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].
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.
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
The
With Mie-scattering application,
In these equations,
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.
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:
The homogeneity of a WLED can be demonstrated by the extent of color deviation that occurs, which can be defined by: ΔCCT = CCT(
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.
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.
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]:
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.
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.