Light-emitting element, light-emitting device, display device, electronic device, and lighting device
View Patent ↗An object is to provide a light-emitting element which uses a plurality of kinds of light-emitting dopants and has high emission efficiency. In one embodiment of the present invention, a light-emitting device, a light-emitting module, a light-emitting display device, an electronic device, and a lighting device each having reduced power consumption are provided by using the above light-emitting element. Focus is placed on Förster mechanism, which is one of mechanisms of intermolecular energy transfer. Efficient energy transfer by Förster mechanism is achieved by employing a combination of molecules which makes it possible to obtain an overlap between an emission spectrum band of the molecule which donates energy and the longest-wavelength-side peak of a characteristic curve obtained by multiplying an absorption spectrum of the molecule which receives energy by a wavelength raised to the fourth power.
1. A light-emitting element comprising:
a first electrode;
a first light-emitting layer over the first electrode comprising:
a first phosphorescent compound; and
a first host material;
a second light-emitting layer over the first light-emitting layer comprising:
a second phosphorescent compound; and
a second host material;
a third light-emitting layer over the second light-emitting layer comprising:
a third phosphorescent compound; and
a third host material; and
a second electrode over the third light-emitting layer,
wherein light emitted from the second phosphorescent compound has a longer wavelength than light emitted from the first phosphorescent compound,
wherein a longest-wavelength-side peak of a function ε(λ)λ 4 of the second phosphorescent compound overlaps with a phosphorescence spectrum F(λ) of the first phosphorescent compound,
wherein light emitted from the third phosphorescent compound has a longer wavelength than the light emitted from the second phosphorescent compound,
wherein a longest-wavelength-side peak of a function ε(λ)λ 4 of the third phosphorescent compound overlaps with a phosphorescence spectrum F(λ) of the second phosphorescent compound,
wherein the second light-emitting layer has a thickness of less than 5 nm and greater than or equal to 1 nm,
wherein λ denotes a wavelength, and
wherein ε(λ) denotes a molar absorption coefficient at the wavelength λ.
2. The light-emitting element according to claim 1 , wherein the second light-emitting layer has a thickness of less than or equal to 2 nm and greater than or equal to 1 nm.
3. The light-emitting element according to claim 1 ,
wherein the first host material has an electron-transport property, and
wherein the second host material and the third host material each have a hole-transport property.
4. The light-emitting element according to claim 1 ,
wherein the first host material has a hole-transport property, and
wherein the second host material and the third host material each have an electron-transport property.
5. The light-emitting element according to claim 1 , wherein the longest-wavelength-side peak of the function ε(λ)λ 4 of the third phosphorescent compound is larger than the longest-wavelength-side peak of the function ε(λ)λ 4 of the second phosphorescent compound.
6. A lighting device comprising the light-emitting element according to claim 1 .
7. A light-emitting device comprising:
the light-emitting element according to claim 1 ; and
a driver circuit configured to control the light-emitting element.
8. A display device comprising:
the light-emitting element according to claim 1 in a display portion; and
a driver circuit configured to control the light-emitting element.
9. An electronic device comprising the light-emitting element according to claim 1 .
10. A light-emitting element comprising:
a first electrode;
a first light-emitting layer over the first electrode comprising:
a first phosphorescent compound; and
a first host material;
a plurality of second island-like light-emitting regions over the first light-emitting layer comprising:
a second phosphorescent compound; and
a second host material;
a third light-emitting layer over the plurality of second island-like light-emitting regions comprising:
a third phosphorescent compound; and
a third host material; and
a second electrode over the third light-emitting layer,
wherein light emitted from the second phosphorescent compound has a longer wavelength than light emitted from the first phosphorescent compound,
wherein a longest-wavelength-side peak of a function ε(λ)λ 4 of the second phosphorescent compound overlaps with a phosphorescence spectrum F(λ) of the first phosphorescent compound,
wherein light emitted from the third phosphorescent compound has a longer wavelength than the light emitted from the second phosphorescent compound,
wherein a longest-wavelength-side peak of a function ε(λ)λ 4 of the third phosphorescent compound overlaps with a phosphorescence spectrum F(λ) of the second phosphorescent compound,
wherein λ denotes a wavelength, and
wherein ε(λ) denotes a molar absorption coefficient at the wavelength λ.
11. The light-emitting element according to claim 10 ,
wherein the first host material has an electron-transport property, and
wherein the second host material and the third host material each have a hole-transport property.
12. The light-emitting element according to claim 10 ,
wherein the first host material has a hole-transport property, and
wherein the second host material and the third host material each have an electron-transport property.
13. The light-emitting element according to claim 10 , wherein the longest-wavelength-side peak of the function ε(λ)λ 4 of the third phosphorescent compound is larger than the longest-wavelength-side peak of the function ε(λ)λ 4 of the second phosphorescent compound.
14. A lighting device comprising the light-emitting element according to claim 10 .
15. A light-emitting device comprising:
the light-emitting element according to claim 10 ; and
a driver circuit configured to control the light-emitting element.
16. A display device comprising:
the light-emitting element according to claim 10 in a display portion; and
a driver circuit configured to control the light-emitting element.
17. An electronic device comprising the light-emitting element according to claim 10 .
18. A light-emitting element comprising:
a first electrode;
a first light-emitting layer over the first electrode comprising:
a first phosphorescent compound; and
a first host material;
a mixed layer over the first light-emitting layer comprising:
a plurality of second island-like light-emitting regions comprising:
a second phosphorescent compound; and
a second host material;
a plurality of third island-like light-emitting regions comprising:
a third phosphorescent compound; and
a third host material; and
a second electrode over the mixed layer,
wherein light emitted from the second phosphorescent compound has a longer wavelength than light emitted from the first phosphorescent compound,
wherein a longest-wavelength-side peak of a function ε(λ)λ 4 of the second phosphorescent compound overlaps with a phosphorescence spectrum F(λ) of the first phosphorescent compound,
wherein light emitted from the third phosphorescent compound has a longer wavelength than light emitted from the second phosphorescent compound,
wherein a longest-wavelength-side peak of a function ε(λ)λ 4 of the third phosphorescent compound overlaps with a phosphorescence spectrum F(λ) of the second phosphorescent compound,
wherein λ denotes a wavelength, and
wherein ε(λ) denotes a molar absorption coefficient at the wavelength λ.
19. The light-emitting element according to claim 18 ,
wherein the first host material has an electron-transport property, and
wherein the second host material and the third host material each have a hole-transport property.
20. The light-emitting element according to claim 18 ,
wherein the first host material has a hole-transport property, and
wherein the second host material and the third host material each have an electron-transport property.
21. The light-emitting element according to claim 18 , wherein the longest-wavelength-side peak of the function ε(λ)λ 4 of the third phosphorescent compound is larger than the longest-wavelength-side peak of the function ε(λ)λ 4 of the second phosphorescent compound.
22. A lighting device comprising the light-emitting element according to claim 18 .
23. A light-emitting device comprising:
the light-emitting element according to claim 18 ; and
a driver circuit configured to control the light-emitting element.
24. A display device comprising:
the light-emitting element according to claim 18 in a display portion; and
a driver circuit configured to control the light-emitting element.
25. An electronic device comprising the light-emitting element according to claim 18 .