ZnO nanoparticles in bettering the color uniformity of phosphor-converted white led lights

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Managing the color uniformity of WLEDs means controlling the size and concentration of ZnO. With the concentration of 10% ZnO, the lumen output of LEDs reaches the highest values. Meanwhile, when the concentration and size of ZnO are 14% and 500 nm respectively, ∆CCT is reduced to the lowest value. Based on the manufacturers' requirements, the most appropriate ZnO concentration and particle size can be determined. However, if requirements include both lumen and color uniformity, the right choice is 14% concentration with 500 nm particle size of ZnO.

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Nội dung Text: ZnO nanoparticles in bettering the color uniformity of phosphor-converted white led lights

VOLUME: 4 | ISSUE: 3 | 2020 | September ZnO Nanoparticles in Bettering The Color Uniformity of Phosphor-Converted White Led Lights 1 2 3,∗ Ming-Jui CHEN , Van Tho LE , Doan Quoc Anh NGUYEN , 3 Thinh Cong TRAN 1 Department of Electrical Engineering, National Kaohsiung University of Science and Technology, Kaohsiung, Taiwan 2 Institute of Tropical Biology, Vietnam Academy of Science and Technology, Ho Chi Minh City, Vietnam 3 Faculty of Electrical and Electronics Engineering, Ton Duc Thang University, Ho Chi Minh City, Vietnam *Corresponding Author: Doan Quoc Anh NGUYEN (Email: nguyendoanquocanh@tdtu.edu.vn) (Received: 14-Feb-2020; accepted: 20-Jun-2020; published: 30-Sep-2020) DOI: http://dx.doi.org/10.25073/jaec.202043.280 Abstract. To make further improvements in most appropriate ZnO concentration and parti- future WLED generation, bettering color unifor- cle size can be determined. However, if require- mity is an important goal manufacturers desire ments include both lumen and color uniformity, to accomplish. One of the most common and ef- the right choice is 14% concentration with 500 fective methods to enhance the color homogene- nm particle size of ZnO. ity is the one focusing on improving scattering in phosphor layer which can be achieved by adding ZnO into the phosphor layer. Based on theoret- Keywords ical application of Mie-scattering, we compute and analyze the scattering characteristics of the WLEDs, Mie-scattering theory, remote diusor particles. From the results, the ZnO phosphor package, color rendering index, particles are proven to have positive inuences lumen output. on the development of lighting quality. Addition- ally, the article analyzed and presented the ef- fects of ZnO concentration which uctuates from 2% to 22% on the color homogeneity. Thus, the color uniformity is inuenced not only by the 1. Introduction particle size but also by the concentration of the added ZnO. Hence, managing the color unifor- Light-emitting diodes (LEDs) have many bene- mity of WLEDs means controlling the size and ts for lighting solution because its component concentration of ZnO. With the concentration of materials are strong and stable, cost-saving, and 10% ZnO, the lumen output of LEDs reaches the environment-friendly. Thus, it has been widely highest values. Meanwhile, when the concentra- used in many indoor and outdoor applications as tion and size of ZnO are 14% and 500 nm re- a solid-state light source which can perfectly re- spectively, ∆CCT is reduced to the lowest value. place the conventional lights such as light bulbs Based on the manufacturers' requirements, the and discharge lamps [1, 2]. So far, white LEDs c 2020 Journal of Advanced Engineering and Computation (JAEC) 173
VOLUME: 4 | ISSUE: 3 | 2020 | September are fabricated from a compound of Indium Gal- it comes to the use of large size LED applications lium Nitride (InGaN) with phosphor materi- in illuminating an important surface. There- als. This combination is capable of converting fore, several studies proposed some solutions to blue light to yellow light [3], for which these minimize DCCTD, including change the dius- WLEDs are also known as phosphor-converted ing method to conformal coating [15], applying white LEDs (pc-WLEDs) [4]. specialized lens [16], re-modelling the phosphor material. Additionally, there are some complex In recent years, many researchers put a lot approaches demonstrated in previous papers to of eorts into improving the light extraction solve this problem such as applying engraved of WLEDs [5]. They have conducted various sapphire substrate structure or a graded refrac- analysis about how the packing aect the light tive index with multiple layers of phosphor [17]. performance of pc-WLEDs through comparisons However, it is essential to consider the impacts among dierent LED structures such as confor- of all the mentioned solutions on DCCTD and mal coating, half-dome glass cover, isolating con- lumen output simultaneously. It turns out that guration, and many more [6]. The main pur- the inuences are not the same for all the CCT pose of analyzing is to gure out the impacts of [18, 19]. Specically, for high CCT, the DCCTD the particle size and particle number of the ap- seems to be stronger than that at low CCT due plied phosphors on the LED output [7]. Beside to the weaker scattered light from phosphor. Re- the perk of being reusable, the particle number garding that issue, this article proposes a detail is one of the most vital factors inuencing lumen analysis of the inuence on the DCCTD of ZnO output and the color temperature of the lighting molecule, an eective scattering particle that is device [8]. This state was explained and proved applied in the lighting structure [20]. Though through experiments and model simulation, ap- there is no discussion about the phosphor re- plying the Monte Carlo method, analyzing the duction, the rise in backward scattering eect of diusion properties, the absorbing capacity, the zirconia seems to be caused by using nano-size converted light, the structural arrangement and particle in remoting conguration [21]. In some the refractive index of materials in the congura- cases, the luminous ux could get benets from tion [9]. These approaches were proposed to an- this increase, basically an eect of emitted light alyze dierent phosphors from YAG, Silicate, to being recycled. However, the issue is only lu- green YAG [10]. Moreover, the back-scattering men output is accounted to the inuences of ZnO event that causes light loss due to the emitted in their study and correlated color temperature light being reabsorbed was also studied. The at- (CCT) deviation was not mentioned [22]. Our tain results allow researchers to succeed in simu- simulated pc-WLEDs package is the one con- lating any type of pc-WLED optical properties. structed with a hemisphere, and this is one of the In the literature of many previous researches, most popular commercially-used structures. Be- the importance of the color uniformity is not sides, we will compare experimental and model- taken into account [11, 12]. There is an obsta- ing results and analyze them in accordance with cle that blocks the way of achieving the high the change of the CCT. Moreover, the analysis color uniformity for white WLEDs, the yellow about not only the chromatic performance but ring phenomenon. This phenomenon occurs also ZnO inuences based on the concentration as the blue light which is strongest surround- of particles at equal color temperature was con- ing the center joins with phosphor molecule ducted and demonstrated. The results showed that has consistent strength in all positions [13]. that the phosphor concentration can directly ad- Once this yellow ring appears, it indicates that just the heating performance of the lighting de- the high temperature color such as blue light vice. Therefore, it seems that having a decrease is stronger in the central region and becomes in the amount of phosphor may become a crucial weaker as it reaches the edge and results in light part of improving the performance of WLEDs. turn more yellow. In addition to that, the an- Generally, phosphor consists of rare earth ele- gular CCT deviation (DCCTD) can reach up to ments which is considered as critical raw ele- 3000 K in lighting conguration that has no ad- ments. Most of them are cerium, and in some justment [14], which becomes more drastic when specic phosphors, those elements are europium, 174 c 2020 Journal of Advanced Engineering and Computation (JAEC)
VOLUME: 4 | ISSUE: 3 | 2020 | September all of which are considered as critical raw ele- Tab. 1: The actual parameters of LED chip. ments. Thus, processes that can reduce the use LED vender Epistar of rare earth element will be a great topic about this concept. LED chip V45H Voltage (V) 3.5∼3.6 Peak Wavelength 453 2. Research method (nm) Power (mW) 320∼340 2.1. Physical and optoelectronic 4.7 mm Lead frame characterization Jentech Size-S Sumitomo Die attach A picture of the actual 9-chip WLEDs and the 1295SA schematic of WLED package with ZnO base are shown in Fig. 1. A GaN LED chip that emits blue light is applied to the model 453 nm emit- ting frequency and peak intensity of 1 W with running power of 339 mA. The LED chip were cover by a composition of YAG:Ce yellow phos- phor (diameter, 13±2 µm ) and silicone. The Bonding diagram actual parameters of LED chip are presented in Tab. 1. In which, c/m indicates the particle distribution of the QDs; c is the amount of QD (mg/cm3 ); D shows the magnitude of the molecule (nm); λ represents wavelength (nm); f (D) is the func- tion for QD particle size distribution; m is the QD weight (mg) of QDCE, that is a results of f (D) integration. Csca (D, λ) is the scattering cross-section of the QD. Besides, Csca (D, λ) can Fig. 1: (a) Photograph of 9W wLED device. (b) be written as: Schematic cross-sectional view of ZnO-doped WLED devices. R Psca (D, λ) Psca (θ, D, λ)dθ Csca (D, λ) = = Pinc (λ) Pinc (λ) (2) 2.2. Mie-scattering analysis In which, Pinc (λ) means the incident irradiance of source (W/m2 ); Psca (D, λ) represents the en- The Monte Carlo simulation of scattering ergy emission (W) as light transmits pass the model can determine the scattering property QD; and Psca (θ, D, λ) is the scattering power of quantum-dot-converted elements (QDCEs), (W). While, the other parameter, the scatter- similar to the white LEDs that convert phosphor ing phase function describes the scattered power material into energy [23]. There are two vital pa- allocation satisfying the standardizing require- rameters in this analysis, scattering coecient ments, which can be expressed by the following and scattering phase function. The scattering equation: coecient µsca presents the scattering probabil- ity, which can express as the following expression ρ(θ, λ) [24]: R f (D)ρsca (θ, D, λ)/Psca (D, λ)Csca (D, λ)dD c Z = R µsca (λ) = f (D)Csca (D, λ)dD (1) f (D)Csca (D, λ)dD m (3) c 2020 Journal of Advanced Engineering and Computation (JAEC) 175
VOLUME: 4 | ISSUE: 3 | 2020 | September 3. Results and analysis The graph in Fig. 2 illustrates the scattering cross-section of the ZnO particles, Csca (D, λ), with dierent sizes from 400 nm to 600 nm. As can be seen, when the size of ZnO particles in- crease, Csca (D, λ) also goes up, and then, lead- ing to stronger scattering ability. It seems that with larger particles of ZnO, lights will transmit straight, and this is an advantage for better lu- minous ux. Meanwhile, when it comes to small Fig. 2: Scattering coecient of ZnO particles with var- particles, there are more light scattering events ious sizes. occurring in all directions, which is benecial to the color uniformity but disadvantageous to the luminous ux. Moreover, dependent values Csca (D, λ) is in inverse proportion to the wave- length values, which means Csca (D, λ) decrease along with the increase of the wavelength values. From all the charts of Fig. 2, Csca (D, λ) reaches the highest value in the wavelength range of 380 nm. In Fig. 3, obviously, µsca (λ) has the same Fig. 3: Scattering cross-sections of ZnO particles with trend as Csca (D, λ), which is directly propor- various sizes. tional to the particles size of ZnO but inversely proportional to the wavelength values. Specif- ically, µsca (λ) rises with the growth of ZnO sizes and decreases when the wavelength range is wider. Moreover, the maximum value of µsca (λ) is also shown in the 380 nm wavelength. That the increase of µsca (λ) with bigger ZnO particle proved the better scattering ability of ZnO. The scattering ability of ZnO can be demonstrated based on Csca (D, λ) and µsca (λ), as follows: Fig. 4: Average cosine of phase function of ZnO parti- 1. It is essential to improve the scattering abil- cles with various sizes. ity in the phosphor layer in order to get the light rays mixed more times and leads to the white light emit color copper. large size of ZnO particles are suitable for LED applications with high demand of color unifor- 2. The scattering is maximum at 380 nm and mity. Thus, ρ(θ, λ), an index describing the then gradually decreases. Then, it reaches angle of scattering intensity, also needs consid- the smallest value at 780 nm. However, the eration. In Fig. 4 are the values of ρ(θ, λ) LED chip has the wavelength of 453 nm; in in connection with dierent sizes of ZnO par- other words, ZnO particle benet the pro- ticles. Obviously, from those charts, the larger cess of increasing the scattering ability of the ZnO particle sizes are, the higher the in- phosphor layers. tensity of scattering becomes. However, with that increase in particle sizes, the scattering an- Clearly, Fig. 2 and Fig. 3 proved that as ZnO gle is narrower. In more detail, ZnO large-sized size is larger, the scattering ability of the phos- particles let the light transmit straight through phor layer becomes better. However, not all the particles, and thus the emitted luminous ux 176 c 2020 Journal of Advanced Engineering and Computation (JAEC)
VOLUME: 4 | ISSUE: 3 | 2020 | September is beneted. Meanwhile, as the particle size is sizes, for accomplishing the greatest lumen out- smaller, the light is distributed in many direc- put. Nevertheless, the study focuses on not only tions, or in other words, the scattering angle is the lumen output, but also the color uniformity larger. Moreover, the back-scattering events of of the WLEDs. Therefore, an analysis of the re- light to the LED chip occurring more inside the sults from Fig. 6 can help to gure out the most LEDs package, leading to a reduction in light suitable concentration and particle size of ZnO energy, and as a result, the lumen output is low- to achieve both targets. ered. However, the more the light scattering happens, the more the time that blue and yellow beams are mixed together, resulting in better quality of white light. Specically, small particle sizes of ZnO help blue rays scatter more times in- side the LEDs, and be distributed to more sides of the LED chip. From this phenomenon, the blue rays combine with "yellow ring" to gener- ate white light, which reduces the "yellow ring" phenomenon, and better the color uniformity. Fig. 6: ∆ CCT of WLED as a function of the size and concentration of ZnO particles. As can be seen, when ZnO concentration is about 14%, ∆CCT declines considerably though its value is not the lowest one. However, with the 500 nm particle size of ZnO and at 14% ZnO concentration, ∆CCT reaches the lowest Fig. 5: Lumen output of WLED as a function of the size point. Nevertheless, when the concentration of and concentration of ZnO particles. ZnO continues to grow from the point of 14%, it seems that ∆CCT also shows its increase. This can be understood by the strong scatter- However, if we just based on the results about ing events in the phosphor layer. As a result, the sizes of ZnO particles, it is impossible to give there are more and more scattered blue light, an exact evaluation about the lumen output of especially with smaller size of ZnO particles. the LEDs since it also is aected by the con- However, to achieve a high color uniformity, the centration of ZnO in the phosphor layer. The balance between the intensity of blue and yel- eects of ZnO concentrations along with its par- low lights is very important, which means that ticle sizes on the lumen output are illustrated in the level of ∆CCT must be lowered. Thus, the Fig. 5. The lumen output shows a downward ZnO particle size and concentration must be ad- trend when the ZnO concentration has an up- justed. If the yellow light emitted is more than ward trend, regardless of the ZnO particle size. the blue one, the "yellow ring" phenomenon will On the other hand, the increase of ZnO concen- occur, leading to generating warm white light. tration is benecial to the scattering capacity, Therefore, it needs to get the scattering of blue from which the energy of emitted light can be light improved in order to reach the reduction of reduced. Thus, the right selections of size and that phenomenon and achieve a balance in the concentration of ZnO particles play a crucial role amount of blue and yellow light. in WLED fabrication. From the results of Fig. 5, it is possible to choose the concentration of By investigating the scattering ability of ZnO at around 10%, regardless of the particle the phosphor layer through values Csca (D, λ), c 2020 Journal of Advanced Engineering and Computation (JAEC) 177
VOLUME: 4 | ISSUE: 3 | 2020 | September µsca (λ) and ρ(θ, λ) with the addition of ZnO, Acknowledgments the suitable ZnO particle parameters could be determined, depending on what the manufactur- This work was supported by the Ministry of Sci- ers require. If the target is to get high a lumen ence and Technology of the Republic of China, output, it is possible to add 14% ZnO into the project 108-2622-E-992-013-CC3. phosphor layer. Meanwhile, the manufacturers want to accomplish a WLED package with high color uniformity, 14% turns out to be the most suitable concentration of ZnO for application. In References case of that both luminous ux and color homo- geneity are the goal, 14% concentration can be [1] Nguyen, D. Q. A., Le Phan, X., & Lee, selected with 500 nm ZnO particle sizes. H. Y. (2019). Enhanced Luminous Flux of White Led using Flat Dual-layer Re- mote Phosphor Conguration. Journal of Advanced Engineering and Computation, 3(2), 425-431. 4. Conclusions [2] Yu, Y., Cao, C., Wu, Z., Wu, Q., Lin, W., Peng, X., ... & Tong, Q. (2019). Improving In short, the inuences of ZnO particles on the the color-rendering index of a tandem warm lumen output and color homogeneity of WLED white organic light-emitting device by em- are performed and demonstrated in this arti- ploying a simple fabrication process. Optics cle. For carrying out experiments and simula- Letters, 44(4), 931-934. tions, ZnO particles whose size is from 400 nm to 600 nm are added into YAG:Ce phosphor layer. [3] Yeh, C. T., Chou, Y. I., Yang, K. S., Wu, The purpose of mixing ZnO and YAG:Ce is to S. K., & Wang, C. C. (2019). Lumines- enhance the scattering ability of the phosphor cence material characterizations on laser- layer, resulting in a better color uniformity. In phosphor lighting techniques. Optics ex- addition, the aim of the studies is to analyze and press, 27(5), 7226-7236. propose the most suitable ZnO size and concen- [4] Chang, Y. P., Chang, J. K., Chen, H. A., tration for WLED applications. Based on the Chang, S. H., Liu, C. N., Han, P., & Cheng, analysis of Csca (D, λ), µsca (λ) and ρ(θ, λ), the W. H. (2019). An advanced laser head- study gives a clear detailed demonstration about light module employing highly reliable glass the scattering eect of ZnO particles by size, in phosphor. Optics express, 27(3), 1808-1815. which the larger the size of ZnO particles is, the greater the scattering ability becomes. Besides, [5] Sharma, S., Brahme, N., Bisen, D. P., & the impacts of ZnO concentration are also in- Dewangan, P. (2018). Cool white light emis- cluded in the paper, so manufactures can make sion from Dy3+ activated alkaline alumino their own choice about how they apply ZnO to silicate phosphors. Optics express, 26(22), their production process. The results showed 29495-29508. that when ZnO concentration is 10%, the lumen output of LEDs has the highest value. Mean- [6] Lee, H., Cho, H., Byun, C. W., Han, J. while, as ZnO concentration and particle size of H., Kwon, B. H., Choi, S., ... & Cho, 14% and 500 nm, respectively, ∆CCT value is N. S. (2018). Color-tunable organic light- the lowest. However, based on the requirements emitting diodes with vertically stacked of the production, manufacturers can decide the blue, green, and red colors for lighting most appropriate parameters of ZnO for appli- and display applications. Optics express, cation. If they want to better both lumen out- 26(14), 18351-18361. put and color uniformity, it is possible to add 14% and 500 nm ZnO into the phosphor layers [7] Kim, W. J., Kim, T. K., Kim, S. H., of WLED bulks. Yoon, S. B., Jeong, H. H., Song, J. O., 178 c 2020 Journal of Advanced Engineering and Computation (JAEC)
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VOLUME: 4 | ISSUE: 3 | 2020 | September [22] Liu, L., Tan, X., Teng, D., Wu, M., & Van Tho LE was born in Thanh Hoa Wang, G. (2015). Simultaneously Enhanc- province, Vietnam. In 2019, he received his ing the Angular-Color Uniformity, Lumi- master degree from the University of Science nous Eciency, and Reliability of White Vietnam National University. His research Light-Emitting Diodes by ZnO SiO2 Modi- interest is optoelectronics. He has worked at ed Silicones. IEEE Transactions on Com- Institute of Tropical Biology, Vietnam Academy ponents, Packaging and Manufacturing of Science and Technology. Technology, 5(5), 599-605. Doan Quoc Anh NGUYEN was born [23] Kim, S. H., Song, Y. H., Jeon, S. R., Jeong, in Khanh Hoa province, Vietnam. He has T., Kim, J. Y., Ha, J. S., ... & Park, been working at the Faculty of Electrical H. J. (2013). Enhanced luminous ecacy and Electronics Engineering, Ton Duc Thang in phosphor-converted white vertical light-University. Quoc Anh received his PhD degree emitting diodes using low index layer. Op- from National Kaohsiung University of Science tics Express, 21(5), 6353-6359. and Technology, Taiwan in 2014. His research [24] Chen, L. Y., Chang, J. K., Wu, Y. R., interest is optoelectronics. Cheng, W. C., Chen, J. H., Tsai, C. C., TRAN has been working & Cheng, W. H. (2013). Optical model for Thinh Cong novel glass-based phosphor-converted white at the Faculty of Electrical and Electronics light-emitting diodes. Journal of Display Engineering, Ton Duc Thang University. Dr Technology, 9(6), 441-446. Thinh received his PhD degree from Technical University of Ostrava, Czechia in 2018. His research interest is optoelectronics. About Authors Ming Jui CHEN was born in Tainan city, Tai- wan. He has been working at the Department of Electrical Engineering, National Kaohsiung Uni- versity of Science and Technology, Kaohsiung, Taiwan. His research interest is optical mate- rial. 180 "This is an Open Access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium provided the original work is properly cited (CC BY 4.0)."