GB2354336A - Liquid crystal displays - Google Patents

Abstract

A liquid-crystal display device comprises a source of excitation light, a liquid-crystal modulator cell adapted to modulate excitation light input from the source on one side of the cell and output elements such as phosphors, emitting display light when struck by the excitation light passing through the modulator. The light source emits excitation light over a range of angles, up to about 20{ from normal incidence, and the liquid-crystal cell contains a liquid crystal of the guest-host (GH) type, preferably planar anti-parallel-rubbed. Such a cell exhibits a respectable contrast ratio over a wider range of input angles than conventional TN or STN cells. The output elements emit display light in proportion to the integral over the range of angles of the excitation light input to the cell and can thus produce a substantial contrast ratio, preferably at least 10:1, in spite of the relatively weak collimation of the input light.

Description

2354336 LIQUID-CRYSTAL DISPLAYS The Photolumine scent Liquid-crystal

Display (PLLCD) depends on light from a suitable backlight being directed by a suitable optical system through a liquid-crystal modulator onto a phosphor which is activated by the light. A complex matrix of modulators and phosphors may be used to give a high-information, full-colour display. Reference may be made for instance to WO 97/25650.

one feature of this scheme is that the phosphor integrates all light falling on it. Thus if light from various angles due to (mis)direction of the light away from normal incidence falls on the phosphor it adds to the total light emission from the phosphor. PLLCDs need to have light collimated to some degree because the device operates by mapping the LCD pixels onto phosphor pixels over a distance of about 1-2mm, the thickness of the front face glass, plus analyser etc., of the liquid-crystal cell. Since the pixel size may be as small as 0.3mm, with sub-pixels (for-RGB displays) of about one third of that and the gaps between pixels:5 0.02mm, to avoid light spilling onto undesired pixels (crosstalk) a highly collimated light source is required. However, some small light spillage can usually be accepted.

Another limitation is the angle over which the contrast attainable with any given electro-optic effect remains at an acceptable level. One of the most commonly used liquid-crystal electro-optic effects is the twisted nematic (TN) effect. This effect shows an asymmetric distribution of transmitted light with angle, which means that the display changes when viewed from other angles, as is well known. In the case of complex displays the TN cell, not being suitable for passive multiplexing, is most useful for devices where thin-film transistors provide the multiplexability and grey levels. This is also true for complex displays using an antiparallel-rubbed planar dye configuration.

According to the invention, there is provided a liquid-crystal display device comprising: a source of excitation light, a liquid-crystal modulator cell adapted to modulate excitation light input from the source on one side of the cell and output elements emitting display light when struck by the excitation light passing through the modulator, in which the light source emits excitation light over a range of angles and the liquid-crystal cell contains a liquid crystal of the guest-host (GH) type, the output elements emitting display light in proportion to the integral over the range of angles of the excitation light input to the cell in such a way as to afford a substantial contrast ratio, preferably at least 10:1.

The display can be a direct-drive display, in which modulation is carried out by application of a voltage across the cell at each modulated area. This would be applicable to a simple alphanumeric display such as that of a cash register, for example. More complex displays, comprising orthogonal arrays of transparent stripe-shaped electrodes addressed in turn with a suitable waveform, can also use the invention; however, currently available GH LCs cannot be multiplexed, so such displays are best made using active addressing, with a thin-film transistor (TFT) at each pixel.

The guest-host configuration is of course well known in itself and is one where the liquid-crystal material ("host") incorporates "guest" molecules of a dye whose absorption varies with direction, i.e. which is dichroic. These molecules follow the movements of the liquid-crystal molecules imposed by the electrod ' es of the cell. Guest-host liquid crystal cells do not transmit as well as TN cells in the ON state, because the dye molecules are not as uniformly aligned as the second polariser (analyser) of a TN cell. However, in the PLLCD configuration this can be compensated for by the increased amount of light that one can pass through the system because of the relaxation of the collimation requirement.

The guest-host system has been suggested before for application with phosphor emitters: see US 4830469 (Breddels, et al., US Philips Corp). However, here the liquid crystal is of the supertwist type - a range of 1800-3600 is mentioned. Supertwisted nematic (STN) cells tend to have a sharp voltage response, which makes them suitable for multiplexing, but the contrast between bright and dark states is not as good as, say, for TN; moreover STN has a very asymmetric angular response. The Breddels document does not contain any discussion of the angular response of the cell, nor of the collimation, or lack of it, of the UV light input, and it is to be doubted that the arrangements shown in it produce a usable integrated contrast.

Liquid-crystal cells following the present invention on the other hand use a liquid crystal whose electro-optic response is substantially uniform across the range of angles impinging from the source. TN with up to 90 twist may be suitable, provided that the light is waveguided though the thickness of the cell (whether or not waveguiding occurs depends on the thickness of the cell as well as the degree of twist).

Preferred embodiments of the invention use the planar aligned (non-twisted) ant iparal lel -rubbed configuration. Planar cells can exhibit improved switching speeds compared to twisted cells: they are not as easily multiplexed but in most embodiments of the invention this is of no concern since multiplexed cells are of less interest in any event.

The excitation light is preferably UV light in the range near the visible region, or perhaps short wavelength blue light, so that visible-emitting RGB phosphors, acting as the output elements, can be excited by this light without too great a quantum efficiency loss.

Thus the invention envisages a PLLCD for which relatively poorly collimated light is used (> about 10-150 collimation angle), but an improved integrated contrast direct-drive display or complex active-matrix display is obtained by using a dye cell, in particular in a planar antiparallel-rubbed configuration, as compared to a conventional TN display. This configuration has the added advantage that it needs only a single polariser and thus can be used with an internal phosphor without the need for a polariser to be placed inside the cell (which would otherwise be the case in a two-polariser configuration) Reference may again be made to US 4830469, discussed above, on this point.

For a better understanding of the invention embodiments of it will now be described, by way of example, with reference to the accompanying drawings, in which:

Figure la is an intensity contour plot for a normally white TN LC; Figure Ib is an intensity contour plot f or a Heilmeier GH LCD; Figure 2 shows integrated contrast vs.

collimation angle using data from simulation for TN and Heilmeier-type GH LCDs; Figure 3 shows integrated contrast vs.

collimation angle as in Fig. 2 but using experimental data; Figure 4 shows an embodiment of the invention; and Figures 5a and 5b show total transmission curves.

As shown in Fig. la, the contrast between "bright" and "dark" states (ratio, of "bright" intensity to "dark" intensity) is quite high at normal incidence and at small angles from normal incidence but falls off rapidly as the angle increases, above, say, 200. The circumferential dashed lines are at intervals of 20'.

In a simple direct-drive configuration TN cells can show good contrast for well collimated light; however, TN displays are of limited value in complex, many-row, passive-matrix displays. This is because the voltages for "on" and "off" states are too dissimilar to allow a high level of multiplexing using normal strobe-type multiplexing techniques in which the RMS voltage on each "on" pixel is equal to the sum of a strobe RMS voltage and a data RMS voltage plus RMS data voltages at all other times. The "off" pixels have an RMS voltage equal to the difference between the strobe RMS voltage and the data RMS voltage plus RMS data voltages at all other times. The result of this is that only a modest number of rows can be addressed in this way and give contrast better than 10:1 (depending on the liquid crystal used; it is usually less than 10) The electro-optic effect is, however, well suited to use in active-matrix displays as this difference between "on" and "off" voltages allows switching to a number of grey shades.

Because the TN displays have only a small high contrast viewing cone they tend to show poor contrast with poorly collimated light in PLLCD displays, because such displays integrate the contrast values over the angles at which the excitation light impinges on the cell. Embodiments of the invention therefore use a planar ant iparal lel -rubbed (top and bottom alignment layers rubbed opposite ways) nematic liquid crystal containing a dichroic dye. This configuration, known as a guest-host liquid crystal, is expected to be less sensitive than the TN case to angular variation away from normal incidence; the contour plots for the dark and bright states are shown in Fig. 1b. It will be evident that, although the total transmission in the bright (ON) state is lower, the dark state is more or less uniformly dark up to angles of 40-50'.

Integrating over, say, 0-300 for the input light gives a greater contrast ratio than for the TN cell of Fig. la.

Fig. 2 shows a simulated comparison of these two effects for a 5pm second-minimum TN cell (dots and dashed line) and a 10pm Heilmeier guest-host dye cell (triangles and solid line). Fig. 3 shows experimental data for two single-pixel cells, one 5ym TN cell (second minimum) and one 20Am dye cell. As usual for the TN cell two polarisers were used; the polarisers were, however, designed for UV light and were less transparent than normal visible-light polarisers. The dye cell used a single polariser with its transmission axis parallel to the maximum- absorption direction of the liquid crystal (Heilmeier et al., Mol. Cryst. Liq.

Cryst. 8, 293-304, 1969). The dye used was G-207 (an azo based dye) from Nippon Kankoh-Shikiso Kenkyusho and the LC was ZLI 1132, a commercial nematic mixture from Merck, Darmstadt.

The intensities are the integrated intensity (summing over all the angles) for light collimated within the range shown, i.e. impinging on the rear face of the cell at angles deviating by no more than the stated angle from the normal to that face. The contrast decreases in both cases, of course, as the collimation worsens. However, that for the TN cell decreases faster and after about 10' the integrated contrast for the TN cell drops below that for the Heilmeier cell. This makes the latter useful because collimation of about 200 from a diffuse source (discharge tubes and TIR plane, say) is achievable with quite high efficiency, whereas 50 or even 10' is much more difficult.

Moreover, in the case of the dye cell, because only a single polariser is used the effect is suitable for use in displays with phosphors contained inside the liquid-crystal cell, since then a simple construction can be used with phosphors in the cell any second polariser (analyser) would have to be internal, giving rise to additional problems of manufacture and operation. Also the geometric problems associated with imperfectly collimated input light are avoided by the use of internal phosphors.

The overall design is shown in Fig. 4, which illustrates a Heilmeier GH-PLLCD. For simplicity only a passive-matrix design is shown, but the principle is similar for an active-matrix (TFT) construction. The liquid-crystal cell contains a nematic liquid-crystal material 1 incorporating a dye between two glass plates 3, 5. On the plates transparent ITO electrodes 7, 9 are laid down in orthogonal strips in the usual way. Over these, though not shown, are also the antiparallel alignment layers. A layer of phosphor pixels 20 in RGB formation is located on the output side (right-hand side in the diagram) of the liquid crystal, inside the cell. This layer must of course be thin enough and flat enough not to disturb the electro optic properties of the liquid crystal, as discussed in WO 97/40416.

At the input (left-hand) side of the cell near-UV light is input as excitation light. Where the pixels of the cell, formed by the intersections of the electrodes, are switched on this light passes through the cell.

As mentioned above, a drawback of the dye cell is its lower transmission in the bright state. Using the cells described here the results shown in Figs. 5a and 5b were obtained for the TN and Heilmeier cells respectively. Fig. 5a shows the total transmission of 5Am TN LCD at normal incidence (bright state), while Fig. 5b shows the total transmission at normal incidence of a 20Am Heilmeier GH LCD (bright state) It can be seen that at the critical wavelength (390nm in preferred embodiments) the TN gives about 23% and the Heilmeier about 9% transmission in the bright state. However, if partial collimation only to about 30' is required then the amount of light available can be several times that when, say, < lo'-collimation is needed, as in the case of the TN cell.

In summary, it has been found that in the case of a PLLCD for which only moderately collimated light is used (>10' collimation angle) an improved integrated contrast direct-drive display or complex active-matrix display is obtained by using a dye in a planar antiparallel rubbed configuration over a conventional TN or STN display. This configuration has the added advantage that it can be used with a single polariser and thus with an internal phosphor without the need for an internal polariser.

Claims (8)

1. A liquid-crystal display device comprising: a source of excitation light, a liquid-crystal modulator cell adapted to modulate excitation light input from the source on one side of the cell and output elements emitting display light when struck by the excitation light passing through the modulator, in which the light source emits excitation light over a range of angles and the liquid-crystal cell contains a liquid crystal of the guest-host (GH) type, the output elements emitting display light in proportion to the integral over the range of angles of the excitation light input to the cell in such a way as to afford a substantial contrast ratio, preferably at least 10:1.

2. A display device according to claim 1, and being a direct-drive display, in which modulation is carried out by application of a voltage across the cell at each modulated area.

3. A display device according to claim 1, and being a multiplexed display using thin-film transistors.

4. A display device according to claims 1 or 2, in which the liquidcrystal cell is planar-aligned and antiparallel-rubbed.

S. A display device according to any of claims 1 to 3, in which the liquid-crystal cell is a TN cell with up to 900 twist.

6. A display device according to any preceding claim, in which the liquid-crystal cell is a dye cell.

7. A display device according to any preceding claim, in which the excitation light is near-visible UV or short wavelength blue light.

8. A display device according to any preceding claim, in which the range of input angles of the excitation light is between 00 and about 200.