WO1999054780A1

WO1999054780A1 - Backlit displays

Info

Publication number: WO1999054780A1
Application number: PCT/GB1999/001157
Authority: WO
Inventors: Karl Pichler
Original assignee: Cambridge Display Technology Ltd.
Priority date: 1998-04-17
Filing date: 1999-04-15
Publication date: 1999-10-28
Also published as: GB9808181D0; AU3436799A

Abstract

A light emissive device comprising: a first electrode layer; a light-switching layer having areas that are controllable to vary the transmission of light therethrough, and non-controllable areas between the controllable areas; a second, light-transmissive electrode layer between the first electrode and the light-switching layer; a light-emissive layer between the first electrode and the second electrode and comprising a light-emissive organic material; and a contrast enhancement layer between the light-emissive layer and the second electrode and comprising areas of reflective material located to underlie the non-controllable areas of the light-switching layer.

Description

BACKLIT DISPLAYS

This invention relates to backlit displays, for example backlit liquid crystal displays. The means of backlighting is preferably an organic light-emissive device.

Electroluminescent devices that employ an organic material for light emission are described in PCT WO90/13148 and US 4,539,507, the contents of both of which are incorporated herein by reference. The basic structure of these devices is a light-emissive organic layer, for instance a film of a poly(p-phenylenevinylene) ("PPV"), sandwiched between two electrodes. One of the electrodes (the cathode) injects negative charge carriers (electrons) and the other electrode (the anode) injects positive charge carriers (holes). The electrons and holes combine in the organic layer generating photons. In PCT/WO90/13148 the organic light emissive material is a polymer. In US 4,539,507 the organic light emissive material is of the class known as small molecule materials, such as (8- hydroxyquinoline)aluminium ("Alq3"). In a practical device, one of the electrodes is typically transparent, to allow the photons to escape the device.

Figure 1 shows the typical cross-sectional structure of an organic light emissive device ("OLED"). The OLED is typically fabricated on a glass or plastic substrate 1 coated with a transparent first electrode 2 such as indium-tin-oxide ("ITO"). Such coated substrates are commercially available. This ITO-coated substrate is coated with at least a layer of a thin film of an electroluminescent organic material 3 and a final layer forming a second electrode 4 which is typically a metal or alloy. Other layers can be added to the device, for example to improve charge transport between the electrodes and the electroluminescent material.

Organic light-emissive materials have great potential for use in various display applications. One such application is as a backlight for transmissive or transreflective liquid crystal displays. In a liquid crystal display there is typically a planar liquid crystal cell which has active regions where the optical properties of the liquid crystal material can be altered by the application of an electric field to vary the transmission of light through the regions. In a transmissive liquid crystal display there is a light source behind the liquid crystal plane; light from the source shines to a viewer through those of the regions through which light can be transmitted. In a transreflective liquid crystal display the light source is supplemented by a reflective mirror, also behind the liquid crystal plane, which can return incident light towards the viewer.

The shape of the active liquid crystal regions is generally defined by the pattern of electrodes in the LCD. Some patterns are specific to alpha-numeric or special character formats. An alternative is a general dot matrix display pattern, in which the active regions are usually arranged to provide pixels of an orthogonal grid layout. The pixels can be controlled by a conventional display controller.

Figure 2 shows a schematic plan view of the basic structure of a passive-matrix LCD. There are orthogonal row 10 and column 11 lines of a transparent conductor such as ITO. These form the electrodes. The row and column lines are separated in the plane of figure 2 by the liquid crystal layer itself. The areas where row and column lines overlap define the active regions (pixels) of the device (e.g. at 13), which can be addressed by applying a voltage between the relevant row and column lines. Because the column lines run across the row lines it is not possible to individually address all the pixels at the same time. Instead, the pixels are addressed with a row-by-row scan. (For simplicity other LCD components such as polarisers, alignment layers, the liquid crystal layer and colour filters are omitted from figure 2).

Figure 2 shows that because adjacent lines are spaced apart a considerable proportion of the display's area is not occupied by active regions, and is "dead space" 12. A device of the type shown in figure 2 can be operated in a reflective mode. A reflective surface is placed behind the LCD plane. Ambient light that passes through the LCD plane is reflected back towards a viewer to generate the required contrast.

Figure 3 shows a cross-section of a transflective active matrix liquid crystal display with an OLED backlight 15. Like parts in figure 3 are numbered as for figures 1 and 2. The LCD display cell 16 is placed between the backlight and the viewer. Arrow A in figure 3 indicates the path of light emitted from the backlight and transmitted through one of the active regions 17 of the liquid crystal plane. In a transreflective display the rear surface of the backlight, and hence the whole display, is reflective to return towards the viewer ambient light which penetrates trough the LCD cell and the backlight. Arrow B indicates the path of ambient light penetrating into the display and being reflected back to the viewer. The reflective surface is provided by layer 4, which is the second electrode of the OLED. This is described in, for example, PCT/GB97/00939.

In most prior art devices the whole area of the LCD cell is constantly illuminated by the backlight. However, this is very often inefficient because, particularly in passive matrix driven displays, not all the LCD pixels are addressed at once. In a passive matrix display only one row is addressed at a time. This means that the light emitted (and hence the power consumed) by the backlight over areas (rows) which are not presently being addressed is wasted.

To avoid this waste, sequentially addressed back-lit transmissive or transreflective LCDs have been developed. In these the backlight is patterned to emit in individually addressable lines (rows); the emissive backlight rows are aligned under the rows of the LCD display; and the operation of the LCD is synchronised with the operation of the backlight rows so that the addressing of an LCD row is synchronised with the provision of backlight for that row. Such a sequentially addressed (powered) backlight for LCD displays is described in PCT/GB96/00924. Figure 4 shows the basic design of such a sequentially addressed backlight with the layers 1 to 4 as in figure 1 but with the metallic and reflecting electrode 4 now patterned in rows. The areas where the electrodes 2 and 4 overlap are the active areas. A potential problem with this design is that not the whole back-area of the display is covered with a reflective layer - this may result in a deterioration in contrast of the back-lit LCD display. It is possible to provide a continuous reflective layer by, for example, laminating an additional reflective layer over the OLED but this requires additional processing steps.

According to the present invention there is provided a light emissive device comprising: a first electrode layer; a light-switching layer having areas that are controllable to vary the transmission of light therethrough, and non-controllable areas between the controllable areas; a second, light-transmissive electrode layer between the first electrode and the light-switching layer; a light-emissive layer between the first electrode and the second electrode and comprising a light- emissive organic material; and a contrast enhancement layer between the light- emissive layer and the light switching layer and comprising areas of reflective material located to underlie the non-controllable areas of the light-switching layer.

A method for forming a light-emissive device comprising the steps of: forming a first electrode layer; forming a light-switching layer having areas that are controllable to vary the transmission of light therethrough, and non-controllable areas between the controllable areas; forming a second, light-transmissive electrode layer between the first electrode and the light-switching layer; forming a light-emissive layer between the first electrode and the second electrode and comprising a light- emissive organic material; forming a contrast enhancement layer between the light- emissive layer and the light-switching layer and comprising areas of reflective material; and mutually locating the contrast enhancement layer and the light switching layer so that the areas of reflective material underlie the non-controllable areas of the light-switching layer.

The steps of the method may be performed in any order, depending on the details of the fabrication method that is chosen. The contrast enhancement layer may be located between the light-emissive layer and the second electrode or between the second electrode and the light-switching layer.

The first electrode layer may be for injecting negative charge carriers (electrons). The first electrode layer preferably has a work function of less than 4.0 eV and most preferably less than 3.5 eV. The material of the first electrode layer is suitably a metal or alloy. Preferred materials include Sm, Yb, Tb, Ca, Ba, Li or alloys of such elements with each other and/or with other metals such as Al.

The light-switching layer may comprise a liquid crystal layer. The light-switching layer may be provided by a liquid crystal device. The variability of the transmissivity of a controllable area is suitably controlled by the application of an electric field across the thickness of the area. The controllable areas of the light- switching layer may be defined by the location of electrodes of the liquid crystal device. The controllable areas may be of the same or different shapes. The controllable areas may be regularly or irregularly spaced apart. The controllable areas are preferably pixels of the display. The non-controllable areas preferably constitute the non-pixel region of the display. Some preferred configurations include providing the controllable areas in an orthogonal grid layout or in an alphanumeric layout. Preferred shapes for the controllable areas include squares and rectangles and shapes that are substantially square or rectangular. On preferred configuration is for the areas of reflective material are linear and mutually parallel.

The second electrode is preferably transparent. The second electrode suitably comprises a conductive oxide such as ITO or tin oxide.

The light-emissive layer preferably contains one or more electroluminescent materials. The electroluminescent material(s) could be polymers (preferably conjugated polymers) or small molecule materials. Examples of suitable polymer/copolymer materials include poly(phenylene vinylene), PPV, or derivatives thereof or polyfluorenes and derivatives. An example of a suitable small molecule material is tris(8-hydroxyquinoline)aluminium ("Alq3"); alternatives are other small molecule electroluminescent materials as generally known in the prior art. Layers of polymer/copolymer materials can be deposited by spin-, blade-, meniscus-, dip-coating or self-assembly, etc. Layers of small molecule materials can be deposited by vacuum sublimation, etc.

The device may comprise additional organic layers and/or materials which aid charge injection and/or transport, improve device efficiency and/or improve device stability and/or operating life. An example could be a layer of a conducting polymer such as polystyrene sulphonic acid-doped polyethylene-dioxythiophene ("PEDOT-PSS") or doped polyaniline deposited between the transparent conductive oxide and the electroluminescent polymer. Where such a transporting layer is a polymer it could be deposited by one of the techniques mentioned above in connection with light-emissive polymers.

There is suitably a film or sheet substrate between the second electrode and the light-switching layer, on to which the second electrode is suitable deposited. The substrate is preferably light-transmissive and most preferably transparent. Suitable materials for the substrate include glass or transparent plastics (preferably non-birefringent plastics).

The reflective material is suitably a metal or alloy, for example Al or Cu or an alloy of one or both of those metals. The reflective material is preferably electrically conductive. The deposition of the reflective material may be done by evaporation or sputtering, for instance. The patterning of the reflective material may be done by lithography combined with wet-chemical or plasma etching, for instance. The reflective material may optionally be covered with a layer of an insulator, which may be an organic or inorganic insulator such as polyimide, aluminium-oxide, silicon-oxide, a nitride or an oxy-nitride. The reflective material is preferably 7

patterned into the form of a grid, suitably defined by crossing sets of parallel lines. The reflective material is suitably opaque. As an alternative to patterning the reflective material after deposition it could be deposited in a patterned state, for instance by shadow masking.

The order of the deposition steps could be reversed. The definitions herein of the locations of the components of the device are (unless the context requires otherwise) definitions of the relative rather than the absolute locations of the components. The cathode and anode electrodes of the device could be exchanged.

The present invention will now be described by way of example with reference to the accompanying drawings, in which: figure 5a shows a schematic cross-sectional view of an OLED device along the line A-A' of figure 5c. figure 5b shows a schematic cross-sectional view of an OLED device along the line B-B' of figure 5c figure 5c shows a schematic plan view of parts (layers 23 and 24 are omitted for clarity) of the structure of figures 5a and 5b and depicts the patterned metallisation on top of the transparent electrode of the OLED device; and figure 6 shows a schematic cross-sectional view of the OLED device of figures 5a to 5c combined with an LCD. The figures are not to scale.

Figures 5a to 5c show schematic views of a part of an OLED backlight. The backlight comprises a transparent glass substrate 21 , which is covered with a substantially unpattemed transparent anode electrode 22. Over the electrode 22 is an opaque low-resistance and reflective layer 25 which is patterned into a grid defined by two orthogonal series of parallel, spaced-apart lines. A thin electrical insulating cover layer (not shown) may lie over the lines. Over this structure is a continuous layer 23 of an organic electroluminescent material. A second, cathode electrode 24 overlies the organic layer 23 and is patterned into rows orthogonal to 8

one set of the lines of the reflective layer 25. Where the electrode 24 overlaps the electrode 22 without the insulating cover layer intervening - i.e. in an orthogonal array of spaced-apart square regions - the light-emissive areas of the backlight are defined.

In a complete installation the OLED backlight would have contacts to a voltage source attached to the electrodes 22 and 24 and would be encapsulated for environmental protection. These features are not shown in the figures. Then, as shown in figure 6, the backlight is fitted to the underside of an liquid crystal display 26, aligned so that each of the light-emissive areas is located under one of the pixels 27 of the LCD. The result is a transflective backlit liquid crystal display in which backlighting is provided only under the LCD pixels (this enhances power efficiency) and in which the lines of the layer 25 act to reflect ambient light back through the LCD. In addition, as will be described below, further advantages are that it is possible to form the lines of the layer 25 by straightforward processing techniques, and that the layer 25 can also act to enhance charge movement through the electrode layer 22 to improve uniformity of emission from the backlight.

There may be one or more additional layers between the LCD cell and a viewer. For instance, there could be a contrast-enhancing film (filter) to help increase contrast.

The substrate 21 could be a glass or transparent plastic film. The first electrode 22 could be of ITO. The lines of the layer 25 could be formed by depositing a continuous layer of Al or Cu or an alloy thereof by sputtering over the first electrode 22, and then patterning by standard lithographic patterning and etch techniques. The organic light-emissive layer 23 could be of PPV and deposited by spin-coating. The electrode 24 could be of Ca or Li and patterned by shadow- masking, laser ablation or lithographic techniques. Throughout the device, other materials and techniques could, of course, be used. The liquid crystal display could be a standard pre-fabricated unit. If the shape or positioning of the controllable pixels of the layer 25 were different then the patterning of the layers 24 and 25 could be altered accordingly. It should also be noted that although in the figures the light-emissive regions of the backlight are shown exactly below the pixels of the LCD this is not essential: the light-emissive regions could be larger or smaller than or slightly offset with respect to the pixels.

It is normally possible for the cathode electrode to be made sufficiently thick and of sufficiently high conductivity that current required by the OLED in the on-state can be supplied through this electrode layer over the display area without significant power dissipation and loss in the electrode layer due to resistive heating. Sheet resistances for these metal/alloy electrode layers of less than 1 Ohms/square can be readily achieved. However, the ITO typically used for the anode electrode has sheet resistances typically of the order of 10 to 30 Ohms/square. These ITO sheet resistances can cause detrimental resistive heating in the ITO layer, particularly for larger displays of areas over several and particularly several tens of square-centimetres. This is aggravated by the fact that the contacts (from the external power supply) to the ITO electrode layer can normally only be made at/to the edge of the display. It is generally known in the prior art that the ITO resistance can be lowered by partly covering the ITO layer with low resistance metal tracking. In the present device the reflective lines 25, if they have a low resistance, can double as such conductive tracks. As an alternative is possible for the layer 25 to be of a material with low conductivity. However, that would not provide the above additional advantage.

Additional layers could be introduced to improve the performance of the backlight. For example, there could be one or more charge carrier transport/blocking layers between the light emissive layer 23 and either or both of the electrodes 22 and 24 to improve charge carrier injection into the light emissive layer and/or inhibit short circuiting. For example, there could be a layer of polystyrene sulphonic acid 10

doped polyethylene dioxythiophene ("PEDOT-PSS") between the electrode 22 and the emissive layer 23.

One or both of the electrode layers could be patterned in lines - for example into strips of conductive electrode material that are linear and mutually parallel - to allow the backlight to be used as a sequentially addressed backlight. A display driver connected to the strips can be used to drive them sequentially.

The present invention may include any feature or combination of features disclosed herein either implicitly or explicitly or any generalisation thereof irrespective of whether it relates to the presently claimed invention. In view of the foregoing description it will be evident to a person skilled in the art that various modifications may be made within the scope of the invention.

Claims

11CLAIMS

1. A light emissive device comprising: a first electrode layer; a light-switching layer having areas that are controllable to vary the transmission of light therethrough, and non-controllable areas between the controllable areas; a second, light-transmissive electrode layer between the first electrode and the light-switching layer; a light-emissive layer between the first electrode and the second electrode and comprising a light-emissive organic material; and a contrast enhancement layer between the light-emissive layer and the light- switching layer and comprising areas of reflective material located to underlie the non-controllable areas of the light-switching layer.

2. A light-emissive device as claimed in claim 1 , wherein the reflective material is electrically conductive.

3. A light-emissive device as claimed in claim 2, comprising an electrical insulating layer between the reflective material and the light-emissive layer.

4. A light-emissive device as claimed in claim 1 , wherein the reflective material is electrically insulating.

5. A light-emissive device as claimed in any preceding claim, wherein the light- switching layer is provided by a liquid crystal device.

6. A light-emissive device as claimed in claim 5, wherein the liquid crystal device is a passive matrix device.

7. A light-emissive device as claimed in any preceding claim, wherein the controllable areas are pixels. 12

8. A light-emissive device as claimed in any preceding claim, wherein the controllable areas are arranged on an orthogonal grid layout.

9. A light-emissive device as claimed in any preceding claim, wherein the second electrode layer is patterned into strips of material that are linear and mutually parallel.

10. A light-emissive device as claimed in claim 9, comprising drive means for sequentially addressing the strips of material of the electrode layer.

11. A light-emissive device as claimed in any preceding claim, wherein the light- emissive layer comprises poly(phenylene vinylene), PPV or a derivative thereof.

12. A light-emissive device as claimed in any preceding claim, comprising a charge-carrier transport layer between the light-emissive layer and the first or second electrode.

13. A light-emissive device as claimed in any preceding claim, wherein the contrast enhancement layer is located between the light-emissive layer and the second electrode.

14. A light-emissive device as claimed in any of claims 1 to 12, wherein the contrast enhancement layer is located between the second electrode and the light-switching layer.

15. A method for forming a light-emissive device comprising the steps of: forming a first electrode layer; forming a light-switching layer having areas that are controllable to vary the transmission of light therethrough, and non-controllable areas between the controllable areas; 13

forming a second, light-transmissive electrode layer between the first electrode and the light-switching layer; forming a light-emissive layer between the first electrode and the second electrode and comprising a light-emissive organic material; forming a contrast enhancement layer between the light-emissive layer and the light-switching layer and comprising areas of reflective material; and mutually locating the contrast enhancement layer and the light switching layer so that the areas of reflective material underlie the non-controllable areas of the light- switching layer.

16. A light-emissive device substantially as herein described with reference to figures 5a, 5b, 5c and 6 of the accompanying drawings.

17. An organic backlight substantially as herein described with reference to figures 5a, 5b, 5c and 6 of the accompanying drawings.

18. A method for forming a light-emissive device substantially as herein described with reference to figures 5a, 5b, 5c and 6 of the accompanying drawings.