Light-emitting element, light-emitting device, display device, electronic appliance, and lighting device
View Patent ↗Provided is a light-emitting element with a small degree of luminance degradation with accumulation of driving time (a long-lifetime light-emitting element). Provided is a light-emitting element in which a light-emitting layer with an electron-transport property is formed with a plurality of layers containing different host materials. Further, the LUMO level of a host material on an anode side is higher than the LUMO level of a host material on a cathode side. With such a structure, it is possible to provide a long-lifetime light-emitting element with little degradation in luminance with accumulation of driving time.
1. A light-emitting element comprising:
an anode;
a cathode; and
an EL layer between the anode and the cathode, the EL layer including a light-emitting layer,
wherein the light-emitting layer includes a first light-emitting layer and a second light-emitting layer,
wherein the first light-emitting layer is between the second light-emitting layer and the anode,
wherein the second light-emitting layer is between the first light-emitting layer and the cathode,
wherein the first light-emitting layer includes a first host material having a higher electron transporting property than a hole transporting property and a first emission center substance,
wherein the second light-emitting layer includes a second host material having an electron transporting property and a second emission center substance,
wherein the first host material and the second host material each have an electron mobility of higher than or equal to 10 −6 cm 2 /Vs, and
wherein a difference between a LUMO level of the first host material and a LUMO level of the second host material is greater than or equal to 0.1 eV and less than or equal to 0.6 eV.
2. The light-emitting element according to claim 1 , wherein the first emission center substance is a same substance as the second emission center substance.
3. The light-emitting element according to claim 1 , wherein the first emission center substance and the second emission center substance are each able to convert a triplet excited state into light emission.
4. The light-emitting element according to claim 1 , wherein the first emission center substance and the second emission center substance are each an iridium complex.
5. The light-emitting element according to claim 1 , wherein neither of the first host material nor the second host material has an aromatic amine skeleton in a skeleton.
6. A light-emitting element comprising:
an anode;
a cathode; and
an EL layer between the anode and the cathode, the EL layer including a light-emitting layer,
wherein the light-emitting layer includes a first light-emitting layer and a second light-emitting layer,
wherein the first light-emitting layer is between the second light-emitting layer and the anode,
wherein the second light-emitting layer is between the first light-emitting layer and the cathode,
wherein the first light-emitting layer includes a first host material having a higher electron transporting property than a hole transporting property and a first emission center substance,
wherein the second light-emitting layer includes a second host material having an electron transporting property and a second emission center substance,
wherein the first host material and the second host material each have an electron mobility of higher than or equal to 10 −6 cm 2 /Vs,
wherein the first host material and the second host material each include a molecular weight of greater than or equal to 300 and less than or equal to 2000, and include a six-membered aromatic ring in an electron-transport skeleton,
wherein nitrogen atoms included in the six-membered aromatic ring in the first host material are fewer than nitrogen atoms included in the six-membered aromatic ring in the second host material, and
wherein a difference between a LUMO level of the first host material and a LUMO level of the second host material is greater than or equal to 0.1 eV and less than or equal to 0.6 eV.
7. The light-emitting element according to claim 6 , wherein the six-membered aromatic ring of the first host material includes a benzene skeleton or a pyridine skeleton, and wherein the six-membered aromatic ring of the second host material includes a diazine skeleton.
8. The light-emitting element according to claim 6 , wherein the first emission center substance is a same substance as the second emission center substance.
9. The light-emitting element according to claim 6 , wherein the first emission center substance and the second emission center substance are each able to convert a triplet excited state into light emission.
10. The light-emitting element according to claim 6 , wherein the first emission center substance and the second emission center substance are each an iridium complex.
11. The light-emitting element according to claim 6 , wherein neither of the first host material nor the second host material has an aromatic amine skeleton in a skeleton.