WO2005073790A1

WO2005073790A1 - Liquid crystal display device

Info

Publication number: WO2005073790A1
Application number: PCT/IB2005/050161
Authority: WO
Inventors: Gerardus P. Karman; Barry Mos
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2004-01-26
Filing date: 2005-01-14
Publication date: 2005-08-11
Also published as: TW200537222A

Abstract

The present invention relates to a liquid crystal display device comprising a plurality of pixels (2). Each pixel comprises first and second electrodes (4, 5), which are used to manipulate a liquid crystal material (8), and are arranged at either side thereof. At least the first electrode (4) comprises a resistive material which allows different potentials to be applied over the first electrode. This entails domains of liquid crystal material with different liquid crystal molecule orientation over the width of the pixel (2), which provides an improved display viewing angle when light is transmitted through the pixel (2). Preferably, this feature may be switched off as desired, to make a displayed image less visible to other persons than the user, placed right in front of the display device.

Description

Liquid crystal display device

TECHNICAL FIELD The present invention relates to a liquid crystal display device.

TECHNICAL BACKGROUND Liquid crystal displays (LCDs) are used in a wide range of applications, such as for instance laptop computers. In many cases a backlighting arrangement is used, and the liquid crystal pixels modulate the light from the backlighting arrangement. Most conventional LCDs provide poor viewing angle properties, i.e. at an oblique viewing angle, the contrast and grey-scale properties of the display are different from the properties observed by a viewer straight in front of the display, in such a way that a displayed image may be unrecognisable. One possible method for improving the viewing angle is to provide slits in the electrodes that control the liquid crystal material in each pixel of a display device, as disclosed in international patent application WO 01/33288 Al. This may provide multiple domains in the pixel, which improves the viewing angle. However, the provision of slits in electrodes defines the liquid crystal molecule orientation pattern in the on state of the pixel once and for all. This pattern may therefore not be altered for obtaining different viewing angle properties needed by the user at different occasions.

SLTMMARY OF THE INVENTION It is an object of the present invention to provide a liquid crystal display device having a liquid crystal molecule orientation pattern which can be altered during operation. This object is achieved by a display device as defined in claim 1. More specifically, according to one aspect the invention relates to a liquid crystal display device comprising a liquid crystal material layer and a plurality of picture elements arranged in an array, wherein a picture element comprises a first electrode and a second electrode arranged at either side of the liquid crystal material layer. At least one of the electrodes comprises first and second end parts and presents a resistance between the first and second end parts. In a first mode, different potentials are applied to said first and second end parts by electrode control means. Such an electrode having an electrical resistance between its end parts will be referred to as a resistive electrode hereinafter. For the purpose of the present invention, the resistance should be such that voltage differences of at least several Volts can be applied between the end parts. hen such a voltage difference is applied, the electric field between the first and second electrodes varies over the width of the pixel, which introduces multiple liquid crystal molecule orientation domains in the liquid crystal material layer within the pixel. This provides an improved viewing angle of the pixel when light is transmitted therethrough. The viewing angle is improved in an adjustable manner. Preferably, the display device is switchable between the first mode and a second mode, in which the potential difference between the first and second end parts is lower than in the first mode. In a preferred embodiment, the first and second end parts are at substantially equal electric potential in the second mode. This provides a useful "privacy"-mode, where the viewing angle in fact is lowered. In this mode, it is difficult for persons at the users sides to view the image displayed by the display device, which allows the user e.g. to work with sensitive information on a laptop computer, comprising the display device, in public spaces. Preferably, the switching device is controllable by a user, so that the user can set the viewing angle of the display device in accordance with his actual situation. In a preferred embodiment, in the first mode the first and second end parts of the first electrode receive a first and a second potential, respectively, and the second electrode receives a third potential in the range between the first and second potentials. This improves the viewing angle in two directions. In another embodiment, also the second electrode comprises first and second end parts and presents a resistance between said first and second end parts. This provides even greater freedom of choice when deciding on the viewing angle properties. For example, the first resistive electrode can be used to control the viewing angle properties in a first direction such as a horizontal direction, while the second resistive electrode can be used to control the viewing angle properties in a second direction different from the first direction, such as a vertical direction. Preferably, the first electrode includes a thin film resistor, comprising a resistive material. Suitable materials include ITO, oxygen enriched ITO, SiCrN, Sn0₂, or NiCrAlSi. Alternatively, the first electrode comprises a plurality of separate electrode segments, which are interconnected by means of resistors. These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS Fig 1 illustrates schematically a display device according to an embodiment of the invention. Fig 2 illustrates a cross-section through a pixel in a display device according to an embodiment of the invention. Fig 3 illustrates, more schematically, the pixel according to a first embodiment of the invention. Fig 4 illustrates the pixel according to a second embodiment of the invention. Fig 5 illustrates a perspective view of the pixel according to the second embodiment of the invention. Figs. 6a and 6b illustrate simulation results for a pixel in a first mode when the pixel is switched off and on, respectively. Figs. 7a and 7b illustrate simulation results for a pixel in a second mode when the pixel is switched off and on, respectively.

DESCRIPTION OF PREFERRED EMBODIMENTS Fig. 1 illustrates a display device 1 according to an embodiment of the invention. The display device comprises a plurality of picture elements 2, hereinafter called pixels, which comprise controllable liquid crystal elements. Each such element operates by controlling the transmission therethrough of light from a backlighting arrangement (not shown). The display device 1 further comprises switching means such as a button 3 in order to allow a user to change the viewing angle properties of the display device 1, as will be discussed later. Fig. 2 illustrates a cross-section through a pixel 2 in a display device according to an embodiment of the invention. The pixel comprises a first and a second electrode 4, 5, which are disposed on a first and second substrate 6, 7 respectively. The substrates 6, 7, which face each other, are preferably made of glass and are normally common to all pixels in the display device. A liquid crystal material 8 comprising liquid crystal molecules 10 is placed between the first and second substrates 6, 7. As is well known to the skilled person, the orientation of the liquid crystal molecules may be changed by applying an electric field E over the liquid crystal material. Moreover, as is well known to the skilled person, these molecules change the polarisation of incoming light in a manner depending on their orientations. This effect, in combination with polarisers (not shown), serves to control the amount of light that passes through the pixel. The display device comprises means, such as orientation layers, (not shown) for providing an orientation field, which orientates the liquid crystal molecules 10 substantially in parallel with the substrates 6, 7 when no field is generated by the electrodes 4, 5. In this state the liquid crystal material, together with the polarisers, substantially stops light from passing through the pixel 2 (off-state). By applying different potentials to the electrodes 4, 5 the orientation field is overcome and the liquid crystal molecules turn out of their parallel alignment with the substrates 6, 7 and allow a greater amount of light to pass through the pixel 2 (on-state). Hence, the pixel 2 may be used to control the amount of light passing therethrough, thus allowing the display device to produce a visible image. In conventional display devices, as mentioned earlier, the liquid crystal molecules, when oriented transversely to the substrates in the "on-state", give the display device a relatively narrow viewing angle. Outside the viewing angle range, the properties of the displayed image deteriorate rapidly with increasing viewing angle. Fig. 3 illustrates, schematically, a pixel of a display device according to a first embodiment of the invention. In this pixel, the first electrode 4 is constituted by a resistive layer, which allows different potentials to be kept at the end parts of the electrode without causing a short circuit. Such potentials are applied to the end parts by control means in the display device. This causes the electric field direction to vary in the pixel along the length of the first electrode between the first end part and the second end part. The different potentials are maintained substantially during an entire period when the pixel is in the on state, i.e. when the pixel is intended to pass light therethrough. In Fig. 3 the first end part 11 of the first electrode 4 is kept at the potential 0 Volts, whereas the second end part 12 is kept at 10 Volts. The second electrode 5, which is conductive, is kept at 5 Volts. As will be shown in connection with Fig. 6b, this arrangement causes the liquid crystal molecules to be oriented parallel with the substrates (not shown in Fig. 3) close to the center C of the pixel and substantially transversally relative to the substrates near the end parts 11, 12. Between the center C and the end parts 11, 12 the devia- tion from the parallel direction increases gradually. This provides a substantially improved viewing angle, since, even at a relatively large viewing angle, a certain domain in the pixel area of each pixel will always contain liquid crystal molecules providing a suitable orientation for conveying light to a viewer's eye. The display device according to the invention thus comprises means for applying two different voltages to the end parts 11, 12 of the electrode 4, as compared to the conventional display where one voltage is applied to a conductive electrode. The user is also capable of reducing the viewing angle. This is useful in circumstances when the user does not want others to be able to read over his shoulder, for instance when he works with a display device on a laptop computer in a public space, such as an airport. With the display device according to the invention, the user, for instance by pressing a button (cf 3 in Fig. 1) may switch the display device into a "privacy mode" (in contrast to the "public mode" when the user wants to share information on the display device with others) when the viewing angle is substantially reduced. When this is done, the first end part 11 receives 10 Volts instead of 0 Volts. The electric field E between the first and second electrodes 4, 5 then becomes homogenous and parallel, since the voltage distribution over the first electrode 4 is the same as if it were entirely conductive. Needless to say, other means may be used as switching means in order to switch to and from the "privacy mode", such as software having a graphical user interface. Fig. 4 illustrates the pixel according to a second embodiment of the display device according to the invention. In this embodiment also the second electrode 5 is resistive in such a way that also first and second end parts 13, 14 of the second electrode 5 may receive different voltages. This provides even greater possibilities for varying the electric field across the pixel and thus obtain a desired light emission pattern. For instance, as illustrated in Fig.4, the pixel may be manipulated to predominantly convey light at a particular angle by applying 5 Volts to the first end part 11 of the first electrode 4, 0 Volt to the second end part 12 of the first electrode 4, 10 Volts to the first end part 13 of the second electrode 5, and 5 Volts to the second end part 14 of the second electrode 5. Fig. 5 illustrates a perspective view of the pixel of the display device according to a second embodiment of the invention. As is clear from the perspective view, the respective end parts 11, 12, 13, 14 of the first and second electrodes 4, 5 each consist of a linear element, covering one edge of the respective electrodes 4, 5. The end parts 11, 12, 13, 14 are conductive so as to ensure that their respective edges of the electrodes 4, 5 receive the same potential along their lengths. Figs. 6a and 6b illustrate simulation results for a pixel in a first mode when the pixel is switched off and on, respectively. The simulation is performed using the 2dimMOS™-SOFTWARE. The first electrode 4 is simulated as a number of series-connected segments 4a, 4b, etc., which are interconnected by resistors. The first electrode is thus "discretized" in the simulation, but a continuous, resistive electrode provides a very similar result. Moreover, such a "discretized" electrode constitutes an alternative embodiment of the invention. The liquid crystal material used in the simulation is known as MERCK ZLI-4792. It is enclosed in a rectangular cell with the width 86μm and the height 5μm. Note that the proportions of the figure are not to scale, they are stretched out in the vertical direction for clarity. This also makes the desired liquid crystal orientation angle θ (for a particular region in the pixel) look larger than it is (in the illustrated case 18°). The thin lines, e.g. 15, illustrate equipotential lines in the pixel. In Fig. 6a, the pixel is still switched off (i.e. the liquid crystal molecules 10 are ordered horizontally and perpendicular to the linear elements 11, 12, 13, 14, constituting the end parts in Fig. 5, by the orientation layer), but the end parts have just received their on-state driving voltages (as illustrated in Fig. 3) in such a way that the liquid crystal molecules will begin to turn. In Fig. 6b the liquid crystal molecules (which of course are magnified in size in order to be visible) have reached their respective orientations in the on-state. As can be seen, the pixel comprises two regions 16, 17. The first region 16 has viewing angle properties that are different from the viewing angle properties of the second region 17. When taken together the two regions provide an optical performance that is a lot less angle-dependent than a conventional display device comprising conductive electrodes only. The molecules of the region in the middle of the pixel do not have the proper orientation. Therefore, a light-shielding layer (black matrix) may be used to block this part of the pixel in order to improve the contrast. Another advantage with the display device is that no pretilt needs to be used to obtain good switching properties, i.e. the liquid crystal molecules may be oriented exactly parallel to the substrates in the off states. Figs. 7a and 7b illustrate simulation results for a pixel in a second mode when the pixel is switched off and on, respectively. In this case both end parts 11, 12 of the first electrode 4 receive the same driving voltage which entails vertically directed liquid crystal molecules (see Fig. 7b). In this case only one domain exists, and the viewing angle is reduced, as mentioned earlier. The slight deviations from vertical of some molecules close to the top electrode 4 are a result of the above mentioned "discretization" in the simulation, and do not exist when a resistive electrode is used. Preferably, the first electrode 4 comprises a thin film resistor, comprising a resistive material. Suitable resistive materials include ITO (Indium Tin Oxide), oxygen enriched ITO, SiCrN, Sn0 , NiCrAlSi. Tests have shown that sputtering of an ITO layer (20nm) in the presence of excessive oxygen followed by heating treatment at 200°C results in a square resistance of about 1.8 kΩ/sq. Such a layer would be suitable in the resistive electrode used. As mentioned, the first electrode may alternatively comprise a plurality of separate electrode segments, which are interconnected by means of resistors. The invention is not restricted to the embodiments described above. It may be varied within the scope of the appended claims. The invention may be used in different types of liquid crystal displays, such as passive matrix displays and active matrix displays. Apart from the first mode and the second mode mentioned, the principle of the invention can be extended to switch the display device between a plurality of viewing angle modes, each optimized for a particular situation. Switching between these modes can either be discrete or continuous, in the latter cases intermediate states in between two different modes can also be chosen. Summarizing, the present invention relates to a liquid crystal display device comprising a plurality of pixels. Each pixel comprises first and second electrodes, which are used to manipulate a liquid crystal material and are arranged at either side thereof. At least the first electrode comprises a resistive material which allows different potentials to be applied over the first electrode. This entails domains of liquid crystal material with different liquid crystal molecule orientation over the width of the pixel which provides an improved display viewing angle when light is transmitted through the pixel. Preferably this feature may be switched off when desired, to make a displayed image less visible to other persons than the user who is placed right in front of the display device.

Claims

CLAIMS:

1. A liquid crystal display device, comprising a liquid crystal material layer (8) and a plurality of picture elements (2) arranged in an array, wherein a picture element (2) comprises a first (4) and a second (5) electrode arranged at either side of said liquid crystal material layer (8), wherein at least said first electrode (4) comprises first and second end parts (11, 12) and presents a resistance between the first and second end parts (11, 12), and wherein the display device comprises means for applying, in a first mode, different potentials to said first and second end parts (11, 12).

2. A display device according to claim 1, wherein the display device is switchable, by means of a switching device (3), between said first mode and a second mode, in which the potential difference between the first and second end parts (11, 12) is lower than in the first mode.

3. A display device according to claim 2, wherein the first and second end parts (11, 12) are at substantially equal electric potential in the second mode.

4. A display device according to claim 1, wherein the switching device (3) is controllable by a user.

5. A display device according to claim 1 , wherein in the first mode the first and second end parts (11, 12) of the first electrode receive a first and a second potential, respectively, and the second electrode (5) receives a third potential in the range between the first and second potentials.

6. A display device according to claim 1, wherein the second electrode (5) comprises first and second end parts (13, 14) and presents a resistance between said first and second end parts.

7. A display device according to claim 1 or 6, wherein one or both of the first electrode (4) and the second electrode (5) comprises a thin film resistor comprising a resistive material.

8. A display device according to claim 1, wherein the first electrode comprises a plurality of separate electrode segments interconnected by means of resistors.