Comparison between SEPs of CaCO and TiO in phosphor layer for better color uniformity and stable luminous flux of WLEDs with 7,000 K

Phosphor-converted white light-emitting diodes (pc-WLEDs), which basically contain a blue LED chip coated with a phosphor layer, usually YAG:Ce phosphor, have offered significant benefits to the lighting industry [1]. Compared to conventional light sources such as fluorescent, incandescent, mercury, and high-pressure sodium, LED can produce higher lighting efficiency that can be up to 160 lm/W [2]. Moreover, pc-LED products provide considerable energy-consumption reduction, lower maintenance costs, high resistance, long-life performance, and limitation of carbon and UV emissions [3]. Thus, LED has been gradually cementing its position in solid-state lighting (SSL) [4]. Yet, deficiencies in the color quality and the light extraction efficiency have been the obstacles preventing the phosphor-based LEDs from attaining a wider application in the SSL market [5]. In general, the phosphor-converted LED, using a YAG:Ce phosphor layer to coat the blue LED chip, generates white light by mixing the blue light from the chip that transmits through the phosphor layer with the converted yellow light produced by phosphors [6]. Particularly, a proportion of the emitted blue light is absorbed by the yellow phosphor, after which a significant amount of the resultant emitted light is yellow light. However, the yellow phosphor layer tends to absorb more scattered blue light, leading to the non-uniform color distribution. This means the yellow-light proportion is higher than that of the blue one, which probably results in the yellow-ring phenomenon [7, 8]. The color inconsistency can be addressed by taking advantage of the different emission characteristics of phosphor materials. In other words, combing one or more phosphor types with the yellow phosphor YAG:Ce could reduce the color deviation or the deviating correlated color temperature (D-CCT) by providing a tunable blended emission of lights [9]. Doping Ca₉Y(PO₄)₇ with Eu²⁺ and Sm³⁺ ions could provide the blue-green and orange-red spectral regions to the photoluminescence, which would be beneficial to the WLED that requires a high color rendering index (CRI) [10]. Another study focusing on the phosphor-in-glass structure demonstrated that the combination of silicone with other phosphor particles, including SiO₂ [11], B₂O₃ [12], PbO [13], and yellow YAG:Ce³⁺, could minimize the color deviation by 590 K at the WLED color temperature (CT) of 6,000 K [14]. Other methods have also resulted in significant color-deviation reduction, such as HfO₂/SiO₂ DBR film providing 1,478 K reduced color differences [15] and micro-patterned structure with 441 K D-CCT reduction [16] at the CT of around 5,000 K. However, the high-cost and complex production process made these methods unfavorable to the manufacturers.

The particles for scattering enhancement (SEPs) can be a more practical and potential approach for the development in color homogeneity and adequacy of WLEDs. Generally, the application of SEPs is to enhance the scattering properties of phosphor layers in the structure of LEDs, thus leading to a more uniform color distribution. The SEPs widely applied can be listed as TiO₂, CaCO₃, microspheres, and SiO₂ nanoparticles [17]. Besides, it is essential to ascertain the concentration and diameter of SEPs that can offer the most satisfying lighting properties. Also, it is necessary to identify the SEP that serves as the most potential material for achieving improvements in LED output and that can be applied in mass production, as well to discuss procedures by which we can carry out this identification. The paper focuses on the two SEPs of TiO₂ and CaCO₃ as they are frequently combined with the phosphor compound of WLED to enhance the package's lighting performance. TiO₂ particles, for example, can be mixed into the phosphor layers with the concentration of 0.1% for better color quality [18], or doped with Eu³⁺ to provide high red light emission efficiency for high-performance GaN-based WLED devices [19]. Besides, CaCO₃ particles have been used as a diffuser in WLED encapsulation to enhance the optical properties of the LED. It was observed that 10% CaCO₃ in the encapsulation showed better color uniformity while preserving the luminous-flux stability [20]. The mentioned studies on TiO₂ and CaCO₃ indicated the importance of the SEPs’ concentration; yet the particle sizes of these particles were not thoroughly investigated. Moreover, the comparison of the performance of various SEPs by simulated scattering parameters is barely demonstrated. Hence, the present study focuses on experimenting with and analyzing the influences of TiO₂ and CaCO₃ via simulation and calculation of four scattering properties at the correlated color temperature (CCT) of 7,000 K. Specifically, the scattering coefficients, anisotropic scattering, the reduced scattering, and scattering amplitudes were calculated based on Mie-scattering theory [21]. A comparison between TiO₂ and CaCO₃ can therefore be drawn, and the most suitable SEP for WLED production can be determined. The results obtained from the present study on CaCO₃ and TiO₂ nanoparticles’ light scattering enhancement could greatly contribute to widening the application of LEDs in other aspects, such as in biomedical fields’ sensing techniques requiring high sensitivity that can be acquired by increasing the light scattered from small particles [22].

Scattering analysis

2.1

Simulated WLED model

Figure 1A presents the actual WLED used in the lighting performance investigation of TiO₂ and CaCO₃. In Figure 1B, the schematic illustration of this WLED is presented. Here, to simulate the required WLED, the commercial LightTools 8.1.0 (Synopsys, Inc., Mountain View, California) and the conformal phosphor coating method, which could result in better color distribution and luminous flux than other coating techniques, were applied. The multi-chip WLED package was prepared with nine LED chips attached to the lead frame by the gold wire bonding process. Given that TiO₂ and CaCO₃ particles are spherical, they were individually blended with the compound of yellow phosphor YAG:Ce³⁺ to form coating films with a 0.08 mm thickness, and were then placed over the LED chips’ surfaces. The reflector of each WLED has a depth and inner and surface diameters of 2.1 mm, 8 mm, and 10 mm, respectively. Besides, the indices of refraction of CaCO₃, TiO₂, YAG:Ce³⁺ particles, and the silicone glue in visible spectral wavelengths are 1.66, 2.87, 1.83, and 1.5, respectively.

(A) pc-LED model used in the research, (B) 2D schematic image of pc-LEDs model. SEPs, particles for scattering enhancement

2.2

Scattering analysis

The numeric computation of light scattering events in the LED structure was conducted with MATLAB (MathWorks, Massachusetts, USA). The theory of Mie scattering was applied to demonstrate the results of the light scattering properties in the LED, using SEPs [23]. The scattering coefficient μ_sca(λ), anisotropy factor g(λ), and reduced scattering coefficient δ_sca(λ) of the phosphor-SEPs layer could be measured using: (1) $μ_{sca} (λ) = \int N (r) C_{sca} (λ, r) dr$ {\mu _{sca}}\left( \lambda \right) = \int {N\left( r \right){C_{sca}}\left( {\lambda ,r} \right)dr} (2) $g (λ) = 2 π \int \int_{- 1}^{1} p (θ, λ, r) f (r) cos θ d cos θ dr$ g\left( \lambda \right) = 2\pi \int {\int\limits_{ - 1}^1 {p\left( {\theta ,\lambda ,r} \right)f\left( r \right)\cos \theta d\cos \theta dr} } (3) $δ_{sca} = μ_{sca} (1 - g)$ {\delta _{sca}} = {\mu _{sca}}\left( {1 - g} \right) where N(r) shows the diffusional-particle distribution density (mm³), C_sca denotes the scattering cross-sections (mm²), p(θ,λ,r) denotes the phase function, θ indicates the scattering angle (°C), and f (r) demonstrates the diffusional particles’ size distribution function in the phosphor-silicone film. λ and r express the wavelength of lights (nm) and diffusional particles’ radius (μm), respectively.

The f (r) is calculated using: (4) $f (r) = f_{dif} (r) + f_{phos} (r)$ f\left( r \right) = {f_{dif}}\left( r \right) + {f_{phos}}\left( r \right) (5) $\begin{array}{l} N (r) & = N_{dif} (r) + N_{phos} (r) \\ = K_{N} \cdot [f_{dif} (r) + f_{phos} (r)] \end{array}$ \matrix{ {N\left( r \right)} \hfill & { = {N_{dif}}\left( r \right) + {N_{phos}}\left( r \right)} \hfill \cr {} \hfill & { = {K_N} \cdot \left[ {{f_{dif}}\left( r \right) + {f_{phos}}\left( r \right)} \right]} \hfill \cr } where N_dif (r) and N_phos(r) present the diffusional-particle and phosphorus-particle densities, respectively. f_dif (r) indicates the diffusional-particle size distribution function, and that datum of the phosphors is represented as f_phos(r). K_N is the diffusional-particle unit quantity for one diffusional-particle concentration, which is calculated using: (6) $c = K_{N} \int M (r) dr$ c = {K_N}\int {M\left( r \right)dr} M(r) – mass distribution of the diffusor unit – is expressed as follows: (7) $M (r) = \frac{4}{3} π r^{3} [ρ_{dif} f_{dif} (r) + ρ_{phos} f_{phos} (r)]$ M\left( r \right) = {4 \over 3}\pi {r^3}\left[ {{\rho _{dif}}{f_{dif}}\left( r \right) + {\rho _{phos}}{f_{phos}}\left( r \right)} \right] where ρ_diff (r) indicates the density of the diffusive spheres and ρ_phos(r) shows the density of phosphor granules.

With Mie-scattering application, C_sca could be shown as: (8) $C_{sca} = \frac{2 π}{k^{2}} \sum_{0}^{\infty} (2 n - 1) ({| a_{n} |}^{2} + {| b_{n} |}^{2})$ {C_{sca}} = {{2\pi } \over {{k^2}}}\sum\limits_0^\infty {\left( {2n - 1} \right)\left( {{{\left| {{a_n}} \right|}^2} + {{\left| {{b_n}} \right|}^2}} \right)} Here, k = 2π/λ , while a_n and b_n can be demonstrated by: (9) $a_{n} (x, m) = \frac{ψ_{n}^{'} (mx) ψ_{n} (x) - m ψ_{n} (mx) ψ_{n}^{'} (x)}{ψ_{n}^{'} (mx) ξ_{n} (x) - m ψ_{n} (mx) ξ_{n}^{'} (x)}$ {a_n}\left( {x,m} \right) = {{\psi _n^\prime \left( {mx} \right){\psi _n}\left( x \right) - m{\psi _n}\left( {mx} \right)\psi _n^\prime \left( x \right)} \over {\psi _n^\prime \left( {mx} \right){\xi _n}\left( x \right) - m{\psi _n}\left( {mx} \right)\xi _n^\prime \left( x \right)}} (10) $b_{n} (x, m) = \frac{m ψ_{n}^{'} (mx) ψ_{n} (x) - ψ_{n} (mx) ψ_{n}^{'} (x)}{m ψ_{n}^{'} (mx) ξ_{n} (x) - ψ_{n} (mx) ξ_{n}^{'} (x)}$ {b_n}\left( {x,m} \right) = {{m\psi _n^\prime \left( {mx} \right){\psi _n}\left( x \right) - {\psi _n}\left( {mx} \right)\psi _n^\prime \left( x \right)} \over {m\psi _n^\prime \left( {mx} \right){\xi _n}\left( x \right) - {\psi _n}\left( {mx} \right)\xi _n^\prime \left( x \right)}} where x = k × r, m denotes the index of refraction, and ψ_n(x) and ξ_n(x) indicate the Riccati–Bessel function. The computations for the relative refractive indices of diffusional sphere (m_dif) and phosphor grain (m_phos) in the silicone are: m_dif = n_dif /n_sil and m_phos = n_phos/n_sil. Subsequently, the phase function p(θ, λ, r) can be reckoned using: (11) $p (θ, λ, r) = \frac{4 π β (θ, λ, r)}{k^{2} C_{sca} (λ, r)}$ p\left( {\theta ,\lambda ,r} \right) = {{4\pi \beta \left( {\theta ,\lambda ,r} \right)} \over {{k^2}{C_{sca}}\left( {\lambda ,r} \right)}} β (θ, λ, r), S₁(θ), and S₂(θ) indicate the angular scattering amplitudes, which are computed using: (12) $β (θ, λ, r) = \frac{1}{2} [{| S_{1} (θ) |}^{2} + {| S_{2} (θ) |}^{2}]$ \beta \left( {\theta ,\lambda ,r} \right) = {1 \over 2}\left[ {{{\left| {{S_1}\left( \theta \right)} \right|}^2} + {{\left| {{S_2}\left( \theta \right)} \right|}^2}} \right] (13) $S_{1} = \sum_{n = 1}^{\infty} \frac{2 n + 1}{n (n + 1)} [\begin{matrix} a_{n} (x, m) π_{n} (cos θ) \\ + b_{n} (x, m) τ_{n} (cos θ) \end{matrix}]$ {S_1} = \sum\limits_{n = 1}^\infty {{{2n + 1} \over {n\left( {n + 1} \right)}}\left[ {\matrix{ {{a_n}\left( {x,m} \right){\pi _n}\left( {\cos \theta } \right)} \cr { + {b_n}\left( {x,m} \right){\tau _n}\left( {\cos \theta } \right)} \cr } } \right]} (14) $S_{2} = \sum_{n = 1}^{\infty} \frac{2 n + 1}{n (n + 1)} [\begin{matrix} a_{n} (x, m) τ_{n} (cos θ) \\ + b_{n} (x, m) π_{n} (cos θ) \end{matrix}]$ {S_2} = \sum\limits_{n = 1}^\infty {{{2n + 1} \over {n\left( {n + 1} \right)}}\left[ {\matrix{ {{a_n}\left( {x,m} \right){\tau _n}\left( {\cos \theta } \right)} \cr { + {b_n}\left( {x,m} \right){\pi _n}\left( {\cos \theta } \right)} \cr } } \right]}

In these equations, θ is the scattering angle (°); a_n and b_n represent the expansion coefficients with even symmetry and odd symmetry, respectively; and π_n(cos θ) and τ_n(cos θ) indicate the angular dependent functions. The computed scattering properties of CaCO₃ and TiO₂ at two different light wavelengths of 450 nm and 550 nm as a function of particle sizes are demonstrated in Figure 2. Previous studies demonstrated that a larger size of nanoparticles probably enhanced the scattering performance, while a lower size induced the adsorption cross-section to outweigh that of the scattering one, stimulating the influences of dissipation loss on the scattering factor [24]. Thus, bigger particle sizes of CaCO₃ and TiO₂ could be remarkably beneficial to the light scattering properties of LED packages. As can be seen from Figures 2A–2D, all the scattering characteristics of CaCO₃ and TiO₂ are promoted significantly as the larger SEP particles are applied. This has reinforced the point of using these two particles to achieve scattering enhancement for white LEDs. Moreover, both TiO₂ and CaCO₃ result in stronger blue-light scattering intensity (at 450 nm) than the intensity observed in the case of yellow light (at 550 nm), indicating that the blue light lost by absorption could be compensated, and leading to the balance in the color distribution. Additionally, the more the light scattering occurs, the more uniform the light mixing becomes. Besides, the blue-light extraction efficiency of blue rays at the right and left edges of the phosphor layer can be boosted as a result of this blue-light scattering enhancement. The extracted blue light then undergoes mixing with the extracted yellow light so as to create the yellow ring that generates white light. Consequently, the yellow ring was diminished, leading to a more adequate color quality for WLED packages. Noticeably, TiO₂ sphere produces higher scattering results than CaCO₃ does, and so it can be assumed that TiO₂ is more beneficial to the color distribution. However, this should be further examined to decide the most suitable SEP for better lighting performance.

Comparison of scattering properties in pc-LEDs using CaCO₃ and TiO₂ with different diameters at 450 nm and 550 nm: (A) Scattering coefficient; (B) Average cosine of phase function; (C) Reduced scattering coefficient; (D) Scattering cross-section

The reduction in the yellow ring is advantageous to the color quality of WLED, and thus TiO₂ and CaCO₃ are appropriate materials for acquiring higher color distribution in the WLED. From the scattering results, it can be observed that the diameter of SEPs is important. The larger sizes of the spherical particles lead to better scattering properties. In addition to the particle size, the concentration of CaCO₃ and TiO₂ in the yellow phosphor layer is relevant as this could influence the particle intensity distribution in the coating film, leading to the changes in the color uniformity and luminescence of the LEDs. Generally, when the light scattering is abundant, the luminous intensity can be significantly reduced. Hence, the scattering effects of CaCO₃ and TiO₂ as a function of their concentrations must be analyzed and discussed in more detail, which will be presented in Section 3. Based on the findings in Section 3 and combined with the results in Section 2 concerning the calculated scattering properties of SEPs, the suitable SEP for the enhancement of the corresponding optical criteria of WLEDs is possibly determined.

Computation and discussion

The benefit of integrating a SEP into the original phosphor-silicone layer is the control over the particle density required to stabilize the CCT of a WLED. Particularly, with SEP, the weight percentage of each element could be balanced, thus maintaining the desired CCT, which can be expressed as follows: (15) $W_{phosphor} + W_{silicone} + W_{SEP} = 100 %$ {W_{phosphor}} + {W_{silicone}} + {W_{SEP}} = 100\% where the weight percentage of phosphor, silicone, and SEPs in the coating layer are presented as W_phosphor, W_silicone, and W_SEP, respectively. Based on this equation, the weight percentage of yellow phosphor YAG:Ce should decline, when the weight percentage of SEP increases as a result of the increase in its concentration, to keep the CCT stable at the determined value of 7,000 K.

The homogeneity of a WLED can be demonstrated by the extent of color deviation that occurs, which can be defined by: ΔCCT = CCT_(Max)−CCT_(Min). Here, CCT_(Max) and CCT_(Min) describe the maximum and minimum CCTs at the viewing angle of 0° and 90°, respectively. The color deviation, which relates to CCT variations in the light-emission angle, is the primary cause of the yellow-ring effect and the non-uniform color distribution. Previous studies pointed out that the main cause of the CCT variance is the difference between the optical properties of each element in the phosphor layer, and the solution to decrease this was to increase the blue-light emission in the white-light spectra band. As the blue-light emission rises, the dissimilarity in the blue and yellow-ray proportions can be addressed. In particular, the white light is comprised of transmitted blue light and converted yellow light; therefore, to obtain the balance in these two lights, there must be enough blue-light proportions for the light transmission and light absorption to be converted into yellow light by the phosphor. When there is a shortage in the blue-light proportion essential to the yellow phosphor's absorption, most of the transmitted and scattered blue light will be absorbed and converted into yellow light. As a result, there is the apparent dominance of yellow-light proportion, causing the yellow ring on the lighting surface to appear, and consequently degrading the overall color quality. Thus, SEPs can be used as a solution to enhance the blue-light emission for a minimization in the CCT variation, which will probably result in the more homogenous chromaticity of the LED.

The CCT deviation of WLEDs with CaCO₃ and TiO₂ in the phosphor layer is demonstrated in Figure 3. As can be seen, both nanoparticles show remarkable performance in decreasing the variations of CCT in the light-emission angle. TiO₂ is relatively better than CaCO₃ in this aspect of comparison, as in Figure 3A the CCT deviation in the pc-LED with 30% TiO₂ declines two-fold, compared to that in the LED without TiO₂. Meanwhile, in Figure 3B, by using 30% CaCO₃ in the phosphor layer, the varied CCT decreases from 2,070 K to 1,680 K, which means there is a 390 K CCT-deviation reduction. Therefore, it is possible to use TiO₂ for enhancing the color quality of WLED; and this has confirmed the assumption mentioned in Section 2.2.

The fluctuation of CCT deviation in pc-LEDs employing (A) CaCO₃ and (B) TiO₂. CCT, correlated color temperature

Besides the color uniformity, the luminous efficiency should also be taken into consideration as it is one of the most crucial factors in WLED quality evaluation. The lumen output of WLEDs integrating SEPs is shown in Figure 4, in which graph (A) illustrates the luminescence of the CaCO₃-doped layer, and graph (B) presents that of the TiO₂-doped one. The concentrations of CaCO₃ and TiO₂ are adjusted in the range of 0%–50%, while their sizes range from 100 nm to 1,000 nm. As can be seen, the luminous flux declines as the concentrations of both SEPs increase. However, the luminous degradation that can be observed when there is a growth in the CaCO₃ concentration is much smaller than that in the case of higher TiO₂ concentrations. In other words, CaCO₃ can maintain the relatively high luminous stability.

Luminous flux yielded from pc-LEDs with (A) CaCO₃ and (B) TiO₂

To analyze the change in luminous efficiency of WLEDs using CaCO₃ and TiO₂, the scattering computation applying Mie theory, and the transmitted light power calculation based on Lambert-Beer law, were utilized [25]: (16) $I = I_{0} exp (- μ_{ext} L)$ I = {I_0}\exp \left( { - {\mu _{ext}}L} \right) where I₀ and L are the incident light power and phosphor layer thickness (mm), respectively. μ_ext indicates the extinction coefficient, and can be defined as: $μ_{ext} = N_{r} \cdot C_{ext}$ {\mu _{ext}} = {N_r} \cdot {C_{ext}} where N_r denotes the number density distribution of particles (mm⁻³) and C_ext (mm²) shows the phosphor-particle extinction cross-section.

According to Eq. (16), the concentration of the SEPs is in inverse proportion to the luminous efficiency while being directly proportional to the scattering properties. In other words, the rise in SEP concentrations causes a reduction in luminescence, despite heightening the scattering occurrences in the WLED packages. Specifically, if the concentrations of CaCO₃ and TiO₂ continuously rise to 50wt.%, the scattering will become redundant and cause more light to be trapped, along with the increasing rate of energy loss owing to backscattering events, thus rendering the luminous flux lower. Hence, the concentration of SEPs should be managed with respect to their particle size to obtain the best results of the color and luminous performances.

Conclusions

The effects of two SEPs – CaCO₃ and TiO₂ – on the CCT uniformity and lumen efficacy are investigated, compared, and demonstrated in this article. The four scattering properties of SEPs in the phosphor layer, including the scattering coefficients, anisotropic scattering, the reduced scattering, and scattering amplitudes, are computed with Mie theory to provide a more thorough analysis of the scattering influences of CaCO₃ and TiO₂. The results display that both CaCO₃ and TiO₂ are suitable for achieving an increase in the chromatic adequacy, as they can increase the blue-light intensity to reduce the CCT variations and yellow-ring phenomenon. TiO₂ proves superior to CaCO₃ in stimulating the CCT-deviation reduction by presenting a two-fold lower CCT variance at a 30wt.% concentration, compared to the results at 0wt.% TiO₂. Although CaCO₃ resulted in a slightly lower decrease in varied CCT, by about 390 K, the luminous flux at 30% CaCO₃ is still higher and more stable than that at 30% TiO₂. Thus, CaCO₃ seems to be the better material for WLED lights, since it enables acquiring both high luminous flux and CCT homogeneity. However, it is noted that the concentrations of TiO₂ and CaCO₃ should be managed at a certain level to avoid the excessive scattering that induces a dramatic degradation in luminous efficacy.

eISSN:: 2083-134X
Język:: Angielski

Częstotliwość wydawania:: 4 razy w roku
Dziedziny czasopisma:: Materials Sciences, other, Nanomaterials, Functional and Smart Materials, Materials Characterization and Properties

Kanał RSS czasopisma

Comparison between SEPs of CaCO₃ and TiO₂ in phosphor layer for better color uniformity and stable luminous flux of WLEDs with 7,000 K

Data publikacji: 25 maj 2022

Zakres stron: 1 - 8

Otrzymano: 11 lip 2021

Przyjęty: 12 paź 2021

DOI: https://doi.org/10.2478/msp-2022-0008

Słowa kluczowe
WLEDs, scattering enhancement, luminous efficiency, color quality, CaCO, TiO

© 2021 Phan Xuan Le et al., published by Sciendo

This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License.

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Fig. 4

Comparison between SEPs of CaCO3 and TiO2 in phosphor layer for better color uniformity and stable luminous flux of WLEDs with 7,000 K