Light-emitting element, display device, electronic device, and lighting device
A light-emitting element having low driving voltage and high emission efficiency is provided. In the light-emitting element, a combination of a guest material and a host material forms an exciplex. The guest material is capable of converting triplet excitation energy into light emission. Light emission from the light-emitting layer includes light emission from the guest material and light emission from the exciplex. The percentage of the light emission from the exciplex to the light emission from the light-emitting layer is greater than 0 percent and less than or equal to 60 percent. The energy after subtracting the energy of light emission from the exciplex from the energy of light emission from the guest material is greater than 0 eV and less than or equal to 0.23 eV.
1. A light-emitting element comprising:
a light-emitting layer comprising:
a first organic compound; and
a second organic compound,
wherein one of the first organic compound and the second organic compound has a LUMO level higher than or equal to a LUMO level of the other of the first organic compound and the second organic compound, and has a HOMO level higher than or equal to a HOMO level of the other of the first organic compound and the second organic compound,
wherein a combination of the first organic compound and the second organic compound forms a first exciplex,
wherein the first organic compound is configured to convert triplet excitation energy into light emission, and
wherein a relation 0 eV<E G _ em −E Ex _ em ≤0.23 eV is satisfied where E G _ em is energy of light emission from the first organic compound and E Ex _ em is energy of light emission from the first exciplex.
2. The light-emitting element according to claim 1 , wherein E G _ em is energy calculated from a wavelength of an emission peak on a shortest wavelength side of an emission spectrum of the first organic compound.
3. The light-emitting element according to claim 1 , wherein E Ex _ em is energy calculated from a wavelength of an emission peak on a shortest wavelength side of an emission spectrum of the first exciplex.
4. The light-emitting element according to claim 1 ,
wherein a lowest triplet excitation energy level of the second organic compound is higher than or equal to a lowest triplet excitation energy level of the first organic compound, and
wherein the lowest triplet excitation energy level of the first organic compound is higher than or equal to a lowest triplet excitation energy level of the first exciplex.
5. The light-emitting element according to claim 1 ,
wherein the light-emitting layer further comprises a third organic compound,
wherein one of the LUMO level of the second organic compound and a LUMO level of the third organic compound is higher than or equal to the other of the LUMO level of the second organic compound and the LUMO level of the third organic compound,
wherein one of the HOMO level of the second organic compound and a HOMO level of the third organic compound is higher than or equal to the other of the HOMO level of the second organic compound and the HOMO level of the third organic compound, and
wherein a combination of the second organic compound and the third organic compound forms a second exciplex.
6. The light-emitting element according to claim 5 ,
wherein a lowest triplet excitation energy level of the second organic compound and a lowest triplet excitation energy level of the third organic compound are higher than or equal to a lowest triplet excitation energy level of the second exciplex, and
wherein the lowest triplet excitation energy level of the second exciplex is higher than or equal to a lowest triplet excitation energy level of the first organic compound.
7. The light-emitting element according to claim 5 ,
wherein one of the second organic compound and the third organic compound is configured to transport a hole, and
wherein the other of the second organic compound and the third organic compound is configured to transport an electron.
8. The light-emitting element according to claim 7 ,
wherein the one of the second organic compound and the third organic compound comprises at least one of a π-electron rich heteroaromatic ring skeleton and an aromatic amine skeleton, and
wherein the other of the second organic compound and the third organic compound comprises a π-electron deficient heteroaromatic ring skeleton.
9. The light-emitting element according to claim 1 ,
wherein light emission from the light-emitting layer comprises the light emission from the first organic compound and the light emission from the first exciplex, and
wherein the light emission from the light-emitting layer comprises an emission component whose time in which emission intensity is reduced to lower than or equal to 1% is shorter than or equal to 37 μs.
10. The light-emitting element according to claim 1 , wherein the first organic compound comprises iridium,
wherein the first organic compound comprises a ligand coordinated to the iridium,
wherein the ligand comprises a nitrogen-containing five-membered heterocyclic skeleton, and
wherein the second organic compound comprises a π-electron deficient heteroaromatic ring skeleton.