WO2011090308A2

WO2011090308A2 - White light-emitting device and fabricating method thereof

Info

Publication number: WO2011090308A2
Application number: PCT/KR2011/000379
Authority: WO
Inventors: Yong-Kwang Kim; Geun Ho Kim; Soonmok Kwon; Hachul Kim
Original assignee: Iljin Semiconductor Co., Ltd.
Priority date: 2010-01-19
Filing date: 2011-01-19
Publication date: 2011-07-28
Also published as: KR20110085206A; WO2011090308A3; KR101144754B1

Abstract

Provided are a white light-emitting device and a method of fabricating the same. The white light-emitting device includes a blue light-emitting diode (LED), and a fluorescent substance layer conigured to absorb light emitted from the blue LED and emit light having a wavelength different from a wavelength of the absorbed light. Here, the blue LED includes a GaN-based semiconductor light-emitting layer and has a light-emission peak in a wavelength region of at least 410 nm and less than 460 nm, and the fluorescent substance layer has a blue fluorescent substance having a light-emission peak in a wavelength region of at least 460 nm and less than 500 nm, a green fluorescent substance having a light-emission peak in a wavelength region of at least 500 nm and less than 550 nm, a yellow fluorescent substance having a light-emission peak in a wavelength region of at least 550 nm and less than 600 nm, and a red fluorescent substance having a light-emission peak in a wavelength region of at least 600 nm and less than 660 nm. Thus, white light is emitted by mixing light emitted from the blue LED and blue, green, yellow and red lights emitted from the fluorescent substance layer.

Description

WHITE LIGHT-EMITTING DEVICE AND FABRICATING METHOD THEREOF

The described technology relates to a white light-emitting device and a method of fabricating the same, and more particularly, to a white light-emitting device having an excellent color rendering property and a small deviation of color coordinates and a method of fabricating the same.

A light-emitting diode (LED) which has lately attracted attention as a leading next-generation light source is a semiconductor device converting electricity into ultraviolet rays, visible rays, infrared rays, etc. using a characteristic of a compound semiconductor. The LED is mainly used for home appliances, remote controls, large electronic display boards, etc. to show a signal. LEDs generating red and green colors were developed long ago and have been widely used to show signals. A high-luminance red LED was developed in the early 1990s, and several years thereafter, a high-luminance blue LED was developed using a GaN-based semiconductor by Nichia Corp. in Japan. Since all LEDs capable of showing the three primary colors of light have been developed, active research is under way to use LEDs as a light source. A white LED light source for general lighting needs to have high efficiency and high color rendering property.

In one aspect, there is provided a white light-emitting device including: a blue light-emitting diode (LED); and a fluorescent substance layer configured to absorb light emitted from the blue LED and emit light having a wavelength different from a wavelength of the absorbed light. Here, the blue LED includes a GaN-based semiconductor light-emitting layer and has a light-emission peak in a wavelength region of at least 410 nm and less than 460 nm, and the fluorescent substance layer has a blue fluorescent substance having a light-emission peak in a wavelength region of at least 460 nm and less than 500 nm, a green fluorescent substance having a light-emission peak in a wavelength region of at least 500 nm and less than 550 nm, a yellow fluorescent substance having a light-emission peak in a wavelength region of at least 550 nm and less than 600 nm, and a red fluorescent substance having a light-emission peak in a wavelength region of at least 600 nm and less than 660 nm. Thus, white light is emitted by mixing light emitted from the blue LED and blue, green, yellow and red lights emitted from the fluorescent substance layer.

In another aspect, there is provided a method of fabricating a white light-emitting device having a blue LED and a fluorescent substance layer, the method including: preparing a blue fluorescent substance having a light-emission peak in a wavelength region of at least 460 nm and less than 500 nm, a green fluorescent substance having a light-emission peak in a wavelength region of at least 500 nm and less than 550 nm, a yellow fluorescent substance having a light-emission peak in a wavelength region of at least 550 nm and less than 600 nm, and a red fluorescent substance having a light-emission peak in a wavelength region of at least 600 nm and less than 660 nm; depositing a first slurry formed by mixing a transparent coating material and at least one kind of fluorescent substance selected from among the blue fluorescent substance, the green fluorescent substance, the yellow fluorescent substance, and the red fluorescent substance on the blue LED; hardening the first slurry to form a first sublayer; depositing a second slurry formed by mixing a transparent coating material and at least one kind of fluorescent substance selected from among the blue fluorescent substance, the green fluorescent substance, the yellow fluorescent substance, and the red fluorescent substance on the first sublayer; and hardening the second slurry to form a second sublayer. Here, the fluorescent substance layer having a plurality of sublayers including the first sublayer and the second sublayer covers the blue LED, and includes all of the blue fluorescent substance, the green fluorescent substance, the yellow fluorescent substance, and the red fluorescent substance.

In an exemplary embodiment, the fluorescent substance layer may have a structure in which at least two sublayers including at least one selected from among the blue fluorescent substance, the green fluorescent substance, the yellow fluorescent substance, and the red fluorescent substance.

In another exemplary embodiment, a emission peak wavelength of a fluorescent substance included in a lower sublayer of the fluorescent substance layer may be the same as or shorter than that of a fluorescent substance included in an upper sublayer.

In exemplary embodiments, a blue LED with high efficiency is used instead of an ultraviolet LED. Thus, it is possible to solve a problem of low excitation efficiency caused when ultraviolet rays are used, and ensure a high color rendering property by combining a plurality of fluorescent substances covering the whole visible light region such as green, yellow, and red fluorescent substances in addition to a blue fluorescent substance of a slightly longer wavelength which can be excited by the blue LED.

Also, since sublayers are stacked to form a fluorescent substance layer, reproducibility can be ensured by solving the problem of enlarged color coordinate distribution and increasing difference between light-emitting device units. Further, a fluorescent substance in an upper sublayer can be additionally excited by a fluorescent substance in a lower sublayer, and thus efficiency of a fluorescent substance in a long-wavelength region can be improved.

FIG. 1 is a schematic cross-sectional view of a white light-emitting device according to an exemplary embodiment.

FIG. 2 illustrates distribution of fluorescent substance particles according to time when fluorescent substances having different particle sizes are mixed with a transparent coating material.

FIG. 3 is a schematic cross-sectional view of a white light-emitting device including a fluorescent substance layer having a multi-layer structure according to an exemplary embodiment.

FIG. 4 is a schematic cross-sectional view of a white light-emitting device including a fluorescent substance layer having a multi-layer structure according to another exemplary embodiment.

FIG. 5 is a flowchart illustrating a method of fabricating a white light-emitting device according to an exemplary embodiment.

Hereinafter, exemplary embodiments of the present disclosure will be described in detail with reference to the accompanying drawings. Like numbers refer to like elements throughout the description of the figures unless the context clearly indicates otherwise. It should be understood that there is no intent to limit exemplary embodiments to the particular forms disclosed, but on the contrary, exemplary embodiments are to cover all modifications, equivalents, and alternatives falling within the scope of the invention. Those of ordinary skill in the art will readily appreciate that components of this disclosure, that is, components generally described herein and illustrated in the drawings, can be arranged, configured, combined, and designed in various different ways, and all the components are clearly designed and constitute a part of this disclosure. In the drawings, the widths, lengths, thicknesses, shapes, etc. of components may be exaggerated to clearly show several layers, regions, and shapes. The drawings are described from a viewpoint of an observer. It will be understood that when an element is referred to as being "on" another element, it can be directly on the other element or intervening elements may be present.

Referring to FIG. 1, a white light-emitting device 100 includes a blue light-emitting diode (LED) 110 and a fluorescent substance layer 101 surrounding the blue LED 110. The fluorescent substance layer 101 includes four kinds of fluorescent substances in a coating portion 120. The four kinds of fluorescent substances are a blue fluorescent substance 132, a green fluorescent substance 134, a yellow fluorescent substance 136, and a red fluorescent substance 138, which are evenly distributed in the coating portion 120. The respective components will be described in further detail below.

The blue LED 110 serves as a light source which emits blue light required for the white light-emitting device 100 to emit white light, and as an excitation source which excites the four kinds of fluorescent substances to emit light of several wavelengths. The blue LED 110 has a semiconductor light-emitting layer formed of a GaN-based material such as GaN, InGaN, AlGaN, or AlGaInN and thus can have a light-emission peak in a wavelength region of at least 410 nm and less than 460 nm. Although not shown in detail, when bias current is provided between an n-GaN layer and a p-GaN layer with the light-emitting layer having a single quantum well structure or multi-quantum well structure interposed between the n-GaN layer and the p-GaN layer, the blue LED 110 can emit blue light from the light-emitting layer. Use of the blue LED 110 as a light source exciting a fluorescent substance in the white light-emitting device 100 has the following advantages in comparison with use of an ultraviolet LED. The blue LED 110 has better efficiency compared to the ultraviolet LED. The ultraviolet LED does not have better chip performance compared to the blue LED 110. When a blue fluorescent substance having a short wavelength is excited, energy of blue light is reduced because the ultraviolet LED depends on conversion efficiency of the fluorescent substance. On the other hand, the blue LED 110 can use blue light emitted from the blue LED 110 itself and thus has higher efficiency than the ultraviolet LED when white light is emitted. Thus, white light emission with high luminous flux is enabled.

The coating portion 120 surrounds and protects the blue LED 110 overall, and contains the

fluorescent substances

132, 134, 136 and 138 converting light emitted from the blue LED 110. As a material of the coating portion 120, a transparent resin such as epoxy resin, acrylic resin, polyimide resin, urea resin, and silicone having excellent weather resistance can be used. When fluorescent substances are formed by thin-film deposition such as thermal evaporation, chemical vapor deposition (CVD), sputtering, and atomic layer epitaxy (ALE), the coating portion 120 may be omitted.

The plurality of

fluorescent substances

132, 134, 136 and 138 constituting the fluorescent substance layer 101 may have four wavelength bands. To be specific, the blue fluorescent substance 132 may have a light-emission peak in a wavelength region of at least 460 nm and less than 500 nm. The blue fluorescent substance 132 can be excited by the blue LED 110 having a shorter wavelength, and have a light-emission peak at a longer wavelength than a light-emission wavelength of the blue LED 110. The green fluorescent substance 134 may have a light-emission peak in a wavelength region of at least 500 nm and less than 550 nm. The yellow fluorescent substance 136 may have a light-emission peak in a wavelength region of at least 550 nm and less than 600 nm. The red fluorescent substance 138 may have a light-emission peak in a wavelength region of at least 600 nm and less than 660 nm.

An example of the blue fluorescent substance 132 may include a silicate-based material such as (Ba_x, Sr_y, Ca_z)₃MgSi₂O₈:Eu (0≤x≤1, 0≤y≤1, 0≤z≤1) or a sulfide-based material such as (Sr_x, Ca_y)S:Ce (0≤x≤1, 0≤y≤1). To be specific, the blue fluorescent substance 132 may be selected from among Sr₃MgSi₂O₈:Eu, Ba₃MgSi₂O₈:Eu, SrS:Ce,CaS:Ce, CaAl₂S₄:Eu,Sr₄Al₁₄O₂₅:Eu, and Ba₂SiO₄:Eu.

An example of the green fluorescent substance 134 may include a silicate-based material such as (Ba_x,Sr_y,Ca_z)₂SiO₄:Eu (0≤x≤1, 0≤y≤1, 0≤z≤1), a thiogallate-based material such as (Ba_x, Sr_y, Ca_z)Ga₂S₄:Eu (0≤x≤1, 0≤y≤1, 0≤z≤1), or a thioaluminate-basedmaterial such as (Ba_x, Sr_y, Ca_z)Al₂S₄:Eu (0≤x≤1, 0≤y≤1, 0≤z≤1). To be specific, the green fluorescent substance 134 may be selected from among Sr₂SiO₄:Eu, Ba₂SiO₄:Eu, Ca₂SiO₄:Eu, SrGa₂S₄:Eu, BaGa₂S₄:Eu, CaGa₂S₄:Eu, Sr₂Ga₂S₅:Eu, SrAl₂S₄:Eu, BaAl₂S₄:Eu, Sr₂Al₂S₅:Eu, SiAlON:Eu, and CaSc₂O₄:Ce.

An example of the yellow fluorescent substance 136 may include a silicate-based material, such as (Ba_x,Sr_y,Ca_z)₂SiO₄:Eu (0≤x≤1, 0≤y≤1, 0≤z≤1), yttrium aluminum garnet(YAG):Ce, or terbiumaluminumgarnet(TAG):Ce.

An example of the red fluorescent substance 138 may include a sulfide-based material, such as (Sr_x,Ca_y)S:Eu (0≤x≤1, 0≤y≤1), a nitride-based material, such as (Ba_x, Sr_y, Ca_z)₂Si₅N₈:Eu (0≤x≤1, 0≤y≤1, 0≤z≤1), Sr₃SiO₅:Eu, CaAlSiN₃:Eu, or SrY₂S₄:Eu.

All the

fluorescent substances

134, 134, 136 and 138 are high-efficiency fluorescent substances that are excited by light emitted from the blue LED 110, which is an excitation source, and emit intense light. The disclosed white light-emitting device 100 can directly use blue light emitted from the excitation source to the outside of the fluorescent substance layer 101. Also, since blue, green, yellow and red lights emitted from the

fluorescent substances

132, 134, 136 and 138 having longer wavelength ranges than the blue LED 110 have various wavelength bands, light emitted from the blue LED 110 and lights emitted from the

fluorescent substances

132, 134, 136 and 138 are mixed, and white light emission with high luminous flux and a high color rendering property is enabled.

A conventional white light-emitting device fabricated by depositing only a yellow fluorescent substance on a blue LED has only two wavelengths of blue and yellow colors. Thus, the conventional white light-emitting device has a low color rendering property and is not appropriate to be used as a backlight for a full-color display. Meanwhile, when an ultraviolet LED is used as the excitation source, a red fluorescent substance has low efficiency because an energy difference between ultraviolet light and red is larger than an energy difference between ultraviolet light and blue or green. However, since the disclosed white light-emitting device 100 uses a blue LED as an excitation source, such a problem can be solved.

To evaluate the quality of light of a white light-emitting device for illumination, a color rendering index (CRI) can be used as a reference. The CRI denotes an evaluation index indicating the degree of recognition of 15 reference colors when an object is illuminated by an artificial luminaire in comparison with a case in which the object is illuminated by the sun (CRI = 100 R_a). Currently, a CRI of an incandescent light bulb is 80 or greater, and that of a fluorescent light bulb is 75 or greater. On the other hand, a CRI of a commercialized white light-emitting device is about 65 to 75, and lately has been improved up to about 80 by finely adjusting a wavelength of a blue excitation light source.

The disclosed white light-emitting device 100 can emit light in a most visible light region when a blue LED and fluorescent substances are combined at an appropriate ratio. Then, the white light-emitting device 100 can have a CRI of 90 or greater, and may have a CRI of 95 or greater by optimization. In an exemplary embodiment, the disclosed white light-emitting device 100 can have a luminous efficiency of 80 lm/W or greater while maintaining a high CRI of 90 or greater.

To fabricate a white light-emitting device, a hole cup or a reflection cup, which is formed of a plastic ejection, including a blue LED chip is filled with a transparent coating material such as an epoxy resin layer or a silicon resin layer. A fluorescent substance is mixed with the transparent coating material in powder form before the transparent coating material hardens. Subsequently, an unhardened slurry including the fluorescent substance powder is deposited on the blue LED chip, and then the transparent coating material is hardened to form a transparent coating layer, thereby fabricating a white light-emitting device.

In general, fluorescent substance particles are randomly distributed in the transparent coating layer. However, it may be difficult for the white light-emitting device having the transparent coating layer including the fluorescent substance particles to obtain uniformity of white light. Such non-uniformity results from non-uniformity of the fluorescent substance particles included in the slurry of the transparent coating material. A phenomenon caused by the non-uniformity of the fluorescent substance particles will be described with reference to FIG. 2.

FIG. 2 illustrates distribution of fluorescent substance particles according to time when fluorescent substances having different particle sizes are mixed with a transparent coating material. Referring to FIG. 2, a light-emitting device in which a fluorescent substance layer 201 is deposited on an LED chip 210 is shown. The fluorescent substance layer 201 includes

fluorescent substance particles

220 and 230 evenly distributed in a transparent coating layer 240. The left drawing of FIG. 2 illustrates that the relatively large fluorescent substance particles 220 and the relatively small fluorescent substance particles 230 are evenly distributed immediately after fluorescent substance powder is mixed in the transparent coating layer 240. After a predetermined time elapses before a hardening process, the relatively large fluorescent substance particles 220 sink faster than the relatively small fluorescent substance particles 230 due to a difference in specific gravity. As a result, the number of the relatively large fluorescent substance particles 220 present near the LED chip 210 increases, and the overall distribution of the

fluorescent substance particles

220 and 230 varies. Subsequently, when the hardening process is performed, the spatial distribution of the

fluorescent substance particles

220 and 230 becomes uneven. According to whether it is an early stage or late stage of a process of mixing a transparent coating material and a fluorescent substance between light-emitting device units, a time from the mixing process to the hardening process differs, and thus the light-emitting device units have different spatial distributions of the fluorescent substance particles after hardening.

Such a difference affects light-emission characteristics of a white light-emitting device. As the unevenness of spatial distribution increases, the degree of color coordinate dispersion increases, and it becomes difficult to realize white light-emitting devices having the same color coordinates. The reason can be explained as follows. Emission light with a longer wavelength from fluorescent substances excited by a portion of the light from a LED chip may be mixed with the other portion of the light from the LED chip, thereby white light may be generated. After the hardening process, many large fluorescent substances may be distributed near the LED chip, and small fluorescent substances may float. When light emitted from fluorescent substances excited by the LED chip is not emitted to the outside but collides with the floating substances, the light may vanish, be reflected or be diffused, or excite the fluorescent substances. Thus, the quantity of light corresponding to the original color may be reduced, and the quantity of light of the floating fluorescent substances may increase. Then, the total quantity of mixed light and color coordinates may be affected.

Such a phenomenon may become severe as the difference in the specific gravity between fluorescent substances increases. As a result, it is difficult to realize white light-emitting devices having the same color coordinates because difference in distribution of fluorescent substances between light-emitting device units may cause difference in color coordinates.

A white light-emitting device having a fluorescent substance layer containing four kinds of fluorescent substances has been described with reference to FIG. 1. The four kinds of fluorescent substances having different wavelength regions have different chemical compositions and different average diameters of fluorescent substance particles. For example, a silicate fluorescent substance generally has an average diameter 20 ㎛ to 25㎛, and a YAG fluorescent substance generally has an average diameter of 7 ㎛ to 10 ㎛. Thus, when several kinds of fluorescent substances having various average diameters are mixed and used, the distribution of the fluorescent substances in one light-emitting device becomes uneven due to difference in sinking speed between the fluorescent substances, and respective light-emitting device units may have difference in color coordinates.

To solve this problem, the disclosed technology provides a fluorescent substance layer having a multi-layer structure. FIG. 3 is a schematic cross-sectional view of a white light-emitting device including a fluorescent substance layer having a multi-layer structure according to an exemplary embodiment. Referring to FIG. 3, a white light-emitting device 300 includes a blue LED 310 and a fluorescent substance layer having a coating portion 320 containing

fluorescent substances

332, 334, 336 and 338 surrounding the blue LED 310. The fluorescent substance layer is divided into two

sublayers

301 and 302. The first sublayer 301 contains the blue fluorescent substance 332 and the green fluorescent substance 334, and the second sublayer 302 contains the yellow fluorescent substance 336 and the red fluorescent substance 338. Thus, mixed light of the blue fluorescent substance 332 and the green fluorescent substance 334 in the first sublayer 301 and mixed light of the yellow fluorescent substance 336 and the red fluorescent substance 338 in the second sublayer 302 are mixed with blue light of the blue LED 310 to be white light. Even when a fluorescent substance layer has a structure having multiple sublayers as mentioned above, the whole fluorescent substance layer may have the same thickness as a general fluorescent substance layer. The whole fluorescent substance layer has a thickness of 10 ㎛ to 1 mm and may have a thickness of 50 ㎛ to 500 ㎛. Since the fluorescent substance layer is divided into the first sublayer 301 and the second sublayer 302, thicknesses of the respective sublayers may be reduced.

Although FIG. 3 shows an example in which blue and green fluorescent substances are included in the same layer and yellow and red fluorescent substances are included in the same layer, each of fluorescent substances contained in the same layer may have a similar size. When fluorescent substances having a similar size are mixed in the same layer, optical characteristics of respective sublayers can be readily controlled because fluorescent substances in each sublayer have similar sinking speeds. When a fluorescent substance layer is fabricated to have at least two sublayers as described above, a thickness of each sublayer can be reduced. Thus, distribution of specific fluorescent substances can be limited to the respective sublayers, and difference in the degree of sinking between fluorescent substances caused by difference in specific gravity between the respective fluorescent substances can be minimized.

The example of FIG. 3 can be modified in various ways. In a white light-emitting device including a blue LED as an excitation source and four kinds of fluorescent substances according to some exemplary embodiments, a fluorescent substance layer may have a structure in which at least two sublayers including at least one selected from among a blue fluorescent substance, a green fluorescent substance, a yellow fluorescent substance and a red fluorescent substance are stacked. For example, one, two or three kinds of fluorescent substances can be mixed in one sublayer, and also one, two or three kinds of fluorescent substances can be mixed in another sublayer. For example, two kinds of blue fluorescent substances may be selected from among the blue, green, yellow and red fluorescent substances, and one kind of each of the green, yellow and red fluorescent substances may be selected, thereby constituting five sublayers. Otherwise, one kind of fluorescent substance having a specific color may be included in different sublayers. The sublayers can be configured in various ways in addition to those mentioned above. The number of sublayers is not limited, but may be appropriately selected in consideration of process efficiency, and a color rendering property, a color reproducibility, etc. of a white light-emitting device.

Although the kinds of fluorescent substances, the number of sublayers, a method of mixing fluorescent substances, etc. can be unrestrictedly selected, a emission peak wavelength of a fluorescent substance included in a lower sublayer in the fluorescent substance layer may be the same as or shorter than that of a fluorescent substance included in an upper sublayer. This is because a part of light which is emitted from the lower sublayer but not transmitted to the outside can somewhat excite a fluorescent substance in a sublayer disposed above the lower sublayer. In other words, a blue LED in the lowermost portion basically excites all fluorescent substances in sublayers present above the blue LED, and also a wavelength region of light emitted from a sublayer in a lower portion may somewhat overlap an absorption wavelength region of a fluorescent substance in a sublayer above the lower sublayer, so that light-emission efficiency of the fluorescent substance in the upper sublayer can be further improved. For example, when a red fluorescent substance is deposited in the uppermost layer among multiple sublayers, a probability that the red fluorescent substance will be excited by light of a fluorescent substance having a shorter wavelength than light of the red fluorescent substance may be increased.

FIG. 4 is a schematic cross-sectional view of a white light-emitting device including a fluorescent substance layer having a multi-layer structure according to another exemplary embodiment. Referring to FIG. 4, a white light-emitting device 400 includes a blue LED 410 and a fluorescent substance layer having a coating portion 420 containing

fluorescent substances

432, 434, 436 and 438 surrounding the blue LED 410. The fluorescent substance layer is divided into four

sublayers

401, 402, 403 and 404. The first sublayer 401 contains a blue fluorescent substance 432 of the shortest wavelength, the second sublayer 402 contains a green fluorescent substance 434, the third sublayer 403 contains a yellow fluorescent substance 436, and the fourth sublayer 404 contains a red fluorescent substance 438 of the longest wavelength. Thus, white light can be obtained from lights emitted from the

respective sublayers

401, 402, 403 and 404 and blue light emitted from the blue LED 410.

When each sublayer contains one kind of fluorescent substance as shown in FIG. 4, it is possible to readily control the distribution of fluorescent substances in each sublayer. Also, when a long-wavelength fluorescent substance is disposed in an upper portion, light-emission efficiency of the long-wavelength fluorescent substance can be maximized.

According to an aspect of the present disclosure, a method of fabricating a white light-emitting device having a blue LED and a fluorescent substance layer is provided.

FIG. 5 is a flowchart illustrating a method of fabricating a white light-emitting device according to an exemplary embodiment. Referring to FIG. 5, in step S1, a blue fluorescent substance, a green fluorescent substance, a yellow fluorescent substance, and a red fluorescent substance are prepared. In step S2, a first slurry formed by mixing a transparent coating material and at least one kind of fluorescent substance selected from among the blue fluorescent substance, the green fluorescent substance, the yellow fluorescent substance, and the red fluorescent substance is deposited on the blue LED. In step S3, the first slurry is hardened to form a first sublayer. In step S4, a second slurry formed by mixing a transparent coating material and at least one kind of fluorescent substance selected from among the blue fluorescent substance, the green fluorescent substance, the yellow fluorescent substance, and the red fluorescent substance is deposited on the first sublayer. In step S5, the second slurry is hardened to form a second sublayer, so that a white light-emitting device including a fluorescent layer having a multi-layer structure can be fabricated. Here, the fluorescent substance layer has a plurality of sublayers including the first sublayer and the second sublayer, covers the blue LED, and includes all of the blue fluorescent substance, the green fluorescent substance, the yellow fluorescent substance, and the red fluorescent substance.

For example, the white light-emitting device 300 shown in FIG. 3 can be fabricated by fabricating the first sublayer 301 and then the second sublayer 302. More specifically, to fabricate the first sublayer 301, a slurry is obtained by mixing the blue fluorescent substance 332 and the green fluorescent substance 334 in a transparent coating material and covers a blue LED while a hole cup or a reflection cup is filled with the slurry. Subsequently, the transparent coating material is hardened to form the coating portion 320, thereby fabricating the first sublayer 301. After this, to fabricate the second sublayer 302, a slurry is obtained by mixing the yellow fluorescent substance 336 and the red fluorescent substance 338 in a transparent coating material and deposited on the first sublayer 301. Subsequently, the transparent coating material is hardened to additionally form the coating portion 320, thereby fabricating the second sublayer 302. In this way, the white light-emitting device 300 can be fabricated.

While the invention has been shown and described with reference to certain example embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.

Claims

A white light-emitting device, comprising:

a blue light-emitting diode (LED); and

a fluorescent substance layer configured to absorb light emitted from the blue LED and emit light having a wavelength different from a wavelength of the absorbed light,

wherein the blue LED includes a GaN-based semiconductor light-emitting layer and has a light-emission peak in a wavelength region of at least 410 nm and less than 460 nm,

the fluorescent substance layer has a blue fluorescent substance having a light-emission peak in a wavelength region of at least 460 nm and less than 500 nm, a green fluorescent substance having a light-emission peak in a wavelength region of at least 500 nm and less than 550 nm, a yellow fluorescent substance having a light-emission peak in a wavelength region of at least 550 nm and less than 600 nm, and a red fluorescent substance having a light-emission peak in a wavelength region of at least 600 nm and less than 660 nm, and

white light is emitted by mixing light emitted from the blue LED and blue, green, yellow and red lights emitted from the fluorescent substance layer.
The white light-emitting device of claim 1, wherein the fluorescent substance layer has a structure in which at least two sublayers including at least one selected from among the blue fluorescent substance, the green fluorescent substance, the yellow fluorescent substance, and the red fluorescent substance are stacked.
The white light-emitting device of claim 2, wherein a light-emission peak wavelength of a fluorescent substance included in a lower sublayer in the fluorescent substance layer is the same as or shorter than that of a fluorescent substance included in an upper sublayer.
The white light-emitting device of claim 1, wherein the fluorescent substance layer includes four sublayers, and

the blue fluorescent substance, the green fluorescent substance, the yellow fluorescent substance, and the red fluorescent substance are stacked in sequence on the blue LED.
The white light-emitting device of claim 1, which has a color rendering index (CRI) of 90 or greater.
The white light-emitting device of claim 5, which has a luminous efficiency of 80 lm/W or greater.
A method of fabricating a white light-emitting device having a blue light-emitting diode (LED) and a fluorescent substance layer, the method comprising:

preparing a blue fluorescent substance having a light-emission peak in a wavelength region of at least 460 nm and less than 500 nm, a green fluorescent substance having a light-emission peak in a wavelength region of at least 500 nm and less than 550 nm, a yellow fluorescent substance having a light-emission peak in a wavelength region of at least 550 nm and less than 600 nm, and a red fluorescent substance having a light-emission peak in a wavelength region of at least 600 nm and less than 660 nm;

depositing a first slurry formed by mixing a transparent coating material and at least one kind of fluorescent substance selected from among the blue fluorescent substance, the green fluorescent substance, the yellow fluorescent substance, and the red fluorescent substance on the blue LED;

hardening the first slurry to form a first sublayer;

depositing a second slurry formed by mixing a transparent coating material and at least one kind of fluorescent substance selected from among the blue fluorescent substance, the green fluorescent substance, the yellow fluorescent substance, and the red fluorescent substance on the first sublayer; and

hardening the second slurry to form a second sublayer,

wherein the fluorescent substance layer has a plurality of sublayers including the first sublayer and the second sublayer, covers the blue LED, and includes all of the blue fluorescent substance, the green fluorescent substance, the yellow fluorescent substance, and the red fluorescent substance.
The method of claim 7, wherein a emission peak wavelength of a fluorescent substance included in a lower sublayer of the fluorescent substance layer is the same as or shorter than that of a fluorescent substance included in an upper sublayer.
The method of claim 7, wherein the fluorescent substance layer includes four sublayers, and

the blue fluorescent substance, the green fluorescent substance, the yellow fluorescent substance, and the red fluorescent substance are stacked in sequence on the blue LED.