U.S. patent application number 10/754701 was filed with the patent office on 2004-09-09 for light emitting device and electrical appliance.
This patent application is currently assigned to Semiconductor Energy Laboratory Co., LTD, a Japan Corporation. Invention is credited to Inukai, Kazutaka, Yamazaki, Shunpei.
Application Number | 20040173811 10/754701 |
Document ID | / |
Family ID | 18656190 |
Filed Date | 2004-09-09 |
United States Patent
Application |
20040173811 |
Kind Code |
A1 |
Yamazaki, Shunpei ; et
al. |
September 9, 2004 |
Light emitting device and electrical appliance
Abstract
There is provided a light emitting device which is bright and
has low electric power consumption and high reliability. A triplet
EL element 203 electrically connected to a current controlling TFT
102 is provided in a pixel portion 201. A luminescent material used
for the triplet EL element 203 has a feature that EL is obtained by
triplet excitation, and exhibits high luminous efficiency at a low
operation voltage as compared with the prior art. Accordingly, the
operation is made at the low operation voltage, so that the light
emitting device which is bright and has low electric power
consumption and high reliability can be obtained.
Inventors: |
Yamazaki, Shunpei; (Tokyo,
JP) ; Inukai, Kazutaka; (Kanagawa, JP) |
Correspondence
Address: |
FISH & RICHARDSON P.C.
1425 K STREET, N.W.
11TH FLOOR
WASHINGTON
DC
20005-3500
US
|
Assignee: |
Semiconductor Energy Laboratory
Co., LTD, a Japan Corporation
|
Family ID: |
18656190 |
Appl. No.: |
10/754701 |
Filed: |
January 12, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10754701 |
Jan 12, 2004 |
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09862680 |
May 21, 2001 |
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6677621 |
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Current U.S.
Class: |
257/103 |
Current CPC
Class: |
H01L 51/0084 20130101;
H01L 51/5016 20130101; H01L 27/3244 20130101; H01L 51/0081
20130101; H01H 2219/037 20130101; G09G 2310/0251 20130101; G09G
3/3607 20130101; H01L 27/1214 20130101; G09G 3/2022 20130101; H01L
51/0078 20130101; G09G 2300/0426 20130101; H01L 51/0077 20130101;
G09G 2300/0842 20130101; G09G 3/3258 20130101 |
Class at
Publication: |
257/103 |
International
Class: |
H01L 033/00 |
Foreign Application Data
Date |
Code |
Application Number |
May 22, 2000 |
JP |
2000-150484 |
Claims
What is claimed is:
1. A light emitting device comprising: an electroluminescent
element using a luminescent material in which electroluminescence
is obtained by triplet excitation; and a semiconductor component
electrically connected to the electroluminescent element.
2. A device according to claim 1, wherein the semiconductor
component is a TFT.
3. An electrical appliance using a light emitting device according
to claim 1.
4. A portable telephone using a light emitting device according to
claim 1.
5. A digital camera using a light emitting device according to
claim 1.
6. An audio equipment using a light emitting device according to
claim 1.
7. A wireless portable equipment using a light emitting device
according to claim 1.
8. A light emitting device comprising: a semiconductor component;
and an electroluminescent element electrically connected to the
semiconductor component, wherein the electroluminescent element
includes a thin film made of a luminescent material expressed by a
following formula: 3wherein Et represents etyl group; and M
represents element belonging to group 8 to 10 of the periodic
table.
9. A device according to claim 8, wherein said M is an element
selected from the group consisting of nickel, cobalt and
palladium.
10. A device according to claim 8, wherein the luminescent material
is a metal complex containing an element selected from the group
consisting of nickel, cobalt and palladium.
11. A device according to claim 8, wherein the luminescent material
is an organic compound containing an element selected from the
group consisting of nickel, cobalt and palladium.
12. A device according to claim 8, wherein the semiconductor
component is a TFT.
13. An electrical appliance using a light emitting device according
to claim 8.
14. A portable telephone using a light emitting device according to
claim 8.
15. A digital camera using a light emitting device according to
claim 8.
16. An audio equipment using a light emitting device according to
claim 8.
17. A wireless portable equipment using a light emitting device
according to claim 8.
18. A light emitting device comprising: a semiconductor component;
and an electroluminescent element electrically connected to the
semiconductor component, wherein the electroluminescent element
includes a thin film made of a luminescent material expressed by a
following formula: 4wherein Et represents etyl group; and M
represents element belonging to group 8 to 10 of the periodic
table.
19. A device according to claim 18, wherein said M is an element
selected from the group consisting of nickel, cobalt and
palladium
20. A device according to claim 18, wherein the luminescent
material is a metal complex containing an element selected from the
group consisting of nickel, cobalt and palladium.
21. A device according to claim 18, wherein the luminescent
material is an organic compound containing an element selected from
the group consisting of nickel, cobalt and palladium.
22. A device according to claim 18, wherein the semiconductor
component is a TFT.
23. An electrical appliance using a light emitting device according
to claim 18.
24. A portable telephone using a light emitting device according to
claim 18.
25. A digital camera using a light emitting device according to
claim 18.
26. An audio equipment using a light emitting device according to
claim 18.
27. A wireless portable equipment using a light emitting device
according to claim 18.
28. A light emitting device comprising: an electroluminescent
element using a luminescent material in which electroluminescence
is obtained by triplet excitation; and a thin film transistor
electrically connected to the electroluminescent element, wherein a
voltage applied to the electroluminescent element through the thin
film transistor is 4 to 6 V.
29. An electrical appliance using a light emitting device according
to claim 28.
30. A portable telephone using a light emitting device according to
claim 28.
31. A digital camera using a light emitting device according to
claim 28.
32. An audio equipment using a light emitting device according to
claim 28.
33. A wireless portable equipment using a light emitting device
according to claim 28.
34. A light emitting device comprising: an electroluminescent
element using a luminescent material in which electroluminescence
is obtained by triplet excitation; a thin film transistor
electrically connected to the electroluminescent element; a source
signal line; and a power supply line connected to a source side of
the thin film transistor, wherein a voltage of the power supply
line is 4 to 6 V.
35. An electrical appliance using a light emitting device according
to claim 34.
36. A portable telephone using a light emitting device according to
claim 34.
37. A digital camera using a light emitting device according to
claim 34.
38. An audio equipment using a light emitting device according to
claim 34.
39. A wireless portable equipment using a light emitting device
according to claim 34.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a light emitting device
including a element (hereinafter referred to as an EL element) in
which a thin film (hereinafter referred to as an EL film) made of a
luminescent material capable of generating electroluminescence
(hereinafter abbreviated as EL) is interposed between a pair of
electrodes. Incidentally, in the present specification, an EL
element using a luminescent material in which EL is obtained by
singlet excitation is called a singlet EL element, and an EL
element using a luminescent material in which EL is obtained by
triplet excitation is called a triplet EL element.
[0003] 2. Description of the Related Art
[0004] In recent years, an EL element using an organic EL film as a
light emitting layer is has been developed, and EL elements using
various organic EL films have been proposed. An attempt to realize
a flat panel display by employing a light emitting device, which
uses an EL element of the kind as a light emitting element, has
been made.
[0005] As a light emitting device using an EL element, a passive
matrix type and an active matrix type are known. The passive matrix
type is a light emitting device using an EL element made of a
structure in which stripe-like anodes and cathodes are provided to
intersect with each other at right angles and an EL film is
interposed between them. The active matrix type is a system in
which a semiconductor component is provided for each of pixels, and
one of an anode and a cathode of an EL element is connected to the
semiconductor component, so that a current flowing through the EL
element is controlled by the semiconductor component.
[0006] However, in both the passive matrix type light emitting
device and the active matrix type light emitting device, since the
light emission performance of the EL element is greatly influenced
by the physical properties of the EL film itself, the development
of the EL element being bright and having high reliability has been
just the development of a luminescent material.
[0007] Although various kinds of luminescent materials from a low
molecular material to a high molecular material are developed, a
theoretical upper limit of luminous efficiency has been always a
problem. Especially, with respect to internal quantum efficiency,
it has been thought that the ratio of the generation efficiency of
singlet exciton to the generation efficiency of triplet exciton is
1:3, and only the singlet exciton contributes to light emission
(fluorescent emission).
[0008] Thus, even if all carriers (electrons and holes) are
recombined, 25% of the whole contributes to the light emission, and
if extracting efficiency to the outside of the component is 20%,
external quantum efficiency becomes 5% in total. That is, according
to the calculation, only 5% of consumed energy can be
extracted.
[0009] However, recently, a material in which light emission
(phosphorescent emission) using triplet exciton is proposed, and
its high luminous efficiency attracts attention. As examples which
the triplet exciton is used and the external quantum efficiency is
improved, there are following reports.
[0010] (1) T. Tsutsui, C Adachi, S. Saito, Photochemical Processes
in Organized Molecular System, ed. K. Honda, (Elsevier Sci. Pub.,
Tokyo, 1991) p. 437.
[0011] (2) M. A. Baldo, D. F. O'Brien, Y. You, A. Shoustikov, S.
Sibley, M. E. Thompson, S. R. Forrest, Nature 395 (1998) p.
151.
[0012] (3) M. A. Baldo, S. Lamansky, P. E. Burrows, M. E. Thompson,
S. R. Forrest, Appl. Phys. Lett., 75 (1999) p.4.
[0013] (4) T. Tsutsui, M.-J. Yang, M. Yahiro, K. Nakamura, T.
Watanabe, T. Tsuji, Y. Fukuda, T. Wakimoto, S. Mayaguchii, Jpn.
Appl. Phys., 38 (12B) (1999) L1502.
[0014] The luminescent materials set forth in the above papers are
examples in which the external quantum efficiency is improved by
obtaining phosphorescence from the triplet exciton.
SUMMARY OF THE INVENTION
[0015] An object of the present invention is to provide a light
emitting device which is bright and has low electric power
consumption, by using an EL element having high luminous
efficiency.
[0016] Another object of the present invention is to provide an
electrical appliance which has a bright display portion and low
electric power consumption, by using the light emitting device.
[0017] Still another object of the present invention is to provide
an electrical appliance having low electric power consumption by
using the light emitting device of the present invention as a light
source (typically, a backlight).
[0018] A light emitting device of the present invention is
characterized in that a triplet EL element is electrically
connected to a semiconductor component and is controlled. That is,
the present invention is characterized by using the triplet EL
element as a light emitting element in an active matrix type light
emitting device. As the semiconductor component, a field effect
transistor (FET), preferably a thin film transistor (TFT) can be
used.
[0019] First, a process up to the present invention will be
described. When a voltage is applied between an anode and a cathode
of an EL element, a carrier (electron or hole) is injected into an
EL film, and luminescence is generated by recombination. Thus, a
proportionality relation is obtained between density of current
flowing through the EL element and luminous brightness.
Incidentally, in the present specification, a voltage applied to an
EL element is called an operation voltage of the EL element.
[0020] FIG. 3 is a graph schematically showing this relation. FIG.
3 shows the relation between the density of current flowing through
an EL element and the luminous brightness. Reference numeral 301
indicates a characteristic of a conventional EL element using
singlet excitation (singlet EL element); and 302, a characteristic
of an EL element using triplet excitation (triplet EL element).
[0021] The characteristic of the singlet EL element shown as 301 is
that although the proportionality relation (linear relation) is
obtained when the current density is low, an inclination becomes
small as the current density becomes high. That is, it is known
that even if the current density increases, the luminous brightness
becomes hard to increase over a certain point. This tendency is
remarkable in the case of the characteristic of the triplet EL
element shown as 302. When the current density is low, it is in the
proportionality relation having an inclination larger than the
singlet EL element with respect to the luminous brightness.
However, when the current density increases, the inclination
becomes extremely small, and there occurs such a state where
luminous brightness is hardly changed even if the current density
increases.
[0022] From the graph of FIG. 3, it is understood that although the
luminous brightness is several times higher than that of the
singlet EL element when the triplet EL element emits light in the
operation region of small current density, it becomes almost equal
to the luminous brightness of the singlet EL element in the
operation region of high current density.
[0023] FIG. 4 shows a relation between the operation voltage of an
EL element and its luminous efficiency. Reference numeral 401
indicates a characteristic of a singlet EL element; and 402, a
characteristic of a triplet EL element. An operation voltage "a"
indicates an operation voltage (8 to 12 V) where the luminous
efficiency of the singlet EL element becomes highest, and an
operation voltage "b" indicates an operation voltage (3 to 5 V)
where the luminous efficiency of the triplet EL element becomes
highest.
[0024] At this time, as shown in the graph of FIG. 4, the triplet
EL element has a feature that its luminous efficiency becomes
highest when the operation voltage is lower than the singlet EL
element. That is, the triplet EL element shows higher luminous
efficiency than the singlet EL element in the operation region of
low current density. This is not contradictory to the graph of FIG.
3 in which the triplet EL element shows higher luminous brightness
than the singlet EL element in the operation region of low current
density.
[0025] Here, from the characteristics of the triplet EL element
shown in the graphs of FIGS. 3 and 4, relations in FIGS. 5 and 6
can be schematically derived. A graph of FIG. 5 shows a relation
between electric power consumption and luminous efficiency in the
triplet EL element, and it is understood that the luminous
efficiency becomes low in the operation region of high electric
power consumption. A graph of FIG. 6 shows a relation between
electric power consumption and luminous brightness in the triplet
EL element, and it is understood that an increasing rate of the
luminous brightness is low in the operation region of high electric
power consumption.
[0026] From the above, the present inventor has considered that it
is desirable to make the triplet EL element emit light in the
operation region of low current density, that is in the operation
region of low operation voltage.
[0027] Here, the driving principle of a passive matrix type light
emitting device becomes a problem. In the case of the passive
matrix type light emitting device, since only one selected pixel
emits light, a time obtained by dividing one frame period (normally
{fraction (1/60)} second) by the number of pixels becomes a
luminous time. That is, as the number of pixels becomes large and
the fineness becomes high, a luminous time per pixel becomes short.
Thus, in order to enable a bright and highly fine image display,
the luminous brightness per pixel is raised, and a high density
current must be made to flow instantaneously.
[0028] Accordingly, in the case where the triplet EL element is
used for the passive matrix type light emitting device, light
emission is made in the operation region of high current density in
FIG. 3, that is, by the operation voltage higher than the operation
voltage "b" in FIG. 4, and the light is made to emit in the
operation region where the luminous efficiency is low. Accordingly,
in order to obtain high luminous brightness, it is necessary that a
higher current is made to flow, and as a result, the electric power
consumption increases and the EL film is deteriorated.
[0029] From the above, the present inventor has considered that the
passive matrix type light emitting device is disadvantageous to the
light emission of the triplet EL element, and has found that an
active matrix type light emitting device is most suitable for the
triplet EL element. This is because a luminous time can be
controlled by a semiconductor component in the active matrix type
light emitting device, so that the density of current made to flow
through the triplet EL element can be greatly suppressed.
[0030] Therefore, the inventor considers that the inventiveness of
the present invention is in the point that the triplet EL element
is combined with the active matrix type light emitting device as
the most suitable light emitting device in view of the electric
characteristics of the triplet EL element.
[0031] In the active matrix type light emitting device using the
triplet EL element according to the present invention, when the
operation voltage of the EL element is low, high luminous
efficiency is obtained, so that it becomes possible to carry out a
bright image display having high luminous brightness. Accordingly,
the operation voltage may be made 10 V or less, preferably 7.5 V or
less, more preferably 5 V or less. Besides, since it is expected
that materials are further developed from now, it is conceivable
that the operation voltage becomes 2.5 to 10 V.
[0032] Besides, in the active matrix type light emitting device,
since the luminous time of the EL element can be made long, the
operation voltage of the EL-element can be set low when the same
brightness (illuminance) as the passive matrix type light emitting
device is secured. That is, the electric power consumption can be
suppressed as compared with the passive matrix type light emitting
device.
[0033] This point is schematically shown in FIGS. 7A and 7B. In
FIG. 7A, the horizontal axis indicates the number of pixels
included in a pixel portion, and the vertical axis indicates the
luminous time in an arbitrary pixel. Incidentally, the luminous
time in an arbitrary pixel is a time when the arbitrary one pixel
continues emitting light, and here, in the case of the active
matrix type (indicated by 701) and the case of the passive matrix
type (indicated by 702), a time necessary to secure the same
luminous brightness is considered as the luminous time.
[0034] The graph of FIG. 7A shows that when the number of pixels
becomes large (becomes high fineness), the luminous times necessary
to secure the same luminous brightness are different from each
other between the passive matrix type and the active matrix type.
That is, since the luminous time can be controlled by a
semiconductor component in the active matrix type, the almost equal
luminous time can be secured irrespective-of the number of pixels.
However, in the passive matrix type, when the number of pixels
increases, the luminous time decreases.
[0035] Thus, in order to secure the same luminous brightness, the
relation between the number of pixels and the current density
becomes as shown in FIG. 7B. That is, in the case of the active
matrix type (indicated by 703), even if the number of pixels
becomes large, the current density may be almost constant and
small. However, in the case of the passive matrix type (indicated
by 704), when the number of pixels becomes large, the current
density necessary to secure the luminous brightness greatly
increases.
[0036] Therefore, when the number of pixels increases and high
fineness is achieved, the active matrix type light emitting device
in which the current density may be low, becomes advantageous in
view of the suppression of electric power consumption as compared
with the passive matrix type light emitting device.
[0037] Therefore, since the triplet EL element can obtain the most
efficient light emission in the region of low operation voltage,
when it is combined with the active matrix type light emitting
device, it is possible to realize a light emitting device which has
low electric power consumption and enables a bright image display.
Further, since the operation voltage of the EL element is low, the
density of current made to flow through the EL element may be low,
so that the light emitting device having a long lifetime (high
reliability) can be obtained.
BRIEF DESCRIPTION OF THE DRAWINGS
[0038] In the accompanying drawings:
[0039] FIG. 1 is a view showing a sectional structure of a light
emitting device;
[0040] FIGS. 2A and 2B are views showing an upper structure and a
sectional structure of a light emitting device;
[0041] FIG. 3 is a view showing a relation between current density
and luminous brightness;
[0042] FIG. 4 is a view showing a relation between operation
voltage and luminous efficiency;
[0043] FIG. 5 is a view showing a relation between electric power
consumption and luminous efficiency;
[0044] FIG. 6 is a view showing a relation between electric power
consumption and luminous brightness;
[0045] FIGS. 7A and 7B are views showing a relation between the
number of pixels and luminous time and a relation between the
number of pixels and current density;
[0046] FIGS. 8A and 8B are views showing a circuit structure of a
light emitting device and a pixel structure;
[0047] FIG. 9 is a view for explaining an operation of gradation
display by a time division method;
[0048] FIG. 10 is a view showing a pixel structure of a light
emitting device;
[0049] FIG. 11 is a view showing a pixel structure of a light
emitting device;
[0050] FIGS. 12A and 12B are views showing timing charts of signals
inputted to gate wiring lines;
[0051] FIGS. 13A to 13F are views showing specific examples of
electrical appliances;
[0052] FIGS. 14A and 14B are views showing specific examples of
electrical appliances;
[0053] FIG. 15 is a sectional view showing an example of an active
matrix type light emitting device;
[0054] FIG. 16 is a top view showing a structure of a pixel portion
of an active matrix type light emitting device;
[0055] FIG. 17 is an equivalent circuit diagram of a pixel portion
of an active matrix type light emitting device;
[0056] FIG. 18 is a graph showing characteristics of current
density versus brightness in a triplet EL element and a singlet EL
element;
[0057] FIG. 19 is a graph showing characteristics of voltage versus
luminous efficiency in a triplet EL element and a singlet EL
element; and
[0058] FIG. 20 is a graph showing characteristics of electric power
consumption versus brightness in a triplet EL element and a singlet
EL element.
[0059] FIG. 21 is a graph showing lifetime of a TFT of the present
invention.
[0060] FIG. 22 is a graph showing characteristics of 10-year
guarantee voltage versus channel length of a TFT of the present
invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0061] [Embodiment Mode 1]
[0062] An embodiment mode of the present invention will be
described with reference to FIG. 1. FIG. 1 is a sectional view of
an active matrix type light emitting device using a triplet EL
element according to the present invention. Although a TFT is used
here as a semiconductor component, the number of TFTs included in
one pixel is not limited.
[0063] In FIG. 1, reference numeral 101 designates a substrate, and
here, a substrate transparent to visible light is used.
Specifically, a glass substrate, a quartz substrate, a crystallized
glass substrate, or a plastic substrate (including a plastic film)
may be used. Incidentally, the substrate 101 includes also an
insulating film provided on the surface of the substrate.
[0064] A pixel portion 201 and a driving circuit 202 are provided
on the substrate 101. First, the pixel portion 201 will be
described.
[0065] The pixel portion 201 is a region where an image display is
carried out, and includes a plurality of pixels. A TFT (hereinafter
referred to as a current controlling TFT) 102 for controlling a
current flowing through an EL element, and a triplet EL element 203
are provided in each of the pixels. Although only the current
controlling TFT 102 is shown here, a TFT (hereinafter referred to
as a switching TFT) for controlling a voltage applied to a gate of
the current controlling TFT is provided.
[0066] Besides, it is preferable that a p-channel TFT is used for
the current controlling TFT 102. Although an n-channel TFT can be
used, in the case where the current controlling TFT is connected to
an anode of the EL element as in the structure of FIG. 1, electric
power consumption can be suppressed when the p-channel TFT is used.
However, the switching TFT (not shown) may be an n-channel TFT or a
p-channel TFT.
[0067] A pixel electrode 103 is electrically connected to a drain
of the current controlling TFT 102. Here, since a conductive
material having a work function of 4.5 to 5.5 eV is used as a
material of the pixel electrode 103, the pixel electrode 103
functions as an anode of the triplet EL element 203. As the pixel
electrode 103, typically, indium oxide, tin oxide, zinc oxide, or a
compound of these may be used.
[0068] An EL layer 104 is provided on the pixel electrode 103.
Incidentally, in the present specification, the EL layer generally
indicates a layer where an organic material functioning as a hole
injecting layer, a hole transporting layer, a hole blocking layer,
an electron injecting layer, an electron transporting layer, or an
electron blocking layer is laminated on an EL film (light emitting
layer). However, the EL layer includes also a case where the EL
film is used as a single layer.
[0069] The present invention is characterized in that a thin film
made of a luminescent material in which light is emitted by triplet
excitation is used as a light emitting layer. As such luminescent
materials, a luminescent material set forth in the papers cited as
the prior art can be used. Besides, as a layer structure of the EL
layer 104, a layer structure set forth in the papers cited as the
prior art can be used.
[0070] Besides, the present inventor considers that in addition to
the luminescent materials set forth in the above papers,
luminescent materials (specifically, metal complex or metal organic
compound) expressed by the following molecular formulae can be
used. 1
[0071] Et: ETYL GROUP
[0072] M: ELEMENT OF 8-10 FAMILIES IN PERIODIC TABLE 2
[0073] M: ELEMENT OF 8-10 FAMILIES IN PERIODIC TABLE
[0074] In the above molecular formulae, M designates an element
belonging to groups 8 to 10 of the periodic table. In the above
papers, platinum or iridium is used. However, the present inventor
considers that iron, nickel, cobalt, or palladium is preferable.
Since iron, nickel, cobalt and palladium are inexpensive as
compared with platinum and iridium, they are effective in reduction
of manufacturing cost of the light emitting device. Especially,
nickel is preferable since a complex is easily formed and the
productivity is also high.
[0075] Next, a cathode 105 is provided on the EL layer 104. As a
material of the cathode 105, a conductive material having a work
function of 2.5 to 3.5 eV is used. As the cathode 105, a conductive
film typically containing an element belonging to group I or group
II of the periodic table may be used.
[0076] The triplet EL element 203 made of the pixel electrode 103,
the EL layer 104, and the cathode 105 is covered with a protection
film 106. The protection film 106 is provided to protect the
triplet EL element 203 against oxygen and water. As a material of
the protection film 106, a silicon nitride film, a silicon nitride
oxide film, an aluminum oxide film, a tantalum oxide film, or a
carbon film (specifically, a diamond-like carbon film) is used.
[0077] Next, the driving circuit 202 will be described. The driving
circuit 202 is a region for controlling timing of signals (gate
signal and data signal) transmitted to the pixel portion 201, and
is provided with a shift register, a buffer, a latch, an analog
switch (transfer gate) or a level shifter. Here, a CMOS circuit
made of an n-channel TFT 107 and a p-channel TFT 108 is shown as a
basic unit of these circuits.
[0078] The circuit structure of the shift register, the buffer, the
latch, the analog switch (transfer gate) or the level shifter may
be a well-known one. In FIG. 1, although the pixel portion 201 and
the driving circuit 202 are provided on the same substrate, it is
also possible to connect an IC or an LSI without providing the
driving circuit 202.
[0079] In the active matrix type light emitting device shown in
FIG. 1, although a top gate type TFT (specifically, a planar type
TFT) is mentioned as an example of the TFT, a bottom gate type TFT
(specifically, a reverse stagger type TFT) may be used.
[0080] Here, although the anode of the triplet EL element 203 is
electrically connected to the current controlling TFT 102, such a
structure may be adopted that the cathode of the triplet EL element
is electrically connected to the current controlling TFT. In that
case, the pixel electrode is formed of a material used in the
cathode 105, and the cathode is formed of a material used in the
pixel electrode 103. In that case, it is preferable that the
current controlling TFT is made as an n-channel TFT.
[0081] Here, the whole structure of the active matrix type light
emitting device shown in FIG. 1 is shown in FIGS. 2A and 2B. FIG.
2A is a top view, and FIG. 2B is a sectional view cut along A-A' of
FIG. 2A. Besides, the reference characters in FIG. 1 are used.
[0082] In FIG. 2A, reference numeral 211 designates a pixel
portion; 212, a gate signal side drive circuit; and 213, a data
signal side drive circuit. Signals transmitted to the gate signal
side drive circuit 212 and the data signal side drive circuit 213
are inputted from an FPC (Flexible Printed Circuit) 215 through an
input wiring line 214.
[0083] At this time, reference numeral 216 designates a cover
member provided above the triplet EL element 203 shown in FIG. 1,
and is bonded by a seal member 217 made of resin. Any material may
be used for the cover member 216 as long as the material is
impermeable to oxygen and water. Here, as shown in FIG. 2B, the
cover member 216 is made of a plastic member 216a, and carbon films
(specifically, diamond-like carbon films) 216b and 216c provided on
the front surface and the back surface of the plastic member
216a.
[0084] Further, as shown in FIG. 2B, the seal member 217 is covered
with an encapsulation member 218 made of resin, and the triplet EL
element 203 is completely sealed in an airtight space 219. At this
time, the airtight space 219 may be filled with an inert gas
(typically, nitrogen gas or rare gas), a resin or an inert liquid
(typically, liquid fluorocarbon typified by perfluoroalkane).
Further, it is also effective to provide a moisture absorbent or a
deoxidation agent.
[0085] The active matrix type light emitting device shown in this
embodiment mode has a feature that a bright image display with low
electric power consumption is enabled by using the triplet EL
element as the light emitting element. Since the necessary
operation voltage of the triplet EL element is low, the device also
has a feature that the lifetime of the EL film is long and the
reliability is high.
[0086] [Embodiment Mode 2]
[0087] In this embodiment mode, the description will be given of a
case where an active matrix type light emitting device using a
triplet EL element according to the present invention is operated
in by digital driving. Incidentally, the digital driving is a
driving method for carrying out an image display by using digital
signals.
[0088] Besides, here, the description will be given of an example
in which a time division method. (time-sharing method) is used as a
method of carrying out a gradation display. The time division
method is a technique for visually enabling a plurality of
gradation displays by combination of luminous times.
[0089] Here, when the digital driving is performed, in this
embodiment mode, an active matrix type light emitting device of a
circuit structure shown in FIG. 8A is adopted. In FIG. 8A,
reference numeral 801 designates a pixel portion; 802, a data
signal side drive circuit; and 803, a gate signal side drive
circuit. The data signal side drive circuit 802 includes a shift
register 802a, a first stage latch 802b, and a second stage latch
802c.
[0090] Besides, a plurality of pixels 804 are formed in the pixel
portion 801. FIG. 8B is an enlarged view of the pixel 804.
Reference numeral 805 designates agate wiring line; 806, a data
wiring line; 807, a switching TFT; 808, a current controlling TFT;
809, a triplet EL element; 810, a capacitor; and 811, a current
supply line. Incidentally, the switching TFT 807 and the current
controlling TFT 808 may be respectively made to have multigate
structure in which a plurality of channel formation regions are
provided between a source region and a drain region. Especially, it
is effective that the switching TFT 807 is made to have the
multigate structure in view of the suppression of an off current
(drain current flowing when the TFT is in an off state).
[0091] In the pixel of the above structure, a data signal
transmitted to the data wiring line 806 is charged in the capacitor
810 when the gate of the switching TFT 807 is opened. Besides, this
data signal is applied to the gate of the current controlling TFT
808, and when the gate of the current controlling TFT 808 opens, a
signal transmitted to the current supply line 811 is applied to the
EL element 809. In this way, a predetermined voltage is applied to
the EL element 809 and light emission occurs.
[0092] Next, an example in which the gradation display is carried
out by the time division method will be described. Here, the
description will be given of an example in which a full color
display of 256 gradations (16.77 million colors) is carried out by
an 8-bit digital driving system.
[0093] First, one frame of an image is divided into eight fields.
Incidentally, one period when data is inputted to all pixels of a
display region is called one frame, and in a normal EL display, an
oscillation frequency is 60 Hz, that is, 60 frames are formed in
one second. Further, a frame obtained by further dividing one frame
into plural parts is called a field.
[0094] One field is divided into an address period (Ta) and a
sustain period (Ts). The address period indicates the whole time
required for inputting data into all pixels in one field period,
and the sustain period (may also be called a lighting period)
indicates a period when an EL element is made to emit light.
[0095] Here, a first field is called F1, and subsequently, a second
field to an eighth field are called F2 to F8. The address periods
(Ta) are constant from F1 to F8. On the other hand, the sustain
periods (Ts) of F1 to F8 are respectively made Ts1 to Ts8.
[0096] At this time, the sustain periods are set to establish
Ts1:Ts2:Ts3:Ts4:Ts5:Ts6:Ts7: Ts8=1/2:1/4:1/8:{fraction
(1/16)}:{fraction (1/32)}:{fraction (1/64)}:{fraction (1/128)}.
However, the sequence of the appearance of F1 to F8 may be random.
By the combination of the sustain periods, a desired gradation
display can be carried out among 256 gradations.
[0097] First, in a state where a voltage is not applied (selected)
to an electrode of the EL element at a side where it is not
connected to the TFT, data signals are inputted to the respective
pixels without making the EL element emit light. This period is the
address period. When the data is inputted to all pixels and the
address period is ended, a voltage is applied (selected) to the
electrode to make the EL element emit light at the same time. This
period is the sustain period. The period of light emission (pixel
is lighted) is any one of the periods Ts1 to Ts8. Here, it is
assumed that a predetermined pixel is lighted in the period
Ts8.
[0098] Next, again in the address period, data signals are inputted
to all pixels, and then the sustain period starts. At this time,
any one of the periods Ts1 to Ts7 becomes the sustain period. Here,
it is assumed that a predetermined pixel is lighted in the period
Ts7.
[0099] Subsequently, a similar operation is repeated with respect
to remaining six fields, and it is assumed that the sustain periods
are sequentially set as Ts6, Ts5 . . . Ts1, and predetermined
pixels are lighted in the respective fields.
[0100] When the eight fields appear, one frame is ended. At this
time, the gradation of the pixel is controlled by the addition of
the sustain periods. For example, in the case where Ts1 and Ts2 are
selected, when the whole lighting is made as 100%, brightness of
75% can be expressed, and in the case where Ts3, Ts5 and Ts8 are
selected, brightness of 16% can be expressed.
[0101] Although the description has been given of the case of 256
gradations, another gradation display can also be carried out.
Next, the case where an n-bit gradation display is carried out will
be described with reference to FIG. 9.
[0102] In the case where a display of n-bit (n is an integer of two
or more) gradations (2.sup.n gradations) is carried out, first, one
frame is made to correspond to the n-bit gradations and is divided
into n fields (expressed by F1, F2, F3 . . . F(n-1), F(n)). As the
number of gradations becomes large, the division number of one
frame is also increased, and the driving circuit must be driven at
high frequency.
[0103] Further, each of these n fields is divided into an address
period (Ta) and a sustain period (Ts). That is, by selecting
whether or not a voltage is applied to an electrode (electrode at
the side where it is not connected to the TFT) common to all EL
elements, the address period and the sustain period are
selected.
[0104] Then, the sustain periods of the respective n fields
(sustain periods corresponding to F1, F2, F3 . . . F(n-1), F(n) are
respectively expressed by Ts1, Ts2, Ts3 . . . Ts(n-1), Ts(n)) are
processed to establish Ts1:Ts2:Ts3: . . .
:Ts(n-1):Ts(n)=2.sup.0:2.sup.-1:2.sup.-2: . . .
:2.sup.-(n-2):2.sup.-(n-1).
[0105] In this state, in an arbitrary field, a pixel is
sequentially selected (strictly speaking, a switching TFT of each
pixel is selected), and a predetermined gate voltage (corresponding
to a data signal) is added to the gate of the current controlling
TFT. At this time, an EL element of pixel in which a data signal to
make the current controlling TFT have a conductive state is
inputted, emits light only in the sustain period allocated to the
field after the address period is ended, that is, a predetermined
pixel is lighted.
[0106] This operation is repeated for all of the n fields, and the
gradation of each pixel is controlled by the addition of the
sustain periods. Accordingly, when attention is paid to one
arbitrary pixel, the gradation of the one pixel is controlled
according to how long the pixel is lighted in each field (via how
many sustain periods).
[0107] By the technique as described above, the time division
gradation display becomes possible. In order to carry out the
gradation display by the time division method, digital driving is
desirable. Since the EL element has high responsibility to an
electric signal, it is advantageous when the time division method
is used.
[0108] However, the present invention is not limited to the above
driving method, but analog driving (driving method using analog
signals) may be used, and in addition to the time division method,
the gradation display may be carried out by using an area division
method (also called an area gradation). In the case of the analog
driving, a voltage gradation method by analog signals may be
used.
[0109] The driving method described in this embodiment mode of the
present invention can be used as a driving method of the light
emitting device described in Embodiment Mode 1.
[0110] [Embodiment Mode 3]
[0111] In this embodiment mode, an example in which the present
invention is applied to an active matrix type light emitting device
having such a pixel structure that three semiconductor components
are provided in one pixel, will be described with reference to FIG.
10.
[0112] In FIG. 10, reference numeral 1001 designates a source
wiring line of a switching TFT 1002; 1003, a gate wiring line of
the switching TFT 1002; 1004, a current controlling TFT; 1005, a
capacitor (may be omitted); 1006, a current supply line; 1007, an
erasing TFT; 1008, an erasing gate wiring line; and 1009, a triplet
EL element. Incidentally, with respect to the operation of the
erasing TFT 1007 and the pixel structure, Japanese Patent
Application No. Hei. 11-338786 can be cited.
[0113] The drain of the erasing TFT 1007 is connected to the gate
of the current controlling TFT 1004, and the gate voltage of the
current controlling TFT 1004 can be forcibly changed. Although the
erasing TFT 1007 may be an n-channel TFT or a p-channel TFT, it is
desirable to make the erasing TFT 1007 have the same structure as
the switching TFT 1002 so that an off current can be made low.
[0114] Besides, in FIG. 10, there is also a feature that the
current supply line 1006 is made common to two pixels. That is, the
two pixels are formed to be axially symmetrical with respect to the
current supply line 1006. In this case, since the number of current
supply lines can be reduced, the pixel portion can be made to have
higher fineness.
[0115] Incidentally, this embodiment mode can be carried out in
combination with Embodiment Mode 1 or Embodiment Mode 2.
[0116] [Embodiment Mode 4]
[0117] In this embodiment mode, an example in which the present
invention is applied to an active matrix type light emitting device
having such a pixel structure that four semiconductor components
are provided for one pixel will be described with reference to FIG.
11.
[0118] In FIG. 11, reference numeral 1101 designates a source
wiring line of a first switching TFT 1102; and 1103, a second
switching TFT connected in series to the first switching TFT 1102
(connected to a drain). Reference numeral 1104 designates a (i+1)th
gate wiring line connected to the gate of the switching TFT 1102;
and 1105, an ith gate wiring line connected to the gate of the
switching TFT 1103.
[0119] Reference numeral 1106 designates an erasing TFT. The source
of the erasing TFT 1106 is connected to a current supply line 1107,
and the gate of a current controlling TFT 1108 is connected to the
drain of the second switching TFT 1103. The gate of the erasing TFT
1106 is connected to an (i-1)th gate wiring line 1109. The source
of the current controlling TFT 1108 is connected to the current
supply line 1107, and its drain is connected to a triplet EL
element 1110.
[0120] At this time, the drain of the erasing TFT 1106 is connected
to the gate of the current controlling TFT 1108, so that the gate
voltage of the current controlling TFT 1108 can be forcibly
changed. The erasing TFT 1106 may be an n-channel TFT or a
p-channel TFT.
[0121] Different points from the pixel structure of Japanese Patent
Application No. Hei. 11-338786 described in Embodiment Mode 3 are
that the switching TFT is divided into the first switching TFT 1102
and the second switching TFT 1103, and the gate of the first
switching TFT 1102 is connected to the gate wiring line of a next
adjacent line, that is, the (i+1)th gate wiring line, and that the
gate of the erasing TFT 1106 is connected to the gate wiring line
of an adjacent prior line, that is, the (i-1)th gate wiring
line.
[0122] In the case where the pixel structure of FIG. 11 is adopted,
by contriving gate signals inputted to the (i-1)th gate wiring
line, the ith gate wiring line, and the (i+1)th gate wiring line, a
similar effect to the pixel structure disclosed in Japanese Patent
Application No. Hei. 11-338786 can be obtained. Here, FIG. 12A is a
timing chart of gate signals when a data signal is written in a
pixel, and FIG. 12B is a timing chart of gate signals when a data
signal is erased from a pixel.
[0123] According to this embodiment mode of the present invention,
since the erasing gate wiring line 1108 shown in FIG. 10 can be
omitted, an effective light emitting region of a pixel can be
increased, and a driving circuit for driving the erasing gate
wiring line 1008 can be omitted, so that such an effect that
manufacturing yield is improved can be expected.
[0124] Incidentally, this embodiment mode of the present invention
can be carried out in combination with Embodiment Mode 1 or
Embodiment Mode 2.
[0125] [Embodiment Mode 5]
[0126] In the structure of a light emitting device of the present
invention, the number of TFTs in one pixel may be arbitrary.
Although Embodiment Mode 3 shows the example in which three TFTs
are provided, and Embodiment Mode 4 shows the example in which four
TFTs are provided, a pixel structure in which more TFTs are
provided may be adopted.
[0127] Incidentally, this embodiment mode of the present invention
can be carried out in combination with Embodiment Mode 1 or
Embodiment Mode 2.
[0128] [Embodiment Mode 6]
[0129] Since the light emitting device of the present invention has
advantages that it is bright and has low electric power consumption
and high reliability, it can be used as a light source for various
electrical appliances.
[0130] Typically, the device can be employed as a light source used
as a backlight or a frontlight of a liquid crystal display device,
or as a light source of an illuminating equipment.
[0131] Incidentally, this embodiment mode of the present invention
can be carried out in combination with any structure of Embodiment
Mode 1 to Embodiment Mode 5.
[0132] Embodiments
[0133] [Embodiment 1]
[0134] In this embodiment, an example of an active matrix type
light emitting device using a triplet EL component is described
referring to FIG. 15. The active matrix type light emitting device
shown in FIG. 15 is formed with a pixel portion 585 and a driver
circuit 584 which outputs a signal to the pixel portion. In the
pixel portion 585 there are provided a switching FT 582 and a
driver TFT 583, and in the driver circuit there are provided a
p-channel TFT 580 and an n-channel TFT 581.
[0135] In the pixel portion 585, the semiconductor layer of the
switching TFT 582 has as an n-type impurity region a source region
560 and drain regions 561 and 568 and LDD regions 562 to 565. This
TFT has a multi gate structure and is formed with channel forming
regions 566 and 567 by the gate electrode 513 on the gate
insulating film 510. Further, LDD regions 562 to 565 are provided
in positions that do not overlap with the gate electrode 513, and
is a structure which aims to decrease the off-current of the
TFT.
[0136] The n-channel TFT 581 of the driver circuit 584 has a source
region 591, a drain region 592, a channel forming region 594, and
on the drain region 592 side an LDD region 593 formed. The LDD
region 593 is provided in a position overlapping the gate electrode
512, and is a structure which suppresses degradation due to a hot
carrier effect. This n-channel TFT 581 does not have to take into
account the off-current very much, but rather is designed with more
importance on the operation speed. Further, to increase the current
driving ability, the LDD region 593 is completely overlapped with
the gate electrode 512, so that serial resistance loss is
suppressed as much as possible. On the other hand, the p-channel
TFT 580 may mostly ignore degradation due to the hot carrier
effect, so that it is not necessary to particularly provide an LDD
region and therefore a single drain structure is used.
[0137] The triplet EL component is formed on an interlayer
insulating film with a lamination structure of a silicon nitride
film 561 with a thickness of 50 nm and an acrylic 562 with a
thickness of 1000 nm. The silicon nitride film is used to prevent
ionic impurity mixing into the semiconductor layer from the
outside. The acryl is used for leveling and other organic resin
material of a thermal hardening type or a light hardening type such
as polyamide, polyamide, and BCB (benzocyclobutene) may be used in
place. An organic resin film such as acryl is hygroscopic so that
it is preferable to reform the surface by a plasma processing to
make it compact. Further, by this processing, an effect of
preventing alkaline metallic material, such as lithium used as a
cathode material of an EL component, from scattering in the
semiconductor layer may be expected.
[0138] An anode 555 of the triplet EL component is formed by indium
and tin oxide (ITO). The anode 555 is connected to the drain wiring
544 of the driver TFT 583. The bank 556 which separates the
adjacent pixels is formed by forming a photosensitive acryl on the
anode 555 and the acryl 562, and forming an opening in accordance
with the position of the anode 555. This bank 556 covers the end
portion of the anode 555 and is formed with an inclination of the
side surface to be 30' or more, preferably 45.degree.. Further,
with the bank 556 provided on the portion to which a drain wiring
544 of the anode 555 and the driver TFT 583 are connected to, the
light emission failure of the EL layer 557 due to the step which
occurs in the portion of the contact hole is prevented. Note that,
by mixing resin or the like to the resin material forming the bank
556, the bank 556 may be used also as a light shielding film.
[0139] Then, an EL layer 557 including a light emitting material
which emits light by triplet excitation as shown in Embodiment Mode
1, and a cathode (an MgAg or AlLi electrode) 558 is formed using a
vacuum evaporation method. These two layers are preferably formed
continuously without exposure to the atmosphere. Note that, the
film thickness of the EL layer 557 is 80 to 200 nm(typically 100 to
120 nm), and the thickness of the cathode 558 is 180 to 300 nm
(typically 200 to 250 .mu.m). Note that, in this embodiment only
one pixel is shown but at the same time, the EL layer emitting ref
light, the EL layer emitting green light and the EL layer emitting
blue light are formed.
[0140] Note that, in this embodiment the EL layer 557 may be a
single layer structure made from only light emitting layers, or may
be provided with such as a hole transporting layer, a hole
injecting layer, an electron transporting layer, an electron
injecting layer, or the like in addition to the light emitting
layer. Such a combination is already reported in various examples
and any structure may be used.
[0141] Further, after the cathode 557 is formed, as a passivation
film 559 a DLC (diamond like carbon) or a silicon nitride film is
formed.
[0142] The active matrix type light emitting device shown in FIG.
15 is further high in air tightness so as not to be exposed to the
outside air, and is packaged (sealed) by a transparent sealing
member or a protecting film with little degassing (a laminate film,
a UV cured resin film or the like). At that time, the reliability
of the EL component is improved when the inside of the sealing
member is made an inert atmosphere or a hygroscopic material (for
example barium oxide) is placed inside.
[0143] Further, when air tightness is heightened by a process such
as packaging, a connector (flexible print circuit: FPC) for
connecting the terminal lead out from the component or the circuit
formed on the substrate and the external signal terminal are
mounted to thereby complete the product. Such a state until the
shipment is referred to as an EL display (EL module) in this
specification.
[0144] [Embodiment 2]
[0145] A circuit diagram of the pixel of the active matrix type
light emitting device is shown in FIG. 16. Reference numeral 1107
denotes a switching TFT, reference numeral 1108 denotes a driver
TFT, reference numeral 1109 denotes an erasure TFT, reference
numeral 1110 denotes an EL element, reference numeral 1111 denotes
an opposing power source, and reference numeral 1112 is a
capacitor. The driver TFT 1108 is one in which two EL driver TFTs
(a first EL driver TFT and a second EL driver TFT) are connected in
parallel. The first EL driver TFT and the second EL driver TFT are
combined and referred to as the driver TFT in this
specification.
[0146] A gate electrode of the switching TFT 1107 is connected to a
write in gate signal line Ga (one of the write in gate signal lines
Ga1 to Gay). One of a source region and a drain region of the
switching TFT 1107 is connected to a source signal line S (one of
the source signal lines S1 to Sx), and the other is connected to a
gate electrode of the driver TFT 1108, to the capacitor 1112 of
each pixel, and to a source region or a drain region of the erasure
TFT 09.
[0147] The capacitor 1112 is provided so as to retain a gate
voltage of the driver TFT 1108 when the switching TFT 1107 is in a
non-selected state (OFF state). Although the structure in which the
capacitor 1112 is provided is shown in this embodiment, the present
invention is not limited thereto and the structure in which the
capacitor 1112 is not provided may also be employed.
[0148] One of a source region of the driver TFT 1108 is connected
to a power source supply line V (any one of V1 through Vx), and a
drain region is connected to an EL element 1110. The power source
supply line V is connected to the capacitor 1112. From among the
source region and the drain region of the erasure TFT 1109, the one
which is not connected to the gate electrode of the driver TFT 1108
is connected to the power source supply line V. The gate electrode
of the erasure TFT 1109 is connected to an erasure gate signal line
Ge (any one of Ge1 through Gey).
[0149] The EL element 1110 includes an anode, a cathode and an EL
layer provided between the anode and the cathode. In the case where
the anode is connected to the drain region of the driver TFT 1108,
the anode serves as a pixel electrode while the cathode serves as
an opposing electrode. On the contrary, in the case where the
cathode is connected to the drain region of the driver TFT 1108,
the cathode serves as a pixel electrode while the anode serves as
an opposing electrode.
[0150] The opposing electrode of the EL element 1110 is connected
to the opposing power source 1111 formed externally to the
substrate having the pixel portion 101, and an opposing electric
potential is always applied. Further, the power source supply line
V is connected to an power source (not shown in the figure) formed
externally to the substrate having the pixel portion 101, and a
power source electric potential is always applied. The opposing
electric potential and the power source electric potential are
always maintained at an electric potential difference on an order
such that the EL element emits light when the power source electric
potential is applied to the pixel electrode.
[0151] For an EL display using a triplet EL element, in the case
where the amount of light per area of a pixel portion is 200
cd/m.sup.2, about 0.5 to 1 mA/cm.sup.2 of a current per area of a
pixel portion is needed. Therefore, when the size of the pixel
portion is increased, it becomes difficult to control the electric
potential applied from the power source to the power source supply
line by a switch. In the present invention, however, the power
source electric potential and the opposing electric potential are
always held constant. Therefore, since it is not necessary to
control the electric potential level applied from the power source
provided in the IC by a switch, the present invention is effective
to realize a panel having a larger screen size.
[0152] The switching TFT 1107, the driver TFT 1108, and the erasure
TFT 1109 can use both n-channel TFTs and p-channel TFTs. However,
it is necessary that the first driver TFT and the second driver TFT
have the same polarity. It is preferable that the driver TFT 1108
be a p-channel TFT for cases in which the anode of the EL element
1110 is the pixel electrode and the cathode is the opposing
electrode. Conversely, it is preferable that the driver TFT 1108 be
an n-channel TFT when the anode of the EL element 1110 is the
opposing electrode and the cathode is the pixel electrode.
[0153] Further, the switching TFT 1107, the driver TFT 1108, and
the erasure TFT 1109 may have, in addition to a single gate
structure, a multi-gate structure such as a double gate structure
or a triple gate structure.
[0154] A top surface diagram of the pixel of the EL display shown
in FIG. 16 of the present invention is described with reference to
FIG. 17. Common reference numerals are used in FIG. 16 and FIG.
17.
[0155] The region 1105 having one each of a source signal line (S),
a power source supply line (V), a write in gate signal line (Ga),
and an erasure gate signal line (Ge) in FIG. 16 is a pixel. The
pixel 1105 has the switching TFT 1107, the driver TFT 1108, and the
erasure TFT 1109. The driver TFT 1108 has the first and the second
EL driver TFT, and the first and the second EL driver TFTs are
connected in parallel.
[0156] The switching TFT 1107 has a semiconductor layer 1107a, and
a gate electrode 1107b which is a portion of the write in gate
signal line (Ga). The driver TFT 1108 has a semiconductor layer
1108a, and a gate electrode 1108b which is a portion of a gate
wiring 1121. The erasure TFT 1109 has a semiconductor layer 1109a,
and a gate electrode 1109b which is a portion of the write in gate
signal line (Ge).
[0157] One of a source region and a drain region of the
semiconductor layer 1107a of the switching TFT 1107 is connected to
the source signal line, and the other of the source region and the
drain region is connected to the gate wiring 1121 through a
connection wiring 1113. Note that, the connection wiring 1113 is
referred to as a source wiring and as a drain wiring, depending on
the electric potential of the source signal line (S).
[0158] One of a source region and a drain region of the
semiconductor layer 1109a of the erasure TFT 1109 is connected to
the source signal line, and the other of the source region and the
drain region is connected to the gate wiring 1121 through a
connection wiring 1115. Note that, the connection wiring 1113 is
referred to as a source wiring and as a drain wiring, depending on
the power source electric potential of power source electric
potential.
[0159] A source region and a drain region of the semiconductor
layer 1108a of the driver TFT 1108 are connected to the power
source supply line V and to a drain wiring 1114, respectively. The
drain wiring 1114 is connected to an anode 1117.
[0160] A capacitor wiring 1116 is formed by a semiconductor film.
The capacitor 1112 is formed by the capacitor wiring 1116, which is
electrically connected to the power source supply line (V), and
between an insulating film (not shown in the figures) on the same
layer as the gate insulating film and the gate wiring 1121.
Further, it is also possible to use a capacitance formed by the
gate wiring 1121, a layer (not shown in the figures) which is the
same as the first interlayer insulating film, and the power source
supply line (V).
[0161] Note that, a bank having an opening 1131 is formed on the
anode 1117 by etching an organic resin film. The EL layers and the
opposing electrode are then laminated in order on the pixel
electrode 1117, although not shown in the figure. The anode 1105
and the EL layer contact in the opening 1131 of the bank, and the
EL layer emits light only in a portion sandwiched by the contacting
opposing electrode and the pixel electrode.
[0162] Note that, the pixel portion of the EL display of the
present invention is not limited to the structure shown in FIG.
16.
[0163] [Embodiment 3]
[0164] The superiority of the triplet EL element in respect to the
singlet EL element is in that a high precision may be obtained with
low voltage. Here, a test example of applying the EL element to the
active matrix type light emitting device is shown.
[0165] As a triplet EL element and an anode, a copper
phthalocyanine (hereinafter referred to as CuPc), 4,4', 4"-tris
(N-3-methylphenyl-N-phen- yl-amino) triphenylamine (hereinafter
referred to as mTDATA) which is an aromatic amine material,
4,4'-bis [N-naphthyl-N pheny-amino] biphenyl (hereinafter referred
to as .alpha.-NPD), 4,4' N,N'-dicaribazole-biphenyl (hereinafter
referred to as CBP) added with tris-(2-phenyl-pyridine and iridium
(hereinafter referred to as Ir (Ppy).sub.3), bathocuproine
(hereinafter referred to as BCP), an aluminum complex of oxine
(hereinafter referred to as Alq.sub.3) are laminated subsequently
to fabricate the sample. CBP added with Ir (ppy).sub.3 is an
organic compound where light emission (fluorescence) may be
obtained from a triplet excited state.
[0166] As a comparative example, a singlet EL element is formed
which is formed with a hole injecting layer made of CuPc, a hole
transporting layer formed of mTDATA and .alpha.-NPD and an electron
injecting layer as well as a light emitting layer formed of
Alq.sub.3 on an anode formed by ITO.
[0167] FIG. 18 is a graph showing the characteristics of a current
density with the light emission brightness. The triplet EL element
obtains high light emission brightness from a low current density,
and in comparison has a brightness five to ten times in the same
current density. This tendency is particularly significant in a
region with low current density. Further, in the range measured
here, the brightness is increases almost in a straight with the
increase of the current density.
[0168] FIG. 19 shows a graph of the relationship of the applied
voltage with the luminous efficiency. The triplet EL element
obtains a luminous efficiency of 9% in the range of 4 to 6 V. On
the contrary, the singlet EL element obtains only a luminous
efficiency of 1 to 2%, in the range of 4 to 6 V and it is possible
to make the triplet EL is element emit brighter light.
[0169] Note that, the luminous efficiency .eta. (ext) has a value
of the below formula. 1 ( ext ) = Le KmhcJ L u F ( ) L U F ( ) y (
) [ Formula 3 ]
[0170] Here, 7 .mu.L is total luminous flux (lm/m.sup.2), e is a
charge. Km is a maximum spectral luminous efficacy (680 lm/W), J is
a current density (A/m.sup.2). F (.lambda.) is an emission spectrum
of the actual measurement, and y (.lambda.) is a standard spectral
luminous efficacy curve.
[0171] FIG. 20 shows the characteristics of consumption power with
luminous efficiency. Luminous efficiency does not increase with
consumption power, but in the operation region with high
consumption power, luminous efficiency is decreasing. However, the
triplet EL element has a much higher luminous efficiency and from
the viewpoint of consumption power, the triplet EL element is shown
to be more superior than the singlet EL element.
[0172] On the other hand, FIG. 21 shows data in respect to the
lifetime of the TFT (n-channel) of the single drain structure. In
the graph, a constant 2V as a gate voltage and the time from when
the drain voltage is steadily applied until the drain current
degrades by 10% is computed and plotted thereon. The drain voltage
is altered to find the time, and the straight line joins the plots,
and the voltage where it crosses with 3.15.times.110 seconds (10
years) is defined as the 10 year guarantee voltage.
[0173] FIG. 22 shows the 10 year guarantee voltage when the channel
length (Li) is changed to 2.8 to 9.8 .mu.m. For example, the TFT of
the single drain structure of a channel length of 3 .mu.m. Is shown
to have a 10 year guarantee voltage of about 7V. That is, it shows
that there is practically no problem when it is driven with a drain
voltage below such value.
[0174] With the above data, in the active matrix type light
emitting device, the driver voltage of the TFT may be made lower by
using the triplet EL element. As shown in FIG. 19, if the triplet
EL element shown in this embodiment is used, the driver voltage may
be set to the range of 4 to 6V. The TFT driven with this voltage
may mostly ignore the degradation due to the hot carrier effect.
Therefore, there is no need to specially provide such as an LDD and
therefore it is possible to reduce the manufacturing steps and the
manufacturing costs.
[0175] As shown in this invention, the driver voltage of the TFT is
lowered by using the triplet EL element, and it is possible to
obtain a bright display device as well as lowering the power
consumption.
[0176] [Embodiment 4]
[0177] The light emitting display device of this invention has a
superior visibility in a bright location in comparison with a
liquid crystal display device since it is a self light emitting
type, and also has a wide angle of view. Therefore it can be used
as a display portion of various electronic equipment.
[0178] The following can be given as electronic equipment of the
present invention: a video camera; a digital camera; a goggle type
display (head mounted display); a navigation system; an audio
device; a notebook type personal computer; a game apparatus; a
wireless portable information terminal (such as a mobile computer,
a cellular phone, a portable game machine, or an electronic book);
and an image playback device equipped with a recording medium.
Specific examples of those electronic devices are shown in FIGS. 13
and 14.
[0179] FIG. 13A is an EL display, and includes a frame 2001, a
support stand 2002, and a display portion 2003. The light emitting
device of the present invention can be used in the display portion
2003. The EL display is a self-light-emitting type, and therefore a
back light is not necessary, and the display portion can be made
thinner than that of a liquid crystal display device.
[0180] FIG. 13B is a video camera, and includes a main body 2101, a
display portion 2102, a sound input portion 2103, operation
switches 2104, a battery 2105, and an image receiving portion 2106.
The light emitting device of the present invention can be used in
the display portion 2102.
[0181] FIG. 13C is a digital camera and includes a main body 2201,
a display portion 2202, an optical system 2203, an operating switch
2204. The light emitting device of the present invention can be
used in the display portion 2202.
[0182] FIG. 13D is an image playback device equipped with a
recording medium, and includes a main body 2301, a recording medium
(such as a CD, LD or DVD) 2302, operation switches 2303, a display
portion (a) 2304, and a display portion (b) 2305, etc. The display
portion (a) is mainly used for displaying image information, and
the display portion (b) is mainly used for displaying character
information, and the light emitting device of the present invention
can be used in the display portion (a) and for the display portion
(b). Note that, the image playback device equipped with the
recording medium includes devices such as CD playback devices and
game machines.
[0183] FIG. 13E is a portable (mobile) computer, and includes a
main body 2401, a display portion 2402, an image receiving portion
2403, operation switches 2404, and a memory slot. The electronic
optical device of the present invention can be used in the display
portion 2402. This portable computer records information onto the
recording medium accumulated with flash memory or volatile memory
and can play back such recordings.
[0184] FIG. 13F is a personal computer, and includes a main body
2501, a frame 2502, a display portion 2503, and a keyboard 2504.
The light emitting device of the present invention can be used in
the display portion 2503.
[0185] Note that, if the brightness of light emitted by EL
materials increases in the future, then it will become possible to
use in a front type or a rear type projector to expand and project
light containing output image information with a lens or the
like.
[0186] Further, the above electronic appliances often display
information distributed through an electronic communication network
such as the Internet and a CATV (cable television) and radio
communication such as a radio wave. In particular, there are more
and more opportunities where the electronic appliances display
dynamic image information. Since the response speed of an EL
material is very high, an EL display device is suitable for dynamic
image display.
[0187] In addition, since the light emitting device consumes power
in the light emitting portion, it is therefore preferable to use
the EL display device for displaying information so as to make the
light emitting portions as few as possible. Consequently, when
using the light emitting device in a display portion mainly for
character information, such as in a portable information terminal,
in particular a cellular phone or an audio playback device, it is
preferable to drive so as to form character information by the
light emitting portions while non-light emitting portions are set
as background.
[0188] FIG. 14A is a cellular phone, and includes a main body 2601,
a sound output portion 2602, a sound input portion 2603, a display
portion 2604, operation switches 2605, and an antenna 2606. The
light emitting device of the present invention can be used in the
display portion 2604. Note that by displaying white color
characters in a black color background, the display portion 2604
can suppress the power consumption of the cellular phone.
[0189] FIG. 14B is an audio playback device, (specifically a car
audio system), and includes a main body 2701, a display portion
2702, and operation switches 2703 and 2704. The light emitting
device of the present invention can be used in the display portion
2702. Further, a car audio system is shown in this embodiment, but
the EL display device of the present invention can be used in a
household audio playback system, too. Note that by displaying white
color characters in a black color background, the display portion
2704 can suppress the power consumption.
[0190] Further, it is effective to incorporate the light sensor, to
provide means to detect the brightness of the environment to be
used, in order for it to function to modulate the luminous
brightness according to the brightness of the environment to be
used. If the user may maintain the brightness with a brightness of
a contrast ratio of 100 to 150 compared to the brightness of the
used environment, there is no problem in recognizing the image or
the character information. That is, when the environment to be used
is bright, the brightness of the image is increased so it is easy
to see, and when the environment to be used is dark, the brightness
of the image may be suppressed to thereby suppress the power
consumption.
[0191] Note that, the embodiments of the present invention may be
implemented with any structure of Embodiment Mode 1 to 5 and
Embodiments 1 to 3.
[0192] By carrying out the present invention, an active matrix type
light emitting device which is bright and has low electric power
consumption and high reliability can be obtained. Further, by using
that sort of light emitting device as a light source or a display
portion, an electrical appliance having a bright display portion
and low electric power consumption can be obtained.
* * * * *