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 hole-transport layer; and
a light-emitting layer over the hole-transport layer,
wherein the light-emitting layer includes a first organic compound having an electron-transport property, a second organic compound having a hole-transport property, and a third organic compound converting triplet excitation energy into light emission,
wherein the hole-transport 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 second organic compound, and
wherein a HOMO level of the fifth organic compound is higher than the HOMO level of the second organic compound.
2. 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 first organic compound.
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 second organic compound.
4. The light-emitting element according to claim 1 ,
wherein the first organic compound is a π-electron deficient heteroaromatic compound,
wherein the 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.
5. A light-emitting device comprising the light-emitting element according to claim 1 .
6. An electronic appliance 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 hole-injection layer;
a hole-transport layer over the hole-injection layer;
a light-emitting layer over the hole-transport 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 first organic compound having an electron-transport property, a second organic compound having a hole-transport property, and a third organic compound converting triplet excitation energy into light emission,
wherein the hole-transport 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 second organic compound, and
wherein a HOMO level of the fifth organic compound is higher than the HOMO level of the second organic compound.
9. The light-emitting element according to claim 8 , wherein a T 1 level of the fourth organic compound is higher than a T 1 level of the first organic compound.
10. The light-emitting element according to claim 8 , wherein a T 1 level of the fourth organic compound is higher than a T 1 level of the second organic compound.
11. The light-emitting element according to claim 8 ,
wherein the first organic compound is a π-electron deficient heteroaromatic compound,
wherein the 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.
12. A light-emitting device comprising the light-emitting element according to claim 8 .
13. An electronic appliance comprising the light-emitting device according to claim 8 .
14. A lighting device comprising the light-emitting device according to claim 12 .