GB2314423A

GB2314423A - Liquid crystal devices

Info

Publication number: GB2314423A
Application number: GB9613032A
Authority: GB
Inventors: Paul Bonnett; Henning Molsen; Michael John Towler; Martin David Tillin
Original assignee: Sharp Corp
Current assignee: Sharp Corp
Priority date: 1996-06-21
Filing date: 1996-06-21
Publication date: 1997-12-24
Also published as: GB9613032D0

Abstract

A liquid crystal device comprises a cholesteric liquid crystal material disposed between electrodes (1, 5) and suitable alignment layers. The liquid crystal material has stable planar and focal conic states and a dielectric anisotropy which is positive for low frequency applied fields and negative for high frequency applied fields. A field generator (2, 6) selectively applies a low frequency voltage to the electrodes for switching the liquid crystal material to the focal conic state and a high frequency field to the electrodes for switching the liquid crystal material to the planar state.

Description

LIQUID CRYSTAL DEVICE.

The present invention relates to a liquid crystal device. Such a device may, for instance, be used as a liquid crystal display panel having applications in direct view and projection displays.

W093/05436 discloses a monostable composite polymer-liquid crystal device which may use cholesteric or smectic liquid crystal materials. In the cholesteric case, a dual frequency addressable cholesteric liquid crystal is cured in the presence of an applied electric field whose frequency is less than a critical frequency at which the dielectric anisotropy changes sign. The liquid crystal adopts the homeotropic alignment which is transparent to light due to the matched refractive indices of the liquid crystal and the polymer. The homeotropic alignment remains stable after the applied field is removed. In order to switch to the homogeneous alignment, a field is applied with a frequency greater than the critical frequency. This results in a scattering coloured state. Upon removal of the applied field, the liquid crystal relaxes to the homeotropic alignment.

G.H. Heilmeier and J.E. Goldmacher, Appl. Phys. Lett. 13, 132(1968) discloses a bistable display using an ion containing cholesteric liquid crystal material. When a field of frequency below the critical frequency is applied, the display switches to an opaque backward scattering texture due to the dynamic scattering mode. This results from ionic movement distorting the alignment of the cholesteric liquid crystal material.

Switching the material to the transparent state is achieved by applying a field whose frequency is greater than the critical frequency, which field only interacts with the negative dielectric anisotropy of the cholesteric liquid crystal material. The cholesteric liquid crystal does not exhibit a change of sign of the dielectric anisotropy with respect to the frequency of the applied field.

G.A. Dir et al, Proceedings SID 13/2, 105 (1972) discloses a bistable selectively reflecting cholesteric liquid crystal display. The display is switchable between a reflective planar or Grandjean alignment or texture and a slightly scattering focal conic texture. Switching to the focal conic texture is achieved by applying a direct or low frequency alternating field, which induces an ion flow to cause the change of state. For fields of higher frequencies, the ions cannot follow the electric field so that the field interacts with the liquid crystal according to its negative dielectric anisotropy. However, switching times are relatively long and the working life of devices of this type is relatively short.

X.-Y. Huang, D.-K. Yang, P.J. Bos and J.W. Doane, SID 95 Digest of Technical Papers, 347 (1995) discloses a dynamic drive scheme for a bistable cholesteric liquid crystal display in which the switching speed of the liquid crystal is determined by relaxation from the field induced homeotropic alignment to the planar or focal conic alignment. Although the state of the liquid crystal material may be changed by application of a relatively short addressing signal, for instance of the order of a millisecond, typical relaxation times are of the order of 100 milliseconds or longer.

Various displays and addressing schemes are disclosed in: D.-K. Yang, J.L. West, L.-C. Chien and J.W. Doane, J. Appl. Phys. 76(2), 1331(1994); P.P. Crooker and D.-K. Yang, Appl. Phys. Lett. 56,2529(1990); and GB44941/73. However, none of these is concerned with dual frequency cholesteric liquid crystals of the type which exhibit a change of sign of the dielectric anisotropy with frequency.

S.E. Day, I.A. Shanks and F.C. Saunders, SID 88 Digest, 57(1988) discloses the use of dual frequency addressing in order to shorten the turnoff time of a super twisted nematic liquid crystal display. However, despite the use of this technique, the turn-off time is still relatively long and of the order of 270 milliseconds.

M.G. Clark, Microelectron. Reliab. 21,887(1981) discloses various techniques which are useful in addressing nonbistable nematic and twisted nematic liquid crystal displays. Very long pitch cholesteric liquid crystals are included.

According to the present invention, there is provided a liquid crystal device as defined in the appended Claim 1.

Preferred embodiments of the invention are defined in the other appended claims.

It is thus possible to provide a liquid crystal device which is switchable between the planar state, in which light in a narrow frequency band and of one handedness of circular polarisation is reflected, and the focal conic alignment, in which the liquid crystal is substantially transparent to light in the visible spectrum. Such a device is bistable and does not require an applied field to maintain either stable state. A switching speed to both states of the order of 20 milliseconds, for instance for each pixel of a pixellated display, can readily be achieved.

It is also possible to provide a reflective cholesteric liquid crystal display or projection display in which the switching time between the stable transmissive focal conic state and the coloured planar state is reduced.

This enhances the visual perception of the display because the pixel switching speed may be reduced below 20 milliseconds instead of being of the order of 100 milliseconds or greater.

An intermediate field induced homeotropic orientation of the liquid crystal is obsolete in dual frequency addressing.

Employing data composed of high and low frequency parts, preferably of equal pulse areas, overcomes the problem of a low threshold voltage and flat electro-optic characteristics in dual frequency addressed cholesteric liquid crystal panels. Hence, data voltages of more than twice the threshoid voltage of single frequency data addressing may be used.

Possible image retention can be accounted for by modifying the data voltage in relation to a previous frame via a frame memory and treating the data with an appropriate algorithm.

Such devices may be used to provide displays, for instance in the form of pixellated panels for direct viewing or for projection. Another possible application is as a switchable notch filter.

The invention will be further described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a schematic diagram of a passive matrix liquid crystal display (LCD) constituting an embodiment of the present invention; Figure 2 is a schematic cross sectional diagram of the display of Figure 1 illustrating the focal conic texture of the liquid crystal; Figure 3 is a schematic cross sectional diagram of the display of Figure 1 illustrating the planar orientation of the liquid crystal; Figures 4 and 5 are graphs of reflectivity in arbitrary units against applied field in volts per 4 micrometers of liquid crystal thickness; and Figure 6 to 8 are waveform diagrams of voltage in arbitrary units against time in arbitrary units illustrating schematically different addressing waveforms of the display of Figure 1.

Figure 1 shows a passive matrix pixellated liquid crystal display (LCD) comprising four columns and four rows of picture elements (pixels). In practice, an actual display would comprise many more pixels arranged as a square or rectangular matrix but the 4x4 array has been shown for simplicity of illustration.

The display comprises four column electrodes 1 connected to respective outputs of a data signal generator 2 so as to receive data signals Vdl to Vd4. The generator 2 has a data input 3 for receiving data to be displayed, for instance one row at a time. The generator 2 has a clock input for receiving clock signals so as to control the timing of the supply of the data signals to the column or data electrodes 1.

The display further comprises four row electrodes 5 connected to respective output of a strobe signal generator 6 so as to receive respective strobe signals Vsl to Vs4. The generator 6 has a clock input which is also connected to receive the clock signals for controlling the timing of supply of the strobe signals to the row or strobe electrodes 5.

A layer of liquid crystal is disposed between the data electrodes 1 and the strobe electrodes 5. The liquid crystal is a dual frequency addressable cholesteric liquid crystal material or mixture whose dielectric anisotropy is positive for applied electric fields of relatively low frequency and negative for applied electric fields of relatively high frequency. The frequency at which the dielectric anisotropy changes sign between positive and negative is referred to as the critical frequency.

Dichroic dyes may be added to the liquid crystal if desired. The intersections between the data and strobe electrodes define individual pixels which are addressable independently of each other.

In use, data signals for refreshing a row of pixels are supplied simultaneously to the column electrodes 1 by the data signal generator.

The strobe signal generator 6 supplies a strobe signal to the strobe electrode 5 of the pixel row to be refreshed. No signals are applied to the other strobe electrodes 5. The simultaneous presence of the strobe signal and the data signals causes an electric field to be applied across the liquid crystal layer of each pixel of the row so that display data are written into the row being refreshed. At the end of the strobe signal, the pixels of the refreshed row remain in the state determined by the data signals until the row is refreshed during the next refresh cycle. Fresh data signals for the next row to be refreshed are then applied by the data signal generator 2 to the column electrodes 1 and the strobe signal generator 6 supplies a strobe signal to the strobe electrode 5 of the next row. The row-by-row refresh cycle is continued until the whole display has been refreshed to complete a frame refresh cycle, which is then repeated.

The column electrodes 1 and the strobe electrodes 5 together with the data signal generator 2 and the strobe signal generator 6 thus constitute an electric field generator for applying electric fields to the liquid crystal material disposed between the electrodes 1 and 5.

As shown in Figures 2 and 3, the LCD comprises a transparent substrate 10 which carries the data electrodes 1 and a transparent substrate 11 which carries the strobe electrodes 5. The electrode directed to the observer is transparent and may be made of indium tin oxide (ITO). The electrodes 1 and 5 are coated with alignment layers 12 and 13, for instance in the form of a silicon oxide coating. The region between the alignment layers 12 and 13 contains the liquid crystal material 14.

When viewed from the observer, at least one of the elements behind the liquid crystal material 14 is at least partially absorbent to visible light.

In one specific example of an LCD, the liquid crystal material 14 has a thickness of 4 micrometers and comprises a mixture of a dual frequency addressable nematic liquid crystal mixture 3333 (available from Hoffmann-LaRoche) and a chiral dopant DL36 amounting to 3.8% by weight of the mixture and disclosed in G. Heppke et al, Z.

Naturforsch.41 ,1 214(1986). The dual frequency addressable nematic liquid crystal mixture 3333 has a dielectric constant AE of +3.9 at 0.5 kHz, -4.5 at 10 kHz, a critical frequency of 4 kHz at 200C, and a An of 0.1. Suitable addressing frequencies for the material are 330 Hz and 10 kHz at 20"C.

Each pixel of the LCD is individually switchable between stable states illustrated in Figures 2 and 3. Figure 2 illustrates the focal conic texture which occurs after the application of a relatively low frequency pulse between the electrodes 1 and 5 of the pixel. The helical molecular structures, for instance illustrated at 15, of the cholesteric liquid crystal formed by the mixture described hereinbefore are randomly oriented to provide the focal conic texture, in which state the pixel is essentially optically transmissive. The focal conic texture or alignment remains stable after the application of the low frequency pulse so that it is not necessary to apply an electric field between the electrodes 1 and 5 to maintain this state.

Figure 3 illustrates the substantially uniform orientation of the helical structure in the planar orientation or alignment, which is achieved by applying a high frequency pulse between the electrodes 1 and 5. The cholesteric material is aligned with the twist axes substantially perpendicular to the alignment layers 12 and 13 so that the pixel becomes reflective to light in a narrow frequency band of one handedness of circular polarisation and transmissive to the remainder of the visible spectrum. The planar orientation remains stable after the application of the high frequency pulse so that no applied field is necessary to maintain this state. A pixellated display panel of this type may therefore be addressed using the passive matrix arrangement illustrated in Figure 1. The mixture described hereinbefore is suitable for reflecting green light in the planar orientation but different amounts of chiral dopant may be used for tuning the band of reflectivity to other colours.

Figure 4 illustrates the response of a pixel of the type described hereinbefore to the amplitude of a square wave voltage applied between the electrodes 1 and 5 at a frequency of 10 kHz. Starting from the focal conic texture, the liquid crystal material 14 is switched towards the reflective planar orientation with a dependence on the voltage as illustrated.

Figure 5 illustrates the reverse switching operation starting from the planar texture and resulting from applying a low frequency square wave of 330 Hz. For this specific example, square waves having amplitudes between 20 and 25 volts are required to ensure substantially complete switching between the planar and focal conic states.

The liquid crystal material 14 may be of other types. For instance, the material may comprise a helical polymer-nematic liquid crystal composite. Another suitable material comprises a polymer dispersed cholesteric liquid crystal. In this case, parallel alignment of the applied electric field and the helix of the cholesteric liquid crystal material in the droplets may be obtained by addressing with a high frequency electric field during polymerisation. The mean refractive index of the cholesteric liquid crystal and the surrounding polymer are adjusted to match each other after phase separation. Thus, a stable selectively reflecting hazefree state is stabilised and the cholesteric liquid crystal can be switched into a colourless scattering state upon application of a low frequency electric field.

Figure 6 schematically illustrates addressing signals in the form of strobe signals 21 and switching and non-switching data signals 22. The signal amplitudes, the number of pulses and the pulse periods are not shown to scale for the sake of clarity. Each row addressing cycle comprises a strobe period 23 and a select period 24. The resultant signals corresponding to the electric field across the liquid crystal material are shown at 25. During the strobe period 23, the data signal 22 is at zero volts whereas a high frequency signal, for instance with an amplitude of 40 volts at a frequency of 10 kHz, is applied to the respective strobe electrode. Accordingly, during the strobe period 23, all of the pixels of the row are reset to the planar orientation. During the stroke period for the selected row, voltages may be applied to pixels in order to address the previously blanked row. However, these voltages do not affect blanking.

During the select period 24, a low frequency strobe signal, for instance having an amplitude of 16 volts and a frequency of 330 Hz, is supplied to the row electrode. A respective data signal having the same frequency of 330 Hz and an amplitude of, for instance, 8 volts is supplied to each of the data electrodes 1. The signal supplied to each data electrode is either in phase with the strobe signal or is inverted with respect thereto so that the pixel of that row is either switched to the focal conic texture or is unswitched so as to remain in the planar orientation, respectively.

The addressing cycle illustrated in Figure 6 is then repeated for each row in turn to complete a frame refresh of the display, which frame refresh is then repeated so as to update the display continuously. Alternatively, resetting and selecting can be achieved by exchanging high and low frequencies, respectively. In general, applied voltages, frequencies and address time have to be optimised according to the material and the operating temperature.

The addressing scheme shown in Figure 7 differs from that shown in Figure 6 in that the strobe period 23 for each row is omitted so that there is no resetting of the row prior to writing of data to the row. For purposes of illustration, the select period 24 is shown having four consecutive sub-periods comprising a first burst of high frequency strobe pulses, a first burst of low frequency strobe pulses, a second burst of high frequency strobe pulses, and a second burst of low frequency strobe pulses. The data signal comprises corresponding bursts of high and low frequency pulses with the high frequency pulses being in phase with the high frequency pulses of the strobe signal and the low frequency data pulses being out of phase with the low frequency pulses of the strobe signal so as to switch to the planar state. Alternatively, in order to switch to the focal conic state, the high frequency data pulses are out of phase with the high frequency strobe pulses and the low frequency data pulses are in phase with the low frequency strobe pulses.

The addressing scheme illustrated in Figure 7 provides increased contrast ratio because of the high data voltage allowed by interlacing the high and low frequency pulse bursts. This is due to the following effect. If the voltage of the high and low frequency components is the same, there is very little effective torque on the cholesteric liquid crystal if no strobe voltage is applied. This is typically the cross-talk problem of all passive matrix displays when the data voltage is applied to address one row at a time but is seen by all of the other rows.

The addressing scheme shown in Figure 8 differs from that shown in Figure 7 in that a strobe period 23 is provided before the select period 24. For purposes of illustration, the strobe period 23 is shown having four sub-periods comprising a first burst of high frequency strobe pulses, a first burst of low frequency strobe pulses, a second burst of high frequency strobe pulses, and a second burst of low frequency strobe pulses. The effect of the interlaced high and low frequency strobe pulse bursts is to ensure that previous pixel information is effectively deleted so as to reduce the dependence of the optical state of the row on the previous optical state.

Although the addressing schemes illustrated in Figures 6 to 8 use rectangular waves, other waveforms such as sine waves may be used.

Further, more than two frequencies may be applied as appropriate.

The LCD may be provided with temperature sensing so as to adjust the high and low frequencies in order to compensate for the exponential dependency of the cross-over frequency with temperature.

Grey scale may be provided by a multi-threshold modulation technique, for instance of the type used with ferroelectric liquid crystal displays and disclosed in JP 61-121087 and JP01-179127.

The tendency of each pixel to be effected by its display state in the preceding frame may be reduced or overcome by correlating the data voltages to the previous pixel states and modifying the voltages accordingly. This may be particularly useful in the addressing scheme shown in Figure 7, in which there is no resetting prior to the writing of new data. For instance, in the case where a pixel is to be switched from one state to the other, the amplitude of the data pulses may be increased as compared with the case where the pixel is to remain in the same state. Thus, any tendency for the pixel not to change fully to the new state can be reduced or eliminated.

Displays of the type disclosed herein may be used for direct view and projection displays. For instance, such displays may be used in applications including electronic newspapers and personal digital assistant.

Claims

1. A liquid crystal device comprising: a cholesteric liquid crystal material having first and second states, both of which states are stable in the absence of an applied electric field, and having a dielectric anisotropy which is positive in the presence of an applied electric field of a first frequency and negative in the presence of an applied electric field of a second frequency greater than the first frequency; and at least one electric field generator for applying to the liquid crystal material an electric field of the first frequency for switching the liquid crystal material to the first state and an electric field of the second frequency for switching the liquid crystal material to the second state.

2. A device as claimed in Claim 1, in which the at least one electric field generator comprises first and second electrodes with the liquid crystal material disposed therebetween and an alternating voltage source connectable across the first and second electrodes.

3. A device as claimed in Claim 1 or 2, in which the first state is the focal conic state and the second state is the planar state.

4. A device as claimed in Claim 1 or 2, in which the liquid crystal material is selectively reflective to visible light in one of the first and second states.

5. A device as claimed in any one of the preceding claims, in which the liquid crystal material comprises a nematic liquid crystal and a chiral dopant.

6. A device as claimed in Claim 5, in which the liquid crystal material further comprises a polymer.

7. A device as claimed in any one of Claims 1 to 4, in which the liquid crystal material comprises a nematic liquid crystal and a helical polymer.

8. A device as claimed in any one of the preceding claims, including at least one alignment layer in contact with the liquid crystal material.

9. A device as claimed in Claim 8, in which the at least one alignment layer comprises a silicon oxide.

10. A device as claimed in any one of the preceding claims of passive matrix type comprising a first set of elongate substantially parallel electrodes and a second set of elongate substantially parallel electrodes substantially perpendicular to the electrodes of the first set.

11. A device as claimed in Claim 10, comprising a data signal generator connected to the first set of electrodes and a scanning signal generator connected to the second set of electrodes.

12. A device as claimed in Claim 11, in which the scanning signal generator is arranged to supply, to each of the electrodes of the second set in turn, a scanning signal comprising a blanking signal of one of the first and second frequencies followed by a select signal of the other of the first and second frequencies.

13. A device as claimed in Claim 12, in which the data signal generator is arranged to supply, to each of the first electrodes, a respective data signal which is simultaneous with the select signal, of the other of the first and second frequencies, and selectively in phase with or of the opposite phase with respect to the select signal.

14. A liquid crystal device substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.