Light-emitting element, light-emitting device, electronic device, and lighting device

1. A light-emitting element comprising:

a light-emitting layer between an anode and a cathode,

wherein the light-emitting layer is a stack of a first light-emitting layer and a second light-emitting layer,

wherein the first light-emitting layer is located at the anode side,

wherein the first light-emitting layer comprises 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,

wherein the second light-emitting layer comprises a second light-emitting substance converting triplet excitation energy into light emission, the second organic compound, and a third organic compound having an electron-transport property,

wherein the first organic compound has a higher lowest unoccupied molecular orbital level than the third organic compound,

wherein the first light-emitting substance emits light with a wavelength shorter than a wavelength of light emitted from the second light-emitting substance,

wherein the first organic compound and the second organic compound form a first exciplex,

wherein the second organic compound and the third organic compound form a second exciplex,

wherein an emission spectrum of the first exciplex is overlapped with an absorption spectrum of the first light-emitting substance, and

wherein light emission from the first exciplex is substantially not observed when the first light-emitting layer emits a first light.

2. The light-emitting element according to claim 1 , wherein the first exciplex has higher excitation energy than the second exciplex.

3. The light-emitting element according to claim 1 ,

wherein the first exciplex is formed from an anion of the first organic compound and a cation of the second organic compound, and

wherein the second exciplex is formed from an anion of the third organic compound and a cation of the second organic compound.

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,

wherein the second light-emitting substance is a phosphorescent compound or a thermally activated delayed fluorescence material,

wherein the first organic compound is a π-electron deficient heteroaromatic compound,

wherein the second organic compound is a π-electron rich heteroaromatic compound or an aromatic amine compound, and

wherein the third organic compound is a π-electron deficient heteroaromatic compound.

5. A light-emitting device comprising the light-emitting element according to claim 1 .

6. An electronic device comprising the light-emitting device according to claim 5 .

7. A lighting device comprising the light-emitting device according to claim 5 .

8. A light-emitting element comprising:

a light-emitting layer between an anode and a cathode;

a hole-transport layer between the anode and the light-emitting layer; and

an electron-transport layer between the cathode and the light-emitting layer,

wherein the light-emitting layer is a stack of a first light-emitting layer and a second light-emitting layer,

wherein the first light-emitting layer is in contact with the hole-transport layer,

wherein the first light-emitting layer comprises 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,

wherein the second light-emitting layer is in contact with the electron-transport layer,

wherein the second light-emitting layer comprises a second light-emitting substance converting triplet excitation energy into light emission, the second organic compound, and a third organic compound having an electron-transport property,

wherein the first organic compound has a higher lowest unoccupied molecular orbital level than the third organic compound,

wherein the first light-emitting substance emits light with a wavelength shorter than a wavelength of light emitted from the second light-emitting substance,

wherein the first organic compound and the second organic compound form a first exciplex,

wherein the second organic compound and the third organic compound form a second exciplex,

wherein an emission spectrum of the first exciplex is overlapped with an absorption spectrum of the first light-emitting substance, and

wherein light emission from the first exciplex is substantially not observed when the first light-emitting layer emits a first light.

9. The light-emitting element according to claim 8 , wherein the first exciplex has higher excitation energy than the second exciplex.

10. The light-emitting element according to claim 8 ,

wherein the first exciplex is formed from an anion of the first organic compound and a cation of the second organic compound, and

wherein the second exciplex is formed from an anion of the third organic compound and a cation of the second organic compound.

11. The light-emitting element according to claim 8 ,

wherein the first light-emitting substance is a phosphorescent compound or a thermally activated delayed fluorescence material,

wherein the second light-emitting substance is a phosphorescent compound or a thermally activated delayed fluorescence material,

wherein the first organic compound is a π-electron deficient heteroaromatic compound,

wherein the second organic compound is a π-electron rich heteroaromatic compound or an aromatic amine compound, and

wherein the third organic compound is a π-electron deficient heteroaromatic compound.

12. A light-emitting device comprising the light-emitting element according to claim 8 .

13. An electronic device comprising the light-emitting device according to claim 12 .

14. A lighting device comprising the light-emitting device according to claim 12 .

15. A light-emitting element comprising:

an anode:

a first light-emitting layer over the anode;

a second light-emitting layer over the first light-emitting layer; and

a cathode over the second light-emitting layer,

wherein the first light-emitting layer comprises 4,6-bis[3-(9H-carbazol-9-yl)phenyl]pyrimidine, N-(1,1′-biphenyl-4-yl)-N-[4-(9-phenyl-9H-carbazol-3-yl)phenyl]-9,9-dimethyl-9H-fluoren-2-amine, and a first light-emitting substance converting triplet excitation energy into light emission,

wherein the second light-emitting layer comprises 2-[3′-(dibenzothiophen-4-yl)biphenyl-3-yl]dibenzo[f,h]quinoxaline, N-(1,1′-biphenyl-4-yl)-N-[4-(9-phenyl-9H -carbazol-3-yl)phenyl]-9,9-dimethyl-9H-fluoren-2-amine, and a second light-emitting substance converting triplet excitation energy into light emission,

wherein the 4,6-bis[3-(9H-carbazol-9-yl)phenyl]pyrimidine and the N-(1,1′-biphenyl-4-yl)-N-[4-(9-phenyl-9H-carbazol-3-yl)phenyl]-9,9-dimethyl-9H-fluoren-2-amine form a first exciplex,

wherein the 2-[3′-(dibenzothiophen-4-yl)biphenyl-3-yl]dibenzo[f,h]quinoxaline and the N-(1,1′-biphenyl-4-yl)-N[4-(9-phenyl-9H-carbazol-3-yl)phenyl]-9,9-dimethyl-9H -fluoren-2-amine form a second exciplex,

wherein an emission spectrum of the first exciplex is overlapped with an absorption spectrum of the first light-emitting substance, and

wherein light emission from the first exciplex is substantially not observed when the first light-emitting layer emits a first light.

16. The light-emitting element according to claim 15 , wherein the first light-emitting substance is tris(2-phenylpyridinato)iridium(III).

17. The light-emitting element according to claim 15 , wherein the second light-emitting substance is bis{4,6-dimethyl-2-[5-(2,6-dimethylphenyl)-3-(3,5-dimethylphenyl)-2-pyrazinyl-κN]phenyl-κC}(2,8-dimethyl-4,6-nonanedionato -κ 2 O,O′)iridium(III).

18. A light-emitting device comprising the light-emitting element according to claim 15 .

19. An electronic device comprising the light-emitting device according to claim 18 .

20. A lighting device comprising the light-emitting device according to claim 18 .