Light-emitting element, light-emitting device, electronic device, and lighting device

1. A display device comprising:

a substrate;

a transistor comprising an oxide semiconductor film over the substrate; and

a tandem light-emitting element comprising a plurality of EL layers between a pair of electrodes, the tandem light-emitting element being over and electrically connected to the transistor,

wherein a charge generation layer is provided between the plurality of EL layers,

wherein one of the pair of electrodes functions as an anode and comprises ITO,

wherein emission colors of the EL layers differ from each other,

wherein white light is obtained from the tandem light-emitting element as a whole,

wherein one of the plurality of EL layers emits blue light and comprises an electron-transport layer comprising a polyazole compound,

wherein another of the plurality of EL layers comprises a first light-emitting layer comprising a first organic compound, a second organic compound and a first iridium complex, and

wherein the first organic compound and the second organic compound form an exciplex.

2. The display device according to claim 1 , wherein the other of the plurality of EL layers comprises a second light-emitting layer comprising a second iridium complex.

3. The display device according to claim 1 , wherein an emission spectrum of the exciplex overlaps with an absorption band on the longest wavelength side of the first iridium complex.

4. The display device according to claim 2 , further comprising:

a second electron-transport layer comprising a polyazole compound,

wherein the first light-emitting layer is in contact with the second light-emitting layer.

5. A display device comprising:

a substrate;

a transistor comprising an oxide semiconductor film over the substrate; and

a tandem light-emitting element comprising a first EL layer and a second EL layer between an anode and a cathode, the tandem light-emitting element being over and electrically connected to the transistor,

wherein a charge generation layer is provided between the first EL layer and the second EL layer,

wherein the anode comprises ITO,

wherein emission colors of the first EL layer and the second EL layer are complementary so that white light is obtained from the tandem light-emitting element as a whole,

wherein the first EL layer emits blue light,

wherein the second EL layer comprises a first light-emitting layer comprising a first organic compound, a second organic compound and a first iridium complex,

wherein an electron-transport layer comprising a polyazole compound is provided on a third light-emitting layer of the first EL layer, and

wherein the first organic compound and the second organic compound form an exciplex.

6. The display device according to claim 5 , wherein the second EL layer comprises a second light-emitting layer comprising a second iridium complex.

7. The display device according to claim 5 , wherein an emission spectrum of the exciplex overlaps with an absorption band on the longest wavelength side of the iridium complex.

8. The display device according to claim 6 , further comprising:

a second electron-transport layer comprising a polyazole compound over the first light-emitting layer,

wherein the first light-emitting layer is in contact with the second light-emitting layer.

9. A display device comprising:

a substrate;

a transistor comprising an oxide semiconductor film over the substrate; and

a tandem light-emitting element comprising a first EL layer, a second EL layer, and a third EL layer between a pair of electrodes,

wherein a first charge generation layer is provided between the first EL layer and the second EL layer,

wherein a second charge generation layer is provided between the second EL layer and the third EL layer,

wherein one of the pair of electrodes functions as an anode and comprises ITO,

wherein emission colors of the first EL layer and the second EL layer differ from each other,

wherein white light is obtained from the tandem light-emitting element as a whole,

wherein any one of the first EL layer and the third EL layer emits blue light,

wherein the second EL layer comprises a first light-emitting layer comprising a first organic compound, a second organic compound and a first iridium complex, and

wherein the first organic compound and the second organic compound form an exciplex.

10. The display device according to claim 9 ,

wherein an electron-transport layer comprising a polyazole compound is provided between a third light-emitting layer of the first EL layer and the first light-emitting layer, and

wherein the second EL layer comprises a second light-emitting layer comprising a second iridium complex.

11. The display device according to claim 9 , wherein an emission spectrum of the exciplex overlaps with an absorption band on the longest wavelength side of the iridium complex.

12. The display device according to claim 10 , wherein the first light-emitting layer is in contact with the second light-emitting layer.

13. The display device according to claim 9 , wherein the first EL layer and the third EL layer emit the same color.

14. The display device according to claim 10 , further comprising:

a second electron-transport layer comprising a polyazole compound between a fourth light-emitting layer of the third EL layer and the first light-emitting layer,

wherein at least one organic compound in the first EL layer and the third EL layer is the same.