Highly efficient narrow-band green and red phosphors enabling wider color-gamut LED backlight for more brilliant displays

Le Wang; Xiaojun Wang; Takahashi Kohsei; Ken-ichi Yoshimura; Makoto Izumi; Naoto Hirosaki; Rong-Jun Xie

doi:10.1364/OE.23.028707

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 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.

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 ²E_g→⁴A_2g 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.

Table 1. Narrow-band phosphor candidates used for wLED backlight.

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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.

Fig. 1 Schematics of the preparation of K₂SiF₆:Mn⁴⁺ by a two-step co-precipitation method. The detailed description is given in the text.

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2. Experimental methods

2.1 Phosphors preparation

β-sialon:Eu²⁺ (Si_6-zAl_zO_zN_8-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.

\begin{array}{l} KF + HF + {KMnO}_{4} + H_{2} O_{2} \to K_{2} {MnF}_{6} + H_{2} O + O_{2} \\ {KHF}_{2} + HF + H_{2} {SiF}_{6} + K_{2} {MnF}_{6} \to K_{2} {SiF}_{6} : {Mn}^{4 +} \end{array}

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 (λ_em = 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 (η_i) quantum efficiencies (QEs) and absorption efficiency (α_abs) were calculated by using the following equations [25]:

\begin{array}{l} η_{0} = \frac{\int λ • P (λ) d λ}{\int λ • E (λ) d λ} \\ η_{i} = \frac{\int λ • P (λ) d λ}{\int λ {E (λ) - R (λ)} d λ} \\ α_{a b s} = \frac{\int λ {E (λ) - R (λ)} d λ}{\int λ • E (λ) d λ} \end{array}

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].

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 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].

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.

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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 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.

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.

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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. 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 ²E_g → ⁴A₂ transition [17,18,22].

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.

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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 [32]. The longer decay time oberserved in K₂SiF₆:Mn⁴⁺ is ascribed to the spin and parity-forbidden ²E_g → ⁴A₂ electronic transitions, and is consistant with those reported in the literature [7,20,23,33].

Fig. 5 Decay curves of K₂SiF₆:Mn⁴⁺ (λ_em = 631 nm) and β-sialon:Eu²⁺ (λ_em = 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⁺.

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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.

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%).

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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% (T_tq) is 247°C (520 K) for K₂SiF₆:Mn⁴⁺, and it is much higher for β-sialon:Eu²⁺ (> 600°C). According to the Arrhenius equntion I_T/I₀ = [1 + C⋅exp(-E_a/κT)]⁻¹ (I₀ is the initial luminescence intensity, I_T 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 (E_a) is calculated as 0.20 and 0.17 eV for K₂SiF₆:Mn⁴⁺ and β-sialon:Eu²⁺, respectively.

Fig. 7 Thermal quenching of K₂SiF₆:Mn⁴⁺ and β-sialon:Eu²⁺ when excited at 450 nm.

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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.

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 β-sialon:Eu²⁺ is superior to KSF:Mn⁴⁺ at higher tempetures (> 200°C).

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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 (λ_em = 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).

Fig. 9 Electroluminescence spcetra of the as-prepared wLEDs (a-c) and of the wLEDs after filtering (d-f).

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Table 2. Optical properties of wLED backlight using K₂SiF₆:Mn⁴⁺ and β-sialon:Eu²⁺

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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).

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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.

Table 3. Optical properties of phosphor-converted wLEDs for LCD backlights

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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.

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 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.

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 (NSFC) (No. 61575182 and No. 51572232).

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Color temperature (K)	Chromatic coordinates		Color gamut (% NTSC)		Luminous efficacy (lm/W)
Color temperature (K)	CIE1931 (x, y)	CIE1976 (u’, v’)	CIE1931	CIE1976	Luminous efficacy (lm/W)
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

Phosphors		Color temperature (K)	Luminous efficacy (lm/W)	Color gamut (% NTSC)		ref
Green	Red	Color temperature (K)	Luminous efficacy (lm/W)	CIE 1931	CIE 1976	ref
β-sialon:Eu²⁺	CaAlSiN₃:Eu²⁺	8620	38	82.1	91.9	1
Sr₃Si₁₃Al₃O₂N₂₁:Eu²⁺	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
YAG:Ce³⁺		8000	105	67.9	N/A	7
YAG:Ce³⁺		4950	59	68.3	71.6	1
β-sialon:Eu²⁺	K₂SiF₆:Mn⁴⁺	8611	94	85.9	96.2	This work

Color temperature (K)	Chromatic coordinates		Color gamut (% NTSC)		Luminous efficacy (lm/W)
Color temperature (K)	CIE1931 (x, y)	CIE1976 (u’, v’)	CIE1931	CIE1976	Luminous efficacy (lm/W)
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

Phosphors		Color temperature (K)	Luminous efficacy (lm/W)	Color gamut (% NTSC)		ref
Green	Red	Color temperature (K)	Luminous efficacy (lm/W)	CIE 1931	CIE 1976	ref
β-sialon:Eu²⁺	CaAlSiN₃:Eu²⁺	8620	38	82.1	91.9	1
Sr₃Si₁₃Al₃O₂N₂₁:Eu²⁺	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
YAG:Ce³⁺		8000	105	67.9	N/A	7
YAG:Ce³⁺		4950	59	68.3	71.6	1
β-sialon:Eu²⁺	K₂SiF₆:Mn⁴⁺	8611	94	85.9	96.2	This work

Highly efficient narrow-band green and red phosphors enabling wider color-gamut LED backlight for more brilliant displays

Abstract

1. Introduction

2. Experimental methods

2.1 Phosphors preparation

2.2 Charaterization of phosphors

2.3 Fabrication of white LEDs

3. Resulsts and discussion

3.1 Microstrural observations

3.2 Photolumienscence

3.3 Temperature-dependent luminescence and quantum efficiency

3.4 Optical properties of white LEDs

4. Conlcusions

Acknowledgment

References and links

Cited By

Figures (10)

Tables (3)

Equations (2)

Optics Express

		Emission maxima / nm	FWHM / nm	stability	EQE / %	ref
Green phosphors	β-sialon:Eu²⁺	535	55	Excellent	N/A	[10,11,14]
	Sr₃Si₁₃Al₃O₂N₂₁:Eu²⁺	525	66	Good	67	[13]
	Sr₂GaS₄:Eu²⁺	540	47	Bad	N/A	[5]
	γ-alon:Mn²⁺	520	44	Good	13	[15]
Red phosphors	CaAlSiN₃:Eu²⁺	650	92	Excellent	78	12
	Sr[LiAl₃]N₄:Eu²⁺	655	52	Bad	52	[16]
	CaS:Eu²⁺	650	64	Bad	N/A	[5]
	K₂SiF₆:Mn⁴⁺	630	3	medium	80	[7,17,18]