FIELD OF THE INVENTION The present invention relates to a liquid crystal display comprising a first pixel electrode, a second pixel electrode; said first and second pixel electrode being made of different conducting materials, and liquid crystal material sandwiched between said first pixel electrode and said second pixel electrode.

BACKGROUND OF THE INVENTION In reflective and transflective liquid crystal displays, it is observed that when the product is exposed to light, e.g. sunlight, for even a modest period of time, like a few hours, non-uniformities in the image can result. For instance, when a direct view reflective LCD of a mobile telephone or PDA (Personal Digital Assistant, an electronic organizer) is lying close to a window, which is partly illuminated by the sun, for a few hours, the user will afterwards be able to identify the illuminated area by slight greyscale variations and optical flicker. This is of course an annoying unwanted effect, also referred to as "the sunburn effect", which will remain visible for weeks. A related effect is observed in reflective LCDs for projection, also referred to as LCoS (Liquid Crystal on Silicon). Here a very intense UHP lamp illuminates the whole active area. Therefore, during the first hours of operation a rapid shift in grey levels will take place, the so-called "burn-in effect". Since this is a uniform shift, this will be harder to identify for the viewer than a non-uniform shift. On the other hand, the LCoS application is much more demanding in terms of image quality (natural reproduction of colors and gamma), so this effect is nevertheless undesirable. The consequence of this problem is that every panel needs a costly additional fabrication step, in which the panel is illuminated to evoke the largest part of the grey level shift. However, this is not a complete solution, because after weeks to months of not using the display, the problem will return. These problems were identified some years ago, but until now, no solution has been suggested. Thus, there is a need for liquid crystal displays which are not deteriorated when exposed to light. SUMMARY OF THE INVENTION An object with the present invention is therefore to obtain reflective and transflective liquid crystal displays, which are less sensitive to light exposure. This object is achieved by the incorporation, in a liquid crystal display, of a pixel structure comprising a first pixel electrode, a second pixel electrode; said first pixel electrode and said second pixel electrode being made of different conducting materials; and liquid crystal material sandwiched between said first pixel electrode and said second pixel electrode, which is characterized in that a dielectric barrier layer is arranged between said liquid crystal material and at least one of said first pixel electrode and said second pixel electrode. The present inventor has found that the cause of the observed problems is the asymmetry in pixel electrode materials. For reflective, and most transflective, liquid crystal displays, the pixel electrodes are made from different conducting materials. Generally, the rear pixel electrode is reflective and arranged for reflecting ambient light back towards a viewer. The front pixel electrode, which is arranged at the viewer's side of the liquid crystal layer, is optically transparent. For example, the rear pixel electrode is made of aluminum (Al) and the front pixel electrode is made of indium tin oxide (ITO). Different conducting materials generally have different work functions. When the displays are illuminated, e.g. by sunlight, an asymmetric change in the electrode work functions arise. As used herein, "work function" relates to the energy barrier for electrons to escape the conductor. It has been found that this asymmetric change in the electrode work functions is responsible for the sunburn and burn-in effects set out in the above. By coating one or both electrodes with a dielectric barrier layer, the asymmetry in the electrodes is compensated, and the resulting device will have a much improved image stability under illumination. The reason for this is that the work function of the stack, now consisting of a dielectric layer on top of a metal, will be determined by the interface between the metal and the dielectric. This means that the critical interface is passivated by the dielectric layer. This results in a much more stable interface since there is no direct contact with the liquid crystal material of the display. Dielectric barrier layers may be arranged both between said first electrode and said liquid crystal material, and between said second electrode and said liquid crystal material. Using barriers on both electrodes will provide a better effect, depending on the electrode materials used. For instance, ITO suffers more from the light-sensitivity than Al, so that ITO passivation only will have the biggest impact, but double sided passivation will be even better. The dielectric barrier layer(s) may be coated onto said first and/or said second electrode before the assembly of the display. This is favorable since the step of coating one or both of the electrodes with a barrier layer can easily be incorporated in existing manufacturing processes. The dielectric barrier layer(s) may be made of a material selected from the group consisting of Al2O3, CaO, Gd2O3, HfO2, La2O3, MgO, Sc2O3, Si3N4, SiO2, Ta2O5, TiO2, Y2O3, ZrO2, and BaZrO3, or a mixture thereof. Metal-oxides have excellent barrier properties and are therefore suitable to be used in connection to the present invention. In order to maintain the optical performance, a dielectric barrier layer is preferably transparent and thin. For example, the dielectric barrier layer may have a thickness in the range of 2-100 nm, or in the range of 2-20 nm. In the range 2-20 nm, the increase of the driving voltage is negligible (<1%). Alternatively, thicker layers (20-100 nm) may be used, since they are also optically acceptable, and usually easier to manufacture pinhole free. The first electrode may be made of a material selected from the group consisting of aluminium, molybdenum, and silver, or mixtures thereof, and the second electrode may be made of a material selected from the group consisting of indium tin oxide (ITO), indium zinc oxide, and aluminium zinc oxide, or mixtures thereof. The pixel structure according to the present invention may be used in all kinds of liquid crystal displays, with different electrodes on top and bottom substrates. This includes reflective and transflective liquid crystal displays for mobile applications, e.g. mobile phones, electronic organizers, game boys, etc. The pixel structure according to the present invention may also be used in reflective projection liquid crystal displays such as front- and rear-projection displays (LCoS) and near-to-eye displays. 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 shows a pixel structure according to the invention. Fig 2 shows the effect of illumination on the DC bias of reflective cells, Fig 3 shows the effect of illumination on the DC bias in LCoS panels. DESCRIPTION OF PREFERRED EMBODIMENTS The present inventor has found that coating one or both electrodes in the pixel structure of a liquid crystal display with a dielectric barrier layer results in an improved image stability under illumination. With reference to Fig 1 , a "pixel structure" (1) of a liquid crystal display relates to the stack of layers generating the electro-optical effect. The pixel structure (1) according to the invention includes two electrodes (2), the liquid crystal material (3) and at least one dielectric barrier layer (6). The pixel structure generally further includes well-known optical layers such as polarizes, diffusers, retardation layers and color filters. As used herein, "liquid crystal material" (3) relates to liquid crystal molecules (4) surrounded by alignment layers (5). The liquid crystal molecules (4) are incorporated for polarization- interaction with light, and the alignment layers (5) are incorporated for orienting the liquid crystal molecules. The alignment layers (5) are in turn surrounded by conducting electrode layers (2) to generate the electric field for controlling the liquid crystal molecules. According to the invention, dielectric barrier layers (6) are incorporated between the alignment layers (5) and the electrodes (2). Thereby, the pixel structure (1) according to the invention is formed. Thus, the heart of a liquid crystal display is a piece of liquid crystal material placed between a pair of electrodes. The liquid crystal changes the polarization direction of the light passing through it and this polarization change can be controlled by the voltage applied between the electrodes. If such a unit is placed between a pair of plane polarizer plates then light can pass through it only if the correct voltage is applied. Liquid crystal displays are formed by integrating a number of such cells, or more usually, by using a single liquid crystal plate and a pattern of electrodes. The simplest kind of liquid crystal displays, those used in digital watches and calculators, contain a common electrode plane covering one side and a pattern of electrodes on the other. These electrodes can be individually controlled to produce the appropriate display. Computer displays, however, require far too many pixels (typically between 50,000 and several millions) to make this scheme, in particular its wiring, feasible. The electrodes are therefore replaced by a number of row electrodes on one side and column electrodes on the other (passive matrix), or mostly by an active matrix consisting of rows and columns on one side with transistors in every pixel, i.e., the crossing point of a row and a column. By applying voltage to one row and several columns the pixels at the intersections are set. Modern liquid displays (LCDs) are complex devices which comprise a large numbers of separate films and layers with optical functions, In addition to the actual liquid crystal mixture that forms the optical switch of the display, other well-known optical components such as polarizes, diffusers, retardation layers and color filter are required in order for the assembled display to deliver the desired optical performance. Reflective LCDs have an internal mirror reflecting incoming ambient light. Backlighting is not required with a reflective LCD. A transmissive LCD is illuminated from one side (backlighted) and viewed from the opposite side. Transflective LCDs are a combination of backlighting and reflection of ambient light, usually by dividing each pixel into a reflective area and a transmissive area, side by side. As stated above, in reflective and transflective liquid crystal displays, the upper and lower electrodes are made from different conducting materials, usually aluminum (Al) and indium tin oxide (ITO), and these materials have different work functions. By a conducting material is meant a material in which electrons or holes are mobile, such that electric potential is transported uniformly across the entire material. In the research work leading to the present invention, it was found that the origin of the defects resulting from light exposure lies in the sensitivity of the electrode work functions to light. As used herein, "work function" relates to the energy barrier for electrons to escape the conductor. The present inventor has surprisingly found that passivating the electrodes (2) by thin metal-oxide dielectric barrier layers (6) strongly improves the light stability. In transmissive LCDs this effect does not play an important role, because of the inherent symmetry of the electrodes (both ITO). That means that any change in the work function will be equal in both electrodes and therefore not result in a net effect. In transflective and reflective LCDs this inherent compensation is not present, because the different electrode materials behave differently under the influence of illumination. This is also the case when the reflective electrode and transmissive electrode have equal work functions (for example molybdenum and ITO). The work functions of materials are very sensitive to the conditions of the surface. For instance the presence of oxygen can greatly influence the resulting work function (see J. Muller, "The effects of oxygen on the work function of metal films", Surface Science, vol. 69, No 2, p 708-711, 1977). In LCDs, the pixel electrodes (2) are normally surrounding the alignment materials (5), which in turn are surrounding the organic liquid crystal molecules (4). The alignment materials (5) are normally organic, e.g. polyimide or polyamide. The liquid crystal molecules (4) may be a mixture of several components that are optimized for a wide range of chemical, electrical and optical properties. Such liquid crystal molecules are known to a man skilled in the art. High intensity illumination of the display, and consequently the interface between the liquid crystal material and the electrodes, causes an asymmetric change in the work functions of the metals. This asymmetric change is avoided or at least reduced by coating at least one of the electrodes with a dielectric barrier layer (6) according to the invention. As referred to herein, "dielectric" refers to a material which is a poor conductor of electricity. The dielectric barrier layer could also be referred to as a passivation layer, when it behaves as an electrical blocking layer, not only for electrons, but also for ions and ions soluted in liquids. So it has to be mechanically intact (no pinholes). The dielectric barrier layer (6) is suitably coated directly onto the pixel electrodes (2). Both sides of the electrode could be coated with a barrier layer, but only the side that faces the liquid crystal material has effect. The dielectric layer (6) is preferably arranged so that it prevents contact between the pixel electrode (2) and the liquid crystal material (3). Suitably, the barrier layer (6) covers the complete interface between the pixel electrode (2) and the alignment layers (5). The dielectric barrier layers (6) according to the invention may be made of all materials that can prevent the permeation of organic molecules to the electrode surface. If the liquid crystal material (3) is in contact with the electrode surface when the display is illuminated, it affects the work function of the metal, causing a deterioration of the display performance. For instance, with ITO, reduction can take place, forming metallic indium with a different work function inside the ITO. (See further: J. Muller, "The effects of oxygen on the work function of metal films", Surface Science, vol. 69, No. 2, p 708-711, 1977). It is known that metal-oxides have intrinsically superior barrier properties compared with fluorocarbons, epoxies and silicones. Only pure metals form better barriers. However, these are not useful because they will determine the work function of the stack, to move the critical interface instead of passivating it. For the above reasons, dielectric metal oxides are very suitable. For instance: Al2O3, CaO, Gd2O3, HfO2, La2O3, MgO, Sc2O3, Si3N4, SiO2, Ta2O5, TiO2, Y2O3, ZrO2, and BaZrO3. Two of the above metal oxides are specifically interesting. First, TiO2 because of its exceptionally high dielectric constant (of about 80). This is an advantage because given a certain layer thickness, the capacitive voltage loss in this passivation layer will be lower with a higher dielectric constant. Secondly, plasma-oxidized Al2O3 because of its low defect density. In addition, the barrier layers (6) should be transparent in order to maintain the optical performance of the display. For this reason a thinner layer is preferred, at maximum 100 nm. With plasma-oxidized Al2O3 layers it is possible to achieve a good passivation layer of only 2nm thin (see first embodiment).

Examples The present invention will now be further illustrated by reference to the following examples.

Example 1 Reflective display In one embodiment this invention has been applied in a (direct view) reflective display. On top of both the aluminium and ITO electrodes very thin (only 2 nanometers) layers of plasma oxidized aluminium-oxide have been applied. In Figure 2 the result can be observed, compared with liquid crystal pixel-structures with different electrode materials (aluminium and molybdenum as reflective electrode and ITO as a transparent electrode), without a barrier layer. Compared to the reference cell, the image stability under illumination has improved ten times. This was measured with 1 MegaLux of illumination, which is about ten times accelerated compared to sunlight. The advantage of this thin layer is that the subsequent fabrication steps of the process are not influenced. Example 2 LCoS display In a second embodiment this invention has been applied in an LCoS display. Here, only the ITO electrode has been passivated, with a lOOnm thick layer of aluminium- oxide. Since in the LCoS device the ITO counter-electrode consists of unstructured ITO the application of a thicker layer causes no problems for the subsequent fabrication steps. As apparent in Figure 3, the LCoS panel with single-sided passivation shows a much improved (roughly five times) image stability. Notice that this has been measured on the extremely severe conditions of 25Mlux (250 times sunlight intensity!) illumination intensity, 50 degrees Celsius sample temperature and hundreds of hours stress time. Example 3 Manufacturing method A pixel structure (1) according to the invention may be manufactured by conventional processing of two substrates, meaning (after fabrication of transistors and color filters) depositing the electrodes (2) lithographically. Then one or both plates are fully covered with the passivation layers (6), and on top of that alignment layers (5) are spin coated or screen-printed. Then the two substrates are assembled, with glue and spacers (to maintain a fixed separation usually of 2-4 microns) inbetween. Only a small opening is left, in which finally the fluid liquid crystal molecules (4) are injected, forming the liquid crystal material (3). Afterwards, the opening is closed. Thus, overall there is only one additional processing step, that does not interfere with all the other steps.