Patent Application
20090015135
1. Field of the Invention
The present invention relates to a light emitting device including a semiconductor light emitting element and fluorescent particles emitting light by light emitted from this semiconductor light emitting element.
2. Description of the Related Art
A light emitting device, called a white LED, formed in order to emit white base light includes a blue LED chip in combination with a fluorescent substance emitting yellow light.
The blue LED chip is formed of a p-n junction chemical semiconductor which primarily includes gallium nitride (GaN), and when a forward current is supplied to the chip, for example, blue base light having a wavelength of 560 nm or less is emitted therefrom. The fluorescent substance emits yellow light using the light emitted from the blue LED chip as exciting light, and an yttrium aluminum garnet (YAG) fluorescent substance is generally used.
A white LED emits white base light generated by synthesis between blur light and yellow light complementary thereto. This white LED is not only able to reduce power consumption by approximately 30% compared to that of a fluorescent lamp but is also superior in terms of environmental adaptation since it uses no mercury unlike a fluorescent lamp. Accordingly, a white LED starts to be used for a backlight for various display devices and for a simple lighting apparatus (for example, see Japanese Unexamined Patent Application Publications Nos. 2005-41941 and 2005-41942).
The above YAG fluorescent substance is a substance which absorbs light emitted from a blue LED chip and is excited thereby to emit yellow light; however, the luminous efficiency by a fluorescent effect depends on a use environmental temperature, and in particular at an environmental temperature of approximately 100° C. or more, the luminous efficiency is seriously decreased. On the other hand, since a white LED is desired to have a higher output of the quantity of light to be emitted, an electric power applied to a blue LED tends to be increased. Hence, the temperature is increased when a blue LED is emitting light, and the luminous efficiency of a YAG fluorescent substance is liable to be decreased. When the luminous efficiency of a YAG fluorescent substance is decreased, the balance between the quantity of light emitted therefrom and the quantity of light emitted from a blue LED chip cannot be maintained, and the wavelength of light emitted from a white LED is liable to shift to a blue color side. As a result, for example, when a white LED is used for a backlight of a display device, unfavorably, the balance of color to be displayed by the display device is not maintained.
Furthermore, when the luminous efficiency of a YAG fluorescent substance is decreased by an increase in temperature thereof, and a blue color component emitted from a blue LED chip is increased, the color temperature of light obtained by synthesis with light emitted from a YAG fluorescent substance is increased, and bluish light having a cold feeling starts to be emitted from a white LED, so that this type of white LED is not easily used as a lighting apparatus.
The present invention has been conceived to solve the above-described problems and provides a light emitting device which can suppress a decrease in luminous efficiency of a fluorescent substance when a use environmental temperature is increased and which can suppress a significant change in luminescent color obtained by synthesis between color of light emitted from a semiconductor light emitting element and color of light emitted from a fluorescent substance.
According to the present invention, there is provided a light emitting device including: a semiconductor light emitting element; electrodes supplying electricity to the semiconductor light emitting element; and a fluorescent layer covering a light emitting side of the semiconductor light emitting element. In the above light emitting device, the fluorescent layer includes: fluorescent particles emitting light by light emitted from the semiconductor light emitting element; transparent fine particles which adhere to the outsides of the fluorescent particles, and air layers formed between the fluorescent particles and the fine particles and between the fine particles.
In the light emitting device according to the present invention, the fine particles adhere to the outsides of the fluorescent particles, and the air layers (which are preferably air layers each forming a perfectly sealed closed space) are formed around the fluorescent particles. Since the air layers each function as a heat insulating layer, even when the use environment temperature is increased, an increase in temperature of the fluorescent particles can be suppressed, and a decrease in luminous efficiency thereof can be suppressed. Hence, the variation in luminescent color obtained by synthesis between the color of light emitted from the semiconductor light emitting element and the color of light emitted from the fluorescent particles can be suppressed.
The air layers preferably have a spatial length of 100 nm or less. Since the mean free path of nitrogen under the atmospheric pressure is approximately 100 nm or is slightly smaller than that, when the spatial length of the air layer is set to be smaller than the above free mean path, the heat insulating effect of the air layer can be enhanced. In addition, the spatial length of the air layer is more preferably 80 nm or less.
In addition, the fluorescent particles provided with the fine particles which adhere to the outsides thereof preferably agglomerate in at least a part of the fluorescent layer.
Since a plurality of fluorescent particles is made to agglomerate, temperature transmission efficiency to the fluorescent particles is decreased, and when the use environment temperature is increased, an increase in temperature of each fluorescent particle can be more easily suppressed.
In addition, according to the present invention, for example, the fluorescent layer preferably further includes a transparent synthetic resin besides the fluorescent particles and the fine particles. The synthetic resin described above includes, for example, an epoxy resin, a polyallylamine (PAA), and a silicone resin.
In addition, according to the present invention, with an intermolecular bonding force generated by applying mechanical energy, preferably, the fluorescent particles and the fine particles are bonded together, and the fine particles are bonded to each other.
For example, according to the present invention, preferably, the semiconductor light emitting element emits blue light, and the fluorescent particles emit yellow light.
In the light emitting device according to the present invention, even when the use environment temperature is increased, the balance of luminescent color is not likely to be degraded. In addition, even when the use environment temperature is increased, an increase in color temperature of emitted light can be suppressed.
The light emitting device 1 includes the chip type semiconductor light emitting element 10. The semiconductor light emitting element 10 is formed by a thin film process. As shown in
On a surface of the n-type clad layer 13, an active layer 14 is formed to have a close contact therewith. This active layer 14 is formed of n-type indium gallium nitride (InGaN) or is formed of a laminate film including n-type InGaN doped with Si and InGaN, and the overall film thickness thereof is approximately 400 Å. On a surface of the active layer 14, a p-type clad layer 15 is formed to have a close contact therewith. The p-type clad layer 15 is formed of aluminum gallium nitride (AlGaN) or is formed of AlGaN and GaN, and the thickness thereof is approximately 0.5 μm. Furthermore, on a surface of the p-type clad layer 15, a p-type contact layer is formed (not shown).
At one side of the semiconductor light emitting element 10, the n-type contact layer 12 is partly exposed, and on the surface of the exposed portion of the n-type contact layer 12, an n electrode 16 is formed. In addition, on the surface of the p-type contact layer, a p electrode 17 is formed at a position which is not located in a light emission region. The n electrode 16 and the p electrode 17 are each formed of Ni/Au (that is, a laminate of nickel and gold).
When a positive potential is applied to the p electrode 17 of the semiconductor light emitting element 10, and a forward current is supplied to the semiconductor light emitting element 10 having a pn-junction, free electrons, which are negative charges in the n-type clad layer 13 and free holes in the p-type clad layer 15 are recombined in the active layer 14, and by the energy generated thereby, light is emitted. The wavelength of light emitted from the semiconductor light emitting element 10 primarily formed of GaN is 530 nm or less, and light in bands from green to blue and further to ultraviolet can be emitted; however, in this embodiment, blue light having a wavelength of 160 to 470 nm is emitted.
In addition, as the semiconductor light emitting element, a transparent electrode formed of indium tin oxide (ITO) or the like may be formed as the p electrode 17 on the surface of the p-type clad layer 15 or the surface of the p-type contact layer covering the p-type clad layer 15.
In the light emitting device 1 shown in
The package material 4 is also used as a reflector, and the surface thereof functions as a reflection surface 4a. This reflection surface 4a is formed so that an opening area thereof is gradually increased toward a light emission direction.
In addition, on the above reflection surface 4a, a fluorescent layer 20 covering the semiconductor light emitting element 10 is provided.
The fluorescent layer 20 is formed of a transparent synthetic resin material, such as an epoxy resin, a polyallylamine (PAA), or a silicone resin, and fluorescent particles 21 mixed therewith. As shown in
The fluorescent particles 21 absorb light emitted from the semiconductor light emitting element 10, and internal molecules are excited by the absorbed light, so that light having a wavelength different from that of the absorbed light is emitted. In this embodiment, the fluorescent particles 21 are formed of a YAG fluorescent substance and emit yellow light since being excited by the light emitted from the semiconductor light emitting element 10. The average particle diameter of the fluorescent particles 21 is approximately 5 to 20 μm.
As shown in
Since many fine particles 22 adhere to the outside of the fluorescent particle 21, between the fluorescent particle 21 and the fine particles 22 and between the fine particles 22, a plurality of air layers 23 is formed. The air layers 23 each function as a heat insulating layer, and when the outside temperature is increased, the temperature of the fluorescent particle 21 is suppressed from being increased. When the air layers 23 are each made to function as a heat insulating layer, almost all air layers 23 are preferably formed in respective closed spaces, that is, the peripheries thereof are each preferably closed. Incidentally, the mean free path of a nitrogen molecule under the atmospheric pressure (1 atmospheric pressure) is approximately 100 nm or is slightly smaller than that. Hence, when the maximum spatial length δmax of one air layer 23 is 100 nm or less, the transmission of heat in the air layer 23 can be decreased, and hence the heat insulating effect of the air layer 23 can be enhanced. In addition, the ratio of the number of air layers having a maximum spatial length δmax of 100 nm or less with respect to that of all the air layers 23 is preferably 50% or more and more preferably 80% or more. Furthermore, it is more preferable that 50% or more or 80% or more of the air layers 23 have a maximum spatial length δmax of 80 nm or less.
After a mixed liquid formed by mixing the fluorescent particles 21 and the fine particles 22 shown in
In this light emitting device 1, when a voltage is applied across the lead terminals 5 and 6, and a forward current is supplied to the semiconductor light emitting element 10, blue or blue base light is emitted therefrom. In this embodiment, a blue light having a wavelength of 460 to 470 nm is emitted. In addition, the fluorescent particles 21 absorb the above light and are excited thereby, so that yellow or yellow base light is emitted. Since the blue or blue base light passing through the layer of the synthetic resin material and the yellow or yellow base light emitted from the fluorescent particles 21 are synthesized, white or white base color is emitted from the light emitting device 1.
When a relatively large current is supplied to the semiconductor light emitting element 10 in order to emit light having a high output, the semiconductor light emitting element 10 is heated, and this heat is transmitted to the fluorescent layer 20. In addition, when the use environment temperature is increased, the fluorescent layer 20 is heated to a high temperature. When the temperature of the fluorescent particles 21 formed of a YAG fluorescent substance or the like is increased, the luminous efficiency is decreased, and as a result, as for the light emitted from the light emitting device 1, the quantity of light emitted from the fluorescent particles 21 is decreased with respect to the quantity of light emitted from the semiconductor light emitting element 10; hence, the chromaticity and the color temperature of light synthesized between the above two types of light are liable to vary. However, in the light emitting device 1, as shown in
In the light emitting device 1 of the example, as the semiconductor light emitting element 10, an element emitting blue light having a wavelength 460 to 470 nm was used. As the fluorescent particles 21, a YAG fluorescent substance having an average particle diameter of 8 μm was used, and as the fine particles 22, silica (SiO2) having an average particle diameter of 0.1 μm was used. By using “Nano-particle composite production system (Model: NC-LAB-P)” manufactured by Hosokawa Micron Group, the fluorescent particles 21 and the fine particles 22 were processed to form a composite.
The bonding state between the fluorescent particles 21 and the fine particles 22, which formed the composite, was observed by a scanning electron microscope (SEM), and it was confirmed that the fine particles 22 adhered to the outsides of the fluorescent particles 21 to form five layers on an average, and that the maximum spatial length δmax of each air layer 23 was in the range of 50 to 60 nm.
After the fluorescent particles 21 provided with the fine particles 22 which adhered to the outsides thereof were mixed in a pre-cured epoxy resin and were then stirred in a ball mill, a liquid thus stirred was potted on the surface of the semiconductor light emitting element 10, and the epoxy resin was cured by a heat treatment, so that the fluorescent layer 20 was formed. The ratio of the fluorescent particles 21 and the fine particles 22 in the mixed liquid including the pre-cured epoxy resin, the fluorescent particles 21, and the fine particles 22 was set to 50 percent by weight. In addition, after the cured fluorescent layer 20 was cut off, the cross-section thereof was observed by a scanning electron microscope, and it was confirmed that almost all fluorescent particles 21 agglomerated to each other. In addition, it was also confirmed that the thickness dimension from the light emitting surface of the semiconductor light emitting element 10 to the surface of the fluorescent layer 20 was 100 μm.
The same light emitting device as that in the above example was used for the comparative example except that the fluorescent particles 21 were not provided with the fine particles 22 so that the fluorescent layer was formed only from the epoxy resin and the fluorescent particles. The ratio of the fluorescent particles 21 in the mixed liquid of the epoxy resin and the fluorescent particles 21 was set to the same as that in the above example. In addition, the thickness of the fluorescent layer was set to the same as that of the above example.
Forward currents of 1 mA, 5 mA, 20 mA, 50 mA, and 100 mA were supplied to the light emitting devices of the example and the comparative example, and the changes, on the chromatic coordinates, in light emitted from the devices of the example and the comparative example were measured at the respective currents by a color meter.
When a forward current of 20 mA was supplied to the light emitting device of each of the example and the comparative example, and the environment temperatures were stabilized at −40° C., −30° C., 0° C., 25° C., 50° C., and 85° C., the changes, on the chromatic coordinates, of light emitted from the devices of the example and the comparative example were measured by a color meter.
According to the evaluation method A shown in
Accordingly, in the example, when a large current is supplied to the semiconductor light emitting element, the color temperature of luminescent color can be suppressed from being increased, and the state in which light having a cold feeling is emitted can be suppressed.
According to the evaluation method B shown in
| Number | Date | Country | Kind |
|---|---|---|---|
| 2006-089026 | Mar 2006 | JP | national |
This application is a continuation of International Application No. PCT/JP2007/054889, filed Mar. 13, 2007, which claims benefit to the Japanese Patent Application No. 2006-089026 filed on Mar. 28, 2006, both of which are incorporated herein by reference in their entirety.
| Number | Date | Country | |
|---|---|---|---|
| Parent | PCT/JP2007/054889 | Mar 2007 | US |
| Child | 12211362 | US |
