FR2728358A1 - ACTIVE MATRIX LIQUID CRYSTAL DISPLAY - Google Patents

Abstract

A display including a cell with a twisted nematic liquid crystal thin film (10) sandwiched between two facing transparent plates (12, 14) supporting electric field generating electrodes (18, 20) connected to a control circuit, as well as two crossed polarisers (26, 28) flanking the cell. The thickness of the liquid crystal thin film is such that it causes a twist of at least 90 DEG in the absence of an electric field. On of the polarisers is substantially at an angle of (2 phi +/- pi )/4 to the orientation of the crystals when they are inactive, and the thickness of the thin film is optimised as far as transmission in the inactive state is concerned. The display further includes at least one positively birefringent delay foil (34, 36) having an optical axis parallel to the plates and being positioned between the liquid crystal thin film and the respective polariser.

Description

ACTIVE MATRIX LIQUID CRYSTAL DISPLAY
The present invention relates to liquid crystal displays and active matrix. Such displays conventionally comprise a cell having a thin layer of helical nematic liquid crystals trapped between two transparent plates having two facing faces carrying electrodes for creating an electric field connected to a control circuit and comprising two polarizers framing the cell.

 The facing faces in contact with the thin layer are treated, in general by depositing a coating and brushing, so as to impose an orientation on the molecules of the liquid crystals along these faces. At present, the thickness of the layer and the treatment are generally provided so that the twist through the cell or twist 'is 900 and that the corresponding spatial delay b, defined as the product of the thickness d of the layer by the birefringence an of the liquid crystal, that is to say about 0.4 μm. This choice is consistent with the model that the orientation of the polarization vector is guided by the helical arrangement of the liquid crystals.

 A display thus formed and with two crossed polarizers, one of which is perpendicular or parallel to the direction of the molecules along the nearest orientation wall, is qualified as "normally white", in the sense that it is transparent when no electric field is applied to the liquid crystal layer, on the contrary the display becomes opaque when an electric field of sufficient value is applied to the layer.

 Such a display has a contrast (ratio between transparency in the idle state, that is to say in the absence of an electric field, and transparency when an electric field is applied) satisfactory when the light reaching the eye of the observer crosses the display substantially perpendicular to the layer. On the other hand, it presents, in oblique incidence, light leaks which reduce the contrast and distort the colors. It has already been proposed to reduce them by using negative birefringence retarding sheets whose optical axis is perpendicular to the liquid crystal cell. The delays introduced by such sheets must be optimized taking into account that the presence of these sheets has an unfavorable effect on transparency at rest, in oblique mink.

 The present invention aims to provide a display that meets the requirements of practice better than those previously known, in particular in that it makes it possible to obtain a contrast comparable to that of a conventional display and improves lateral vision while not putting in work that simple means.

To achieve this result, it was first necessary to realize that it is not necessary for the liquid crystal layer to have a twist of 900 in order for it to act as a half-wave retarder. In the ideal case of polarizers crossed to within plus or minus a few degrees and of a liquid crystal layer whose molecules have no inclination relative to the faces which delimit the layer, the transmission T of the cell at rest can be written
T = sin k + .sin (+ X / 2) / k + cos k + .cos (++ X / 2))
+ sink # .cos (# -2P1 + # / 2) .u (1 + u) (1) where
P1 is the angle made by the transmission axis of the input polarizer with respect to the orientation in the boundary layer of crystals located on its side,
- u = zdÔn / À. (2)
- A being the wavelength of the light, - k = (l + u2).

We note that, in this case, the transmission at rest is maximum if we choose P so that the argument of the term cos2 (± 2PI + X / 2) is equal to (0, +1, -1, +2, -2, .. ...) s i.e. for P1 = (2 ###) / 4 (3)
For an observation in light perpendicular to the cell, the choice of + x / 4 or -X / 4 in formula (3) is indifferent.

On the other hand, in oblique vision, and in particular as soon as one reaches or exceeds an incidence of light of 300, it appeared that one obtains a better color independence if one chooses, for the input polarizer, an angle
P1 which is equal to
P1 = (2 + 2) / 4.

For a + angle, crossed polarizers and a choice of
P1 in accordance with formula (3) above, the transmission coefficient also depends on the spatial delay A = d.6n imposed by the layer of liquid crystals. Equation (1) above makes it possible to make a choice of A leading to the best possible transparency at rest and promoting contrast.

 The invention notably proposes, consequently, a display of the kind defined above, in which: the liquid crystal layer causes a + twist of less than 900.

 The polarizer advantageously makes the angle at rest of the crystals in the boundary layer an angle which is of the order of (2 ++ X) / 4.

 Particularly advantageous results are achieved for an angle + of 00.

 It must generally be taken into account that the real liquid crystal cells are not ideal, in the sense that the change of orientation is not uniformly helical and that there is a pre-tilt or zpre-tilt along the walls.

The optimization of will therefore be carried out using not the formula (1) above, but a process involving first of all a simulation of the orientation profile in the liquid crystals as a function of the applied voltage. There are commercially available software usable for this purpose, including DIMOS software, available from Autronic-Melchers GmbH,
Karlsruhe, Germany. This software is used with representative values for the constants of the liquid crystals and in particular the constants corresponding to the ZLI-3771 liquid crystals, supplied by the company Merck,
Darmstadt, Germany. The software was used on the assumption of a pre-tilt angle on the faces of 30, which can be considered as representative.

Optimization is then carried out so as to obtain simultaneously a high transmission in the rest state and a transmission as weak as possible in the event of application of an electric field, using the formalism of matrix calculation Berreman 4 * 4 . On this subject, we can refer to the article by AH Potma et al, Optimization of
Foil-Compensated STN Displays, Eurodisplay 90, Amsterdam.

 It should be noted that the value found for the optimized thickness of the liquid crystal layer must be considered as a virtual value ", since the significant optical parameter is in fact the delay given by the liquid crystals, which is the product of thickness 4 of the layer of liquid crystals and of the birefringence An of the liquid crystals.

 In the calculations, the birefringence of the liquid crystals used is that of the ZLI-3771, which is equal to 0.1067 for a wavelength of 550 nm.

 If another birefringence was used, the resulting value for the optimized thickness of the liquid crystal layer would have been changed accordingly.

 The table below gives, by way of example, the results calculated for a configuration of nematic liquid crystals in a helix and with crossed polarizers and an orientation according to equation (3), for a twist t of 300, in incident light orthogonal.

WHITE STATE (REST)
Thickness of
Layer (micron) Delay (at 1 = 550 nm) Y (%)
2.40 0.2561 43.4
2.50 0.2668 44.1
2.55 0.2721 44.4
2.60 0.2774 45.5
2.70 0.2881 44.4
STATE "BLACK1 (ELECTRIC FIELD APPLIED)
Layer (micron) Delay (at 550 nm) Y (%)
2.55 0.2721 1.25
2.50 0.2668 1.21
2.40 0.2561 1.12
On the table, Y is the perceived luminance, taking into account the spectral distribution of the light source and the sensitivity curve of the eye. For ideal crossed polarizers, the maximum transmission cannot exceed 50%.

By way of comparison, the transparency for a conventional configuration (twist of 900, normally white), with a delay A = 0.4 is
- 44% in the white state ",
- less than 0.05% in the black state ".

 The comparison carried out has shown that the optical characteristics in the "white" state do not depend more on the spatial delay λ than in the case of the conventional constitution. On the other hand, the transmission coefficient in the "black" state under normal incidence is higher for the same applied voltage.

 The transmission coefficient, greater in the "black" state for the configuration with 300 of torsion, would normally lead to significantly lower contrast values than with the configuration with torsion of 900 and in normally "white" state.

 The invention makes it possible to arrive at an architecture which provides a maximum contrast of the same order as those obtained with the configuration with 900 of torsion.

 Two reasons can be mentioned to explain the increased black transmission of the configuration at 300 of torsion. On the one hand, when the angle of torsion of the liquid crystal layer is reduced from 900, possibly to 00, the transmission curve as a function of the voltage becomes less steep. This implies that at a given voltage (above the threshold voltage) the transmission coefficient, and therefore the luminance, is all the more important the smaller the angle of torsion.

On the other hand, the transmission in the "black" state is essentially due to the residual delay of the boundary layers of liquid crystals. For the configuration with 900 of torsion and normally "white" state, the effects of these layers compensate each other.

 To take account of these effects, the device advantageously comprises at least one delay sheet with positive birefringence, placed between the layer of liquid crystals and one of the polarizers, the optical axis of the sheet being parallel to the wall of the cell. . The sheet is advantageously placed so that its optical axis is perpendicular to the orientation of the liquid crystal molecules along the nearest wall. In general, two sheets will be used, each placed on one side of the liquid crystal layer.

 A very weak spatial delay introduced by each sheet is sufficient, typically less than or equal to one fifth of the spatial delay introduced by the liquid crystal layer. Owing to their short delay, the sheets practically do not affect the optical quality in the "white" state, both in incident light normal to the cell, and in oblique incidence. On the other hand, these sheets make it possible to obtain a transmission coefficient in the "black" state at normal incidence which is practically as low as with a twist angle of 900. The delay of each sheet will generally be less than 50 nm and often at 20 nm.

 In general, the spatial delay values due to the liquid crystal layer which will be chosen to implement the invention range from approximately 280 nm, for a twist angle of 00, up to approximately 400 nm, for an angle of twist of 800.

 It is noted that the implementation of the invention brings in all cases an improvement from the point of view of the contrast in lateral vision, due to the maintenance of a low transmission coefficient in the "black" state.

The improvement is all the more important as the angle of torsion approaches zero. This does not result in an annoying degradation of the transmission coefficient in the "white" state, for a suitable choice of the spatial delay given by the liquid crystal layer.

 It is also possible to reduce light leakage in oblique vision, in the "black" state, by adding at least one negative birefringence sheet, as in the case of a display having a twist angle of 900. Again, it is best to use two negative birefringence sheets each placed on one side of the cell. The optical axis of the delay sheet is perpendicular to the plates.

 The total spatial delay given by the sheets will always be less than the delay introduced by the liquid crystal layer.

 It is finally possible to substitute, for a positive birefringence sheet and a negative birefringence sheet placed on the same side of the layer, a single biaxial retardation sheet having the same optical properties as the combination of the two separate sheets.

The invention will be better understood on reading the following description of particular embodiments, given by way of nonlimiting examples. The description refers to the accompanying drawings, in which
- Figure 1 shows a possible constitution of a display according to 1invention
- Figure 1A is a diagram showing the orientations
P1 and P2 of the input and output polarizers
FIG. 2 is a diagram showing an optimal zone for choosing the spatial delay λ for given values of the angle of torsion +, on the one hand without a positive birefringence compensation sheet and on the other hand with such sheets
- Figures 3 and 4 are diagrams showing the variation curves of the transmission coefficient, in polar coordinates, for a display whose layer has a twist angle of 300, with and without sheets with positive birefringence
- Figure 5, similar to Figure 3, shows curves of variation of the transmission coefficient in the black state for a display according to the invention at 00 of torsion and a known display, with an obliquity of 600;
- Figure 6 shows iso-contrast curves of 10: 1 for a display with = 0 and sheets with positive birefringence (in solid lines) and for = 900 (in dashes)
- Figure 7 is similar to Figure 6, and shows the effect of a decrease in delay for = 00
The display, the basic structure of which is shown in FIG. 1, comprises a layer 10 of nematic liquid crystals, a few μm thick. The layer 10 is trapped between two transparent plates 12 and 14, generally made of glass. The interval between the plates is kept constant by spacer callets 15. When no electric field is applied to the layer, the crystals are oriented by coatings 16, provided so that the angle of twist or "twist" is less than 900. As a general rule, the angle will not exceed 850. It is possible to descend to 00.

 The plates 12 and 14 carry electrodes intended to constitute elementary capacitors each corresponding to a pixel. For example, the internal surface of the plate 12 can carry control electrodes constituting an array 18 and each connected to a thin film transistor. The transistors are distributed in rows and columns and controlled by a circuit not shown, via a connector 30. The other transparent plate 14 carries a counter electrode 20 which is usually a thin film of indium tin oxide . The liquid crystal layer is separated from the plates by brushed orientation layers 16. In a color display, colored filters 25 are interposed between one of the glass plates 14 for example, and the liquid crystal layer. The cell thus formed is placed between two crossed polarizers 26 and 28.

 In the particular embodiment of the invention shown in FIG. 1, two sheets with positive birefringence 34 and 36, of optical axis parallel to the plates are each placed between one of the plates 12 and 14 and the polarizer which is its closer.

Characteristics are generally determined by looking for an optimal value of b. We deduce for liquid crystals of nature, therefore from an, given, the value of d. The delay A and of the angle + will generally correspond to the domain between the curves A and B of
Figure 2.

The favorable values for A are believed to increase in
same time as +. In the figure, we go from the range 0.175
0.325 µm for # = 0 to 0.27-0.49 µm for = 850.

We will now give some examples of implementation
of the invention and indications of the characteristics
displays obtained.

Example 1:
configuration with twist of 300 and two delay sheets
with positive birefringence each placed so that its
optical axis be perpendicular to the direction of the crystals
liquids at the limit with the corresponding plate.

Without leaf With leaves
Delay of
liquid crystal 255.2 nm 299.8 nm
Sheet delay 17.6 nm
"White" transmittance 43.4% 45.1%
"Black" transmittance 1.11% 0.01%
The polarizers were crossed with P1- <2x300 + 1800) / 4 = 600.

Coor transmittance variation curves
polar data have been plotted, for oblique incidence
600. In the "white" state, the results with and without
leaves are practically identical. The interest of the invention
tion appears in particular in Figure 3, which shows the variation of the transmission coefficient in the "black" state for a conventional liquid crystal display with a
torsion angle of 900 (dashed curve) and for a
optimized display at 300 torsion angle, with
delay sheets. The gain obtained in terms of transmission coefficient in the "black" state is found in terms of iso-contrast.

Example 2:
Configuration with a twist angle of 300 and sheets with positive birefringence, oriented with their optical axis perpendicular to the direction of orientation of the molecules in the median plane of the liquid crystal layer.

- Layer delay: 296.7 nm - Delay of each sheet: 13.5 nm - Transmittance in the white state: 44.4 R - Transmittance in the "black" state: 0.04%
FIG. 4 is a comparison of the black states between the transmittance in the case of example 2 (dashed curve) and in the case of sheets whose optical axes are placed perpendicular each to the direction of the molecules at the limit (example 1 ).

 In the case of such a display, the configuration with two sheets oriented so that their optical axes are perpendicular to the direction of the molecules along the walls gives more favorable results in oblique (and no longer perpendicular) vision as regards transmission in the white state ". It has also appeared that the configuration with optical axes of the sheets perpendicular to the direction in the median plane gives rise to a slight yellow coloration in the" white "state.

Compensation3
Configuration at 300 of angle of torsion and sheets with positive birefringence oriented so that the optical axes are perpendicular to the respective directions of the liquid crystals near the plates.

 In this example, the optimization procedure implemented differs from the previous ones. We first choose the virtual thickness of the liquid crystal layer. Then, for each thickness, the delay of the sheets is determined such that the transmission coefficient in the "black1" state is less than 0.05%.

 Results obtained are given in the table below. The first table corresponds to the optimized value.

The other tables show the possibility of keeping favorable results with modified values in the domain defined by FIG. 2.

"BLACK" STATUS "WHITE" STATUS
Layer: 2809.7 nm -> Delay
the layer: 299.8 nm
Delay of each
sheet: 17.6 nm
COEFFICIENT OF
TRANSMISSION: 45.1% 0.01%
Layer: 2600 nm ->> Delay
the layer: 277.4 nm
Delay of each
sheet: 14.5 nm
COEFFICIENT OF
TRANSMISSION: 43.3% 0.04%
Layer: 2400 nm ->> Delay
the layer: 256.1 nm
Delay of each
sheet: 12.5 nm
COEFFICIENT OF
TRANSMISSION: 41.8% 0.05%
Layer: 2200 nm ->> Delay
the layer: 234.7 nm
Delay of each
sheet: 12.0 nm
COEFFICIENT OF
TRANSMISSION: 37.8% 0.03 t
Layer: 2000 nm ->> Delay
the layer: 213.4 nm
Delay of
sheet: 11.0 nm
COEFFICIENT OF
TRANSMISSION: 34.0% 0.03%
In oblique incidence, the reduction in transmission in the "black" state is proportionally greater than the loss of transmission in the "white" state, when the delay of the layer is reduced. Consequently, a tilt gain with given iso-contrast is obtained, in return for a loss of transmission in the white state ".

EXAMPLE 4
Configuration with a twist angle of 0 and sheets with positive birefringence placed with their optical axes perpendicular to the long orientation of the adjacent plate.

 The liquid crystals used were also those designated by the reference ZLI 3771 and crossed polarizers, with P1 = (00 + 1800) / 4 = 450.

The results obtained were as follows
WITHOUT SHEET WITH SHEETS
Layer delay: 253.1 nm 282.9 nm
Delay of each sheet: 15.7 nm
Transmission in "white" state: 44.4 z 44.8%
Transmission in "black" state: 1.36% 0.02%
The same results are obtained by rotating the polarizers by 900.

 The above data are in normal incidence. A more complete comparison between this example and a configuration having a twist of 900 with the same nematic liquid crystals, again with a "white" state in the absence of applied voltage, shows that the "white" state at 0 is as good as with a twist of 900.

 Figure 5 shows the transmission curves in the "black" state, for an obliquity of 600, on the one hand for the torsional configuration of 900 (in dashes) and on the other hand for the optimized torsional configuration of 0 following the example (in solid lines). It shows a reduced transmission in a ratio at least equal to two for almost all of the angles.

 The 10: 1 iso-contrast curves of FIG. 6 for the torsional configuration of 900 (in dashed lines) and for the optimized torsional configuration of 00 (in solid lines) show a substantial gain by implementing the invention: the obliquities for the same contrast are much higher in the case of the invention.

 As in the case of Example 3, favorable results remain even with values of λ which are not optimal. This appears for example by comparing the tables below to that given above and which corresponds to the optimal configuration. The additional tables correspond to slightly different choices. In all cases, the delay of the sheets is chosen so that the transmission coefficient in the "black" state is less than 0.05%.

"WHITE" STATUS "BLACK" STATUS
Liquid crystal layer: 2.6 µm -> Delay
the layer: 277.4 nm
Delay of each
sheet: 15 nm
COEFFICIENT OF
TRANSMISSION: 44.4% 0.02 t
Layer: 2.4 wm ->> Delay
the layer: 256.1 nm
Delay of each
sheet: 13 nm
COEFFICIENT OF
TRANSMISSION: 41.2% 0.04%
Layer: 2.2 um ->> Delay
the layer: 234.7 nm
Delay of each
sheet: 13 nm
COEFFICIENT OF
TRANSMISSION: 38.2% 0.01 t
Layer: 2.0 um ->> Delay
the layer: 213.4 nm
Delay of each
sheet: 11 nm
COEFFICIENT OF
TRANSMISSION: 35.3% 0.02 t
Again, in oblique incidence, the "black" state is all the more markedly improved when the delay of the layer is low.

 FIG. 7 shows the angular gain in isocontrast for a reduction in the delay. The dashed curves correspond to a layer delay of 213.4 nm, and the solid line curves to the optimized values (retad of the layer of 282.9 nm and of each sheet of 15.7 nm).

However, this gain in iso-contrast is offset by a loss of transmission in the "white" state.

Claims (9)

 1. Liquid crystal and active matrix display, comprising a cell having a thin layer (10) of helical nematic liquid crystals trapped between two transparent plates (12,14) facing each other, carrying creation electrodes (18,20) electric field connected to a control circuit and comprising two crossed polarizers (26,28) framing the cell,  characterized in that: the liquid crystal layer is such that it causes a + twist of less than 900.  2. Display according to claim 1, characterized in that the input polarizer (Pl) makes with the orientation at rest of the crystals in the adjacent boundary layer an angle of the order of (2 ++ x) / 4 , advantageously (2X) / 4,  3. Device according to claim 1, characterized in that the angle + is between 0 and 850.  4. Display according to claim 1, 2 or 3, characterized in that the liquid crystal layer has a delay As a function of the torsion + and varying from a value between 0.175 and 0.325 µm for + = 0 up to a value between 0.27 and 0.49 µm for + = 85.  5. Display according to any one of claims 1 to 4, characterized in that it also comprises a delay sheet (34,36) with positive birefringence placed between the layer of liquid crystals and one of the polarizers or two sheets each placed between the layer and a respective polarizer, the optical axis of the sheet or sheets being parallel to the plates.  6. Display according to claim 5, characterized in that the spatial delay given by the sheet is less than 50 nm, usually 20 nm.  7. Display according to claim 5 or 6, characterized in that the optical axis of the sheet or of each delay sheet is perpendicular to the direction of the liquid crystals at the limit with the face of the plate to which it is closest. .  8. Display according to claim 5 or 6, characterized in that it also comprises a negative birefringence sheet placed between the layer of liquid crystals and one of the polarizers so that its optical axis is perpendicular to the plates or two such sheets each placed between the layer and a respective polarizer.  9. Display according to claim 5 or 6, characterized in that the sheet or each sheet also has a negative birefringence, of optical axis perpendicular to the plates, so that the sheet or each sheet is of the bi-axial type.