Light-emitting element, light-emitting device, electronic appliance, and lighting device
A light-emitting element which has low driving voltage and high emission efficiency, is provided. The light-emitting element includes, between a pair of electrodes, a hole-transport layer and a light-emitting layer over the hole-transport layer. The light-emitting layer contains a first organic compound having an electron-transport property, a second organic compound having a hole-transport property, and a light-emitting third organic compound converting triplet excitation energy into light emission. A combination of the first organic compound and the second organic compound forms an exciplex. The hole-transport layer contains at least a fourth organic compound whose HOMO level is lower than or equal to that of the second organic compound and a fifth organic compound whose HOMO level is higher than that of the second organic compound.
1. A light-emitting element comprising:
a layer; and
a light-emitting layer over the layer,
wherein the light-emitting layer includes a mixture of an electron-transport first organic compound, a hole-transport second organic compound, and a third organic compound converting triplet excitation energy into light emission,
wherein the hole-transport second organic compound comprises a plurality of carbazole skeletons,
wherein the layer includes a fourth organic compound and a fifth organic compound,
wherein a HOMO level of the fourth organic compound is lower than or equal to a HOMO level of the hole-transport second organic compound, and
wherein a HOMO level of the fifth organic compound is higher than the HOMO level of the hole-transport second organic compound.
2. The light-emitting element according to claim 1 , wherein a combination of the electron-transport first organic compound and the hole-transport second organic compound forms an exciplex.
3. The light-emitting element according to claim 1 , wherein a T 1 level of the fourth organic compound is higher than a T 1 level of the electron-transport first organic compound.
4. The light-emitting element according to claim 1 , wherein a T 1 level of the fourth organic compound is higher than a T 1 level of the hole-transport second organic compound.
5. The light-emitting element according to claim 1 , wherein the electron-transport first organic compound is a π-electron deficient heteroaromatic compound,
wherein the hole-transport second organic compound, the fourth organic compound, and the fifth organic compound are π-electron rich heteroaromatic compounds or aromatic amine compounds, and
wherein the third organic compound is a phosphorescent compound.
6. A light-emitting device comprising the light-emitting element according to claim 1 .
7. An electronic appliance comprising the light-emitting device according to claim 6 .
8. A lighting device comprising the light-emitting device according to claim 6 .
9. The light-emitting element according to claim 1 , wherein the plurality of carbazole skeletons is 2.
10. A light-emitting element comprising:
a hole-injection layer;
a layer over the hole-injection layer;
a light-emitting layer over the layer;
an electron-transport layer over the light-emitting layer; and
an electron-injection layer over the electron-transport layer,
wherein the light-emitting layer includes a mixture of an electron-transport first organic compound, a hole-transport second organic compound, and a third organic compound converting triplet excitation energy into light emission,
wherein the hole-transport second organic compound comprises a plurality of carbazole skeletons,
wherein the layer includes a fourth organic compound and a fifth organic compound, wherein a HOMO level of the fourth organic compound is lower than or equal to a HOMO level of the hole-transport second organic compound, and
wherein a HOMO level of the fifth organic compound is higher than the HOMO level of the hole-transport second organic compound.
11. The light-emitting element according to claim 1 , wherein an absorption band of the third organic compound on the longest wavelength side and an emission spectrum of the electron-transport first organic compound and the hole-transport second organic compound overlap with each other.
12. The light-emitting element according to claim 10 , wherein the plurality of carbazole skeletons is 2.
13. The light-emitting element according to claim 10 , wherein a combination of the electron-transport first organic compound and the hole-transport second organic compound forms an exciplex.
14. The light-emitting element according to claim 10 , wherein a T 1 level of the fourth organic compound is higher than a T 1 level of the electron-transport first organic compound.
15. The light-emitting element according to claim 10 , wherein a T 1 level of the fourth organic compound is higher than a T 1 level of the hole-transport second organic compound.
16. The light-emitting element according to claim 10 , wherein the electron-transport first organic compound is a π-electron deficient heteroaromatic compound,
wherein the hole-transport second organic compound, the fourth organic compound, and the fifth organic compound are π-electron rich heteroaromatic compounds or aromatic amine compounds, and
wherein the third organic compound is a phosphorescent compound.
17. A light-emitting device comprising the light-emitting element according to claim 10 .
18. An electronic appliance comprising the light-emitting device according to claim 17 .
19. A lighting device comprising the light-emitting device according to claim 17 .
20. The light-emitting element according to claim 10 , wherein an absorption band of the third organic compound on the longest wavelength side and an emission spectrum of the electron-transport first organic compound and the hole-transport second organic compound overlap with each other.