GB2217089A

GB2217089A - Display device

Info

Publication number: GB2217089A
Application number: GB8905095A
Authority: GB
Inventors: Sally Elizabeth Day; Ian Alexander Shanks
Original assignee: Thorn EMI PLC
Current assignee: EMI Group Ltd
Priority date: 1988-03-28
Filing date: 1989-03-06
Publication date: 1989-10-18
Anticipated expiration: 2009-03-06
Also published as: GB2217089B; GB8807333D0; GB8905095D0; JPH0210313A

Abstract

In a display device including an LC cell 2 of the 'super-twist' type, grey scale variations are achieved by driving the cell at two frequencies. The cell or all the pixels in a matrix are initially set in the transmissive state by application of drive voltages from source 7 at a frequency below that corresponding to zero dielectric anisotropy. Then a selected voltage from a range of drive voltages at a higher frequency above zero anisotropy is applied on line 8 to the pixels to set them in the desired transmissive states. The LC cell may be associated with a pair of polarisers. Alphanumeric or television type pictures may be obtained in monochrome or colour. <IMAGE>

Description

DISPLAY DEVICE The present invention relates to a display device incorporating super-twisted liquid crystal material.

Super-twisted liquid crystal materials are produced by adding a suitable amount of chiral dopant to conventional twisted nematic materials; in this way, a liquid crystal cell with a twist of typically 1800 to 2700 can readily be achieved. A display panel formed by a matrix of such cells has an increased multiplexing capability, as well as improved contrast and viewing angle characteristics as compared to a standard twisted-nematic display. Additionally, a super-twisted liquid crystal display can have reduced temperature sensitivity, as distinct from twisted-nematic displays wherein frequent re-adjustment of drive voltages are often necessary in order to compensate for temperature fluctuations.

Various types of super-twist display are known, e.g.

super-twisted nematic or STN displays (see Proc. SID Vol 25/4 p261-4, 1984), super-twisted birefringence effect or SBE displays (see Appl Phys Lett 45 ply21, 1984), and optical mode of interference or OMI displays (see Appl Phys Lett 50 p236, 1987).

The possibility of providing enhanced multiplexibility in such a super-twisted nematic cell can be seen by considering the optical transmissivity against voltage characteristic, this having a much steeper gradient as compared to that of a standard twisted nematic cell. However, this steep gradient is associated with the fact that such a cell has a bi-stable nature, i.e. it has only two states (essentially transmissive and non-transmissive) in which feasibly it can be driven, thereby preventing the possibility of realising grey scales by selection of voltage driving levels.

The present invention provides a display device comprising at least one liquid crystal cell with liquid crystal material in a super-twisted form, means selectively to switch the at least one cell between three or more states of optical transmissivity, the switching means including means to provide driving of the cell at at least two frequencies.

Thus, according to the invention, the Applicant has discovered that by appropriate two-frequency driving, it is possible to realise grey scale operation of a super-twisted liquid crystal cell with due selection of the voltage levels at which driving occurs to control the slope and degree of bi-stability obtained in the electro-optic (i.e. transmission against voltage) characteristics.

It is not evident from the behaviour of super-twisted materials known in the prior art that grey scale generation by such driving is feasible nor indeed that it can be achieved by the technique set forth in the present invention, namely two-frequency driving.

Preferably, the switching means includes means to apply a signal at a first frequency and at a voltage such as to put one said cell in a specified transmissive state. Accordingly, the Applicant has determined that an important feature of the technique embodied in the present invention is the setting of the cell(s) firstly to a given state of transmissivity; advantageously, when the device incorporates a number of cells, then the setting is achieved simultaneously in one or more groups of cells. In a preferred embodiment, the signal at the first frequency is at a voltage in the range of typically from 1 to 10 volts, advantageously from 4.5 volts to 7 volts, to effect switching of one such cell to a specified transmission state.

Preferably, the switching means includes means to apply a signal at a second frequency and at any one of two or more voltages selected in a range of voltages from that producing a substantially non-transmissive state of such a cell to that producing a substantially completely transmissive state of such a cell. In this way, there can be achieved a state of transmissivity in the cell which is in dependence on that selected voltage of the signal in the second frequency. Thus an image with numerous grey levels can be achieved by selectively driving the cells of the display device at appropriate voltages of the second frequency.

Preferably, the first frequency is lower than the second frequency. In this way, any deleterious harmonics effects can be substantially reduced or eliminated, as the switching of the drive voltage levels to the cells is done at the higher frequency and hence the data-dependent upper harmonics of the drive waveform merely have the same effect as the fundamental of the high frequency waveforms.

Preferably the liquid crystal material comprises a long-pitch chiro-nematic liquid crystal material in a super-twisted form.

Preferably, the display device comprises means to polarise the light entering and exiting the cell. Alternatively, the display device comprises di-chroic dye in liquid crystal material to effect an appropriate change of transmission within the cell as required.

Preferably, any polarisers are oriented at approximately 300 and 600 to the director at the surfaces and the liquid crystal layer thickness is chosen such that 8 nd Cos2 fez 0.8 where t n is the bi-refringence of the liquid crystal, d is the layer thickness in pm and 6? is the average mid-layer director tilt angle in the super-twisted state.

The present invention also provides a method of operating a display device having at least one liquid crystal cell with liquid crystal material in a super-twisted form, the method comprising selectively switching the at least one cell between three or more states of optical transmissivity, the switching step including providing driving of the cell at at least two frequencies.

Preferably, the switching step includes applying a signal at a first frequency and at a voltage such as to put one said cell in a specified transmissive state.

Advantageously, the method comprises setting simultaneously a group of cells using a signal at the first frequency. The signal at the first frequency is preferably at a voltage in the range typically of from 1 to 10 volts, advantageously of from 4.5 volts to 7 volts to effect switching of one such cell to a transmission state.

Preferably, the switching includes applying a signal at a second frequency and at any one of two or more voltages selected in a range of voltages from that producing a substantially non-transmissive state of such a cell to that producing a substantially completely transmissive state of such a cell.

Preferably, a pixel comprises display elements of one or more liquid crystal cells.

The present invention is applicable to colour displays and to monochrome displays. Also the present invention is applicable to a wide variety of displays, including simple images (for example a line of letters/words or of numbers) and complex images (for example sophisticated pictograms and television-standard pictures).

The present invention also embodies equipment for the generation, and/or storage, and/or transmission, and/or reception, and/or processing, of signals suited and/or designed for a display device as herein defined.

In order that the invention may more readily be understood, a description is now given, by way of example only, reference being made to the accompanying drawings, in which: Figure 1 is a schematic diagram of part of a liquid crystal display unit embodying the present invention; Figure 2 shows the grey levels of the display unit of Figure 1; Figure 3 is a graph showing the measured transmission-against voltage characteristics for various low frequencies of a liquid crystal cell in the unit of Figure 1; Figure 4 is a graph showing typical transmission-againstvoltage characteristics of a liquid crystal cell embodying the present invention; and Figure 5 shows an alternative grey level arrangement derived from the characteristics curves of Figure 4.

Figure 1 shows schematically a liquid crystal display unit 1 comprising a matrix 2 of liquid crystal cells 3 selectively addressable with appropriate driving by row driver circuits 4 and column driver circuits 5 under the control of multiplexing control unit 6 in accordance with a data signal input to the unit 1 and clock waveforms (not shown).

Matrix 2 is made of a layer of super-twisted liquid crystal between two glass or plastic slides onto each of which an electrode structure consisting of row electrodes 9 or column electrodes 10 are formed to allow an electric field to be selectively applied across the liquid crystal layer of the cells. An alignment layer is applied over the electrodes on the slide. Normally a layer inducing high tilt in the director of the liquid crystal close to the surface is used. A typical type of high tilt alignment layer is achieved by evaporating SiO at an oblique angle (5 ) to the surface of the glass. The cell thickness is typically 6-7)cm (limits are between 2 to 50 pin).

The liquid crystal material has a dielectric anisotropy tE which changes sign (at a frequency f0 of the order of 100Hz to the order of lo kHz), being positive at low frequencies and negative at high frequencies, and is typically Tx2A, available from BDH Ltd.

To provide the liquid crystal material in super-twist form, cholesteric dopant, eg CB15 (available from (BODE Ltd.), is added to the liquid crystal to define the pitch of the liquid crystal.

The ratio of thickness of the cell to the pitch (d/P) should be in the range 0.5 to 1.0. The total twist through the cell is 2700 and the d/P ratio is 0.7, achieved by adding 1.2% CB15 to TX2A.

A super-twist cell can have a total twist in the range 1800 to 360 .

In the first step of setting all the cells 3 of matrix 2, the multiplexing control unit 6 uses a low frequency source 7 to apply a signal of 5.5V at a low frequency of 500 Hz simultaneously to all the cells in order to switch the cell ON. In this state the mid-layer director is aligned approximately parallel to the normal to the cell. In variants, the value of the low frequency can be any appropriate value below frequency f at which the dielectric 0 anisotropy change (being 5.1 kHz at 200C for TX2A) and above the frequency at which the cell starts to respond to the r.m.s. value of the applied voltage, and the low frequency voltage may have any reasonable value above the voltage required to switch the cell ON, limited by dielectric breakdown of the liquid crystal.

In the next concurrent setting step, the multiplexing control unit 6 uses a high frequency supply line 8 to apply to matrix 2 a high frequency (i.e. above fO) signal of 20key to align the liquid crystal director approximately perpendicular to the cell normal, thereby switching the pixels OFF, after which the cells may be sequentially or concurrently switched to one of a number of different transmissive states, by reducing the high frequency voltage difference developed at each pixel, the value of the eventual pixel voltage value determining the degree of transmissability of each pixel. Thus, as indicated in Figure 2, application of voltage V4 would make a pixel fully transmissive, V2 half transmissive, and VO fully opaque.

Figure 3 shows the transmission against voltage characteristics at the high frequency (being 20kHz) for various different values of voltage at the low frequency (500Hz), the measurements being made on a liquid crystal cell as in Figure 1.

As can be seen, it is only for voltages (at the low frequency) in excess of about 6V that the hysteresis/bi-stability is removed and the curve enables grey level driving to be used. The grey level scheme of Figure 2 is representative of the above-threshold upper section of the curve representing the low frequency voltage of 6V shown in Figure 3.

The values of high frequency at which the cell switches are determined by the physical properties of the liquid crystal as well as the voltage applied at the low frequency, dependence on the low frequency voltage being shown by Figure 4 which indicates unexpected characteristics, namely that, as the curve moves to higher voltages the threshold for switching increases, and the re-entrant behaviour is reduced, being substantially zero for a low frequency voltage of 6 volts.

Above a low frequency voltage of about 6 volts, the slope of the curve decreases in a controllable way as the low frequency voltage is increased. This contrasts with published work on twisted nematic cells where the slope of the characteristic curve increases under similar conditions and is both unexpected and useful in obtaining control of grey scale. For a fully effective grey level scheme, the liquid crystal properties and the operating parameters (including values of low and high frequencies, and low frequency voltage) are chosen substantially to eliminate any re-entrant behaviour and associated hysteresis.

The information in Figure 4 is based on a liquid crystal cell with the data essentially that of the unit Figure 1 except that d/P ratio is 0.55, the low frequency is 500 Hz and the high frequency is 20 kHz.

Figure 5 shows an alternative grey level scheme wherein there is a binary relationship between the transmission states which may be more suited to particular matrix multiplexing techniques.

Logarithmic greyscales may also be obtained. Figure 5 is based on a representation of the above-threshold section of the curve corresponding to the low frequency voltage of 7 volts in Figure 3.

By driving the matrix at the high frequency, any harmful effects of harmonics of the driving frequency are eliminated.

Clearly, if some other technique for avoiding harmonics problems is utilised, then a different driving arrangement can be used.

The value of f for the super-twist liquid crystal material 0 is temperature dependent (as f varies experimentally with 0 absolute temperature), and so the operating range of a display unit embodying the present may be limited to between O and 600 for practical choices of low and high frequencies. This may be acceptable for consumer applications, but for applications which require a wider operating range some form of heater/cooler may be required to stabilize the display temperature. Other problems are concerned with the supply of voltage at the high frequency. A liquid crystal cell is essentially a parallel plate capacitor, and so larger currents and power than normal will have to be supplied by the drivers at the high frequency.In addition the row and column electrodes (or their equivalents) have a finite resistance/unit length so the high frequency current flowing along them cause a voltage drop along their lengths which may make it impossible adequately to control all of the pixels uniformly.

This may necessitate metallising the electrodes to reduce their resistance/unit length and thus reduce the voltage gradients.

Alternatively the low frequency voltage level may be made to vary across the display to compensate for these different levels of high frequency voltage in different areas.

Preferably, the display includes red/green/blue coloured pixels. This may cause some problems with an STN display because of the inherent colouration of the liquid crystal material in certain states; thus for example an STN material may be operated in a blue/white mode or a black/yellow mode. However a multigap construction may be used giving a different thickness for each colour of pixel, so that the correct portion of the spectrum is transmitted for the particular colour of the filter used on each pixel. Such a display can be used in transmission illuminated from behind using a fluorescent lamp or tube whose emission spectrum is matched to the spectrum of the colour filters.Thus, the present invention allows the implementation of a full colour video grey-scale liquid crystal display without the need to resort to active layers of TFTs or MIM diodes with their problematic costs and yields, or to the very thin cells (1 to 2un) required for ferroelectric LCDs. A display embodying the present invention is in contrast to the ferroelectric displays, based on well understood and verified theories which can be accurately modelled using a computer and on conventipnal (or nearly conventional in the case of multigap colour pixellated liquid crystal displays) cell fabrication methods (about 6 thickness), which should allow them to be cheaply mass manufactured.

The major advantages of using two-frequency super-twist cells embodying the present invention are as follows: the response time of the switching operation is shortened, the cell being driven into an OFF state as well as the ON state thereby ensuring that the cell can be readily rapidly updated; and the undesirable re-entrant behaviour in the transmission/voltage characteristic of some liquid crystals can be removed or reduced by the use of two-frequency addressing, leading to curves for which control of the grey-scale is possible.

Another advantageous aspect is the suppression of the formation of scattering textures. The high frequency field tends to align the director perpendicular to the cell normal which tends to stabilise the liquid crystal director and prevent the scattering textures, formed by a periodic variation of the director, with parallel alignment in some regions. If the scattering textures are suppressed then the high tilt alignment may not be necessary in the construction of the super-twist cell, allowing cheaper low-tilt alignment methods such as rubbed polyimide layers, already used in the mass production of liquid crystal displays to be employed.

Claims

1. A display device comprising at least one liquid crystal cell with liquid crystal material in a super-twisted form, means selectively to switch the at least one cell between three or more states of optical transmissivity, the switching means including means to provide driving of the cell at at least two frequencies.

2. A display device according to Claim 1 wherein the switching means includes means to apply a signal at a first frequency and at a voltage such as to put one said cell in a specified transmissive state.

3. A display device according to Claim 2 incorporating a number of cells wherein said means to apply a signal at a first frequency comprises means to apply the signal at the first frequency simultaneously to one or more groups of cells.

4. A display device according to Claims 2 or 3 wherein said voltage is in the range of from 1 to 10 volts.

5. A display device according to Claim 4 wherein said voltage is in the range of from 4.5 to 7 volts.

6. A display device according to any one of Claims 2 to 5 wherein the switching means includes means to apply a signal at a second frequency and at any one of two or more voltages selected in a range of voltages from that producing a substantially non-transmissive state of such a cell to that producing a substantially completely transmissive state of such a cell.

7. A device according to Claim 6 wherein the first frequency is lower than the second frequency.

8. A display device according to any one of the preceding claims wherein the liquid crystal material comprises a long-pitch chiro-nematic liquid crystal material in a super-twisted form.

9. A display device according to any one of the preceding claims further comprise means to polarise the light entering and exiting -the cell.

10. A display device according to Claim 9 wherein the polarising means are oriented at approximately 300 and 600 to the director at the surfaces and the liquid crystal layer thickness is chosen such that #nd Cos < # > # 0.8 m where & n is the bi-refringence of the liquid crystal, d is the layer thickness in m m and < 6 > is the average mid-layer director tilt angle in the super-twisted state.

11. A display device according to any one of Claims 1 to 8 wherein di-chroic dye is included in the liquid crystal material to effect an appropriate change of transmissive state within the cell as required.

12. A method of operating a display device having at least one liquid crystal cell with liquid crystal material in a super-twisted form, the method comprising selectively switching the at least one cell between three or more states of optical transmissivity, the switching step including providing driving of the cell at at least two frequencies.

13. A method according to Claim 12 wherein the switching step includes applying a signal at a first frequency and at a voltage such as to put one said cell in a specified transmissive state.

14. A method according to Claim 13 wherein the signal at the first frequency is applied simultaneously to a group of cells.

15. A method according to any one of Claims 12 to 14 wherein said voltage is in the range of from 1 to 10 volts.

A A method according to Claim 15 wherein said voltage is in the range of from 4.5 to 7 volts.

17. A method according to any one of Claims 12 to 16 comprising the step of applying a signal at a second frequency and at any one of two or more voltages selected in a range of voltages from that producing a substantially non-transmissive state of such a cell to that producing a substantially completely transmissive state of such a cell.

18. A method according to Claim 17 wherein the first frequency is lower than the second frequency.