1. Introduction

White light-emitting diode (WLED)-based solid-state lighting systems have been rapidly replacing conventional incandescent and fluorescent lamps in general lighting situations because of their high luminous efficacy, environmental friendliness, and long lifetime [1]. The most common type of WLED consists of a blue InGaN chip coated with the yellow phosphor Y₃Al₅O₁₂:Ce³⁺ (YAG:Ce³⁺) [2]. However, there remain several major issues regarding emission color when these WLEDs are used as illumination sources.

The first issue is the poor color rendering index (CRI) of WLEDs because of the lack of spectrum in the red region [3]. This issue has been addressed by the development of phosphors such as CaAlSiN₃:Eu²⁺ (CASN:Eu²⁺) and Ca₂Si₅N₈:Eu²⁺, which emit red luminescence, and β-SiAlON:Eu²⁺, which emits green luminescence [4–6]. Consequently, WLEDs have achieved a sufficient CRI using these phosphors [7]; however, the luminous efficacy of multi-phosphor WLEDs tends to be low because of the cascade excitation among phosphors and inelastic scattering among the phosphor particles arising from the increase in the mounted quantity of phosphors [8]. Cascade excitation is the reabsorption of shorter-wavelength luminescence (blue or green) by phosphors with longer-wavelength luminescence, such as by red luminescence-emitting nitride phosphors [9].

The second issue is the low color uniformity within the illuminated area. This is ascribed to the variation in chromaticity depending on the irradiation angle caused by differences in directivity and by cascade excitation. A decline in the angular uniformity of color due to differences in directivity is observed in WLEDs with a blue chip and YAG:Ce³⁺. The emission from blue chips exhibits a strong directivity, whereas that from yellow phosphors is omnidirectional. Because of this difference in directivity, yellow emissions in the lateral direction increase, resulting in the formation of a yellow ring around the irradiation area. In addition, a decline in the angular uniformity of color due to cascade excitation is observed in multi-phosphor WLEDs for obtaining superior CRIs. We previously reported that a red ring was observed in a WLED containing a violet chip and three primary-color phosphors [10]. Various WLED structures have been studied to ensure the angular uniformity of color; however, complete uniform color irradiation remains to be realized [11–14].

The final issue is the fluctuation in emission chromaticity in individual WLED devices [15]. Conventional light sources (i.e., incandescent and fluorescent lamps) emit light at a specific chromaticity, whereas WLEDs emit light that fluctuates in chromaticity. This fluctuation is caused by variations in the mounted quantity of phosphors and in the emission wavelength of the chip. The variation in the mounted quantity of phosphors affects the amount of blue light leakage through the phosphor layer and, in the case of multi-phosphors, the degree of color shift because of cascade excitation among the phosphors. Because the emission from conventional WLEDs comprises yellow emission from the phosphor layer and blue light leakage through the phosphor layer, in order to obtain a constant amount of blue light leakage, the phosphor layer must contain a precise quantity of yellow phosphors. Studies have been conducted examining the use of a pulse spraying method, phosphor-in-glass, and phosphor ceramics plate, which allow precise control over the quantity of phosphors in the phosphor layer [16–18]. In multi-phosphor WLEDs, as fluctuations in emission chromaticity occur by cascade excitation, WLEDs with complex structures, such as stacked layers of separate phosphors, in order to address this issue have been reported [19,20].

As another cause of individual chromaticity fluctuation, there is the variation in the emission wavelength of the chip. The emission wavelength of InGaN-based chips varies depending on the In content in the active layer. With growth processes such as metalorganic vapor phase epitaxy, it is difficult to ensure a uniform distribution of In on the epitaxial substrate. Thus, such chips, with the peak emission wavelength of a distribution from 10 to 20 nm, have been achieved. Accompanying this, the absorption of blue light by YAG:Ce³⁺ fluctuates by about 10%. As a result, the amount of blue light leakage fluctuates, and a large color shift occurs. Thus, it is necessary to adjust the mounted quantity of phosphors according to the emission wavelength of each chip. With a strict process control for individual chips, WLEDs are manufactured to maintain chromatic stability; however, they require the classification of chromaticity in their production despite the many efforts being made.

Here, we report a WLED that suppresses individual chromaticity variations. In our previous works, we developed three novel phosphors, i.e., (CaI₂:Eu²⁺/SiO₂) blue-emitting nanocomposite phosphor (NCP), (Ca_1–x–y,Sr_x,Eu_y)₇(SiO₃)₆Cl₂ yellow-emitting phosphor (Cl_MS:Eu²⁺), and K₂Ca(PO₄)F:Eu²⁺ red-emitting phosphor (FOLP:Eu²⁺) [10,21,22]. When these phosphors were admixed to obtain white luminescence, no cascade excitation was observed. The WLEDs combining the mixed phosphor with a violet chip in this study were not affected by the variation in the mounted quantity of phosphors and in the emission wavelength of the chips and, consequently, could emit white light while maintaining a certain chromaticity. We demonstrate that an encapsulated WLED with a thick and low concentration mixed-phosphor layer improved not only the CRI but also the luminous flux, and we elucidate that the reduction in inelastic scattering in the phosphor layer was the primary reason.

2. Experiment

NCP, Cl_MS:Eu²⁺, and FOLP:Eu²⁺ were synthesized via solid-state reactions using the flux [10,21,22]. Nemoto Lumi-Materials (Japan) provided the yellow phosphor YAG:Ce³⁺, Denka (Japan) provided the green phosphor β-SiAlON:Eu²⁺, and Mitsubishi Chemical (Japan) provided the blue phosphor Ca₅(PO₄)₃Cl:Eu²⁺ (APT:Eu²⁺) and red phosphor CASN:Eu²⁺. The weight ratios for the two mixed phosphors were as follows: mixed-phosphor 1: NCP/Cl_MS:Eu²⁺/FOLP:Eu²⁺ = 14/16/70; mixed-phosphor 2: APT:Eu²⁺/β-SiAlON:Eu²⁺/ CASN:Eu²⁺ = 38/40/23. GeneLite (Japan) and Mitsubishi Chemical (Japan) provided the blue and violet chips, respectively. The chips were mounted in Al₂O₃ casings (cavity diameter: 2.3 mm; cavity depth: 0.5 mm) or on Al₂O₃ substrates using a silicone bonding material and Au wires. The Au wires were protected by potting with a transparent silicone resin. A phosphor sheet made from a transparent silicone resin and either YAG:Ce³⁺ or the mixed phosphors was then attached to the casing. Encapsulated WLEDs were fabricated by encapsulating the chip mounted on the substrate using a phosphor paste consisting of a translucent silicone resin and the relevant phosphor; the phosphor concentration in the paste ranged from 0.2 to 1.3 vol.%.

Photoluminescence (PL) spectra under continuous excitation and photoluminescence excitation (PLE) spectra of the phosphor samples were measured at room temperature with a multichannel optical analyzer (PMA C5966-31, Hamamatsu Photonics, Shizuoka, Japan). A CAS 140B-152 spectrometer (Instrument Systems, Munich, Germany) was used to examine the electroluminescence characteristics of WLEDs operating in an integrated sphere. Emission color distributions for the encapsulated WLEDs were measured using a two-dimensional color analyzer (CM-1500M, Minolta, Tokyo, Japan).

3. Results and discussion

Previously, we reported three novel phosphors for violet chips [10,21,22]. WLEDs using violet chips can be expected to exhibit not only superior CRIs in principle but also high efficiencies. This is because the content of In incorporated in the active layer of the violet chips is lower than that in blue chips, indicating that the efficiency droop is less likely to occur in the violet chips because of their high crystallinity [23].

3.1 Luminescence properties of our developed phosphors

Figure 1 presents the PLE and PL spectra of our three developed phosphors. The peak wavelengths of the respective phosphors in the PL spectra with 400-nm excitation are 471 nm for NCP (blue emission), 564 nm for Cl_MS:Eu²⁺ (yellow emission), and 658 nm for FOLP:Eu²⁺ (red emission). The external quantum efficiencies of each phosphor are 83% for NCP, 85% for Cl_MS:Eu²⁺, and 65% for FOLP:Eu²⁺ [10,21,22]. A common feature of these phosphors is that the PLE spectra have an excitation band edge around 420 nm; therefore, the phosphors have a high excitation intensity in the region below 410 nm, and light with a wavelength longer than that is not absorbed or converted. As a result of these properties, the mixed phosphor does not suffer from cascade excitation. WLEDs containing a violet chip and mixed-phosphor 1 succeed in effectively suppressing the chromaticity variation in individual WLED devices, even when the phosphor layer thickness or emission wavelength of the chip fluctuates. Their details are described below.

 

Fig. 1 PLE (solid curves) and PL (dashed curves) spectra of NCP, Cl_MS:Eu²⁺, and FOLP:Eu²⁺ at room temperature.

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3.2 Influence of mounted quantity of phosphors

We investigated two of the above-mentioned fluctuation factors on chromaticity using WLEDs fabricated with the structure shown in the inset of Fig. 2(a). In the case of evaluating the influence of the mounted quantity of phosphors, a blue (λ_p = 451 nm) or violet (λ_p = 405 nm) chip was mounted at the bottom of an alumina casing. The mounted chip was sealed with a transparent silicone resin, and phosphor sheets of various thicknesses were then attached to the top of the casing. The concentration of phosphors in the phosphor sheet was 15 vol.% when the phosphor was YAG:Ce³⁺ and 30 vol.% when mixed phosphors were used.

 

Fig. 2 (a) CIE color coordinates (cx, cy) of WLEDs fabricated by using phosphor sheets of various thicknesses. Blue triangles, YAG:Ce³⁺ combining with a blue chip; red circles, mixed-phosphor 1 combining with a violet chip; green squares, mixed-phosphor 2 combining with a violet chip (emission peak 405 nm). Inset shows the structure of the WLED used in this experiment. (b) 471nm normalized electro-luminescence spectra of WLEDs containing a violet chip and a mixed-phosphor 1 sheet with different thicknesses. Inset shows enlarged spectra of violet region indicating violet leakage-light.

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We preliminarily examined the effect of blue light leakage on the emission chromaticity using WLEDs containing a blue chip and a YAG:Ce³⁺ sheet of various thicknesses (60–460 µm). As the thickness of the YAG:Ce³⁺ sheet increases (i.e., as blue light leakage decreases), the Commission Internationale de l'Eclairage color coordinates (CIE; cx, cy) markedly increase, that is, a yellow shift is observed as the blue light leakage decreases (Fig. 2(a)).

We then examined the effect of cascade excitation on the emission chromaticity using a WLED containing a violet chip and a mixed-phosphor 2 sheet, which consisted of three conventional primary-color phosphors. A violet chip with a low luminous sensitivity was adopted to reduce the influence of light leakage from the chip. In the same manner, when the thickness of the mixed-phosphor 2 sheet increased from 60 to 460 μm, the emission chromaticity exhibited a large color shift in the red direction. This shift was a redshift, which has higher cx value fluctuations than the cy value induced by cascade excitation.

In contrast, in the evaluation using a WLED containing a violet chip and a mixed-phosphor 1 sheet, only small increases in the cx and cy values are observed with the increase in the phosphor sheet thickness because of the low luminous sensitivity of the violet light leakage and low cascade excitation. Figure 2(b) presents the 471-nm normalized electroluminescence spectra of the WLEDs with the mixed-phosphor 1 sheet of different thicknesses. Each spectrum is almost identical except for the amount of violet light leakage. This fact indicates that violet light leakage with a low luminous sensitivity does not contribute primarily to the formation of white light. Further, the reason these WLEDs have different quantity of phosphors exhibiting the same emission spectrum except for the violet region is that mixed-phosphor 1 does not cause cascade excitation.

3.3 Influence of variation in the chip emission wavelength

In the case of evaluating the influence of the variation in the chip emission wavelength on the emission chromaticity, we used WLEDs containing either a blue chip and a 1-vol.% YAG:Ce³⁺ sheet or a violet chip and a 3-vol.% mixed-phosphor 1 sheet. The phosphor sheets were 1-mm thick. For the WLED containing a blue chip and YAG:Ce³⁺ sheet, both the cx and cy coordinates are observed to increase with the peak chip emission wavelength (Figs. 3(a) and 3(b)). The peak wavelength of the PLE spectrum for YAG:Ce³⁺ ranges from 455 to 460 nm (Fig. 4). The absorption of the chip emission by YAG:Ce³⁺ decreases with respect to emission wavelengths shorter than 450 nm; consequently, more blue light is leaked at shorter wavelengths. In contrast, for the WLEDs containing a violet chip and mixed-phosphor 1, there is almost no change observed in the cx and cy values, as the chip emission wavelength increases (Figs. 5(a) and 5(b)). Because our developed phosphors exhibit a high, stable PLE intensity against excitation light with a wavelength below 410 nm (Fig. 1), there is no change in the absorption of each phosphor against the variation in chip emission wavelengths. As a result, the luminescent chromaticity is stable.

 

Fig. 3 (a), (b) CIE color coordinates (cx, cy) of WLEDs containing a blue chip and a YAG:Ce³⁺ sheet as a function of the peak wavelength of the blue chip.

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Fig. 4 PLE spectrum of YAG:Ce³⁺ with monitoring at 558 nm.

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Fig. 5 (a), (b) CIE color coordinates (cx, cy) of WLEDs containing a violet chip and a mixed-phosphor 1 sheet as a function of the peak wavelength of the violet chip.

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3.4 Suppression of inelastic scattering

Mixed-phosphor 1 was required in the WLEDs to convert most of the emission from the violet chip. A larger quantity of phosphors than is normally used in WLEDs with a blue chip was thus necessary. However, a large quantity of phosphors degrades the luminous flux because of the increasing inelastic scattering loss among the phosphor particles [8]. In order to address this issue, we fabricated an encapsulated WLED (Fig. 6, upper inset) in which a violet chip was encapsulated in a thick, hemispherical dome-shaped layer of silicone paste containing a low phosphor concentration. Figure 6 shows the effect of the thick and low-concentration phosphor layer on the luminous flux of WLEDs. We used the encapsulated WLED containing mixed-phosphor 1 of 1.3 vol.% and conventional WLEDs with mixed-phosphor 1 sheets of various thicknesses in this experiment. All WLEDs possess a constant quantity of mixed-phosphor 1, that is, the thicker the phosphor layer, the lower the phosphor concentration. The linear increase in luminous flux with the thickness of the phosphor layer (decrease in phosphor concentration) indicates that the inelastic scattering loss was reduced as a result of the formation of a wider-spread light pathway among the phosphor particles at lower phosphor concentrations. When the encapsulated WLED was driven at 100 mA, a luminous flux of 18.0 lm, correlated color temperature of 3650 K, and CRI of 92 were achieved.

 

Fig. 6 Luminous flux of WLEDs containing a violet chip and mixed-phosphor 1 as a function of phosphor layer thickness containing a constant amount quantity of phosphors. The insets show the structures of the WLEDs used in this experiment.

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3.5 Color uniformity within the emission area

Finally, we examined the color uniformity within the emission area of the encapsulated WLEDs. Figures 7(a) and 7(b) present the cx and cy values, respectively, along the centerline of the hemispheric dome, and the electroluminescence spectrum and a representative emission image of the encapsulated WLED are shown in Fig. 7(c). The encapsulated WLED containing a violet chip and mixed-phosphor 1 has a uniform hue with little difference in chromaticity between the center and outer edge of the hemispheric dome. In contrast, in the WLED containing a blue chip and YAG:Ce³⁺, there is a large difference in chromaticity between the center and outer edge of the hemispheric dome, and a yellow ring is observed around the circumference of the dome [10]. The main cause is attributed to the difference in the radiation directivity of the blue chip and of YAG:Ce³⁺. Similarly, in the WLED containing a violet chip and mixed-phosphor 2, there is a large difference in chromaticity, and a red ring is observed around the circumference of the dome because of cascade excitation among the phosphors [10]. The emission color uniformity of the encapsulated WLED using mixed-phosphor 1 is attributed to the emission of omnidirectional white light consisting primarily of luminescence emitted by the phosphors, and to the suppression of cascade excitation among the phosphors.

 

Fig. 7 (a), (b) CIE color coordinates (cx, cy) along the centerline of an encapsulated WLED as a function of projection position from the center point of the hemispherical dome. Blue triangles: YAG:Ce³⁺ combined with a blue chip; red circles: mixed-phosphor 1 combined with a violet chip; green squares: mixed-phosphor 2 combined with a violet chip. (c) Electroluminescence spectrum of an encapsulated WLED consisting of a violet chip and mixed-phosphor 1 driven at 100 mA. The inset is a representative emission image of the encapsulated WLED.

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Because the emission image is projected to an irradiation area, this encapsulated WLED enables irradiation with a uniform color.

4. Conclusions

In this study, we succeeded in suppressing chromaticity fluctuations in individual WLEDs by combining a violet chip (emission peak: 405 nm) with three newly developed phosphors that generate no cascade excitation. These WLEDs were not affected by fluctuations in the mounted quantity of phosphors, because the luminous sensitivity of the violet light leakage was low and cascade excitation did not occur among the phosphors. In addition, this WLED was not affected by variations in the emission wavelength of the violet chip, because these phosphors exhibited a broad excitation band below 410 nm. These results rendered the classification of chromaticity in WLD production unnecessary. When these phosphors were applied to an encapsulated WLED with a thick resin layer containing a low phosphor concentration, the WLED exhibited not only a uniform color within its irradiation area but also an improved luminous flux via the enhanced formation of light pathways among the phosphor particles. The CRI of this WLED was excellent ( = 92), and its luminous flux, when driven at 100 mA, was 18.0 lm. The technology of the encapsulated WLED containing the violet chip and mixed-phosphor 1 provides an appropriate solution for major issues regarding emission color in WLEDs.