CaO:Tb 3+ green-emitting phosphor for white light-emitted diode-phosphor applications: the improvement of light output intensity

ABSTRACT


INTRODUCTION
Solid-state lighting (SSL) has been immersed as an environmental-friendly and high-efficient illuminating solution that could replace coal-powered and gas-powered light bulbs [1], [2].A diode emits white light-emitting diode (WLED), a typical semiconductor illuminating supply of SSL, has been recognized for excellent properties for example, outstanding shock resistance, low power consumption, and long-term operation that are suitable for wide-range lighting applications, from indoor to outdoor illumination.For the fabrication of the WLED, using phosphor to convert light emitted from a blue lightemitting diode (LED) chip for white light production is more favorable than using three chromatic LED chips of red, blue, and green due to its simplicity.The universal combination for phosphor-conversion LED is the yellow-emitting phosphor of YAG:Ce 3+ and GaN-blue chip [3]- [5].The YAG:Ce 3+ has been reported to be the effective phosphor material for high-efficiency WLED owing to its excellent quantum efficiency and high chemical stability [6].Other phosphor materials are proposed to combine with yellow-emitting phosphor to further enhance its advantages and improve its disadvantages in color adequacy by enhancing the scattering effectiveness of the phosphor layers [7], [8].
The phosphor with rare-earth ions, especially the lanthanide type, have been investigated and applied in several aspects such as SSL, sensors, imaging techniques, and lasers [9], [10].Tb 3+ is one of the most attractive lanthanide ions for doping in the phosphor composition owing to its efficient green emission provided by the 4f-5d transitions.Besides, the Tb 3+ ion-doped glass matrix is reported to exhibit four emission regions of green, blue, yellow, and red [11], [12].Thus, Tb 3+ ion-doped phosphors can be potential for increasing the luminous output of a phosphor-converted WLED (pc-WLED).In this paper, the Tb 3+ -doped CaO phosphor is applied to fabricate the pc-WLED package with high scattering efficiency [13]- [15].The impacts of CaO:Tb 3+ on the chromaticity and lumen performances of the WLEDs is investigated with various concentration of the green-phosphor concentration.The study performs the simulations of three phosphor packages, consisting of in-cup phosphor, conformal phosphor, and two-layered remote phosphor.The Mie-scattering theory is applied to validate the phosphor layers' diffusing [16]- [18].The study demonstrates that this CaO:Tb 3+ phosphor considerably promotes the illuminating beam and hue homogeneity of all three pc-WLEDs.Meanwhile, the color accuracy evaluated by the quality scale can be around 66 with appropriate control over the concentration of CaO:Tb 3+ .

METHOD 2.1. Green-emitting CaO:Tb 3+ phosphor composition and preparation
The composition of green-emitting phosphor CaO:Tb 3+ includes three chemical ingredients of CaCO3, CaF2, and Tb4O7, as shown in Table 1.The preparation process of CaO:Tb 3+ is performed through 5 steps, starting from making a chemical solution and ending with storing the product in a dry container [19], [20].Initially, the required chemical components of CaO:Tb 3+ composition are blended well in water or methanol for a complete solution.The combination is then dried in air and subsequently powdered.The powder is latterly heated up in a capped quartz tube with CO at 1200 °C and for 1 hour.As the firing time is over, the product is ground into powders and subsequently stored in a well-closed container.Note that, it is essential to keep the product dry to preserve its chemical characteristics.The product-CaO:Tb 3+ phosphor-has a light green emission color with the highest intensities near 2.26-2.28eV.

Structure of pc-WLEDs
The modeling process of three WLED models, including the conformal, in-cup, and remote phosphor packages, is performed with the LightTools simulation software and the Monte Carlo method [21].The phosphor sheets covering the LED chip's surface are simulated with a flat silicon thin film.Two essential points that need to perform in the modeling process are the establishment of the structures and lighting features of each pc-WLED model, and the control over phosphor compounds' influences on the determined lighting features of the white illumination.Specifically, the performance of WLEDs' optical properties is monitored with the changing concentration of CaO:Tb 3+ in the phosphor compound.Consequently, the green spectral proportion will be different, leading to significant changes in color uniformity and rendition, and luminescence intensity of the three pc-WLED packages.Additionally, the effects of CaO:Tb 3+ on the chromatic and luminous features of the WLEDs are examined and demonstrated utilizing the light scattering by Mie-scattering theory.
The WLED models are simulated with a pre-set correlated color coordinate (CCT) of 8,500 K. Figure 1(a) shows the photograph of a real WLED using nine LED chips and the conformal phosphor coating approach.The illustrations of the conformal, the in-cup, and the remote phosphor structures are presented in Figures 1(b)-(d), respectively.Nine LED chips, with 1.14×0.15mm in square base and altitude, respectively, is attached to the lead frame in three parallel columns.All the chips emit the radiant energy of 1.14 W with the maximum wavelength centered at 453 nm.The phosphor film with a 0.08 mm thickness is diffused on the LED chips' surface.The phosphor particles in spherical are 14.5 μm in their diameter.The reflector of the WLED model has the dimension of 8×2.07×9.85mm (bottom×height×top).In the conformal (CPP) and in-cup (IPP) phosphor models, green-emitting phosphor CaO:Tb 3+ is mixed with the yellow-emitting phosphor YAG:Ce 3+ (Figures 1(b) and (c)).Meanwhile, in the two-layer remote (RPP) phosphor model, the CaO:Tb 3+ phosphor and YAG:Ce 3+ phosphor are separated into two layers with the green-emitting layer above the yellow-emitting film (Figure 1 (d)).In Figures 2 (a

RESULTS AND DISCUSSION
Scattering computation for CaO:Tb 3+ phosphor is performed applying the Mie-scattering theory [22]- [25].By recognizing the scattering coefficient   , the effects of this green-emitting phosphor on the lighting efficiency can be validated.Particularly,   , depends on the diameter of the phosphor sphere and the wavelength, which can be expressed as ( 1)-( 4): In ( 1)-( 4), the wavelength and the phosphor's diameter are indicated by  and , respectively.In (1)  shows the concentration of the green-emitting phosphor (g/cm 3 ), ̄ shows the mass of the phosphor, and  ̄ () shows the scattering cross-section of the phosphor sphere.In ( 2)-( 4), () indicates the dimension dispersion,  , indicates the diffusing cross-section connected with diameter  of the phosphor sphere,   () presents the diffusing energy of the phosphorus photon, and   () presents the degree of the phosphorus irradiation.
Besides, to ensure the stability of the WLED devices at the pre-set CCT value of 8,500 K, the change in yellow-phosphor concentration must be monitored under the adjustment of the green-phosphor concentration.The decrease in weight percentage of the yellow-emitting phosphor is required as that of the green-emitting phosphor CaO:Tb 3+ increase, according to the following expression [26].
,   ,   and   present the weight percentages of the phosphor film package, the silicone, the yellow and green phosphor, respectively.The values of μsca of CaO:Tb 3+ are demonstrated in line charts in Figures 3(a) and (b) in CPP and IPP respectively.Three WLED structures show a similar trend of μsca with the increasing CaO:Tb 3+ concentration.In particular, the μsca, at first, slightly decreases when the concentrations of CaO:Tb 3+ increase to a certain number, 4 wt% in CPP and 0.2 wt% in IPP, and starts to increase significantly as the concentration of green phosphor continuous to rise.From this demonstration, the scattering enhancement can be obtained with the use of CaO:Tb 3+ .The scattering stimulation helps to redirect the light path and reduce the light loss by internal reflection for significantly promoted light extraction [27].Moreover, the scattering enhancement is reported to be an important feature for attaining better color performance of WLED devices.Hence, there is a high probability of achieving noticeable improvements in WLED performance in both luminous and color characteristics.Via the demonstrated scattering coefficient by Mie theory, the effectiveness in enhancing the luminescence and the chromaticity of the WLED structures.To further examine the impacts of green-emitting phosphor CaO:Tb 3+ with highly-efficient scattering properties, the paper presents specific results of each optical feature, including luminous fluxes (Figure 4), color deviation of CPP, IPP, RPP (Figures 5(a)-(c)) for the evaluation of color uniformity (CRI), and color quality scales (CQS) for the evaluation of color rendition and fidelity.In Figures 6(a

CONCLUSION
The study demonstrates the impacts of CaO:Tb 3+ green-emitting phosphor on the color performance and luminous efficiency of three different WLED packages of CPP, IPP, and RPP.The scattering factor is analyzed to realize the changes in these optical features of WLEDs in connection with the modification of CaO:Tb 3+ concentration.The scattering enhancement is obtained with the increasing concentration of this green phosphor, which encourages blue-light conversion and green-light generation.Moreover, the improved scattering boosts the light extraction efficiency for greater luminous intensity.The color consistency is also better since the color variation is reduced.Nevertheless, it is quite disadvantageous to the CQS as the increasing concentration of CaO:Tb 3+ decreases the CQS values in all three WLED packages.Thus, if the IPP and RPP are applied for fabrication, it is advisable to keep the concentration of CaO:Tb 3+ below specific levels (>0.4 wt% and ≥8 wt%, respectively) for good CQS while attaining lower color deviation and higher luminous flux.The findings of the study indicate that the CaO:Tb 3+ is potential for high-efficiency WLED applications.