Electroluminescent device

1. An electroluminescent device comprising:

a pair of electrodes, and

an electroluminescent layer comprising at least a luminescent layer, situated between the electrodes,

the luminescent layer comprising a matrix material comprising at least one organic compound, and quantum dots whose surfaces are protected by a protective material and that are dispersed in the matrix material, wherein

the quantum dots have cores and shells surrounding the cores,

the protective material comprises a first protective material, and

the absolute value of the ionization potential Ip(h), the absolute value of the electron affinity Ea(h), and the band gap Eg(h) of the first protective material, the absolute value of the ionization potential Ip(m), the absolute value of the electron affinity Ea(m), and the band gap Eg(m) of the organic compound, and the band gap Eg(q) of the quantum dots fulfill all of the following conditions (A) to (C):

(A) Ip(h)<Ip(m)+0.1 eV,

(B) Ea(h)>Ea(m)−0.1 eV, and

(C) Eg(q)<Eg(h)<Eg(m)

wherein

the first protective material comprises in one molecule, one or more hydrophilic group and one or more hydrophobic group,

the hydrophobic group in the first protective material, the organic compound, and the quantum dots are in at least one of the following combinations (1) to (3):

(1) the hydrophobic group in the first protective material being a residue of 3-(2-benzothiazoleyl)-7-(diethylamino)coumarin,

the organic compound being 4,4′-bis(carbazol-9-yl)biphenyl, and

the quantum dots being composed of the cores made of CdSe and the shells made of ZnS, having an emission wavelength of 620 nm,

(2) the hydrophobic group in the first protective material being a residue of 5,6,11,12-tetraphenylnaphthacene,

the organic compound being 4,4′-bis(carbazol-9-yl)biphenyl,

and

the quantum dots being composed of the cores made of CdSe and the shells made of ZnS, having an emission wavelength of 620 nm,

(3) the hydrophobic group in the first protective material being a residue of 2,5,8,11-tetra-tert-butylperylene,

the organic compound being 4,4′-bis(carbazol-9-yl)biphenyl,

and

the quantum dots being composed of the cores made of CdSe and the shells made of ZnS, having an emission wavelength of 520 nm.

2. The electroluminescent device according to claim 1 , wherein the electroluminescent layer further comprises at least one layer adjoining the luminescent layer, and

the band gap Eg(h) is smaller than the band gap Eg(a) of the material making up the layer adjoining the luminescent layer.

3. The electroluminescent device according to claim 1 , wherein the hydrophobic group in the first protective material, the organic compound, and the quantum dots are in combination (1).

4. The electroluminescent device according to claim 1 , wherein the hydrophobic group in the first protective material, the organic compound, and the quantum dots are in combination (2).

5. The electroluminescent device according to claim 1 , wherein the hydrophobic group in the first protective material, the organic compound, and the quantum dots are in combination (3).

6. The electroluminescent device according to claim 1 , wherein the protective material has the property of transporting charges and further comprises a second protective material whose molecular weight is higher than the molecular weight of the first protective material.

7. The electroluminescent device according to claim 6 , wherein the second protective material has a longer chain length than the chain length of the first protective material.