Light-emitting element, light-emitting device, and method for manufacturing the same
An object is to provide a light-emitting element with high emission efficiency. Another object is to provide a light-emitting element with a long lifetime and high reliability. Another object is to provide a light-emitting element driven at low voltage. A first light-emitting layer whose one surface is in contact with a hole-transport layer, and a second light-emitting layer which is in contact with the other surface of the first light-emitting layer and includes a bipolar host material and a light-emitting substance are provided, where the hole-transport property of the first light-emitting layer is higher than that of the second light-emitting layer. A recombination region of holes and electrons is preferably provided in the light-emitting layer. The hole-transport layer preferably includes an anti-reducing substance.
1. A light-emitting element comprising:
a first electrode;
a second electrode;
a hole-transport layer between the first electrode and the second electrode;
an electron-transport layer between the first electrode and the second electrode; and
a light-emitting layer between the hole-transport layer and the electron-transport layer,
wherein the light-emitting layer comprises:
a first light-emitting layer comprising a first light-emitting substance and being in contact with the hole-transport layer; and
a second light-emitting layer comprising a second light-emitting substance and being in contact with the first light-emitting layer,
a third light-emitting layer comprising a third light-emitting substance and being in contact with the second light-emitting layer and the electron-transport layer,
wherein the first light-emitting layer and the second light-emitting layer are bipolar,
wherein a hole-transport property of the first light-emitting layer is higher than a hole-transport property of the second light-emitting layer,
wherein the hole-transport layer comprises a first organic compound and an anti-reducing substance,
wherein the first light-emitting substance is the same as the second light-emitting substance, and
wherein a concentration of the second light-emitting substance is higher than a concentration of the third light-emitting substance, and the concentration of the third light-emitting substance is higher than a concentration of the first light-emitting substance.
2. A light-emitting element comprising:
a first electrode;
a second electrode;
a hole-transport layer between the first electrode and the second electrode;
an electron-transport layer between the first electrode and the second electrode; and
a light-emitting layer between the hole-transport layer and the electron-transport layer,
wherein the light-emitting layer comprises:
a first light-emitting layer comprising a first light-emitting substance and being in contact with the hole-transport layer;
a second light-emitting layer comprising a second light-emitting substance and being in contact with the first light-emitting layer; and
a third light-emitting layer comprising a third light-emitting substance and being in contact with the second light-emitting layer and the electron-transport layer,
wherein the first light-emitting layer and the second light-emitting layer are bipolar,
wherein the hole-transport layer comprises a first organic compound and an anti-reducing substance,
wherein the first light-emitting substance is the same as the second light-emitting substance, and
wherein a HOMO level (highest occupied molecular orbital level) of the third light-emitting substance is equal to a HOMO level of the electron-transport layer.
wherein a concentration of the second light-emitting substance is higher than a concentration of the third light-emitting substance, and the concentration of the third light-emitting substance is higher than a concentration of the first light-emitting substance.
3. A light-emitting element comprising:
a first electrode;
a second electrode;
a hole-transport layer between the first electrode and the second electrode;
an electron-transport layer between the first electrode and the second electrode; and
a light-emitting layer between the hole-transport layer and the electron-transport layer,
wherein the light-emitting layer comprises:
a first light-emitting layer comprising a first light-emitting substance and being in contact with the hole-transport layer; and
a second light-emitting layer comprising a second light-emitting substance and being in contact with the first light-emitting layer,
a third light-emitting layer comprising a third light-emitting substance and being in contact with the second light-emitting layer and the electron-transport layer,
wherein the first light-emitting layer and the second light-emitting layer are bipolar,
wherein the hole-transport layer comprises a first organic compound and a metal oxide,
wherein the first light-emitting substance is the same as the second light-emitting substance, and
wherein a HOMO level (highest occupied molecular orbital level) of the third light-emitting substance is equal to a HOMO level of the electron-transport layer.
wherein a concentration of the second light-emitting substance is higher than a concentration of the third light-emitting substance, and the concentration of the third light-emitting substance is higher than a concentration of the first light-emitting substance.
4. A light-emitting element comprising:
a first electrode;
a second electrode;
a hole-transport layer between the first electrode and the second electrode;
an electron-transport layer between the first electrode and the second electrode;
a light-emitting layer between the hole-transport layer and the electron-transport layer,
wherein the light-emitting layer comprises:
a first light-emitting layer comprising a first light-emitting substance and being in contact with the hole-transport layer; and
a second light-emitting layer comprising a second light-emitting substance and being in contact with the first light-emitting layer,
a third light-emitting layer comprising a third light-emitting substance and being in contact with the second light-emitting layer and the electron-transport layer,
wherein the hole-transport layer comprises a first organic compound and an anti-reducing substance,
wherein the first light-emitting substance is the same as the second light-emitting substance, and
wherein a HOMO level (highest occupied molecular orbital level) of the third light-emitting substance is equal to a HOMO level of the electron-transport layer.
wherein a concentration of the second light-emitting substance is higher than a concentration of the third light-emitting substance, and the concentration of the third light-emitting substance is higher than a concentration of the first light-emitting substance.
5. The light-emitting element according to claim 2 ,
wherein the third light-emitting layer has an electron-transport property.
6. The light-emitting element according to claim 2 ,
wherein the first light-emitting layer further comprises a first host material,
wherein the second light-emitting layer further comprises a second host material,
wherein the third light-emitting layer further comprises a third host material, and
wherein a HOMO level (highest occupied molecular orbital level) of the third host material is deeper than a HOMO level of the second host material.
7. The light-emitting element according to claim 2 ,
wherein the third light-emitting layer further comprises a host material, and
wherein a HOMO level of the third light-emitting substance is shallower than a HOMO level of the host material.
8. The light-emitting element according to claim 2 or 4 ,
wherein the anti-reducing substance is a metal oxide.
9. The light-emitting element according to claim 1 ,
wherein the anti-reducing substance is a metal oxide.
10. The light-emitting element according to any one of claims 1 , 2 , and 4 ,
wherein the first light-emitting layer further comprises a first host material,
wherein the anti-reducing substance is a second organic compound,
wherein a LUMO level (lowest unoccupied molecular orbital level) of the second organic compound is shallower than a LUMO level of the first host material,
wherein a band gap of the second organic compound is wider than a band gap of the first light-emitting substance, and
wherein a HOMO level of the second organic compound is deeper than a HOMO level of the first organic compound.
11. The light-emitting element according to any one of claims 2 , 3 , and 4 ,
wherein a hole-transport property of the first light-emitting layer is higher than a hole-transport property of the second light-emitting layer.
12. The light-emitting element according to any one of claims 1 , 2 , 3 , and 4 ,
wherein the first light-emitting layer further comprises a first host material,
wherein the second light-emitting layer further comprises a second host material,
wherein a difference between a HOMO level of the first light-emitting substance and a HOMO level of the second light-emitting substance is less than or equal to 0.2 eV,
wherein a difference between a HOMO level of the second host material and a HOMO level of the first host material is less than or equal to 0.2 eV, and
wherein the HOMO level of the first light-emitting substance is shallower than the HOMO level of the first host material.
13. The light-emitting element according to any one of claims 1 , 2 , 3 , and 4 ,
wherein the first light-emitting layer further comprises a first host material,
wherein the second light-emitting layer further comprises a second host material, and
wherein the first host material and the second host material are the same.
14. The light-emitting element according to any one of claims 1 , 2 , 3 , and 4 ,
wherein the first electrode serves as an anode, and
wherein the hole-transport layer is in direct contact with the anode.
15. The light-emitting element according to any one of claims 1 , 2 , 3 , and 4 ,
wherein the hole-transport layer is in contact with the first electrode.
16. A lighting device to which the light-emitting element according to any one of claims 1 , 2 , 3 , and 4 is applied.
17. An electronic device to which the light-emitting element according to any one of claims 1 , 2 , 3 , and 4 is applied,
wherein the electronic device is one selected from the group consisting of a camera, goggle type display, a navigation system, an audio reproducing device, a computer, a game machine, a portable information terminal, an image reproducing device, and television.