Semiconductor device manufacturing: process – Making device or circuit emissive of nonelectrical signal – Making emissive array
Reexamination Certificate
2000-08-17
2002-12-24
Thompson, Craig (Department: 2813)
Semiconductor device manufacturing: process
Making device or circuit emissive of nonelectrical signal
Making emissive array
C257S089000
Reexamination Certificate
active
06498049
ABSTRACT:
This invention relates to display devices, especially ones that use an organic material for light emission.
One type of electroluminescent display device is described in PCT/WO9013148, the contents of which are incorporated herein by reference. The basic structure of this device is a light-emitting polymer film (for instance a film of a poly(-phenylenevinylene)—“PPV”) sandwiched between two electrodes, one of which injects electrons and the other of which injects holes. The electrons and holes excite the polymer film, emitting photons. These devices have potential as flat panel displays.
Another type of organic light-emitting device is a small molecule device, details of which are given in U.S. Pat. No. 4,539,507, the contents of which are incorporated herein by reference. These have a light-emitting layer which comprises at least one small molecule material such as tris(8-hydroxyquinoline)aluminium (“Alq
3
”) sandwiched between the two electrodes.
In an organic light-emitting device the organic light-emitting layer is generally divided into individual pixels, which can be switched between emitting and non-emitting states by altering the current flow through them. The pixels are generally arranged in orthogonal rows and columns. Two alternative arrangements for controlling the pixels are generally used: passive matrix and active matrix. In a passive matrix device one of the electrodes is patterned in rows and the other in columns. Each pixel can be caused to emit light by applying an appropriate voltage between the row and column electrodes at whose intersection it lies. This calls for high peak brightnesses from the pixels, because each pixel can only be powered for a fraction of the scan cycle. In an active matrix display the high peak brightness is not required because each pixel can be left in an emitting state whilst another pixel is addressed.
FIG. 1
illustrates a circuit for driving one pixel in a thin-film transistor (“TFT”) active matrix display. The circuit comprises the pixel itself, illustrated as diode
1
, which is connected between electrodes
2
and
3
. Electrodes
2
and
3
are connected to all the pixels of the device and a voltage sufficient for emission from the pixel is applied constantly between the electrodes
2
and
3
. At least part of the switch circuit
4
, which in practice is embodied by thin-film transistors, lies between electrode
3
and the pixel
1
. (There may also, or alternatively, be circuitry between the pixel/diode
1
and the electrode
2
). The switch circuit is controlled by way of row and column electrodes
5
,
6
. To cause the pixel
1
to emit light, voltages are applied to the electrode
6
, to switch the switching transistor
7
on, and to electrode
5
to charge the storage capacitor
8
. Electrodes
6
is then turned off. Since the capacitor
8
is charged the current transistor
9
is switched on and the voltage applied at electrode
3
is applied to the pixel, causing it to emit. Although it requires a more complex circuit than a passive matrix device this arrangement has the advantage that the pixel can be held in an emitting state by means of the capacitor
8
whilst other pixels on different rows and columns are addressed by their row and column electrodes.
FIG. 2
shows a schematic plan view of typical switching circuitry associated with a pixel of an organic light-emitting device and
FIG. 3
shows a cross-section of the circuitry of
FIG. 2
on the line
1
A-
1
A′. The circuitry comprises a scan (or gate) line
10
(which corresponds to the electrode
6
in FIG.
1
), a signal (or data) line
11
(which corresponds to the electrode
5
in FIG.
1
), a common line
12
(which corresponds to the electrode
3
in FIG.
1
), a switching thin film transistor shown generally at
13
(which corresponds to the transistor
7
in FIG.
1
), a storage capacitor shown generally at
14
(which corresponds to the capacitor
8
in
FIG. 1
) and a current transistor shown generally at
15
(which corresponds to the transistor
9
in FIG.
1
). As
FIG. 3
shows, insulating layers
16
of SiO
2
separate the component parts of the circuitry, and the circuitry is deposited on a glass substrate
17
. At the output of transistor
15
is a contact region
29
which constitutes the output terminal of the circuit.
Banks
30
of insulating material (not shown in
FIG. 2
) are formed to constrain the edges of the light-emitting region itself. To connect between the output terminal of the TFT circuit and the light-emitting material of the pixel there is an electrode
19
of transparent indium-tin oxide (“ITO”). This makes contact with the contact region
29
and provides a wide pad which forms the anode of the emitting device. A layer
33
of light-emitting material is deposited on the pad (corresponding to pixel
1
in
FIG. 1
) and finally a cathode
31
(corresponding to electrode
2
in
FIG. 1
) is deposited on top of it. Light emission from the pixel towards a viewer is generally in the direction into the page in FIG.
2
and as shown by arrow B in FIG.
3
. Therefore, to prevent it obscuring the emitted light, the TFT circuitry is located generally to the side of the light-emitting material
33
.
ITO has good transparency, low sheet resistance and established processing routes, and it has a low resistance which makes it especially useful in passive matrix displays where, because each pixel can only emit for part of the time, high peak through-currents are needed. However, the processing of the ITO can cause problems. Typically, the ITO is deposited as a continuous layer (e.g. by sputtering or evaporation) over the entire device. It must then be pattemed to give separate pads
19
for each pixel of the device. The patterning is typically lithographic, with the ITO being etched to remove the unwanted areas. This causes problems because the materials used for the etching can easily seep into the TFT structure, through voids between the component regions, and damage the circuitry. Damage to the circuitry of only one pixel of a display may cause the entire display to be rejected.
Devices have been made more stable by using a layer of a conductive polymer between the ITO and the light emitting layer (see, for example, J Carter et al., Appl. Phys. Lett. 71 (1997) 34), and in other fields, such as the field of non-emissive devices the ITO layer has been omitted (see, for example, A Lien et al., “Conducting Polyaniline as a Potential ITO Replacement for Flat Panel Applications”, Proceedings of the International Display Research Conference, Society of Information Display, Toronto, Sep. 15-19, 1997, p. 1).
According to the present invention there is provided a method for forming a display device, comprising: depositing a thin-film transistor switch circuit on a substrate; depositing by ink-jet printing an electrode layer of light transmissive conductive organic material in electrical contact with the output of the thin-film transistor circuit; and depositing an active region of the device.
Preferably the active region is also deposited by ink-jet printing.
The active region may preferably be in the form of a layer.
The active region may be a light-emitting region (for instance comprising an organic light-emitting material) or a region the passage of light through which can be controlled (for instance comprising a liquid crystal material).
The said organic light emitting material may be a polymer material. The organic light-emitting material is preferably a conjugated material. A suitable material is a semiconductive conjugated polymer such as PPV, or a derivative thereof. The light-emitting material suitably is or comprises PPV, poly(2-methoxy-5(2′-ethyl)hexyloxyphenylene-vinylene) (“MEH-PPV”), a PPV-derivative (e.g. a di-alkoxy derivative), a polyfluorene and/or a co-polymer incorporating polyfluorene segments, PPVs and/or related co-polymers. As an alternative to ink-jet printing it could be deposited by spin-coating, dip-coating, blade-coating, meniscus-coating, self-assembly etc. The constituent of the light-emitting region and/
Carter Julian C.
Friend Richard H.
Heeks Stephen K.
Pichler Karl
Towns Carl R.
Cambridge Display Technology
Thompson Craig
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