Electric lamp and discharge devices – With luminescent solid or liquid material – Solid-state type
Reexamination Certificate
2002-01-25
2004-12-07
Patel, Ashok (Department: 2879)
Electric lamp and discharge devices
With luminescent solid or liquid material
Solid-state type
Reexamination Certificate
active
06828725
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a light emitting panel in which a light emitting element formed on an insulating surface is sealed between a substrate and a cover member. The invention also relates to a light emitting module obtained by mounting a driving circuit to the light emitting panel. ‘Light emitting device’ is herein the generic term for the light emitting panel and for the light emitting module. Specifically, the present invention relates to improving efficiency in taking out light from a light emitting element.
2. Description of the Related Art
In recent years, the technique of forming TFTs on a substrate has made a great advance and application of TFTs to active matrix display devices (light emitting devices) is being developed. TFTs formed of polysilicon films, in particular, have higher field effect mobility (also referred to as mobility) than conventional TFTs that use amorphous silicon films and accordingly can operate at high speed. Therefore pixels now can be controlled by a driving circuit formed on the same substrate on which the pixels are formed instead of a driving circuit external to the substrate as in the past.
Thus having various circuits and elements formed on the same substrate, an active matrix light emitting device provides a lot of advantages including reduction of manufacture cost, reduction in size of electro-optical device, raise in yield, and improvement of throughput.
An active matrix light emitting device using a light emitting element as a self-luminous element is particularly actively researched.
In this specification, a light emitting element has an organic compound layer sandwiched between a pair of electrodes (an anode and a cathode). The organic compound layer may take a laminate structure. As an example, a laminate structure consisting of a hole transporting layer, an organic compound layer, and an electron transporting layer can be given. The term organic compound layer in this specification includes layers for carrier injection, layers for carrier transportation, and layers for carrier recombination all. Luminescence obtained from an organic compound layer is classified into light emission upon return to the base state from singlet excitation (fluorescence) and light emission upon return to the base state from triplet excitation (phosphorescence). The present invention is applicable to a light emitting device using fluorescence and a light emitting device using phosphorescence both.
Heat, light, moisture, oxygen, and the like accelerate degradation of an organic compound layer of a light emitting element. For that reason, in general, a light emitting element is formed after a wiring line and a TFT are formed in a pixel portion in manufacturing an active matrix light emitting device.
After the light emitting element is formed, the substrate on which the light emitting element is formed is bonded to a cover member and sealed (packaged) using a seal member or the like so as not to expose the light emitting element to the outside air.
Once the airtightness is enhanced by packaging or other processing, a connector (FPC, TAB, or the like) is attached to connect a terminal led out of the light emitting element or a circuit formed on the substrate to an external signal terminal. The active matrix light emitting device is thus completed.
Now, a description is given on refraction of light with reference to FIG.
15
. As shown in
FIG. 15
, the angle of refraction of light is determined by the angle of incident light (angle of incident) and index of refraction of the medium thereof. This relation follows Mathematical Expression 1 (Snell's Law) below. When light (incident light) enters, at an angle of &thgr;
1
, a medium
801
having an index of refraction of n
1
and exits a medium
802
having an index of refraction of n
2
, the light (refracted light) has an angle of &thgr;
2
satisfying the Expression 1 below.
Mathematical Expression 1
n
1
*sin &thgr;
1
=n
2
*sin &thgr;
2
(1)
The angle of incident &thgr;
1
that makes the angle &thgr;
2
of the refracted light or transmitted light 90° is called a critical angle. When the angle of incident &thgr;
1
to the medium
802
is larger than the critical angle, the incident light is totally reflected. In other words, the light is trapped in the medium
801
.
FIG. 16
shows the relation between angle of incident and reflectance when the medium
801
is glass (n
1
=1.52) and the medium
802
is air (n
2
=1.00).
As can be seen in
FIG. 16
, the reflectance sharply rises once the angle of incident to the interface reaches 35° or larger. When the angle of incident to the interface is 41° or larger, the light is totally reflected and cannot reach outside of the glass that is the medium
801
.
A critical angle refers to the minimum angle at which total reflection of light at the interface between a medium
1
and a medium
2
takes place, and any angle larger than the critical angle causes total reflection. The magnitude of critical angle varies between media. For example, the critical angle is 41° when the medium
801
is glass and the medium
802
is air whereas it is 42.2° if the medium
801
is acrylic and the medium
802
is air.
Next, reference is made to FIG.
17
. Reference numeral
202
denotes an organic compound layer. An arrow starting from the organic compound layer
202
shown in
FIG. 17
indicates a direction in which light emitted from the organic compound layer
202
travels. The light emitted from the organic compound layer
202
is dispersed in every direction and enters the interface between the bottom face of a substrate
208
and air
209
. Since light travels by nature toward a medium having higher index of refraction, only light that has a small angle of incident with respect to the interface between the substrate
208
and the air
209
can reach the air
209
.
Assume here that the substrate
208
shown in
FIG. 17
is a glass substrate (having an index of refraction of 1.52). Then, of the light emitted from the organic compound layer
202
, one having an angle of incident of 35° or larger and 41.1° or smaller is reflected at the interface with abruptly increased reflectance. Furthermore, light having an angle of incident larger than 41.1° is totally reflected at the interface and therefore cannot be taken out the substrate
208
. Accordingly, the efficiency is low in taking out light emitted from the organic compound layer
202
to the exterior.
To simplify the explanation, this specification focuses on light refracted or reflected at the interface between the substrate
208
and the air
209
while ignoring other light emitted from the organic compound layer
202
, namely, one that is refracted or reflected at the interface of solid thin films such as a gate insulating film and an interlayer insulating film. In actuality, light is always totally reflected or refracted at the interface between different media. For example, the interface between a transparent electrode and an interlayer insulating film, or the interface between an interlayer insulating film and a gate insulating film causes total reflection or refraction of light. However, these are ignored in this specification.
SUMMARY OF THE INVENTION
The present invention has been made to solve the above problem, and an object of the present invention is therefore to provide a light emitting device in which light emitted from a light emitting element can be taken out efficiently.
The structure of a light emitting device according to the present invention will be described with reference to FIG.
1
A. In
FIG. 1A
, an arrow started from an organic compound layer
202
represents light emitted from the organic compound layer
202
.
In
FIGS. 1A and 1B
, reference numeral
201
denotes a transparent electrode (pixel electrode);
202
, the organic compound layer; and
203
, a cathode. An area in which the transparent electrode
201
, the organic compound layer
202
, and the cathode
203
overlap one another corresponds to a light emitti
Cook Alex McFarron Manzo Cummings & Mehler, Ltd.
Krishnan Sumati
Patel Ashok
Semiconductor Energy Laboratory Co,. Ltd.
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