There are three forms of mixed-type WLED (white-light emitting diode) devices with considerable performances, such as pc/WLED, QD-WLED, and pc/R-WLED. In reality, researchers have been extensively working on achieving the remarkable chromatic generation and great performance of the natural lights for the artificial illumination sources. Thus, when it comes to the advancing development of white LEDs, especially the mixed-type models, the color rendering index (CRI) or R
Since R
Our LE model consists of R
Additionally, this research displayed the desirable indexes of spectrum for a chromatic element, along with photometric and colorimetric efficiencies, for pc/R-WLED to exhibit greater strength of LE under CCT values ranging from 5000 K to 8000 K with R
First, CdS:In phosphor was prepared by putting the dry CdS and In2O3 powders with suitable amounts (see Table 1) into a quartz ampoule and then thoroughly mixed with a Vortex mixer. Then the quartz ampoule was sealed under vacuum at roughly 1 × 10 torr. The ampoule was tempered for 10 hours at the temperature of 900°C. The ampoule was opened and then the substance was pulverized into mortar. After the process, the obtained material had a uniform light green appearance. In the case that the color was not consistent, the substance underwent the same tempering step again in an ampoule under vacuum conditions. The resulting phosphor was greenish-yellow in color with a green emission peak at approximately 519 nm, and can be excited by either any ultraviolet (UV) or observable blue lighting, with an exponential decay time <1 s [19].
The composition of CdS:In
CdS | 99% | 1 g |
In2O3 | 1% | 7.95 mg |
We recreated the layer of phosphor in the WLEDs having flat layers of silicone using the LightTools 9.0 and the Monte Carlo technique. The recreation procedure is a two-stage process in which the initial step is to define the desired WLED structure and simulate it accordingly. The next step is to adjust and monitor the change of CdS:In concentration to analyze its impacts on optical-efficiency parameters of the WLED. To assess how the YAG:Ce3+ and CdS:In phosphor compounding can initiate the changes to WLED's optical parameters, comparisons among the WLEDs’ outputs need to be demonstrated and it is necessary to create certain contrasts. Particularly, the investigation on the phosphor compounds at the four different average CCT levels of 5000 K, 5700 K, 6500 K, and 8000 K, and the two-layer remote phosphor is carried out. In Figure 1, we can see a depiction of WLED lamps having conformal phosphor compounding at a high CCT of 8500 K. It is also indicated that the facets of WLEDs’ recreation do not involve CdS:In. The reflector's bottom length, height, and top surface length are measured at 8 mm, 2.07 mm, and 9.85 mm, respectively. The batch of nine similar LED chips is covered in conformity with a yellow-phosphor sheet and is attached to the reflector's cavity. The chip has a default 0.08 mm thickness, dimension of 1.14 mm long and 0.15 mm tall, a radiated flux of 1.16W, and a peak wavelength of 453 nm.
The formula below determines the relative spectral power distribution (RSPD) in the pc/RWLED device that contains LEDs treated with phosphor, along with phosphors in yellow and green colors, under blue and red LEDs’ excitation [20]:
In Eq. (8), V(
In Eq. (9), j is equal to the values of 1, 2, 3, 4, 5, 6, 7, and 8 that represent the CCT values of 5000 K, 5700 K 6500 K, and 8000 K. The mixed-type LED devices’ color appeared to remain across the Planckian locus (CCT < 5000 K) or daylight CCT point (CCT > 5000 K). The color distinction between the Planckian and daylight point on the 1960 uv color graph, D
The YAG:Ce3+ has been recognized for the high luminescent feature but the high doping concentration resulted in low homogenous white light. Since it is essential to have both blue and red spectral energies to generate good white light, the YAG:Ce3+ phosphor is deficient in pumping red ones. Not only does the color property decrease but also the total light extraction efficiency when high doping YAG:Ce3+ amount is utilized. This could be ascribed to a large amount of light trapped in the gap between the phosphor sheet and the chip, leading to a higher probability of light reabsorption that finally promotes the energy loss of the WLED. In other words, the luminous output of the WLED probably decreases. Such disadvantages could be improved by lowering the doping concentration of YAG:Ce3+. However, this concentration reduction could not completely solve the chromatic issue related to red-spectral deficiency. Therefore, the use of red LED is to improve the red color elements for the white light. Regarding the use of green phosphor CdS:In, it aims at enhancing the color uniformity via balancing the blue, red, and green colors, while reducing the effects of high YAG:Ce3+ concentration. The green spectral is enhanced, and the yellow phosphor concentration is reduced when the doping concentration of CdS:In increases, as can be seen in Figures 2 and 3.
Simulated results of yellow-phosphor concentration when varying green-phosphor CdS:In concentration with increasing particle size:
The simulated WLEDs’ emission power when adding CdS:In phosphor
Particularly, Figure 2 shows that the YAG:Ce3+ yellow-phosphor concentration displays a decline linked to the heightened CdS:In green-phosphor concentration. Such opposite introduces key functions of CCT-stability maintenance and influences internal absorption and scattering abilities of phosphor sheets. As the absorption and scattering features are impacted, the color attributes of white light will change consequently. Hence, the choice of CdS:In concentration is one of the decisive factors to determine the LE and chromatic performance of the pc/R-WLED devices. As the said concentration raises from 5 wt% to 15 wt%, the concentration of YAG:Ce3+ goes down to maintain the average CCT levels, in all four CCT cases.
Figure 3 demonstrates the influence of green-phosphor CdS:In concentration on the emission strength of the pc/WLED. Based on the given data, as well as the objectives of manufacturers, the suitable concentration for CdS:In to be integrated would be determined. If the chroma properties are the priority, the luminescent flux can display a minor decline in its intensity, and vice versa. From Figure 3, it can be inferred that white-light generation is possible to obtain from the combination of three spectral zones, including blue, yellow, and green-orange. Here, the emission strengths are recorded at 5000 K, 5700 K, 6500 K, and 8000 K CCTs. The 420–480 nm and 500–640 nm emission regions show enhancements as the corresponding CCT is higher, which denotes the improvement in luminescent flux. Moreover, these data demonstrate the promoted scattering intensity of blue light within the WLED. Greater scattering chances would lead to greater uniformity of the chroma scale. Regulating good color uniformity at a high CCT point is a significantly challenging task for remote phosphor packages in WLEDs. So, the observed enhancements in emission spectra and scattering probability when using CdS:In are noticeable and important to the advancement of WLEDs.
Color uniformity is one of the critical chromatic features of a white-light source. It is noted that human eyes are more sensitive to chromatic gradients or deviations than the illuminance differences [26]. Therefore, minimizing the color deviations is crucial to enhance the color quality of the LED emitting white light, implying the probability of increasing WLED's prices in the lighting market due to its heightened color uniform adequacy and fidelity. The CdS:In phosphor, on the other hand, is one of the most popular phosphor materials for pc-WLED devices, and it could offer the economical factor; as such it may have widespread application. Notably, when adding the green phosphor CdS:In, the color deviation is significantly reduced, as can be seen in Figure 4. The variation among the essential chroma elements displays a notable reduction connected to the green-phosphor CdS:In addition, at four specific CCTs. This reduction in color differences could be a function of the absorption characteristic in green-phosphor CdS:In film. Generally, the chromaticity follows the RGB color basis, thus adding CdS:In is beneficial to the green light enhancement. The granules of green phosphor convert the blue light into green light as the said phosphor absorbs the blue light generated by the chip of LED. Besides the blue light mentioned, the granules of CdS:In also introduce the yellow-light absorption but have weaker strength. In other words, the absorption of blue light perform by this green phosphor is more significant. Furthermore, with CdS:In in the phosphor configuration, the scattering features could be improved considerably, which is also crucial to the heightening of chromatic homogeneity. As mentioned above, the reduction of yellow phosphor amount with increasing concentration of CdS:In promotes the scattering of lights, this mechanism acts as an integrator to redistribute the color elements and stimulate the combination efficiency to produce white light. Moreover, with the enhanced scattering ability, the lights from the chip and phosphor layer are transmitted in multiple directions to sufficiently reach the right and left edges of the WLED structure, offering a greater chance of light mixing in these regions to reduce the yellow-ring effects. In short, the improved scattering efficiency and green light components with CdS:In contribute to optimizing the chromatic uniformity of the WLED.
The simulated angular-dependent CCTs when adding CdS:In green phosphor. CCT, correlated color temperature
Determining the WLEDs’ chromatic performance is related to other factors, not solely the chromatic uniformity; they are the rendering properties of the light source. It is undeniable that great chromatic uniformity helps to ensure visual comfort for users but the overall adequacy of white-light chromaticity cannot be fulfilled. Consequently, other chromatic indices are proposed to access the unreached aspects of lighting color performance, color rendition. The CRI is one of the popular parameters to evaluate the chromatic rendering ability of white-light source on tested objects. It can assess the effectiveness in the ability to reproduce an object's color of the white light when it lightens that object. However, the CRI is somehow ineffective to present an accurate color fidelity evaluation as this parameter just allows a small color gamut, about eight color samples, for color reproduction. The color quality scale (CQS), on the other hand, allows color generation to perform on 15 color samples, thus more color shades are examined, resulting in higher accuracy. Besides, when applying CQS for color tests, it means all the features of CRI, human visuality, and chromatic coordinates are specifically analyzed. Therefore, the CQS can be regarded as a powerful and efficient metric to provide a more accurate assessment for the color-recreation ability of a light source [23].
The CRI and CQS values of the WLED with increasing weight percentages of CdS:In can be observed in Figures 5 and 6, respectively. As can be seen in Figures 5 and 6, the concentration of 15% CdS:In presents the lowest CRI and CQS, regardless of the CCT values. This means the concentration increase of CdS:In can cause the CRI and CQS to reduce. This might be the result of the supplemented green spectral energy being too excessive to keep the stability and consistency among the essential colors, which are blue, yellow, and green-orange, owing to the over-added concentration of CdS:In. This imbalance will initiate the degradation in color rendition and reproduction of WLED light. However, when the increasing CdS:In concentration is introduced and kept <10 wt%, there is no notable reduction in CQS. More than 10 wt% CdS:In apparently causes degradation in performances of either CRI or CQS, according to the mentioned explanation of color imbalance by redundant green-light energy. This also reinforces the importance of selecting an adequate weight percentage of green-phosphor CdS:In.
Simulated CRI values when varying green-phosphor CdS:In concentration with increasing particle size:
Simulated CQS values when varying green-phosphor CdS:In concentration with increasing particle size:
Generally, in a WLED, the enhanced color properties may get the LE decreased. Thus, the luminescent flux intensity in the remote phosphor configuration using a dual-layer packet with the presence of CdS:In should be investigated and demonstrated. Figure 7 shows that the produced lumen is offered a substantial boost as the CdS:In concentration goes from 5 wt% to 15 wt% at all determined CCTs (5000–8000 K). This improvement is attributed to the lower yellow phosphor concentration, as mentioned above. The back-scattering and light trapped events are common to the WLED package with a high concentration or thicker layer of yellow YAG:Ce3+, thus the reabsorption and energy loss are likely to be significant.
Simulated luminous flux when varying green-phosphor CdS:In concentration with increasing particle size:
Therefore, when the smaller yellow phosphor doping concentration is presented, the higher drawback connected to the backscattering-causing low extraction efficiency can be addressed. Thus, CdS:In has fulfilled its responsibility in offering the LE enhancement. Last but not least, it is essential to take both lumen and color performances into consideration before determining the concentration of CdS:In for WLED applications.
Our research demonstrates how the green phosphor CdS:In can affect the light attributes in the two-layer phosphor package. Through the Monte Carlo recreation on the computer, our research confirms that CdS:In can be chosen to boost the chromatic homogeneity, which applies to the WLED devices at a small color temperature of 5000 K as well as color temperature >8000 K. The attained results of the study are significant, one of which shows the reduction in color deviation, the key factor to bettering the color homogeneity. Additionally, the CRI and CQS exhibit greater values when the CdS:In concentration is ≤ 10 wt%. More than 10 wt% CdS:In can cause the reduction in both rendering parameters due to the degraded color balance initiated by redundant green-light proportion. Meanwhile, the luminous flux is heightened with the rise in CdS:In concentration, thanks to the effective minimization of back-scattered and trapped lights. Therefore, the discovery of the research has met its goal of boosting the chromatic performance and lumen, a complex task for the remote phosphor package. Finally, the decision of suitable green-emitting CdS:In concentration must not be ignored and should depend on the manufacturer's goals. This article can provide vital data to be used for reference when it comes to getting better chromatic homogeneity and lumen in WLED devices.