Light-emitting element, light-emitting device, electronic device, and lighting device
Provided is a light-emitting element having a light-emitting layer which contains at least a host material and a plurality of guest materials, where the host material has a lower T1 level than that of at least one of the plurality of guest materials. The emission of the one of the plurality of guest materials exhibits a multicomponent decay curve, and the lifetime thereof is less than or equal to 15 μsec, preferably less than or equal to 10 μsec, more preferably less than or equal to 5 μsec, where the lifetime is defined as a time for the emission to decrease in intensity to 1/100 of its initial intensity.
1. A light-emitting device comprising:
a first electrode;
a light-emitting layer over the first electrode, the light-emitting layer comprising a host material, a first guest material, and a second guest material; and
a second electrode over the light-emitting layer,
wherein the light-emitting layer comprises a first light-emitting layer and a second light-emitting layer which are in contact with each other,
wherein the host material and the first guest material are included in the first light-emitting layer,
wherein the host material and the second guest material are included in the second light-emitting layer,
wherein a T1 level of the host material is lower than a T1 level of the first guest material and higher than a T1 level of the second guest material, and
wherein a difference in T1 level between the host material and the first guest material is larger than 0 eV and less than or equal to 0.2 eV.
2. The light-emitting device according to claim 1 ,
wherein the first guest material and the second guest material each are an organometallic compound.
3. The light-emitting device according to claim 1 ,
wherein a rate constant of a radiative transition of the second guest material is larger than 5×10 5 sec −1 .
4. The light-emitting device according to claim 1 ,
wherein a rate constant of a non-radiative transition of the host material is smaller than 1×10 2 sec −1 .
5. The light-emitting device according to claim 1 ,
wherein an emission from the light-emitting layer exhibits a multicomponent decay curve, and
wherein a lifetime of the emission is less than or equal to 15 μsec where the lifetime is a time required for the emission to decrease in intensity to 1/100 of an initial value thereof.
6. The light-emitting device according to claim 5 ,
wherein the lifetime of the emission is less than or equal to 10 μsec.
7. The light-emitting device according to claim 5 ,
wherein the lifetime of the emission is less than or equal to 5 μsec.
8. An electronic device comprising the light-emitting device according to claim 1 .
9. A lighting device comprising the light-emitting device according to claim 1 .
10. The light-emitting device according to claim 1 ,
wherein the light-emitting layer further comprises an organic compound,
wherein a T1 level of the organic compound is lower than the T1 level of the first guest material and higher than the T1 level of the second guest material.
11. The light-emitting device according to claim 1 ,
wherein the first light-emitting layer consists of the host material and the first guest material, and
wherein the second light-emitting layer consists of the host material and the second guest material.
12. A light-emitting device comprising:
a first electrode;
a first EL layer over the first electrode;
a charge generation layer over the first EL layer;
a second EL layer over the charge generation layer; and
a second electrode over the second EL layer,
wherein at least one of the first EL layer and the second EL layer comprises a light-emitting layer which comprises a host material, a first guest material, and a second guest material,
wherein the light-emitting layer comprises a first light-emitting layer and a second light-emitting layer which are in contact with each other,
wherein the host material and the first guest material are included in the first light-emitting layer,
wherein the host material and the second guest material are included in the second light-emitting layer,
wherein a T1 level of the host material is lower than a T1 level of the first guest material and higher than a T1 level of the second guest material, and
wherein a difference in T1 level between the host material and the first guest material is larger than 0 eV and less than or equal to 0.2 eV.
13. The light-emitting device according to claim 12 ,
wherein the first guest material and the second guest material each are an organometallic compound.
14. The light-emitting device according to claim 12 ,
wherein a rate constant of a radiative transition of the second guest material is larger than 5×10 5 sec −1 .
15. The light-emitting device according to claim 12 ,
wherein a rate constant of a non-radiative transition of the host material is smaller than 1×10 2 sec −1 .
16. The light-emitting device according to claim 12 ,
wherein an emission from the light-emitting layer exhibits a multicomponent decay curve, and
wherein a lifetime of the emission is less than or equal to 15 μsec where the lifetime is a time required for the emission to decrease in intensity to 1/100 of an initial value thereof.
17. The light-emitting device according to claim 16 ,
wherein the lifetime of the emission is less than or equal to 10 μsec.
18. The light-emitting device according to claim 16 ,
wherein the lifetime of the emission is less than or equal to 5 μsec.
19. An electronic device comprising the light-emitting device according to claim 12 .
20. A lighting device comprising the light-emitting device according to claim 12 .
21. The light-emitting device according to claim 12 ,
wherein the light-emitting layer further comprises an organic compound,
wherein a T1 level of the organic compound is lower than the T1 level of the first guest material and higher than the T1 level of the second guest material.
22. The light-emitting device according to claim 12 ,
wherein the first light-emitting layer consists of the host material and the first guest material, and
wherein the second light-emitting layer consists of the host material and the second guest material.