Bright and High-Color-Rendering White Light-Emitting Diode Using

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Bright and High-Color-Rendering White Light-Emitting Diode Using Color-Tunable Oxychloride and Oxyfluoride Phosphors Pengpeng Dai, Jian Cao, Xintong Zhang, and Yichun Liu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b03177 • Publication Date (Web): 18 Jul 2016 Downloaded from http://pubs.acs.org on July 19, 2016

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The Journal of Physical Chemistry

Bright and High-Color-Rendering White Light-Emitting Diode Using Color-Tunable Oxychloride and Oxyfluoride Phosphors Pengpeng Dai,†,‡ Jian Cao,† Xintong Zhang,†* and Yichun Liu† †

Center for Advanced Optoelectronic Functional Materials Research, and Key Laboratory of UV Emitting

Materials and Technology of Ministry of Education, Northeast Normal University, 5268 Renmin Street, Changchun 130024, China ‡

Key Laboratory for Mineral Luminescence and Microstructure of Autonomous, Xinjiang Normal

University, 102 New Hospital Street, Urumchi 830054, China

*

Corresponding author. Tel./Fax: +86-431-85099772; E-mail: [email protected]. 1

ACS Paragon Plus Environment


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Abstract White-lighting converted by tricolor phosphor and near ultraviolet light-emitting diode (NUVLED) demonstrates great potential in the general lighting.

However, a major problem

with this approach is the lower luminous efficiency of the resulting white-lighting by the re-absorption among tircolor phosphors.

In this work, we propose a general but effective

method for achieving high-efficacy white light-emitting diodes (WLEDs), by using two-color phosphors having the same host that, can lower energy loss by re-absorption. Sr7.95Si4O12Cl8:0.05Eu2+

(SSO_Cl:0.05Eu2+)

phosphors

are

synthesized

Color-tunable by

cationic

substitution strategy, the emission colors can be tuned from blue-greenish to blue/yellow by Sr→Mg/Ca substitution.

The red-shifted emission is attributed to the increased Eu2+

d-orbital splitting owing to the smaller size of Ca2+, whereas blue-shifted Eu2+ emission is unusual in Mg-substitution for Sr2+. WLEDs with luminous efficacies (ηL) of 28.9-56 lm/W and CRIs of 79-90.2 are demonstrated using the two-color phosphors.

By employing a

narrow-line Mn4+ emission deep red oxyfluoride phosphor, a high CRI (Ra = 90.3, R9 = 94) WLED with ηL of 39.3 lm/W is obtained.

These findings provide a possible way in

developing high CRI WLED by using two-color phosphors. Keywords: two-color phosphor, mixed-ligand, high CRIs, WLEDs

2


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1. Introduction Phosphor-converted WLEDs (pc-WLEDs) are considered to be the simplest, cost-effective and more reliable solid-state lightings, and are penetrating deeply into various market segments.1-4

Presently the commercially available WLEDs are based on the blue LED chip

combined with yellow-emitting phosphor (YAG:Ce3+).1,2

However, this type of WLED has

poor CRI (Ra ≤ 80) due to the blue-greenish and red deficiency.3,4

An alternative method is

by pumping tricolor phosphors with near ultraviolet LEDs (NUVLEDs).4

Although this

strategy can promise WLEDs with a high color rending index (Ra>90) and homogenous color distribution, the low luminous efficiency owing to re-absorption has not been fully overcome: this limits commercial use of this type of WLEDs.5-6

In this regard, dichromatic strategy,

having great advantage in suppressing re-absorption and simplifying the fabrication process, may be a good choice to achieve a high-luminous efficacy WLED.7,8

Therefore, great efforts

have been made to design and investigate novel dichromatic phosphors for WLED. Conventional strategy for two-color pc-WLEDs is to use the physical mixtures of dichromatic phosphors with different host families, which usually result in a lack of consistency in physical/chemistry properties.9

In this work, we propose a general but

effective method for achieving high-quality pc-WLEDs, by using two-color phosphors having the same host lattice that, can lower energy loss by re-absorption.

In our search for a proper

host lattice, inorganic compounds with a mixed-ligand environment as host materials for phosphors are highly desired, since Eu2+/Ce3+ activators can produce desired spectral properties and enhanced luminous output when Eu2+/Ce3+ ions are doped into the distorted co-ordination fields.10,11

Recently, some new phosphors and novel luminescence phenomena

have been identified in carbidonitride,12 oxynitride,13-16 oxychloride,10,17 oxyfluoride,18 fluorosulfide19 and nitridophosphate halides.20

Inspired by naturally occurring minerals

structure such as silicates and aluminate and meanwhile considering the aforementioned structure characteristics, chlorosilicate, Sr8Si4O12Cl8, which can be synthesized at a relatively moderate temperature (~1000 °C) in comparison with the above compounds,21-24 are chose as host for Eu2+ activator.

It is reported that Sr8Si4O12Cl8 (SSO_Cl) consists of a

three-dimensional framework made from SiO4 tetrahedrons as well as irregular SrO4Cl4 3



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polyhedron.23,24

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The SiO4 tetrahedra link each other by corners sharing and form an unusual

four-member ring (Figure 1a).24

The rings are separated by an irregular eightfold

polyhedrons SrO4Cl4 (Figure1b), which exhibits one long Sr-Cl bond distances at 3.37(1) Å, three short Sr-Cl bond distances from 2.940 Å to about 2.98(8) Å, and four Sr-O bond distances from 2.48(1) Å to 2.71(2) Å.24

More recent works have been done on

SSO_Cl:Eu2+ phosphors about luminescence properties under excitation at X-ray/Vacuum ultraviolet light (VUV) and temperature-dependent luminescence characteristics.23,24 However, as far as we know, there is no report about the performance of WLED using the two-color oxychloride phosphor.

In this work, color-tunable Sr7.95-xMxSi4O12Cl8:0.05Eu2+

(M = Mg, Ca) oxychloride phosphors is synthesized by using cationic substitution strategy, and the emission color can be tuned from blue-greenish to blue/yellow by partial substitution of Sr2+ by Mg2+/Ca2+. In this way, as-fabricated WLEDs show a wide range of luminous efficacies (28.9-56 lm/W) and CRIs (79.9-90.2).

By employing a deep red phosphor

Mg4(GeSn)O5.5F:Mn4+ (MGSO_F:Mn4+), the resulting WLED shows high CRIs (Ra = 90.3, R9 = 94) with ηL of ~39.3 lm/W, which is higher than that of reported counterparts.

In short,

color-tunable oxychlioride phosphors may have a potential application in WLEDs, and the mixing strategy of two-color phosphors having the same host may offer efficient solutions for developing high-quality WLED.

2. Experimental 2.1 Materials synthesis All samples Sr7.95-xMxSi4O12Cl8:0.05Eu2+ (M = Mg, Ca) were synthesized using a conventional solid-state reaction.

Starting materials were used: Sr2CO3 (99.99%),

SrCl2·4H2O (99.95%), CaCO3 (99.99%), MgO (99.95%), Eu2O3 (99.99%).

The powder

mixtures were quantitatively mixed using an agate mortar and pestle and subsequently sintered for 3 h at 1000 °C in reducing atmosphere (5% H2/95% N2).

Obtained samples

were reground and fired again for one hour at 1000 °C in the same muffle furnace. that, samples were then ground well for further characterization.

After

Deep red phosphor

MGSO_F:Mn4+ patented by our group (Patent No.: ZL200910218132.0) was made via high-temperature solid-state in muffle furnace under air atmosphere. 4


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2.2 Characteristic methods The structure of samples was identified via a Rigaku D/max-2500 X-ray diffractometer with Cu Kα radiation (λ = 1.5406 Å). analysis system (GSAS) program.

The Rietveld refinement used a general structure

The PL spectra were measured by a home-made PL

measurement system consisting of a 400 nm NUVLED array as excitation source and an Ocean Optics USB 4000 plug-and-play spectrometer as detector, and a glass cell (active area of 1.5 x 1.5 cm2) to contain pressed powder samples.

The detection system of spectrometer

was calibrated using a standard tungsten-halogen lamp source.

The PLE spectra of powders

were measured with a LS-55 fluorescence spectrophotometer equipped with a 150 W Xe lamp. The external quantum yield (QY) was performed using a PL quantum-yield measurement system (C9920-02, Hamamatsu Photonics). (JESFE3AX, Japan).

EPR spectra were recorded by a spectrometer

All measurements were performed at room temperature unless

otherwise mentioned. The WLEDs were fabricated using a mixture of transparent silicon resin and blends of as-prepared two-color oxychloride and a deep red oxyfluoride phosphors, which was dropped onto a 395 nm LED chip (chip size: 12×12 mil, forward voltage: 3.0-3.8 V, work current: 20 mA, power: 9-11 mW).

The electroluminescence (EL) spectra, luminous efficacies (ηL),

commission International de I’Eclairage (CIE) color coordinates and CRIs of as-prepared WLEDs and the commercial YAG-based WLED were characterized by using the same plug-and-play spectrometer equipped with an integrating sphere and a standard tungsten-halogen lamp source.

3. Results and Discussion X-ray diffraction patterns of Sr7.95-xMxSi4O12Cl8:0.05Eu2+ (M = Mg and Ca) with the different compositions are illustrated in Figure 2.

At x = 0, the major diffraction peaks

correspond to the standard data of Sr8Si4O12Cl8 (JCPDS 37-0616),21 indicating that single-phase Sr7.95Si4O12Cl8:0.05Eu2+ powder sample was synthesized successfully.

For

SSO_Cl doped with different content of Mg/Ca, all the samples almost maintain the features Given that ion radius of Mg2+ (r = 0.89Å, CN

of the tetragonal structure (space group I4/m).

= 8) /Ca2+ (r = 1.12 Å, CN = 8) is smaller than that of Sr2+ (r =1.26 Å, CN = 8), an obvious 5



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shifting to higher angle aside is observed with increasing the Mg2+/Ca2+content.

In addition,

the FWHM of diffraction peaks are also broadened gradually with increasing the doping content, indicating the compositional disordering of Sr2+/Mg2+ or Sr2+/Ca2+ in the host lattice. There is also an evidence for the formation of byproducts, which implies that the actual amounts of Mg and Ca incorporated in SSO_Cl may deviate somewhat from the amounts present in the starting materials. It is usually assumed that Eu2+ would take up the Sr site in the SSO_Cl lattice, since there is only one cationic site available.23

In our case, only one emission around 485 nm is observed

under excitation at 395 nm (Figure 3a), and thus supports the earlier reported.24 When doping Mg2+ ions at the level of 0.2, a shoulder on the short wavelength around 440 nm is observed under excitation at the same wavelength (Figure 3a), which gradually strengthens with increasing the Mg2+ content.

At x = 0.4, the external QY of sample is close to 72 % for

400 nm excitation. The emission spectra thus can be deconvoluated into two Gaussian components with maxima at 494 nm (referred to Eu(1)) and 439 nm (referred to Eu(2)), as shown in Figure S1 in ESI†.

Likewise, a distinct shoulder on the longer wavelength around

557 nm is observed in Ca-doped SSO_Cl:0.05Eu2+ samples under excitation at 395 nm (Figure 3b), which gradually intensifies with increasing the Ca2+ content. for SSO_Cl:0.05Eu2+, 0.5Ca2+ is 96 % under excitation at 400 nm.

The external QY

The additional emission

peak is located about 550 nm on the basis of the Gaussian fitting of PL spectra of SSO_Cl:0.05Eu2+, xCa2+ (x = 0, 0.2, 0.4, 0.5 and 0.6), as shown in ESI†, Figure S2. However, the two emission peaks in this compound is rather surprising, since the crystal structure contains a single substitution site (Sr1) for Eu ions.

We hypothesize that addition

of Mg2+/Ca2+ possibly induces distinct Eu2+ centers, which will be discussed in detail in EPR section later.

Additionally, a continuous red shift in the PL spectra is observed with

increasing Ca2+ content.

It indicates that introduction of Ca2+ possibly results in a

highly-distorted Eu2+ sites due to the smaller Ca2+ ions, which reduce the distances of metal-ligand and thereby increases Eu2+ 5d-orbital splitting.

However, unexpected ‘blue

shift’ emission is observed in the PL spectra of Mg2+-doped sample. can be found in other works.12,14

Similar phenomenon

We speculate that the incorporation of smaller Mg2+ into 6


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the host lattice results in an internal lattice strain and thereby enlarges EuO4Cl4 polyhedrons. To release the lattice strain, the Eu-O/Cl length increases gradually with increasing the Mg2+ content, and thereby decreasing the crystal field splitting of Eu2+. Figure 4a shows the normalized PLE spectra of the SSO_Cl:0.05Eu2+, 0.4Mg2+ by monitoring the emissions at 446 and 491 nm, respectively. to the 4f-5d transition of Eu2+ ions.

Both PLE spectra are attributed

However, their spectral profiles are different, which

indicates that the two transitions originate from different emission centers.

The normalized

PLE spectra of the SSO_Cl:0.05Eu2+, xMg2+ (x = 0.2, 0.4, 0.6 and 0.8) samples by monitoring the higher-energy emission are shown in Figure 4b.

We found that the spectral profiles are

almost the same, suggesting these higher-energy emission bands are all from one Eu center. Similarly, by monitoring the emissions at 497 and 557 nm, different PLE profiles are also found in the SSO_Cl:0.05Eu2+, 0.5Ca2+ samples (Figure 4c).

This result indicates that

additional shoulder around 557 nm indeed originates from distinct Eu2+ centers.

Moreover,

it is clearly seen that the onset of the 4f65d1 band in the PLE spectra shifts to the lower energy side as increasing Ca2+ content, along with profile changes when monitoring the lower energy emission band, which further demonstrates the change of the crystal field strength acting upon the Eu2+ ions.25 Based on the crystal structure analysis above, two Eu2+ centers in the compound is rather surprising when taking the only one cationic site for Eu2+ ion into account.

To explore

further the luminescence mechanism of the SSO_Cl:0.05Eu2+, Mg2+/Ca2+ phosphors, EPR spectra are carried out to investigate the local environment of Eu2+ ions. Figure 5 illustrates the EPR spectra for the host, SSO_Cl:0.05Eu2+, xMg2+ (x = 0, 0.2 and 0.4) and SSO_Cl:0.05Eu2+, xCa2+ (x = 0.2, 0.5), respectively.

As for the host lattice, a weak EPR

signal is detected by enlarging the EPR spectrum, indicating some types of paramagnetic defects appear in the host lattice.

However, the weak EPR signal can be nearly neglected in

comparing with that of Eu2+-doped SSO_Cl. attributed to unpaired electrons of Eu2+.

These EPR signals thus can be totally

As for the Mg/Ca-doped samples, it is clearly seen

that their local Eu2+ environments are different from that of undoped sample.

In contrast, the

difference in the EPR signals (marked by dash line) is likely to be caused by the change of Eu2+ sites in the host lattice with Mg/Ca addition, and thereby yield different emissions 7



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around 445 and 580 nm. Generally, the EPR signals of Eu2+ in the lower symmetry sites can present fine structure consisting of seven sharp lines resulting from the splitting of J = 7/2 state into 2j+1 levels.26 The separations between these lines are dependent on the symmetry around Eu2+ ion in the host lattice.26

However, we do not find the hyperfine structure of Eu2+ in Ca-doped samples

instead of a wide EPR lineshape.

It is reported that the wide EPR lineshape suggests (1)

Eu2+ ions are located at the sites with an exceedingly asymmetrical or highly-distorted coordinate environment.27

(2) Eu2+ sites have a broad distribution of symmetry.27-29

Moreover, (3) strong interaction between Eu2+ and host lattice also has contribution to the increasing EPR lineswidth.30 Herein, we consider that the larger EPR lineswidth of Ca-doped samples in comparison with that of Mg-doped sample is mainly attributed to (1) and (3). The thermal quenching behaviors for the SSO_Cl:0.05Eu2+, 0.4Mg2+/0.5Ca2+ samples are investigated.

As shown in figure 6, the PL intensity of SSO_Cl:0.05Eu2+, 0.4Mg2+ sample

decreases with increasing the temperature under excitation at 400 nm, and the relative PL intensity at 150 °C and 210 °C fell 20 % and 26 % of the initial values at room temperature, respectively.

In contrast, SSO_Cl:0.05Eu2+, 0.5Ca2+ sample shows better thermal stabilities,

the relative PL intensity fell only 8 % and 12 % of the initial values under excitation at the same wavelength.

Unfortunately, the definite reason is unclear at present stage because of

the lack of refinement results of crystal structure.

Herein, we tentatively consider that it is

assigned to the increase of lattice rigidity and the quenching barrier height. Conventional phosphor-converted white-lighting sources suffer from a trade-off between a high CRI and the maximum achievable luminous efficacy.31,32

More recently, Schubert. E et

al. demonstrate that employing dual-wavelength blue-emitting phosphor can maximize the luminous efficacy while significantly increasing the CRIs.31

In our case, as-prepared

SSO_Cl:0.05Eu2+, 0.4Mg2+ blue phosphor, which just presents two emission bands at ca. 450 and 480 nm, may be an ideal blue component for pc-WLED in this case (Figure 7a).

A

broadband yellow phosphor with a larger Stokes shift (∆S) is very helpful to improving the CRI of two-phosphor pc-WLED.33,34

Herein, SSO_Cl:0.05Eu2+, 0.5Ca2+ yellow phosphor

presents a broadband emission, with FWHM of 4857 cm-1 and a large ∆S of 7589 cm-1. 8


It is

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important to note that the PLE spectrum of the yellow phosphor exhibits a strong absorption at 400 nm, but the intensity of absorption rapidly decreases at wavelength longer than 420 nm (Figure 7b).

In this way, it can be imaged that the re-absorption process will be lowered to

some extent if the two-color phosphors are mixed, because blue light at ~ 450 nm or longer only weakly absorbed by the yellow phosphor.

To evaluate the device performance for the

two-color phosphors, we fabricate three typical WLEDs by using the two-color phosphor systems and NUVLED chips (~ 400 nm). WLEDs under a drive of 20 mA current.

Figure 8 shows the EL spectra of the resulting Four emission bands at 400, 450, 480 and 554 nm

can be clearly seen in EL spectra, which are attributed to the NUVLED chip, blue-greenish phosphor SSO_Cl:0.05Eu2+, xMg2+ (x = 0.2, 0.4 and 0.6), and yellow phosphor SSO_Cl:0.05Eu2+, 0.5Ca2+, respectively. 56 lm/W and CRI of 79.

At x = 0.2, the as-prepared WLED 1 shows ηL of

With increasing x and the weight ratio of blue component, the

emissions intensity at 450 and 480 nm increase gradually (Figure 8a-c), and the corresponding color coordinates (x,y) shifts gradually from (0.317, 0.417) of x = 0.2 to (0.303, 0.348) of x = 0.6, and meanwhile the Ra value is greatly improved to 90.3. be 56, 37.7 and 28.9 lm/W, respectively.

The ηL values are measured to

Herein, we emphasize that the ηL of as-prepared

WLED 3 will reach ~60 lm/W or more if the external QY of UV LED chip is improved as well as the commercial blue LED chip. Recently, an additional R9 value, describing the rendering of deep red, is requested to especially notice in the lighting society since the average CRI (Ra) cannot promise good saturated colors of illuminated objects.1,4,35

It is known that the R9 value, for the

commercial YAG-based WLED, is very low due to red deficiency.

In our case, although the

R9 value for the as-prepared WLED is 66.3, the amount of deep red emission is insufficient for high-quality white-lighting source.

To improve further the performance of the WLED, a

deep red phosphor with sharp emission line are introduced, that are excepted to improve the R9 value of WLED without compromising ηL.3,36,37

By introducing the deep-red phosphor,

the emission in deep red region for WLED 1 is enhanced dramatically (Figure 9a), and the R9 value increases to 94 (ESI†, Table S2 and S3).

Although a slight decrease in ηL is found in

the resulting WLED, the ηL (>39 lm/W) is sufficiently high for a high CRI (Ra = 90.3, R9 = 94) WLED, and even much higher than that of the reported counterparts.8,17,39,40 9


This result


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demonstrates that deep red phosphor with appropriate band-gap and Stokes shift is indispensable in improving the CRI of WLED. In comparison with the commercial WLEDs (Ra = 80.2, R9 = 14.3), as-prepared WLED (Figure 9d) presents a higher CRI (Ra = 90.3, R9 = 94), which is ascribed not only to enriched deep red emission, but a greater amount of blue-greenish (450 - 490 nm) and a wider spectral distribution at 570 nm (ESI†, Table S2 and S3). Figure 9e shows EL spectra of the resulting WLED under different currents.

With

increasing the applied forward bias currents, a continuous enhanced EL emission is observed, indicating the phosphor blends has little spectral saturation properties.

In addition, the slight

variation of CIE color coordinate of WLED operated under different currents is found (inset of Figure 9f). stability.

This indicates that as-fabricated WLED shows excellent color coordinate

These findings show that the blends of two-color oxychloride and the deep red

oxyfluoride phosphors have potential application in pc-WLEDs. Figure 10 illustrates the CIE chromaticity coordinates of SSO_Cl:0.05Eu2+ and SSO_Cl:0.05Eu2+, Mg2+/Ca2+ with different contents, respectively.

The color tone of the

SSO_Cl:0.05Eu2+ phosphor shifts gradually from blue-greenish (0.156, 0.391) to blue (0.151, 0.112) by increasing Mg2+ content, and from blue-greenish (0.156, 0.391) to yellow (0.434, 0.512) by increasing Ca2+ content.

The corresponding digital photographs (λex = 365 nm)

are displayed in the inset of Figure 10.

As a reference, the photograph (λex=365 nm) and the

CIE chromaticity coordinate of the deep red phosphor MGSO_F:Mn4+ are also presented in the inset of Figure 10.

Moreover, a photograph and the chromaticity coordinate of

as-prepared WLED are illustrated in the inset of Figure 10.

The chromaticity coordinate of

(0.320, 0.323) is much close to that (0.333, 0.333) of the equal energy white point.

In

summary, these results show that the as-prepared WLEDs using the two-color oxychloride and deep red emission oxyfluoride phosphors could be a promising candidate for solid state lighting.

4. Conclusions Color-tunable Sr7.95-xMxSi4O12Cl8:0.05Eu2+ (M = Mg, Ca) phosphors are synthesized via high-temperature solid-sate reaction by utilizing cationic substitution strategy. 10


The emission

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color can be tuned from blue-greenish to blue/yellow by partial replacing Sr2+ with Mg2+/Ca2+. In this contribution, we propose an effective approach for achieving high-efficacy WLEDs, based on the two-color oxychloride phosphors having a similar host.

In this way,

as-prepared WLEDs presented a wide range of Ra (79-90.3) and ηL (56-28.9 lm/W).

By

employing a narrow-line Mn4+-activated deep red phosphor, a high CRI (Ra = 90.3, R9 = 94) WLED with ηL of ~39.3 lm/W is obtained, which is higher than that of reported WLEDs. These findings suggest the phosphors system have a potential application for WLEDs.

Our

study also emphasizes the feasibility of the strategy of the two-color phosphors having the same host for developing high-quality WLED.

Associated Content Supporting Information The XRD patterns and the resolved emission spectra of Sr7.95Si4O12Cl8:0.05Eu2+ with different Mg/Ca contents are presented in Figure S1-S4. information is shown in Table S1.

The detailed PL spectra

The full set of 14 CRI values for the commercial WLED

and the as-prepared WLED are presented in Table S2-S3.

This information is available free

of charge via the Internet at http:pubs.acs.org.

Acknowledgements The work was supported by the Science and Technology Development Project of Jilin province (Grant No. 20080329), National Natural Science Young Foundations of China (Grant No. 51302034), and the Doctoral/Postdoctoral Fundamental Research Funds of Xinjiang Normal University (Grant No. XJNUBS1542).

We appreciate the help of Dr Meng

Zhang of Northeast Normal University in performing EPR measurements.

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Figure captions:

Figure 1 a) Crystal structure of Sr8Si4O12Cl8 unit cell viewed in z-direction and b) the coordination environment of Sr site in Sr8Si4O12Cl8 viewed in x-direction.

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Figure 2 a) XRD patterns of SSO_Cl:0.05Eu2+ doped with different content of Mg2+/Ca2+. As a reference, the standard XRD data (JCPDS 37-0616) of Sr8Si4O12Cl8 is also shown in Figure 2a. b) Magnified XRD patterns in the region between 40° and 41.5° for SSO_Cl:Eu2+ doped with different content of Mg2+/Ca2+.

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Figure 3 a) - b) The PL spectra of SSO_Cl:0.05Eu2+ doped with different Mg2+/Ca2+ contents under excitation at 400 nm.

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Figure 4 a) The normalized PLE spectra of SSO_Cl:0.05Eu2+, 0.4Mg2+ phosphor by monitoring the emissions at 446 and 491 nm.

b) The normalized PLE spectra of SSO_Cl:0.05Eu2+, xMg2+ (x = 0.2, 0.4,

0.6 and 0.8) by monitoring the higher energy emissions.

c) The normalized PLE spectra of

SSO_Cl:0.05Eu2+, 0.5Ca2+ by monitoring the emissions at 497 and 557 nm. d) The normalized PLE spectra of SSO_Cl:0.05Eu2+, xCa2+ (x = 0.2, 0.4, 0.5 and 0.6) by monitoring the lower energy emissions.

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Figure 5 The EPR spectra of Sr8Si4O12Cl8 host (SSO_Cl), SSO_Cl:0.05Eu2+, and SSO_Cl:0.05Eu2+ doped with the different contents of Mg2+/Ca2+.

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Figure

6

Temperature

dependence

of

integrated

emission

SSO_Cl:0.05Eu2+,0.4Mg2+/0.5Ca2+ samples under excitation at 400 nm.

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intensity

for

the

selected


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Figure 7 a) The normalized PL (λex = 400 nm) and PLE spectrum (λem = 492 nm) of SSO_Cl:0.05Eu2+, 0.4Mg2+ sample.

b) The normalized PL (λex = 400 nm) and PLE spectrum (λem = 558 nm) of

SSO_Cl:0.05Eu2+, 0.5Ca2+ phosphor.

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Figure 8 a) - c) EL spectra of as-fabricated three typical WLEDs composed of a NUVLED chip (λem = 400 nm) and phosphor blends of SSO_Cl:0.05Eu2+, 0.5Ca2+ (CSSO_Cl:Eu2+) and SSO_Cl:0.05Eu2+, xMg2+ (x = 0.2, 0.4 and 0.6) phosphors with different weight ratios.

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Figure 9 a) The EL spectra of as-prepared WLED using the two-color oxychloride and deep red oxyfluoride phosphors MGSO_F:Mn4+. b) The digital photographs of the resulting WLED and c) the WLED operated at 20 mA. d) CRIs of as-prepared WLED and the commercial YAG-based WLED. e) EL spectra of the resulting WLED operated under different forward bias currents. The inset of Fig. 9e displays the variation in CIE chromaticity coordinates of the WLED operated under different forward bias currents.

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Figure 10 Color coordinates of SSO_Cl:0.05Eu2+ blue-greenish phosphor (No.0), SSO_Cl:Eu2+, xMg2+ (x = 0.2, 0.4 and 0.6) blue phosphor (No.1, x = 0.6), SSO_Cl:Eu2+, xCa2+ (x = 0.2, 0.4, 0.5 and 0.6) yellow phosphor (No.2, x = 0.5) and deep red phosphor Mg4(Ge,Sn)O5.5F0.5:Mn4+ under excitation at 365 nm in the CIE chromaticity diagram. The insets show the digital photographs (λex = 365 nm) and corresponding CIE coordinate values of the selected samples. In addition, the photograph of the resulting WLED operated at 20 mA and the corresponding coordinate value are also presented in the inset of Figure 10.

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TOC

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Bright and High-Color-Rendering White Light-Emitting Diode Using

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