Tuning the diurnal natural daylight with phosphor converted white LED – Advent of new phosphor blend composition

Tuning the diurnal natural daylight with phosphor converted white LED – Advent of new phosphor blend composition

Materials Science and Engineering B 193 (2015) 4–12 Contents lists available at ScienceDirect Materials Science and Engineering B journal homepage: ...

3MB Sizes 46 Downloads 106 Views

Materials Science and Engineering B 193 (2015) 4–12

Contents lists available at ScienceDirect

Materials Science and Engineering B journal homepage: www.elsevier.com/locate/mseb

Tuning the diurnal natural daylight with phosphor converted white LED – Advent of new phosphor blend composition Yoon Hwa Kim a,b , Paulraj Arunkumar a , Seung Hyok Park b , Ho Shin Yoon b , Won Bin Im a,∗ a b

School of Materials Science and Engineering, Chonnam National University, 300, Yongbong-dong, Buk-gu, Gwangju 500-757, South Korea Research Institute, Force4 Corp., Daechon-dong, Buk-gu, Gwangju 500-470, South Korea

a r t i c l e

i n f o

Article history: Received 18 June 2014 Received in revised form 20 October 2014 Accepted 5 November 2014 Keywords: Natural daylight Phosphor blend Full spectrum

a b s t r a c t We demonstrate the feasibility of developing phosphor converted white LED (pc-WLED) that mimics diurnal natural daylight with the newly designed phosphor blend in the color temperature (CCT) 2700–6000 K for health benefits. Natural daylight (sunlight) spectrum possesses broad emission in the visible region and closely approximates black body radiator, with color rendition index (CRI) of 100 under wide CCT (2500–6500 K). Current white light LEDs although are efficient and durable, they are not broad enough compared to daylight. We report new phosphor blend based on Sr3 MgSi2 O8 :Eu2+ blue phosphor with broad emission and high CRI ≥ 96 under both near UV and blue excitation. The fabricated WLED has exhibited ∼91% spectral resemblance with natural daylight compared to 39.2% for YAG:Ce3+ white LED at 4500 K. The developed phosphor blend tunes the spectrum in wider CCT and would be a prospective candidate for full spectrum daylight WLED. © 2014 Published by Elsevier B.V.

1. Introduction Life expectancy in the 21st century has improved due to the advancement in public health technology and medicine; however more interestingly, longevity of human life has recently been investigated with the influence of light [1,2]. In the technology-based society, illuminated door environment operates 24 h/day with artificial electric lighting. Light technology has traveled beyond the efficiency by understanding the physiological effect of artificial light on the human body in comparison with natural daylight. The most ideal source of illumination is the light from sun (natural daylight), with broad emission ranging UV to IR ( = 250–1800 nm) and closely approximates black body radiator with color rendition index (CRI) of 100; nevertheless, natural daylight was not always accessible for the indoor lighting [3]. Typical natural daylight has color temperature (CCT) of 5500 K, the longest part of the day where it stays the same, but daylight ranges from 2000 to 3000 K at the sunrise through to ∼6500 for an overcast day. Natural daylight significantly contributes to human health and harmonizes the body’s circadian rhythm that assists the brain wave activity, hormone secretion, eating and sleeping patterns. Natural daylight having varying color temperature (2700–6500 K) ranging from warm to cool light is applied for specific uses. Cool light (4000–6500 K)

∗ Corresponding author. Tel.: +82 625301715; fax: +82 625301699. E-mail address: [email protected] (W.B. Im). http://dx.doi.org/10.1016/j.mseb.2014.11.001 0921-5107/© 2014 Published by Elsevier B.V.

which has high contrast compared to warm light is preferred for clear light in kitchens and restroom, while warm light (<3000 K) is preferred for indoor lighting especially in living rooms which creates welcoming atmosphere and offers health benefits [4]. Hence, to improve health outcomes and energy savings, it is quintessential to either deploy natural daylight in an energy efficient manner or mimic the full spectrum of natural lighting with environmental benign and high efficient artificial lighting system. The lighting industry is exploring the ways of creating more naturalistic indoor environment to facilitate the human biological rhythms and cognitive functioning. Ever since the advent of solid state lighting, the lighting industry has not only been revolutionized through its versatile light sources with high energy-efficiency, durability, and reasonable cost, but also with its ability to mimic the full spectrum of natural lighting [3]. Few lighting products are available in the market which supplies “full spectrum” lighting in the form of CFL lamps, delivers noontime sunlight spectrum and not the full spectrum of diurnal daylight, in terms of color temperature and spectral light intensity. Organic light emitting diodes (OLED), another solid state lighting, closely mimic natural light with any desirable color, due to the broad and diffused emission spectra of electro-luminescent materials in the visible region [5]. Recently, OLED based artificial light that mimics sunset hues was developed and exhibits a CRI of 92 with 87% spectral resemblance to sunset hue at 2745 K [3]. However, phosphor converted white LEDs (pc-WLEDs) were least investigated for mimicking full spectrum of diurnal natural light due to the underlying challenge in

Y.H. Kim et al. / Materials Science and Engineering B 193 (2015) 4–12

generating a broad emission from phosphors. Conventional way of fabricating pc-WLED by additive color mixing of yellow-emitting Y3 Al5 O12 :Ce3+ (YAG:Ce3+ ) phosphor exciting with blue LED faces challenge of low efficiency and poor CRI value even after the inclusion of deficient red emitters [6,7]. Few literature report on designing phosphors with broad emission by tuning the emission of single phosphor, through multi-emissive sites, but they were unsuccessful in attaining full spectrum in the complete visible region [8,9]. In the current generation of LED revolution, researchers are customizing the phosphor blends to create white LED with broader emission bands resulting in improved color rendition index and high efficiency [10]. The quality of lighting device may be quantified by metrics including CRI, color temperature (CCT) and resemblance to natural lighting for full spectrum broadband white LEDs [11]. Recently, CRI was reported insufficient to render the color objects precisely with high fidelity and ability to chromatic contrast, when the light sources contain narrow band electroluminescent components and due to the associated drawbacks of treating color shifts irrespective of the direction. These limitations of the CRI led to the development of new color rendering metrics like color fidelity index and color saturation index [12,13]. In the current work, we have developed a phosphor blend (with minimal overlap of excitation/emission of constituent phosphors) that can mimic the full spectrum of diurnal natural daylight emission under near UV with high spectral matching ratio. The phosphor blends were assessed using CRI metric instead of the updated metrics (CFI, CSI), as we employed broad emitters as phosphor constituents. The major phosphor component for near UV LED was chosen as Merwinite Sr3 MgSi2 O8 :Eu2+ (SMS:Eu2+ ). SMS:Eu2+ is the promising blue-emitting phosphor which had been investigated for its application in lighting industry for a decade due to high efficiency and broad absorption ranging from near UV to visible region [14,15]. SMS:Eu2+ is also commercialized blue phosphor with high internal and external quantum efficiency of 79% and 70% respectively and especially exploited for near UV excitation [16]. Due to these optical properties, SMS:Eu2+ phosphor was exploited in the current investigation to fabricate the WLED to mimic full spectrum of diurnal natural daylight. SMS:Eu2+ phosphor was synthesized and its optical properties are investigated. The possibility of tuning natural daylight emission with broad emission of the developed SMS:Eu2+ based phosphor blend was investigated using pc-WLED with particular color temperature (2700–6000 K), for human health and well-being. The developed pc-WLED fairly mimics natural daylight (spectral matching ratio ≥90%) for diurnal full spectrum lighting. The structural and optical properties of SMS:Eu2+ blue phosphor were investigated for its application in pc-WLED. Phosphor blend with ideal composition was optimized and the fabricated WLED shows high CRI (∼97) under near UV and CRI ∼95 under blue excitation. Consistency of the color chromaticity results was confirmed by fabricating 10,000 WLED packages with this phosphor blend and compared with the standard color coordinates.

2. Experimental 2.1. Materials and synthesis Sr3−x MgSi2 O8 :Eu2+ (SMS:Eu2+ ) phosphor was synthesized through solid-state reaction with SrCO3 (Aldrich, 99.99%), MgO (Aldrich, 99.9%), SiO2 (Aldrich, 99.99%), in stoichiometric quantities with different concentrations of Eu for optimization. All the precursors were well grinded in a mortar and annealed at 1100 ◦ C for 4 h in reducing atmosphere of H2 /N2 (5%/95%). All the remaining phosphors were used from Force4 Corporation, Gwangju, South Korea

5

where three of our authors are affiliated. Powder X-ray diffraction (XRD) was measured using CuK␣ radiation over the angle range of 10◦ ≤ 2 ≤ 100◦ with step size of 0.026◦ . Rietveld analyses were performed under general structure analysis system (GSAS) software for the crystal structure refinement [17]. Photoluminescence spectra under room temperature were measured with Hitachi F-4500 fluorescence spectrometer over the wavelength range of 200–750 nm. The temperature dependent fluorescence spectra in the temperature range 25–200 ◦ C were measured using the integrated heater, temperature controller and thermal sensor with the Hitachi F-4500 fluorescence spectrometer. 2.2. Measurement and characterization Prototype WLED was fabricated by integrating the phosphor blend in the presence of transparent silicone resin under near UV (max = 400 nm) and blue (max = 450 nm) LED chip. For electroluminescence (EL) measurements, discrete LEDs were then encapsulated with phosphor/silicone mixture, and cured at 150 ◦ C for 1 h. The cured LED devices were introduced into integrating sphere and EL measurements were performed. Surface mount device (SMD) type 3528 was employed for WLED prototype fabrication under blue LED excitation. The 3528 SMD LED chip is the commercially available surface mount device (SMD) with single LED chip with dimension of 3.5 mm × 2.8 mm (that are represented by numbers as 3528 without the decimal). This SMD chip is comprised of printed circuit board with epoxy resin, which is used to mount the emitting components (phosphor). The true color of an object (flower vase) was viewed under the fabricated WLED as lighting source, which was performed in Force4 Corporation. The mass production of 10,000 WLED package was carried out using highly automated equipment with auto-dispenser for dispensing the phosphor blend, followed by trimming of LED chip typically high power SMD 5450 package under blue LED excitation. Then, trimming, testing and sorting of phosphor were performed and finished with taping and packing the WLED. The tester used for mass production of WLED is Withlight, LEOS OPI-100, and dispenser employed is PROTEC, Phantasm-MSS, along with Nihon Garter, NCS-3100 sorting machine. 3. Results and discussion 3.1. Structural analysis and optical properties of SMS:Eu2+ phosphor Rietveld refinement of the XRD data profiles of SMS:Eu2+ obtained with Rwp = 12.61% and goodness of fit parameter (2 = 3.754) is displayed in Fig. 1. The refinement pattern clearly reveals the high crystallinity and phase purity of the SMS:Eu2+ phosphor irrespective of the Eu2+ concentration. Although Rp and Rwp are large, we could not find any additional impurity peak from the X-ray diffraction pattern. SMS phosphor crystallizes into monoclinic phase with space group P21 /a and the cell parameters are ˚ b = 5.457 A˚ and c = 9.450 A, ˚ with the calculated volume a = 13.875 A, of 715.588 A˚ 3 , that are listed in Table 1. Strontium sites in the SMS:Eu2+ are occupied by the dopant europium due to the close ˚ and Eu2+ (1.12 A) ˚ [18]. proximity of the ionic radii of Sr2+ (1.26 A) SMS:Eu2+ belongs to Merwinite mineral with formula of A3 MgSi2 O8 (A = Ba, Sr, Ca) and three different Sr sites are identified in a unit cell containing one 12-coordinated Sr(I) site and two10-coordinated Sr(II, III) sites similar to A3 MgSi2 O8 [19]. Optical properties of the blue-emitting SMS:Eu2+ phosphor are described in Fig. 2. The excitation and emission spectra of the optimized SMS:Eu2+ (x = 0.15) were measured at room temperature and are shown in Fig. 2(a). The excitation spectra measured

6

Y.H. Kim et al. / Materials Science and Engineering B 193 (2015) 4–12

Fig. 1. Rietveld refinement pattern of Sr2.85 Eu0.15 MgSi2 O8 under CuK␣ with data (dots), fit (lines) profile along with profile difference.

under em = 461 nm show broad excitation band in the wavelength range of 200–450 nm, that reveals that SMS:Eu2+ may be excited under UV, near UV and blue lighting source. The excitation spectra comprise two major peak maxima appearing at 275 and 365 nm, where both the peaks correspond to the transitions of Eu2+ from 4f7 ground state to 4f6 5d1 excited states [20]. SMS:Eu2+ phosphor shows a broad emission at 461 nm, when excited at 400 nm, with a full-width at half-maximum is around ∼55 nm. Blue emission of SMS:Eu2+ was attributed to the weak crystal field exerted on the Eu2+ occupying the Sr(I) site with 12 coordination, that has the longest Sr(Eu2+ ) O bond length [21], while a green emission would arise from Sr(II) and Sr(III) sites with strong crystal field and short Sr(Eu2+ ) O bond length. Optimization of Eu2+ concentration in the SMS host was studied under different concentrations of x = 0–0.30 as shown in Fig. 2(b). SMS:Eu2+ with x = 0.15 was the optimal concentration with high emission intensity and further studies were performed with this Eu2+ composition. Dependence of the emission peak position and intensity on varying Eu2+ is shown in Fig. 2(c). As the Eu2+ concentration increases, the emission band (ex = 400 nm) shifts toward longer wavelength which is attributed to the reabsorption of the Eu2+ ions. The internal and external quantum efficiency of blue-emitting SMS:Eu2+ phosphor is 76% and 70%, respectively, under near UV excitation. Thermal quenching characteristics of the fabricated LED with SMS:Eu2+ phosphor were compared with commercial Sr2 SiO4 :Eu2+ phosphor in the temperature range RT to 200 ◦ C as depicted in Fig. 3. Both the phosphors suffer from sharp decrease in the conversion efficiency with respect to the temperature of LED operation, due to the increase in non-radiative transition [22]. The emission intensities of phosphors decreased by 26% and 18% of the RT emission intensity for commercial Sr2 SiO4 :Eu2+ and SMS:Eu2+ respectively

Fig. 2. (a) PL emission and excitation spectra of Sr2.85 Eu0.15 MgSi2 O8 under ex = 400 and em = 461 nm respectively, (b) relative emission intensity, and (c) emission peak shift as a function of Eu2+ concentration under ex = 400 nm in Sr3−x Eux MgSi2 O8 phosphor.

Table 1 Rietveld refinement parameters of Sr2.85 Eu0.15 MgSi2 O8 . Formula

Sr2.85 Eu0.15 MgSi2 O8

Radiation type 2 range (◦ ) T/K Symmetry Space group a/Å b/Å c/Å Volume/Å3 Z Rp Rwp 2

CuK␣ 20–100 293 Monoclinic P21 /a 13.8754 5.4570 9.4507 715.588 4 9.03% 12.61% 3.754

Fig. 3. Temperature dependent emission intensity of Sr2.85 Eu0.15 MgSi2 O8 and commercial Sr2 SiO4 :Eu2+ phosphor in the temperature range 25–200 ◦ C.

Y.H. Kim et al. / Materials Science and Engineering B 193 (2015) 4–12

7

Table 2 Composition of blended phosphor LED matching with sunlight spectrum and their optical properties under near UV excitation. Target CCT (K)

3000 4500 6000

Phosphor composition

Blue

Green

Yellow

Orange

Red

Sr3 MgSi2 O8 : Eu2+ (461 nm)

Ba2 SiO4 : Eu2+ (505 nm)

(Sr,Ba)2 SiO4 : Eu2+ (565 nm)

Sr3 SiO5 : Eu2+ (583 nm)

CaAlSiN3 : Eu2+ (650 nm)

CaAlSiBN: Eu2+ (660 nm)

50.0 (%) 72.9 (%) 72.7 (%)

18.9 (%) 13.3 (%) 11.1 (%)

21.0 (%) 10.8 (%) 11.1 (%)

4.1 (%) 1.0 (%) 0.0 (%)

4.0 (%) 1.5 (%) 5.1 (%)

2.0 (%) 0.5 (%) 0.0 (%)

as shown in Fig. 3. This reveals high thermal stability of the fabricated SMS:Eu2+ phosphor than commercial Sr2 SiO4 :Eu2+ phosphor. 3.2. Optical properties of the phosphor blend under near UV excitation (max = 400 nm) mimicking natural daylight Phosphor blend using blue-emitting SMS:Eu2+ phosphor was fabricated with the optimized composition of phosphors emitting green (Ba2 SiO4 :Eu2+ ), yellow ((Sr,Ba)2 SiO4 :Eu2+ ), orange (Sr3 SiO5 :Eu2+ ), and red (CaAlSiN3 :Eu2+ and CaAlSiBN:Eu2+ ). Two red emitting phosphors, namely CaAlSiN3 :Eu2+ and CaAlSiBN:Eu2+ , were used because CaAlSiN3 :Eu2+ with a emission maxima at 650 nm that could extend only till 680 nm and, does not cover the entire red region (620–720 nm). Hence CaAlSiBN:Eu2+ with the emission maxima at 660 nm, extending to up to 720 nm was also employed as red component. The composition of the phosphor constituents was initially derived through simulations using “minitab” software. With this rudimentary information, further optimization was carried out experimentally to get the desired color temperature. For higher color temperature, the blue composition of the phosphor blend is increased and for lower color temperature it is vice versa. An optimized composition of these phosphors forming ideal phosphor blend was tabulated in Table 2 with the optical properties of the fabricated WLED device under near UV excitation. The average particle size of the blend phosphor is in the range 10–20 ␮m. The EL measurements were performed on the fabricated WLEDs at warm (∼3000 K), natural (∼4500 K) and cool (∼6000 K) light color temperatures in comparison with standard CIE illuminant under near UV excitation and they are depicted in Fig. 4(b–d). The spectral resemblance ratio of the fabricated phosphor blend containing white LED with standard CIE illuminants depicting full spectrum under corresponding color temperature was checked in terms of spectral intensity as well as peak position. The spectral resemblance ratio was calculated from the integral area of the phosphor blend spectrum and the integral area of the standard illuminant under respective color temperature. The following equation was employed for the calculation of spectral resemblance ratios: Spectral resembance ratio



= 100% −

(ref · area) − (sample area)



(ref · area)

White LED with commercial YAG:Ce3+ phosphor was fabricated under blue LED excitation and used as reference for its comparison with spectral power distribution of the CIE standard illuminant D65, which represents average daylight with ∼6500 K as shown in Fig. 4a. The YAG:Ce3+ phosphor exhibited color temperature of 6500 K; hence, D65 illuminant was used as standard in Fig. 4a. However, YAG:Ce3+ phosphor was also investigated under different color temperature illuminants, namely warm, natural and cool light and their spectral resemblance ratios are tabulated in Table 3. The resemblance ratio of commercial YAG:Ce3+ with

CRI

Spectral matching ratio (%)

97 88 88

92.0 91.5 90.9

spectral power distribution of CIE standard illuminant D65 was too low with ∼39.5%, which does not give the true color of the object. The commercial YAG:Ce3+ phosphor under blue LED excitation exhibited spectral resemblance ratios of 39.2% and 36.9% at 4500 K and 6000 K, respectively, as shown in Table 3, while higher resemblance ratio of 73.7% was observed under warm light (3000 K). The resemblance ratios of YAG:Ce3+ were taken as standard and to evaluate the properties of developed phosphor blend. The developed phosphor blend composition was examined under near UV excitation for its spectral resemblance ratio with the daylight spectrum under wide color temperature range (2700–6000 K). For tuning the color temperature of phosphor blend, the composition of constituent blue phosphor was increased by certain percentage, since the increase in blue phosphor yields high color temperature. The blue component (SMS:Eu2+ ) for 3000 K target was optimized to 50% and its constituent in the phosphor blend was increased by ∼23% for 4500 and 6000 K. Fig. 4(b) describes the fabricated warm white LED device with the phosphor blend under near UV excitation that show 92.0% spectral resemblance with the spectral distribution of CIE standard illuminant A, which represents typical incandescent lamp with Planckian radiator at ∼2800 K. This high spectral resemblance ratio represents the closeness of the white light mimicking natural daylight with impressive CRI of 97 at ∼2874 K compared to commercial YAG:Ce3+ with CRI of 73.7% which also approximates closely with tungsten filament lamp (black body radiator). Ideally, warm light (∼3000 K) with high color rendition (>90) is suitable for specific indoor lighting, namely living rooms for welcoming atmosphere, but current existing WLED device shows bluish cool white. Hence, the developed phosphor blend that closely mimics the natural daylight under warm color temperature may be promising candidate for indoor lighting. The spectral resemblance ratio of phosphor blend at ∼4500 K under near UV compared with CIE standard illuminant D45 (∼4500 K) was 91.5%, with CRI of 88 as depicted in Fig. 4(c) which is very high compared to the commercial YAG:Ce3+ (CRI of 39.2) at 4500 K as shown in Table 3, while spectral resemblance for phosphor blend under cool white was 90.9% for WLED in comparison with CIE standard illuminant D60 (∼6000 K) that showed CRI of 88 as shown in Fig. 4(d) compared to the YAG:Ce3+ (CRI of 36.9) at 6000 K as shown in Table 3. All the phosphor blends exhibited high CRI > 88 and spectral resemblance well over 90% when compared with respective color temperature standard illuminant, which is very promising note for its implementation in pc-WLED that

Table 3 Spectral resemblance ratio of YAG:Ce3+ phosphor under different standard illuminants. Illuminant

Color temperature (K)

Spectral resemblance ratio (%)

D30 A D45 D60

3000 4500 6000

73.7 39.2 36.9

8

Y.H. Kim et al. / Materials Science and Engineering B 193 (2015) 4–12

Fig. 4. Embodiment of sunlight spectrum with the EL spectra of the fabricated LED for (a) commercial grade YAG:Ce3+ under blue LED excitation compared with (b–d) Sr2.85 Eu0.15 MgSi2 O8 based blended phosphor for (b) warm light (∼3000 K), (c) natural light (∼4500 K) and (d) cool light spectrum (∼5500 K) under near UV LED excitation.

mimics natural daylight under near UV excitation. The fabricated WLED with phosphor blend under near UV is highly favorable for generating warm white light with high CRI (97) and spectral resemblance of 92.0% which is the highlight of the present work, that meets the consumer needs of LED industry intended for indoor lighting applications. 3.3. Optical properties of the phosphor blend under blue LED excitation (max = 450 nm) that mimic natural daylight A new blend composition without SMS:Eu2+ blue phosphor was developed for WLED and tested under blue LED excitation and their CRI values are tabulated in Table 4. The WLED with phosphor composition was studied under CCT ranging 2700–6000 K and their EL spectra were depicted in Fig. 5. The sharp blue emission at 450 nm corresponds to InGaN blue LED chip, while the broad band in the wavelength ranged 480–760 nm, due to the mixing of emissions of all phosphor constituents. Fabricated WLED for warm white light with CCT∼3000 K, showed CRI 96 (Fig. 5a) which is mainly envisioned for indoor lighting, which remains a target for the current LED lighting industry, while CRI of 95 each was observed at ∼4500 K and ∼6000 K (cool white) as shown in Fig. 5b and c. These optical results are very impressive for their application in WLED industry for different color temperature applicability areas. The concentration of phosphor to silicone ratio was very critical in obtaining high CRI for these fabricated WLED and these values are obtained with highest optimized ratios. The reason for high CRI for the white

LED fabricated with phosphor blend was attributed to the selective choice of phosphor constituent with high efficiency and minimal overlapping of excitation/emission spectra. The true color of an object was investigated with fabricated WLED containing phosphor blend, as lighting source under blue LED excitation using SMD 3528 LED packages. Two LED devices were fabricated for warm (∼3000 K) and cool white (∼6000 K) with CRI 70 and 97 and are probed to find effectiveness of the lighting source, on the object and the ability to deliver true color of the object. Warm white LED was fabricated by assembling 28 flexible SMD 3528 LED chips and cool white WLED with 20 SMD 3528 LED chips which can excite at 450 nm as depicted in Fig. 6a and b. Flower vases with white roses and yellow solution were viewed with our fabricated WLED (CRI 70 and 97) in the Force4 corporation laboratory as depicted in Fig. 6. The image under CRI ∼97 shows the true color of the flower vase with detailed white colors in rose petals. Although the color temperature plays significant role in the quality of lighting, for instance, warmer colors look best under warmer light (∼3000 K), while cooler colors look best under cool light (∼6000 K), and the true color of the image of our fabricated WLED was perceived by the human eyes. EL spectra of the fabricated WLED with CRI 70 and 97 for warm (3000 K) and cool (6000 K) white light excited by blue LED chip are depicted in Fig. 7a and c respectively which are used for rating the effectiveness of light on true color of the flower vase (Fig. 6). It discloses that EL spectra of WLED with high CRI ∼97 show broad emission spectra compared to CRI 70 spectra for both CCT ∼3000

Y.H. Kim et al. / Materials Science and Engineering B 193 (2015) 4–12

9

Fig. 5. Electroluminescent spectra of the fabricated LED with high CRI for (a) warm light (∼3000 K), (b) natural light (∼4500 K), and (c) cool light (∼6000 K) spectrum under 450 nm blue LED excitation.

Fig. 6. LED packaging design of 3528 SMD type with CCT target of (a) ∼3000, (b) ∼6000 K, and (c) image of flower vase under CCT ∼3000 and ∼6000 K with CRI of 70 and 97 each under 450 nm blue LED excitation.

10

Y.H. Kim et al. / Materials Science and Engineering B 193 (2015) 4–12

Table 4 Composition of blended phosphor and their optical properties under blue LED excitation. Phosphor composition

Target CCT (K)

GREEN

3000 4500 6000

YELLOW

CRI

1:5 1:7 1:6

96 95 95

RED

Ba2 Si2 O2 N: Eu2+ (495 nm)

Ba2 SiO4 : Eu2+ (505 nm)

YAG: Ce3+ (559 nm)

CaAlSiN3 : Eu2+ (650 nm)

CaAlSiBN: Eu2+ (660 nm)

25.5% 25.5% 24.3%

29.3% 29.3% 26.6%

29.3% 29.3% 34.7%

13.4% 13.4% 12.1%

2.5% 2.5% 2.3%

and ∼6000 K. The CCT of the fabricated WLED was tuned by varying the phosphor: silicone ratio, which invariably affects the intensity of light falling on the phosphor. For instance more bluish light (from 450 nm emitting blue LED chip) is needed to produce cool white light (∼6000 K), while lesser blue light emitting would produce a warm white light (∼3000 K). Low blue emission of blue LED chip is observed for warm white LED and vice versa for cool white LED as observed in Fig. 7a and c. The corresponding averaged CRI values of fabricated 15 LEDs are depicted in Fig. 7b and d for CCT ∼3000 and ∼6000 K respectively. The average CRI measurement from 15 different fabricated LEDs simultaneously to obtain reliable averaged CRI value. The feasibility of this phosphor blend and reproducibility of optical properties under blue LED excitation were performed using mass production of 10,000 WLED packages. For the mass

Ratio (%) (phosphor: silicone)

production of the device, phosphors blend composition with less number of phosphor components are tabulated in Table 5. The CIE chromaticity of these fabricated WLED with mass production for warm (∼3000 K), natural (∼4500 K) and cool (∼6000 K) white light was measured and compared with the commercial CIE standard rank and is shown in Fig. 8. The color coordinate of warm white LED falls within the range of commercial CIE white standards confined to narrow window shows high reproducibility. The color coordinates of natural and cool white light apart from displaying a narrow band of coordinates lying well within the commercial white CIE standards, it covers very small space in the large window of standards depicting by red box. All the optical properties of the fabricated WLED with the ideal phosphor blend composition are very promising and are edging closer to the ideal optical properties of sun. The fabricated WLED shows high CRI for warm, natural and

Fig. 7. Electroluminescent spectra of phosphor blend with CRI ∼70 and ∼97 under (a) 3000 K and (c) 6000 K. The corresponding representation on CRI averaging of 15 LEDs (R1–R15) with CRI ∼70 and ∼97 at (b) 3000 K and (d) 6000 K under 450 nm blue LED excitation. The CRI averaging is represented in the circular form with 15 LEDs and the concentric circles with radius are CRI values with 30, 60 and 90 as depicted. The chord extending the radius of the circle from the center to the R1–R15 describes the CRI value for the particular LED.

Y.H. Kim et al. / Materials Science and Engineering B 193 (2015) 4–12

11

Table 5 Composition of phosphor blend for mass production of LED with high CRI > 90 and material ratio under blue LED excitation. Phosphor composition

Target CCT (K)

3000 4500 6000

Green

Yellow

Red

Ba2 SiO4 :Eu2+ (505 nm)

YAG:Ce3+ (562 nm)

CaAlSiN3 :Eu2+ (650 nm)

Ratio (%) (phosphor:silicone)

CRI

29.5:70.5 22.0:78.0 17.5:82.5

93.6 91.9 91.7

Fig. 8. CIE chromaticity diagram investigated for mass production of LED with phosphor blend containing high CRI >90 with CCT target of (a) ∼3000 K, (b) ∼4500 K and (c) ∼6000 K in comparison with CIE and commercial rank under 450 nm blue LED excitation.

cool white light, and high spectral matching ratio with the spectral power distribution of sun. 4. Conclusions In summary, we have investigated the feasibility of tuning the diurnal daylight with pc-WLED through the advent of newly developed phosphor blend under near UV and blue excitation. The objective of obtaining a diurnal daylight matching pc-WLED was achieved with (a) developed phosphor blend to mimic natural daylighting in full spectrum, under changing color temperature and light intensity in par with the standard CIE illuminant, (b) high CRI (97) for developed blend and its high resemblance (in intensity) to natural daylight (91%) under near UV excitation compared to 39.2% for commercial YAG:Ce3+ based WLED at 4500 K, and (c) created broad emission covering complete visible spectrum. The prime stature of this diurnal daylight was realized with the development of new phosphor blend with wide excitation ranging near UV to

visible. Mass production of WLEDs with developed phosphor blend under blue LED excitation showed narrow color chromaticity compared with CIE standards. The developed phosphor blend may be the potential candidate for diurnal full spectrum white LED with high CRI (97) for indoor lighting applications. Acknowledgement This research is supported by the Ministry of Education, Science, and Technology (MEST) and the National Research Foundation of Korea (NRF) through the Human Resource Training Project for Regional Innovation, and supported by the Strategic Key-Material Development and the Materials and Components Research and Development bodies, funded by the Ministry of Knowledge Economy (MKE, Korea). This work was also financially supported by Basic Science Research Program through the NRF, funded by MEST. The authors appreciate the financial support from Joint Research Project funded by MKE, Korea.

12

Y.H. Kim et al. / Materials Science and Engineering B 193 (2015) 4–12

References [1] J.H. Oh, S.J. Yang, Y.R. Do, Light Sci. Appl. 3 (2014) e141. [2] E.V. Ellis, E.W. Gonzalez, D.A. Kratzer, D.L. McEachron, G. Yeutter, ARCC Conference Repository, 2014. [3] E.F. Schubert, J.K. Kim, Science 308 (2005) 1274–1278. [4] J.-H. Jou, K.-Y. Chou, F.-C. Yang, A. Agrawal, S.-Z. Chen, J.-R. Tseng, C.-C. Lin, P.-W. Chen, K.-T. Wong, Y. Chi, Appl. Phys. Lett. 104 (2014). [5] J.-H. Jou, R.-Z. Wu, H.-H. Yu, C.-J. Li, Y.-C. Jou, S.-H. Peng, Y.-L. Chen, C.-T. Chen, S.-M. Shen, P. Joers, ACS Photonics 1 (2013) 27–31. [6] J.R. Oh, S.-H. Cho, Y.-H. Lee, Y.R. Do, Opt. Express 17 (2009) 7450–7457. [7] S. Pimputkar, J.S. Speck, S.P. DenBaars, S. Nakamura, Nat. Photonics 3 (2009) 180–182. [8] A. Dobrowolska, E. Zych, J. Phys. Chem. C. 116 (2012) 25493–25503. [9] Y. Liu, X. Zhang, Z. Hao, X. Wang, J. Zhang, J. Mater. Chem. 21 (2011) 6354–6358. [10] F. Rahman, Opt. Photonics News 24 (2013) 26–32. [11] M.S. Rea, J.P. Freyssinier-Nova, Color Res. Appl. 33 (2008) 192–202.

[12] A. Zˇ ukauskas, R. Vaicekauskas, M.S. Shur, J. Phys. D: Appl. Phys. 43 (2010) 354006. ¯ e, ˙ A. Petrulis, M. Shur, Opt. [13] A. Zˇ ukauskas, R. Vaicekauskas, P. Vitta, A. Zabiliut Express 21 (2013) 26642–26656. [14] J.K. Park, K.J. Choi, C.H. Kim, H.D. Park, H.K. Kim, Electrochem. Solid-State Lett. 7 (2004) H42–H43. [15] X. Sun, J. Zhang, X. Zhang, S. Lu, X. Wang, J. Lumin. 122–123 (2007) 955–957. [16] H.-D. Nguyen, I.-H. Yeo, S.-I. Mho, ECS Trans. 28 (2010) 167–173. [17] A.C. Larson, R.B. Von Dreele, Los Alamos National Laboratory Report LAUR, 1994. [18] R. Shannon, Acta Crystallogr. Sect. A: Found. Crystallogr. 32 (1976) 751–767. [19] J.S. Kim, P.E. Jeon, J.C. Choi, H.L. Park, S.I. Mho, G.C. Kim, Appl. Phys. Lett. 84 (2004) 2931–2933. [20] G.J. Talwar, C.P. Joshi, S.V. Moharil, S.M. Dhopte, P.L. Muthal, V.K. Kondawar, J. Lumin. 129 (2009) 1239–1241. [21] J.S. Kim, A.K. Kwon, Y.H. Park, J.C. Choi, H.L. Park, G.C. Kim, J. Lumin. 122–123 (2007) 583–586. [22] G. Blasse, B. Grabmaier, Luminescent Materials, Springer-Verlag, Berlin, 1994.