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 phosphor-cpnverted 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
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 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 . 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 . 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 . 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 . 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.
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+ 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 . 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+ . 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 . 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.
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 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.
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 temperature-dependent 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 :
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 . The chromaticity coordinates defined in Commission Internationale de 1’Eclairage (CIE) can be calculated based on .
3. Resulsts and discussion
3.1 Microstrural observations
K2SiF6 crystallizes in a cubic system with a space group of Fm[REMOVED EQ FIELD]m . 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 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 . 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 β-sialon 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.
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. β-sialon: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].
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 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 β-sialon: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 . 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].
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 temperature-dependent 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 + 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) , the activation energy for thermal quenching (Ea) is calculated as 0.20 and 0.17 eV for K2SiF6:Mn4+ and β-sialon:Eu2+, respectively.
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 β-sialon: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.
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).
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 . 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.
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 two-step 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 β-sialon:Eu2+ phosphors can be considered as the most suitable luminescent materials for use in large color gamut and high efficiency wLED backlights.
This work was financially supported in part by Grants-in-Aid for Scientific Research from KAKENHI (No. 15K06448), National Natural Science Foundation of China (NSFC) (No. 61575182 and No. 51572232).
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