Green and red phosphor for LED backlight in wide color gamut LCD

Green and red phosphor for LED backlight in wide color gamut LCD

Journal Pre-proof Green and red phosphor for LED backlight in wide color gamut LCD Yunpeng Zhang, Lin Luo, Guantong Chen, Yuanhong Liu, Ronghui Liu, X...

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Journal Pre-proof Green and red phosphor for LED backlight in wide color gamut LCD Yunpeng Zhang, Lin Luo, Guantong Chen, Yuanhong Liu, Ronghui Liu, Xiaochun Chen PII:

S1002-0721(19)30402-8

DOI:

https://doi.org/10.1016/j.jre.2019.10.005

Reference:

JRE 631

To appear in:

Journal of Rare Earths

Received Date: 28 May 2019 Revised Date:

7 August 2019

Accepted Date: 24 October 2019

Please cite this article as: Zhang Y, Luo L, Chen G, Liu Y, Liu R, Chen X, Green and red phosphor for LED backlight in wide color gamut LCD, Journal of Rare Earths, https://doi.org/10.1016/ j.jre.2019.10.005. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © [Copyright year] Published by Elsevier B.V. on behalf of Chinese Society of Rare Earths.

Green and red phosphor for LED backlight in wide color gamut LCD a*

Yunpeng Zhang a, Lin Luo b, Guantong Chen b, Yuanhong Liu b, Ronghui Liu b*, Xiaochun Chen Beijing University of Chemical Technology, No.15, North 3rd Ring East Road, Chaoyang District, Beijing 100029, China b National Engineering Research Center for Rare Earth Materials, General Research Institute for Nonferrous Metals, and Grirem Advanced Materials Co. Ltd. Beijing 100088, China a

Abstract: :Wide color gamut (WCG) backlight for liquid crystal display (LCD) utilizing white light-emitting diodes (LED) has attracted considerable attention for their high efficiency and color reduction. In this review, recent developments in crystal structure, luminescence and applications of phosphors for wide color gamut LED backlight are introduced. As novel red phosphors, Mn4+ activate fluoride and aluminate phosphors are advanced in quantum efficiency, thermal quenching and color saturation for their characteristic spectrum with broad excitation band and linear emission. The crystal structure and fluorescence properties of Mn4+ doped fluosilicate, fluorogermanate, fluotitanate, as well as Sr4Al14O25, CaAl12O19 and BaMgAl10O17 phosphors are discussed in detail. A serial of narrow-band red-emitting Eu2+, Eu3+ and Pr3+-doped nitride silicates and molybdate phosphors are also introduced. Rare-earth-doped oxynitride and silicate green-emitting phosphors have attracted more and more attention because of the wide excitation, narrow emission, high quenching temperature, high quantum efficiency, such as β-sialon:Eu2+, Ba3Si6O12N2:Eu2+, MSi2O2N2:Eu2+ (M=Ca, Sr, Ba), γ-AlON: Mn2+ and Ca3Sc2Si3O12:Ce3+. All above phosphors demonstrate their adaptability in wide color gamut LCD display. Especially for Mn4+ doped fluosilicate red phosphor and β-sialon: Eu2+ green phosphor. To achieve an ultra-high color gamut in white LED backlight and against the OLED, innovative narrow-band emission red and green phosphor materials with independent intellectual property rights are continuously pursed. Keywords: :Wide color gamut; Backlight; Liquid crystal display; Phosphor; White LED; Rare earths 1. Introduction Nowadays, white light-emitting diodes (LEDs) are the maturest approach to acquiring a general lighting and display device owing to their high efficiency, long lifetime, and excellent stability, which are considered as the fourth lighting generation light source[1,2]. With economics and high quality of LEDs, it has replace 40% market shares conventional lamps for general lighting and 80% market shares for backlight. Many emerging materials such as CCFL, w-LED, quantum dots, and perovskites have been reported to be used as backlight for LCD[3-6]. With the rapid development of games and HD video industry, people put forward to higher requirements for liquid crystal display technology, one of the important indicators is color gamut coverage and a triangular area in color coordinates, which can also be represented by adopting National Television Standard Committee (NTSC) standard [7]. The traditional cold cathode fluorescent lamps

(CCFL) backlight display can only reach to 65%–75% NTSC, which is similar to normal LED backlight display, which is far behind the 100% NTSC value of organic light emitting diode (OLED)[8,9]. Therefore, wide color gamut with more than 92% NTSC for LED backlight is desperately needed. DCI-P3 standard and Rec. 2020 standard are the promising color gamut standard for mobile phones and ultra-high definition TVs in the future. A LED backlight for liquid crystal display (LCD) is generally utilizing a blue LED and one or two kinds of phosphors. Therefore, phosphors play an important role on the NTSC value [10-14]. At present, there are four methods of fabricating w-LED with various color gamut: (i) using a blue GaInN LED chip with Y3Al5O12:Ce3+ (YAG:Ce) yellow phosphor; (ii) using a blue GaInN LED chip with Lu3Al5O12:Ce3+(LuAG:Ce) or β-SiAlON:Eu green phosphor and CaAlSiN3:Eu2+ red phosphor; (iii) using a blue GaInN LED chip with β-SiAlON:Eu green phosphor and K2SiF6:Mn4+/K2GeF6:Mn4+ red phos-

*Foundation item: Project supported by National Key Research and Development Program (2017YFB0404301). *aCorresponding author: Xiaochun Chen (Email: [email protected]; Tel: +86-10-64441280); *bCorresponding author: Ronghui Liu (E-mail: [email protected]; Tel: +86-10-82241180).

phor;(iv) using a blue GaInN LED chip with green quantum dot phosphor and K2SiF6:Mn4+/K2GeF6:Mn4+ red phosphor[15-21]. Normal color gamut LED backlight for LCD is fabricated by blue LED and YAG:Ce yellow phosphor, whose color gamut coverage is restricted by secondary colors and low color purity, and NTSC value is 70%. To achieve wide color gamut and high efficiency, phosphors should have high x value or y value in chromaticity coordinate, as well as their spectrum should match well with RGB color filters of LCDs. In method (ii), LED package can reach a maximum color gamut of 85% NTSC by adopting green and red phosphors with high luminous efficiency and high saturation [22]. Nevertheless, it is found that mutual absorption caused by wide excitation band of CaAlSiN3:Eu2+ red phosphor and emission band of green phosphor can sharply reduce luminous efficiency of LED package. In order to solve this problem, a novel Mn4+ activated K2(Si/Ge/Ti)F6 fluoride red phosphors and quantum dot phosphors were introduced in method (iii) and (iv). LED package with high luminous efficiency and wide color gamut of 92%–110% NTSC is invented. Novel fluoride red phosphors and quantum dot phosphors are thermally unstable and sensitive to moisture or oxygen, especially their huge light attenuation in ambient atmosphere. Phosphors for wide color gamut LCDs are supposed to have high efficiency, narrow emission and high stability. In general, transition metal Mn4+, rare earth Eu3+ and Eu2+ activated phosphors match well with requirements of wide color gamut. Luminescence characteristics of Mn4+ lies in its 3d3 electronic state, caused by the crystal field splitting of 3d energy level orbitals and electron distribution within octahedron structure. Its luminous properties are mainly dependent on spin forbidden transitions between 2Eg and 4A2g energy levels, and typically exhibit broadband excitation and peak line spectral emission ranging from 600 to 650 nm[23]. The electron configuration 5p64f65d0 of the Eu3+ ion exist a typical f-f transition characteristic and a linear emission spectrum. The emission properties of Eu3+ ions mainly depend on the electron transition between 5D0 and 7Fj energy levels [24, 25]. In this review, we discussed the development of red and green phosphors with host of aluminate, nitride and oxynitride, fluoride and silicate for wide color gamut LCDs, including their crystal structure luminescent properties and applications.

sists of a LCD panel and a w-LED backlight module. Driven by the big demand and rapid development of wide color gamut LED display, red phosphors with high luminous efficiency, high color saturation and narrow full width at half maximum (FWHM) have been attracted more and more attentions. Various novel aluminate, nitride, fluoride and silicate phosphors activated by Mn4+, Eu3+, Eu2+ and Pr3+ for wide color gamut LCDs were investigated in recent years. For instance, w-LED with > 92 NTSC% were fabricated by combining an InGaN blue LED chip peaked at 445nm and β-SiAlON:Eu green and Mn4+ doped red phosphors. 2.1. Mn4+ doped aluminate red phosphors Mn4+-doped red phosphors mainly including oxide and fluoride. High-temperature solid-state reaction is the most favorable method to prepare the Mn4+ doped aluminate phosphors. In general, the excitation peak in the oxide host is in the range of ultraviolet-near and ultraviolet excitation light, while the emission peak is after 650 nm. The first phosphor host discussed here is the well-known aluminate model, whose composition derivatives have a wide variety of applications. For red emission, Mn4+ is an ideal activator in aluminate hosts since host lattices influence its emission. Generally, Mn4+ doped Sr4Al14O25, CaAl12O19 and BaMgAl10O17 phosphors, exhibit advantages on simple preparation, luminescence efficiency and chemical stability [26-31]. For example, they crystallize as an orthorhombic structure, with space group Pmma (51) (shown in Fig.1) [26], which is mainly composed of (AlO6) octahedrons and (AlO4) tetrahedrons. The octahedral layers separated in an orderly manner by a tetrahedron (AlO4), and the interaction between Mn4+ in octahedrons is weak. Under the excitation of UV or blue light, Sr4Al14O25 gives off a sharp red emission (651 nm) due to 2E→4A2 transition (shown in Fig.2).

Fig.1 the structure of Sr4Al14O25 crystal

2. Wide color gamut display with red phosphor The proposed WCG prototype display system con-

Herein, Mn4+ occupies the Al3+ sites, however, the charge of the system is unbalanced and the luminous

intensity is low. Peng et al. enhanced the luminescence intensity of Mn4+ by using Mg2+ as the charge compensator in the synthesis reaction to reduce the probability of non-radiative transition and optimize the particle morphology [27]. Long et al. reported that when Na+ was used as the charge compensator, Na+-Mn4+ bond could replace Sr2+-Al3+ bond. The excitation peak (450 nm) is significantly improved in the blue region due to the Na+ doping [28]. Hence, the luminescence intensity of phosphors enhanced about 160% compared with the Mn4+-doped phosphor.

Fig.3 Crystal structure (a), excitation and emission spectra of CaAl12O19: Mn4+ (b)

Murata et al. reported that the relative fluorescence intensity of Mn4+ in CaAl12O19 is enhanced by adding CaF2 and MgF2 [31]. The effect of CaF2 and MgF2 on the improvement of the relative fluorescence intensity can be explained by the synergetic effect of flux and charge compensation of CaF2 and MgF2, respectively: CaF2 would accelerate the crystal growth of CaAl12O19:Mn4+, and Mg2+ ions would compensate the local charge balance surrounding Mn4+ ions instead of Mn2+. In addition, the emission in CaAl12O19:Mn4+ is enhanced by 81% with introduction of GeO2 flux [32]. The introduced of GeO2 helps to form new Ge4+–Mn4+–O2− pairs instead of Mn4+–Mn4+ pairs. Liu fabricated a w-LED combining CaAl12O19: Mn4+ with yellow YAG:Ce (Fig.4) [33]. Under the 36 mA forward bias current operation, the w-LED exhibits bright warm white light with a high CRI of 88.5, CCT of 4553 K, and (0.3603, 0.3340) in CIE coordinates. These results suggest that CaAl12O19:Mn4+ phosphors would have desirable potential applications in warm w-LED.

Fig.2 Excitation and emission spectrum of Sr4Al14O25:Mn4+

Pure hexagonal CaAl12O19 phase with space group of P63/mmc (194)[29]. CaAl12O19:Mn4+ exhibits band emission ranging from 600 to 700 nm, which is consisted of several peaks, including 643, 656, 666 and 671 nm, due to the 2E→4A2 transitions of Mn4+.The PLE spectrum shows broadband absorption in the range of 250 to 550 nm (Fig.3) [30].

Fig.4 EL spectrum (a) and CIE chromaticity diagram (b) of the w-LED fabricated with blue InGaN chip, YAG:Ce yellow and CaAl12O19: Mn4+ red phosphors

BaMgAl10O17:Mn4+ crystallized in a hexagonal space group P63/mmc (Fig.5(a)) [34]. There are two kinds of Al related polyhedrons, i.e., [AlO6] octahedron and [AlO4] tetrahedron. Mn4+ is stabilized in an octahedral environment. It has also been pointed out that the Mn4+-Mn4+ interaction would be greatly reduced in z direction for the layered structure, indicating that BaMgAl10O17 is an appropriate host to achieve efficient Mn4+ red emission. BaMgAl10O17:Mn4+ system presents two broad excitation bands in the ultraviolet and blue spectral. The PL spectrum of the phosphor exhibits a double-peak structure between 620 and 700 nm with two strong bands at about 651 and 662 nm (Fig.6) [35]. SrMgAl10O17:Mn4+ has the similar structure and luminescence with BaMgAl10O17:Mn4+. Meng et al. reported that Li+, Na+ and Cl- could significantly improve the luminescent properties of SrMgAl10O17:Mn4+, and the color coordinate of SrMgAl10O17:Mn4+ was calculated to be (0.72, 0.28), which was similar to that of 3.5MgO 0.5MgF2 GeO2:Mn4+ phosphor, indicating its application in warm white light emitting diode[36].

Fig. 5 Crystal structure of BaMg2Al16O27 (a) and CaMg2Al16O27(b)

The crystal structure of CaMg2Al16O27 consists of two parts, i.e. M(CaAl12O19) and S(Mg2Al4O8). CaMg2Al16O27: Mn4+ phosphors have a sharp excitation peak at 468 nm and the strongest emission peak is at 655 nm with a half-peak width of about 20 nm [37]. SrMgAl10O17:Mn4+has a similar spectral shape to CaMg2Al16O27:Mn4+, and the luminescence intensity of the system can be improved by the incorporation of Li+, – Na+, Cl .

Fig. 6 PL and PLE of BaMg2Al16O27: Mn4+

2.2. Mn4+ doped fluoride red phosphors Compared with the traditional commercial nitride red phosphors, whose emission spectra show a wide FWHM and low color purity, Mn4+ activated fluoride A2BF6:Mn4+ (A=K, Na, B=Si, Ge, Ti) exhibit a narrow red emission band with high luminous effiency and excellent thermal quenching behavior [32]. The octahedral coordination of SiF62–/GeF62–/TiF62– may be more preferred in fluoride than oxide, caused by the electronegativity difference between F– ion and O2–, which made Mn4+, can be easily compressed into a small lattice site in fluoride matrix. In fluoride, due to the low crystal field intensity provided by ligands, the excitation peak is around 460 nm in the blue region, while the emission peak is around 630 nm [39,40]. The relative phonon energy of fluoride is lower and the luminescence efficiency of Mn4+ ions in fluoride is higher than that of oxide matrix. Meanwhile, without using many expensive alkaline earth metal nitride and oxide, rare earth oxides in raw materials, high temperature and high pressure in synthesis, which reduce preparation difficulty and production costs of A2BF6:Mn4+ phosphors. There exits four approaches to synthesize fluoride red phosphors, including chemical etching, hydrothermal, co-precipitation and cation exchange. Among them, these two co-precipitation and cation exchange methods possess more application prospects in product performance and industrialization [41, 42]. There are three kinds of fluorides according to the valence of the cation, A2XF6, BXF6 and A3MF6, and the A2XF6 is the most investigated [43-46]. Novel fluoride phosphors are frequently considered as Mn4+ doped K2SiF6 (KSF), K2GeF6 (KGF), K2TiF6 (KTF), have been proved to be excellent red phosphors in luminous efficacy and color gamut [47, 48]. This series of fluoride phosphors are composed of [XF6]2– (X=Si/Ge/Ti) and K+ ion. Six-coordinated [XF6]2– group forming octahedral geom-

etry and X is located in the center of the octahedron[49]. KSF has a face-centered cubic structure with a space group of Fm 3 m, KGF and KTF has a hexagonal structure with a space group of P 3 m1, as shown in Figure 7[38]. In this fluorides system, activator Mn4+ substitutes for Si4+/Ge4+/Ti4+ sites in the SiF62− octahedral. The excitation spectrum of K2SiF6:Mn4+ has two excitation peaks at 350 nm and 450 nm corresponding to A2→4T1 and A2→ 4 T2 transitions, respectively[50]. The emission spectrum has a sharp emission peak, and the strongest emission is located at 630 nm, corresponding to 2E→A2 transition[39]. KGF and KTF have a hexagonal structure with the same space group of P 3 m. The excitation spectrum of K2GeF6:Mn4+ shows two excitation bands peaking at 364 and 458 nm, and the strong emission band in K2GeF6:Mn4+ red phosphor reveals six main vibronic related peaks [51]. The K2TiF6:Mn4+ belongs to trigonal system, stokes shift line in the emission spectrum will split. This splitting may be attributed to the trigonal distortion, resulting in the axial splitting of the spin orbital degeneracy of the 2E energy state [42].

Fig. 7 Crystal structures of (a) KSF (b) KGF (c) KTF

Zhu reported a highly efficient K2TiF6:Mn4+ red phosphor by a novel strategy of cation exchange, which can reach an extremely high PL QY up to 98% [52]. Jin reported a pure K2GeF6 phase with P63mc space group other than P 3 m1 space group just by incorporation of Si in K2GeF6 at room temperature through co-precipitation method [53]. The K2(Ge,Si)F6:Mn4+ had a 630 nm emission peak, which could be assigned to the 2Eg→4A2 transitions. These findings verified the phase changes from room temperature of hexagonal P 3 m1 to high temperature of hexagonal p63mc in K2GeF6. A novel non-rare-earth K2XF7:Mn4+ (X = Ta, Nb) was reported by Lin[54], this phosphors exhibiting an admirable quantum efficiency of 93.5%, an extremely narrow FWHM of 2.3 nm, and a high color purity of 99.6%. The excitation and emission spectra of Na2SnF6:Mn4+ and Cs2SnF6:Mn4+ phosphors can be effectively excited by UV and blue light and emit in 600–660 nm range [55]. In the A2XF6 family, K2SnF6·H2O is the only hydrate phosphor, the K2SnF6·H2O can be dehydrated by vacuuming or heating, thereby inducing lattice deformation, leading to splitting of the Mn4+ emission peak and enhancing the emission intensity of the zero-phonon lines[56]. Use 27% K2SiF6:Mn4+ and 63% YAG:Ce package can be used to obtain a warm w-LED with a color temperature of 3466K and a color rendering index of 90, the radiant luminous efficiency is 337 lm/w. However, its thermal stability is poor. When the temperature is 400 K, the temperature quenching begins to occur [57]. Hu reported a brand-new oxyfluoride Na2WO2F4: Mn4+(NWOF:Mn4+) which shows unprecedented intense red ZPL at ~620 nm under blue light excitation, and the energy transfer from “WO2” groups to Mn4+ ions to enhance the emission intensity[58,59]. It is found that All Mn4+ activated phosphors lifetimes are in the range of microseconds, caused by the forbidden transitions in intra-d-shell of Mn4+ ion. This long fluorescence decay time (τ>5 ms) may cause the image-retention phenomenon in backlight displays, which may impose restrictions on some application such as pulse-width-modulated backlight[60]. 2.3. Rare-earth-doped nitride and silicate red phosphors

Fig. 8 PL spectra of the Mn4+-activated fluoride phosphors KSF, KTF, KGF and KGSF

Inorganic luminescent material nitride phosphors, discovered in the 1990, are considered to be the most commercialized LED red phosphors for its excellent physical and chemical stability, wide spectral tuning, high luminous efficiency, and high quenching temperature. Among them Eu2+ doped M2Si5N8, MAlSiN3 (M=Ca, Sr, Ba) are the hot spots of science and application at present, which wide FWHM over 80nm mis-

match the requirements of wide color gamut light-emitting diode backlight. Therefore, novel nitride phosphors with narrow FWHM are in demand. Due to the strong structural rigidity of the [SiN4] tetrahedron, nitride phosphors have high quantum efficiency and good thermal quenching performance [61]. Schnick reported the narrow-band red-emitting Eu2+-doped Sr[LiAl3N4] phosphor[62,63]. Sr[LiAl3N4] compounds crystallize in a triclinic lattice with the space group of P 1 . The structure of Sr[LiAl3N4] consists of AlN4 and LiN4 tetrahedron share edges and corners forming a dense and stable structure. The M2+ ions distributed in two different gap channels. Depending on the radius and the degree of valence matching, the Eu2+ activated ion occupies the position of M2+ in the structure. Fig.9 shows the crystal structure of Sr[LiAl3N4]:Eu2+.

Fig.10 Excitation and emission spectra of Sr[LiAl3N4]:Eu2+

Schnick et al. reported a narrow-band red-emitting phosphor based on Eu2+-doped Ca18.75Li10.5[Al39N55]. Ca18.75Li10.5[Al39N55] has a cubic crystal structure with the space group of Fd 3 m, and consists of two interspersed supertetrahedra which is composed of AlN4 tetrahedra. The Ca2+ and Li+ fill the channels between the supertetrahedra frameworks[65], as shown in Fig.11. The Ca18.75Li10.5[Al39N55]:Eu2+ phosphor shows a single emission band centered at 647 nm and a FWHM of 56 nm, and the excitation spectrum shows also a broad band from 350 to 550 nm corresponding to the 4f7→4f65d1 transition of Eu2+ .

Fig.9 Crystal structure of Sr[LiAl3N4]:Eu2+

By doping Ca[LiAl3N4] with Eu2+, the maximum emission is located at 668 nm with a half-width of 60 nm, and the excitation shows a broad band from 450 nm to 580 nm [64]. By doping Sr[LiAl3N4] with Eu2+, the maximum emission is located at 650 nm with a half-width of 55 nm, and the excitation band covers the range of 400 to 600 nm (Fig.10)[62]. The Sr[LiAl3N4]:Eu2+ phosphor shows excellent thermal stability (its relative quantum efficiency is more than 95% at 200 ℃), because of the high coordination number of the activated ions and the high symmetry environment[61].

Fig.11 PL and PLE of Ca18.75Li10.5[Al39N55]:Eu2+ phosphors

The structural transformations in silicate phosphor ensure the abundant emission colors of nitride when incorporated rare earth ions, which also exhibit excellent chemical stability, and long-wavelength excitation properties. Hence, rare earth doped silicate phosphors can be applied to various fields such as display devices, detector systems, and scintillates of phosphor marking. These interesting features make them very suitable for use as down-conversion materials in w-LED. Eu3+ shows linear peaks of PL and PLE in silicates. For example, Li2ZnSiO4:Eu3+ is crystallized in a tetragonal system, which is connected by [SiO4] tetrahedron and [ZnO4] tetrahedron to form a (Zn-SiO4)2-

mesh structure, and Li+ ions are located at the gap position (Fig.12) [66]. The PL and PLE of such red phosphor are consisted with linear peaks due to the 4f–4f transitions of Eu3+ (Fig.13)[66]. The PLE spectrum shows that the phosphor can effectively excited by ultraviolet and blue light, indicating its application.

is enhanced by incorporating a charge compensator Li+ or Na+. Pr3+ occupies the position of Ba in BaMoO4 crystal [73]. The excitation spectrum contains three sets of excitation peaks, namely 448, 473, and 486 nm, which assigned to 3H4-3P2, 3H4-3P1, and 3H4-3P0 energy level transition of Pr3+, respectively. Therefore, the phosphor can be excited by blue light. The emission spectrum consists of a set of sharp emission peaks at 643 nm, which attributed to the 3P0-3F2 transition of the Pr3+ ion. Mahlik et al. [74] reported the luminescence properties of CaWO4:Pr3+. CaWO4 with tetragonal system and Pr3+ ions occupy the Ca lattice in the crystal. 3. Wide gamut display with green phosphor

Fig.12 Crystal structure of Li2ZnSiO4

Wang reported a novel red phosphor Sr3B2SiO8:Eu3+, which be efficiently excited by long-UV (about 393 nm) and blue light (464 nm), and exhibits red emission peaked at 611 nm [67]. The red emission aroused from the excitation of 393 nm has comparable integral intensity to that of the commercial Y2O3:Eu3+ phosphor, indicating its potential application for w-LED. SrZnMoO6:Eu3+, Sr9R2W4O24:Eu3+ (R=Gd, Y), Y6W2O15:Eu3+, La2MgTiO6:Eu3+, MgTiO3:Eu3+ can also be effectively excited by blue light, and emit red light with a narrow half-peak width and a short peak wavelength[68-72].

Fig.13 Excitation and emission spectra of Li2ZnSiO4:Eu3+

He reported the luminescence properties of BaMoO4:Pr3+ phosphor, whose luminescence intensity

3.1. Rare-earth-doped oxynitride green phosphor In recent years, rare-earth-doped oxynitride green-emitting phosphor has attracted more and more attention because of its excellent performance, such as wide excitation, narrow emission, high quenching temperature, high quantum efficiency, etc [75]. Eu2+ activated MSi2O2N2(M=Ca, Sr and Ba) and Ba3Si6O12N2 were research hotspot for their simple synthetic technique. Nevertheless, poor thermal quenching properties and thermal stability impose restrictions on their commercial applications. β-SiAlON:Eu green phosphor has been large-scale application in the field of high-end LCD backlight display. Subject to stringent synthesis equipment and key technology of preparation, only Japan electric chemistry and Mitsubishi Chemical to realize industrialization for now. The SiA1ON ceramic is a silicon oxynitride compound with the unique (Si,Al)(O,N) tetrahedron structure unit. It includes two kinds of crystal forms, α-type and β-type namely. It is reported that Eu2+-doped and Mn2+-doped α-sialon emits blue and orange-red emission under the ultraviolet excitation, respectively[76,77]. β-SiAlON, with the chemical formula Si6-zAlzOzN8-z (where z represents the number of Al-O substituting for Si–N), is derived from β-Si3N4 by partially replacing Si–N bonds with Al–O bonds. It has a hexagonal crystal system with the space group of P63 or P63/m. The activator Eu2+ resides along the [001] channel in the β-SiAlON lattice [78, 79], as shown in Fig14.

The LED using the synthesized β-sialon: Eu2+ green emitting exhibited high color gamut (102% NTSC) (shown in Fig.16) [82].

Fig. 14 Crystal structure of β-SiAlON:Eu2+

The excitation and emission spectra of β-SiAlON: Eu2+ shown in Fig.15. The β-SiAlON: Eu2+ phosphor shows a green emission band centered at 538 nm with a FWHM of 55 nm. The broad excitation band enables β-SiAlON: Eu2+ can be activated effectively under NUV (400–420 nm) or blue (420–470 nm) light [78]. Xie investigated the influences of z values in the crystal structure and luminescent properties. The XRD pattern shows that the samples with smaller z value obtained higher phase purity [80], because the second phase usually appears with the substitution of Al-O for Si-N. With the z increases, the wavelength of the emission peak red shift gradually.

Fig.15 Excitation and emission spectra of β-SiAlON: Eu2+

Green emitting β-sialon:Eu2+ phosphor shows an extremely high reliability. At 150 °C, the luminescence intensity remained 84%–87% of that measured at room temperature and the emission peak position barely changes. The luminescence intensity of the coated phosphor increased about 39% compared with the uncoated sample. Li reported that the luminescent properties of synthesized phosphor was enhanced by thermally post-treated in a reducing atmosphere N2/H2 at high temperature, due to the removal of Eu3+ in the lattice[76]. Wang et al. investigated found the oxide and fluoride additives increase the luminescence intensity because of the increased crystallinity of the particles [81].

Fig.16 Color gamut of LCD devices obtained from β-SiA1ON:Eu2+ phosphor

Ba3Si6O12N2 has a trigonal crystal system with the space group of P 3 with the SiO3N tetrahedron structure unit [83], as shown in Fig 17. The Ba3Si6O12N2:Eu2+ phosphor shows a green emission band centered at 525 nm and a broad excitation band from 250 nm to 500 nm. The emission peak will display a red shift as the amount of Eu2+ doping increases. At 200 °C, the luminescence intensity remains 90% of that measured at room temperature. The average efficiency of the packaged white LED is 60.6–75.5 lm/w [84] . Wang et al. reported that the introduction of Mg2+ could effectively improve the luminescence intensity of Ba3Si6O12N2:Eu2+ [85]. With the concentration of Mg2+ increasing, the luminescence intensity increases and the FMHW decreases, because the introduction of Mg2+ in the matrix lattice increases the distance between Eu2+ and Eu2+, thereby reduces the interaction between ions. Han reported the effect of different fluxes on the luminescent properties of Ba3Si6O12N2:Eu2+ phosphors [86]. It shows that the sample doped 1% H3BO3 obtains the strongest luminescence intensity, which is attributed to higher crystallinity.

Fig. 17 Crystal structure of Ba3Si6O12N2:Eu2+

MSi2O2N2 (M=Ca, Sr, Ba) compounds are a

series of alkaline earth metal oxynitridosilicates in monoclinic lattice with different space groups, P21/c for M=Ca, P21/m for M=Sr, and P2/m for M=Ba, respectively [87-89], as shown in Fig 18. The excitation spectra of MSi2O2N2:Eu2+ show a broad band. CaSi2O2N2:Eu2+ shows a yellowish emission centered at 562 nm, SrSi2O2N2:Eu2+ shows a green emission centered at 543 nm, BaSi2O2N2:Eu2+ shows a blue-green emission centered at 491 nm, as shown in Fig 19. Fig. 19 Excitation and emission spectra of MSi2O2N2:Eu2+ (M=Ca, Sr, Ba)

Fig. 18 Crystal structure of MSi2O2N2 (M=Ca, Sr, Ba)

Hassan et al. proved that the sample synthesized by the solid phase method has better luminescence properties [90]. Yang et al. reported the effect of co-doped La3+ on the luminescence properties of CaSi2O2N2:Eu2+ [91]. With La3+ doping, the luminescence intensity first increases because of the decreases of Eu3+ in the lattice, and then reduces due to the non-radiative transition between Eu2+ and Eu3+. Huo et al. reported the effects of different co-solvents and co-doped Gd3+ on the luminescence properties of CaSi2O2N2:Eu2+[92]. Among the co-solvents used in the experiments, CaCl2 can significantly increase the crystallinity and luminescence intensity of the sample, and the luminescence intensity of the sample doped with Gd3+ is twice as strong as that of the host. The white LED obtained by the sample encapsulation had a color temperature of 5100 K, a luminous efficiency of 41 lm/W, and a color-rendering index of 76.

Xie et al. reported the crystal structure and luminescence properties of γ-AlON:Mn2+, which has a single crystal spinel phase, in which two lattice positions can be substituted by cations, namely tetrahedral and octahedral, and Mn2+ tends to replace the tetrahedral of Al3+ [93-95]. The excitation and emission spectra of γ-AlON:Mn2+ are shown in Fig.20. The γ-AlON:Mn2+ phosphor shows a green emission band centered at 520 nm with a FWHM of 45 nm under the excitation of 455 nm, and its FWHM is narrower than that of the commercial β-SiAlON:Eu2+ phosphor. Mg2+-doping in the lattice can improve the luminescence intensity [96]. Under the excitation of 455 nm, the quantum efficiencies of the Mg2+-doped sampled and the undoped sample was 62% and 53%, respectively. At 150 °C, the luminescence intensity of the Mg2+ doped sample remained 88% of that measured at room temperature [97].

Fig. 20 Excitation and emission spectra of γ-AlON: Mn2+

Li et al. synthesized a new type of Mn2+ doped green-emitting transparent ceramic MgAlON[96]. The emission spectrum of the Mg0.21Al2.57O3.8N0.2:0.03Mn2+ phosphor shows a narrow-band green light with the peak at 513 nm, which attributed to 4T1→6A1 transition of Mn2+, as shown in Fig.21. At 150 °C, the internal

quantum efficiency remained 47% and the luminescence intensity was still 82.8% of that measured at room temperature.

Fig. 21 Excitation and emission spectra of the Mg0.21Al2.57O3.8N0.2:0.03 Mn2+

Fig. 23 CIE coordinates of Ca3Sc2Si3O12:Ce3+ under excitation at 440 nm

3.2. Rare-earth-doped silicate green phosphor In order to get green emission, silicate is an ideal host since its stable crystal structure and wildly transformation. Ca3Sc2Si3O12:Ce3+ is a garnet type phosphor. As shown in Fig.22, the structure can be illustrated by a unit cell of an A3B2C3O12 garnet crystal [98]. The A, B, C, and O atoms occupy Wyckoff positions 12c, 8a, 12d, and 48h. The image to the right shows a close-up of the local coordination of an A atom, which is oriented in a way that the neighboring polyhedra can be clearly seen, showing the substitution of Ce3+ for an A ion.

Fig. 22 Crystal structure of Ca3Sc2Si3O12:Ce3+

The phosphor shows a broadband emission centered at 504 nm, and the spectra possess excitation maxima at approximately 440 nm, with a full width at half maximum (FWHM) of 73 nm. The performance of Ca3Sc2Si3O12:Ce3+ was evaluated by applying the phosphor on a blue InGaN LED. The system shows a luminous efficacy of optical radiation of 243 lm/W and a linear response with increasing applied voltage, indicating its application for w-LED (Fig.23). Ca8Mg(SiO4)4Cl2:Eu2+ phosphor was reported by Yang [99] . The phosphor gives off a green emission centered at 505 nm, with a broad PLE spectrum ranging from 300 to 460 nm. Zhang et al. reported the a green emission in Ca8Zn(SiO4)4Cl2:Eu2+[100]. The excitation spectra corresponding to transition of Eu2+ cover the spectral range of 370–470 nm, matching the UV and/or blue LEDs. In addition, the emission spectra of the phosphor shows a green band at 505 nm of Eu2+, demonstrating a potential application in phosphor-converted w-LED.

Important photoelectric parameters of above mentioned red and green phosphors were summarized in table 1.

requirements. β-SiAlON:Eu green phosphor, the only commercial green phosphors in the WCG display market, subject to its rigorous synthesis condition, which has product and technology monopoly by foreign industries. Mn2+, Mg2+ coped γ-AlON with higher efficiency is considered to be the most promising oxynitride green phosphors for WCG or ultra WCG on the foundation of luminous efficiency improvement. Eu2+ doped alkali earth metal oxynitride phosphors usually confront thermal and humidity issues, for instance, poor thermal quenching for Ba3Si6O12N2:Eu2+ and CaSi2O2N2:Eu2+, bad waterproof for Sr[LiAl3N4]:Eu2+. Unfortunately, patents of phosphors or LED devices containing phosphors for Mn4+ doped fluoride and β-SiAlON:Eu oxynitride were monopolized by foreign enterprises, which is in common with other commercial phosphor. In short, phosphors for WCG display in China are in its preliminary stage,

4. Conclusion and Prospect In order to meet the challenges of Organic Light Emitting Diode (OLED) in display color gamut, it is necessary to develop narrow-band emission phosphors, which are applicable in WCG LED display. Recently, China has achieved rapid technical development in narrow-band emission phosphors research and application for WCG liquid crystal LED backlight. Many novel phosphors, such as Mn4+ doped aluminates and fluoride red, rare earth doped nitride silicates and molybdate red, as well as Eu2+ and Mn2+ oxynitride and silicate green have attractive more and more attentions. Among them Mn4+ doped KSF/KGF fluoride and β-SiAlON:Eu are the most matched red and green

Table 1 Important photoelectric parameters of above mentioned red and green phosphors

Red phosphor

Green phosphor

phosphors

Excitation

emission

Sr4Al14O25:Mn4+ CaAl12O19:Mn4+ BaMgAl10O17:Mn4+ SrMgAl10O17:Mn4+ CaMg2Al16O27:Mn4+ K.2SiF6: Mn4+ K.2GeF6: Mn4+ K.2TiF6: Mn4+ Na2SnF6: Mn4+ K2SnF6-H2O: Mn4+ Cs2SnF6: Mn4+ Sr[LiAl3N4]:Eu2+ Sr3B2SiO8:Eu3+ Sr2ZnMoO6:Eu3+ Sr9Y2EuxW4O24:Eu3+ Y6W2O15:Eu3+ MgTiO3:Eu3+ β-Sialon: Eu2+ Ba3Si6O12N2:Eu2+ CaSi2O2N2:Eu2+ SrSi2O2N2:Eu2+ γ-AlON:Mn2+ MgAlON:Mn2+ Ca3Sc2Si3O12:Ce3+ Ca8Mg(SiO4)4C12:Eu2+

330/450 416/466 335/468 320/460 345/390/468 350/450 357/461 360/460 360/470 365/465 370/470 485/560 393/464 395/465 395/465 466/538 394/465 450 250–450 300–450 250–450 445 445 440 370–470

652 659 660 660 655 630 631 633 626 632 633 668 611/620 597/624 618 612 614 540 525 556 530 527 513 504 505

Internal Quantum efficiency

FWHM / nm 7 20 30 20 5

54 (80) (98).

71.3(96.5)

phosphors in fabricating WCG LED displays. However, compared with nitride red phosphors, poor temperature and moisture resistance properties of fluoride red phosphors and low luminous efficiency of Mn4+ doped aluminates impede widely applications in general lighting and LED backlight display. Novel aluminates red phosphors with high luminous efficiency synthesized by high temperature solid-state method can solve the above problems. Eu3+ activated phosphors with linear spectral emission should promote the luminous efficiency to match the practical application

(47)

60.3 25 20 10 10 12 54 68 70 65 75 50 50 60

stability good good normal normal good good normal good good normal normal normal normal normal normal normal normal excellent good normal normal excellent excellent excellent normal

Expected LCD color gamut

Luminous efficiency 35 99.2

92–105 92–105 92–105

92–105 85–90

100–105 100–105 75–80

107 143.82

136

reference 26 30 34 35 37 40 41 42 55 56 55 62/63 67 68 69 70 73 76 84 90 91 95/96 97 98 100

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Graphical abstract:

Transition-metal-ions Mn4+ activated fluoride phosphors such as K2MF6:Mn4+ (M=Si/Ge/Ti), which show high luminescence efficiency and thermal stability, are of particular interest for high-end white LEDs. Luminescence properties of Mn4+ activated KSF/KGF/KTF phosphor by coprecipitation method were presented. A novel K2(Ge,Si)F6:Mn4+ phosphor with P63mc space group of pure K2GeF6 phase other than P 3 m1 space group was affirmed by incorporation part Si in K2GeF6.

Conflict of Interest Statement We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work, there is no professional or other personal interest of any nature or kind in any product, service and company that could be construed as influencing the position presented in the manuscript “Green and red phosphor for LED backlight in wide color gamut LCD”.

Sincerely yours, Ronghui Liu