A multi-layer electroluminescent element so constructed that the overlapping area between a second voltage applying electrode and a transparent conductive layer is larger than the overlapping area between a first voltage applying electrode and the transparent conductive layer, a further decrease of the driving voltage is possible.

FIELD OF THE INVENTION 
This invention relates to a multi-layer electroluminescent element 
including a plurality of voltage applying electrodes at the back of an 
electroluminescent layer so that on application of an alternating voltage 
to the voltage applying electrodes, an alternating electric field is 
applied to the display surface of the electroluminescent layer via a 
transparent conductive layer forming an equal potential surface, to 
thereby cause the electroluminescent layer to emit light. 
BACKGROUND OF THE INVENTION 
One form of prior art multi-layer electroluminescent elements is shown in 
FIG. 4 in which a transparent electrode 2, a first insulative layer 3, an 
electroluminescent layer 4, a second insulative layer 5 and an opposed 
electrode 6 are accumulated in this order on a transparent glass substrate 
1. The multi-layer electroluminescent element is configured to emit light 
when active ions in the electroluminescent layer 4 are energized by 
application of an alternating electric field of several decahertz to 
several kilo hertz approximately between the transparent electrode 2 and 
the opposed electrode 6. The multi-layer electroluminescent elements are 
used more and more for display of various devices. 
In the prior art multi-layer electroluminescent elements, however, it is 
necessary to externally extend electrodes from upper and lower portions of 
the electroluminescent layer, and this electrode extension process is 
significantly complicated Along with an increased demand of 
electroluminescent elements for display of various devices, it is desired 
to improve their display resolving power. However, one of the electrode 
layers for extraction of electroluminescence (it is normally the electrode 
layer nearer to the transparent glass substrate) must be a transparent 
conductive layer which has the specific resistance of about 
2.times.10.sup.-4 .OMEGA..cm as far as the present technical level 
permits. If the pattern width is decreased in the attempt to improve the 
display resolving power, its conductive resistance increases and fails to 
improve the display resolving power. 
In this connection, the present inventor proposed a multi-layer 
electroluminescent element shown in FIGS. 5 and 6 (see Japanese patent 
applications 123880/1985, 132881/1985 and 123882/1985). The 
electroluminescent element includes a transparent conductive layer 12, an 
insulative layer 13, an electroluminescent layer 14, an insulative layer 
15 and voltage applying electrodes 16 and 17 all accumulated in this order 
on a transparent glass substrate 11. The voltage applying electrodes 16 
and 17 consist of at least one pair of electrodes which are not 
electrically connected. The voltage applying electrodes 16 and 17 overlap 
the transparent conductive layer 12. When an alternating voltage is 
applied between one pair of voltage applying electrodes 16 and 17, an 
electric field produced by the alternating voltage is applied between the 
voltage applying electrodes 16-17 and the transparent conductive layer 12 
which forms an equivalent potential surface. As a result, the 
electroluminescent layer 14 between the voltage applying electrodes 16-17 
and the transparent conductive layer 12 emits light. The light emitting 
portion of the electroluminescent layer 14 is shown by a hatching and 
designated by S in FIG. 6 where the voltage applying electrodes 16 and 17 
overlap the transparent conductive layer 12. 
The inventor's proposal, however, six layers excluding the transparent 
conductive layer 12 are interposed between the voltage applying electrodes 
16 and 17. More specifically, three layers, i.e. the insulative layer 15, 
electroluminescent layer 14 and insulative layer 13, exist between the 
voltage applying electrode 16 and the transparent conductive layer 12, and 
three layers, i.e. the insulative layer 13, electroluminescent layer 14 
and insulative layer 15 exist between the transparent conductive layer 12 
and the voltage applying electrode 17. Therefore, a significantly large 
voltage is required between the voltage applying electrodes 16 and 17. 
In this connection, the present inventor further proposed a multi-layer 
electroluminescent element shown in FIGS. 7 and 8 (see Japanese patent 
application 7014/1986). The multi-layer electroluminescent element 
includes a transparent conductive layer 12 provided on a transparent 
substrate 11, an electroluminescent layer 14 provided on a part of the 
transparent conductive layer 12, a first voltage applying electrode 16 
provided above the electroluminescent layer 14 via an insulative layer 15, 
and a second voltage applying electrode 17 provided above a part of the 
transparent conductive layer 12 via the insulative layer 15 and not 
overlapping the electroluminescent layer 14. When an alternating voltage 
is applied between the first and second voltage applying electrodes 16 and 
17, an electric field produced by the alternating voltage is applied 
between the first and second voltage applying electrodes 16-17 and the 
transparent conductive layer 12 which forms an equivalent potential 
surface. As a result, the hatched portion S of the electroluminescent 
layer 14 at which the first voltage applying electrode 16 overlaps the 
transparent conductive layer 12 emits light. 
The multi-layer electroluminescent element requires a decreased driving 
voltage because the number of layers is decreased between the first and 
second voltage applying electrodes 16 and 17. However, the decrease of the 
driving voltage is still insufficient. Further, since the transparent 
conductive layer 12 is spaced from the second voltage applying electrode 
17 by only one layer, i.e. the insulative layer 15, it causes a leak 
between them which often damages the element. 
OBJECT OF THE INVENTION 
It is therefore an object of the invention to provide an electroluminescent 
element including voltage applying electrodes both provided on the back 
surface thereof but reliably operative at a decreased driving voltage. 
SUMMARY OF THE INVENTION 
According to the invention, there is provided a multi-layer 
electroluminescent element comprising: 
a transparent conductive film provided on a transparent substrate; 
an electroluminescent layer provided directly or indirectly via an 
insulative layer on a part of said transparent conductive layer; 
a first voltage applying electrode provided directly or indirectly via said 
insulative layer on said electroluminescent layer; and 
a second voltage applying electrode provided above said transparent 
conductive layer via said insulative layer at a portion where said 
electroluminescent layer does not extend, the overlapping area between 
said transparent conductive layer and the second voltage applying 
electrode being larger than the overlapping area between the transparent 
conductive layer and the first voltage applying electrode. 
With this arrangement, when an alternating voltage is applied between the 
first voltage applying electrode and the second voltage applying 
electrode, an electric field produced by the alternating voltage is 
applied between the first and second voltage applying electrodes and the 
transparent conductive layer forming an equivalent potential surface. As a 
result, the electroluminescent layer between the first volta9e applying 
electrode and the transparent conductive layer emits light. 
According to the specific features of the invention, the first voltage 
applying electrode is spaced from the transparent conductive layer by the 
electroluminescent layer and by the insulative layer when required, and 
the transparent conductive layer is spaced from the second voltage 
applying electrode only by the insulative layer. Therefore, the number of 
layers is decreased between the first and second voltage applying 
electrodes. This enables a decrease of the driving voltage of the element. 
Beside this, since the overlapping area between the transparent conductive 
layer and the first voltage applying electrode is larger than the 
overlapping area between the transparent conductive layer and the second 
voltage applying electrode, a further decrease of the driving voltage is 
possible. Additionally, since the thickness of the insulative layer 
between the transparent conductive layer and the second voltage applying 
electrode may be increased without inviting an increase of the driving 
voltage, a leak between the transparent conductive layer and the second 
voltage applying electrode is prevented to ensure a reliable operation of 
the element. 
Furthermore, since all electrodes can be taken out from the back surface of 
the element, the electrode extension process is very easy. 
In a more preferred embodiment of the invention, the overlapping area 
between the transparent conductive layer and the second voltage applying 
electrode is larger by 1.5 times or more than the overlapping area between 
the transparent conductive layer and the first voltage applying electrode. 
If the former overlapping area is less than 1.5 times with respect to the 
latter overlapping area, a sufficient effect of the invention is not 
obtained. 
In another preferred embodiment of the invention, the ratio of the 
overlapping area between the transparent conductive layer and the second 
voltage applying electrode with respect to the overlapping area between 
the transparent conductive layer and the first voltage applying electrode 
is equal to the ratio of the thickness of the insulative layer interposed 
between the transparent conductive layer and the second voltage applying 
electrode with respect to the thickness of the insulative layer interposed 
between the transparent conductive layer and the first voltage applying 
electrode. With this arrangement, the insulative layer can be increased in 
thickness between the transparent conductive layer and the second voltage 
applying electrode to ensure a reliable operation of the element.

DETAILED DESCRIPTION 
FIG. 1 shows a multi-layer electroluminescent element embodying the 
invention. The multi-layer electroluminescent element has the same basic 
arrangement as that shown in FIGS. 7 and 8. 
A transparent conductive layer 12 of In.sub.2 O.sub.3 -S.sub.n O.sub.2 
material is deposited up to about 2000.ANG. thickness on a transparent 
glass substrate (Corning #7059) on market by sputtering, and a pattern is 
formed by etching. Subsequently, an electroluminescent layer 14 made from 
zinc sulfide with manganese doping (ZnS:Mn, Mn=0.3 at %) is deposited up 
to about 6000.ANG. thickness on the transparent conductive layer 12 by 
sputtering. In this case, the electroluminescent layer 14 is shaped to a 
pattern partly covering the transparent conductive layer 12. Further, an 
insulative layer made from Ta.sub.2 O.sub.5 is deposited on these layers 
by reactive sputtering up to about 3000.ANG. thickness. The insulative 
layer includes a portion 15a covering the electroluminescent layer 14 and 
another portion 15b directly covering the transparent conductive layer 12 
where the electroluminescent layer 14 does not extend. Further, an 
aluminum layer for voltage applying electrodes is deposited on the 
insulative layers 15a and 15b by sputtering up to about 2000.ANG. 
thickness. Etching is effected to the aluminum layer to form a first 
voltage applying electrode 16 above the electroluminescent layer 14 and a 
second voltage applying electrode 17 not overlapping the 
electroluminescent layer 14. The first and second voltage applying 
electrodes 16 and 17 are configured to overlap the transparent conductive 
layer 12 respectively. 
In the illustrated embodiment, the overlapping area between the first 
voltage applying electrode 16 and the transparent conductive layer 12 is 
twice the overlapping area between the second voltage applying electrode 
17 and the transparent conductive layer 12. 
With this arrangement, when an alternating voltage is applied between the 
first voltage applying electrode 16 and the second voltage applying 
electrode 17, an electric field produced by the alternating voltage is 
applied between the first and second voltage applying electrodes 16-17 and 
the transparent conductive layer 12 forming an equivalent potential 
surface. As a result, the electroluminescent layer 14 located between the 
first voltage applying electrode 16 and the transparent conductive layer 
12 emits light. In this case, the light is emitted from a portion of the 
electroluminescent layer 14 where the first voltage applying electrode 16 
overlaps the transparent conductive layer 12. 
In this multi-layer electroluminescent element, two layers, i.e. the 
insulative layer 15a and the electroluminescent layer 14, exist between 
the first voltage applying electrode 16 and the transparent conductive 
layer 12, and only one layer, i.e. the insulative layer 15b, exists 
between the transparent conductive layer 12 and the second voltage 
applying electrode 17. Therefore, totally 3 layers are interposed between 
the first and second voltage applying electrodes 16 and 17, excluding the 
transparent conductive layer 12. 
Assuming that the insulative layer 15a, electroluminescent layer 14 and 
insulative 15b are dielectric layers respectively, a circuit diagram shown 
in FIG. 3 is established. In FIG. 3, C.sub.1 refers to the capacitance of 
the insulative layer 15a, C.sub.2 to the capacitance of the 
electroluminescent layer 14, C.sub.3 to the capacitance of the insulative 
layer 15b, C to the sum of C.sub.1, C.sub.2 and C.sub.3, V.sub.1 to a 
voltage applied to the insulative layer 15a, V.sub.2 to a voltage applied 
to the electroluminescent layer 14, V.sub.3 to a voltage applied to the 
insulative layer 15b, V to the sum of V.sub.1, V.sub.2 and V.sub.3, and Q 
to an electric charge. 
From this circuit, the following equations (1) through (5) are established. 
EQU V=V.sub.1 +V.sub.2 +V.sub.3 . . . (1) 
##EQU1## 
From these equations, V.sub.2 is expressed by: 
##EQU2## 
Further, the capacitance C of a dielectric member is generally expressed 
by: 
##EQU3## 
In the aforegoing equations, d is the thickness of a layer, S is the area 
of an electrode .epsilon..sub.0 is the space dielectric constant, and e is 
the specific dielectric constant. Therefore, expressing the thickness of 
the insulative layer 15a a by d.sub.1, the thickness of the 
electroluminescent layer 14 by d.sub.2, the thickness of the insulative 
layer 15b by d.sub.3, the overlapping area between the first voltage 
applying electrode and the transparent conductive layer 12 by S, the 
overlapping area between the second voltage applying electrode and the 
transparent conductive layer 12 by S.sub.3, the specific dielectric 
constant of the insulative layer 15a by .epsilon..sub.1, the specific 
dielectric constant of the electroluminescent layer 14 by .epsilon..sub.2, 
and the specific dielectric constant of the insulative layer 15b by 
.epsilon..sub.3, the following equations are established: 
##EQU4## 
As described above, since the insulative layers 15a and 15b have 
thicknesses d.sub.1 and d.sub.3 of 3000.ANG. whereas the 
electroluminescent layer 14 has a thickness d.sub.2 of 6000.ANG. their 
relationships are expressed by 2d.sub.1 =d.sub.2 =2d.sub.3, d.sub.1 =d. 
Since the insulative layers 15a and 15b are made from Ta.sub.2 O.sub.5, 
their specific dielectric constants .epsilon..sub.1 and .epsilon..sub.3 
are 24. Further, since the electroluminescent layer 14 is made from 
ZnS:Mn, its specific dielectric constant .epsilon..sub.2 is 8. Therefore, 
their relationships are expressed by .epsilon..sub.1 =.epsilon..sub.3 
=3.epsilon..sub.2, .epsilon..sub.2 =.epsilon.. 
From these relationships, equations (8) through (11) may be replaced by: 
##EQU5## 
Combining equations (8)' through (10)' with equation (6), the following 
equation is obtained: 
##EQU6## 
If S=S.sub.3, equation (11) results in V.sub.2 =0.75 V. Therefore, when the 
overlapping area S between the first voltage applying electrode 16 and the 
transparent conductive layer 12 equals the overlapping area S.sub.3 
between the second voltage applying electrode 17 and the transparent 
conductive layer 12, 75% of the supplied voltage is applied to the 
electroluminescent layer 14. 
However, if 2S=S.sub.3, equation (1) results in V.sub.2 =0.8 V. Therefore, 
when the overlapping area S.sub.3 between the second voltage applying 
electrode 17 and the transparent conductive layer 12 is twice the 
overlapping area S between the first voltage applying electrode 16 and the 
transparent conductive layer 12, 80% of the supplied voltage is applied to 
the electroluminescent layer 14. 
As explained, by increasing the overlapping area S.sub.3 between the second 
voltage applying electrode 17 and the transparent conductive layer 12, the 
ratio of the voltage effectively applied to the electroluminescent layer 
14 can be increased to reduce the driving voltage supplied to the 
FIG. 2 shows a further embodiment of the invention. 
This embodiment is basically equal to the embodiment of FIG. 1 except that 
the thickness d.sub.3 of the insulative layer 15b interposed between the 
second voltage applying electrode 17 and the transparent conductive layer 
12 is 6000.ANG. which is twice the thickness of the insulative layer 15b 
in the embodiment of FIG. 1. The overlapping area S.sub.3 between the 
second voltage applying electrode 17 and the transparent conductive layer 
12 is twice the overlapping area S between the first voltage applying 
electrode 16 and the transparent conductive layer 12 as in the first 
embodiment. 
In the multi-layer electroluminescent element according to the second 
embodiment, it is understood from equation (10) repeated below: 
##EQU7## 
that the capacitance C.sub.3 does not change regardless of the double 
value of d.sub.3 because the overlapping area S.sub.3 is also twice. 
Additionally, it is understood from equation (6) that the voltage ratio to 
the electroluminescent layer 14 does not change at a uniform value C.sub.3 
as far as C.sub.1 and C.sub.2 are equal. 
Therefore, regardless of the double value of the thickness d.sub.3 of the 
insulative layer 15b, light emission is performed with the same supply of 
driving voltage provided that the overlapping area S.sub.3 between the 
second voltage applying electrode 16 and the transparent conductive layer 
12 is twice the overlapping area S between the first voltage applying 
electrode 16 and the transparent conductive layer 12. Additionally, by 
doubling the thickness of the insulative layer 15b, pin holes or other 
undesired phenomenon of the insulative layer 15b are prevented. This 
contributes to prevention of a leak between the second voltage applying 
electrode 17 and the transparent conductive layer 12 and improves the 
reliability of the element. 
As described above, according to the invention arrangement, the number of 
layers interposed between the first and second voltage applying electrodes 
to decrease the driving voltage to the element. Further, since the 
overlapping area between the second voltage applying electrode and the 
transparent conductive layer is larger than the overlapping area between 
the first voltage applying electrode and the transparent conductive layer, 
a further decrease of the driving voltage is possible. Additionally, 
without increasing the driving voltage, the thickness of the insulative 
layer interposed between the second voltage applying electrode and the 
transparent conductive layer is increased to prevent a leak of the 
insulative layer and improve the reliability of the element. Furthermore, 
all electrodes are configured or located to permit external extension 
thereof from the back surface of the element to facilitate the electrode 
extension or connection process. Finally, since the transparent conductive 
layer does not directly receive any voltage but merely serves to form an 
equivalent potential surface, no limitation is imposed to the pattern 
width, and the display resolving power is further improved.