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Cite This: Chem. Mater. 2018, 30, 5137−5147
Improving Quantum Efficiency and Thermal Stability in BlueEmitting Ba2−xSrxSiO4:Ce3+ Phosphor via Solid Solution Xiaoyu Ji,†,‡ Jilin Zhang,*,†,‡ Ying Li,†,‡ Shuzhen Liao,*,§ Xinguo Zhang,∥ Zhiyu Yang,⊥ Zhengliang Wang,⊥ Zhongxian Qiu,†,‡ Wenli Zhou,†,‡ Liping Yu,†,‡ and Shixun Lian*,†,‡
Chem. Mater. 2018.30:5137-5147. Downloaded from pubs.acs.org by REGIS UNIV on 10/21/18. For personal use only.
†
Key Laboratory of Chemical Biology and Traditional Chinese Medicine Research (Ministry of Education of China) and ‡Key Laboratory of Sustainable Resources Processing and Advanced Materials of Hunan Province College, Hunan Normal University, Changsha 410081, China § School of Chemistry and Chemical Engineering, Hunan Institute of Engineering, Xiangtan 411104, China ∥ Guangdong Provincial Key Laboratory of New Drug Screening, School of Pharmaceutical Sciences, Southern Medical University, Guangzhou 510515, China ⊥ Key Laboratory of Comprehensive Utilization of Mineral Resource in Ethnic Regions, School of Chemistry & Environment, Yunnan Minzu University, Kunming 650500, China S Supporting Information *
ABSTRACT: Ba1.8−xSrxSiO4:0.1Ce3+,0.1Na+ (x = 0−1.8) phosphors were prepared by a high-temperature solid-state reaction. The emission peaks of Ba1.8−xSrxSiO4:0.1Ce3+,0.1Na+ shift from 391 to 411 nm with increasing Sr2+ content under excitation by a UV light at around 360 nm. Ba0.4Sr1.4SiO4:0.1Ce3+,0.1Na+ phosphor exhibits the best performance of luminescence, whose absolute quantum efficiency is 97.2%, and the emission intensity at 150 °C remains 90% of that at room temperature. The effect of replacing Ba2+ by Sr2+ on the red shift of the emission band and the increase of quantum efficiency (QE) and thermal stability (TS) was investigated in detail based on the Rietveld refinements, Raman spectra, thermoluminescence, and decay curves, etc. The performance of UV chip-based pc-LEDs indicates that Ba0.4Sr1.4SiO4:0.1Ce3+,0.1Na+ can be a promising blue phosphor for white-emitting pc-LEDs.
1. INTRODUCTION White light-emitting diodes (w-LEDs) have recently received much attention as solid-state lighting devices due to the advantages such as high brightness, long lifetime, environmental friendliness, and low power consumption.1−7 The most common method is the phosphor-converted LED (pc-LED) by combining a Y3Al5O12:Ce3+ (YAG:Ce3+) yellow phosphor with an InGaN blue LED chip.8 However, the color rendering index (CRI) of this device is low ( 100 °C).60 As a comparison, the TL spectra of Ba1.8SiO4:0.1Ce3+,0.1Na+ and Sr1.8SiO4:0.1Ce3+,0.1Na+ are also measured, which are shown in Figure S18 in the Supporting Information. Similar phenomena are found, which suggest that thermal ionization is also responsible for thermal quenching of Ba1.8SiO4:0.1Ce3+,0.1Na+ and Sr1.8SiO4:0.1Ce3+,0.1Na+. It should be noted that the TL peaks for these two phosphors also situate at ∼50 and 185 °C, which suggests the similar traps for these samples. The curves of PL intensity versus temperature in the 100−250 °C range exhibit a similar tendency for these three phosphors with similar traps, which may supply some evidence for the thermalionization-induced thermal quenching at high temperature. Temperature-dependent absorbance, IQE, and EQE are collected by using the QE-2100 spectrophotometer from 5143
DOI: 10.1021/acs.chemmater.8b01652 Chem. Mater. 2018, 30, 5137−5147
Article
Chemistry of Materials
Figure 10. (a) Temperature-dependent PL spectra for IQE and EQE. (b) IQE, EQE, and absorbance versus temperature.
Otsuka Photal Electronics, which are shown in Figure 10. Results indicate that heating treatment really results in the degradation of absorbance that also contributes to thermal quenching. Therefore, the thermal quenching at high temperature (T > 100 °C) should originate from both thermal ionization and degradation of absorbance upon increasing temperature. However, the activation energy for thermal quenching is around 0.2 eV, which is obtained from the fitting of temperature-dependent PL intensity. This value is much smaller than the trap depth (1.113 eV). Therefore, we think other reasons for thermal quenching should not be ignored, such as the displacement between 4f and lowest 5d excited state in the configuration coordinate diagram, and the existence of quenching defects and impurities. The process of charging for TL measurement and thermal quenching induced by thermal ionization can by illustrated as the following diagram in Figure 11. Higher temperature benefits detrapping and thermal ionization.
Figure 12. Schematic configurational coordinate diagram for Ba1.8−xSrxSiO4:0.1Ce3+,0.1Na+ phosphors, showing the variation of the lowest 5d energy level relative to 4f level. Eex, excitation energy at PLE peak; Eem, emission energy at PL peak; ΔR, offset between the parabolas of excited and ground levels; Ea, activation energy for thermal quenching.
(EL) spectra of a pc-LED composed with a UV chip and the Ba0.4Sr1.4SiO4:0.1Ce3+,0.1Na+ phosphor under 50−350 mA forward bias current, whose intensity increases with the current. Figure 13b is the EL spectra of a warm white pc-LED by combining a UV LED chip with Ba0.4Sr1.4SiO4:0.1Ce3+,0.1Na+, commercial (Ba,Sr)2SiO4:Eu2+, and CaAlSiN3:Eu2+ phosphors under 50−350 mA forward bias current. The weight ratio of blue, yellow, and red phosphor and silica gel is 7:2:1:50. The performance of the pc-LEDs and a naked 365 chip (EL in Figure S19) is listed in Table 4. The warm white pc-LED exhibits an excellent CRI value (>90), and the correlated color temperature (CCT) value is 3042−3151 K. Figure 13c shows the CIE coordinates obtained from the EL spectra at different currents and the photographs of the two pc-LEDs. These results indicate that the Ba0.4Sr1.4SiO4:0.1Ce3+,0.1Na+ phosphor can be used as a promising blue phosphor for whiteemitting pc-LEDs.
Figure 11. Illustration of charging for TL measurement and thermal quenching induced by thermal ionization. The 254 and 365 nm radiation are turned on simultaneously. CB: conduction band.
A schematic configurational coordinate diagram is used to explain the phenomenon of evolution of luminescent properties with the change of host composition, which is shown in Figure 12. Refinement results indicate the reduction of the size of MO9 polyhedra, which leads to the decrease of energy difference between 5d and 4f levels. The average Uiso obtained from Rietveld refinement suggests the order of offset (ΔR) between the parabolas. Furthermore, the decrease of MO9 size relates to a larger force constant (k) between M and O atoms, which narrows the parabolas. All of the above reasons lead to the sequence of PLE, PL peak, and activator energy (Ea) for thermal quenching as shown in Figure 12, which are also based on the data mentioned above. Finally, Ba0.4Sr1.4SiO4:0.1Ce3+,0.1Na+ phosphor with the best performance is used to fabricate pc-LEDs with an InGaN LED chip (λmax = ∼365 nm) to further prove the potential application for w-LEDs. Figure 13a is the electroluminescence
4. CONCLUSIONS Blue-emitting Ba1.8−xSrxSiO4:0.1Ce3+,0.1Na+ phosphors were synthesized by a high-temperature solid reaction method. A red shift was observed in the emission spectra when the composition changes from Ba1.8SiO4:0.1Ce3+,0.1Na+ to Sr1.8SiO4:0.1Ce3+,0.1Na+. However, Ba0.4Sr1.4SiO4:0.1Ce3+,0.1Na+ with intermediate host composition exhibits the best emission intensity, quantum efficiency, and thermal stability among them. The absolute quantum efficiency for Ba0.4Sr1.4SiO4:0.1Ce3+,0.1Na+ is 97.2%. The PL intensity of the phosphor at 150 °C remains 90% of that at room temperature. Rietveld refinement results indicate 5144
DOI: 10.1021/acs.chemmater.8b01652 Chem. Mater. 2018, 30, 5137−5147
Article
Chemistry of Materials
Figure 13. EL spectra of (a) Ba0.4Sr1.4SiO4:0.1Ce3+,0.1Na+ converted LED, and (b) a warm white pc-LED combined with a ∼365 nm UV chip and Ba0.4Sr1.4SiO4:0.1Ce3+,0.1Na+, a commercial green phosphor, and a commercial red phosphor under 50−350 mA forward bias currents. (c) CIE coordinates obtained from the EL spectra at different currents and photographs of the two pc-LEDs.
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Table 4. Performance of the Two pc-LEDs and a Naked 365 nm Chip for Comparison current (mA) blue pc-LED
50 100 150 200 250 300 350
white pc-LED
50 100 150 200 300 350
365 nm chip
100
CIE 0.1609, 0.0528 0.1553, 0.0498 0.1603, 0.0529 0.1620, 0.0560 0.1610, 0.0549 0.1613, 0.0557 0.1621, 0.0559 0.4363, 0.4203 0.4373, 0.4217 0.4366, 0.4224 0.4373, 0.4239 0.4402, 0.4221 0.4418, 0.4194
CCT (K)
Ra
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.8b01652.
luminance efficiency (lm/W)
100 000
0.8
100 000
1.5
100 000
2.3
100 000
2.2
100 000
1.9
100 000
2.0
100 000
1.9
ASSOCIATED CONTENT
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Rietveld refinement, crystallographic data, QE measurement, XRD, PL spectra, temperature-dependent PL spectra, TL spectra, and EL spectra (PDF)
AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. *E-mail:
[email protected]. ORCID
3140
94.5
26.3
3134
94.1
23.9
3151
93.9
21.3
3150
93.2
19.0
3088
92.0
12.8
3042
91.5
12.2
Jilin Zhang: 0000-0001-7235-341X Xinguo Zhang: 0000-0002-8950-0831 Wenli Zhou: 0000-0002-6975-2206 Shixun Lian: 0000-0001-6524-2703 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work is financially supported by the National Natural Science Foundation of China (Grants 21505038, 51402105, 21601081, 21471055), National Key Research and Development Program (Grant 2016YFB0302403). We thank Dr. Xuhui Xu and Mr. Xiaotong Fan (College of Materials Science and Engineering, Kunming University of Science and Technology, China) for their help with TL measurements.
0.3
that the size of MO9 polyhedra decreases with increasing Sr2+ content, which supports the red shift of the emission band. Rietveld refinement results also suggest that the rigidity of Ba0.4Sr1.4SiO4:0.1Ce3+,0.1Na+ is the highest one among the Ba1.8−xSrxSiO4:0.1Ce3+,0.1Na+ phosphors, which is responsible for the highest quantum efficiency and thermal stability. Analyses indicate that thermal ionization, thermal induced degradation of absorbance, and resonance of the 4f and lowest 5d state should be responsible for the thermal quenching. The performance of the pc-LEDs based on the Ba0.4Sr1.4SiO4:0.1Ce3+,0.1Na+ phosphor suggests that it could be a candidate as a blue phosphor for white pc-LEDs.
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DOI: 10.1021/acs.chemmater.8b01652 Chem. Mater. 2018, 30, 5137−5147
