Influence of scattering enhancement particles CaCO3, CaF2, SiO2 and TiO2 on color uniformity of white LEDs

APPLIED PHYSICS VOLUME: 14 |NUMBER: 5 |2016 |DECEMBER Influence of Scattering Enhancement Particles CaCO3, CaF2, SiO2and TiO2on Color Uniformity of White LEDs Nguyen Huu Khanh NHAN, Tran Hoang Quang MINH, Nguyen Doan Quoc ANH Faculty of Electrical and Electronics Engineering, Ton Duc Thang University, 19 Nguyen Huu Tho Street, Tan Phong Ward, District 7, Ho Chi Minh City, Vietnam nguyenh[email protected], [email protected], nguy[email protected] DOI: 10.15598/aeee.v14i5.1884 Abstract. In this paper, the influence of scattering enhancement particles CaCO3, CaF2, SiO2and TiO2, adding to YAG:Ce phosphor compounding, on color uniformity of white LEDs (W-LEDs) was presented. Firstly, the physical model of multi-chip W-LEDs is simulated and demonstrated by using commercial LightTools 8.1.0 program. After that, the influence of scattering enhancement particles on color uniformity is calculated and analyzed. With using the Monte Carlo simulation and the Mie-scattering theory, the color uniformity improvement of an 8500 K W-LEDs is demonstrated convincingly. From the researched results, the best color uniformity can be accomplished with TiO2particles. The results and discussions provided a practical approach for higher-quality manufacturing W-LEDs. Keywords CaCO3, CaF2, color uniformity, SiO2, TiO2, white LEDs. 1. Introduction Nowadays, W-LEDs are becoming increasingly important light sources for illumination applications, because they are long-life, compact, mercury-free and energy-efficient. Color uniformity is the main optical properties of W-LEDs and it could be improved in many previous papers [1], [2] and [3]. All these studies started from the scattering enhancement in phosphor-converted white-LEDs (PC-LEDs). In fact, the structure of PC-LEDs is the combination of YAG:Ce phosphor and silicone glue. The YAG:Ce phosphor absorbs the exciting blue light from the chips to stimulate the yellow light and thus result in white light with the desired color temperature [4]. In other words, in these studies, the color uniformity of LEDs was improved by optimizing the state of the phosphor or the optical structure of PC-LEDs. In conclusions, the spatial color uniformity of PC-LEDs can be controlled by the thickness and the concentration of the phosphor [9]. Moreover, the location of phosphor material in the silicone layer significantly effects on the color performance. The color temperature of PC-LEDs has demonstrated the strong influence of the refractive indexes of the silicone matrix and the phosphor materials and the size of phosphor particles [10]. In this study, we concentrated on finding one particle from scattering enhancement particles CaCO3, CaF2, SiO2and TiO2, which is employed for manufacturing higher-quality W-LEDs. The target of study is an improvement the color uniformity of W-LEDs. This research paper can be divided into three main sections: In Section 2. , the physical model of 8500 K W-LEDs is simulated and demonstrated by using commercial LightTools 8.1.0 program. In Section 3. , by adding one of scattering enhancement particles CaCO3, CaF2, SiO2and TiO2to YAG:Ce phosphor compounding, the color uniformity is simulated, calculated and analyzed: In Section 4. , the simulation can be convinced by using the Monte Carlo simulation and the Mie-scattering theory. In this study, the results demonstrated that the best color uniformity of 8500 K W-LEDs could be accomplished with TiO2particles. This results can consider the prospective solution for higher-quality manufacturing W-LEDs in the near future. c 2016 ADVANCES IN ELECTRICAL AND ELECTRONIC ENGINEERING 615 APPLIED PHYSICS VOLUME: 14 |NUMBER: 5 |2016 |DECEMBER 2. Physical Model In this work, an 8500 K W-LEDs with the conformal phosphor structure is simulated by using the commercial LightTools software based on the Monte Carlo raytracing method. To perform optical simulations, we built 3-D models (Fig. 1). In this research, W-LEDs has commonly configured: •The reflector has a bottom length of 8 mm, a height of 2.07 mm and a length of 9.85 mm at its top surface. •The conformal phosphor layer with a fixed thickness of 0.08 mm covers the 9 LED chips. •Each LED chip with a square base of 1.14 mm and a height of 0.15 mm is bound in the cavity of the reflector (Fig. 1(b)).The radiant flux of each blue chip is 1.16 W at wavelength 455 nm. Silicone lens Gold wire Y O :Eu 23 3+ Reflector Silicone substrate LED chip (a) The conformal phosphor structure. (b) The original lamps and physical model. Fig. 1: W-LEDs structure. To maintain the average Correlated Color Temperature (CCT) of 8500 K, the YAG:Ce concentration changes to the concentration of CaCO3, CaF2, SiO2 and TiO2.The refractive index of the diffusors such as CaCO3, CaF2, SiO2and TiO2are chosen as 1.66, 1.44, 1.47 and 2.87, respectively. The diffusers are assumed to be spherical and have radius 0.5 µm. The average radius of the phosphor particles are 7.25 µm and have a refractive index of 1.83 at all wavelengths of light. The refractive index of the silicone glue is 1.5. The diffusional particle density is varied for optimizing illumination CCT uniformity and output efficiency by the expression: Wphosphor +Wsilicone +Wdiffusor = 100 %,(1) where Wsilicone,Wphosphor and Wdiffusor are the weight percentages of the silicone, phosphor and diffuser of the W-LEDs, respectively. To maintain the mean CCT value of 8500 K, the weight of YAG:Ce phosphor should be decreased when the weight percentage of the diffuser is increased. 3. Results and Discussion For improving the light quality of the W-LEDs, the difference of angular CCT Deviation (D-CCT) between the normal and large angle is an important standard to evaluate in the solid-state lighting application [9]. The larger D-CCT can cause the yellow ring phenomenon and generate the non-uniform white color at the different angle [14]. In this study, the D-CCT is expressed as D-CCT = CCT (Max) – CCT (Min). Here CCT (Max) and CCT (Min) are the maximal CCT at the zero degree of viewing angle and minimal CCT at the 70 degree of viewing angle, respectively. The scattered light of each particle in PC-LEDs is different, resulting in varying the optical properties of W-LEDs. If the scattered blue light is enhanced enough, the D-CCT can be reduced significantly. Conversely, the D-CCT should be increased with lack or redundancy of the scattered blue light in W-LEDs. The scattered blue light not only combines with the converted yellow but also combine the yellow ring for emitting white light, resulting in a reduction of yellow ring phenomenon of W-LEDs. It can be seen in Fig. 2, where the D-CCT of CaCO3and TiO2cases have a downward trend. Meanwhile, the D-CCT of CaF2and SiO2cases grow with their concentration. 0 5 10 15 20 25 30 35 1000 1500 2000 2500 3000 3500 4000 Wt. (%) D−CCT (K) CaCO3 CaF2 SiO2 TiO2 Fig. 2: The impact of the diffusive particles concentration on CCT deviations. c 2016 ADVANCES IN ELECTRICAL AND ELECTRONIC ENGINEERING 616 APPLIED PHYSICS VOLUME: 14 |NUMBER: 5 |2016 |DECEMBER 4. Scattering Description Simulation results can be investigated and demonstrated by Matlab software using Mie-scattering theory [11]. The scattering coefficient µsca(λ), anisotropy factor g(λ)and reduced scattering coefficient δsca(λ)are calculated by expression Eq. (2), Eq. (3) and Eq. (4): µsca (λ) = ZN(r)Csca(λ, r)dr, (2) g(λ) = Z Z 1 −1 p(θ, λ, r)f(r) cos θd cos θdr, (3) δsca =µsca(1 −g),(4) where N(r)is the number density distribution of diffusional particles (per cubic millimeter), Csca is the scattering cross sections (per square millimeter), p(θ, λ, r) is the phase function, λis the wavelength of the incident light (nanometers), ris the radius of particles (micrometers), θis the scattering angle (degree) and f(r)is the size distribution function of the diffusers in the phosphor layer. f(r) = fdif (r) + fphos(r),(5) N(r) = Ndif (r) + Nphos(r) = =KN·[fdif (r) + fphos(r)],(6) where N(r)is composed of the diffusive particle number density Ndif (r)and the phosphor particle number density Nphos(r).fdif (r)and fphos(r)are the size distribution function data of the diffusor and phosphor particle. If the phosphor concentration c (milligrams per cubic millimeter) of the mixture is known, KNdenotes the number of the unit diffusor for one diffuser concentration and KNcan be obtained by: c=KNZM(r)dr. (7) To obtain KN, we should first know the mass distribution M(r)(milligrams) of the unit diffusor. Below equation can calculate M(r): M(r) = 4 3πr3[ρdif fdif (r) + ρphosfphos(r)] ,(8) where ρdif and ρphos are the density of diffusor and phosphor crystal. In Mie theory, Csca is normally presented: Csca =2π k2 ∞ X 0 (2n−1)(|an|2+|bn|2),(9) where kis the wavenumber (2π/λ)and anand bnare the expansion coefficients with even symmetry and odd symmetry, respectively. These coefficients can be calculated by equations below: an(x, m) = Ψ0 n(mx)Ψn(x)−mΨn(mx)Ψ0 n(x) Ψ0 n(mx)ξn(x)−mΨn(mx)ξ0 n(x),(10) an(x, m) = mΨ0 n(mx)Ψn(x)−Ψn(mx)Ψ0 n(x) mΨ0 n(mx)ξn(x)−Ψn(mx)ξ0 n(x),(11) where xis the size parameter (= k·r),mis the refractive index of the scattering diffusive particles. Ψn(x) and ξn(x)are the Riccati - Bessel function. According to Eq. (3), the theoretical results of g(λ)are calculated and shown in Fig. 3, Fig. 4 and Fig. 3. Results show that the variation of the diffuser concentration has a slight impact on the anisotropy factor g(λ)and the increase of g(λ)by the diffusional particle density is so small that the increase can be neglected. The anisotropy factor of particles for a long wavelength should be larger than that of a short wavelength. It means that the particles should present stronger a scattering effect for a short wavelength. This theoretical result can be modified in the following angular scattering amplitudes simulation shown in Fig. 3, Fig. 4 and Fig. 5. Fig. 3: The angular scattering amplitudes of the various diffusional particles with sphere diameter = 1 µm for blue light = 455 nm. Fig. 4: The angular scattering amplitudes of the various diffusional particles with sphere diameter = 1 µm for yellow light = 595 nm. c 2016 ADVANCES IN ELECTRICAL AND ELECTRONIC ENGINEERING 617 APPLIED PHYSICS VOLUME: 14 |NUMBER: 5 |2016 |DECEMBER Fig. 5: The angular scattering amplitudes of the various diffusional particles with sphere diameter = 1 µm for red light = 680 nm. In the mixture of phosphor, diffusor and silicone, the refractive index of embedded silicone (nsil) is 1.53 and the refractive index of diffusor (ndif ) are 1.66, 1.44, 1.47 and 2.87 respectively. Silicone and diffusors are considered to be transparent for the blue light and the yellow light. The refractive index of the phosphor particle (nphos) has a complex form. Therefore, the relative refractive indices of diffusor (mdif ) and phosphor (mphos) in the silicone are mdif =ndif ·n−1 sil and mphos =nphos ·n−1 sil . For small spheres, the phase function p(θ, λ, r)can be calculated according to the following equation [12] and [13]: p(θ, λ, r) = 4πβ(θ, λ, r) k2Csca(λ, r),(12) where β(θ, λ, r)is the dimensionless scattering function, which is obtained by the scattering amplitude functions S1(θ)and S2(θ): β(θ, λ, r) = 1 2h|S1(θ)|2+|S2(θ)|2i.(13) S1= ∞ X n=1 2n+ 1 n(n+ 1) an(x, m)πn(cos θ) +bn(x, m)τn(cos θ).(14) S2= ∞ X n=1 2n+ 1 n(n+ 1) an(x, m)τn(cos θ) +bn(x, m)πn(cos θ).(15) In equations Eq. (14) and Eq. (15), the angular dependent functions and are expressed in the angular scattering patterns of the spherical harmonics. 5. Conclusion In this research, the influence of CaCO3, CaF2, SiO2 and TiO2on color uniformity of 8500 K MCW-LEDs was presented, calculated, analyzed and demonstrated. From the researched results, some conclusions are proposed: •The CCT deviation has a decreasing tendency when the concentration of CaCO3and TiO2increases. •Meanwhile the CCT deviation of CaF2and SiO2 cases grow with their concentration. •The best color uniformity of W-LEDs can be obtained in TiO2case. In summary, TiO2particles should be chosen for improving the color uniformity of W-LEDs. This research provided an important technical implication for the selection of phosphors in WLED manufacturing and development of phosphor materials for WLED applications. In further research, color rending index and luminous efficiency of MCW-LEDs by adding CaCO3, CaF2, SiO2and TiO2particle into the phosphor compounding is necessary to analyze and demonstrate. Acknowledgment This paper was supported by Professor Hsiao-Yi Lee, Department of Electrical Engineering, National Kaohsiung University of Applied Sciences, Kaohsiung, Taiwan. References [1] LIU, Z., S. LIU, K. WANG and X. LUO. 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Detailed Study on PulseSprayed Conformal Phosphor Configurations for LEDs. Journal of Display Technology. 2013, vol. 9, iss. 6, pp. 433–440. ISSN 1558-9323. DOI: 10.1109/JDT.2012.2225019. [19] SCHRATZ, M., C. GUPTA, T. J. STRUHS and K. GRAY. Reducing energy and maintenance costs while improving light quality and reliability with led lighting technology. In: Pulp and Paper Industry Technical Conference (PPIC). Charlotte: IEEE, 2013, pp. 43–49. ISBN 978-1-4673-5100-3. DOI: 10.1109/PPIC.2013.6656043. c 2016 ADVANCES IN ELECTRICAL AND ELECTRONIC ENGINEERING 619 APPLIED PHYSICS VOLUME: 14 |NUMBER: 5 |2016 |DECEMBER About Authors Nguyen Huu Khanh NHAN defended his Ph.D. thesis at Institute of Research and Experiments for Electrical and Electronic Equipment, Moscow, Russian Federation. He is working as Lecturer in Faculty of Electrical and Electronic Engineering, Ton Duc Thang University, Ho Chi Minh City, Vietnam. His research interests include VLSI, MEMS and LED driver chips. Tran Hoang Quang MINH defended his Ph.D. thesis at Tomsk Polytechnic University, Tomsk City, Russian Federation. The author’s major fields of study are High-voltage Power System, Relay Protections and Optoelectronics. He is working as Lecturer in Faculty of Electrical and Electronics Engineering, Ton Duc Thang University, Ho Chi Minh City, Vietnam. Nguyen Doan Quoc ANH was born in Khanh Hoa province, Vietnam. He has been working at the Faculty of Electrical and Electronics Engineering, Ton Duc Thang University, Ho Chi Minh City, Vietnam. He received his Ph.D. degree from National Kaohsiung University of Applied Sciences, Taiwan in 2014. His research interest is optoelectronics (such as Multi-chip white light LEDs, free-form lens and optical material). c 2016 ADVANCES IN ELECTRICAL AND ELECTRONIC ENGINEERING 620