WO1996018931A1 - 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° in the absence of an electric field. On of the polarisers is substantially at an angle of (2ζ±π)/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 in such a way that the twist through the cell or "twist" is 90 ° and that the corresponding spatial delay Δ, defined as the product of the thickness . of the layer by the birefringence δn of the liquid crystal, that is to say about 0.4 u-m. 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 retardatri¬ these sheets with negative birefringence whose optical axis is perpenicular 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.

The liquid crystal layer does not have to have a 90 ° twist for it to act as a half-wave retarder. Document EP-A-0 448 173 discloses a liquid crystal display in which the twist can go down to 40 °.

In the ideal case of polarizers crossed more or less a few degrees, and 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 s to write :

T = i sin kφ.sin (φ + π / 2) / k + cos kφ.cos (φ + π / 2) I + sin ² kφ.cos ² (φ-2P ₁ + π / 2) .u ² / (l + u ² ) (1)

or

P-, 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 = π.d.δn / λ.φ (2 ) - λ being the wavelength of light,

- k = (1 + u ² ) ^.

We note that, in this case, the transmission at rest is maximum if we choose P so that the argument of the term cos (φ-2P ₁ + π / 2) is equal to:

(0, +1, -1, +2, -2, ...) π i.e. for:

(3)

For a light observation perpendicular to the cell, the choice of + π / 4 or -π / 4 in formula (3) is indifferent.

Pour un angle φ, des polariseurs croisés et un choix deOn the other hand, in oblique vision, and in particular as soon as one reaches or exceeds an incidence of light of 30 °, it appeared that one obtains a better color independence if one chooses, for the input polarizer, a angle P ^ which is equal to:

For an angle φ, crossed polarizers and a choice of

P-L in accordance with formula (3) above, the transmission coefficient still depends on the spatial delay Δ≈d.δn imposed by the layer of liquid crystals. Equation (1) above makes it possible to make a choice of Δ leading to the best possible transparency at rest and favoring the contrast.

The invention notably proposes, consequently, a display of the kind defined above, in which the liquid crystal layer causes a twist φ between 0 ° and 85 ° and has a delay Δ as a function of the twist φ and varying from a value between 0.175 and 0.325 μm for φ = 0 ° up to a value between 0.27 and 0.49 μm for φ = 85 °, said display also comprising a positive birefringence delay sheet placed between the layer of liquid crystals and one of the polarizers or two sheets with positive birefringence each placed between the layer and a respective polarizer, the optical axis of sheet or sheets being parallel to the plates.

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

Particularly advantageous results are achieved for an angle φ of 0 °.

The actual liquid crystal cells are not ideal, in the sense that the change in orientation is not uniformly helical and that there is a pre-tilt or "pre-tilt" along the walls.

The optimization of Δ is consequently carried out by 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 3 °, which can be considered as representative.

Optimization is then carried out so as to simultaneously obtain a high transmission in the idle state and a transmission as low as possible in the event of an electric field being applied, using the Berreman 4 matrix calculation formalism. * 4. On this subject, we can refer to the article by A.H. Potma et al, Optimization of Foil-Compensated ST 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 para è- being significant optics is in fact the delay given by the liquid crystals, which is the product of the thickness d. of the liquid crystal layer and the birefringence of 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 with liquid helical liquid crystals and crossed polarizers and an orientation according to equation (3), for a torsion φ of 30 °, in orthogonal incident light.

"WHITE" STATUS (REST

Layer thickness (micron) Delay (at λ = 550 nm) Y (%)

2.40 0.2561 43 2.50 0.2668 44 2.55 0.2721 44 2.60 0.2774 45 2.70 0.2881 44

"BLACK" STATUS (APPLIED ELECTRIC FIELD)

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 90 °, normally white), with a delay Δ = 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 Δ 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, which is greater in the "black" state for the 30 ° twist configuration, would normally lead to significantly lower contrast values than with the 90 ° twist configuration and in the 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 at 90 ° of twist. Two reasons can be mentioned to explain the increased transmission in the "black" state of the configuration at 30 ° torsion. On the one hand, when the angle of twist of the liquid crystal layer is reduced from 90 °, possibly down to 0 °, 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, transmission in the "black" state is essentially due to the residual delay of the boundary layers of liquid crystals. For the 90 ° twist configuration and normally "white" state, the effects of these layers compensate each other.

The device takes these effects into account by the presence of at least one delay sheet with positive birefringence, placed between the liquid crystal layer and one of the polarizers, the optical axis of the sheet being parallel to the wall of the cell. The sheet is 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 small spatial delay introduced by each sheet is sufficient, typically less than or equal to one fifth of the spatial delay introduced by the layer of liquid crystals. 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 90 °. The delay of each sheet will generally be less than 50 nm and often 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 0 °, up to approximately 400 nm, for an angle 80 ° twist. 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 still possible to reduce light leakage in oblique vision, in the "black" state, by adding at least minus a negative birefringence sheet, as in the case of a display having a twist angle of 90 °. 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 the invention;

- Figure 1A is a diagram showing the orientations P-L and P2 of the input and output polarizers; FIG. 2 is a diagram showing an optimal zone for choosing the spatial delay Δ for given values of the angle of twist φ, 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 30 °, 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 0 ° of torsion and a known display, with an obliquity of 60 °;

- Figure 6 shows iso-contrast curves of 10: 1 for a display with 0 = 0 ° and sheets with positive birefringence (in solid lines) and for 0 = 90 ° (in dashes); - Figure 7 is similar to Figure 6, and shows the effect of reducing the delay for 0 = 0 °

The display, the basic structure of which is shown in FIG. 1, comprises a layer 10 of nematic liquid crystals, having a thickness of a few μm. 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 braces 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 90 °. As a rule, the angle will not exceed 85 °. It is possible to go down to 0 °.

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, by the intermediary of a connector 30. The other transparent plate 14 carries a counter electrode 20 which is usually a thin film of indium oxide -tin. 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, with an optical axis parallel to the plates, are each placed between one of the plates 12 and 14 and the polarizer which is closest thereto.

Characteristics are generally determined by looking for an optimal value of Δ. We deduce for liquid crystals of nature, therefore from δn, given, the value of d .. The delay Δ and the angle φ will generally correspond to the domain between curves A and B in Figure 2. We believe that the favorable values for Δ increase at the same time as φ. In the figure, we go from the range 0.175-0.325 μm for φ = 0 ° to 0.27-0.49 μm for φ = 85 °.

We will now give some exemplary embodiments of the invention and indications on the characteristics of the displays obtained.

Example 1:

Configuration with 30 ° twist and two positive birefringence delay sheets, each placed so that its optical axis is perpendicular to the direction of the liquid crystals at the limit with the corresponding plate.

Without sheet With sheets Liquid crystal delay 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

P ₁ = (2x30 ° + 180 °) / 4 = 60 °.

Variation curves of the transmittance in polar coordinates have been drawn, for an oblique incidence of 60 °. In the "white" state, the results with and without leaves are practically identical. The advantage of the invention appears in particular in FIG. 3, which shows the variation of the transmission coefficient in the "black" state for a conventional liquid crystal display with a twist angle of 90 ° (dashed curve ) and for a display optimized at 30 ° 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 30 ° twist angle 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% - 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 the 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. Example 3:

Configuration at 30 ° angle of twist 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. First, the virtual thickness of the layer of liquid crystal is chosen. Then, for each thickness, the delay of the sheets is determined such that the transmission coefficient in the "black" 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 -> Layer delay: 299.8 nm Delay of each sheet: 17.6 nm TRANSMISSION COEFFICIENT: 45.1% 0.01%

Layer: 2600 nm - >> Layer delay: 277.4 nm Delay of each sheet: 14.5 nm

TRANSMISSION COEFFICIENT: 43.3% 0.04%

Layer: 2400 nm - >> Layer delay: 256.1 nm Delay of each sheet: 12.5 nm COEFFICIENT OF

TRANSMISSION: 41.8% 0.05%

Layer: 2200 nm - »Layer delay: 234.7 nm Delay of each sheet: 12.0 nm COEFFICIENT OF

TRANSMISSION: 37.8% 0.03%

Layer: 2000 nm - >> Layer delay: 213.4 nm

Sheet delay: 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.

Claim 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 Pl = (0 ° + 180 °) / 4 = 45 °.

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% 44.8% Transmission in "black" state 1.36% 0 .02%

The same results are obtained by rotating the polarizers 90 °.

The above data are in normal incidence. A more complete comparison between this example and a configuration having a twist of 90 ° 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 90 ° twist.

FIG. 5 shows the transmission curves in the "black" state, for an obliquity of 60 °, on the one hand for the configuration with torsion of 90 ° (in dashes) and on the other hand for the configuration with torsion of 0 ° optimized according to 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 90 ° twist configuration (in dashes) and for the 0 ° twist configuration (in solid lines) show a substantial gain by implementing 1 inven¬ tion: the obliquities for the same contrast are significantly higher in the case of the invention.

As in the case of Example 3, favorable results remain even with values of Δ 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 -> Layer delay: 277.4 nm

Delay of each sheet: 15 nm COEFFICIENT OF

ION TRANSMISSION: 44.4% 0.02%

Layer: 2, 4 μm - »Layer delay: 256.1 nm Delay in each sheet: 13 nm TRANSMISSION COEFFICIENT: 41.2% 0.04%

Layer: 2.2 μm - >> Layer delay: 234.7 nm Delay of each sheet: 13 nm

TRANSMISSION COEFFICIENT: 38.2% 0.01%

Layer: 2.0 μm - >> Layer delay: 213.4 nm Delay in each sheet: 11 nm COEFFICIENT OF

TRANSMISSION: 35.3% 0.02%

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 iso¬ contrast for a reduction in the delay. The curves in dashes correspond to a layer delay of 213.4 nm, and the curves in solid lines with 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

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, in which the layer of liquid crystals is such that it causes a twist φ between 0 ° and 85 ° and has a delay Δ function of torsion φ and varying from a value between 0.175 and 0.325 μm for φ = 0 ° up to a value between 0.27 and 0.49 μm for φ = 85 °, said display also comprising a sheet delay (34,36) with positive birefringence placed between the liquid crystal layer and one of the polarizers or two sheets with positive birefringence each placed between the layer and a respective polarizer, the optical axis of the sheet or sheets being parallel to the plates. 2. Display according to claim 1, characterized in that the input polarizer (P ^) makes with the orientation at rest of the crystals in the adjacent boundary layer an angle of the order of (2φ ± π) / 4, advantageously (2φ + π) / 4. 3. Display according to claim 1, characterized in that the spatial delay given by the sheet is less than 50 nm. 4. Display according to claim 3, characterized in that the spatial delay is 20 nm. 5. Display according to claim 1,2 or 3, caracté¬ ized 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 which it is the closest. 6. Display according to claim 1,2 or 3, character- laughed in that it also comprises a sheet with negative birefrin¬ gence 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. 7. Display according to claim 1,2 or 3, caracté¬ ized in that the sheet or each sheet also has a negative birefringence, with an optical axis perpenicular to the plates, so that the sheet or each sheet is of the type biaxial. 8. Display according to claim 1, 2 or 3, caracté¬ ized in that the sheet or each sheet is of the uniaxial type.