Organic electroluminescence element, image-forming apparatus, display apparatus, and imaging apparatus
Provided is an organic electroluminescence element improved in emission efficiency. The organic electroluminescence element includes: a reflective electrode; a light exiting side electrode; an emission layer provided between the reflective electrode and the light exiting side electrode; and at least one low-refractive index layer provided between the reflective electrode and the emission layer, the low-refractive index layer having a refractive index lower than the refractive index of the emission layer. An optical path L 1 between the maximum emission surface of the emission layer and the reflection surface of the reflective electrode, and an optical path L 2 between the reflection interface on an emission layer side of the low-refractive index layer and the maximum emission surface of the emission layer satisfy specific expressions.
1. An organic electroluminescence element comprising:
a reflective electrode;
a light exiting side electrode;
an emission layer provided between the reflective electrode and the light exiting side electrode;
at least one low-refractive index layer provided between the reflective electrode and the emission layer, a refractive index of the low-refractive index layer being lower than that of the emission layer; and
a charge injection/transport layer disposed between the reflective electrode and the low-refractive index layer, a refractive index of the charge injection/transport layer being higher than that of the low-refractive index layer,
wherein an optical path L 1 between a maximum emission surface of the emission layer and a reflection surface of the reflective electrode satisfies the following expression (1):
(
4
m
-
1
-
ϕ
90
)
×
(
λ
8
)
<
L
1
<
(
4
m
+
1
-
ϕ
90
)
×
(
λ
8
)
(
1
)
in the expression (1), λ represents a maximum peak wavelength of an emission spectrum, φ represents a phase shift [deg] on the reflection surface of the reflective electrode, and m represents a positive integer; and
wherein an optical path L 2 between an interface on an emission layer side of the low-refractive index layer closest to the emission layer and the maximum emission surface of the emission layer satisfies the following expression (2′):
0
≦
L
2
<
λ
8
(
2
′
)
in the expression (2′), λ represents the maximum peak wavelength of the emission spectrum.
2. The organic electroluminescence element according to claim 1 , wherein the optical path L 1 satisfies the following expression (1′):
(
8
m
-
1
-
ϕ
45
)
×
(
λ
16
)
≦
L
1
≦
(
8
m
+
1
-
ϕ
45
)
×
(
λ
16
)
.
(
1
′
)
3. The organic electroluminescence element according to claim 1 , wherein the m represents 1.
4. The organic electroluminescence element according to claim 1 , wherein the optical path L 2 satisfies the following expression (2″):
0
≦
L
2
≦
λ
16
(
2
″
)
in the expression (2″), λ represents the maximum peak wavelength of the emission spectrum.
5. The organic electroluminescence element according to claim 1 , wherein an optical path L 3 between the charge injection/transport layer and the low-refractive index layer, and the maximum emission surface of the emission layer satisfies the following expression (3):
(
4
q
+
1
)
×
λ
8
<
L
3
<
(
4
q
+
3
)
×
λ
8
.
(
3
)
6. The organic electroluminescence element according to claim 5 , wherein the optical path L 3 satisfies the following expression (3′):
(
8
q
+
3
)
×
λ
16
≦
L
3
≦
(
8
q
+
5
)
×
λ
16
(
3
′
)
in the expression (3′), λ represents the maximum peak wavelength of the emission spectrum and q represents 0 or a positive integer.
7. The organic electroluminescence element according to claim 5 , wherein the q represents 0.
8. The organic electroluminescence element according to claim 1 , further comprising another low-refractive index layer disposed between the reflective electrode and the charge injection/transport layer, a refractive index of the another low-refractive index layer being lower than those of the emission layer and the charge injection/transport layer.
9. The organic electroluminescence element according to claim 8 , wherein the optical path L 2 between an interface of the another low-refractive index layer and the charge injection/transport layer, and the maximum emission surface of the emission layer satisfies one of the following expression (2) and the following expression (2a):
(
4
p
-
1
)
×
λ
8
<
L
2
<
(
4
p
+
1
)
×
λ
8
(
2
)
(
8
p
-
1
)
×
λ
16
≦
L
2
≦
(
8
p
+
1
)
×
λ
16
(
2
a
)
and a relationship of p≧1 is satisfied in one of the expression (2) and the expression (2a).
10. The organic electroluminescence element according to claim 9 , wherein the p represents 1.
11. The organic electroluminescence element according to claim 1 , further comprising an intervening layer disposed between the low-refractive index layer and the emission layer.
12. The organic electroluminescence element according to claim 1 , wherein a thickness of the low-refractive index layer is more than ⅛λ and less than ⅜λ in terms of an optical path length.
13. An image-forming apparatus comprising:
the organic electroluminescence element according to claim 1 ;
a photosensitive member on a surface of which a latent image is formed by the organic electroluminescence element; and
a charging unit for charging the photosensitive member.
14. A display apparatus comprising:
the organic electroluminescence element according to claim 1 ; and
a drive circuit for driving the organic electroluminescence element.
15. An imaging apparatus comprising:
the display apparatus according to claim 14 ; and
an imaging element.