fig. 5. (a) Total output power and (b) the CCT of the mixed light versus the particle number. All the cases with various thickness (0.2 mm, 0.4 mm, 0.6 mm, 0.8 mm, 1.0 mm, 1.2 mm, and 1.5 mm) and concentrations (5%, 6%, 8%, 10%, 15%, 20%, 25%, and 30% by weight) of the phosphors locate on the same locus.

where Vplt and plt are the total volume and the density of the phosphor plate. Besides, from Eqs. (2) and (5), the particle number, Npho , is obtained Npho ¼ Pk

Vpho

 3 j¼1 6 dj pðdj Þ

¼P

h

k  3 j¼1 6 dj pðdj Þ

Ah 1þ

pho sil



i :  1 wpho 1

(6)

According to (6), the total output power of the mixed light in terms of the particle number is shown in Fig. 5(a), which is rather than the thickness and the concentration of the phosphors as shown in Fig. 5. Now, all the curves in Fig. 3 become into the same one in Fig. 5(a). It implies that the particle number is much more realistic in the performance evaluation on the phosphor excitation. The particle number actually has successfully replaced with the roles of the phosphor plate thickness and the phosphor concentration in the exploration of the phosphor excitation. We believe how many times of the collision are possible to happen for blue light photons meeting with the phosphor particles is much more essential in the excitation processes. In addition, the correlated color temperature (CCT) of the mixed light is also related to the particle number in a simple way for all the cases of the different thickness and the different concentration, as shown in Fig. 5(b). Here, it also implies that a useful definition of the phosphor particle size might be given in the way of weighted average on volume. That qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi P 3 3 dj pðdj Þ such that the particle is, the phosphor particle size, dN , based on Eq. (6) is given as dN  size distribution can be replaced with a single equivalent particle size dN . Meanwhile, we also find that there could exist an optimal particle number ð1:8  107 Þ of the phosphors for maximal light output power from Fig. 5(a). The optimal particle number should be corresponding to the most proper condition for the phosphor particles uniformly suspending in the silicone resins. For the cases of the particle number less than the optimal one, the phosphor particles are more like sparse in the resins. To increase the particle number is to increase the chances of the conversion from the blue light into the yellow light. In the sparse cases, all the light is seldom to be scattered by more other phosphor particles. Therefore, it is easy for the light to escape out of the phosphor plate such that the output flux increases and the CCT reduces as the particle number increasing. On the other hand, for the cases of the particle number larger than the optimal one, the phosphor particles are more like too dense in the resins. To increase the particle number is also to increase the chances of the conversion from the blue into yellow light. However, in the dense cases, all the light become difficult to escape out of the phosphor plate such that the output flux decreases and the CCT almost saturates as the particle number increasing. To verify the effectiveness of the particle number, we further perform some more experiments for different kinds of packaging. First, we change the degree of the photon recycling on the backward

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Factor for Determining Optical Output of LED

Fig. 6. (a) Total output power and (b) the CCT of the mixed light versus the particle number. All the cases with various thickness (0.2 mm, 0.4 mm, 0.6 mm, 0.8 mm, 1.0 mm, 1.2 mm, and 1.5 mm) and concentrations (5%, 6%, 8%, 10%, 15%, 20%, 25%, and 30% by weight) locate on the same locus. All the solid lines in (a) are the fitting curves based on (7).

reflection from the phosphor plate. For a packaging cavity of white LED, the enhancement of the photon recycling could extract much more backward photons and increase the interaction between the phosphor particles and the recycled blue photons. In fact, photon recycling can effectively recycle the backward photons with change of the cavity geometry or increase of cavity reflectivity [28]–[30]. In the current case of photon recycling, we controlled the reflectivity of the cavity wall and the bottom surface to change the recycling condition for the backward reflected lights. By painting the white and the silver paints on the surfaces, we have successfully enhanced the reflectivity of the black acrylic plate from R ¼ 0:1 to 0.8 and 0.9, respectively. We measured the corresponding light output with covering different phosphor plates on the special-designed packaging cavities one by one. Again, the dependence of the light output flux and the CCT both upon the thickness and the concentration by weight can be converted into upon the particle number as a single factor. All the cases with various thicknesses and concentrations of the phosphors locate at the same curve for each case of different reflectivity. Fig. 6(a) and (b) presents the output flux and the CCT individually in terms of the particle number only. Higher the photon recycling is, the more the output flux presents. Furthermore, it is interesting that the optimal particle numbers for the maximal output flux slightly increase as the degree of the photon recycling increasing (higher reflectivity of the cavity surfaces). More fortunately, the relation between the output flux and the particle number can be further empirically modeled. In Fig. 6(a), all the data of three different cases actually follow the same one mathematical function as shown in the following:



ln P  ðms ln Npho þ bs Þ ln P  ðml ln Npho þ bl Þ ¼ k (7) where P denotes the output flux of light; ms , ml , bs , bl , and k are five fitting parameters. In some sense, and are corresponding to the change rate of the output power to the particle number for the cases of less and larger particle number than the optimal one, respectively. tells the curvature around the optimal particle number. As a matter of fact, as the cases away off the optimal particle numbers, the output flux of light follows the power law in terms of the particle number, that is mx P ¼ bx  Npho (x ¼ s, or l). However, the real physical meanings of these five fitting parameters should be paid much more attention to study in another work.

4. Effectiveness of Particle Number So far, we have applied the particle number to the yellow YAG phosphors very well. Next, the particle number is to test on two more different phosphors. One is the green YAG phosphors from Chi Mei [31], and the other is the red nitride phosphors from Intermatix [32]. Fig. 7(a) shows the results of the output flux in terms of the particle number for the green YAG phosphors, and Fig. 7(b) presents the results of the corresponding CCTs. In Fig. 7(a), we directly change the electric driving

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Factor for Determining Optical Output of LED

Fig. 7. (a) Total output power and (b) the CCT of the mixed light versus the particle number. All the cases with various thickness and concentrations of the phosphors locate on the same locus. All the solid lines in (a) are the fitting curves based on (7).

Fig. 8. (a) Total output flux of the mixed light versus the particle number. All the cases with various thicknesses and concentrations of the phosphors locate on the same locus. The red solid line is the fitting curve based on (7).

current of the blue LED chip instead of the degree of the photon recycling. Two driving conditions are observed, 350 mA and 20 mA. Again, the particle number and Eq. (7) describe the excitation features of the green YAG phosphors very well. The third kind of the phosphors we have tried is the red nitride phosphors. Fig. 8 shows the measurement and analysis results for verification further. It provides us with more confidence about the effectiveness of the particle number and the Eq. (7). It is worthy to note that there is no the optimal particle number to exist. The output flux monotonically decreases as the particle number increasing. Even so, the Eq. (7) still gives a good description for the changing tendency of the output flux upon the particle number. Due to out of the definition range of CCT, there is no corresponding CCT in the cases of the red phosphor pumped by blue LED chip.

5. Conclusion In this work, we have explored an essential parameter, the particle number, for phosphor excitation in white LEDs in details. First, the particle number simplifies the phosphor recipe to exclude the complexity from the combination between the thickness and the concentration of the phosphors both. Second, for any specific phosphors, there might exist an optimal particle number for the maximal

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output flux. In addition, the CCT of the output light is also highly related to the particle number. It indicates that the specific CCT and the corresponding maximal output flux can be obtained with appropriate recipe control on the particle number. Empirically, we also propose Eq. (7) for the prediction on the relation between the output flux and the particle number and verifies its effectiveness. Up to now, we cannot yet give a complete theoretical derivation for the curves of the optic power versus the particle number. Despite this, we ideally believe that any particle size distribution equivalent to a smaller weighted average particle size dN should introduce a larger optimal particle number, and vice versa, for the cases of the same phosphor materials. That is, the curve for the power versus particle number relationship will shift to right for the particle size distribution with smaller equivalent dN , and thus the optimal particle number becomes larger. In a raw picture, when the particle number is smaller than the optimal one, the real phosphor particles are sparse in the resins. To increase the particle number implies to increase the hitting rate of the blue photons to the real phosphor particles, and hence increase the chances of the conversion from the blue light into the phosphor emission light. On the other hand, when the particle number comes larger than the optimal one, the real phosphor particles are dense in the resins. All the light become difficult to escape out of the phosphor plate such that the output optic power decreases as the particle number increasing. In the meantime, the CCT comes close to saturate. In addition, the degree of sparse or/ and dense condition seems to be dependent on the photon numbers of the input excitation lights. As the input photon number increases, it is equivalent to be more sparse condition for phosphor particles and the optimal particle number is thus increased. In summary, it is the first time to clearly figure out the role of the particle number in white LED packaging. It is important to LED packing to control the appropriate phosphor particle number to achieve high flux rate, stabilize the CCT, and even to reduce the usage of phosphor.

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