CH710226A2 - Hybrid display assembly comprising a solar cell for portable object. - Google Patents

Abstract

The invention relates to a display unit for a portable object, said display unit (1) comprising a first at least partially transparent emitting display device (2) located on the side of an observer, a second device for reflective display (6) and a solar cell (10) being arranged in this order under the first emissive display device (2), the second reflective display device (6) being able to switch between a transparent state in which it displays no information and a reflective state when activated.

Description

Field of the invention

The present invention relates to a hybrid display assembly comprising a solar cell. More specifically, the present invention relates to such a display assembly comprising two superimposed display devices in which is arranged a solar cell.

Technological background of the invention

The readability of the information displayed by the display devices such as LCD display cells or display devices with organic electroluminescent diodes is very dependent on ambient light conditions. With some display devices, the information displayed can be read in good conditions in a bright environment, but are difficult to read in a dark environment. Conversely, other categories of display devices provide a good quality display in dim light or dark, but are difficult to read in daylight.

By way of example, consider the liquid crystal display cells of transflective type, that is to say liquid crystal display cells capable of displaying information that will be visible by day thanks to to a phenomenon of reflection, and which will also be visible in the dark by transmission using a backlight device. Such transflective type liquid crystal display cells are optimized to be able to best reflect the sunlight and thus ensure good readability of the information displayed under conditions of high ambient light. However, so that such transflective liquid crystal display cells can best reflect sunlight, their transmission efficiency is greatly limited. Thus, when the backlighting device is activated in order to read the information displayed in the penumbra, a major part of the light emitted by the backlighting device is lost in absorption phenomena. The energy efficiency is therefore poor. In addition, the optical qualities of the information displayed by the liquid crystal cell are strongly dependent on the angle of view.

With regard to the emissive type display devices such as display devices with organic electroluminescent diodes, they have optical qualities that are greater than those of the liquid crystal display cells, these optical qualities do not depending in particular not on the angle of view. Nevertheless, these high quality emissive type display devices do not allow operation in reflective mode. The information they display is therefore very readable in the dark or dark, but become difficult to read as soon as they are observed outdoors. To remedy this problem, it is possible to increase the amount of current supplied to the emissive display devices in order to increase the brightness of the light-emitting diodes and thus guarantee a minimum of readability. However, even under normal use conditions, these emissive display devices consume more than a reflective type liquid crystal cell. Their power consumption is such that it is difficult to envisage keeping them on permanently, especially when they are embedded in a portable object of small dimensions such as a wristwatch whose only source of energy is a battery of which we usually want it to last at least a year.

Summary of the invention

The present invention aims to remedy the aforementioned problems as well as others by providing a display assembly whose energy requirements can be met even when it is embedded in a portable object of small dimensions such as a wristwatch whose energy reserves are limited. The present invention also provides a display assembly whose operation is satisfactory both in a brightly lit environment and in a dark environment.

For this purpose, the present invention relates to a display assembly for a portable object, this display assembly comprising a first at least partially transparent emitting display device located on the side of an observer, a second device reflective display and a solar cell being arranged in this order under the first emissive display device, the second reflective display device being capable of switching between a transparent state when it is at rest and a reflective state when it is activated.

According to an additional characteristic of the invention, the transparent emissive display device is fixed on the reflective display device.

According to another characteristic of the invention, the transparent emissive display device is glued to the reflective display device by means of a film adhesive or a layer of liquid glue.

With these features, the present invention provides a display assembly for a portable object such as a wristwatch whose operation is optimal regardless of ambient lighting conditions. In broad daylight, the information will preferably be displayed by the reflective display device. Indeed, this reflective display device, taking advantage of a reflection phenomenon of sunlight to display information, is energy efficient. It can therefore remain permanently lit and offers good readability of the information. Conversely, in dim light or darkness, the information will be displayed by the emissive display device. Such an emissive display device consumes more current than a reflective display device, but the information it displays is visible at night or in the dark with very good optical properties which are in particular independent of the angle of view. Moreover, if this type of emissive display device is mainly used at night or in poorly lit environments, its electrical power consumption is nevertheless limited because it is not necessary, under such conditions, to feed it to full power. On the contrary, a low electric power is already sufficient to allow easy reading of the information. Thus, unlike a transflective liquid crystal display cell which seeks to achieve a compromise between the reflectivity of its reflective mode, and the electrical energy consumption of its transmissive backlight device, the display assembly according to the invention proposes to combine two display devices, one purely reflective and the other purely emissive, without compromising the performance of either of these two devices. display. In addition, by teaching to have a solar cell under the two superimposed display devices, the present invention allows the integration of such an assembly in a portable object of small dimensions whose reserves of electrical energy are necessarily limited. Indeed, it has been found that the amount of light that reaches the solar cell through all of the two superimposed display devices is sufficient to provide, by photoelectric conversion phenomenon, the amount of electrical energy required for the operation of the solar cells. two superimposed display devices. Therefore, the electrical energy reserves of the portable object are little, or not solicited by the operation of the two superimposed display devices.

According to one embodiment of the invention, the first display device comprises a transparent emissive display cell, and the second display device comprises a helical nematic type reflective liquid crystal display cell. or super-nematic helical or even vertical alignment. The transparent emissive display cell is optionally an electroluminescent organic diode transparent emissive display cell or an inorganic transparent emissive display cell.

According to a complementary feature of the invention, the electroluminescent organic diode display cell is disposed between a circular polarizer and a quarter wave plate, the circular polarizer being placed on the side of the observer.

Addressing the electroluminescent zones of electroluminescent organic diode display cells is provided by transparent electrodes most often made using a metallic material or a metal oxide. These electrodes therefore often cause slight optical reflection phenomena that induce a degradation of the contrast, which affects the readability of the information displayed by the display cells with organic electroluminescent diodes. To overcome this drawback, the present invention teaches arranging the electroluminescent organic diode display cell between a circular polarizer and a quarter wave plate, the circular polarizer being placed on the observer side. Thus, one of the polarization components of the ambient light that enters the display assembly according to the invention is absorbed by the circular polarizer, while the other polarization component of the light is circularly polarized. When passing through the organic electroluminescent diode display cell, the circularly polarized ambient light is partially reflected by the transparent electrodes of the organic light emitting diode display cell, this reflected light is phase shifted, which has the effect of transforming its circular polarization into circular polarization of opposite direction of rotation. Thus, when the reflected light crosses again the circular polarizer, it is absorbed by the latter. In this way, it is possible to eliminate the stray light reflected on the electrodes of the organic electroluminescent diode display cell, and to retain only light passing through the electroluminescent organic diode display cell without modification. . As a result, the light is again linearly polarized after passing through the quarter-wave plate placed beneath the organic electroluminescent diode display cell and will eventually be absorbed or reflected by the reflective liquid crystal display cell according to that one looks for a display with positive or negative contrast.

Brief description of the figures

Other features and advantages of the present invention will emerge more clearly from the detailed description which follows of an embodiment of the display assembly according to the invention, this example being given purely for illustrative purposes and not limiting only in connection with the appended drawing in which: <tb> Fig. 1 <SEP> is a schematic sectional view illustrating a display assembly according to the invention comprising a first at least partially transparent emitting display device located on the observer side, a second reflective display device and a cell solar array being arranged in this order under the first emissive display device; <tb> fig. 2 <SEP> is a sectional view of an exemplary embodiment of a display assembly according to the invention in which the first display device is a transparent emissive display cell with organic electroluminescent diodes, and the second display device is a reflective liquid crystal display cell of helical nematic type, a solar cell being disposed under this display assembly; <tb> figs. 3A to 3D <SEP> schematically illustrate the operating mode of the display assembly illustrated in FIG. 2 according to whether the electroluminescent organic diode display cell and the helical nematic liquid crystal display cell are active or passive; <tb> fig. 4 <SEP> is a view similar to that of FIG. Wherein the second display device is a vertically aligned reflective liquid crystal display cell; <tb> figs. 5A to 5D <SEP> schematically illustrate the operating mode of the display assembly illustrated in FIG. 4 depending on whether the organic electroluminescent diode display cell and the vertically aligned liquid crystal display cell are active or passive, and <tb> fig. 6 <SEP> is a view similar to that of FIG. 2, except that, in order to suppress the parasitic reflections and thereby improve the display contrast, above the transparent organic light-emitting diode display cell, on the observer's side, is placed circular polarizer which consists of a second absorbing polarizer and a first quarter wave plate.

Detailed description of an embodiment of the invention

The present invention proceeds from the general inventive idea of providing a display assembly that is able to legibly display information both in daylight and in twilight or darkness and whose electrical energy consumption is optimal. To achieve this objective, the present invention teaches combining an emissive type display device with a display device which is arranged to be able to switch between a state of rest in which it is transparent and an active state in which it is capable of to reflect the ambient light. The emissive type display device is typically an organic electroluminescent diode display cell, while the reflective type display device is typically a liquid crystal display cell. For displaying information in broad daylight, the use of the reflective type of display device which, by reflection of sunlight, makes it possible to display the information in a clear and readable manner while consuming few electric energy. For the display of information in dim light or darkness, the use of the emissive display device is preferred. Thanks to its excellent optical properties, especially in terms of contrast and color reproduction, such an emissive display device makes it possible to display a large amount of information in a very readable manner. In particular, the readability of the information displayed is not dependent on the angle of view. In addition, in the dark or dark, it is possible to significantly reduce the power consumption of such an emissive display device while ensuring good readability of the information displayed. Thus, there is provided a display assembly which includes a reflective display device placed at the base of the stack and which is capable of displaying information continuously while consuming very little energy, and a display device. emissive display placed on the top of the stack and which is able to display information on demand in a very readable way in dim light or darkness. It was further realized that by placing a solar cell under the two superposed display devices, the solar cell produced, by photoelectric conversion effect, an electric current sufficient to allow the operation of the two superimposed display devices. . It is therefore possible to integrate such a display assembly in a portable object of small dimensions such as a wristwatch whose storage capacity in electrical energy are however limited.

FIG. 1 is a schematic sectional view of a display assembly according to the invention. Designated as a whole by the general numerical reference 1, this display assembly comprises a first at least partially transparent emitting display device 2 arranged on the side of an observer 4, and a second reflective display device 6 also at the less partially transparent disposed under the first emissive display device 2. For the purposes of the present invention, the first emissive display device 2 is capable of switching between a passive state in which it is at least partially transparent, and an active state in which it emits light to display information. As for the second reflective display device 6, it is able to switch between a passive state in which it is transparent and an active state in which it is able to reflect the ambient light.

Preferably, but not absolutely, the first emissive display device 2 is secured to the second reflective display device 6 by means of a transparent adhesive layer 8. This transparent adhesive layer 8 may be formed of an adhesive in film or a layer of acrylic or silicone liquid glue. This transparent adhesive layer 8 is intended to avoid the problems of spurious reflections that would occur if the two display devices 2, 6 were separated by a layer of air and which would degrade the optical quality of the display assembly 1 according to the invention.

Finally, a solar cell 10 capable of supplying electrical energy by exploiting the photoelectric conversion phenomenon is disposed under the second reflective display device 6. As a preferred but non-limiting example, the solar cell 10 is stuck under the second reflective display device 6 by means of a transparent adhesive layer 12.

In all that follows, the invention will be described with reference to a transparent emissive display cell of the electroluminescent organic diode type. It will be understood, however, that this example is given purely by way of illustration and is not limiting only and that other types of transparent emissive display cells such as transparent inorganic emissive display cells still known by their Anglo-Saxon name Electro Luminescent Display or ELD can be envisaged without departing from the limits of the invention as defined by the appended claims.

FIG. 2 is a detailed sectional view of an exemplary embodiment of the display assembly 1 according to the invention in the case where the first emissive display device 2 comprises a transparent emissive display cell 20 with organic electroluminescent diodes. which will be referred to in the following as the Transparent Organic Light Emitting Diode (TOLED) transparent display cell. As for the second reflective display device 6, it comprises a reflective liquid crystal display cell 60 of helical nematic type, also known by its Anglo-Saxon name Twist Nematic or TN.

More specifically, the transparent display unit TOLED 20 comprises a transparent substrate 21 made of glass or a plastic material and an encapsulation cover 22 which extends parallel to and away from the transparent substrate 21. The substrate 21 and the encapsulation cover 22 are joined together by a sealing frame 23 which delimits a closed volume protected from air and moisture for the confinement of a stack of electroluminescent layers generally designated by 24. A transparent upper electrode 25 made for example of tin-indium oxide or ITO and a transparent lower electrode 26 made for example by means of a metallic material such as aluminum or silver or of a metal oxide such as ITO or zinc-indium oxide are structured on either side of the stack of the electroluminescent layers 24. electrodes 25, 26, made of a metallic material, are slightly reflective. The organic electroluminescent light-emitting diode display cells are available either with direct addressing in cases where it is simply a question of displaying icons or segments, or with a passive matrix type addressing in the case of a dot matrix display. In the case of a dot matrix display, it is also possible to deal with an active matrix type addressing combined with transparent transistors of the Thin Film Transistor or TFT type intended to control the current and which are arranged in the pixels of the matrix. display.

On the other hand, the reflective liquid crystal display cell 60 comprises a front substrate 61 disposed on the side of the observer 4 and a rear substrate 62 which extends parallel to and away from the front substrate 61. The front 61 and rear 62 substrates are joined together by a sealing frame 63 which delimits a sealed enclosure 64 for the confinement of a liquid crystal whose optical properties are modified by applying an appropriate voltage to a crossover point considered. between transparent electrodes 65a formed on a lower face of the front substrate 61 and transparent counter-electrodes 65b formed on an upper face of the rear substrate 62. The electrodes 65a and the counter-electrodes 65b are made of an electrically conductive transparent material such that of indium-zinc oxide or indium-tin oxide, the latter material being better known under its name. appointment Anglo-Saxon Indium Tin Oxide or ITO.

In the case of the present invention, all the liquid crystal phases such as helical nematic (Twist Nematic or TN in English terminology), super-nematic helix (Super Twist Nematic or STN in English terminology). ) or else vertically aligned (Vertically Aligned or VA in English terminology) can be envisaged. Similarly, all types of addressing such as direct addressing, addressing by active matrix or multiplex addressing of a passive matrix can be envisaged.

An absorbent polarizer 30 is adhered to an upper face of the front substrate 61 of the reflective liquid crystal display cell 60 by means of an adhesive layer 32. This adhesive layer 32 may be formed of a film adhesive or a layer of liquid glue. The adhesive used to secure the absorbing polarizer 30 on the reflective liquid crystal display cell 60 may be transparent or slightly diffusing depending on whether one seeks to obtain specular or diffuse reflection. As for the absorbing polarizer 30, it may be, for example, of the iodine or dye type.

An absorbent reflective polarizer 34 is bonded to a lower face of the rear substrate 62 of the reflective liquid crystal display cell 60 by means of an adhesive layer 36 which may be transparent or slightly diffusing depending on whether one is looking for to obtain specular or diffuse reflection.

We now examine in conjunction with FIGS. 3A to 3D the operating principles of the display assembly 1 according to the invention according to whether the TOLED transparent display cell 20 and the reflective liquid crystal display cell 60 are in use or not. It will be assumed, by way of purely illustrative and in no way limiting example, that the reflective liquid crystal display cell 60 is a nematic TN helical liquid crystal cell and that the transmission axis of the absorbing polarizer 30 and the reflection axis of the reflective polarizer 34 are parallel.

In FIG. 3A, the TOLED transparent display cell 20 and the TN 60 reflective liquid crystal display cell are both turned off. The ambient light, denoted by the numeral 46, passes through the transparent display cell TOLED 20 without modification, and is then polarized linearly by the absorbing polarizer 30. The ambient light 46 is then rotated by 90 ° as it passes through the cell reflective liquid crystal display TN 60, so that when it falls on the reflective polarizer 34, its polarization direction is perpendicular to the reflection axis of the reflective polarizer 34 and therefore crosses the latter before being absorbed by the solar cell 10. The reflective liquid crystal display cell TN 60 therefore appears dark when off, which means that the information it will display will appear in the light on a dark background. The display of information is therefore in negative contrast. Of course, an information display in positive contrast could be achieved simply by ensuring that the transmission axis of the polarizer 30 and the reflection axis of the reflective reflective polarizer 34 are perpendicular. In this case, the ambient light 46 reaches the solar cell only in the areas of the TN reflective liquid crystal display cell 60 that are switched.

In FIG. 3B, the transparent display unit TOLED 20 is activated, while the reflective liquid crystal display unit TN 60 is deactivated. The light emitted by the TOLED transparent display cell 20 reaches the observer 4 without modification, while the TN 60 reflective liquid crystal display cell appears dark. The information displayed by the transparent display unit TOLED 20 therefore stands out against a dark background.

In FIG. 3C, the transparent display unit TOLED 20 is off, while the reflective liquid crystal display unit TN 60 is activated. As already explained above, the unswitched areas of the TN 60 reflective liquid crystal display cell appear dark. On the other hand, in the switched areas of the TN 60 reflective liquid crystal display cell, the ambient light 46 passes through these zones without modification, so that the ambient light 46 falls on the reflective polarizer 34 with a direction of polarization parallel to the reflection axis of this reflective polarizer 34. The ambient light 46 is reflected back and successively passes through the reflective liquid crystal display unit TN 60, the absorbing polarizer 30 and the transparent display cell TOLED 20 unchanged , so that it is perceptible by the observer 4. The information is displayed in clear on a dark background.

In FIG. 3D, the TOLED transparent display cell 20 and the TN 60 reflective liquid crystal display cell are both activated. The light emitted by the transparent display unit TOLED 20 is directly perceptible by the observer 4. The ambient light 46 which passes through the unswitched areas of the TN 60 reflective liquid crystal display cell is absorbed by the solar cell 10 , so these areas appear dark. Finally, the ambient light 46 which passes through the switched areas of the TN 60 reflective liquid crystal display cell is reflected by the reflective polarizer 34, so that these areas appear clear.

FIG. 4 is a sectional view of an exemplary embodiment of the display assembly 1 according to the invention in the case where the first display device 2 comprises a transparent emissive display cell 20 with organic electroluminescent diodes which will be hereinafter referred to as Transparent Organic Light Emitting Diode (TOLED) Transparent Display Cell. As for the second display device 6, it comprises a reflective liquid crystal display 600 vertically aligned, also known by its Anglo-Saxon Vertically Aligned or VA.

We now examine in conjunction with Figs. 5A to 5D the principles of operation of the display unit 1 according to the invention according to whether the transparent display unit TOLED 20 and the reflective liquid crystal display unit VA 600 are in use or not. It will be assumed, by way of purely illustrative and in no way limiting example, that the transmission axis of the absorbing polarizer 30 and the reflection axis of the reflective polarizer 34 are perpendicular.

In FIG. 5A, the TOLED transparent display cell 20 and the VA 600 reflective liquid crystal display cell are both turned off. The ambient light, denoted by the reference numeral 46, passes successively through the transparent display unit TOLED 20 and the reflective liquid crystal display unit VA 600, so that when it falls on the reflective polarizer 34, its polarization direction is perpendicular to the axis of reflection of the reflective polarizer 34. It crosses the latter without modification and is ultimately absorbed by the solar cell 10. The TN 60 reflective liquid crystal display cell appears dark when it is off, which means that the information it will display will appear in the clear on a dark background. The display of information is therefore in negative contrast. Of course, a display of information in positive contrast can be achieved simply by ensuring that the transmission axis of the absorbing polarizer 30 and the reflection axis of the reflective polarizer 34 are parallel.

In FIG. 5B, the transparent display unit TOLED 20 is activated, while the reflective liquid crystal display unit VA 600 is deactivated. The light emitted by the TOLED transparent display cell 20 reaches the observer 4 without modification, while the TN 60 reflective liquid crystal display cell appears dark. The information displayed by the transparent display unit TOLED 20 therefore stands out against a dark background.

In FIG. 5C, the transparent display unit TOLED 20 is off, while the reflective liquid crystal display unit VA 600 is activated.

In a vertically aligned liquid crystal cell, the alignment layers are oriented at 45 ° with respect to the polarization axes of the polarizers. On the other hand, the product result between the birefringence of the liquid crystal molecules and the distance between the front and back substrates is chosen so that, when the liquid crystal is switched, it behaves vis-à-vis the direction polarization of light as a half-wave plate. Therefore, since this half-wave plate is placed at 45 ° with respect to the polarization axis of the absorbing polarizer, it causes a 90 ° rotation of the polarization direction of the light. Thus, the ambient light 46 is rotated by 90 ° as it passes through the switched areas of the VA 600 reflective liquid crystal display cell, so that when it falls on the reflective polarizer 34, its polarization direction is parallel to the reflection axis of the reflective polarizer 34 and is therefore reflected by the latter. As for the ambient light 46 which passes through the unswitched zones of the reflective liquid crystal display unit VA 600, it is absorbed by the solar cell 10. The information is displayed in clear on a dark background, it is to say with a negative contrast.

In FIG. 5D, the TOLED transparent display cell 20 and the TN reflective liquid crystal display cell 60 are both enabled. The light emitted by the transparent display unit TOLED 20 is directly perceptible by the observer 4. The ambient light 46 which passes through the unswitched areas of the reflective liquid crystal display unit VA 600 is absorbed by the solar cell 10 , so these areas appear dark. Finally, the ambient light 46 passing through the switched areas of the VA 600 reflective liquid crystal display cell is reflected by the reflective polarizer 34 so that these areas appear clear.

FIG. 6 is a view similar to that of FIG. 2 except that, for the purpose of suppressing parasitic reflections and thus to improve the display contrast, a polarizer is placed above the transparent display unit TOLED 20, on the side of the observer 4 circular 38 which consists of a second absorbing polarizer 40 and a first quarter wave plate 42. On the other hand, a second quarter wave plate 44 is placed under the transparent display cell TOLED 20. second quarter wave plate 44 is parallel to the first quarter wave plate 42 or else disposed at 90 ° with respect to the first quarter wave plate 42. It will be assumed that the transmission axis of the second absorbing polarizer 40 and the reflection axis of the reflective polarizer 34 are perpendicular.

Addressing the electroluminescent zones of electroluminescent organic diode display cells is provided by transparent electrodes most often made using a metallic material or a metal oxide. These electrodes therefore often cause optical reflection phenomena which induce a degradation of the contrast, which affects the readability of the information displayed by the organic light emitting diode display cells. To overcome this drawback, the present invention teaches to have a circular polarizer 38 above the TOLED transparent display cell 20 and a second quarter wave plate 44 under the transparent display cell TOLED 20. Thus, the ambient light 46 which enters the display assembly 1 according to the invention is polarized linearly by the second absorbing polarizer 40, and then circularly polarized by the first quarter wave plate 42. When crossing the transparent display cell TOLED 20, the circularly polarized ambient light 46 is partially reflected by the transparent upper and lower electrodes 25, 26 of the transparent display cell TOLED 20, this reflected light is phase shifted, which has the effect of transforming its circular polarization into circular polarization of opposite direction of rotation. Thus, when the reflected light crosses again the circular polarizer 38, it is absorbed by the latter. In this way, it is possible to eliminate the stray light that is reflected on the electrodes 25,26 of the transparent display cell TOLED 20. The remaining ambient light 46 passes through the transparent display cell TOLED 20 without modification. is polarized linearly as it passes through the second quarter wave plate 44 in a direction perpendicular to the transmission axis of the second absorbing polarizer 40. It is assumed that the first and second quarter wave plates 42 and 44 are parallel to each other. As ambient light 46 passes through the reflective liquid crystal display cell 60, the direction of polarization of the ambient light 46 is rotated 90 °, so that it is finally absorbed by the solar cell 10.

The display is done in clear on a dark background. In other words, the display unit 1 is in negative contrast. Indeed, in the switched areas of the reflective liquid crystal display cell 60, the ambient light 46 passes through the reflective liquid crystal display cell 60 without modification, so that it falls on the reflective polarizer 34 in accordance with FIG. direction of polarization which is parallel to the axis of reflection of the latter. The ambient light 46 is thus reflected by the reflective polarizer 34, then passes through the reflective liquid crystal display cell 60 without modification. The ambient light 46 is then circularly polarized by the second quarter-wave plate 44, then passes through without modification the circular polarizer 38 and is perceptible by the observer 4.

As a variant, it is possible to envisage arranging the second quarter-wave plate 44 between the reflective liquid crystal display cell 60 and the absorbent reflective polarizer 34.

It goes without saying that the present invention is not limited to the embodiments which have just been described and that various modifications and simple variants can be envisaged by those skilled in the art without departing from the scope of the invention. as defined by the claims appended to this patent application. It will be understood in particular that to say that the ambient light passes through without modification the reflective liquid crystal display cell or the transparent display cell TOLED is somewhat of a misnomer. Indeed, when the ambient light passes through these display cells, there always occurs minimal phenomena of parasitic reflection of the light. These parasitic reflections are nevertheless quite negligible in the context of the present invention. It will also be understood from what has been said above that speaking of "transparent" electrodes is also somewhat of an abuse of language. Indeed, although made of a transparent electrically conductive material, these electrodes are still very slightly reflective. The reflective liquid crystal display cell is selected from the group consisting of helical nematic-type liquid crystal display cells, helical super-nematic type liquid crystal display cells, and display cells. liquid crystal type vertical alignment. The reflective liquid crystal display cell may be a bistable display cell.

Nomenclature

[0042] <tb> Display Set <SEP> 1 <tb> First emissive display device <SEP> 2 <Tb> Observer <September> 4 <tb> Second Reflective Display <SEP> 6 <tb> Transparent adhesive layer <SEP> 8 <tb> Solar cell <SEP> 10 <tb> Transparent Adhesive Layer <SEP> 12 <tb> TOLED Transparent Display Cell <SEP> 20 <tb> Transparent Substrate <SEP> 21 <tb> Encapsulation cover 22 Seal frame <SEP> 23 <tb> Stack of electroluminescent layers <SEP> 24 <tb> Transparent Upper Electrode <SEP> 25 <tb> Transparent lower electrode <SEP> 26 <tb> Absorbent polarizer <SEP> 30 <tb> Adhesive layer <SEP> 32 <tb> Reflective Reflective Polarizer <SEP> 34 <tb> Adhesive layer <SEP> 36 <tb> Circular Polarizer <SEP> 38 <tb> Second Absorbing Polarizer <SEP> 40 <tb> First quarter-wave plate <SEP> 42 <tb> Second quarter wave plate <SEP> 44 <tb> Ambient light <SEP> 46 <tb> Reflective liquid crystal display panel TN <SEP> 60 <tb> Substrate before <SEP> 61 <tb> Rear Substrate <SEP> 62 <tb> Sealing framework <SEP> 63 <tb> Watertight enclosure <SEP> 64 <tb> Transparent Electrodes 65a <SEP> 65a <tb> Transparent counter electrodes 65b <SEP> 65b <tb> Reflective liquid crystal display unit VA 600 <SEP> 600

Claims (11)

1. A display unit for a portable object, said display unit (1) comprising a first at least partially transparent emitting display device (2) located on the side of an observer (4), a second device reflective display (6) and a solar cell (10) being arranged in this order under the first emissive display device (2), the second reflective display device (6) being able to switch between a transparent state in which it displays no information and a reflective state when activated. 2. Display assembly according to claim 1, characterized in that the emissive display device (2) is stuck on the reflective display device (6). Display assembly according to claim 2, characterized in that the emissive display device (2) is adhered to the reflective display device (6) by means of a film adhesive or a film coating. liquid glue. Display assembly according to one of claims 1 to 3, characterized in that the first emissive display device (2) comprises a transparent emissive display cell, and in that the second display device reflective (6) comprises a reflective liquid crystal display cell (60). Display assembly according to claim 4, characterized in that the transparent emissive display cell is a light emitting organic electroluminescent diode display cell (20) or an inorganic transparent emissive display cell. 6. Display assembly according to any one of claims 4 or 5, characterized in that the electroluminescent organic diode transparent emissive display cell (20) comprises a stack of electroluminescent layers (24) sandwiched between an electrode transparent upper (25) and a transparent lower electrode (26) 7. Display assembly according to any one of claims 4 to 6, characterized in that the reflective liquid crystal display cell (60) is selected from the group consisting of helical nematic type liquid crystal cells. , the helical super-nematic type liquid crystal cells and the vertically aligned liquid crystal display cells, and that the addressing of these liquid crystal display cells can be of the direct type, of the type by active matrix or multiplex addressing type of a passive matrix. Display assembly according to one of Claims 4 to 7, characterized in that an absorbing polariser (30) is arranged on an upper surface of the reflective liquid crystal display cell (60), and a reflective polarizer (34) is disposed under a bottom face of the rear substrate (62) of the reflective liquid crystal display cell (60). 9. Display assembly according to any one of claims 4 to 8, characterized in that a circular polarizer (38) which consists of a second absorbing polarizer (40) and a first quarter wave plate (42) is placed on top of the organic electroluminescent diode transparent emissive display cell (20), and that a second quarter wave plate (44) is placed under the transparent emissive display cell at organic electroluminescent diodes (20). Portable object comprising a display assembly according to any one of the preceding claims. 11. Portable object according to claim 11, characterized in that it is a wristwatch.