Active solid-state devices (e.g. – transistors – solid-state diode – Organic semiconductor material
Reexamination Certificate
2002-01-28
2003-10-07
Crane, Sara (Department: 2811)
Active solid-state devices (e.g., transistors, solid-state diode
Organic semiconductor material
C257S098000, C313S504000
Reexamination Certificate
active
06630684
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an organic light emitting device, and more particularly, to an organic light emitting device in which a photonic crystal concavo-convex structure is formed in a transparent substrate and an transparent electrode layer to increase a light extraction efficiency.
2. Description of the Related Art
Recently, display devices based on organic light emitting materials become the center of attraction due to flatness, high definition, portability, low power consumption and so forth.
FIG. 1
is a schematic view of a conventional organic light emitting device. Referring to
FIG. 1
, the conventional organic light emitting device has a structure in which a transparent electrode layer
20
, a hole conduction layer
30
, an electron conduction layer
40
and a cathode layer
50
are sequentially stacked on a transparent substrate
10
.
Herein, a glass substrate is typically used as the transparent substrate
10
. An ITO (Indium-Tin-Oxide) layer is mainly used as the transparent electrode layer
20
. In addition, an Mg—Al alloy layer may be used as the cathode electrode layer
50
. The hole conduction layer
30
and the electron conduction layer
40
are comprised of an organic EL(electroluminescent) material. Typically, the hole conduction layer
30
is comprised of N,N′-diphenyl-N,N′-bis-(3-methylphenyl)-4,4′-diamine (hereinafter, “TPD”) or polyethylenedeoxythiophene (PEDOT), and the electron conduction layer
30
is widely comprised of tris (8-hydroxyquinolino) aluminum (hereinafter, “Alq3”). The typical material of each layer has an absolute refractive index, i.e., n(glass)=1.46, n(ITO)=1.8, n(TPD)=1.76 and n(Alq3)=1.7.
As shown in
FIG. 1
, when a negative voltage is applied to the cathode electrode layer
50
and a positive voltage is applied to the transparent electrode layer
20
, the combination of a hole and an electron is occurred in a junction portion (
35
: hereinafter, “active area”) of the hole conduction layer
30
and the electron conduction layer
40
. Thus, light is spontaneously radiated.
The light generated at the active area
35
is radiated through in turn an interface of the hole conduction layer
30
and the transparent electrode layer
20
and an interface of the transparent electrode layer
20
and the transparent substrate
10
to air. Since the absolute refractive index (n(ITO)=1.8) of the transparent electrode layer
20
is larger than that (n(Alq3)=1.7) of the electron conduction layer
40
, at the interface of the electron conduction layer
40
and the transparent electrode layer
20
, most of the light is refracted toward the transparent electrode layer
20
and then transmitted through the transparent electrode layer
20
.
However, since the absolute refractive index (n(glass)=1.8) of the transparent electrode layer
20
is larger than the refractive index of substrate layer
10
(n(glass)=1.46), the light, at an angle larger than a critical angle, is totally reflected so as to be not transmitted to the glass. Further, since the absolute refractive index of the transparent substrate
10
is 1.46 and the absolute refractive index of the air is 1, the same phenomenon occurrs at the interface of the transparent substrate
10
and the air.
In the drawing, a reference symbol &thgr;cc designates a critical angle between the transparent electrode layer
20
and the transparent substrate
10
, and a reference symbol &thgr;c is a critical angle between the transparent substrate
10
and the air, and &thgr;o is an incident angle of the light which is incident to the transparent substrate
10
to be converted into the angle of &thgr;c.
Assuming that the distribution of light generated from a specific radiation point of the active area
35
is spacially isotropic and the light is not reabsorbed, the amount of light, that is totally reflected from the transparent substrate, can be calculated by a following equation:
∫
θ
o
θ
cc
T
glass
(
θ
)
sin
θ
ⅆ
θ
.
It is about 31.5%, wherein T
glss
(&thgr;) is a transmittance of the transparent substrate
10
. And, in the same condition, the amount of light, that is totally reflected from the transparent electrode layer
20
, can be calculated by a following equation:
∫
θ
cc
90
T
ITO
(
θ
)
sin
θ
ⅆ
θ
.
It is about 51%, wherein T
ITO
(&thgr;) is a transmittance of the transparent electrode layer
20
. To summarize, the total-reflected light amount is about 80%.
Therefore, in conventional organic light emitting devices, the light extraction efficiency is only about 20%. There is a big room for improvement. Because of the low light extraction efficiency, the power dissipation should be large, and the life time of the arrayed light emitting device is reduced. Therefore, one need to make the light extraction efficiency as large as possible.
In order to increase the light extraction efficiency, several schemes have been proposed. For example, a cone-shaped array is formed on a glass substrate, such that the light, entering at larger angles than the critical angle, can be transmitted to an outside (cf. High external quantum efficiency organic light emitting device, G. Gu, D. Z. Garbuzov, P. E. Burrows, S. Venkatesh, S. R. Forrest, Optics Letters, 22, 396, 1997). Or a laminated lens array is formed on a glass substrate to reduce the incident angle, thereby increasing the light extraction efficiency (cf. Improvement of output coupling efficiency of organic light emitting diodes by backside substrate modification, C. F. Madigan, M. H. Lu, J. C. Sturm, Applied Physics Letters, 27, 1650, 2000). In these methods, however, there are some problems related to fabricating methods and the image quality is poor.
SUMMARY OF THE INVENTION
Therefore, the object of the present invention is to provide an organic light emitting device in which a photonic crystal concavo-convex structure is formed in the transparent substrate
10
and the transparent electrode layer
20
, thereby increasing the light extraction efficiency.
To achieve the aforementioned object of the present invention, the photonic crystal organic light emitting device includes a transparent substrate having a concavo-convex structure in an upper surface thereof, a transparent electrode layer formed on the transparent substrate, a hole conduction layer comprised of an organic EL material and formed on the transparent electrode layer, an electron conduction layer comprised of an organic EL material and formed on the hole conduction layer, and a cathode layer formed on the electron conduction layer.
Preferably, the lattice constant of the concavo-convex structure formed in the upper surface of the transparent substrate ranges from ⅓&lgr; to 2&lgr;, where &lgr; is the wavelength of light in the active area. And transparent electrode layer has a thickness of 30-200 nm. Further, the depth of a concave potion is formed as deep as possible within the extent that the electrical properties of the transparent electrode layer are acceptable.
The photonic crystal of the concavo-convex structure formed in the upper surface of the transparent substrate can be periodically and repeatedly arrayed in a square lattice type, a triangular lattice type or a honeycomb lattice type. The concavo-convex structure can be formed by etching the upper surface of the transparent substrate. However, it is clear that other methods such as a wet etching or a micro-imprinting also can be used.
If one wants to obtain the constant diffraction angle irrespective of the color of light generated at the interface between the electron conduction layer and the hole conduction layer, it is better to have a constant value &lgr;/&Dgr;, wherein &Dgr; is a period of the photonic period due to the concavo-convex structure formed in the transparent subs
Lee Yong-Hee
Lee Yong-jae
Bachman & LaPointe P.C.
Crane Sara
Korea Advanced Institute of Science of Technology
LandOfFree
Photonic crystal organic light emitting device having high... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Photonic crystal organic light emitting device having high..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Photonic crystal organic light emitting device having high... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3135814