Light-emitting device including light-emitting diode

• US 20090242917A1
• Filed: 03/06/2009
• Published: 10/01/2009
• Est. Priority Date: 03/28/2008
• Status: Active Grant

• Current Assignee: Toyoda Gosei Company Limited (Toyota Motor Corporation)
• Original Assignee: Toyoda Gosei Company Limited (Toyota Motor Corporation)
• Inventors: Kato, Satoru, Kachi, Tetsu, Inoue, Mitsuhiro, Kozawa, Takahiro, Tomita, Kazuyohi, Ito, Kenji, Ichikawa, Tadashi
• Granted Patent: US 7,897,987 B2
• Time in Patent Office: Days
• US Class Current: 257/98
• CPC Class Codes: H01L 2224/48091 Arched
• CPC Class Codes: H01L 2924/00014 the subject-matter covered by the group, the symbol of which is combined with the symbol of this group, being disclosed without further technical details
• CPC Class Codes: H01L 33/504 Elements with two or more wavelength conversion materials

Abstract

A light-emitting device includes a light-emitting diode, a red light-emitting phosphor layer, a yellow light-emitting phosphor layer, and a blue light-emitting phosphor layer. These layers are stacked in the stacking sequence of the yellow, blue, and red phosphor layers in order of increasing distance from the LED. The stacking sequence of the yellow and blue phosphor layers is first determined in such a manner that these layers do not interact with each other. The stacking sequence of the red and yellow phosphor layers and the stacking sequence of the red and blue phosphor layers are determined by the discriminant D. This determination of the stacking sequence suppresses a reduction in the conversion efficiency of the phosphors due to concentration quenching, improving the emission efficiency of the light-emitting device.

Claims

1. A light-emitting device comprising:

a light-emitting diode; and

a plurality of stacked phosphor layers, each of the phosphor layers being configured to emit light having a color different from the color of light emitted from another phosphor layer, and the stacking sequence of the plurality of phosphor layers being a sequence such that phosphors constituting the phosphor layers do not interact with each other,

wherein among the phosphor layers, any two phosphor layers A and B that do not interact with each other are arranged in such a manner that the phosphor layer A is further from the light-emitting diode than the phosphor layer B when a discriminant D>

1 and that the phosphor layer B is further from the light-emitting diode than the phosphor layer A when the discriminant D<

1, and the discriminant D is given by

D=k_B+(k_A−

k_B)/B+(1−

k_A)/AB

where k_A represents the rate of reduction in the conversion efficiency of the phosphor layer A when the stacking sequence of the phosphor layers A and B is changed from the stacking sequence of the phosphor layers A and B to the stacking sequence of the phosphor layers B and A in order of increasing distance from the light-emitting diode;

k_B represents the rate of reduction in the conversion efficiency of the phosphor layer B when the stacking sequence of the phosphor layers A and B is changed from the stacking sequence of the phosphor layers A and B to the stacking sequence of the phosphor layers B and A in order of increasing distance from the light-emitting diode;

A represents the ratio of the intensity of excitation light transmitted through the phosphor layer A to the intensity of excitation light incident on the phosphor layer A; and

B represents the ratio of the intensity of excitation light transmitted through the phosphor layer B to the intensity of excitation light incident on the phosphor layer B, with the proviso that each of the phosphor layers A and B has a phosphor content such that the emission intensity is unchanged when the stacking sequence of the phosphor layers A and B is changed.

2. The light-emitting device according to claim 1, wherein the light-emitting diode is configured to emit ultraviolet light.

3. The light-emitting device according to claim 1, wherein the colors of light emitted from the phosphor layers are mixed together to form white.

4. The light-emitting device according to claim 2, wherein the colors of light emitted from the phosphor layers are mixed together to form white.

5. The light-emitting device according to claim 3, wherein the phosphor layers are a red phosphor layer configured to emit red light, a yellow phosphor layer configured to emit yellow light, and a blue phosphor layer configured to emit blue light.

6. The light-emitting device according to claim 4, wherein the phosphor layers are a red phosphor layer configured to emit red light, a yellow phosphor layer configured to emit yellow light, and a blue phosphor layer configured to emit blue light.

7. The light-emitting device according to claim 1, wherein a yellow phosphor layer configured to emit yellow light, a blue phosphor layer configured to emit blue light and a red phosphor layer configured to emit red light are disposed in order of increasing distance from the light-emitting diode.

8. A method for determining the stacking sequence of phosphor layers of a light-emitting device including a light-emitting diode and a plurality of stacked phosphor layers, each of the phosphor layers being configured to emit light having a color different from the color of light emitted from another phosphor layer, and the stacking sequence of the phosphor layers being a sequence such that phosphors constituting the phosphor layers do not interact with each other, the method comprising:

determining the stacking sequence of any two phosphor layers A and B that do not interact with each other among the phosphor layers in such a manner that the phosphor layer A is further from the light-emitting diode than the phosphor layer B when a discriminant D>

1 and that the phosphor layer B is further from the light-emitting diode than the phosphor layer A when the discriminant D<

1,

wherein the discriminant D is given by

D=k_B+(k_A−

k_B)/B+(1−

k_A)/AB

where k_A represents the rate of reduction in the conversion efficiency of the phosphor layer A when the stacking sequence of the phosphor layers A and B is changed from the stacking sequence of the phosphor layers A and B to the stacking sequence of the phosphor layers B and A in order of increasing distance from the light-emitting diode;

k_B represents the rate of reduction in the conversion efficiency of the phosphor layer B when the stacking sequence of the phosphor layers A and B is changed from the stacking sequence of the phosphor layers A and B to the stacking sequence of the phosphor layers B and A in order of increasing distance from the light-emitting diode;

A represents the ratio of the intensity of excitation light transmitted through the phosphor layer A to the intensity of excitation light incident on the phosphor layer A; and

B represents the ratio of the intensity of excitation light transmitted through the phosphor layer B to the intensity of excitation light incident on the phosphor layer B, with the proviso that each of the phosphor layers A and B has a phosphor content such that the emission intensity is unchanged when the stacking sequence of the phosphor layers A and B is changed.