Triple-layer remote phosphor structure: A potential packaging configuration to enhance both color quality and lumen efficiency of 6,000–8,500 K WLEDs

Solid-state lighting (SSL) has been widely applied in modern illuminations as it consumes less energy and generates less heat than other traditional lighting sources, such as incandescent bulbs and fluorescent tubes [1]. Therefore, white light-emitting diodes (WLEDs), the main light source of SSL, have received significant recognition due to their ability to offer higher lighting efficiency, lower energy consumption, and lower production cost [2]. WLEDs have also been reported to be used under various environmental conditions due to their robustness and long life span [3]. However, the luminous efficiency of WLEDs must be further improved for a wide range of applications in SSL. Generally, the lights generated from traditional LEDs are composed of the blue lights from the LED chip and the converted yellow light from the yellow-emitting phosphor layer [4, 5]. One of the most popular techniques to fabricate a WLED package is freely dispersed phosphor coating. This method utilizes the combination between encapsulated transparent resin and the phosphor powder, which is then dispersed onto the LED chip. Though this method offers a simple fabricating process, affordable production cost, and flexible modification of the thickness of the phosphor layer, the angular color uniformity of the WLED is poor due to the inhomogeneity in the distributions of blue and yellow lights [6, 7]. Thus, it is not suitable for today’s high-power WLEDs. The conformal phosphor coating method has been proposed to address the problems of the previous method [8]. Conformal phosphor coating can give uniform light distribution, leading to more homogeneous angular correlated color temperatures (CCTs) [9, 10]. Yet, this method yields low lumen output due to the strong backscattering effect.

Several studies have previously analyzed the remote phosphor package for WLEDs, in which separation between the phosphor film and the LED chip is used to minimize the heating damages to the phosphor materials and to promote better lumen efficiency [11]. Enhanced light extraction by an internal reflection luminaire structure is introduced in this approach, and it is capable of enhancing the light extraction efficiency of WLEDs by at least 26% [12]. Additionally, another structure with an embedded air gap provides sufficient reflection of downward light to promote better luminescence [13]. These studies also pinpoint the importance of phosphor materials, besides the packaging design, in determining the luminous efficiency of a WLED device. Particularly, once the phosphor concentration increases, the reabsorption loss in the phosphor film becomes significant and, as a result, causes considerable degradation in the emitted luminous flux, which is more noticeable in the low-CCT WLED packages [14,15,16,17]. Therefore, it is crucial to obtain higher blue and yellow light emissions while minimizing the loss of backscattered and reflected light to achieve optical improvements in high-quality WLED lamps.

Considering the above-mentioned issues, this research paper presents a triple-layer remote phosphor structure as a more practical solution for higher WLED optical performances. The WLED models used in the experiments have an average CCT range of 6,000K–8,500K. The phosphors selected for the fabrication of the triple-layer phosphor LED configuration are red-emitting CaMoO₄:Eu³⁺ phosphor, green-emitting CaAl₂O₄:Mn²⁺ phosphor, and yellow-emitting YAG:Ce³⁺ phosphor. The CaAl₂O₄:Mn²⁺ phosphor – with a broad range of green emission spectra – can help increase the green-light components in the packages and thus is beneficial to the luminous flux. Meanwhile, the red-emitting CaMoO₄:Eu³⁺ phosphor can improve the color homogeneity with its high red spectral intensity. The results from experiments and calculations in the study point out that the color quality of the LED package is optimized owing to the balanced distribution among the red, green, and yellow colors, while obtaining minimum reduction in lumen output. We believe that this study can contribute greatly to the enhancement of LED optical performances, especially in terms of color uniformity and light extraction efficiency.

Figure 2 shows the changes of CRI in connection with the concentration of red-emitting and green-emitting phosphor layers in the range of 2wt%–20wt%. In particular, when the concentration of red CaMoO₄:Eu³⁺ layer increases, the CRI also goes up. When 20% CaMoO₄:Eu³⁺ is applied, the structure achieves the highest CRI values. Conversely, the green-emitting CaAl₂O₄:Mn²⁺ phosphor is not beneficial to the color rendering index. As can be seen, the CRI decreases continuously as the concentration of CaAl₂O₄:Mn²⁺ increases to 20wt%, regardless of the increase in red-emitting phosphor concentration. Thus, to get better CRI, a higher red-emitting CaMoO₄:Eu³⁺ phosphor concentration is required to strengthen the red emission spectrum. However, the green-emitting phosphor concentration must be limited since high concentration of CaAl₂O₄:Mn²⁺ apparently leads to high proportion of green-light components, which is not favorable for CRI. Besides, the increase in green-emitting phosphor concentration degrades the red-emitting phosphor conversion energy, as the lights emitted from the chip reach the green-emitting phosphor layer before being transmitted to the red layer due to the order of the phosphor layers in the triple-layer structure. Consequently, it is essential to limit the concentration of the CaAl₂O₄:Mn²⁺ phosphor to the lowest to obtain better CRI for the LED package. Though WLED devices with high CRI are somehow desirable and sold with high prices in the market, this parameter is not able to evaluate all the aspects of white-light color quality. In fact, CRI is used to determine the ability of a light source to show the true color of the illuminated objects. In addition, two other factors are important in the evaluation of color quality: the preference of viewers and the color coordinates; the parameter that can cover all the three criteria is the color quality scale (CQS) [19]. This implies that CQS is more powerful and difficult to manage than CRI.

Fig. 2

Figure 3 displays the CQS of the triple-layer WLED package with different concentrations of red-emitting CaMoO₄:Eu³⁺ phosphor and green-emitting CaAl₂O₄:Mn²⁺ phosphor layers. The point of this experiment is to figure out whether it is possible to achieve high CQS by adjusting the phosphor concentration. Similar to the CRI, red-emitting CaMoO₄:Eu³⁺ phosphor can improve the CQS when its concentration increases. Moreover, as can be seen in Figure 3, the modification of proportion of green-emitting CaAl₂O₄:Mn²⁺ phosphor also benefits the CQS. Particularly, considering the CQS values at a certain red-emitting phosphor concentration, the difference between the CQS values on changing the green-emitting phosphor concentration is insignificant. This proves that CaAl₂O₄:Mn²⁺ and CaMoO₄:Eu³⁺ are appropriate to enhance the CQS of the generated white lights. The triple-layer packaging structure can heighten the CQS for WLED devices because it has the ability to balance the yellow, green, and red light colors, the three crucial colors of white-light formation. Specifically, the yellow YAG:Ce³⁺ concentration decreases to keep the stability of color temperature since the concentration of the red CaMoO₄:Eu³⁺ or green CaAl₂O₄:Mn²⁺ increases.

Fig. 3

The color balance is critical to the performance of CQS, as proven above. Thus, to maintain high CQS, it is essential to control the proportion of green-emitting CaAl₂O₄:Mn²⁺ phosphor to attain sufficient amount of green light color for the best color balance among the green, red, and yellow components. From Figure 3, we can observe the increase in CQS with the green-emitting CaAl₂O₄:Mn²⁺ phosphor in the 2wt%–10wt% concentration range. Moreover, the CQS reaches the highest values when CaAl₂O₄:Mn²⁺ is from 10wt% to 14wt%, since the abundant yellow-light amounts are reduced. In other words, at 10wt%–14wt% of CaAl₂O₄:Mn²⁺, the proportion of green light is sufficient to maintain a balance among the three colors, and the CQS is enhanced consequently. When the concentration of CaAl₂O₄:Mn²⁺ is >14wt%, the CQS starts to decline due to the excessive green-light proportion, damaging the color balance.

In fact, compared to the conformal phosphor coating or in-cup phosphor packaging structure, accomplishing improvement in color uniformity of remote phosphor structure is more difficult and complex, especially the one with high CCT ranging from 7,000K to 8,500K. The use of triple-layer remote phosphor structure aims to address this problem since the results indicate that better CQS is observed in not only low CCT-WLED but also high CCT-WLED packages. Besides having a balanced distribution among the color elements, as mentioned herein, the triple-layer phosphor structure also boosts light scattering in the LED package, leading to better light mixing and thus enhancement of the white-light quality. However, the problem is that when the scattering is better, the lumen output tends to be lower. Thus, the analysis of lumen output in connection with the increase in light scattering is essential.

To carry out the analysis on the WLED efficiency with triple-layer remote phosphor structure, calculation of its transmitted blue light and converted yellow light is carried out. The computation is based on the Mie scattering theory and starts with the expressions of dual-layer remote phosphor structure to demonstrate the better efficiency of the triple-layer structure.

For the dual-layer structure, its transmitted-blue-light and converted-yellow-light computation is presented as follows. The thickness of each phosphor layer in this package is set as h [19, 20]. (1) PB2=PB0e−αB2he−αB2h=PB0e−2αB2h {{PB}_{2}}={{PB}_{0}}{{e}^{-{{\alpha }_{{{B}_{2}}}}h}}{{e}^{-{{\alpha }_{{{B}_{2}}}}h}}=P{{B}_{0}}{{e}^{-2{{\alpha }_{{{B}_{2}}}}h}} (2) PY2=12β2PB0αB2−αY2[e−αY2h−e−αB2h e−αY2h]+ 12β2PB0αB2−αY2[e−αY2h−e−αB2h−e−αB2h]e−αY2h+ 12β2PB0αB2−αY2[e−αY2h−e−αB2h]= 12β2PB0αB2−αY2[e−2αY1h−e−2αB1h] \begin{array}{*{35}{l}} P{{Y}_{2}} & =\frac{1}{2}\frac{{{\beta }_{2}}P{{B}_{0}}}{{{\alpha }_{{{B}_{2}}}}-{{\alpha }_{{{Y}_{2}}}}}\left[ {{e}^{-{{\alpha }_{{{Y}_{2}}}}h}}-{{e}^{-{{\alpha }_{{{B}_{2}}}}h}}\ {{e}^{-{{\alpha }_{{{Y}_{2}}}}h}} \right] \\ {} & +\ \frac{1}{2}\frac{{{\beta }_{2}}P{{B}_{0}}}{{{\alpha }_{{{B}_{2}}}}-{{\alpha }_{{{Y}_{2}}}}}\left[ {{e}^{-{{\alpha }_{{{Y}_{2}}}}h}}-{{e}^{-{{\alpha }_{{{B}_{2}}}}h}}-{{e}^{-{{\alpha }_{{{B}_{2}}}}h}} \right]{{e}^{-{{\alpha }_{{{Y}_{2}}}}h}} \\ {} & +\ \frac{1}{2}\frac{{{\beta }_{2}}P{{B}_{0}}}{{{\alpha }_{{{B}_{2}}}}-{{\alpha }_{{{Y}_{2}}}}}\left[ {{e}^{-{{\alpha }_{{{Y}_{2}}}}h}}-{{e}^{-{{\alpha }_{{{B}_{2}}}}h}} \right] \\ {} & =\ \frac{1}{2}\frac{{{\beta }_{2}}P{{B}_{0}}}{{{\alpha }_{{{B}_{2}}}}-{{\alpha }_{{{Y}_{2}}}}}[{{e}^{-2{{\alpha }_{{{Y}_{1}}}}h}}-{{e}^{-2{{\alpha }_{{{B}_{1}}}}h}}] \\\end{array} Next, in the triple-layer structure, the thickness of each phosphor film is 2h3 \frac{2\text{h}}{3} ; therefore, the calculation of its transmitted blue light and converted yellow light can be expressed as follows: (3) PB3=PB0e−αB22h3e−αB22h3e−αB22h3=PB0e−2αB3h \begin{array}{*{35}{l}} {{PB}_{3}} & ={{PB}_{0}}{{e}^{-{{\alpha }_{{{B}_{2}}}}\frac{2h}{3}}}{{e}^{-{{\alpha }_{{{B}_{2}}}}\frac{2h}{3}}}{{e}^{-{{\alpha }_{{{B}_{2}}}}\frac{2h}{3}}} \\ {} & ={{PB}_{0}}{{e}^{-2{{\alpha }_{{{B}_{3}}}}h}} \\\end{array} (4) PY3′=12β3PB0αB3−αY3[e−αY32h3−e−αB32h3]e−αY32h3+ 12β3PB0e−αB32h3αB3−αY3 [e−αY32h3−e−αB32h3]= 12β3PB0αB3−αY3[e−αY34h3−e−2αB34h3] \begin{array}{*{35}{l}} PY_{3}^{'} & =\frac{1}{2}\frac{{{\beta }_{3}}P{{B}_{0}}}{{{\alpha }_{{{B}_{3}}}}-{{\alpha }_{{{Y}_{3}}}}}\left[ {{e}^{-{{\alpha }_{{{Y}_{3}}}}\frac{2h}{3}}}-{{e}^{-{{\alpha }_{{{B}_{3}}}}\frac{2h}{3}}} \right]{{e}^{-{{\alpha }_{{{Y}_{3}}}}\frac{2h}{3}}} \\ {} & +\ \frac{1}{2}\frac{{{\beta }_{3}}P{{B}_{0}}{{e}^{-{{\alpha }_{{{B}_{3}}}}\frac{2h}{3}}}}{{{\alpha }_{{{B}_{3}}}}-{{\alpha }_{{{Y}_{3}}}}}\ \left[ {{e}^{-{{\alpha }_{{{Y}_{3}}}}\frac{2h}{3}}}-{{e}^{-{{\alpha }_{{{B}_{3}}}}\frac{2h}{3}}} \right] \\ {} & =\ \frac{1}{2}\frac{{{\beta }_{3}}P{{B}_{0}}}{{{\alpha }_{{{B}_{3}}}}-{{\alpha }_{{{Y}_{3}}}}}\left[ {{e}^{-{{\alpha }_{{{Y}_{3}}}}\frac{4h}{3}}}-{{e}^{-2{{\alpha }_{{{B}_{3}}}}\frac{4h}{3}}} \right] \\\end{array} (5) PY3=PY3′e−αY32h3+PB0e−2αB34h312β3αB3−αY3[e−αY32h3−e−αB32h3]=12β3PB0αB3−αY3[e−αY34h3−e−αB34h3]e−αY32h3+ 12β3PB0e−αB34h3αB3−αY3[e−αY32h3−e−αB32h3]=12β3PB0αB3−αY3[e−αY3h−e−2αB3h] \begin{array}{*{35}{l}} P{{Y}_{3}} & =P{{Y}^{'}}_{3}{{e}^{-{{\alpha }_{{{Y}_{3}}}}\frac{2h}{3}}}+P{{B}_{0}} \\ {} & {{e}^{-2{{\alpha }_{{{B}_{3}}}}\frac{4h}{3}}}\frac{1}{2}\frac{{{\beta }_{3}}}{{{\alpha }_{{{B}_{3}}}}-{{\alpha }_{{{Y}_{3}}}}}\left[ {{e}^{-{{\alpha }_{{{Y}_{3}}}}\frac{2h}{3}}}-{{e}^{-{{\alpha }_{{{B}_{3}}}}\frac{2h}{3}}} \right] \\ {} & =\frac{1}{2}\frac{{{\beta }_{3}}P{{B}_{0}}}{{{\alpha }_{{{B}_{3}}}}-{{\alpha }_{{{Y}_{3}}}}}\left[ {{e}^{-{{\alpha }_{{{Y}_{3}}}}\frac{4h}{3}}}-{{e}^{-{{\alpha }_{{{B}_{3}}}}\frac{4h}{3}}} \right]{{e}^{-{{\alpha }_{{{Y}_{3}}}}\frac{2h}{3}}} \\ {} & +\ \frac{1}{2}\frac{{{\beta }_{3}}P{{B}_{0}}{{e}^{-{{\alpha }_{{{B}_{3}}}}\frac{4h}{3}}}}{{{\alpha }_{{{B}_{3}}}}-{{\alpha }_{{{Y}_{3}}}}}\left[ {{e}^{-{{\alpha }_{{{Y}_{3}}}}\frac{2h}{3}}}-{{e}^{-{{\alpha }_{{{B}_{3}}}}\frac{2h}{3}}} \right] \\ {} & =\frac{1}{2}\frac{{{\beta }_{3}}P{{B}_{0}}}{{{\alpha }_{{{B}_{3}}}}-{{\alpha }_{{{Y}_{3}}}}}\left[ {{e}^{-{{\alpha }_{{{Y}_{3}}}}h}}-{{e}^{-2{{\alpha }_{{{B}_{3}}}}h}} \right] \\\end{array} Here, in Eqs (1–5), h indicates the phosphor layer’s thickness. The triple layer and dual layer are described by the subscripts “3” and “2”, respectively; β is the conversion coefficient when blue light is converted to yellow light in the package, while γ indicates the yellow-light reflection coefficient. PB is the blue light intensity and PY is the intensity of yellow light, both of which combine to give PB₀ – the light intensity from the LED chip. Besides, PY₃ represents the yellow light transmitted through the two red-emitting and green-emitting phosphor layers; α_B and α_Y describe the fractions of blue-light and yellow-light energy loss, respectively [21, 22].

Higher lighting performance of the triple-layer structure can be observed, compared to that of the dual-layer structure, as demonstrated in the following expression: (6) (PB3−PY3)−(PB2+PY2)(PB2+PY2)>e−2αB3h−e−2αB2he−2αY3h−e−2αB2h \begin{array}{*{35}{l}} \frac{\left( P{{B}_{3}}-P{{Y}_{3}} \right)-\left( P{{B}_{2}}+P{{Y}_{2}} \right)}{\left( P{{B}_{2}}+P{{Y}_{2}} \right)} \\ >\frac{{{e}^{-2{{\alpha }_{{{B}_{3}}}}h}}-{{e}^{-2{{\alpha }_{{{B}_{2}}}}h}}}{{{e}^{-2{{\alpha }_{{{Y}_{3}}}}h}}-{{e}^{-2{{\alpha }_{{{B}_{2}}}}h}}} \\\end{array} The scattering property of the utilized phosphor materials and the scattering cross section C_sca for the phosphor spheres are analyzed and calculated using Mie scattering theory [23, 24]. The Beer–Lambert law is applied to compute the transmitted light power [25, 26]: (7) I=I0exp(−μextL) I={{I}_{0}}\text{exp}(-{{\mu }_{ext}}L) where I₀, L, and µ_ext are the incident light power, the thickness of the phosphor layers, and the extinction coefficient, respectively. Here, the extinction coefficient can be calculated using the equation µ_ext = N_r · C_ext, in which N_r expresses the number density distribution of the particles (in particles per cubic millimeter), and C_ext (in square millimeters) shows the spherical phosphor’s extinction cross section.

Expression (6) implies that more phosphor layers will result in higher lumen output for WLED packages. Enhancement in lumen efficiency means that the concentrations of the red-emitting and green-emitting phosphor layers increase. As mentioned earlier, the reduction in yellow-emitting phosphor concentration, as a function of the higher concentration of green-emitting and red-emitting phosphors, is necessary for CCT stability. The decline in yellow-emitting phosphor concentration minimizes the energy loss of lights by scattering to promote luminous intensity. Based on Beer’s law and Eq. (7), it can be demonstrated that the higher the light transmission energy, the more is the reduction in the yellow-emitting phosphor concentration, and consequently, the luminous output is intensified. Thus, the increase in either green-emitting CaAl₂O₄:Mn²⁺ or red-emitting CaMoO₄:Eu³⁺ phosphor film is beneficial to WLED lumen efficiency. However, continuous increase in the concentration of red-emitting or green-emitting phosphor probably ruins the color balance as there are excessive amounts of red or green lights; thus, the considerable degradation in CQS is unavoidable.

Figure 4 demonstrates the increase in luminous flux values following the increase of green-emitting CaAl₂O₄:Mn²⁺ phosphor concentration. Specifically, the luminous output can be enhanced by 40% as the green light increases in its proportion and its scattering events, regardless of the changes in red-emitting CaMoO₄:Eu³⁺ phosphor concentration. Moreover, Figure 5 demonstrates the increase in blue, green, and red spectral intensities. This indicates that the blue light transmission and conversion are better, and the color distribution is enhanced, leading to boosted color uniformity for the WLED package. This means that it is possible to achieve improvement in both CQS and luminous efficiency at the same time using the triple-layer structure. However, it is inadvisable to increase the green-emitting phosphor concentration continuously, as the color quality deteriorates sharply. According to the experimental results of CQS investigation, the suitable amount of CaAl₂O₄:Mn²⁺ phosphors is in the range of 10wt%–14wt%, while the red-emitting CaMoO₄:Eu³⁺ phosphor can be from 2wt% to 20wt%.

Fig. 4

Fig. 5

The application and investigation of the influences of the triple-layer remote phosphor packaging structure for WLEDs are demonstrated in this article. In addition to the yellow-emitting YAG:Ce³⁺ phosphor, the other two materials used to fabricate the triple-layer structure are red-emitting CaMoO₄:Eu³⁺ and green-emitting CaAl₂O₄:Mn²⁺ phosphors. The experimental and analyzed results showed that the triple-layer remote phosphor is suitable and has potential for the enhancement of LED optical performances, especially the color quality and lumen efficiency. In particular, with appropriate concentrations of red-emitting and green-emitting phosphors, it is possible to attain higher color uniformity while stabilizing the luminous intensity at high values. The research showed that the concentration of the green-emitting CaAl₂O₄:Mn²⁺ phosphor should remain between 10wt% and 14wt%, and the red-emitting phosphor concentration may be adjusted from 2% to 20%. Hence, this study can be a useful reference for manufacturers to obtain a color balance among the green, red, and yellow components of white lights. In other words, application of the triple-layer remote phosphor structure can be considered in WLED manufacturing to accomplish high-color-quality WLEDs with better luminous efficiency.