Light emitting device

A light emitting device of the present invention includes an LED substrate and a sealing resin portion which seals the LED substrate, the sealing resin portion having a silicone resin having a refractive index n3 to which a fluorescent material having a refractive index n1 and fine particles having a refractive index n2 are added. In the light emitting device, a relationship of n2>n1>n3 holds in the refractive indexes n1 to n3, and a particle size of the fine particles is not more than 1/10 of a wavelength of light emitted from the LED substrate.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority from prior Japanese Patent Application No. 2005-236006, filed Aug. 16, 2005, the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a light emitting device which can be used as various illuminating light sources, and particularly, to a light emitting device in which light extraction efficiency is improved.

2. Description of the Related Art

An LED lamp (light emitting device) in which an LED (light emitting diode) is used can be formed in a compact size with food electric power efficiency and the LED lamp emits bright color light with long life. Furthermore, the LED lamp has vibration-resistant property and is strong in a repeat of turn on/off. Therefore, the LED lamp is frequently used as various illuminating light sources (for example, refer to Jpn. Pat. Appln. KOKAI Publication No. 2005-19663).

FIG. 4shows a conventional LED lamp. An LED lamp100includes an LED substrate101serving as a light emitting element and a sealing resin102made of silicone or the like for sealing the LED substrate101. A fluorescent material103is mixed in the sealing resin102, and various colors can be developed by combination of a color of the LED substrate101and a color of the fluorescent material103.

In the conventional LED lamp, there is the following problem. The fluorescent material103for use in a white LED has a refractive index n ranging from 1.7 to 1.8. On the other hand, the sealing resin102in which the fluorescent material103is dispersed has a refractive index n ranging from 1.4 to 1.5. For this reason, the emitted light is scattered by the fluorescent material103, the light impinges on a wall surface of a package or the fluorescent material103of itself many times, and light intensity is attenuated, which results in a decrease in light extraction efficiency.

In order to suppress the scattering of the fluorescent material103, there is a method of forming the fluorescent material103having particle sizes not more than 50 nm. This is because, when the fluorescent material103is formed in the particle sizes not more than 50 nm, the scattering is hardly generated even if the difference in refractive index is generated. There is also a method of decreasing the difference in refractive index between the sealing resin and the fluorescent material to suppress the scattering with a resin such as epoxy having the high refractive index instead of silicone. There is also a method in which the resin is not used but the fluorescent material is used by sintering the fluorescent material.

However, in the above methods, there are the following problems. That is, the highly efficient fluorescent material having the small particle size is hardly produced, the epoxy resin is easily degraded by heat or an ultraviolet ray compared with silicone, and the sintered body is hardly realized in a Sr2SiO4material.

On the other hand, in a conventional LED lamp110shown inFIG. 5, usually the refractive index of the LED substrate111is higher than that of the sealing resin112. As a consequence, in the light emitted from an active layer of the LED substrate111, total reflection is generated at an interface between the LED substrate111and the sealing resin112, which results in the problem that the light is confined in the LED substrate111to decrease the light extraction efficiency.

BRIEF SUMMARY OF THE INVENTION

In view of the foregoing, an object of the invention is to provide a light emitting device, in which the total reflection is suppress when light is outputted to a resin from a light emitting element substrate while light scattering is suppressed by a fluorescent material and thereby the light extraction efficiency can be improved.

In order to solve the problem and achieve the object, a light emitting device according to the invention is configured as follows.

A light emitting device according to one aspect of the invention comprises: a light emitting element substrate; and a sealing resin portion which seals the light emitting element substrate, the sealing resin portion having a resin to which a fluorescent material having a refractive index n1and fine particles having a refractive index n2are added, the resin having a refractive index n3, wherein a relationship of n2>n1>n3holds in the refractive indexes n1to n3, and a particle size of the fine particles is not more than 1/10 of a wavelength of light emitted from the light emitting element substrate.

A light emitting device according to another aspect of the invention comprises: a light emitting element substrate having a refractive index n4; a sealing resin portion which seals the light emitting element substrate, the sealing resin portion having a resin to which fine particles having a refractive index n5are added, the resin having a refractive index n6, wherein a relationship of n5>n6holds in the refractive indexes n5and n6, and a volume ratio a of the fine particles to the resin satisfies n42=a·n52+(1−a)·n52.

According to the invention, the total reflection is suppress when the light is outputted to the resin from the light emitting element substrate while the light scattering is suppressed by the fluorescent material. Therefore, the light extraction efficiency can be improved.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1shows an LED lamp (light emitting device)10according to a first embodiment of the invention. The LED lamp10includes an LED substrate (light emitting element substrate)20and a sealing resin portion30which seals the LED substrate20. The LED substrate20is formed by a sapphire substrate (refractive index is 1.77). In the LED substrate20, an InGaN active layer emits a blue light having a wavelength of 460 to 480 nm or an ultraviolet ray having a wavelength of 360 to 400 nm. The sealing resin portion30is formed by a silicone resin31, a fluorescent material32, and a fine particle33. The fluorescent material32is added to the silicone resin31.

The fluorescent material32includes a Eu:Sr2SiO4or Ce:YAG fluorescent material having a diameter of 1 to 10 μm, and the fine particle33includes TiO2(titanium oxide) having a diameter of 30 nm. A refractive index n1of the fluorescent material32ranges from about 1.7 to about 1.8, and a refractive index n3of silicone resin used in sealing the LED (light emitting element) usually ranges from 1.4 to 1.5.

In order to eliminate the light scattering to improve the light extraction efficiency of the white LED, the fine particles33are used to bring the refractive index of the sealing resin portion30close to the refractive index of the fluorescent material32. That is, because the refractive index of the resin in itself is hardly increased and brought close to the refractive index of the fluorescent material32, the effective refractive index is increased by dispersing the fine particles33having a high refractive index in the silicone resin31having a low refractive index.

A refractive index n0of the sealing resin portion30having the fine particles33dispersed therein is expressed by n02=a·n22+(1−a)·n32, where “a” is a volume ratio of the fine particles33to the silicone resin31, n2is a refractive index of the fine particle, and n3is a refractive index of the silicone resin31.

On the other hand, the light scattering is increased as the particle size of the fine particle33becomes larger. For this reason, it is necessary that the fine particle33is formed in the sufficiently smaller particle size when compared with the light wavelength of 400 to 800 nm. Specifically, the particle size of the fine particle33is formed not more than 50 nm, and desirably in the range of 10 to 20 nm. This is because as shown inFIG. 2, when the size of the fine particle (refractive index n2) dispersed in the silicone resin (refractive index n3) is not more than 50 nm, the scattering for the visible light is eliminated at the particle boundary in the sealing resin portion30, and the sealing resin portion30apparently exhibits characteristics as a bulk material in which a bulk resin and the fine particles33are mixed. InFIG. 2, a thickness of the sealing resin portion30is 600 nm, the light wavelength is wavelength 500 nm, and data is obtained in the case where the volume ratio a ranges from 0.1 to 0.4.

In this case, the refractive index n0of the bulk material, i.e., the whole of the sealing resin portion30becomes n0=√(a·n22+(1−a)·n32), where “a” is a volume ratio of the fine particle33to the silicone resin31. When the refractive index n0of the bulk material is equalized to the refractive index n1of the fluorescent material32, the light extraction efficiency is improved because the scattering is not generated. Accordingly, in order to improve the light extraction efficiency, it is necessary to satisfy both the conditions of n12=a·n22+(1−a)·n32and n2>n1>n3.

When TiO2(refractive index n2=2.7) is used as the fine particle33while silicone (refractive index n3=1.44) is used as the resin, nx=1.8 for a=0.22. When Ce=YAG (n1=1.8) is used as the fluorescent material32, the scattering caused by the fluorescent material32can be suppressed at a minimum.

Furthermore, a difference in refractive index is small between the LED substrate20(refractive index 1.77) and the sealing resin portion30(refractive index 1.8), so that a reflection generated between the LED substrate20and the sealing resin portion30can also be suppressed at a minimum.

Thus, according to the LED lamp10of the first embodiment, the scattering caused by the fluorescent material32can be suppressed at a minimum in the sealing resin portion30, and the reflection generated between the LED substrate20and the sealing resin portion30can also be suppressed at a minimum. As a consequence, the light extraction efficiency to the outside from the LED substrate20can be increased. Furthermore, because the refractive index is adjusted by adding the fine particle to the resin, the production cost is low and the technical difficulty is also low.

FIG. 3is a vertical sectional view schematically showing an LED lamp40according to a second embodiment of the invention. The LED lamp40includes an LED substrate50and a sealing resin portion60which seals the LED substrate50. The LED substrate50is formed by a sapphire substrate (refractive index n4=1.77). In the LED substrate50, a InGaN active layer emits a blue light having a wavelength of 460 to 480 nm or an ultraviolet ray having a wavelength of 360 to 400 nm. The sealing resin portion60is formed by a silicone resin61, a fluorescent material62, and a fine particle63. The fluorescent material62is added to the silicone resin61. The fine particle62is made of TiO2having the diameter of 30 nm.

When the size of the fine particle62(refractive index n5) dispersed in the silicone resin61(refractive index n6) is not more than 50 nm, the scattering for the visible light is eliminated at the particle boundary, and the sealing resin portion60apparently exhibits the characteristics as the bulk material in which the bulk resin and the fine particles are mixed. In this case, a refractive index nyof the bulk material, becomes ny2=(a·n52+(1−a)·n62), where “a” is a volume ratio.

When the refractive index nyof the bulk material, i.e., the sealing resin portion60is approximated to the refractive index n4(=1.77) of the LED substrate50, the light extraction efficiency to the sealing resin portion60from the LED substrate50is improved because the total reflection is eliminated at the interface between the LED substrate50and the sealing resin portion60. Accordingly, in order to improve the light extraction efficiency, it is necessary to satisfy both the conditions of n42=a·n52+(1−a)·n52and n5>n6.

In the case where TiO2(refractive index n5=2.7) having the particle size of 30 nm is used as the fine particle62while silicone (refractive index n6=1.44) is used as the resin, the refractive index nyof the sealing resin portion60can be set at 1.77 by setting the volume concentration of the fine particle62at 20% (a=0.2). Therefore, the refractive index ny=1.77 of the sealing resin portion60is approximated to the refractive index n4of the LED substrate50.

Thus, according to the LED lamp40of the second embodiment, the reflection generated between the LED substrate50and the sealing resin portion60can also be suppressed at a minimum by decreasing the difference in refractive index between the LED substrate50(refractive index is 1.77) and the sealing resin portion60(refractive index is 1.8). Furthermore, because the refractive index is adjusted by adding the fine particle to the resin, the production cost is low and the technical difficulty is also low.

In the above embodiments, TiO2is used as the fine particles33and62. However, ZrO2(zirconium oxide), ZnO (zinc oxide), and HfO2(hafnium oxide) which have the refractive index of 2.0 and Al2O3(aluminum oxide) having a refractive index of 1.7 may be used as the fine particles33and62.

The invention is not limited to the above embodiments, but various modifications could be made without departing from the scope and spirit of the invention. Various modifications and changes could be made by appropriately combining the plural components disclosed in the embodiments. For example, some components may be removed from all the components shown in the embodiments. The components in the embodiments may appropriately be combined.