Computer graphics processing and selective visual display system – Single display system having stacked superimposed display...
Reexamination Certificate
1998-09-28
2001-08-21
Hjerpe, Richard (Department: 2774)
Computer graphics processing and selective visual display system
Single display system having stacked superimposed display...
C313S509000, C315S169300
Reexamination Certificate
active
06278417
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
This application relates to a method of driving a display device which has a display element disposed between two electrode layers. The method is a passive addressing method, in which a first voltage, known as a “data voltage”, is applied to one of the electrodes and a second voltage, known as a “strobe voltage”, a “select voltage” or a “scanning voltage” (hereinafter referred to as a “strobe voltage” for convenience), is applied to the other electrode. The invention also relates to a display device. The invention particularly relates to electro-luminescent display devices.
DESCRIPTION OF THE RELATED ART
An electro-luminescent device (“EL device”) generates light as a result of electron-hole recombination. An EL device typically has a multilayer structure, in which an emitter layer is confined between an electron transporting layer (ETL) and a hole transporting layer (HTL). Charge carrier recombination occurs in the emitter layer and photons are generated. It is possible for one layer to act as both the ETL and the emitter layer, in which case recombination of electrons and holes occurs near the interface between the HTL and the ETL/emitter layer.
It is possible to vary the wavelength of light emitted by an EL device by using different materials for the emitter layer, and it is also possible to manufacture an electro-luminescent device that emits white light. Both inorganic materials and organic materials can be used as the emitter layer.
FIGS.
1
(
a
) and
1
(
b
) schematically show the structure of two conventional EL devices, which are described in “Journal of Applied Physics”, 65 (9), May 1989, pp 3610-3616. Both devices have an ITO (indium tin oxide) electrode layer
1
disposed on a substrate (not shown). A diamine layer
2
constitutes the HTL in both devices. In the device shown in FIG.
1
(
a
) the emitter layer is formed by a layer
3
of 8-hydroxyquinoline aluminium, Alq, which can be either doped or undoped. Electrons are injected from an Mg:Ag electrode
4
. In the device shown in FIG.
1
(
b
) the emitter layer is formed by a doped layer of Alq
5
located within the Alq layer
3
. Alternatively, it would be possible to put the doped Alq layer adjacent to the HTL layer
2
. As in the device of
FIG. 1
(a), electrons are injected from a Mg:Ag electrode
4
.
In a known colour EL display, each pixel is laterally divided into three sub-pixels, one for each primary colour. The three colours can be obtained by using three different emitter materials in the three sub-pixels, or by using an EL device which emits white light and providing each sub-pixel with an appropriate colour filter. Alternatively, if an EL device with an emitter material which generates blue light is used, red and green sub-pixels can be obtained by using fluorescent colour filters.
These known colour EL displays are unsatisfactory. Complicated masking techniques are required when applying the emitter materials in order to provide different emitter materials in adjacent sub-pixels. On the other hand, if colour filters are used, they will absorb radiation and so reduce the brightness of the display. They will also heat up as they absorb light and thus heat up the display device.
One way of addressing these problems is to use transparent EL elements, and stack two or more elements capable of emitting light of different wavelengths. A display of this type is disclosed in “Applied Physics Letters” 68 (19) May 6, 1996, pp2606-2608.
FIG. 2
shows the structure of such an electro-luminescent element. The device is grown on a glass substrate
6
coated with an indium tin oxide (ITO) film
7
which acts as a transparent, hole injecting contact. The HTL
8
is a 200 Å (20 nm) thick layer of the hole conducting material N.N′-diphenyl-N.N′-bis(3-methylphenyl) 1-1′biphenyl-4,4′diamine (TPD), which is deposited by thermal evaporation in a vacuum. The ETUemitter layer
9
is a 400 Å (40 nm) thick layer of the electron conducting and light emitting material tris-(8-hydroxyquinoline) aluminium (Alq
3
). Typically, EL devices have thick, opaque upper electrodes composed of a low work-function alloy such as Mg:Ag for injection of electrons. For a transparent EL device, this is replaced with a layer
10
of Mg:Ag that is thinner than the optical skin depth, followed by an overlayer of ITO
11
. This results in a highly transparent top electrode with injection characteristics similar to that of a conventional upper electrode.
To form this contact, a thin Mg-Ag layer (with a thickness of 50-400 Å (540 nm)) is deposited through a shadow mask by co-evaporation of Mg and Ag in a ratio of 30:1. The sample is then immediately transferred through a load lock to a sputtering chamber. The ITO film is then deposited by low-power sputtering. The target, housed in a magnetron sputtering gun, is 10% SnO
2
and 90% In
2
O
3
by weight with 99% purity. The sputtering gas is a mixture of 99.9999% pure argon and 99.998% pure oxygen.
FIG. 3
shows a further prior art EL display device, which consists of a red EL element
13
stacked on a blue EL element
12
. This is described in “Applied Physics Letters” 69 (20) November 1996, pp2606-2608. The device has a 1 mm glass substrate
14
, on which a 2000 Å (200 nm) ITO electrode
15
is formed. Layers
16
,
17
and
18
are a 600 Å (60 nm) TPD layer, an 800 Å (80 nm) Alq′
2
OPh layer and a 360 Å (36 nm) Alq
3
layer, and these layers constitute the blue EL element
12
.
Layers
19
and
20
are electrode layers: a 130 Å (13 nm) Mg:Al layer and a 550 Å (55 nm) ITO layer.
21
denotes a 600 Å (60 nm) TPD layer, and
22
is a 590 Å (59 nm) TPP/Alq
3
layer. Finally, a 1500 Å (150 nm) Mg:Al electrode layer
23
and a 500 Å (50 nm) Ag layer
24
are provided. Layers
20
-
24
constitute the red EL element.
While EL devices formed of two stacked, transparent EL devices overcome some of the problems outlined above, there are still a number of problems associated with them. Three separate EL elements are required for a full colour display. In a passively addressed device, each EL element would need its own strobe electrode layer and data electrode layer. If the elements were simply stacked one above another, the strobe electrode layer of one element would be adjacent to the data electrode of another EL element. It would thus be necessary to provide additional insulating layers in the device if the three elements were to be driven simultaneously, and this would complicate the structure of the device. It would be possible to drive the three elements sequentially without providing intermediate insulating layers, but this would reduce the brightness of the display. While the brightness can be restored to the required level by using larger driving voltages, it is undesirable to do this because using larger driving voltages will shorten the lifetime of the device.
GB-A-2 194 376 discloses a display device having an electrode layer, a first light-emitting layer, a second electrode layer, a second light-emitting layer, and a third electrode layer disposed in this order. The first light emitting layer is driven by applying a voltage between the first and second electrode layers, and the second light-emitting layer is driven by applying a voltage between the second electrode layer and the third electrode layer. The first and second light-emitting layers are driven sequentially, so that this device suffers from the disadvantages outlined above.
U.S. Pat. No. 4,416,494 discloses an EL device having a first electrode, a first EL layer, a second electrode layer, a second EL layer, and a third electrode layer. In operation, the second electrode layer is earthed, and an A.C. voltage is applied between the first and third electrode layers. This driving method is adopted so that the voltages applied across the two EL layers are
180
° out of phase with one another to suppress the generation of piezoelectric noise.
U.S. Pat. No. 4,777,402 discloses a device having two stacked
Bock Harald Reinhart
Hudson Andrew James
Rorison Judy Megan
Fatahi Yar M
Hjerpe Richard
Renner Otto Boisselle & Sklar
Sharp Kabushiki Kaisha
LandOfFree
Method of driving a display device, and a display device does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method of driving a display device, and a display device, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method of driving a display device, and a display device will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2545804