Triple-layer remote phosphor geometry: an excellent selection to improve the optical properties of white light-emitted diodes

ABSTRACT


INTRODUCTION
The white-light-emitting diodes (WLEDs) capable of converting white illumination from phosphor substances has been a desirable illumination means when it comes to lighting stability, durability, and ecofriendliness.As a result, this solid-state lighting solution began to replace older lighting methods and aim for more demanding lighting applications [1]- [3].According to previous research results, the color rendering index is a key indicator to human perception of a light source, however, most WLEDs have a chroma rendition indicator (CRI) below 80 and only the highest quality can reach 80 as the consumer review in 2016 showed [4], [5].So, to apply WLEDs in advanced lighting, the CRI must be above 90, which the current WLEDs fabricating method cannot obtain.To achieve this result, a solution of adding more phosphor materials (Lu3Al5O12:Ce 3+ and CaAlSiN3:Eu 2+ ) to produce constricted blue light spectrum, green, and redlight spectrum was introduced, which effectively create WLEDs with CRI over 90 [6]- [8].As of late, phosphor-in-glass (PiG) WLEDs with CRI over 90 were shown on papers, however, the creation process, the optical properties, and materials set up to achieve such result have not been published [9], [10].The PiG with stable heat performance in chromaticity coordinates shift and the ability to preserve lumen output in high energy apparatuses like laser exhibitor as well as laser illuminator [11].Although there are many advantages in PiG, the temperature in which the PiG is crafted is the issue [12]- [14].The 700 °C during the fabrication process of PiG creates an obscure area at the junction of the glass and red phosphor (CaAlSiN3:Eu 2+ ) and induce inter-diffusion between these materials.Besides, the high temperature during sinister cause the ISSN: 2088-8708  Triple-layer remote phosphor geometry: an excellent selection to improve the optical … (Thuc Minh Bui) 5119 emitted light and transparency of the red PiG to decrease, which is detrimental to the CRI of PiG making the PiG unsuitable for high-demand lighting applications as it is required by international commission on illumination (CIE) that the appropriate WLEDs must reach 90 in CRI [15]- [17].Considering these circumstances, improving the CRI of co-doped PiG by adjusting temperature in the sintering process is an essential step to enhance the WLED.
To achieve high CRI co-doped PiG, Zn2SiO4:Mn 2+ , As 5+ and YAl3O4B12:Eu 3+ were distributed on the glass matrix (B2O3-Sb2O3-SiO2-Ta2O5).Before conducting the experiments, the sintering temperature and refractive index need to be adjusted by balancing the percentage of B2O3 and SiO2 glass components.On the other hand, the glass matrix also received adjustments through Sb2O3 and Ta2O5 for the purpose of eliminating gaseous contents as well as attaining significant refraction indicator.The correct procedure to produce glass matrix is to start by mixing the ingredients evenly, then baked at 1,080 °C, and leave to rest in normal air temperature.The calculation outcomes suggest that the glass matrix has good lighting transparency, significant refraction indicator reaching 1.8, exhibits the glass shift heat level of 527 °C, and thawing heat level reaching 593 °C.As a result, glass matrix with Sb2O3 and Ta2O5 is optimal for dually-incorporated PiG, able to reach significant CRI chroma transmutation sheet applicable to demanding interior illuminating utilization.Low sintering temperature that is employed for lessening cross-dispersion among the materials from the dually-incorporated PiG WLED apparatuses yielding CRI=94 will be illustrated.First, the green as well as red phosphor samples are diffused onto the glass latticework.Afterwards, the sintering heat level, fraction as well as mass proportion for said samples would receive appropriate adjustment, completing the goal of high CRI.Though dually-incorporated PiG thickness management, the correlated chroma temperatures (CCTs) in the WLED apparatuses would be between 3,900 and 5,300 K.The innovative co-doped PiG with great heat generation consistency can be the solution for WLEDs with optic attributes suiting demanding illumination purposes.

RESEARCH METHOD 2.1. Preparation of phosphor materials
The following section will demonstrate the process to obtain the phosphor materials used in our experimental tests.The starting materials would be Zn2SiO4:Mn 2+ , As 5+ .The constituents for this green substance would be blended by milling within water for 2 hours.After the ingredients are mixed into one mixture, leave it to dry.Once the compound is dry, grind it into powder.The final step is placing the powder from the last process within a closed quartz tube then firing under 1,200 °C within an hour.The final substance should emit green color and has a discharge apex reaching 2.35 eV.The Table 1 displays the chemical constituents that create the Zn2SiO4:Mn 2+ , As 5+ phosphor [18].
The red phosphor YAl3B4O12:Eu 3+ fabricated through 6 steps: mixing, 4 consecutive firing steps, and washing.Like green phosphor, the ingredients of red phosphor would be blended via pulverizing.Secondly, put the compound within unclosed quartz boats and fire within sixty minutes at 500 °C, the substance from this step will be grinded into powder before moving on to the next step.The next three steps are the compound being placed on open alumina crucibles and fire for 1 hour under 900, 1,100, and 1,200 °C.The product will be grinded to attain powder when the firing process is finished.Following the final heating step, clean the final substance in boiling water for multiple instances to remove residue then leave it till dry.The completed red YAl3B4O12:Eu 3+ phosphor will have red emission color and 2.01 and 2.035 eV emission peaks.The chemical composition is shown in Table 2

Recreation
Figure 1 displays the organization inside a WLED utilized in our research.Figures 1(a) and 1(b) show there are remote half dome to cover below, a remote-plate, and a conformal-coating pc-WLED package.The blue area at the base is the blue light-emitting diodes (LEDs), the chromatic plate are the phosphor materials, and the unoccupied zones between the plates and silicone.The substrate is aluminum nitride.The yellow phosphor would be YAG:Ce 3+ .5,600 K is the default color temperature for all structures when observing along the z-axis.Each phosphor plate is limited to 0.08 mm in density, guaranteeing precise calculation data.The concentration of yellow YAG:Ce 3+ also has an important role in which it must be changed accordingly to other chromatic phosphor proportions.This is to prevent the color temperature of WLED from changing and affect the results of experiments.The YAG:Ce 3+ content within the settings and average correlated color temperatures (ACCT) [20] will have disparate amount and that results in the diversity of scattering and optical properties of WLEDs.
In Figures 2(a) and 2(b), the YAG:Ce 3+ concentration in DL and TL are compared, which leads to the results that DL at all ACCTs will have higher amount of yellow phosphor.More yellow phosphor at all ACCTs means the light emission capacity is reduced due to the back-scattering effect.In addition, the color quality is also damaged as the balance between the primary colors contributes to white light is distorted because the yellow phosphor increases.Therefore, the reduction of back-scattering, luminous flux enhancement, and chromatic performance improvement are the obstacles when enhancing WLEDs, which can be solved by adding the red and green phosphor to enforce control with the extra chromatic light components.Considering these requirements, the three-sheet configuration appears to be a good solution.To verify this theory, other optical properties are examined for reference.Figure 3 displays the discharge spectra of each setting across a range of wavelengths.The spectra intensity in the TL setting noticeably surpasses that in the DL setting, indicating greater proficiency from the TL setting.

RESULTS AND DISCUSSION
The color rendering index between dual-layer and triple-layer are examined side by side in Figures 4(a 5121 keeps increase as the color temperature rises and reach it peak at 8,500 K. Considering how hard achieving good CRI in WLEDs with color temperature higher temperature higher than 7,000 K is, this finding in DL structure makes it the optimal choice for WLEDs that aim for high CRI.The high CRI in DL is resulted from YAl3O4B12:Eu 3+ added into the structure, which certifies DL as the structure with best CRI even greater than TL structure.This result shows that DL can be effective if employed in the production process to create WLEDs with high CRI.However, CRI is not the only quality indicator that is important to the WLEDs quality evaluation.In fact, many researchers have utilized color quality scale (CQS) instead CRI due to CRI having limited evaluation scope while CQS including CRI, viewers' preference, and chromaticity coordinate is much more valuable.The CQS of the structures applied in this research are determined then displayed by Figure 5. Contrary to CRI, the CQS from TL is higher than DL as the TL is benefited by 3 different chromatic phosphor layers.The results from Figure 5 prove that if the color quality is what the manufacturers are after, the TL structure is the suitable option.However, in WLEDs with advanced chromatic performance, the light output usually is damaged in the process.So, is the light output in triple-layer remote phosphor also negatively impacted as side effect of good color quality? the luminous flux of both TL and DL must be compared to clarify this question.Therefore, the next part will demonstrate the calculation equations used to estimate propagated blue illumination as well as transmuted yellow illumination within the three-sheet setting.Various computational expressions were utilized to assess the illumination mechanism in each setting.The transmitted blue illumination and converted yellow illumination in the case of double-sheet phosphor setting having the phosphor sheet breadth (ℎ) would be expressed as formulas ( 1) and ( 2) [21]- [23].The propagated blue illumination as well as transmuted yellow illumination in the case of three-sheet setting having phosphor sheet breadth reaching (1) ℎ signifies the phosphor sheet's breadth.The subscripts "1" as well as "2" signify double-sheet as well as triple-sheet structures. signifies the transmutation coefficient applying to blue illumination transmuting into yellow illumination. signifies the reflectivity coefficient for yellow illumination.The intenseness for blue () as well as yellow illumination () would be the illumination intenseness generated by blue LED, signified via PB0.αB; αY signify the fractions for the power penalty applying to blue and yellow illumination when being propagated within the phosphor sheet.Besides, in subscript "4", '3 signifies the propagated yellow illumination bypassing the pair of phosphor sheets.In the case of three-sheet structure, the illumination proficiency for pc-LED device would be augmented substantially compared to the two-sheet variant [24].As such, it could become the preferred structure in WLEDs.The formula below demonstrates the superiority of proficiency in the three-sheet setting to that in the double-sheet setting.
Mie hypothesis will be employed for assessing phosphor granules' dispersion.Additionally, via a formula derived from said hypothesis, we can determine the dispersion cross section Csca applying to globular granules.It is possible to determine the propagated illumination energy via the Lambert-Beer principle [25]: 0 signifies the incident illumination energy. signifies the phosphor sheet breadth (mm).µ  signifies the extinction coefficient, equal to Nr.Cext, with   signifying the number denseness allocation for granules (mm -3 ).Cext (mm 2 ) signifies the extinction cross-section for phosphor granules.From (6), it can be concluded that more phosphor layers do not harm the lumen output of the device, on the contrary, the emitted light from TL is higher than DL. Figure 6, which demonstrates the calculated flux of TL and DL, can be viewed for better understanding with Figures 6(a) and 6(b) displaying the lumen output in the DL and TL settings, respectively.The lumen output in the TL setting surpasses that in the DL setting in certain zones.Thus, the TL structure is the configuration that can achieve high CQS while maintaining the emitted light at a high amount.This result is yielded from the more enhanced emission rate from 500 to 600 nm wavelength in TL structure because the yellow phosphor concentration in this structure is reduced, thus sustaining ACCT.From these circumstances, the TL setting reduce the illumination loss from internal reflection and let the emitted light from blue light source to move through phosphor layers more effectively, which can also be known as allow the light conversion to occur more effective.The TL structure, with higher emission intensity in the spectrum will achieve better results in terms of light output.
Besides other optical properties, the chromatic consistency of emitted light is also an important index that needs attention.There are many solutions to enhance the color uniformity from using conformal coating method to adding phosphor particles such as SiO2, and CaCO3.These solutions are effective regarding the primary goal of improving color uniformity, however, the light output is also reduced.On the other hand, applying both Zn2SiO4:Mn 2+ , As 5+ and YAl3O4B12:Eu 3+ into the structure augmented the chroma proficiency through scattering particles as well as boosting light emission.The remote phosphor configuration of the TL structure is also another advantage because the gap between the phosphor plates can lower the back-scattering event.The Lambert-Beer principle displayed by (7) proves that the concentrations for the phosphor materials still need to be adjusted in order to achieve the best result.Looking at the 5123 variations of color expression of both DL and TL in Figure 7, we can see that TL offers superior color consistency to DL because the color deviation of TL is lower, and the discrepancy become more significant towards higher color temperature such as 8,500 K.This is because the TL has more phosphor layers, which will generate more scattering events that support the chromatic consistency of emitted light.However, more scattering events also means more light loss due to back-scattering.However, the reduction in light emission can be omitted considering the advantages that TL brings to WLEDs, and how the TL manages to achieve highest performance in both chromatic performance and light output.

CONCLUSION
In conclusion, the method of lowering the temperature during fabrication procedure to create WLEDs with high CRI and diverse correlated color temperature from 3,900 to 5,300 K were shown and discussed.Lowering the sintering temperature 620 °C for the purpose of limiting the cross-dispersion among the red phosphor as well as glass yielded high CRI.The CRI from WLEDs that follow this method can surpass the highest result, which is 80 as demonstrated by previous studies.This co-doped PiG that can adapt to multiple difficult demands of the lighting industry can be a potential solution for higher performance lighting devices that are enhanced in optical properties and suitable for advanced lighting applications.

Figure 2 .Figure 3 .
Figure 2. YAG:Ce 3+ content within all settings (a) DL and (b) TL ) and 4(b).The resulted CRI from DL is certainly higher than TL at all ACCTs.The CRI in DL Int J Elec & Comp Eng ISSN: 2088-8708  Triple-layer remote phosphor geometry: an excellent selection to improve the optical … (Thuc Minh Bui)

Figure 4 .Figure 5 .
Figure 4. CRI in all settings corresponding to ACCT levels (a) DL and (b) TL

Figure 6 .Figure 7 .
Figure 6.Lumen proficiency in all settings corresponding under various ACCT values (a) DL and (b) TL