Light-emitting element, light-emitting device, electronic device, and lighting device
A light-emitting layer, which is a stack of a first light-emitting layer and a second light-emitting layer, is provided between an anode and a cathode. The first light-emitting layer is formed on the anode side and contains a first light-emitting substance converting triplet excitation energy into light emission, a first organic compound having an electron-transport property, and a second organic compound having a hole-transport property. The second light-emitting layer contains a second light-emitting substance converting triplet excitation energy into light emission, the first organic compound, and a third organic compound having a hole-transport property. The second organic compound has a lower HOMO level than the third organic compound. The first light-emitting substance emits light with a wavelength shorter than that of light emitted from the second light-emitting substance. The first and the second organic compounds form an exciplex. The first and the third organic compounds form an exciplex.
1. A light-emitting element comprising:
an anode and a cathode;
a first light-emitting layer over the anode, the first light-emitting layer comprising a first light-emitting substance, a first organic compound, and a second organic compound; and
a second light-emitting layer over the first light-emitting layer, the second light-emitting layer comprising a second light-emitting substance, the first organic compound, and a third organic compound,
wherein a HOMO level of the second organic compound is lower than a HOMO level of the third organic compound,
wherein the first organic compound and the second organic compound form an exciplex, and
wherein an energy of the exciplex is transferred from a singlet state to the first light-emitting substance.
2. The light-emitting element according to claim 1 , wherein an emission spectrum of the exciplex overlaps with an absorption band having the longest wavelength of the first light-emitting substance.
3. The light-emitting element according to claim 1 , wherein the first light-emitting layer is in contact with the second light-emitting layer.
4. The light-emitting element according to claim 1 , wherein the first light-emitting substance is a phosphorescent compound or a thermally activated delayed fluorescence material.
5. The light-emitting element according to claim 1 , wherein the second light-emitting substance is a phosphorescent compound or a thermally activated delayed fluorescence material.
6. A light-emitting device comprising:
a transistor over a substrate; and
the light-emitting element according to claim 1 over the transistor.
7. A light-emitting element comprising:
an anode and a cathode;
a first light-emitting layer over the anode, the first light-emitting layer comprising a first light-emitting substance, a first organic compound, and a second organic compound; and
a second light-emitting layer over the first light-emitting layer, the second light-emitting layer comprising a second light-emitting substance, the first organic compound, and a third organic compound,
wherein a HOMO level of the second organic compound is lower than a HOMO level of the third organic compound,
wherein the first organic compound and the third organic compound form an exciplex, and
wherein an energy of the exciplex is transferred from a singlet state to the second light-emitting substance.
8. The light-emitting element according to claim 7 , wherein an emission spectrum of the exciplex overlaps with an absorption band having the longest wavelength of the second light-emitting substance.
9. The light-emitting element according to claim 7 , wherein the first light-emitting layer is in contact with the second light-emitting layer.
10. The light-emitting element according to claim 7 , wherein the first light-emitting substance is a phosphorescent compound or a thermally activated delayed fluorescence material.
11. The light-emitting element according to claim 7 , wherein the second light-emitting substance is a phosphorescent compound or a thermally activated delayed fluorescence material.
12. A light-emitting device comprising:
a transistor over a substrate; and
the light-emitting element according to claim 7 over the transistor.
13. A light-emitting element comprising:
an anode and a cathode;
a first light-emitting layer over the anode, the first light-emitting layer comprising a first light-emitting substance and a first organic compound; and
a second light-emitting layer over the first light-emitting layer, the second light-emitting layer comprising a second light-emitting substance, the first organic compound, and a second organic compound,
wherein a LUMO level of the first organic compound is lower than a LUMO level of the second organic compound, and
wherein the first organic compound and the second organic compound form an exciplex.
14. The light-emitting element according to claim 13 , wherein an emission spectrum of the exciplex overlaps with an absorption band having the longest wavelength of the second light-emitting substance.
15. The light-emitting element according to claim 13 , wherein the first light-emitting layer is in contact with the second light-emitting layer.
16. The light-emitting element according to claim 13 , wherein the first light-emitting substance is a phosphorescent compound or a thermally activated delayed fluorescence material.
17. The light-emitting element according to claim 13 , wherein the second light-emitting substance is a phosphorescent compound or a thermally activated delayed fluorescence material.
18. A light-emitting device comprising:
a transistor over a substrate; and
the light-emitting element according to claim 13 over the transistor.
19. A light-emitting element comprising:
an anode and a cathode;
a first light-emitting layer over the anode, the first light-emitting layer comprising a first light-emitting substance, a first organic compound, and a second organic compound; and
a second light-emitting layer over the first light-emitting layer, the second light-emitting layer comprising a second light-emitting substance, the first organic compound, and a third organic compound,
wherein a HOMO level of the second organic compound is lower than a HOMO level of the third organic compound,
wherein the first organic compound and the second organic compound form a first exciplex,
wherein the first organic compound and the third organic compound form a second exciplex,
wherein an energy of the first exciplex is transferred from a singlet state to the first light-emitting substance, and
wherein an energy of the second exciplex is transferred from a singlet state to the second light-emitting substance.
20. The light-emitting element according to claim 19 , wherein an emission spectrum of the first exciplex overlaps with an absorption band having the longest wavelength of the first light-emitting substance.
21. The light-emitting element according to claim 19 , wherein the first light-emitting layer is in contact with the second light-emitting layer.
22. The light-emitting element according to claim 19 , wherein the first light-emitting substance is a phosphorescent compound or a thermally activated delayed fluorescence material.
23. The light-emitting element according to claim 19 , wherein the second light-emitting substance is a phosphorescent compound or a thermally activated delayed fluorescence material.
24. A light-emitting device comprising:
a transistor over a substrate; and
the light-emitting element according to claim 19 over the transistor.