Electric lamp and discharge devices: systems – Plural power supplies – Plural cathode and/or anode load device
Reexamination Certificate
2001-01-31
2003-08-19
Wong, Don (Department: 2821)
Electric lamp and discharge devices: systems
Plural power supplies
Plural cathode and/or anode load device
C345S077000
Reexamination Certificate
active
06608448
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an organic light emitting device, and in particular, to a drive scheme for an organic light emitting device.
Light emitting devices are becoming more popular as an image source in both direct view and virtual image displays. The popularity is due, at least in part, to the potential of generating relatively high luminance at relatively low power levels. For example, reflective liquid crystal displays can only be used in high ambient light conditions because they derive their light from the ambient light. Also, liquid crystal displays with back lights may be used in low ambient light conditions because they primarily derive their light from the back light. However, such liquid crystal displays are generally too large for practical use in very small devices.
Organic light emitting devices are especially suitable for use in very small devices, such as pagers, cellular and portable telephones, two-way radios, data banks, radios, etc. Organic light emitting devices are capable of generating sufficient light for use in displays under a variety of ambient light conditions, from no ambient light to high ambient light. Also, organic light emitting devices can be fabricated relatively cheaply and in a variety of sizes from very small (less than a tenth of a millimeter in diameter) to relatively large. In addition, light emitting devices have the added advantage that their emissive operation provides a very wide viewing angle.
Generally, organic light emitting devices include a first electrically conductive layer (or first contact), an electron transporting and emission layer, a hole transporting layer, and a second electrically conductive layer (or second contact). The light can be transmitted either way but typically exits through one of the conductive layers. There are many ways to modify one of the conductive layers for the emission of light there-through but it has been found generally that the most efficient light emitting device includes one conductive layer which is transparent to the light being emitted. Also, one of the most widely used conductive, transparent materials is indium-tin-oxide (ITO), which is generally deposited in a layer on a transparent substrate such as a glass plate.
Referring to
FIG. 1
, a conventional driving system for driving a luminous element is shown. The driving system shown in
FIG. 1
is generally referred to as a simple matrix driving system in which anode lines A
1
through A
m
and cathode lines B
1
through B
n
are arranged in a matrix (grid). In the driving system shown in
FIG. 1
luminous elements E
1,1
through E
m,n
are connected at each intersection of the anode lines and cathode lines. The driving system causes the luminous element at an arbitrary intersection to emit light by selecting and scanning one of the anode lines and the cathode lines sequentially at fixed time intervals and by driving the other of the anode and cathode lines by current sources
52
1
through
52
m
, i.e., driving sources in synchronism with the scan.
Thus, there are traditionally two systems for driving luminous elements by means of the driving sources: (1) a system of scanning the cathode lines and driving the anode lines, and (2) a system of scanning the anode lines and driving the cathode lines.
FIG. 1
illustrates the former case of scanning the cathode lines and driving the anode lines.
As shown in
FIG. 1
, the cathode line scanning circuit
51
is connected to the cathode lines B
1
through B
n
and the anode line driving circuit
52
comprising the current sources
52
1
through
52
m
is connected to the anode lines A
1
through A
m
. The cathode line scanning circuit
51
applies a ground potential (0 volts) sequentially to the cathode lines B
1
through B
n
by scanning these lines while switching switches
53
1
through
53
n
to the side of a ground terminal at fixed time intervals. The anode line driving circuit
52
connects the current sources
52
1
through
52
m
with the anode lines A
1
through A
m
by controlling ON/OFF of switches
54
1
through
54
m
in synchronism with the scanning of the switches of the cathode line scanning circuit
51
to supply driving current to the luminous element at the desired intersection. In essence, a potential is imposed across or a current passed through the light emitting material.
When the luminous elements E
2,1
and E
3,1
are to emit light, for example, the switches
54
2
and
54
3
of the anode line driving circuit
52
are switched to the side of the current sources to connect the anode lines A
2
and A
3
with the current sources
52
2
and
52
3
. At the same time the switch
53
1
of the cathode lines scanning circuit
51
is switched to the ground side so that the ground potential is applied to the first anode line B
1
. The luminous elements are controlled so that the luminous element at an arbitrary position emits light and so that each luminous element appears to emit light concurrently by quickly repeating such scan and drive.
A reverse bias voltage V
cc
, which is equal to the source voltage, is applied to each of the cathode lines B
2
through B
n
. The reverse bias voltage V
cc
is not applied to the cathode line B
1
being scanned in order to prevent erroneous emission. It should be noted that although the current sources
52
1
through
52
m
are used as the driving sources in
FIG. 1
, the same effect may be realized also by using voltage sources.
Each of the luminous elements E
1,1
through E
m,n
connected at each intersection may be represented by a luminous element E having a diode characteristic and a parasitic capacitor C connected in parallel, as shown by the equivalent circuit in FIG.
2
. Traditional driving systems described above have had problems due to the parasitic capacitor C within the equivalent circuit. The problems are described as follows.
FIGS. 3A and 3B
illustrate each of the luminous elements E
1,1
through E
1,n
using only the parasitic capacitors C described above by excerpting the part of the luminous elements E
1,1
through E
1,n
connected to the anode line A
1
in FIG.
1
. When the cathode line B
1
is scanned and the anode line A
1
is not driven, the parasitic capacitors C
1,2
through C
1,n
of the other luminous elements E
1,2
through E
1,n
(except the parasitic capacitor C
1,1
of the luminous element E
1,1
connected to the cathode line B
1
currently being scanned), are charged by the reverse bias voltage V
cc
applied to each of the cathode lines B
1
through B
n
, in the direction as shown in FIG.
3
A.
Next, when the scanning position is shifted from the cathode line B
1
to the next cathode line B
2
and the anode line A
1
is driven in order to cause the luminous element E
1,2
to emit light, for example, the state of the circuit is shown in FIG.
3
B. Thus, not only is the parasitic capacitor C
1,2
of the luminous element E
1,2
, which emits light changed, but the parasitic capacitors C
1,1
and C
1,3
through C
1,n
of the luminous elements E
1,1
and E
1,3
through E
1,n
connected to the other cathode lines B
1
and B
3
through B
n
, also are charged because currents flow into the capacitors in the direction as indicated by arrows.
In general, luminous elements can not emit light normally unless a voltage between both ends thereof builds up to a level which exceeds a specified value. In the traditional driving system, not only is the parasitic capacitor C
1,2
changed when E
1,2
is to emit light, but the parasitic capacitors C
1,3
through C
1,n
of the other luminous elements E
1,3
through E
1,n
are charged as well. As a result, the end-to-end voltage of the luminous element E
1,2
connected to the cathode line B
2
can not build up above the specified value until the charging of all of these parasitic capacitors of the luminous elements is completed.
Accordingly, such a system has the limitation that the build up speed until emission is slow. Also no fast scan can be attained due to the parasitic capacitors described above. Further, because the parasitic capacitors o
A Minh D
Chernoff Vilhauer McClung & Stenzel LLP
Planar Systems Inc.
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