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Abstract
A blue Sr5(PO4)3Cl:Eu2+ phosphor was synthesized by the solid state reaction method. Higher luminescence intensity was observed by increasing the NH4Cl content to twice the stoichiometric ratio. Then a near UV excited warm white LED was fabricated by using an InGaN/GaN chip (395 nm) coated with tricolor phosphors, including (Sr,Ca)AlSiN3: Eu2+ (red), (Ba,Sr)2SiO4: Eu2+ (green), and Sr5(PO4)3Cl:Eu2+ (blue). Moreover, a two-layer LED fabrication structure was designed to weaken the secondary absorption of blue emission by the red and green phosphors. A good luminescence performance was achieved with the correlated color temperature (CCT) of 3150 K, a color rendering index (CRI) of 90.7, CIE chromatic coordinates of (0.4354, 0.4196), and luminous efficiency of 104.86lm/W.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
With the emerging global energy and environment crisis, white light-emitting diodes (WLEDs) have made tremendous progress in the past decades because of their high luminous efficiency, low power consumption, reliability, and environmental friendliness [1,2]. The present commercial WLEDs are fabricated by coating a yellow-emitting Y3Al5O12:Ce3+ (YAG:Ce3+) phosphor on the InGaN/GaN blue LED chips [3–5]. However, such type of white LED has a poor color rendering index (CRI) caused by color deficiency in the red region [6–8]. Moreover, some scholars hold that too much blue light has a negative impact on human health [9–12]. One alternative approach is using InGaN based near UV LED chip coated with blue/green/red tricolor phosphors [13]. Chiu et al. [14] reported a green phosphor Ca3Si2O4N2 and a 20 lm/W WLED. Liu et al. [15] reported a WLED using NUV chip (395 nm) encapsulated with cyan-emitting Ba9Lu2Si6O24:Ce3+ and red-emitting CaAlSiN3:Eu2+ phosphors, the luminous efficiency achieves at 32.2lm/W. In the previous work of our group, a WLED was also fabricated by using a InGaN-based near-UV LED chip (395 nm) coated with tricolor phosphors of blue-emitting Sr5(PO4)3Cl:Eu2+, green-emitting (Ba,Sr)2SiO4:Eu2+ and red-emitting CaAlSiN3:Eu2+ phosphors [16,17]. This WLED had CRI of 95, CCT of 3567.84 K, and luminous efficiency of 33.25lm/W. However, the luminous efficiency of WLEDs for indoor lighting is usually higher than 100lm/W.
In this work, in order to obtain a high luminous efficiency WLED, the blue Sr5(PO4)3Cl:Eu2+ was synthesized by adding the NH4Cl with its content over the stoichiometric ratio. A two-layer structure was designed for the LED fabrication. A warm WLED with good luminescent performances was fabricated by coating tricolor phosphorus on the surface of a single 395 nm near-UV chip.
2. Experimental
2.1 Filter phosphors for near-UV excited warm WLED
For comparison, the luminescent properties of different commercial phosphors are evaluated. The phosphor candidates are listed in Table 1.
Table 1. High efficiency phosphors with different emission wavelengths
2.2 Synthesis process optimization of Sr5-x(PO4)3Cl:xEu2+
The Sr5-x(PO4)3Cl:xEu2+ (x = 2.5% in molar fraction) sample was prepared by solid state reactions method. SrCO3 (AR), NH4H2PO4 (AR), NH4Cl (AR) and Eu2O3 (AR) were mixed by stoichiometric ratio except for changing the content of NH4Cl. The mixture was ground evenly and transferred to a corundum crucible. The sintering process was carried out in a tube furnace under conditions of 95%N2+5%H2 and atmospheric pressure. The process is as follows: raising the temperature to 600°C in 1 h, and up to 850°C in 1 h, then heating to 1050°C in 5 h, finally natural cooling down to room temperature (25°C±5°C). In the end, the obtained sample was ground again for analysis.
3. Results and discussion
3.1 Filtration of phosphors for near-UV LEDs
As reported by Yamada et al [31], the EL output power of InGaN n-UV chips increased with increasing peak emission wavelength from 380 to 400 nm and decreased above 400 nm. Furthermore, the fabrication process of 395 nm chip (In0.09Ga0.91N) has good stability and low cost. Therefore, for commercial application, a single InGaN near UV LED chip (395 nm) was used to fabricate WLED. The tricolor phosphors candidates were evaluated as follows:
- (2) The PL spectra of three green phosphors excited by 395 nm were plotted in Fig. 2. Among these three phosphors, the β-SiAlON:Eu2+ phosphor was developed for display application [26] in recent years. The (Ba,Sr)2SiO4:Eu2+ phosphor has its main emission peak at 550 nm, and it is a promising green color excited by near-UV [24]. And the Y3Al5O12:Ce3+ (YAG:Ce3+) was usually used as an efficient yellow phosphor with the main emission peak locating at 563 nm [29]. The (Sr, Ba)2SiO4:Eu2+ phosphor has the highest luminous intensity under the excitation of 395 nm.
- (3) The BaMgAl10O17:Eu2+ phosphor (in Fig. 3) was a common commercial blue phosphor with an emission peak at 450 nm excited by 395 nm. The intensity of BaMgAl10O17:Eu2+, Mn2+ was enhanced by adding Mn2+ ion as co-doped element, which created two emission peaks at 450 nm and 515 nm. Among these three blue candidates, the Sr5(PO4)3Cl:Eu2+ phosphor has the highest luminous intensity at 445 nm.
Fig. 1. PL emission spectra excited at 395 nm of (Ba,Sr)3SiO5:Eu2+, (Sr,Ca)2Si5N8:Eu2+ and (Sr,Ca)AlSiN3:Eu2+.
Fig. 2. PL emission spectra excited at 395 nm of (Ba,Sr)2SiO4:Eu2+, β-SiAlON:Eu2+, Y3Al5O12:Ce3+(YAG:Ce3+)
Fig. 3. PL emission spectra excited at 395 nm of Sr5(PO4)3Cl:Eu2+, BaMgAl10O17:Eu2+, BaMgAl10O17:Eu2+,Mn2+ and LiCaPO4:Eu2+.
Fig. 4. The CIE chromatic coordinates three phosphors, including point R for (Sr, Ca)AlSiN3:Eu2+, point G for (Ba,Sr)2SiO4:Eu2+ and point B for Sr5(PO4)3Cl:Eu2+.
3.2 Optimization of Sr5(PO4)3Cl:Eu2+ phosphor efficiency
In our previous work, NH4Cl was used as the source of element Cl. Since NH4Cl was easy pyrolyzed to NH3 and HCl in the high temperature solid-state reaction process, and part of HCl gas was evaporated, so that the content of Cl was probably smaller than the stoichiometric ratio (str). In this work, as a compensation, more than the stoichiometric ratio of NH4Cl was added in the solid-state reaction process.
The crystal structure of Sr5(PO4)3Cl (ICSD-80085) is shown in Fig. 5(a). It is hexagonal with a space group of P63/m (176), featuring the lattice parameters of a = b = 9.878 Å, c = 7.1892 Å, and V = 607.47Å3 [32].
Fig. 5. (a) The crystal structure of Sr5(PO4)3Cl (ICSD-80085) and (b) two different coordination structures of Sr2+ ion
The Sr2+ in crystal Sr5(PO4)3Cl is located on two different symmetry lattice sites (Fig. 5(b)), one is coordinated by 6 atoms, the other is coordinated by 5 oxygen atoms and 2 chlorine atoms.
The XRD patterns of the prepared samples were plotted in Fig. 6.
Fig. 6. XRD patterns of Sr5(PO4)3Cl:Eu2+ phosphor samples with the NH4Cl additive amount equal to 1, 2, and 4 times of the stoichiometric ratio (str).
As shown in Fig. 6, when the added content of NH4Cl was equal to stoichiometric ratio in sample 1, a nearly pure (Sr4.99Eu0.01)(PO4)3Cl (ICSD-83253) phase was obtained, together with a weak Sr3(PO4)2 (ICSD-18109) phase being observed. The Sr3(PO4)2 phase disappeared in sample 2 and sample 3, where the content of NH4Cl is twice and four times of the stoichiometric ratio, respectively. Simultaneously, a new SrCl2(H2O)6 (ICSD-48810) phase was created. In order to further quantify approximately the content of element Cl remain in the samples, the compositions of the 3 samples were analyzed by means of whole pattern fitting (WPF) refinement with XRD analysis based on the Inorganic Crystal Structure Database (ICSD). The results were listed in Table 2.
Table 2. The approximate estimation of element content by WPF refinement
As shown in Table 2, when the ratio of NH4Cl is 1, after synthesis process, the ratio of element Cl is approximately estimated to be 0.964, less than the stoichiometric ratio. As to sample 2 and sample 3, when NH4Cl is added over the the stoichiometric ratio, the remain element Cl after synthesis is greater than 1, which is sufficient for forming Sr5(PO4)3Cl structure. Therefore, the second phase Sr3(PO4)2 was disappeared. However, the excessive Cl results in generating a new second phase SrCl2(H2O)6.
The luminescent properties of the above three samples were measured and shown in Fig. 7.
Fig. 7. The PLE (λem = 445 nm) and PL (λex = 395 nm) spectra of Sr5(PO4)3Cl:Eu2+ phosphor samples with the NH4Cl additive amount equal to 1, 2, and 4 times of the stoichiometric ratio (str)
The peak-differentiation-imitating analysis is shown in the inset graph in Fig. 7, The asymmetric emission spectra are formed by overlapping emissions from two luminous centers of Sr5(PO4)3Cl:Eu2+ phosphor samples with peaks at around 447 nm (Fitting curve 1) and 460 nm (Fitting curve 2), respectively. Since Eu2+ substitutes Sr2+ into different lattice sites (in Fig. 5(b)), thus two different luminescence centers are formed due to the different crystal fields surrounding it. As shown in Fig. 7, among the three samples, sample 2 had the highest emission intensity, which is almost twice higher than sample 1. Therefore, it was confirmed that there was a positive effect on the luminescence of blue Sr4.99(PO4)3Cl:0.01Eu2+ by increasing the NH4Cl additive amount over the stoichiometric ratio. However, sample 3 with more NH4Cl addition did not have higher luminous intensity than sample 2. The mechanism was still unclear and seemed complicated, further theory investigation and more experimental research will be carried out in our future works.
3.3 Fabrication structure designation of WLED
As shown in Fig. 8, all of the three phosphor candidates, including the red-emitting (Sr, Ca)AlSiN3:Eu2+, the green-emitting (Ba,Sr)SiO4:Eu2+, and the blue-emitting Sr5(PO4)3Cl:Eu2+ phosphors, have wide excitation spectra.
Fig. 8. PL (λex = 395 nm) and PLE (λem = 445 nm) spectrum of Sr5(PO4)3Cl:Eu2 blue phosphor (B), PLE (λem = 533 nm) spectrum of (Ba,Sr)2SiO4:Eu2+ green phosphor (G) and PLE (λem = 610 nm) spectrum of CaAlSiN3:Eu2+ red phosphor (R)
The three excitation spectra overlap each other in the range of 340 nm and 420 nm. Therefore, it is feasible to use a single n-UV chip (395 nm) to excite three different phosphors.
Unfortunately, the excitation spectra of the red (Sr, Ca)AlSiN3:Eu2+ and green (Ba,Sr)SiO4:Eu2+ phosphors overlap with the whole blue Sr4.99(PO4)3Cl:0.01Eu2+ phosphor under the excitation of 395 nm. So the blue emission is easily absorbed by the red and green phosphors, and this secondary absorption will reduce the blue-emitting intensity greatly and results in a poor performance of the final WLED. In order to weaken the secondary absorption influence, a specific structure (Fig. 9(a)) is designed for WLED fabrication.
Fig. 9. Fabrication structure of WLED, (a) specific two-layer structure, (b) conventional mix-phosphors structure
The new designed structure is a two-layer structure (Fig. 9 (a)), where the red and green phosphors are mixed and located at the bottom and the blue phosphor is covered on the top. As a comparison, three type of phosphorus (red, green and blue) are mixed together and fabricated on the n-UVLED (395 nm) (Fig. 9(b)). The experiments were repeated several times, and the results are listed in Table 3. The highest luminous efficiencies of the LEDs with the above two different fabrication structures are 99.76 lm/W and 89.06 lm/W, respectively. As a comparison with the same Ra value, the luminous efficiencies of two-layer structure are much higher than that of one mixture of tricolor phosphors. Therefore, the secondary absorption of red and green phosphors is reduced obviously on the new designed two-layer structure.
Table 3. The performances of WLED fabricated by two different structures
3.4 Optimization of phosphor proportion
As shown in Fig. 10, when changing the proportion of the red, green and blue phosphors, the emission spectra and optical properties of the fabricated WLEDs are difference. The optical properties of the WLED are listed in Table 4 and the corresponding CIE chromatic coordinates are marked out in Fig. 11.
Table 4. The optical properties of the WLED with different CCT
Generally, the higher the color rendering index, the stronger the color restoration ability of the object is. As shown in Fig. 10 and Table 4, the color indexes (Ra) of the four samples are all higher than 90, so they are good at displaying real color. And sample No.(b) shows warm white color and presents a good luminescence performance with Ra of 90.7%, CCT of 3150 K, the CIE chromatic coordinates is (0.4354, 0.4196), and the luminous efficiency is up to 104.86lm/W.
4. Conclusion
In this work, a warm WLED was fabricated by using a InGaN based blue chip coated with tricolor phosphors, including (Sr,Ca)AlSiN3: Eu2+ (red), (Ba,Sr)2SiO4: Eu2+ (green), and Sr5(PO4)3Cl:Eu2+ (blue). The Sr5(PO4)3Cl:Eu2+ phosphor is synthesized by solid-state reaction method, where the additive content of NH4Cl is twice of the stoichiometric ratio, and a higher luminescence intensity was achieved. A specific two-layer fabrication structure was designed to weaken the secondary absorption of blue emission by the red and green phosphors. Coupled with the component optimization of the three phosphors, the fabricated WLED presents a good luminescence performance with Ra of 90.7%, CCT of 3150 K, and CRI of 90.7, the CIE chromatic coordinates is (0.4354, 0.4196), and the luminous efficiency is up to 104.86lm/W.
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