Organic electroluminescent element

1. An organic electroluminescent element comprising, between an anode and a cathode, at least a luminescent layer and an adjacent layer that is adjacent to the cathode side of the luminescent layer, wherein said adjacent layer is formed directly on the luminescent layer, and

wherein the luminescent layer contains a phosphorescent material and a host material, the adjacent layer contains an electron transport material and a hole transport material, said luminescent layer does not contain said electron transport material and said hole transport material,

the thickness of the adjacent layer is 1 nm to 12 nm, and

the electron affinity Ea (HT) of the hole transport material and the electron affinity Ea (ET) of the electron transport material satisfy the relation of 1 eV≦Ea (ET)−Ea (HT)≦2.8 eV.

2. The organic electroluminescent element of claim 1 , wherein the electron affinity Ea (HT) of the hole transport material and the electron affinity Ea (ET) of the electron transport material satisfy the relation of 1.2 eV≦Ea (ET)−Ea (HT)≦2.8 eV.

3. The organic electroluminescent element of claim 1 , wherein the ionization potential Ip (HT) of the hole transport material and the ionization potential Ip (ET) of the electron transport material satisfy the relation of 1 eV≦Ip (ET)−Ip (HT)≦2.8 eV.

4. The organic electroluminescent element of claim 1 , wherein the electron mobility of the electron transport material is 1×10 −4 to 10 cm 2 /Vs.

5. The organic electroluminescent element of claim 1 , wherein the concentration of the hole transport material contained in the adjacent layer is 5% by mass to 90% by mass based on the total solid content of the adjacent layer.

6. The organic electroluminescent element of claim 1 , wherein the concentration of the hole transport material contained in the adjacent layer is 20% by mass to 80% by mass based on the total solid content of the adjacent layer.

7. The organic electroluminescent element of claim 1 , wherein the hole transport material is an aryl amine compound.

8. The organic electroluminescent element of claim 1 , wherein the electron transport material is a compound selected from the group consisting of an azole compound, an azine compound, an organic silane compound, and a fluorine-substituted aromatic hydrocarbon ring compound.

9. The organic electroluminescent element of claim 1 , wherein the electron transport material is an azole compound or an azine compound.

10. The organic electroluminescent element of claim 1 , wherein the electron transport material is a compound represented by formula (A-1) or (B-1):

wherein Z A1 represents an atomic group necessary for forming a nitrogen-containing heterocycle, L A1 is a linking group, and n A1 is an integer of 2 or more;

wherein Z B1 represents an atomic group necessary for forming an aromatic hydrocarbon ring or an aromatic heterocycle, L B1 is a linking group, and n B1 is an integer of 2 or more.

11. The organic electroluminescent element of claim 10 , wherein L A1 and L B1 are linking groups selected from the group consisting of a single bond, a carbon atom, a silicon atom, a boron atom, an oxygen atom, a sulfur atom, a germanium atom, an aromatic hydrocarbon ring, and an aromatic heterocycle.

12. The organic electroluminescent element of claim 10 , wherein L A1 and L B1 are linking groups selected from the group consisting of a 1,3,5-benzene triyl group, a 1,2,5,6-benzene tetrayl group, a 1,2,3,4,5,6-benzene hexayl group, a 2,2′-dimethyl-4,4′-biphenylene group, a 2,4,6-pyridine triyl group, a 2,3,4,5,6-pyridine pentayl group, a 2,4,6-pyrimidine triyl group, a 2,4,6-triazine triyl group, and a 2,3,4,5-thiophene tetrayl group.

13. The organic electroluminescent element of claim 1 , wherein the electron transport material contains three or more nitrogen atoms.

14. The organic electroluminescent element of claim 1 , wherein the phosphorescent material is a complex containing a transition metal atom or a lanthanoid atom.

15. The organic electroluminescent element of claim 1 , wherein the phosphorescent material is a complex containing at least one metal selected from the group consisting of ruthenium, rhodium, palladium, tungsten, rhenium, osmium, iridium, and platinum.

16. The organic electroluminescent element of claim 1 , wherein the energy level of the lowest multiplet excited state of the host material is higher than the energy level of the lowest multiplet excited state of the phosphorescent material.

17. The organic electroluminescent element of claim 1 , wherein the phosphorescent material is contained in an amount of 0.1 to 20% by mass in the luminescent layer.

18. The organic electroluminescent element of claim 1 , wherein a hole injecting layer and a hole transport layer are further disposed between the anode and the luminescent layer, and a dielectric layer and an electron injecting layer are further disposed between the cathode and the luminescent layer.

19. The organic electroluminescent element of claim 1 , wherein the phosphorescent material comprises Ir(ppy) 3 .