GB2228128A

GB2228128A - LC display device

Info

Publication number: GB2228128A
Application number: GB8902997A
Authority: GB
Inventors: Stanley David Brotherton
Original assignee: Philips Electronic and Associated Industries Ltd
Current assignee: Philips Electronics UK Ltd
Priority date: 1989-02-10
Filing date: 1989-02-10
Publication date: 1990-08-15
Also published as: GB8902997D0

Abstract

In an LC display device, the display elements 20 and associated switching elements 35-40 are formed on one base plate 22 and light shielding elements 46 for the switching elements are formed on the other base plate 24 and arranged to have a light absorbing side 47 and a light reflective side 45 respectively facing towards and away from the switching elements. Problems due to heat being generated in picture elements by high intensity light e.g. in projection systems, incident on the plate 24 and being absorbed by the light absorbing layers 47 are thus avoided by use of reflective layers 45. The layer 47 absorbs the small amount of light reaching the back side of reflective layers by internal reflections. A further light blocking element may be provided over the switching element. Reflective layer 45 may be formed on the outer surface of plate 24 A projection system may include the LC device and a lens-mirror assembly to provide a monochrome or a full colour display e.g. of T.V. pictures. <IMAGE>

Description

LIQUID CRYSTAL DISPLAY DEVICES AND PROJ ECTIO,I SYSTEMS INCORPORATIfJG SUCH DEVICES This invention relates to a liquid crystal display device comprising a pair of spaced plates with liquid crystal material therebetween, electrodes carried on the plates and defining a plurality of picture elements each of which is associated with a switching element carried on one of the plates, light shielding means carried on the other plate situated between picture elements and overlying the switching elements.

Active matrix addressed liquid crystal display devices of the above kind, employing for example MOSFETs or thin film transistors, (TFTs), as switching elements for the picture elements, and which, in operation, are illuminated from the side remote from the one plate carrying the switching elements are well known. âuch devices can be used to display monochrome or colour pictures, for example TV pictures. The photoconductive properties of the semiconductor material used in the addressing transistors, particularly amorphous silicon or polysilicon materials, can influence significantly the off resistance of the transistor resulting in poor display quality.In a monochrome display device, the light shielding means, which may take the form of strips of light absorbing material mixed with, for example, polyimide, overlie the transistors to reduce unwanted induced photocurrents in the transistors. Similar problems can be encountered with other types of switching elements such as diodes or Mimes.

In colour display devices, the picture elements are arranged in groups forming colour triplets consisting of three adjacent picture elements, each providing a display output in a respective one of the three primary colours, red, green and blue, by means of a micro-filter array carried on the plate remote from the transistors. In this case the light shielding means can take the form of a grid of light absorbing material which fills the gaps between adjacent picture element colour filters. The grid commonly comprises photolithographically formed black filter material, for example a combination of the red, green and blue filter material used for colouring the picture elements, which, in addition to shielding the transistors to inhibit photocurrent effects, serves also to improve display contrast and colour purity when viewed directly.

Recently, projection systems using matrix liquid crystal display devices as light valves have been developed in order to provide larger area displays. Such systems may have a single display device providing a monochrome display or, with colour filters, a full-colour display, or alternatively three display devices each illuminated with a respective primary colour light whose outputs are combined to give a full-colour display. In their simplest form, these projection systems have a source of intense illumination at their one side and a projection lens on the other which projects an image of the display device output onto a screen. A typical example of a full-colour projection system is described in the article entitled "LCD Full-Colour Video Projector" by S. tiorozumi et al in SID '86, Digest, pages 375-378.

It is an object of the present invention to provide a liquid crystal display device having improved light shielding means and which is suitable for use in a projection system.

According to one aspect of the present invention, there is provided a liquid crystal display device as described in the opening paragraph which is characterised in that the light shielding means for each switching element comprises a light reflective side facing away from the switching element and a light absorbing side facing towards the switching element.

The light shielding means preferably comprises a light reflective layer and a light absorbing layer overlying one another with the light absorbing layer being disposed closer to the switching element.

In operation of the display device with light being directed thereon from the side of the device, then light incident generally orthogonally on the plate carrying the light shielding means and in the direction of the switching elements is blocked and reflected away from the device by the reflective layer.

Thus light which would otherwise fall upon the switching elements, possibly giviny rise to excess photocurrent, is prevented from reaching the switching elements and instead is re-directed away from the display device. This reflection of light at the input side of the display device is important to the operation of the device when used in a projection system especially and offers a significant advantage over devices utilising conventional black absorbing material. It has been found that with black absorption layers the hiyh intensity light levels experienced in projection systems leads to heat being generated in the picture elements through absorption of the light by the black material. This heating can have an adverse effect on the performance of the liquid crystal material and other components of the display device.In using a reflective layer in the above manner the possibility of such a problem is avoided.

Light entering the display device non-orthogonally and which could otherwise be internally reflected by a reflective layer towards a switching element possibly causing excess photocurrent is prevented from doing so by the absorbing layer covering the reflective layer internally of the device. The amount of light which normally could be expected to reach this absorbing layer is comparatively small and its absorption should not produce any significant heating effect.

The reflective layer preferably comprises metal. For ease of manufacture and good performance chromium is highly suited.

The absorbing layer may comprise a combination of polyimide and a light absorbing material, for example, a black dye, of a kind used previously as a light absorbing material in liquid crystal display devices. Of course other known suitable reflective and absorbing materials could be used as will be apparent to persons skilled in the art.

The display device may be a monochrome or multi-colour device. In a r;lulti-colour device, each picture element may include, in conventional manner, a respective colour filter element. For a full colour display the filter elements are arranged in a regular pattern with three adjacent filter elements consisting respectively of red, green and blue filter material.

The filter elements may, in accordance with known practices, be formed by a printing and dyeing process, by a photolithographic process or by using photosensitive emulsions with portions thereof being exposed selectively to differently coloured light.

In the case particularly of a multi-colour display device, the light shielding means advantageously may be extended beyond the regions overlying the switching elements so as to separate adjacent filter elements, in similar manner to a conventional black filter matrix, thereby improving colour purity.

The switching elements may comprise transistors such as TFTs formed on the one plate with their gate electrodes arranged either between their channel regions and the one plate or on the side of channel regions remote from the one plate. In either case, but more beneficially in the former arrangement, a further light blocking element may extend over the channel region of each transistor on the one plate between the transistors and the light shielding means to inhibit further the possibility of undesirable photocurrent response in operation. In the later arrangement, the gate electrode itself acts as a light shield.

According to another aspect of the present invention, there is provided a projection display system comprising a liquid crystal display device in accordance with the first aspect of the invention, a light source arranged on the side of the display device remote from the plate thereof carrying the switching elements for illuminating the display device, and a projection lens arranged on the other side of the display device.

Embodiments of a liquid crystal display device, and projection display systems incorporating the display devices in accordance with the invention will now be described, by way of example, with reference to the accompanying drawings in which: Figure 1 is a schematical perspective cross-sectional view, not to scale, through a part of an embodiment of liquid crystal display device according to the invention intended for full colour displays and showing a few, typical, picture elements; Figure 2 is a schematic cross-sectional view, not to scale, through a part of a picture element of the display device of the device of Figure 1;; Figure 3 shows schematically a first embodiment of a projection display system incorporating the display device of Figures 1 and 2, and Figure 4 shows schematically a second embodiment of a projection display system using modified forms of the display device of Figures 1 and 2.

Referring to Figure 1, the display device, which is suitable for displaying full colour pictures, for example colour TV pictures, comprises a row and column array of individually-operable, generally rectangular, picture elements 1O, only nine of which are shown for simplicity. In practice the total number of picture elements may be 10,000 or more.

Each picture element is associated with a respective thin film transistor, TFT, 16 and is located adjacent the intersection of sets of row and column address conductors 12 and 14 with the boundaries of the picture elements being determined by the spacing between adjacent pairs of the row and column conductors.

For the sake of simplicity, the conductors 12 and 14 and the TFTs 16 are represented in Figure 1 by plain lines and symbols respectively.

The gate electrodes of all TFTs associated with picture elements in the same row are connected to a common row conductor 12 to which, in operation switching (gating) signals are supplied. Likewise, the source electrodes associated with all picture elements in the same column are connected to a common column conductor 14 to which data (video) signals are applied.

The drain electrodes of the TFTs are connected to respective transparent picture element electrodes 20 of ITO forming part of, and defining, their associated picture elements.

The row and column conductors 12 and 14, TFTs 16 and electrodes 2-3 are all carried on a transparent plate 22, for example of glass.

Parallel to and spaced from this plate, is a further transparent plate 24, for example also of glass, on which is formed a continuous transparent conductive layer of ITO constituting a common electrode 26 for all the picture elements of the device. Liquid crystal material 28 is disposed between the two plates, the two plates being sealed around their periphery to contain the liquid crystal material.

In use, the display device, operating in transmissive mode, is illuminated by a light source disposed on the side of the device facing the plate 24 so that Light, indicated by the arrow A, enters the device through this plate and exits through the plate 22 duly modulated according to the transmission characteristics of the picture elements 10.

The operation of this kind of display device is generally well known and as such will not be described here in detail.

Briefly, however, the liquid crystal material serves to modulate light through the device depending on a voltage applied thereacross with each picture element 10, as defined by a picture element electrode 2, an opposing portion of the common electrode 26 and the liquid crystal material therebetween, being individually operable to vary light transmission through the device in accordance with a drive voltage applied across its electrodes.

Row addressing of the array of picture elements is achieved by applying a gating signal to each row conductor 12 in turn which turns on all TFTs in that row. Video data signals are applied to the column conductors 14 for each row of picture elements in turn in synchronism with the gating signals, these data signals being transferred to the appropriate row of picture elements via the on TFTs of that row. By addressing each row of picture elements in turn, a complete TV picture is built up.

A cross-section of a typical picture element and its associated TFT is illustrated schematically in Figure 2. The TFT 16 in this example comprises an amorphous silicon TFT having a gate electrode 35 carried on the plate 22, and connected to a row conductor 12 (not visible), over which lies an insulative layer 36 such as silicon nitride followed by silicon semiconductor material defining a channel region 37 and metal source and drain contacts 39 and 40 connected respectively to a column conductor 14 (not visible) and the ITO electrode 20 of the picture element formed directly on the plate 22. A passivating layer 41 extends continuously over the TFTs and picture element electrode structures on the plate 22.

Referring to both Figures 1 and 2, the display device is further provided with a filter mosaic layer carried on the plate 24 over the electrode 26. The layer 30 consists of individual colour filter elements 30 which are each in registration with a respective picture element and which are arranged in a repeating pattern of groups of three, each group having a red, green and blue filter element forming a colour triplet in known fashion.

For a monochrome device, such filter a mosaic layer is omitted.

The liquid crystal material 28 is bounded by continuous molecular alignment layers 42 and 43 of suitable insulative material such as polyimide carried on the two plates, only the layer 43 associated with the plate 24 being shown in Figure 1.

The TFTs in the array are associated with light shielding means consisting of individual light shielding elements, generally referenced at 45, carried on the plate 24 and overlying respective TFTs, or at least the channel regions thereof. It will be appreciated that the comparative size of the TFTs 16 and the light shielding elements 45 are shown greatly exaggerated in Figure 1 for ease of identification. Each light shielding element 45 comprises a light reflecting layer 46 of chromium facing outwardly of the display device and a directly superimposed, co-extensive light absorbing layer 47 of black dyed polyimide material facing inwardly. The two layers 46 and 47 are formed in succession over the electrode 26 and fill completely the space between adjacent light filter elements 3ü.

Light arriving from the light source generally orthogonally to the plate 24 and directed towards a TFT 16 is reflected by the reflective layer 46 away from the display device. Light entering the display device generally orthogonally to the plate 24 at the region of a picture element defined by the electrode 2(3 is filtered by the associated filter element 30 before being modulated by the picture element and transmitted through the plate 22.

Light entering the display device at oblique angles with respect to the plate 24 may suffer internal reflections. Any such internally reflected light travelling in a direction towards the reflective layer 46 is substantially absorbed by the absorbing layer 47.

The majority of light which would otherwise adversely affect operation of the TFTs is therefore reflected away or absorbed.

The arnount of light absorbed by the layer 47 is comparatively minor and such absorption produces negligible heating effect even when a high intensity light source is used.

In order to enhance colour purity and display contrast, the two layers of the light shielding means are extended beyond the region 45 overlying the TFTs to form a grid-like structure surrounding, and segregating, the colour filter elements 30, as shown in Figure 1, so that the adjacent borders of filter elements are separated by narrow masking strips of light shielding material which fill completely the space therebetween and whose affect is similar to conventional black masking strips used heretofore.

In a monochrome display device in which the filter mosaic layer is absent, the light shielding means need extend only over the regions of the TFTs.

As a further precaution against unwanted photocurrent effects in the TFTs 16, an opaque light blocking element, for example of metal, may be provided between the layers 41 and 42 on the plate 22 overlying the channel regions of the TFTs, as shown referenced at 49 in Figure 2. Where alternative forms of TFTs are utilised in which their gate electrodes are spaced from the plate 22 with the channel regions located between the gate electrodes of the plate 22, these additional light blocking elements are not necessary as the gate electrodes themselves act to block light.

Referring now to Figure 3, there is shown schematically a simple form of projection display system using the above-described embodiment of colour display device. In this system a suitable source of white light 60, for example a tungsten halogen or arc lamp is combined with a reflector and disposed to one side of the display device, here referenced 59, and directs light through a condensing lens 61 towards the plate 24, via a polarising sheet, 62. A further polarising (analysing) sheet 63, is disposed on the other side of the device 59 adjacent the plate 22. Light transmitted by the display device then passes through a projection lens 64 system which focusses the ima3e from the display device onto a projection screen 65.

If only a black and white display is required then the monochrome version of the display device, from which the filter mosaic layer is omitted, is used instead.

A full colour projection display may be obtained by using three such monochrome display devices, each of which is illuminated by respectively coloured light and with their outputs being combined. Figure 4 shows schematically this system.

In the system, white light from a lamp/ref lector assembly 60 is directed through a condensing lens 61 to a crossed pair of dichroic mirrors 70 where it is broken into red, green and blue portions of the spectrum. These three colour portions are then passed to three monochrome type display devices, 62, as previously described, both directly and by reflection from mirrors 71. Again, each display device has input and output polarising sheets although these are not shown in Figure 4.

After modulation by their respective display devices, the three colour portions are recombined by way of a dichroic prism 74.

The recombined light is projected by a projection lens 64 to a projection screen 65.

It will be apparent to persons skilled in the art that various modifications to the above-described display devices and projection systems are possible. For example, the TFTs 16 could comprise polysilicon devices with the plate 22 being of quartz.

Also materials different to those described may be used to form the layers 46 and 47 of the light shielding means. For example, where a layer of photosensitive emulsion is used to provide the colour filter elements 30, portions of this layer could be rendered black. The order of the common electrode 26 and the filter mosaic and light shielding layers could be reversed.

Moreover, the reflector layer 47 of the light shielding means could perhaps be formed on the outer surface of the plate 24 so as to simplify fabrication of the filter mosaic and absorbing layer 46 on the inner surface.

It is envisaged also that the invention can be applied to liquid crystal display devices comprising switching elements other than TFTs, for example non-linear elements such as diodes or MIMs.

Claims

CLAIM(S)

1. A liquid crystal display device comprising a pair of spaced plates with liquid crystal material therebetween, electrodes carried on the plates and defining a plurality of picture elements each of which is associated with a switching element carried on one of the plates, light shielding means carried on the other plate situated between picture elements and overlying the switching elements, characterised in that the light shielding means for each switching element comprises a light reflective side facing away from the switching element and a light absorbing side facing towards the switching element.

2. A liquid crystal display device according to Claim 1, characterised in that the light shielding means comprises for each picture element a light reflective layer and a light absorbing layer overlying one another with the light absorbing layer being disposed closer to the switching element.

3. A liquid crystal display device according to Claim 2, characterised in that the reflective layer comprises metal.

4. A liquid crystal display device according to Claim 3, characterised in that the metal comprises chromium.

5. A liquid crystal display device according to any one of Claims 2 to 4, characterised in that the absorbing layer comprises polyimide material containing a light absorbing material.

6. A liquid crystal display device according to any one of the preceding claims, characterised in that colouring filter elements associated with the picture elements are carried on said other plate and in that the light shielding means extends so as to separate adjacent filter elements.

7. A liquid crystal display device according to any one of the preceding claims, characterised in that the switching elements comprise thin film transistors and in that each thin film transistor has associated therewith a further light blocking layer which is carried on the one plate and which overlies the channel region of the transistor.

8. A projection display system comprising a liquid crystal display device, a source of illumination, and a projection lens, characterised in that the display device comprises a device according to any one of the preceding claims, the source of illumination being arranged to the side of the device remote from the one plate carrying the switching elements and the projection lens being at the opposite side.

9. A liquid crystal display device substantially as hereinbefore described with reference to, and as shown in, Figures 1 and 2 of the accompanying drawings.

10. A projection display system substantially as hereinbefore described with reference to, and as shown in, the accompanying drawings.