Transparent electrodes of thin-film electroluminescence panel

A thin-film EL panel of the layer structure with transparent electrodes and back electrodes sandwiching therebetween insulative layers and a light-emitting layer and forming display picture elements on the panel is characterized as having the transparent electrodes shaped differently at these display picture elements and other places.

BACKGROUND OF THE INVENTION 
This invention relates to the structure of transparent electrodes of a 
thin-film electroluminescence (EL) panel. 
The structure of conventional thin-film EL panels is shown in FIGS. 22 and 
23, FIG. 22 relating to a thin-film EL panel with a small display area, 
for example, of 512.times.128 dots and FIG. 23 relating to another with a 
somewhat larger display area, for example, of 640.times.400 dots. As shown 
in these figures, conventional thin-film EL panels have a plurality of 
transparent electrodes 11 comprising indium oxide (In.sub.2 O.sub.3) 
(hereinafter referred to as ITO films) formed transversely on a glass 
substrate 10 at specified intervals and a first insulative layer 12, a 
light-emitting layer (not shown), a second insulative layer 13 and back 
electrodes 14 of Al or the like stacked on top of these ITO films in this 
order, and the ITO films 11 are generally so formed as to have uniform 
thicknesses and widths. Numerals 15 indicate Al-Ni terminals which are 
formed on the glass substrate 10 near its edges for connecting the end 
parts of the ITO films 11, numeral 16 indicates a display part and 
numerals 17 indicate display picture elements. With conventional thin-film 
EL panels of this type, breakdown points (hereinafter abbreviated into BP) 
18 frequently occur in the display picture elements 17 during the 
operation. 
SUMMARY OF THE INVENTION 
It is therefore an object of the present invention to significantly reduce 
the occurrence of breakdown points in a thin-film EL panel to thereby 
improve the quality of its display. 
A thin-film EL panel embodying the present invention with which the above 
and other objects are achieved, has the conventional layer structure with 
transparent electrodes, a first insulative layer, a light-emitting layer, 
a second insulative layer and back electrodes stacked in this order on a 
substrate, but is characterized as having its transparent electrodes 
shaped differentily in the display picture element parts and in other 
parts.

DETAILED DESCRIPTION OF THE INVENTION 
Breakdown points damage the quality of a display when their size exceeds a 
certain critical magnitude such as 1/4 of the area of a picture element 
and the function of a display device is thereby seriously affected. The 
tendency for their occurrence generally depends on the structure of the 
thin-film EL panel such as its display area and the resistance of its 
transparent electrodes. Before embodiments of the present invention are 
explained, therefore, experiments conducted by the present inventors for 
determining the relationship between the length (or resistance) of ITO 
films and large breakdown points (defined as breakdown points with 
diameters greater than 100 .mu.m and hereinafter abbreviated into LBP) 
will be described in detail. 
In the first experiment, various thin-film EL panels (hereinafter referred 
to as samples) with different display areas and ITO films with different 
resistances were prepared as shown in Table 1 and after a continuous aging 
process of 20 hours at temperature 75.degree. C. and operating voltage of 
V.sub.th +100 V, the size (maximum diameter) of each LBP that was 
generated and its distance from the base end part of the ITO films (the 
end at which they are connected to the Al-Ni terminal) were measured. FIG. 
4 shows the positions at which measurements were taken of each sample. The 
ITO film resistance was calculated by assuming that its specific 
resistivity .rho.=2.1.times.10.sup.-4 .OMEGA.cm and the thickness=1400 
.ANG.. 
TABLE 1 
__________________________________________________________________________ 
Line ITO ITO 
Picture Capacity Film 
Film 
Sample 
Display 
Element 
Number 
of ITO 
L.sub.3 
L.sub.4 
resist- 
resist- 
No. Area Size of dots 
film mm mm ance 
ance 
__________________________________________________________________________ 
1 120 .times. 90 
0.275 .times. 0.225 
320 .times. 240 
866 6.0 
96.0 
330 5.2 
2 192 .times. 96 
0.275 .times. 0.225 
512 .times. 256 
923 4.5 
100.5 
250 5.5 
3 179 .times. 45 
0.250 .times. 0.200 
512 .times. 128 
373 5.5 
50.5 
330 3.0 
4 192 .times. 60 
0.230 .times. 0.200 
640 .times. 200 
536 4.5 
64.5 
290 4.2 
5 192 .times. 120 
0.180 .times. 0.300 
640 .times. 200 
630 4.0 
125.0 
420 10.4 
6 192 .times. 96 
0.185 .times. 0.280 
640 .times. 200 
604 4.5 
100.5 
360 8.1 
7 192 .times. 120 
0.220 .times. 0.220 
640 .times. 400 
1129 4.0 
125.0 
340 8.5 
8 192 .times. 84 
0.185 .times. 0.240 
640 .times. 200 
518 10.5 
94.5 
290 7.7 
__________________________________________________________________________ 
FIG. 7 shows results of this experiment. It shows that LBP occurred 
frequently in the neighborhood of the ITO film resistance (calculated 
value)=500.OMEGA. in the case of Samples 2, 4 and 7 and that LBP occurred 
frequently near the opposite end distal from the base end part in the case 
of Sample 5 were the ITO film resistance is greater than 10k.OMEGA.. 
In order to understand clearly the cause of occurrence of LPB where the ITO 
films resistance is high, the following series of experiments was carried 
out. Table 2 shows the samples used for these experiments wherein the 
samples used in the first experiment are identified by the same Sample 
Nos. Samples 2', 3' and 4' are the same as Samples 2, 3 and 4, 
respectively, except the sputtered area of the insulative layers is 
enlarged. In Table 2, 1 indicates the distance between the insulative 
layer and the first picture element and W is the widths of the ITO films 
as shown in FIG. 5. S indicates the area of each picture element and n 
indicates the number of picture elements in one line. 
TABLE 2 
______________________________________ 
Sam- Number 
ple of Snl/ 
No. dots l(mm) W(mm) S(mm.sup.2) 
n W 
______________________________________ 
1 320 .times. 240 
6 0.275 0.275 .times. 0.225 
240 324 
2 512 .times. 256 
4.5 0.275 0.275 .times. 0.225 
256 259 
2' 512 .times. 256 
6.5 0.275 0.275 .times. 0.225 
256 374 
3 512 .times. 128 
5.5 0.250 0.250 .times. 0.200 
128 141 
3' 512 .times. 128 
12.0 0.250 0.250 .times. 0.200 
128 307 
4 640 .times. 200 
4.5 0.230 0.230 .times. 0.200 
200 180 
4' 640 .times. 200 
6.5 0.230 0.230 .times. 0.200 
200 260 
5 640 .times. 200 
4.0 0.180 0.180 .times. 0.300 
200 240 
6 640 .times. 200 
4.5 0.185 0.185 .times. 0.280 
200 252 
7 640 .times. 400 
4.0 0.220 0.220 .times. 0.220 
400 352 
8 320 .times. 256 
4.14 0.220 0.220 .times. 0.220 
256 233 
9 320 .times. 256 
4.14 0.190 0.220 .times. 0.220 
256 270 
______________________________________ 
In the second experiment, distribution of LBP and BP was measured. For this 
purpose, the display part 16 of each sample described in Table 2 was 
divided into six sections A-F in the transverse direction and four 
sections I-IV in the longitudinal direction, or into a total of 24 regions 
as shown in FIG. 6, depending on the difference in ITO film resistance, 
and the number of LBP and BP inside the display part was obtained and the 
relationship between the measured number and the ITO film resistance was 
studied. 
In the third experiment, the relationship between the ITO film resistance 
or external resistance and the size of BP caused by a direct current (DC) 
was investigated. For this experiment, a DC was applied with the ITO film 
at a positive voltage to generate BP and the relationship between the ITO 
film resistance (length) and the size of BP was investigated. In addition, 
various external resistors were inserted between the DC source and the 
sample on the side of the base end part of the ITO film to generate BP and 
the relationship between such external resistance and the size of BP was 
studied. 
The four experiment was comprised of BP acceleration tests at 75.degree. C. 
with applied voltage of V.sub.th +100 V and the rate of increase in BP, 
their average size, etc. were studies. After each test, the relationship 
between the ITO film resistance and the number and size of BP was studies 
for each sample. 
Tables 3-6 show the distribution of LBP and BP on four kinds of Sample 5 
after an aging process. In Tables 3-6, ITO(long) indicates LBP and BP 
generated on the part within the region distal from the base end part of 
the ITO film and ITO(short) indicates those generated on the part within 
the region proximal to the base end part of the ITO film. In regions from 
C-I to C-IV and from D-I to D-IV, no distinction is made because the 
lengths of the ITO films are nearly equal from both end parts. 
TABLE 3 
__________________________________________________________________________ 
##STR1## 
__________________________________________________________________________ 
ITO(long)/ITO(short): Numbers inside () indicate LBP of 100 .mu.m or 
greater 
TABLE 4 
__________________________________________________________________________ 
##STR2## 
__________________________________________________________________________ 
ITO(long)/ITO(short): Numbers inside () indicate LBP of 100 .mu.m or 
greater 
TABLE 5 
__________________________________________________________________________ 
##STR3## 
__________________________________________________________________________ 
ITO(long)/ITO(short): Numbers inside () indicate LBP of 100 .mu.m or 
greater 
TABLE 6 
__________________________________________________________________________ 
##STR4## 
__________________________________________________________________________ 
ITO (long)/ITO(short): Numbers inside () indicate LBP of 100 .mu.m or 
greater 
Tables 3-6 show that 2-3 times more LBPs are generated as a whole where the 
ITO film resistance is high than where it is low. By contrast, nearly the 
same number of or about 50% more BPs are generated as a whole where the 
ITO film resistance is low. This agrees with the result with Sample 5 that 
LBPs and missing picture elements occur frequently where the ITO film 
resistance is high. It may be concluded,therefore, that in the case of a 
sample like this with high ITO film resistance (10.4k.OMEGA.), the 
probability of BPs growing (in propagating mode, hereinafter abbreviated 
into P mode) is high in regions where the ITO film resistance is high. 
To study the relationship between the ITO film resistance and the size of 
BP caused by a DC, FIG. 8 shows the relationship between the ITO film 
resistance (calculated from specific) resistance of 2.1.times.10.sup.-4 
.OMEGA.cm) of Sample 5 and the size of BP based on tests on Samples 5-1, 
5-2, 5-3, 5-4 and 5-5. Although there are some differences among these 
five samples, it is observed with all of them that the size of BP 
increases suddenly when the ITO film resistance exceeds 5k.OMEGA. and that 
is also tends to increase when the ITO film resistance is below 1k.OMEGA.. 
It is also observed that when the ITO film resistance is greater than 
8-9k.OMEGA. (that is, the ITO film length is over 9-10 cm), the BP mode 
has a strong tendency to switch from the self-healing mode (hereinafter 
abbreviated into S mode) to the P mode, or the fraction of P mode 
increases. 
FIG. 9 shows the relationship between the ITO film resistance and the size 
of BP with Sample 5 when the widths of the ITO films is 220 .mu.m. For 
this purpose, five samples 5-1', 5-2', 5-3', 5-4' and 5-5' were used. It 
is noted that there are few regions where BP is large as a whole compared 
to the samples used for FIG. 8. It is also noted that the relationship 
between the size of BP and the ITO film resistance is nearly the same 
between these two sets of samples and that the size of BP at the distal 
end part of the ITO film depends on the value of resistance of the ITO 
films. Thus, with the other conditions kept the same, the resistance at 
the distal end part of the ITO film decreases if the widths of the ITO 
film is increased from 180 .mu.m to 220 .mu.m and size of BP caused by a 
DC become smaller. 
FIGS. 10A and 10B show the relationships of the size of BP with the ITO 
film length and resistance in the case of Sample 5 when the film thickness 
is changed from 1400 .ANG. to 1700 .ANG. to thereby reduce its ITO film 
resistance. In these figures, Sample 5-1" has film thickness of 1700 .ANG. 
and measured ITO film resistance of 8.2k.OMEGA.. Samples 5-6 and 5-7 have 
the usual film thickness of 1400 .ANG., their measured ITO film resistance 
being 14.5k.OMEGA. and 17.2k.OMEGA., respectively. FIG. 10A shows that 
these three samples exhibit a similar relationship between the size of BP 
and the ITO film length but FIG. 10B shows that there is hardly any 
similarity in relationship between the size of BP and the ITO film 
resistance. This seems to imply that the size of BP is influenced not only 
by the ITO film resistance but also by the length of the ITO film, that 
is, the position of breakdown within the sample. 
FIG. 11 shows the relationship between the calculated ITO film resistance 
and the size of BP obtained from Samples 2, 6 and 7. There are some 
differences from the result with Sample 5 but it can be seen that the 
largest size of BP is influenced by the ITO film resistance. 
Table 7 shows the results of experiment for studying the relationship 
between external resistance used with different samples and the size of 
BP. Measurements were all taken at the picture element closest to the base 
end part of the ITO film where the ITO film resistance is the smallest. 
Table 7 shows as a whole that BP becomes larger as the external resistance 
is increased, and that this tendency is most conspicuous with Sample 5 
while it is weak with Samples 3-1, 3-2 and 2-1 with small display areas. 
Among samples with the same display area such as Samples 5-1 through 5-3, 
5-1' through 5-3', 5-8 and 7-1, those with a greater 
vertical-to-horizontal ratio such as Samples 5-1 through 5-3 and 5-8 
produce larger BP for the same external resistance. 
TABLE 7 
__________________________________________________________________________ 
Sample 
External Resistance 
No. 0.47 
1 2.2 
3.3 
4.7 
5.6 
10 39 56 100 
470 
__________________________________________________________________________ 
5 - 1 34 152 112 
173 
5 - 3 
152 
84 154 
168 
211 
225 
252 225 
5 - 4 
129 
92 167 
220 
161 
235 
251 217 
220 
237 
5 - 1' 57 67 136 
162 
246 
5 - 2' 64 101 
99 83 95 
5 - 3' 131 148 128 
101 203 
179 
5 - 8 124 177 
128 169 
135 
236 
2 - 1 65 104 126 
105 
160 
7 - 1 42 48 80 134 
91 
3 - 1 46 74 69 121 113 
154 
3 - 2 68 156 117 
178 131 
154 
6 - 1 111 120 128 
162 224 
138 
(Unit: .mu.m) 
__________________________________________________________________________ 
FIG. 12 shows the relationship between the external resistance and the size 
of BP with Samples 5-3 and 5-4. The size of BP increases within the range 
of several k.OMEGA. to 10k.OMEGA. and becomes saturated in the range 
therebeyond. FIG. 13 shows the relationship between the external 
resistance and the ratio of picture elements which enter the P mode upon 
breakdown. The ratio of P mode varies widely between 2k.OMEGA. and 
10k.OMEGA.. The degree of change is greater than in FIG. 11. It may 
therefore be concluded that the size of BP increases with each example of 
Sample 5 as the external resistance is increased because a resistance 
greater than a certain value between the power source and the picture 
element can strongly influence the picture element going into the P mode 
upon breakdown. FIGS. 14 and 15 show the relationship between the length 
of ITO film and the size of BP obtained by moving the position of 
measurement gradually from the base end part of the ITO film to the 
opposite end for studying the relationship between the position of BP in 
Samples 5-1 and 5-4 and the external resistance. It is noted with both 
Samples 5-1 and 5-4 that the size of BP increases by the insertion of an 
external resistance greater than 10k.OMEGA. only within about 10 mm from 
the base end of the display part and that there is hardly any difference 
near the center of the display part whether an external resistance is 
inserted or not. This seems to suggest that the BP does not become large 
simply because an external resistance is inserted but that is become large 
if a resistance greater than a certain value is inserted between the 
picture element and the DC source when the ITO film in the peripheral 
regions of the display part is more deteriorated than at the center 
region. Although the results explained above relate to BP generated by a 
DC, it is believed that similar conclusions will be obtained in the case 
of actual AC operations. 
FIG. 16 shows the relationship between the time of aging and the number of 
BP obtained from Samples 5-1, 5-2, 5-5 and 5-7. FIG. 7 shows the same 
relationship obtained from Samples 5-6' and 5-7' with ITO film thickness 
changed from 1400 .ANG. to 1700 .ANG. and Sample 5-9 having the normal 
film thickness of 1400 .ANG.. Measurements were taken for this figure at 
the left-hand and right-hand ends, that is, at both ends of the ITO film 
in the direction of its widths. FIG. 18 shows this relationship obtained 
from Samples 6 and 7. Table 8 shows the results of BP acceleration tests 
on these samples. 
TABLE 8 
__________________________________________________________________________ 
Rate of 
Number of BP 
Sample No. 
Test time (H) 
Increase 
(/1000) 1/.lambda. (.mu.m) 
Remarks 
__________________________________________________________________________ 
5 - 1 132.0 0.76 96.5 
(132 H) 
18.1 
5 - 2 94.8 0.43 315.9 
(95 H) 
28.7 Many LBP where ITO 
film is long 
5 - 5 82.2 0.80 20.8 
(82 H) 
18.3 
5 - 6' 
82.2 1.03 194.5 
(82 H) 
14.6 
5 - 7 82.2 0.32 24.1 
(82.2) 
30.9 Some LBP 
5 - 8 82.2 0.46 15.2 
(82 H) 
16.5 
5 - 9 89.8 0.27 14.9 
(89 H) 
20.9 (ref.) 
5 - 10 
89.8 0.25 2.6 (89 H) 
23.5 Ni removed 
5 - 11 
89.8 0.43 6.3 (89 H) 
17.4 ITO film thickness 
= 1700.ANG. 
6 132.0 0.39 18.6 
(132 H) 
14.1 
7 192.3 0.52 501.5 
(192 H) 
24.1 Some LBP 
__________________________________________________________________________ 
FIG. 16 shows as a whole that both the nubmer of BP and the rate of its 
increase are fairly large. It is also to be noted that LBP occurred in the 
case of Sample 5-2 at the distal end part of the ITO film. The number of 
BP shown in FIG. 17 is also large but the average size of BP which 
occurred (1/.lambda. in Table 8) is sufficiently small and there is not 
LBP. It is to be noted that Sample 5-6' obtained by removing Ni by etching 
shows no difference from Sample 5-9 having the normal film thickness. On 
the other hand, there is no occurrence of LBP on Sample 5-9 although its 
ITO film resistance is as large as 16.9k.OMEGA.. This seems to indicate 
that the LBP-producing mode which is peculiar to Sample 5 does not occur 
even with the ITO film resistance of about 17k.OMEGA. if the quality and 
composition of the thin-film EL panel are not deteriorated. In FIG. 18, 
the number of BP is large and there are not a few occurrences of LBP but 
the number is not large. 
FIGS. 19A, 19B, 20A, 20B, 21A and 21B show the effects of the length of ITO 
film on the number of BP and the distribution of its size after a BP 
acceleration test. It is noted that the number of BP decreases as a whole 
as the length of ITO film (film resistance) increases and that the size of 
BP increases if the ITO film length exceeds 80 mm. These tendencies are no 
more conspicuous than in the case of the aforementioned second experiment 
but they agree in that the size of BP increases as the ITO film resistance 
increases. 
From the results of the second, third and fourth experiments described 
above, it may be concluded that the principal cause for the occurrence of 
LBP where the ITO film resistance is high (that is, near the distal end of 
the ITO film) is that the breakdown of picture elements do not stop (not 
going into the S mode) but propagates (entering the P mode) because the 
current which provides breakdown energy for picture elements becomse 
limited at the time of occurrence of BP because the LTO film resistance is 
higher than a specified value such that the BP becomes larger. Altogether, 
it may be concluded that the occurrence of LBP can be reduced 
significantly within the display part if the calculated value of ITO film 
resistance at the display part of the thin-film EL panel is set in the 
range of 1k.OMEGA.-9.5k.OMEGA. (or measured value in the ragne of 
500.OMEGA.-13k.OMEGA.). 
With the experimental results thus interpreted, the present invention 
discloses transparent electrodes of a thin-film EL panel of the general 
structure described above, characterized as having different shapes in the 
display element area and in other areas. In other words, the occurrence of 
BP is reduced according to the present invention by changing the shape of 
transparent electrodes (or ITO films), for example, by varying their 
widths such that the calculated resistance of the ITO film becomes between 
1k.OMEGA. and 9.5k.OMEGA.. In what follows, the present invention is 
explained by way of figures which show some embodiments of the present 
invention. 
In FIG. 1 which is a plan view of a portion of a thin-film EL element 
according to the present invention, numeral 1 indicates a glass substrate 
and numerals 2 indicate transparent electrodes (ITO films) comprising 
indium oxide (In.sub.2 O.sub.3) and formed transversely on the glass 
substrate 1. On these ITO films 2 are a first insulative layer 3, a 
light-emitting layer (not shown) and a second insulative layer 4 
sequentially stacked, and many back electrodes 5 of Al or the like are 
disposed above the second insulative layer 4 perpendicularly to the ITO 
films 2. Numerals 6 Al-Ni terminals, numeral 7 indicates a display part 
and numerals 8 indicate display picture elements. This layer structure, 
therefore, is not different from the conventional examples. The embodiment 
of the present invention in FIG. 1 is characterized wherein each of the 
ITO films 2 is made narrower between its connecting end part 2a to the 
Al-Ni terminal 6 and the display part 7 than inside the display part 7, 
thereby reducing its area outside the display part 7 and hence increasing 
its calculated ITO film resistance between the connecting end part 2a and 
the display part 7 so as to be within the range between 1k.OMEGA. and 
9.5k.OMEGA.. This has the effect of increasing the voltage drop along the 
ITO films 2 between the connecting end part 2a and the display part 7 and 
hence of reducing the occurrence of BP near the base end part. 
FIG. 2 shows another thin-film EL panel embodying the present invention of 
which the layer structure is as explained by way of FIG. 1 above. 
Components which are substantially equivalent to or at least similar to 
those in FIG. 1 are indicated by the same numerals. This embodiment is 
characterized as having each of its ITO film 2 wider between its 
connecting end part 2a and the display part 7 than inside the display part 
7, thereby increasing its area outside the display part 7 and hence 
reducing the calculated ITO film resistance so as to be in the range 
between 1k.OMEGA. and 9.5k.OMEGA.. This has the effect of reducing the 
voltage drop along the ITO film 2 between the connecting end part 2a and 
the display part 7 and hence of reducing the occurrence of BP and, in 
particular, of LBP near the distal end parts of the ITO films 2. 
Another embodiment of the present invention shown in FIG. 3 may be 
considered as a variation of the one explained above by way of FIG. 2. 
This embodiment is useful when the resistance of the ITO film 2 is too 
large even after its width is increased between its connecting end part 2a 
and the display part 7. More in detail, the ITO film 2 according to this 
embodiment is identical to those shown in FIG. 2 except its width is 
increased inside the display part 7 and between two mutually adjacent 
picture elements 8, that is, at the positions of the gaps between the 
mutually adjacent pairs of back electrodes 5. The ITO film resistance can 
be further reduced by this design. 
In summary, transparent electrodes of a thin-film EL panel according to the 
present invention are so designed that their shapes outside the areas of 
the display picture elements are modified and their areas are so adjusted 
that the occurrence of BP can be significantly reduced. Accordingly, the 
display quality of the thin-film EL panel can be improved. 
The foregoing description of preferred embodiments of the invention have 
been presented for purposes of illustration and description. It is not 
intended to be exhaustive or to limit the invention to the precise form 
disclosed, and many modifications and variations are possible in light of 
the above teaching. Any modifications and variations that may be apparent 
to a person skilled in the art are intended to be included within the 
scope of this invention.