Active solid-state devices (e.g. – transistors – solid-state diode – Organic semiconductor material
Reexamination Certificate
2000-03-23
2004-09-21
Flynn, Nathan J. (Department: 2826)
Active solid-state devices (e.g., transistors, solid-state diode
Organic semiconductor material
C257S501000, C257S506000
Reexamination Certificate
active
06794675
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an organic electroluminescence (EL) display DEVICE which has organic EL elements and thin film transistors (hereinafter referred to as TFTs) acting as switching elements.
2. Description of Related Art
FIG. 1
is a plan view illustrating the display pixel section of a conventional electroluminescence display.
FIG. 2A
is a cross sectional view illustrating the EL display taken along the line A—A of the FIG.
1
.
FIG. 2B
is a cross sectional view illustrating the EL display taken along the line B—B of FIG.
1
.
Referring to
FIG. 1
, the area, surrounded by gate signal lines
51
and drain signal lines
52
, defines a display pixel. A first thin film transistor
30
is formed adjacent to the intersection of the signal lines. A capacitor is formed between the source
13
s
of the thin film transistor
30
and a storage capacitor electrode line
54
. The source
13
s
of the thin film transistor
30
acts as a capacitor electrode
55
and is connected to the gate
41
of a second thin film transistor
40
. The second thin film transistor
40
has the source
43
s
connected to the anode
61
of an organic EL element
60
and the drain
63
d
connected to a power source line
53
for driving the organic EL element.
The storage capacitor electrode line
54
is disposed in parallel to the gate signal line near the thin film transistor. The storage capacitor electrode line
54
is formed of, for instance, chromium. A capacitor is formed between the storage capacitor electrode line
54
and the capacitor electrode
55
connected to the source
13
s
of the thin film transistor, sandwiching the gate insulating film
12
, to accumulate charges. This storage capacitor holds the voltage applied to the gate
41
of the second thin film transistor
40
.
In order to fabricate an organic EL display, display pixels, which each include the organic EL element
60
and the thin film transistors
30
and
40
, are arranged in a matrix form on the substrate
10
.
Referring to
FIGS. 2A and 2B
, an organic EL display is fabricated by forming, in order, thin film transistors and organic EL elements on a glass or resin substrate
10
or a conductive or on a semiconductor substrate
10
. When the conductive or semiconductor substrate is used as the substrate
10
, an insulating film of Sio
2
or SiN is formed on the substrate
10
. Then, the thin film transistors
30
and
40
and the organic EL display element
60
are formed over the insulating film.
First, the first thin film transistor
30
being a switching thin film transistor will be explained below.
Referring to
FIG. 2A
, the gate signal line
51
acting as the gate electrode
11
is formed of a high melting point metal (refractory metal) of chromium (Cr), molybdenum (Mo), or the like deposited on an insulating substrate
10
of quartz glass, non-alkali glass, or the like. The drain signal line
52
is formed of aluminum (Al) on the insulating substrate. The power source line
53
, which acts as a drive power source for the organic EL element and is formed of aluminum, is disposed over the insulating substrate
10
.
A gate insulating film
12
and an active layer
13
of a poly-crystalline silicon (hereinafter referred to as p-Si) film are formed in order. The so-called LDD (Lightly Doped Drain) structure is formed in the active layer
13
. The source
13
s
and the drain
13
d
are disposed outside the LDD regions.
An inter-layered insulating film
15
is formed all over the surface of the gate insulating film
12
, the active layer
13
, and the stopper insulating film
14
. The inter-layered insulating film
15
is formed by depositing a Sio
2
film, a SiN film, and a Sio
2
film, sequentially. In order to form the drain electrode
16
, the contact hole formed toward the drain
13
d
is filled with a metal such as aluminum. Moreover, a planarization insulating film
17
of organic resin is formed all over the surface to provide a flat surface.
Next, the second thin film transistor
60
being a drive thin film transistor for the organic EL element
60
will be explained below.
Referring to
FIG. 2B
, a gate electrode
41
of a refractory metal such as Cr or Mo is formed on the insulating substrate
10
of quartz glass or non-alkali glass.
The gate insulating film
12
and the active layer
43
formed of a p-Si film are deposited one after another.
In the active layer
43
, an intrinsic or a substantially intrinsic channel
43
c
is formed above the gate electrode
41
. Ions are doped into the sides of the channel
43
c
to form the source
43
s
and the drain
43
d.
A SiO
2
film, a SiN film, and a SiO
2
film are deposited in order all over the gate insulating film
12
and the active layer
43
to form an inter-layered insulating film
15
. The contact hole formed toward the drain
43
d
is filled with a metal such as aluminum. Thus, a power source line
53
connected to the power source
50
is formed, a planarization insulating film
17
of organic resin is formed to flatten the whole surface. A contact hole is formed through the planarization insulating film
17
and the inter-layered insulating film
15
at the position corresponding to the source
43
s
. A transparent electrode, that is, the anode
61
of the organic EL element, is formed on the planarization insulating film
17
. The transparent electrode is formed of an ITO (In
2
O
3
—SnO
2
or Indium Tin Oxide) being in contact with the source
13
s
via the contact hole.
The organic EL element
60
has the laminated structure, which is formed by sequentially depositing an anode
61
, an emissive element layer
62
, and a cathode
63
. The anode
61
is formed of an ITO transparent electrode. The emissive layer
62
is formed of a first hole transport layer, a second hole transport layer, an emissive layer, and an electron transport layer. The cathode
63
is formed of a magnesium indium alloy. The cathode
63
is formed on the whole surface (that is, corresponding to the whole surface of the paper) of the substrate
10
forming an organic EL display, shown in FIG.
3
.
In the organic EL element, holes injected from the anode and electrons injected from the cathode are recombined inside the emissive layer. Thus, the recombination acts to excite the organic molecules included in the emissive layer to create excitons. The emissive layer emanates light when the excitons lose radiant energy. The resultant light emanates to the outside via the anode and the transparent insulating substrate.
In order to prevent a short circuit between the cathode
63
and the anode
61
caused by breakage of the emissive layer because of a step difference in thickness of the anode, the insulating film
64
(except the area surrounded by the dotted lines) is formed on the fringes of the anode
61
after formation of the anode
61
. Thereafter, the emissive element layer
62
and the cathode
63
are formed. The emissive element layer
62
radiates light via the insulating substrate
10
.
The organic EL element
60
, as described above, is an element of the type emanating light by supplying current from the power source line
53
to the organic EL element according to the voltage applied to the gate
41
of the second thin film transistor
30
. This element is a current control driven element because of the charge injection type radiation where holes injected from the anode and electrons injected from the cathode recombine inside the emissive layer to radiate light from the emissive layer. For that reason, a current density of 1 mA/cm
2
is required to provide, for example, a luminance of 100 cd/cm
2
. A current density of 100 mA/cm
2
is required to provide, for example, a luminance of 10000 cd/cm
2
.
Such an organic EL element requires a large current for driving. Hence, for the purpose of good displaying, the source
13
s
, being of a semiconductor film, and the ITO must be in reliable electric contact.
However, in the direct contact region between the source
13
s
, being of the p-Si film, and the ITO, a Sio
2
f
Oda Nobuhiko
Suzuki Koji
Yamada Tsutomu
Yamaji Toshifumi
Andújar Leonardo
Cantor & Colburn LLP
Flynn Nathan J.
Sanyo Electric Co,. Ltd.
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