FR2737315A1 - Liquid crystal electro-optical display - has slabs flanking liquid crystal molecules and supporting pixel electrodes opposite electrode divided into bands separated by equal gaps - Google Patents

Abstract

The display includes two parallel transparent slabs (10,12) each provided with transparent electrodes (14,16). The slabs enclose a liquid crystal (18) flanked by alignment layers (20,22). One of the electrodes (16) is divided into a number of bands (34) separated by equal gaps (36). The gap size is equal to the distance between the pixel electrodes (14) provided on the opposite slab. The bands are orientated along a direction parallel to the pixel electrodes. The whole structure is slightly shifted transversally relative to the pixel electrodes.

Description

 The invention relates to the field of electrooptical liquid crystal displays.

 These displays include a geometric network of elementary electrooptical cells each forming a pixel, the state of which is selectively controlled so as to obtain the appearance on the display of the desired characters or graphic patterns.

 They can be used either in direct vision (observation in ambient light by reflection or with backlighting) or by transparency in projectors, the display being then used as an occultation element in the focal plane of an optical system comprising by elsewhere a light source, a lens and a projection screen.

 More specifically, the general structure of these electrooptical liquid crystal displays comprises two parallel blades arranged opposite, one carrying a series of discrete pixel electrodes in the form of parallel elongated strips and the other a common counter electrode extending opposite the pixel electrodes. These blades enclose between them a material containing molecules of liquid crystals whose orientation of the texture is sensitive to the application of an electric field between pixel electrode and counter-electrode, and carry on their internal face a layer of alignment proper to impose, before application of the electric field, a pre-tilt in the transverse plane to the liquid crystal molecules immediately adjacent to this layer.

 One of the drawbacks of these known displays, which will be described in more detail below with reference to FIG. 1, is the frequent presence of a "misalignment" appearing mainly along one of the sides of each pixel and due to the fact that the direction of the electric field at this location, taking into account an edge effect, is not exactly rectilinear and perpendicular to the plane of the screen, as would be necessary for a theoretically optimal operation of the electrooptical cell.

 This local deformation of the electric field can introduce areas of retro-tilt or reverse tilt, which strongly affects the contrast and the dynamic behavior of the display.

 One of the aims of the invention is to remedy this drawback in particular by proposing a modified electro-optical display structure making it possible to eliminate or, at the very least, to very greatly attenuate the edge defect, and therefore the effects which are attached to it, and create a preferential direction for rocking molecules.

 To this end, the display of the invention, which is of the general type described above, is characterized in that the surface of the counter electrode is divided into a series of parallel strips which, in longitudinal direction, extend in the same direction as the bands forming the pixel electrodes, and in a transverse direction have a pitch equal to that of the latter, and, with respect to the intervals formed between pixel electrodes, the intervals formed between the counter bands electrodes are offset in transverse direction, this offset having a direction and a value making it possible to counter the transverse component of the electric field due to the edge effect in the vicinity of the interval between strips of pixel electrodes; in this way, the local retro-tilt of the liquid crystal molecules is compensated for, which, in the absence of an interval between bands of counter-electrode, would appear due to this transverse component of the electric field due to the edge effect.

 The counter-electrode strips advantageously have, in transverse direction, a width substantially equal to that of the pixel electrode strips.

 The liquid crystals can in particular be crystals of the nematic helical type, or other type, the blades respectively carrying a crossed polarizer and an analyzer.

 The pixel electrodes can in particular be addressed by an active matrix of thin film transistors, a control gate electrode being arranged in the gaps formed between pixel electrodes, or being located under one of the pixel electrodes.

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Other characteristics and advantages of the invention will appear on reading the detailed description below, made with reference to the accompanying drawings (in the different figures, the same reference numerals designate similar elements).

 Figure 1 schematically illustrates the structure and operation of an electrooptical display of the prior art, so as to illustrate the defect which the invention aims to remedy.

 Figure 2 is homologous to Figure 1, for a modified structure according to the invention.

 FIG. 3 represents a result of simulation of the distribution of the molecular orientation in a display of the prior art.

 FIG. 4 is equivalent to FIG. 3, for this same display modified according to the teachings of the invention.

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FIG. 1 illustrates the general principle of operation of an electrooptic liquid crystal display, whether used in direct vision or in a projection system, and comprising pixels selectively controlled so as to compose the image or the characters to display.

 Essentially, this display comprises two parallel blades 10, 12 transparent forming substrates and each carrying electrodes, also transparent.

 The electrodes 14 of the blade 10, which will be called "pixel electrodes", define by their extent the geometry of the pixel and are connected to an addressing system (in itself known and not illustrated) composed of lines ( "selection lines") and columns ("data lines") at the intersections of which are located each of the pixel electrodes. The control of the latter can in particular be achieved by an active matrix of thin film transistors (technology called "TFI" "'): the gates of these transistors form the selection lines and are controlled by peripheral circuits which scan the lines and make the transistors of each line pass; by the data lines connected to other peripheral control circuits, it is thus possible to selectively polarize the pixel electrodes and modify the optical properties of the liquid crystal located opposite this electrode, thus allowing the formation of the picture on the screen.

 The electrode 16 of the blade 12, which will be called "counter-electrode", is common to several pixels; in the case of a screen addressed by an active matrix, it generally covers the entire screen.

 The blades 10, 12 enclose between them a material 18 containing liquid crystal molecules whose optical properties can vary as a function of the local electric field, therefore of a bias voltage applied between the electrodes 14 and 16.

 These liquid crystals may in particular be of the "helical nematic" type, the light polarization characteristics of which vary according to the ambient electric field. The invention is not however limited to this type of liquid crystal and also applies to other materials, such as for example liquid crystals with a cholesteric structure, which are also susceptible to an electrooptical effect.

 The blades 10 and 12 further carry, on their face in direct contact with the material containing the liquid crystals, so-called alignment layers 20, 22 capable of imposing in the absence of any electric field a pre-tilt in the plane transverse liquid crystal molecules immediately adjacent to this layer (deposited on the surface of the electrodes).

 The need for these alignment layers is due to the fact that in general the liquid crystal molecules tend to align in the direction of the electric field (this case is the most general case, but is not limiting, because it other materials exist which are aligned perpendicular to the direction of the field; the invention is also applicable to them mutatis mutandis, the rest state corresponding to the active state in the general case, and vice versa). To avoid the formation of domains where the molecules would be tilted in the opposite direction, a slight pre-tilt angle is imposed (shown diagrammatically at 24, 26 in FIG. 1) to give a preferential direction to the deformation of the state of alignment of liquid crystal molecules when the electric field is applied.

 These alignment layers 20, 22 can be, for example, in itself known, polyimide layers having undergone a light brushing or a slight impression, or else layers of silicon monoxide deposited obliquely (in this regard, it will be noted that, in the figure, the inclination of the hatching of the layers 20 or 22 corresponds to the direction of pre-inclination which it is desired to impose on the molecules).

 Finally, the blades 10, 12 carry a polarizer and an analyzer, for example in the form of films, respectively 28, 30, each deposited on their external surface and making it possible to reveal the electrooptical phenomenon of modification of the texture of the liquid crystals.

 This known structure has the following drawback.

 As can be seen in FIG. 1, taking into account the edge effects, at the periphery of each pixel the electric field E (produced by the application of a voltage between pixel electrodes and counter-electrode) is not directed exactly perpendicular to the blades 10, 12 and, near one of the edges of each pixel electrode (the right edge in Figure 1, taking into account the pre-tilt in the anticlockwise direction given to liquid crystal molecules by the alignment layers 20 and 22), the electric field has a horizontal component which opposes the preferential direction of deformation of the alignment state, mentioned above, which was precisely sought to be imposed by the alignment layers.

 The effect of this phenomenon is to locally create a domain, obviously at 32 in FIG. 1, in which the inclination of the molecules is in the opposite direction to the slight pre-inclination imparted by the layer 22.

 These so-called "back-tilt" domains usually form along only one of the four sides of the pixel electrode, and their size and exact location depend in particular on the value of the pre-tilt angle. .

 They significantly affect the contrast of the display at the edge of the pixel and the dynamic behavior of the display by slowing down the changes of state.

 To remedy this drawback, the present invention proposes to modify the structure of the counter-electrode as illustrated schematically in FIG. 2.

 The counter electrode, which was a continuous electrode 16 in the case of the known structure illustrated in FIG. 1, is now divided into a series of strips 34 parallel to each other and separated by respective intervals 36, so as to form a periodic structure having the same pitch as the pixel electrodes 14 located opposite.

 The width of the gap 36 separating the strips 34 is preferably, but not limited to, of the same order of magnitude as the gap 38 separating the pixel electrodes, advantageously with substantially equal widths.

 The strips 34 are oriented parallel to the edge of the pixel electrode subject to the phenomenon of back-tilting, and are aligned with respect to the pixel electrodes, in transverse direction with a slight offset (i.e. a slight phase shift of the two periodic structures) of the bands 34 relative to the pixel electrodes 14.

 The slight offset 40 makes it possible to locally deform the electric field by reversing the direction of its transverse component in the vicinity of the domain 32 where the defect which one wishes to remedy occurs. In other words, the strips 34 are positioned relative to the pixel electrodes so that the direction of the electric field in the vicinity of the edges corresponds to the preferred direction defined by the pre-tilt angle of the liquid crystal molecules at 1 location of the alignment layer.

 In this way, as can be seen at 32 in FIG. 2, the back-tilt defect has disappeared and the liquid crystal molecules have the desired pre-tilt over the entire extent of the pixel, thus making the defect disappear. prior.

 FIGS. 3 and 4 are results of simulations of the molecular orientation of the liquid crystals, respectively in the case of the prior art and in that of the invention.

 In these figures, for clarity of the representation, the figure has been expanded by a factor of two in the vertical direction. Also shown in this figure is the gate electrode 42, located in the middle of the interval 38 separating two successive pixel electrodes 14, and whose width d, which is of the same order as the thickness of the cell, is about a third of the 38 interval.

 In the example illustrated, the pre-tilt angle is of the order of 1.50, the control voltage on the pixel electrode is more or less twice the threshold voltage of the liquid crystal, the voltage negative on the grid line is six times this same threshold voltage, and the counter electrode is at zero potential.

 In FIG. 3, we can see at 32 a back-tilt domain.

 FIG. 4 illustrates this same cell structure, modified according to the teachings of the invention.

 The counter electrode is now shaped as a series of parallel strips 34 separated by an interval 36 substantially of the same width as the interval 38 separating the pixel electrodes 14, the transverse offset 40 being of the order of half the cell thickness d.

 The figure clearly shows the disappearance in region 32, by the implementation of the present invention, of the back-tilting domain of the molecules.

Claims (4)

 1. An electrooptical liquid crystal display, comprising two parallel blades (10, 12) arranged opposite, one carrying a series of discrete pixel electrodes (14) in the form of parallel elongated strips and, the other, a common counter-electrode extending opposite the pixel electrodes,  these plates enclosing between them a material (18) containing liquid crystal molecules whose texture orientation is sensitive to the application of an electric field (E) between pixel electrode and counter electrode,  the blades carrying on their internal face an alignment layer (20, 22) suitable for imposing, before application of the electric field, a pre-inclination in the transverse plane (24, 26) to the liquid crystal molecules immediately adjacent to this layer ,  display characterized in that the surface of the counter electrode is divided into a series of parallel strips (34) which, in longitudinal direction, extend in the same direction as the strips forming the pixel electrodes, and, in transverse direction , present a pitch equal to that of the latter,  and in that, with respect to the intervals (38) formed between pixel electrodes, the intervals (36) formed between the counter-electrode strips are offset in transverse direction, this offset (40) having a direction and a value making it possible to counter the transverse component of the electric field due to the edge effect in the vicinity of the interval between strips of pixel electrodes,  so as to compensate for the local back-tilt of the liquid crystal molecules which, in the absence of an interval between bands of counterelectrode, would appear due to this transverse component of the electric field due to the edge effect.  2. The electrooptical display of claim I, in which the counter-electrode strips have, in transverse direction, a width substantially equal to that of the pixel electrode strips.  3. The electrooptical display of claim 1, in which the liquid crystals are crystals of the nematic helical type, the blades carrying respectively a crossed polarizer and an analyzer.  4. The electrooptical display of claim 1, in which the pixel electrodes are addressed by an active matrix of thin film transistors, a control gate electrode (42) being arranged in the intervals (38) formed between electrodes of pixel.