Highly efficient narrow-band green and red
phosphors enabling wider color-gamut LED
backlight for more brilliant displays
Le Wang1, Xiaojun Wang,2 Takahashi Kohsei,2 Ken-ichi Yoshimura,3 Makoto Izumi,3
Naoto Hirosaki,2 and Rong-Jun Xie2,*
1
College of optical and Electronic Technology, China Jiliang University, Hangzhou 310018, China
2
Sialon Unit, National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japan
3
Sharp Corporarion, 2613-1 Ichinomoto-ch, Tenri, Nara 632-8567, Japan
*Xie.Rong-Jun@nims.go.jp
Abstract: In this contribution, we propose to combine both narrow-band
green (β-sialon:Eu2+) and red (K2SiF6:Mn4+) phosphors with a blue InGaN
chip to achieve white light-emitting diodes (wLEDs) with a large color
gamut and a high efficiency for use as the liquid crystal display (LCD)
backlighting. β-sialon:Eu2+, prepared by a gas-pressure sinteing technique,
has a peak emission at 535 nm, a full width at half maximum (FWHM) of
54 nm, and an external quantum efficiency of 54.0% under the 450 nm
excitation. K2SiF6:Mn4+ was synthesized by a twe-step co-precipitation
methods, and exhibits a sharp line emission spectrum with the most
intensified peak at 631 nm, a FWHM of ~3 nm, and an external quantum
efficiency of 54.5%. The prepared three-band wLEDs have a high color
temperature of 11,184 - 13,769 K (i.e., 7,828 - 8,611 K for LCD displays),
and a luminous efficacy of 91 – 96 lm/W, measured under an applied
current of 120 mA. The color gamut defined in the CIE 1931 and CIE 1976
color spaces are 85.5 - 85.9% and 94.3 - 96.2% of the NTSC stanadard,
respectively. These optical properties are better than those phosphorcpnverted wLED backlights using wide-band green or red phosphoprs,
suggesting that the two narrow-band phosphors investigated are the most
suitable luminescent materials for achieving more bright and vivid displays.
©2015 Optical Society of America
OCIS codes: (230.3670) Light-emitting diodes; (250.5230) Photoluminescence; (160.2540)
Fluorescent and luminescent materials; (230.3720) Liquid-crystal devices.
References and links
1.
2.
3.
4.
5.
6.
7.
8.
9.
R.-J. Xie, N. Hirosaki, and T. Takeda, “Wide color gamut backlight for liquid crystal displays using three-band
phosphor-converted white light-emitting diodes,” Appl. Phys. Express 2, 022401 (2009).
E. Jang, S. Jun, H. Jang, J. Lim, B. Kim, and Y. Kim, “White-light-emitting diodes with quantum dot color
converters for display backlights,” Adv. Mater. 22(28), 3076–3080 (2010).
Y. Fukuda, N. Matsuda, A. Okada, and I. Mitsuishi, “White light-emitting diodes for wide-color-gamut backlight
using green-emitting Sr-Sialon phosphor,” Jpn. J. Appl. Phys. 51(12), 122101 (2012).
Z. Luo, Y. Chen, and S.-T. Wu, “Wide color gamut LCD with a quantum dot backlight,” Opt. Express 21(22),
26269–26284 (2013).
Y. Ito, T. Hori, T. Kusunoki, H. Nomura, and H. Kondo, “A phosphor sheet and a backlight system providing
wider color gamut for LCDs,” J. Symp. Dig. 22(8), 419–428 (2014).
S.-H. Lee, K.-H. Lee, J.-H. Jo, B. Park, Y. Kwon, H. S. Jang, and H. Yang, “Remote-type, high-color gamut
white light-emitting diode based on InP quantum dot color converters,” Opt. Mater. Express 4(7), 1297–1302
(2014).
J. H. Oh, H. Kang, M. Ko, and Y. R. Do, “Analysis of wide color gamut of green/red bilayered freestanding
phosphor film-capped white LEDs for LCD backlight,” Opt. Express 23(15), A791–A804 (2015).
Y. Shirasaki, G. J. Supran, M. G. Bawendi, and V. Bulovic, “Emergence of colloidal quantum-dot light-emitting
technologies,” Nat. Photonics 7(1), 13–23 (2013).
S. Coe-Sullivan, W. Liu, P. Allen, and J. S. Steckel, “Quantum dots for LED downconversion in display
applications,” ECS J. Solid State Sci. Technol. 2(2), R3026–R3030 (2013).
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© 2015 OSA
Received 1 Sep 2015; revised 9 Oct 2015; accepted 11 Oct 2015; published 23 Oct 2015
2 Nov 2015 | Vol. 23, No. 22 | DOI:10.1364/OE.23.028707 | OPTICS EXPRESS 28707
10. N. Hirosaki, R.-J. Xie, K. Kimoto, T. Sekiguchi, Y. Yamamoto, T. Suehiro, and M. Mitomo, “Characterization
and Properties of Green-Emitting β-SiAlON:Eu2+ Powder Phosphors for White Light-Emitting Diodes,” Appl.
Phys. Lett. 86, 211905 (2009).
11. R.-J. Xie, N. Hirosaki, H. L. Li, Y. Q. Li, and M. Mitomo, “Synthesis and photoluminescence properties of βsialon:Eu2+ (Si6-zAlzOzN8-z:Eu2+) – A promising green oxynitride phosphor for white light-emitting diodes,” J.
Electrochem. Soc. 154, J314–J319 (2008).
12. K. Uheda, N. Hirosaki, Y. Yamamoto, A. Naito, T. Nakajima, and H. Yamamoto, “Luminescence properties of a
red phosphor, CaAlSiN3:Eu2+, for white light-emitting diodes,” Electrochem. Solid-State Lett. 9(4), H22–H25
(2006).
13. Y. Fukuda, K. Ishida, I. Mitsuishi, and S. Nunoue, “Luminescence properties of Eu2+-doped green-emitting SrSialon phosphor and its application to white light-emitting diodes,” Appl. Phys. Express 2(1), 012401 (2009).
14. Y. Q. Li, N. Hirosaki, R.-J. Xie, T. Takeda, and M. Mitomo, “Crystal and electronic structures, luminescence
properties of Eu2+-doped Si6-zAlzOzN8-z and MySi6-zAlz-yOz+yN8-z-y (M = 2Li, Mg, Ca, Sr, Ba,” J. Solid State Chem.
181(12), 3200–3210 (2008).
15. R.-J. Xie, N. Hirosaki, X.-J. Liu, T. Takeda, and H.-L. Li, “Crystal structure and photoluminescence of Mn2+Mg2+ codoped gamma aluminum oxynitride (γ-AlON): A promising green phosphor for white light-emitting
diodes,” Appl. Phys. Lett. 92(20), 201905 (2008).
16. P. Pust, V. Weiler, C. Hecht, A. Tücks, A. S. Wochnik, A. K. Henß, D. Wiechert, C. Scheu, P. J. Schmidt, and
W. Schnick, “Narrow-band red-emitting Sr[LiAl3N4]:Eu2+ as a next-generation LED-phosphor material,” Nat.
Mater. 13(9), 891–896 (2014).
17. T. Takahashi and S. Adachi, “Synthesis of K2SiF6:Mn4+ red phosphor from Silica Glasses by wet chemical
etching in HF/KMnO4 sollution,” Electrochem. Solid-State Lett. 12(8), J69–J71 (2009).
18. S. Adachi and T. Takahashi, “Direct Synthesis and properties of K2SiF6:Mn4+ phosphor by wet chemical etching
of Si wafer,” J. Appl. Phys. 104(2), 023512 (2008).
19. C. X. Liao, R. P. Cao, Z. J. Ma, Y. Li, G. P. Dong, K. N. Sharafudeen, and J. R. Qiu, “Synthesis of K2SiF6:Mn4+
phosphor from SiO2 powders via redox reaction in KF/KMnO4 solution and their application in warm-white
LEDs,” J. Am. Ceram. Soc. 96(11), 3552–3556 (2013).
20. H. Zhu, C. C. Lin, W. Luo, S. Shu, Z. Liu, Y. Liu, J. Kong, E. Ma, Y. Cao, R. S. Liu, and X. Chen, “Highly
efficient non-rare-earth red emitting phosphor for warm white light-emitting diodes,” Nat. Commun. 5, 4312
(2014).
21. J. H. Oh, H. Kang, Y. J. Eo, H. K. Park, and Y. R. Do, “Synthesis of narrow-band red-emitting K2SiF6:Mn4+
phosphors for a deep red monochromatic LED and ultrahigh color quality warm-white LEDs,” J. Mater. Chem. C
Mater. Opt. Electron. Devices 3(3), 607–615 (2015).
22. A. G. Paulusz, “Efficient Mn (IV) emission in fluorine coordination,” J. Electrochem. Soc. 120(7), 942–947
(1973).
23. A. A. Setlur, E. V. Radkov, C. S. Henderson, J.-H. Her, A. M. Srivastava, N. Karkada, M. S. Kishore, N. P.
Kumar, D. Aesram, A. Deshpande, B. Kolodin, L. S. Grigorov, and U. Happek, “Energy-efficient, high-colorrendering LED lamps using oxyuoride and fluoride phosphors,” Chem. Mater. 22(13), 4076–4082 (2010).
24. H. Bode, H. Jenssen, and F. Bandte, “Uber eine neue darstellung des kalium-hexafluoromanganats(IV),” Angew.
Chem-Ger. Ed. 65, 304–304 (1953).
25. K. Ohkubo and T. Shigeta, “Absolute fluorescent quantum efficiency of NBS phosphor standard samples,” J.
Illum. Eng. Inst. Japan 83, 87–93 (1999).
26. J. H. Lee, D. N. Liu, and S. T. Wu, Introduction to Flat Panel Displays (Wiley, 2008).
27. R. Hunt, Measuring Colour, 3rd Edition (Fountain Press, 1998).
28. J. H. Loehlin, “Redetermination of the structure of potassium hexafluorosilicate, K2SiF6,” Acta Crystallogr. C
40(3), 570 (1984).
29. K. H. Jack and W. I. Wilson, “Ceramics based on the Si-Al-O-N and related systems,” Nat. Phys. Sci (Lond.)
238(80), 28–29 (1972).
30. J. L. Sommerdijk, J. M. P. J. Verstegen, and A. Bril, “Luminescence of MeFX:Eu2+ (Me = Sr, Ba; X = Cl, Br),”
J. Lumin. 8(6), 502–506 (1974).
31. R. F. Sosa, E. R. Alvarez, M. A. Camacho, A. F. Munoz, and J. O. Rubio, “Time-resolved spectroscopy of the
Eu2+ luminescence in KCl:Ba2+,Eu2+ KCl:Sr2+,Eu2+ and KBr:Sr2+,Eu2+,” J. Phys. Condens. Matter 7(32), 6561–
6568 (1995).
32. S. S. Wang, W. T. Chen, Y. Li, J. Wang, H. S. Sheu, and R. S. Liu, “Neighboring-cation substitution tuning of
photoluminescence by remote-controlled activator in phosphor lattice,” J. Am. Chem. Soc. 135(34), 12504–
12507 (2013).
33. M. Kim, W. B. Park, B. Bang, C. H. Kim, and K.-S. Sohn, “Radiative and non-radiative decay rate of
K2SiF6:Mn4+ phosphors,” J. Mater. Chem. C Mater. Opt. Electron. Devices 3(21), 5484–5489 (2015).
34. S. Bhushan and M. V. Chukichev, “Temperature dependent studies of cathodoluminescence of green band of
ZnO crystals,” J. Mater. Sci. Lett. 7(4), 319–321 (1988).
35. K. Takahashi, R.-J. Xie, and N. Hirosaki, “Toward higher color purity and narrower emission band β-sialon:Eu2+
by reducing the oxygen concentration,” Electrochem. Solid-State Lett. 14(11), E38–E40 (2011).
1. Introduction
The ever-changing technologies make it come true or get better in the image quality and color
saturation of liquid crystal displays used in televisions, mobile phones, computer, tablet PCs
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2 Nov 2015 | Vol. 23, No. 22 | DOI:10.1364/OE.23.028707 | OPTICS EXPRESS 28708
and car navigators, of which the backlight technology contributes greatly to these
improvements. Recently, phosphor-converted white light-emitting diodes (wLEDs) and
quantum dot (QD) backlights are considered as emerging backlight units for replacing
conventional CCFL (cold cathode-fluorescence lamps) ones because they promise a thinner,
lighter, brighter, and more vivid display [1–9]. In comparison to CCFL having a color gamut
of ~75% of the National Television Standard Committee (NTSC) standard, the three-band
wLEDs can reach a color gamut of > 90% NTSC (CIE 1976), whereas the QD backlights
promise a larger color gamut of > 100% NTSC. Although QD backlights have the most widest
color space, there are many drawbacks for them: size, cost, toxicity and lifetime [7–9]. As an
alternative to the Cd-containing QDs, the non-toxic InP/ZnS QDs only yield a color gamut of
87% NTSC, far below the toxic counterparts [6]. On the other hand, phosphor-converted
wLED backlights, which combine a blue LED chip with a single or multiple phosphors, are
mostly used due to their large size, cost effectiveness, robustness and high efficiency. In this
technology, phosphors are one of key components that make a great influence in the color
saturation and brightness of LCDs.
wLED backlight in its early stage was prepared by pumping a broadband yellow-emitting
YAG:Ce3+ phosphor with a blue LED. However, this type backlight only shows a color gamut
of 72% of the NTSC standard, which is hard to provide clean red and rich picture quality [1].
The small color gamut using YAG:Ce3+ is dominantly ascribed to the large overlap between
the green and the red emission spectra after the wLED emission spectrum passes through
RGB color filters used in LCDs for balancing the image quality and power consumption. To
overcome this problem, an alternative option is to use a dichromatic phosphor blend
consisting of a green- and a red-emitting phosphor. For example, with the discovery and
application of promising β-sialon:Eu2+ (green) and CaAlSiN3:Eu2+ (red) phosphors [10–12],
we produced a three-band wLED backlight that have a color gamut as high as 92% NTSC [1].
Fukuda et al. reported an interesting green Sr3Si13Al3O2N21:Eu2+ phosphor, and succeeded in
using it to fabricate a wLED backlight with a color gamut of 94.2% [3,13]. Ito et al. attempted
the application of SrGa2S4:Eu2+ (green) and CaS:Eu2+ (red) phosphor sheets in wLED
backlights, and obtained a color gamut of 90% NTSC [5]. These results suggest that the color
gamut of the backlight is largely dependent on the luminescence spectrum of both green and
red phosphors. In general, phosphors for backlights are required to have a narrow-band
emission and a specific emission maximum.
Table 1. Narrow-band phosphor candidates used for wLED backlight.
β-sialon:Eu2+
Green
phosphors
Red
phosphors
Sr3Si13Al3O2N21:Eu
Sr2GaS4:Eu2+
γ-alon:Mn2+
CaAlSiN3:Eu2+
Sr[LiAl3]N4:Eu2+
CaS:Eu2+
K2SiF6:Mn4+
2+
Emission
maxima / nm
FWHM
/ nm
stability
535
55
Excellent
525
540
520
650
655
650
630
66
47
44
92
52
64
3
Good
Bad
Good
Excellent
Bad
Bad
medium
EQE
/%
N/A
67
N/A
13
78
52
N/A
80
ref
[10,11,14]
[13]
[5]
[15]
12
[16]
[5]
[7,17,18]
From a viewpoint of materials design, narrow-band phosphors can be achieved by (i)
accommodating activators, such as Eu2+, in a highly symmetric structure; (ii) using activators
having spin- or parity-forbidden electron transitions, such as Mn2+ or Mn4+. The first case is
evidenced in β-sialon:Eu2+ where Eu2+ is coordinated to six O/N atoms at a same distance, and
in Sr[LiAl3]N4:Eu2+ where Eu2+ is bonded to eight N atoms forming a cuboid-like polyhedron
[14,16]. The high symmetry of the structure in those hosts finally results in a quite small
FWHM (full width at half maximum) that is much narrower than that of Eu2+ usually
observed in most hosts (55 nm vs ~90 nm). As seen in Table 1, an extremely small bandwidth
is seen for both Mn2+ and Mn4+, typically the FWHM is of several nanometers in Mn4+-doped
K2SiF6 due to the spin- and parity-forbidden 2Eg→4A2g transition. This enables γ-alon:Mn2+
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Received 1 Sep 2015; revised 9 Oct 2015; accepted 11 Oct 2015; published 23 Oct 2015
2 Nov 2015 | Vol. 23, No. 22 | DOI:10.1364/OE.23.028707 | OPTICS EXPRESS 28709
and K2SiF6:Mn4+ to be promising narrow-band green and red phosphors for backlights,
respectively. In addition, exept bandwidth other important parameters, such as the quantum
efficiency, the peak position of the emission, decay time, and the stability against thermal
and/or chemical attacks, also need to be considered for selecting phosphors used in wLED
backlights. With this in regard, the moisture-sensitive Sr[LiAl3]N4:Eu2+, Sr2GaS4:Eu2+ and
CaS:Eu2+ are hardly used unless their stability is significantly enhanced.
To date, both the narrow-band β-sialon:Eu2+ and the deep-red CaAlSiN3:Eu2+ are accepted
as the most suitable phosphors for wLED backlights due to their high efficiency, high stability
and reliability. On the other hand, CaAlSiN3:Eu2+ has some drawbacks that prevent it from
achieving much larger color gamut and higher brightness of the backlight: (i) a broader
emission spectrum covering a considerable amount of the spectral energy that is lost after
filtering, and (ii) a large spectral overlap between the excitation spectrum of CaAlSiN3:Eu2+
and the emission spectrum of β-sialon:Eu2+ that increases the usage amount of the green
phosphor (Fig. 1). Therefore, it is essential to find an alternative narrow-band red phosphor to
further enhance the color reproducibility and brightness of the wLED backlight. K2SiF6:Mn4+
is such a narrow-band red phosphor that possesses five sharp line spectra at 609, 613, 631,
634, and 648 nm, respectively [17–22]. Till now, investigations on K2SiF6:Mn4+ and its
deviates are almost devoted to the synthesis and its application in warm white LEDs with high
color rendering index for general lighting [19–21,23]. Qiu et al. used KSF:Mn4+ and
YAG:Ce3+ to prepare a warm white LED with color temperature of ~3510K, Ra = 91 and a
luminous efficacy of 82 lm/W [19]. Do et al. reported an ultrahigh color rendition warm white
LED (CRI = 94, R9 = 93, CCT = 2700K, and 107 lm/W) by using KSF:Mn4+ [21]. These
results validate the role of the narrow-band red phosphor in enhancing the color rendition and
luminous efficiency of white LEDs. To the best of our knowledge, KSF:Mn4+ has been rarely
reported and demonstrated for use in wLED backlight. Oh et al., used K2SiF6:Mn4+ as a red
phosphor and Sr2GaS4:Eu2+ as a green phosphor to prepare wLEDs for use in LCD backlights
[7]. Howover, Sr2GaS4:Eu2+ is hardly used in practicle displays due to its moisture sensitivity.
In this work, we attempted to combine both of the narrow-band KSF:Mn4+ and β-sialon:Eu2+
phosphors with an InGaN blue LED, and fabricated a higher brightness (95 lm/W) and larger
color gamut (> 96% NTSC) wLED backlight in comparison to previous studies.
2. Experimental methods
2.1 Phosphors preparation
β-sialon:Eu2+ (Si6-zAlzOzN8-z:Eu2+, z = 0.5, 0.5 at% Eu) was prepared by usinig a gas pressure
sintering method. An appropriate amount of α-Si3N4 (SN-E10, Ube Industries, Japan), AlN
(Type F, Tokuyama Corp., Japan), Al2O3 (TAIMICRON, Daimei Chemicals Co. Ltd., Tokyo,
Japan), and Eu2O3 (Shin-Etsu Chemical Co. Ltd., Japan) were weighed out and well mixed in
a motar by hand. A total of 2 g powder mixture was then packed into a boron nitride crucible,
and fired in a gas-pressure sintering furnace (FVPHR-R-10, FRET-40, Fujidempa Kogyo Co.
Ltd., Osaka, Japan) with a graphite heater. The sample was heated at a constant heating rate of
600°C/h in vacuum (< 10−3 Pa) from room temperature to 800°C. At 800°C, a nitrogen gas
(99.999% purity) was introduced into the chamber, and simultaneously the temperature was
raised up to 2050°C. The sample was heated at the temperature for 12 h under a nitrogen gas
pressure of 1.0 MPa. After firing, the power was shut off, and the samples were cooled down
with furnace. The fired phosphor powder was ground, washed and sieved for further use.
K2SiF6:Mn4+ was synthesized by a two-step co-precipitation method (see Fig. 1) [24],
following the reaction as below.
KF + HF + KMnO 4 + H 2 O 2 → K 2 MnF6 + H 2 O + O 2
KHF2 + HF + H 2SiF6 + K 2 MnF6 → K 2SiF6 : Mn 4+
(1)
Synthesis of K2MnF6. High-purity KF (92 g) was firstly dissolved in aqueous HF (49 wt%,
400 ml), followed by dissolving KMnO4 (12 g). The mixed solution was stirred and cooled to
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© 2015 OSA
Received 1 Sep 2015; revised 9 Oct 2015; accepted 11 Oct 2015; published 23 Oct 2015
2 Nov 2015 | Vol. 23, No. 22 | DOI:10.1364/OE.23.028707 | OPTICS EXPRESS 28710
15°C. A yellow powder K2MnF6 was precipitated by slowly droping H2O2 (30 wt%). After
fast filtering and washing by ethanol, the yellow powder was oven-dried at 100°C for 2 h.
Synthesis of K2SiF6:Mn4+. Solution I was prepared by dissovling high-purity KHF2 (4.9 g)
in aqueous HF (49 wt%, 10 ml) at room temperature. Solution II was formed by firstly mixed
H2SiF6 (35 wt%, 10 ml) in HF (49 wt%, 40 ml), followed by adding K2MnF6 (1.48 g).
Solution I was then added dropwise to the brown Solution II, stirred continuously until the
brown solution became almost colorless, and yellow powders were finally precipitated at the
bottom of the beaker. After filtering and washing with ethanol for three times, the yellow
K2SiF6:Mn4+ powder was dried in an oven at 100°C for 1 h.
Solution I
KHF2 + HF
K2MnF6
KMnO4+KF+HF
Solution II
H2SiF6+HF +
K2MnF6
K2SiF6:Mn4+
Fig. 1. Schematics of the preparation of K2SiF6:Mn4+ by a two-step co-precipitation method.
The detailed description is given in the text.
2.2 Charaterization of phosphors
The morphology of phosphors particles was observed by a scanning electron microscope
(Hitachi S5000). CL measurements of KSF:Mn4+ were done by a field emission SEM
(Hitachi, S4300) equipped with a CL system (Horiba, MP32S/M). The beam current was
fixed at 100 pA and the e-beam energy at 5 kV. Energy-dispersed x-ray spectroscopy (EDS)
measurements were carried out at room temperature using a high-resolution field emission
scanning electron microscope (Hitachi, S4800).
Photoluminescence spectra were measured at room temperature using a fluorescent
spectrophotometer (F-4500, Hitachi Ltd., Tokyo, Japan) with a 200 W Xe lamp as an
excitation source. The emission spectrum was corrected for the spectral response of a
monochrometer and Hamamatsu R928P photomultiplier tube by a light diffuser and a
tungsten lamp (Noma, 10 V, 4 A). The excitation spectrum was also corrected for the spectral
distribution of xenon lamp intensity by measuring rhodamine-B as reference.
Time-resolved PL measurements were conducted using a time-correlated single-photon
counting fluorometer (TemPro, Horiba Jobin-Yvon) equipped with a Nano LED (λem = 370
nm) with the pulse duration full width at half-maximum of ~1ns. Thermal quenching was
evaluated by measuring the temperature-dependent photoluminescence in the Hamamatsu
MPCD-7000 multichannel photodetector with a 200 W Xe-lamp as an excitation source. The
phosphor powder was loaded in a hot plate connected to MPCD-7000, and then was heated to
the desired temperature with a heating rate of 100 °C/min. The sample was held at a certain
temperature for 5 min to reach thermal equilibrium, which will guarantee an uniform
temperature distribution both in the surface and interior of the samples. The temperaturedependent quantum efficiency was evaluated by using a QE-1100 phosphor quantum yield
spectrophotometer (Otsuka Electronics, Japan). External (η0), internal (ηi) quantum
efficiencies (QEs) and absorption efficiency (αabs) were calculated by using the following
equations [25]:
#249252
© 2015 OSA
Received 1 Sep 2015; revised 9 Oct 2015; accepted 11 Oct 2015; published 23 Oct 2015
2 Nov 2015 | Vol. 23, No. 22 | DOI:10.1364/OE.23.028707 | OPTICS EXPRESS 28711
η
0
η
i
α
=
=
abs
• P (λ )d λ
λ
λ
• E (λ )d λ
• P (λ )d λ
λ
 λ {E (λ ) − R (λ )}d λ
 λ {E ( λ ) − R ( λ ) }d λ
=
 λ • E (λ )d λ
where E(λ)/hν, R(λ)/hν and P(λ)/hν are the number of photons in the spectrum of excitation,
reflectance and emission of the phosphor, respectively.
2.3 Fabrication of white LEDs
Three-band phosphor-converted wLED backlights were prepared by combining a blue GaInN
LED chip with β-sialon:Eu and K2SiF6:Mn4+ phosphors. Three color temperatures were
targeted by controlling the ratio of the green to red phosphors. The electroluminescent
spectrum, luminescous efficacy, color temperature and color rendering of the wLED were
measured by using an integrating sphere spectroradiometer system (LHS-1000, Everfine Co.,
Hangzhou, China).
The color gamut is mainly determined by the purity of three primary colors, and can be
computed according to trichromatic color space theory [26]. The chromaticity coordinates
defined in Commission Internationale de 1’Eclairage (CIE) can be calculated based on [27].
Intensity (a.u.)
XRD
10
20
30
40
50
60
70
80
SEM
2θ (degree)
K
Si
F
Mn
Fig. 2. (a) XRD patttens, (b) SEM iamge, and (c-f) elemental mapping of K, Si, F, and Mn. The
XRD partterns reveal a cubic phase of KSF. The polyhedron shape of the phosphor indicates
well crystallized particles that contributes to high luminescence.
3. Resulsts and discussion
3.1 Microstrural observations
K2SiF6 crystallizes in a cubic system with a space group of Fm[REMOVED EQ FIELD]m
[28]. All the XRD diffraction peaks of K2SiF6:Mn4+ can be indexed to this cubic phase
(JCDPC 01-075-0694), with no trace of other impurity phases [Fig. 2(a)]. The K2SiF6:Mn4+
phosphor shows a typical polyhedron morphology and a particle size of 20-30 μm [Fig. 1(b)].
The elemental mapping clearly indicates an uniform distribution of elements K, Si, F and Mn
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© 2015 OSA
Received 1 Sep 2015; revised 9 Oct 2015; accepted 11 Oct 2015; published 23 Oct 2015
2 Nov 2015 | Vol. 23, No. 22 | DOI:10.1364/OE.23.028707 | OPTICS EXPRESS 28712
in each phosphor particle. These data verify that the phase assemblage, morphology and
chemical composition of the prepared K2SiF6:Mn4+ are in a good agreement with previous
studies [19,21].
β-sialon has a hexagonal crystal system with a space group of P63/m [29]. As shown in
Fig. 3, β-sialon:Eu2+ displays a characteristic rod-like shape and has a size of ~5 μm in
diameter and 10 - 30 μm in length. These elongated particles are usually oberserved in βsialon with low z values [10,11]. The XRD pattens of the prepared β-sialon:Eu2+ clearly
indincate a single phase, and the sharp diffraction peaks match well the good crystallinity of
the phosphor particles.
(a)
Intensity (a.u)
(b)
10
15
20
25
30
35
40
45
50
55
60
Degree (2θ)
Fig. 3. (a) SEM image of rod-liked β-sialon:Eu2+ prepared by gas-pressure sintering, and (b)
XRD patterns ofβ-sialon:Eu2+, indicative of a phase pure β-sialon:Eu2+ with high crystallinity.
3.2 Photolumienscence
Figure 4 presents the photoluminescence spectra and absorption efficiency of β-sialon:Eu2+
and K2SiF6:Mn4+. For comparison, those data of CaAlSiN3:Eu2+ are also included. βsialon:Eu2+ has a relatively narrow emission band centered at 535 nm and a FWHM of 53 nm,
which is due to the 4f→5d electronic transition of Eu2+. For K2SiF6:Mn4+, three intense
excitation bands centered at 250, 352 and 455 nm are observed. The 250 nm band can be
considered as the charge transfer transition between F- and Mn4+ ions, whereas the 352 and
455 nm bands are ascribed to the spin-allowed 4A2 → 4T1 and 4A2 → 4T2 transitions,
respectively [17,18,22]. The emission spectrum of K2SiF6:Mn4+ consists of five sharp lines
with the strongest peak at 631 nm, which originates from the spin-forbidden 2Eg → 4A2
transition [17,18,22].
2+
PL intensity (a.u.)
3
200
CaAlSiN3:Eu2+
β-sialon:Eu2+
300
400
100
K2SiF6:Mn4+
500
600
Wavelength (nm)
(emission)
700
800
Absorption efficiency (%)
:Eu
(a) CaAlSiN
(excitation)
2+
CaAlSiN3:Eu
emission spectrum
of β−sialon:Eu2+
80
60
40
535 nm
K2SiF6:Mn4+
20
(b)
0
300
350
400
450
500
550
600
Wavelength (nm)
Fig. 4. (a) Excitation and Emission spectra of K2SiF6:Mn4+, β-sialon:Eu2+ and CaAlSiN3:Eu2+;
(b) absorption efficiency of K2SiF6:Mn4+ and CaAlSiN3:Eu2+. The emission spectra of all
samples were exicted at 450 nm, and the excitation spectra of K2SiF6:Mn4+ and CaAlSiN3:Eu2+
were monitored at 631 and 650 nm, respectively.
By comparing the excitation, emission and absorption spectra of K2SiF6:Mn4+ with those
of CaAlSiN3:Eu2+, one can conclude that K2SiF6:Mn4+ is superior to CaAlSiN3:Eu2+ in the
following aspects: (i) sharp line spectra and free of self-absorption enabling high efficiency
after filtering; (ii) very low absorption of the green emission from β-sialon:Eu (~18% vs
66%@535 nm), allowing for the less use of β-sialon:Eu, (iii) extremely small spectral overlap
between the emission spectra of K2SiF6:Mn4+ and β-sialon:Eu, leading to a higher color
#249252
© 2015 OSA
Received 1 Sep 2015; revised 9 Oct 2015; accepted 11 Oct 2015; published 23 Oct 2015
2 Nov 2015 | Vol. 23, No. 22 | DOI:10.1364/OE.23.028707 | OPTICS EXPRESS 28713
saturation of the backlight, and (iv) almost no photons at wavelengths > 700 nm in
K2SiF6:Mn4+, indicative of no wasted photons for human vision and color perception (whereas
CaAlSiN3:Eu2+ losses about 10% photons). It thus implies that the use of K2SiF6:Mn4+ can
yield higher efficiency and larger color gamut wLED backlighting.
Figure 5 shows the photoluminescence decay curves of K2SiF6:Mn4+ and β-sialon:Eu2+.
Both phosphors have a single exponential decay behavior, and the decay time is determined to
be 0.91 μs and 7.8 ms for β-sialon:Eu2+ and K2SiF6:Mn4+, respectively. The decay time of βsialon:Eu2+ falls in the range usually for Eu2+ 4f65d1→4f7 emission in solids (i.e., 0.2 – 2 μs)
[30,31]. For example, CaAlSiN3:Eu2+ has a decay time of 0.76 μs [32]. The longer decay time
oberserved in K2SiF6:Mn4+ is ascribed to the spin and parity-forbidden 2Eg → 4A2 electronic
transitions, and is consistant with those reported in the literature [7,20,23,33].
1000
PL intensity (a.u.)
PL intensity (a.u.)
10000
1000
100
β-sialon:Eu2+
τ = 0.912 μs
100
10
1
0
2
4
6
8
10
Time (μs)
4+
KSF:Mn
10
τ = 7.8 ms
1
0
20
40
60
80
100
Time (ms)
Fig. 5. Decay curves of K2SiF6:Mn4+ (λem = 631 nm) and β-sialon:Eu2+ (λem = 535 nm)
measured under 370 nm excitation. Both show a single exponential decay, but different decay
times. The decay time of Eu2+ is much shorter than that of Mn4+.
100
K2SiF6:Mn4+
β-sialon:Eu2+
60
40
20
0
300
350
400
450
500
Wavelength (nm)
550
600
80
K2SiF6:Mn4+
(b)
β-sialon:Eu2+
60
40
20
0
300
350
400
450
500
550
100
External
quantum efficiency (%)
(a)
80
Internal
quantum efficiency (%)
Absorption efficiency (%)
100
(c)
K2SiF6:Mn4+
β-sialon:Eu2+
80
60
40
20
600
Wavelength (nm)
0
300
350
400
450
500
550
600
Wavelength (nm)
Fig. 6. Absorption efficiency (a), internal quantum efficiency (b) and external quantum
efficiency (c) as a function of the excitation wavelength of β-sialon:Eu2+ and KSF:Mn4+. It
indicates that (i) both phosphpors have the similar quantum efficiency under 450 nm excitation;
(ii) the absorption of green light (i.e. from β-sialon:Eu2+) by KSF is quite small (less than 20%).
The absorption and quantum efficiency as a function of the exicted wavelength of
K2SiF6:Mn4+ and β-sialon:Eu2+ are given in Fig. 6. Upon the 450 nm excitation, the absorption
efficiency, internal and external quantum efficiency is 70, 77.5 and 54.5% for K2SiF6:Mn4+,
respectively. The luminescence efficiency of the as-prepared K2SiF6:Mn4+ is lower than that
of the commercially available CaAlSiN3:Eu2+, but agrees well with the reported value. Great
efforts should still be made to enhance the efficiency of the narrow-band red phosphor for
commercial purpose. The β-sialon:Eu2+ phosphor has the corresponding absorption efficiency,
internal and external quantum efficiency of 69, 78, and 54% respectively, which is quite
equivalent to those of K2SiF6:Mn4+ under the blue light irradiation.
3.3 Temperature-dependent luminescence and quantum efficiency
Thermal quenching behavior of the phosphor can be evaluated by measuring the temperaturedependent emission intensity. As shown in Fig. 7, the luminescence of K2SiF6:Mn4+ declines
faster than that of β-sialon:Eu2+, which remains 76 and 84% of the initial intensity at 150°C
for K2SiF6:Mn4+ and β-sialon:Eu2+, respectively. The thermal quenching temperature at which
the luminescence intensity reduces by 50% (Ttq) is 247°C (520 K) for K2SiF6:Mn4+, and it is
much higher for β-sialon:Eu2+ (> 600°C). According to the Arrhenius equntion IT/I0 = [1 +
#249252
© 2015 OSA
Received 1 Sep 2015; revised 9 Oct 2015; accepted 11 Oct 2015; published 23 Oct 2015
2 Nov 2015 | Vol. 23, No. 22 | DOI:10.1364/OE.23.028707 | OPTICS EXPRESS 28714
C⋅exp(-Ea/κT)]−1 (I0 is the initial luminescence intensity, IT is the intensity at a given
temperature T, C is a constant, and κ is Boltzman’s constant) [34], the activation energy for
thermal quenching (Ea) is calculated as 0.20 and 0.17 eV for K2SiF6:Mn4+ and β-sialon:Eu2+,
respectively.
Normalized intensity
1.0
2+
β-sialon:Eu
0.8
0.6
K2SiF6:Mn
0.4
4+
0.2
0.0
0
50
100
150
200
250
300
o
Temperature ( C)
Fig. 7. Thermal quenching of K2SiF6:Mn4+ and β-sialon:Eu2+ when excited at 450 nm.
The temperature-dependent quantum efficiency of β-sialon:Eu2+ and K2SiF6:Mn4+ is
plotted in Fig. 8. At room temperature, β-sialon:Eu2+ and K2SiF6:Mn4+ have an internal
quantum efficiency of 86.1 and 70.2% under the 450 nm excitation, respectively. The
corresponding external quantum efficiency is 54.8% for β-sialon:Eu2+and 45.7% for
K2SiF6:Mn4+. As the temperature increases up to 200°C, the luminescence efficiency of βsialon:Eu2+ declines by 7.5%, whereas it remains almost unchanged for K2SiF6:Mn4+ below
200°C. These indicate that both of the luminescence efficiency and the thermal stability of the
narrow-band β-sialon:Eu2+ and K2SiF6:Mn4+ phosphors are quite high and good enough for
practical applications.
80
80
Internal QE
70
60
50
External QE
40
30
20
0
2+
β-sialon:Eu
(a)
10
0
50
100
150
200
o
Temperature ( C)
250
300
Quantum efficiency (%)
Quantum efficiency (%)
90
K2SiF6:Mn
70
60
4+
Internal QE
50
40
External QE
30
20
10
0
(b)
0
50
100
150
200
250
300
o
Temperature ( C)
Fig. 8. Temperature-dependent quantum efficiency of β-sialon:Eu2+ and K2SiF6:Mn4+ when
excited at 450 nm. This indicates that both samples have high thermal stability, and βsialon:Eu2+ is superior to KSF:Mn4+ at higher tempetures (> 200°C).
3.4 Optical properties of white LEDs
wLEDs with three color tempetatures (11184, 11992, and 13769 K) were fabricated by
coating the phosphor blends of K2SiF6:Mn4+ and β-sialon:Eu2+ on a blue LED (λem = 450 nm),
and their electroluminescence spectra before and after filtering are given in Fig. 9. The
transmittance spectra of commercial RGB color filters are also included in Figs. 9(d)-(f). The
color gamut is calculated with the white LED spectrum [Figs. 9(a)-(c)] using the transmission
spectrum of each color filter. As shown in Table 2, the wLED shows a luminous efficacy of
91-96 lm/W. For wLEDs with varying color temperatures, the color gamut defined in the
CIE1931 and CIE1976 color spaces is 85.5-85.9% and 94.3-96.2% of the NTSC space,
respectively (Fig. 10).
#249252
© 2015 OSA
Received 1 Sep 2015; revised 9 Oct 2015; accepted 11 Oct 2015; published 23 Oct 2015
2 Nov 2015 | Vol. 23, No. 22 | DOI:10.1364/OE.23.028707 | OPTICS EXPRESS 28715
400
450
500
550
600
650
700
(b)
400
11992 K
450
550
600
650
700
(e)
500
550
600
Wavelength (nm)
650
700
450
500
550
600
650
700
650
700
Wavelength (nm)
(f)
8114 K
Relative intensity
450
400
13769 K
8611 K
Relative intensity
7828 K
Relative intensity
400
(c)
Wavelength (nm)
Wavelength (nm)
(d)
500
Relative intensity
11184 K
Relative intensity
Relative intensity
(a)
400
450
500
550
600
650
Wavelength (nm)
700
400
450
500
550
600
Wavelength (nm)
Fig. 9. Electroluminescence spcetra of the as-prepared wLEDs (a-c) and of the wLEDs after
filtering (d-f).
(a)
(b)
Fig. 10. The CIE 1931 (a) and CIE 1976 (b) color coordinates of the NTSC standard (black
dotted triangles) and the wLED with the color tempetaure of 8611 K (white triangles).
As summarized in Table 3, the current wLEDs have comparable optical properties to those
using Sr2GaS4:Eu2+ and K2SiF6:Mn4+, but possess larger color gamut (96% vs 92% NTSC)
and higher efficiency (91-96 lm/W vs 38 lm/W) than wLED backlights using β-sialon:Eu2+
and CaAlSiN3:Eu2+, validating its applicable and value for use in wLEDs for LCD backlights.
In addition, the wLED using K2SiF6:Mn4+ shows much less red emissions above 650 nm,
indicative of the significant reduction of wasted photons insensitive to the human eye.
Recently, we have reported a blueshifted oxygen-less β-sialon:Eu2+ which has a FWHM of
47 nm and a peak emission of 525 nm [35]. Although the oxygen-less β-sialon:Eu2+ has
smaller quantum efficiency than the standard β-sialon:Eu2+, its narrower band width and
blueshifted emission enable it to be a more suitable green phosphor for use in wLED
backlighting. One thus can anticipate that a much higher color gamut (> 100% NTSC) would
be achieved if both the oxygen-less β-sialon:Eu2+ and K2SiF6:Mn4+ are combined.
#249252
© 2015 OSA
Received 1 Sep 2015; revised 9 Oct 2015; accepted 11 Oct 2015; published 23 Oct 2015
2 Nov 2015 | Vol. 23, No. 22 | DOI:10.1364/OE.23.028707 | OPTICS EXPRESS 28716
Table 2. Optical properties of wLED backlight using K2SiF6:Mn4+ and β-sialon:Eu2+
Color
temperature (K)
Chromatic coordinates
Luminous
efficacy
(lm/W)
Color gamut (% NTSC)
CIE1931 (x, y)
CIE1976 (u’, v’)
CIE1931
CIE1976
7828
(0.2920, 0.3228)
(0.1857, 0.4619)
85.5
94.3
94
8114
(0.2881, 0.3216)
(0.1834, 0.4607)
85.8
95.0
95
8611
(0.2847, 0.3123)
(0.1843, 0.4549)
85.9
96.2
91
Table 3. Optical properties of phosphor-converted wLEDs for LCD backlights
β-sialon:Eu2+
CaAlSiN3:Eu2+
8620
38
Color gamut
(% NTSC)
CIE
CIE
1931
1976
82.1
91.9
Sr3Si13Al3O2N21:Eu
CaAlSiN3:Eu2+
12723
41
83.8
92.4
3
Sr2GaS4:Eu2+
K2SiF6:Mn4+
8330
105
86.4
N/A
7
Sr2SiO4:Eu2+
CaAlSiN3:Eu2+
8000
103
74.7
N/A
7
3+
YAG:Ce
8000
105
67.9
N/A
7
YAG:Ce3+
4950
59
68.3
71.6
1
8611
94
85.9
96.2
Phosphors
Green
Red
2+
β-sialon:Eu2+
K2SiF6:Mn4+
Color
temperature
(K)
Luminous
efficacy
(lm/W)
ref
1
This
work
4. Conlcusions
Narrow-band phosphors, β-sialon:Eu2+ (green) and K2SiF6:Mn4+ (red), were synthesized and
used to fabricate wide color gamut wLED backlights. K2SiF6:Mn4+ was prepared by a twostep co-precipitation approach, and has an absorption, internal and external quantum
efficiencies of 80.2, 70.2 and 45.7% under the 450 nm excitation, respectively. β-sialon:Eu2+
was obtained by using a gas pressure sintering method, and has the corresponding absorption,
internal and external quantum efficiencies of 63.6, 86.1 and 54.8%, respectively. The thermal
quenching temperature is ~250°C for K2SiF6:Mn4+ and > 600°C for β-sialon:Eu2+. The
temperature-dependent quantum efficiency reveals high thermal stability of both K2SiF6:Mn4+
and β-sialon:Eu2+.
Three-band wLEDs with high color temperatures of 11,186, 11992 and 13,769 K
(corresponding to color temperatures of 7828, 8114 and 8611 K for LCD dispalys) were
fabricated by combining the phosphor blends of K2SiF6:Mn4+ and β-sialon:Eu2+ with an
InGaN blue chip to achieve white balance. The wLEDs have a luminous efficacy of 91 – 95
lm/W, measured under an applied current of 120 mA. The calculated color gamut is ~86% and
94 - 96% relative to the NTSC standard in the CIE 1931 and CIE 1976 color space,
respectively. Both of the luminous efficacy and color gamut of current wLEDs using the
narrow-band K2SiF6:Mn4+ red phosphor are higher than those of wLEDs using the broad-band
and deep-red CaAlSiN3:Eu2+. It indicates that both of the narrow-band K2SiF6:Mn4+ and βsialon:Eu2+ phosphors can be considered as the most suitable luminescent materials for use in
large color gamut and high efficiency wLED backlights.
Acknowledgment
This work was financially supported in part by Grants-in-Aid for Scientific Research from
KAKENHI (No. 15K06448), National Natural Science Foundation of China (No. 61575182
and No. 51572232).
#249252
© 2015 OSA
Received 1 Sep 2015; revised 9 Oct 2015; accepted 11 Oct 2015; published 23 Oct 2015
2 Nov 2015 | Vol. 23, No. 22 | DOI:10.1364/OE.23.028707 | OPTICS EXPRESS 28717