Active solid-state devices (e.g. – transistors – solid-state diode – Incoherent light emitter structure – With reflector – opaque mask – or optical element integral...
Reexamination Certificate
2001-05-04
2003-02-04
Flynn, Nathan J. (Department: 2826)
Active solid-state devices (e.g., transistors, solid-state diode
Incoherent light emitter structure
With reflector, opaque mask, or optical element integral...
C257S257000, C257S091000, C257S099000, C257S744000, C257S745000, C257S749000, C257S313000, C257S491000, C257S492000, C257S503000
Reexamination Certificate
active
06515310
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a device (hereinafter referred to as “light-emitting device”) having a n element (hereinafter referred to as “light-emitting element”) in which a thin film made of a light-emissive material is interposed between a pair of electrodes (an anode and a cathode). In particular, the invention relates to a light-emitting device having a light-emitting element (hereinafter referred to as “EL element”) using a thin film that is made of a light-emissive material capable of EL (electroluminescence). The organic EL display and the organic light-emitting diode (OLED) are included in the light-emitting device according to the invention.
All light-emissive materials that emit light (phosphorescence and/or fluorescence) through singlet excitation or triplet excitation or both are part of the light-emissive materials that can be used in the invention.
2. Description of the Related Art
Light-emitting devices (hereinafter referred to as “EL light-emitting devices”) having an EL element are now being developed. EL light-emitting devices are classified into the passive matrix type and the active matrix type, and EL light-emitting devices of both types operate on the principle that a current is caused to flow through an EL element and thereby a thin film (light-emitting layer) made of a light-emissive material capable of EL emits light.
FIG. 2
shows the structure of a general EL element. As shown in
FIG. 2
, an EL element
200
is formed by laying an anode
202
, a light-emitting layer
203
, and a cathode
204
one on another on an insulator
201
. In general, the cathode
204
as an electron supply source is a metal electrode having a small work function and the anode
202
as a hole supply source is an oxide conductive film (typically, an ITO film) that has a large work function and is transparent to visible light. This is because the metal electrode as the cathode
204
is opaque to visible light and hence light (hereinafter referred to as EL light) that is generated in the light-emitting layer cannot be observed unless the anode
202
is transparent to visible light.
The EL light
205
is observed after directly passing through the anode
202
or being reflected by the cathode
204
and then passing through the anode
202
. That is, an observer
206
can observe the EL light
205
that originates from pixels where the light-emitting layer
203
emits light and that then passes through the anode
202
.
However, there is a problem that incident ambient light (i.e., light outside the light-emitting device)
207
is reflected by the back surface (i.e., the surface adjacent to the light emitting layer) of the cathode
204
in pixels that are not emitting light and the back surface of the cathode
204
acts like a mirror, whereby an external view appears on the observation surface (i.e., the surface on the observer
206
side). One measure against this problem is to attach a circularly polarizing film to the observation surface of an EL light-emitting device. However, a problem still exists that the circularly polarizing film is very expensive and hence its employment increases the manufacturing cost.
SUMMARY OF THE INVENTION
The present invention has been made in view of the above problems in the art, and an object of the invention is therefore to prevent an EL light-emitting device from acting like a mirror surface without using a circularly polarizing film and thereby provide an inexpensive EL light-emitting device through reduction of its manufacturing cost.
Another object of the invention is to provide an inexpensive electric apparatus having such an EL light-emitting device as a display unit.
In a light-emitting device according to the invention, it is characterized that both of a pair of electrodes (an anode and a cathode) that constitute a light-emitting element (typically, an EL element) are formed from the conductive film that is transparent or semitransparent to visible light, and that a light shield film is provided adjacent to the light-emitting element or provided above or below the light-emitting element with an insulating film or a conductive film interposed in between. That is, it is characterized that a light-emitting layer and the light shield film are provided with one of the pair of electrodes interposed in between, or that the light shield film is provided on the anode side or the cathode side of the light-emitting element.
In this specification, the term “transparent to visible light” means that the transmittance for visible light is 80-100% and the term “semitransparent to visible light” means that the transmittance for visible light is 50-80%. Naturally, the transmittance depends on the film thickness. The film thickness may be designed properly so that the transmittance falls within the above range.
FIG. 1A
shows the character of a light-emitting device according to the invention. As shown in
FIG. 1A
, a light shield film
104
is provided adjacent to an EL element
100
that consists of an anode
101
, an EL layer
102
, and a cathode
103
. The anode
101
and the cathode
103
are transparent or semitransparent to visible light. To this end, it is preferable that the anode
101
be an oxide conductive film having a work function of 4.5-5.5.
The cathode
103
needs to be a conductive film (typically, a metal film containing a group-1 or group-2 element in the periodic table) having a work function of 2.0-3.5. Since in many cases such a metal film is opaque to visible light, it is preferable to employ a structure shown in
FIG. 1B
, which is an enlarged view of the EL element
100
. As shown in
FIG. 1B
, the cathode
103
consists of a semitransparent electrode
103
a
and a transparent electrode
103
b.
The semitransparent electrode
103
a
is a metal film containing a group-1 or group-2 element in the periodic table. Being as thin as 5-70 nm (preferably, 10-30 nm), the metal film is semitransparent to visible light. The transparent electrode
103
b
is an oxide conductive film that is transparent to visible light.
The EL layer
102
may have a known structure. That is, the EL layer
102
may be an undoped light-emitting layer or a light-emitting layer containing a dopant (e.g., an organic material that emits light through triplet excitation).
A lamination structure may be formed in which a carrier (electron or hole) injection layer, a carrier transport layer, or a carrier stop layer is laid on a light-emitting layer.
Although in
FIG. 1A
the light shield film
104
is provided adjacent to the anode
101
, it may be provided adjacent to the cathode
103
. An insulating film or a conductive film may be provided between the light shield film
104
and the anode
101
(or between the light shield film
104
and the cathode
103
).
In the invention, the light shield film may be a thin film that is made of a material having a large absorption coefficient for visible light. Typical examples of the light shield film are an insulating film (preferably, a resin film) dispersed with metal particles or carbon particles, a metal film (preferably, a titanium film, a titanium nitride film, a chromium film, a molybdenum film, a tungsten film, a tantalum film, or a tantalum nitride film) having a small reflectance value, and a semiconductor film.
With the structure of
FIG. 1A
, EL light
105
can pass through the cathode
103
and hence is observed directly by an observer
106
. Most of ambient light
107
reaching the light shield film
104
is absorbed by the light shield film
104
and hence reflection light is weak enough to be unproblematic. That is, the reflection light does not reach the observer
106
and hence the problem that an external view appears on the observation surface can be solved.
Capable of solving the problem in the prior art without the need for using an expensive circularly polarizing film, the invention makes it possible to provide inexpensive light-emitting devices. Further, inexpensive electric apparatuses can be provided by using a light-emitting device according t
Koyama Jun
Yamazaki Shunpei
Fish & Richardson P.C.
Flynn Nathan J.
Fordë Remmon R.
Semiconductor Energy Laboratory Co,. Ltd.
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