Light-emitting element, light-emitting device, and electronic appliance
In a light-emitting element including an EL layer between a pair of electrodes, between an electrode functioning as an anode and a fourth layer having a light-emitting property (light-emitting layer), the EL layer includes at least a first layer having a hole-injecting property (hole-injecting layer), a second layer having a hole-transporting property (first hole-transporting layer), and a third layer having a hole-transporting property (second hole-transporting layer). The absolute value of the highest occupied molecular orbital level (HOMO level) of the second layer is larger than the absolute value of the highest occupied molecular orbital level (HOMO level) of each of the first layer and the third layer. With such a structure, the rate of transport of holes injected from the electrode functioning as an anode is reduced and emission efficiency of the light-emitting element is improved.
1. A light-emitting element comprising:
a first electrode;
a second electrode; and
an EL layer between the first electrode and the second electrode, the EL layer comprising:
a first layer having a hole-injecting property over the first electrode, the first layer comprising a first substance;
a second layer having a hole-transporting property over the first layer, the second layer comprising a second substance;
a third layer having a hole-transporting property over the second layer, the third layer comprising a third substance; and
a fourth layer having a light-emitting property over the third layer,
wherein the second layer is provided between the first layer and the third layer,
wherein an absolute value of a highest occupied molecular orbital level of the second substance included in the second layer is smaller than each of absolute values of highest occupied molecular orbital levels of the first substance included in the first layer and of the third substance included in the third layer,
wherein the first substance has a hole-transporting property,
wherein the third substance has a hole-transport property, and
wherein the first substance and the third substance have a same skeleton.
2. The light-emitting element according to claim 1 , wherein the absolute value of the highest occupied molecular orbital level of the second substance is smaller than each of the absolute values of the highest occupied molecular orbital levels of the first substance and the third substance by 0.1 eV or more.
3. The light-emitting element according to claim 1 , wherein the same skeleton is aromatic amine skeleton.
4. The light-emitting element according to claim 1 , wherein the fourth layer comprises a fourth substance having an electron-transporting property.
5. The light-emitting element according to claim 1 , further comprising:
a fifth layer controlling transport of electrons between the second electrode and the fourth layer,
wherein the fifth layer comprises a first organic compound having an electron-transporting property and a second organic compound having a hole-transporting property, and a content of the second organic compound is less than 50% of the total in mass ratio.
6. The light-emitting element according to claim 5 , wherein a difference between an absolute value of a lowest unoccupied molecular orbital level of the second organic compound and an absolute value of a lowest unoccupied molecular orbital level of the first organic compound is 0.3 eV or less.
7. The light-emitting element according to claim 5 , wherein a thickness of the fifth layer is greater than or equal to 5 nm and less than or equal to 20 nm.
8. The light-emitting element according to claim 5 ,
wherein the first organic compound comprises a metal complex, and
wherein the second organic compound comprises an aromatic amine compound.
9. The light-emitting element according to claim 1 , further comprising:
a fifth layer controlling transport of electrons between the second electrode and the fourth layer,
wherein the fifth layer comprises a first organic compound having an electron-transporting property and a second organic compound having an electron-trapping property, and a content of the second organic compound is less than 50% of the total in mass ratio.
10. The light-emitting element according to claim 9 , wherein an absolute value of a lowest unoccupied molecular orbital level of the second organic compound is larger than an absolute value of lowest unoccupied molecular orbital level of the first organic compound by 0.3 eV or more.
11. The light-emitting element according to claim 9 , wherein a thickness of the fifth layer is greater than or equal to 5 nm and less than or equal to 20 nm.