Abstract:
The present invention is directed to an integrated, reflective bistable, cholesteric liquid crystal display and solar cell assembly providing electrical energy to power display electronics. The liquid crystal display includes the layer of cholesteric liquid crystal material sandwiched between first and second transparent substrates. An inner surface of the first substrate and an inner surface of the second substrate bound the layer of liquid crystal material and the first substrate is closest to a viewer of the display. A first set of conductive electrodes is disposed on the inner surface of the first substrate and a second set of conductive electrodes is disposed on the inner surface of the second substrate. Display driver circuitry is electrically coupled to the first and the second sets of conductive electrodes for generating desired voltage differentials between electrodes of the first set of conductive electrodes and electrodes of the second set of conductive electrodes. A solar cell assembly that is provided includes a solar cell or solar panel positioned behind the second substrate and electrically coupled to a rechargeable energy storage device, such as a rechargeable battery. The solar cell receives illumination passing through the first substrate, liquid crystal material and the second substrate and converts the illumination incident on the solar cell to electrical energy to supply power to the rechargeable energy storage device. The cholesteric liquid crystal material permits transmission of incident light regardless of the configuration of the liquid crystal material.

Description:
FIELD OF THE INVENTION 
     The present invention relates to liquid crystal display device which functions both as a cholesteric reflective flat-panel display and a solar cell assembly to provide electrical energy to power the display and associated electronics and, more particularly, to a liquid crystal display device including a passive matrix, bistable, cholesteric liquid crystal display having one or more solar cells disposed in alignment with the display and adjacent a substrate bounding a layer of cholesteric liquid crystal material, the one or more solar cells providing electrical energy to power display electronics. 
     BACKGROUND OF THE INVENTION 
     Typically, a reflective liquid crystal display comprises a single layer of liquid crystal material sandwiched between adjacent inner surfaces of generally planar substrates. In matrix type liquid crystal displays, on the inwardly facing surface of one of the substrates is disposed a set or array of parallel column electrode segments (column electrodes) and on an inwardly facing surface of the other of the substrates is a set or array of parallel row electrode segments (row electrodes), extending generally perpendicular to the column electrodes. The row and column electrode segments (also referred to as “row and column electrodes”) are spaced apart by the thin layer of liquid crystal material. Display picture elements or pixels are defined by regions of liquid crystal material adjacent the intersection of the row and column electrode segments. 
     Upon application of a suitable electric field, a pixel of a display will assume either a reflective or a non-reflective state. A pixel, P(xi,yj), formed at the overlapping or intersection of the ith row electrode segment and the jth column electrode segment is subject to an electric field resulting from the potential difference between a voltage applied to the ith row electrode segment and a voltage applied to the jth column electrode segment. 
     Recent advances in liquid crystal material research have resulted in the discovery of bistable cholesteric (also referred to as chiral nematic) liquid crystal display devices. Cholesteric liquid crystal display materials are able to maintain a given reflective state (reflective or nonreflective) without the need for the constant application of an electric field. In a reflective cholesteric liquid crystal display, the reflectivity of an image pixel depends upon the configuration or texture of the liquid crystal material (e.g., planar, focal conic, homeotropic configurations) defining the image pixel. Moreover, the state of the liquid crystal material may be changed upon imposing an appropriate electric field across the liquid crystal material for an appropriate period of time. This is accomplished by appropriately energizing the row and column electrodes defining an image pixel so as to generate an electric field having a desired magnitude (that is, a desired root mean square (rms) voltage) for a desired period of time. If the panel or substrate furthest from the viewer is painted with a black material, a pixel with a low reflectance or nonreflective state will appear as a black area to the viewer. A pixel in a high reflectance state will appear to the viewer as a visible colored area in the display. 
     Display driver circuitry is coupled to the vertical and horizontal electrodes. Operating under the control of a logic and control unit, the display driver circuitry energizes the row and column electrodes with appropriate voltage waveforms such that an appropriate voltage across each pixel is generated. The voltage across a pixel will either cause it to remain in its present state of reflectance or change its state of reflectance. The image generated by the display pixels may be modified by changing the state of selected pixels. In this way, text or image data can be presented for viewing on the display. 
     Certain prior art calculators and watches have included both a liquid crystal display and a solar cell assembly to provide electrical energy to the device electronics. However, in such devices, the display and the solar cell or cells have been disposed in different areas of the device, that is, the display area and the solar cell area do not overlap. This requires a device with a surface area large enough to accommodate both the solar cell area and the area of the display. This type of configuration is disadvantageous in small sized hand held devices were surface area is at a premium. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to a cholesteric liquid crystal display utilizing a solar cell assembly as a power source for powering the display electronics. The display includes a front substrate, closest to a viewer, a back substrate and a thin layer of cholesteric liquid crystal material sandwiched therebetween. On an inner surface of the front substrate (that is, the surface of the substrate adjacent the liquid crystal material) is disposed a set or array of electrode segments and on an inner surface of the back substrate is a set or array of electrode segments, extending generally perpendicular to the column electrode array. In one configuration of the present invention, the display is a matrix display and one set of electrode segments comprises a set of parallel row electrode segments and the other set of electrode segments comprises a set of parallel column electrode segments. The row and the column electrode segments are substantially orthogonal. This arrangement of perpendicular row and column electrodes results in an orthogonal pattern of image pixels (orthogonal display). It should also be appreciated that the present invention is equally suited to providing power to other types of liquid crystal displays in addition to orthogonal displays such as, for example, segmented displays. While a segmented display also includes electrodes which may be fabricated in various shapes and disposed in orthogonal or non-orthogonal orientations to generate desired image configurations. For example, in a segmented liquid crystal display electrode segments may advantageously be disposed to create a seven segment numerical display. In other segmented displays, electrode segments of irregular shape may be used to generate an icon on the display. Moreover, it should additionally be appreciated that the concept of the present invention of using a solar cell assembly in a cholesteric liquid crystal display to provide power to display electronics is equally applicable to active matrix cholesteric liquid crystal displays in addition to passive matrix cholesteric liquid crystal displays. An actively driven matrix cholesteric liquid crystal display is one in which where each of the image pixels is driven individually by an active circuit component, e.g., a transistor. An active matrix Ch-LCD is disclosed in an article entitled “Amorphous Silicon Thin-Film Transistor Active-Matrix Reflective Cholesteric Liquid Crystal Display,” authored by J. Y. Nahm, T. Goda, B. H. Min, T. K. Chou, J. Kanicki, X. Y. Huang, N. Miller, V. Sergan, P. Bos and J. W. Doane and published in the Proceedings of the 18th International Display Research Conference, Seoul, Korea, September 1998, pages 979-982. The aforesaid active matrix Ch-LCD article is incorporated herein in its entirety by reference. 
     In one embodiment of an orthogonal Ch-LCD, display drive electronics include a set of row driver electronics electrically coupled to the row electrode segments that controls energization of all of the row electrodes and a set of column driver electronics electrically coupled to the column electrode segments that controls energization of all of the column electrode segments in the plurality of sets of column electrodes. The sets of row and column driver electronics constitute a single set of drive electronics. 
     The cholesteric or chiral nematic liquid crystal material is unique and advantageous in that it permits illumination incident on the display to pass through the liquid crystal material and impinge upon the solar cell. Typical nematic liquid crystal material displays such as twisted nematic (TN) or supertwisted nematic (STN) are dissimilar in that they function as a light shutter wherein light that passes through the nematic material is reflected toward the viewer by a reflector in back of the layer of nematic material. However, cholesteric liquid crystal material permits a percentage of incident radiation to pass through the liquid crystal material whether the material is in its highly reflective state (corresponding to the twisted planar configuration of the material) or in its low reflectance state (corresponding to focal conic configuration), or any state therebetween. Radiation that passes through pixels of the display in the focal conic state is absorbed by a black layer at the back of the display for providing contrast with light reflected from pixels in the reflective twisted planar state. 
     The cholesteric liquid crystal display (Ch-LCD) technology, be it an active matrix, passive matrix or segmented display, is ideal for the incorporation of a solar cell assembly in that most of the light incident on the display is available for generating electrical energy. This is not true for other reflective liquid crystal display technologies where only a few percent, if any, is available. In a Ch-LCD, the incident light is reflected by the liquid crystal material itself. In the reflective state, incident light is decomposed into its right and left circular components with only one of the components or 50% of the light being reflected. The other circular component of incident light passes through the liquid crystal material. A monochrome Ch-LCD therefore only reflects 50% of one color with a bandwidth of about 100 nanometers (nm.), the rest of the visible light, i.e., the other colors and the other non-reflected circular component are available for conversion to electrical power by the solar cell assembly. All of the incident light that passes though the cholesteric liquid crystal material can be equal to about 75% of incident light (or incident light intensity) for a monochrome Ch-LCD. Even in a full color Ch-LCD, which is comprised of stacked liquid crystal cells, those colors that are not being reflected in a particular image pixel are available for conversion to electrical power by the solar cell assembly and in such Ch-LCDs, about 65% to 75% of the incident light can be available for conversion to electrical energy by the solar cell assembly, depending on the image. 
     The cholesteric Ch-LCD display of the present invention advantageously utilizes the solar cell assembly in contrast to twisted nematic (IN), supertwisted nematic (SIN), ferroelectric (FLC) and other liquid crystal displays which use polarizers to generate an image on the display. In such displays that make use of polarizers, 50% of the incident light is absorbed by a polarizer and not available for conversion to electrical power. Also, such displays use a mirror on the backplane or back substrate to reflect the incident light (as opposed to the cholesteric liquid crystal material reflecting a specific color) and that reflected light is similarly not available for conversion to electrical power. Likewise, guest host type liquid crystal displays absorb light not being reflected and are, therefore, unsuitable for advantageously utilizing a solar cell assembly to supply power to the display electronics. 
     Cholesteric liquid crystal displays also have another advantage over other display technologies in that they possess bistable memory and do not require any electric power at all to maintain an image on the display, electric power is only required to change the image, i.e., change the reflective state of selected image pixels. Liquid crystal displays utilizing TN and STN technologies need to be refreshed about 60 times per second to maintain an image on the display. In devices that are not required to show moving video images, the power consumption is substantially less in Ch-LCDs than other liquid crystal technologies. Thus, electrical energy provided by a solar cell assembly theoretically does not have to be “used” to refresh the display image if the image does not change, instead, such electrical energy is available for powering other electronics of the display. 
     The foregoing features make Ch-LCD uniquely advantageous for utilizing a solar cell assembly. The combination of LCD and solar cell assembly is especially attractive for hand held and other portable devices wherein the combination of Ch-LCD and solar cell assembly will facilitate size reduction of the device. 
     In a first preferred embodiment of the present invention, a light absorbing solar panel assembly comprising one or more solar cells is affixed to the outer surface of a transparent back substrate (that is, the surface of the back substrate away from the liquid crystal material). This approach is useful for solar cells whose front surface is an appropriate black or dark color as to provide contrast for pixels in the focal conic configuration (low reflectance state). Preferably, a thin layer of index matching optical material is applied between the solar panel assembly and the back substrate to provide suitable optical coupling and to reduce reflections at the substrate/solar cell interface. 
     In a second preferred embodiment of the present invention, a visibly blackened or colored but infrared (IR) transmissive layer or coating comprising IR transmissive ink is applied to the outer surface of the back substrate and a light absorbing solar cell is affixed to the outer surface of the back substrate, that is, the IR transmissive layer is sandwiched between the outer surface of the back substrate and the solar cell. The IR transmissive layer is provided upstream of the solar cell to ensure proper contrast in the event that a solar cell having an undesirable reflectivity is used. The IR transmissive layer provides a dark background for pixels in the focal conic state and absorbs visible light (in the range of 0.38 to 0.78 micrometers (μm.)) but allows a majority of radiation having wavelengths in the near infrared range (typically 0.75 μm. to 1.5 μm.) as well as radiation of the middle and far infrared ranges (typically 1.5 μm. to 1000 μm.) and greater to be absorbed by the solar cell assembly. 
     In a third preferred embodiment of the present invention, the solar cell assembly of the display includes a solar panel assembly which functions as the back substrate or panel of the display. Advantageously, the solar panel assembly includes one or more solar cells comprising a plastic or glass base material with a solar radiation absorbing material coated or bonded on an outwardly facing surface of the base material. The opposite surface of the base material will include horizontally spaced apart ITO electrode segments affixed thereto such that the solar cell and the electrode segments share an opposite side of a common base material. 
     In a fourth preferred embodiment of the present invention, the display is a stacked reflective cholesteric liquid crystal display, for example a triple stacked display providing a color display an RGB (red, green and blue) color display with a solar cell assembly functioning as a back substrate or panel of the display. Alternately, the display could be a double stacked display offering the advantage of improved brightness for the pixels in the “on” or reflective state or for providing night vision capabilities. 
     In one aspect of the present invention, a liquid crystal display is disclosed comprising: a) a layer of chiral nematic liquid crystal material; b) first substrate and a spaced apart second substrate, an inner surface of the first substrate and an inner surface of the second substrate bounding said liquid crystal material layer, the first substrate being closer than the second substrate to a viewer of the display; c) a first set of conductive electrodes disposed on the inner surface of the first substrate and a second set of conductive electrodes spaced apart from the first set of electrodes and disposed on the inner surface of the second substrate bounding said liquid crystal material layer; d) display driver circuitry electrically coupled to the first and the second set of conductive electrodes for generating desired voltage differentials between electrodes of the first set of conductive electrodes and electrodes of the second set of conductive electrodes; and e) a solar cell assembly including one or more solar cells electrically coupled in series and positioned adjacent said second substrate and electrically coupled to the display driver circuitry, the solar cell or cells receiving illumination passing through the first substrate and said liquid crystal material and converting the illumination incident on the solar cell or cells to electrical energy to supply power to the display driver circuitry. 
     The liquid crystal display includes an energy storage device coupled to and providing power to the display driver circuitry. The solar cell assembly is electrically coupled to and supplies power to the energy storage device. Preferably, the solar panel assembly, comprising one or more solar cells, is disposed adjacent an outer surface of the second substrate or, alternatively, comprises the second substrate, i.e., the solar cell is comprised of a plastic or glass material that functions as the second substrate. 
     In another aspect of the present invention, a liquid crystal display device is disclosed comprising chiral nematic liquid crystal material, cell wall structure communicating with liquid crystal material to form focal conic and reflective twisted planar textures that are stable in the absence of an electric field, a solar cell device for converting electromagnetic radiation that has passed through liquid crystal material into electrical energy, and means for applying an electrical field to liquid crystal material to place at least a portion into at least one of the focal conic and twisted planar textures. 
    
    
     These and other objects, features and advantages of the invention will become better understood from the detailed description of the preferred embodiments of the invention which are described in conjunction with the accompanying drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic exploded perspective view of a first preferred embodiment of a bistable, cholesteric liquid crystal display and solar cell assembly of the present invention; 
     FIG. 2 is a sectional view of the display and solar cell assembly of FIG. 1 as seen from a plane indicated by the line  2 — 2  in FIG. 1; 
     FIG. 3 is a schematic exploded perspective view of a second preferred embodiment of a bistable, cholesteric liquid crystal display and solar cell assembly of the present invention; 
     FIG. 4 is a sectional view of the display and solar cell assembly of FIG. 3 as seen from a plane indicated by the line  4 — 4  in FIG. 3; 
     FIG. 5 is a schematic diagram of the display and solar cell assembly of FIG. 1 and a block diagram representation of display driver electronics connected to the display; 
     FIG. 6 is a schematic block diagram representation of the display driver and solar cell electronics of the display and solar cell assembly; 
     FIG. 7 is a schematic representation of a graph illustrating the spectral response of a particular solar cell material; 
     FIG. 8 is a schematic representation of a graph illustrating the transmitivity of a infrared (IR) transmissive ink utilized in an IR transmissive layer or coating of the display; 
     FIG. 9 is a schematic exploded perspective view of a third embodiment of a bistable, cholesteric liquid crystal display and solar cell assembly of the present invention; and 
     FIG. 10 is a schematic exploded perspective view of a fourth embodiment of a stacked, bistable, cholesteric liquid crystal display and solar cell assembly of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Turning now to the drawings, the combination reflective liquid crystal display and solar cell assembly  20  (referred to hereinafter as “display”) of the present invention includes a bistable, cholesteric liquid crystal display including a layer of cholesteric liquid crystal material  40  sandwiched between light transmissive substrates  22 ,  24  (FIGS.  1  and  2 ). The display  20  may be a passive matrix display or an active matrix display. Advantageously, the display  20  further includes a solar cell assembly  300  (shown schematically in FIG. 6) in alignment with the display to provide electrical energy to a rechargeable energy storage device  340  powering display driver circuitry  200  of the display  20 . The solar cell assembly  300  includes a solar panel assembly  310  comprising a pair of solar cells  312 ,  314  electrically coupled in series. Each of the solar cells  312 ,  314  comprises a thin silicon photovoltaic cell that converts electromagnetic radiation directly into electrical energy. 
     The energy generated by the solar cell assembly  300  provides charging power to the energy storage device  340  (schematically shown in FIG.  6 ). This increases the operational time of the display  20  between required charging of the energy storage device  340 . Moreover, regardless of whether the display  20  is energized or not, the solar cell assembly  300  is operational in that it continues to charge or provide energy to the energy storage device  340 . If the energy storage device  340  is fully charged the flow of electrical current from the solar cell assembly  300  to the energy storage device  340  will effectively drop to zero amps given that the output voltage of the solar cell assembly  300  is equal to or greater than the output voltage of the energy storage device  340  at full charge. Depending on the characteristics of the display  20 , such as the size of the liquid crystal display area, the required updating speed of the displayed image (i.e. how often each image pixel is updated) and further depending upon the lighting conditions in which the display  20  is used, the solar cell assembly  300  may, in fact, provide sufficient power to the energy storage device  340  to operate the display  20  without the need for periodic recharging of the energy storage device via nonsolar methods, e.g., use of a AC/DC battery charger to charge the energy storage device. Even if the display  20  is not continuously operable on electrical power generated by the solar cell assembly  300  alone, the electrical power generated by the solar cell assembly  300  and input to the energy storage device  340  provides an extended operating time between energy storage device rechargings compared to not utilizing solar energy. 
     Important to the present invention is the fact that the display  20  uses a layer of cholesteric liquid crystal material  40  (also known as chiral nematic liquid crystal material) which is incident light transmissive (for purposes of reaching the solar cell assembly  300  thereby permitting the conversion of electromagnetic radiation to electrical power) regardless of the configuration of the liquid crystal material  40 . This means that whether the liquid crystal material  40  is in the twisted planar configuration (light reflecting state) or in the focal conic state (non reflecting state) or in any configuration intermediate the reflective and non-reflective states (i.e., gray scale values intermediate reflective and non-reflective states as disclosed in U.S. Pat. No. 5,453,863, issued on May 4, 1993 and entitled “Multistable Chiral Nematic Displays,” which is incorporated herein in its entirety by reference), at least a portion of the light incident on the liquid crystal material  40  is transmitted through the material to the solar cell assembly  300 . For example, a cholesteric liquid crystal layer of a thickness of 5 micrometers (μm.) and first and second substrates  22 ,  24  subject to perpendicularly incident visible light transmits about 92% of the intensity of the incident light through the liquid crystal material and the substrates  22 ,  24  to the solar cell assembly  300  in the focal conic configuration (assuming approximately 4% reflection from each of the first and second substrates  22 ,  24 ). In the twisted planar configuration, approximately 75% of the intensity of the incident light passes through the liquid crystal material and the substrates  22 ,  24  to the solar cell assembly in the twisted planar configuration. 
     In contrast, twisted nematic (TN) or super twisted nematic (STN) liquid crystal material does not effectively transmit an appreciable proportion of incident visible light in all states of the material. Furthermore, cholesteric liquid crystal material also effectively transmits to the solar cell assembly  300  electromagnetic radiation having wavelengths in other than the visible light portion of the electromagnetic spectrum. For example, cholesteric liquid crystal material effectively transmits infrared radiation in the focal conic and twisted planar configurations at about the same percentages set forth above for visible light. Various types and amounts of suitable chiral nematic material, additives and substrate surface treatments, are disclosed in U.S. patent application Ser. No. 08/862,561, filed on May 23, 1997 entitled “Low Viscosity Liquid Crystal Material.” Application Ser. No. 08/862,561 is assigned to the assignee of the present invention and is incorporated herein in its entirety by reference. 
     Overall Configuration of the Display  20   
     It should be appreciated that the concept of the present invention is not limited to any particular display size nor is it limited to a matrix or segmented display. In one typical example, the display  20  may be a VGA (video graphics array) sized liquid crystal display comprising 480 rows by 640 columns having a display viewing area of about 3.8 by 5 inches with 127 dots or pixels per inch (dpi) and is supported in a suitable housing (not shown) consisting of a high impact, durable plastic. The VGA size means that the display  20  comprises 307,200 image pixels (480 rows×640 columns). As noted above, it should be understood that the display  20  of the present invention may be fabricated in other that the VGA display size. While the display  20  is a matrix display, it should be appreciated by one skilled in the art that the display could also be a segmented display. In a segmented display, the electrodes may be fabricated in various shapes and disposed in desired orientations to generate desired image configurations, for example, in a segmented liquid crystal display short electrode segments may advantageously be disposed to create a seven segment numerical display. In other segmented displays electrode segments of irregular shape may be used to generate an icon on the display. 
     The bistable liquid crystal display  20  includes a layer  40  comprised of chiral nematic liquid crystal material which is advantageous in that it maintains its reflective state even in the absence of an electric field. The display  20  is preferably a passive matrix in which the horizontal (row) and vertical (column) electrode segments  37 ,  35 , which define the image pixels of the display are passive, resistive circuit elements. The electrode segments may also comprise active circuit elements such as transistors, that is, circuitry for an active matrix display. A passive matrix display, such as the display  20 , is advantageous because of lower cost compared to active matrix displays. Passive matrix displays have a much simpler design compared to active matrix displays and are thus easier to manufacture and can be produced in greater yields. 
     The display  20  comprises the transparent rectangular front substrate  22  (closest to a viewer V of the display  20  viewing the display in a direction shown in the arrow in FIGS. 2 and 4) and the transparent rectangular back substrate  24 . Between the substrates  22 ,  24  is disposed the thin layer of cholesteric liquid crystal material  40 . The substrates  22 ,  24  may be formed preferably of glass (e.g., 0.5 millimeters (mm.) to 1.5 mm. thick) or of plastic (e.g., 0.18 mm. or approximately 7 mils thick), which materials are well known in the liquid crystal art. The liquid crystal material  40  may be filled into a cell spacing between substrates  22 ,  24  of, for example, from about 4 micrometers to about 6 μm. or to about 10 μm. or greater. 
     Sets of Display Electrodes  35 ,  37   
     The first substrate  22  includes a set of 640 column electrode segments  35  affixed to an inwardly facing surface  22   a  of the substrate  22 , that is, the surface of the substrate facing or adjacent the liquid crystal material  40 . Preferably, the set of column electrodes  35  comprise an array of parallel indium tin oxide (ITO) traces coated on the substrate inwardly facing surface  22   a . The second substrate  24  includes a set of 480 row electrode segments  37  affixed to an inwardly facing surface  24   a  of the substrate  24 . Preferably, the set of row electrodes  37  comprise an array of parallel ITO traces coated on the substrate inwardly facing surface  24   a.    
     Display Driver Circuitry  200   
     Display driver circuitry is shown schematically in FIGS. 5 and 6 at  200  and comprise a set of row driver circuitry (shown schematically at  202  mounted on a row driver board  202   a ) and a set of column driver circuitry (shown schematically at  204  mounted on a column driver board  204   a ). The set of row driver circuitry  202  is connected to the row electrodes  37 . Likewise, the set of column driver circuitry  204  is connected to the column electrodes  35 . 
     A full discussion of an appropriate drive scheme is disclosed in U.S. patent application Ser. No. 08/868,709, filed Jun. 4, 1997 and entitled “Cumulative Drive Scheme And Method For A Liquid Crystal Display”. Application Ser. No. 08/868,709 is assigned to the assignee of the present invention and is incorporated herein in its entirety by reference. In the invention disclosed in U.S. patent application Ser. No. 09/063,907, filed Apr. 21, 1998, and entitled “Unipolar Waveform Drive Method And Apparatus For A Bistable Liquid Crystal Display,” a method and display driver circuitry for activating a bistable, cholesteric liquid crystal display using unipolar waveforms and a pipelining scheme for addressing multiple rows of the display to provide high speed updating of the display is disclosed. Application Ser. No. 09/063,907 is assigned to the assignee of the present invention and is incorporated in its entirety herein by reference. Other suitable drive schemes for a cholesteric liquid crystal display are well known to those skilled in the art. It should be appreciated that the display  20  of the present invention is not limited to any particular drive scheme. 
     The row driver circuitry  202  is mounted on the row driver board  202   a  and has its output channels coupled to respective different row electrode segments  37  via suitable edge connections  203  (shown schematically in FIG.  5 ). Similarly, the column driver circuitry  204  is mounted on the column driver board  204   a  and has its output channels coupled to respective different column electrode segments  35  via suitable edge connections  205  (also shown schematically in FIG.  5 ). 
     The row and column driver circuitry  202 ,  204  is electrically connected to the logic and control unit  150  which includes circuitry that controls the presentation of data on the display  20  by controlling the reflectance state of each pixel in the array of 307,200 pixels that make up the display  20 . A microprocessor controls operations of the circuitry of the control and logic unit  150 . 
     First, Second, Third and Fourth Embodiments of the Display and Solar Cell Aassembly 
     Three preferred embodiments of the display  20  include a first embodiment shown in FIGS. 1 &amp; 2 (designated as  20 ), a second embodiment illustrated in FIGS. 3 &amp; 4 (designated as  20 ′) and a third embodiment illustrated in FIG.  9 . Common reference numbers will be used for identical components in the three embodiments. 
     First Preferred Embodiment 
     In the first embodiment of the display  20 , the solar panel assembly  310  of the solar cell assembly  300  is disposed adjacent an outwardly facing surface  24   b  of the second substrate  24  (away from the direction of the viewer V shown in FIG.  2 ). The solar panel assembly  310  is usually black in color and is light absorbing. The solar panel assembly  310  must be very dark or black in color so that image pixels of the display when in their non-reflective state appear as black or dark to the viewer V. For packaging considerations and maximum energy recovery, the solar panel assembly  310  selected should have a photovoltaic light receiving area that closely matches the display active area. Another important consideration is to select a solar cell assembly  300  whose voltage output is such that a minimum of circuitry will be needed to utilize the output voltage from the solar cell assembly  300  to charge the energy storage device  340 . 
     It should be understood, of course, that the solar panel assembly  310  may be comprised of two (or more) solar cells electrically connected in series to increase the solar cell assembly output voltage. Given that for the VGA display  20  the display area is approximately 3.8×5 inches and because of the desired voltage output of the solar cell assembly  300  is at least 0.85 volts, a suitable solar panel assembly  310  would consist of two solar cells  312 ,  314  electrically connected in series. A suitable solar cell is part no. SP4-200-8 manufactured by Plastecs Company, P.O. Box 578, Webster, Mass. 01570 which is comprised or a single crystalline solar cell material and has an nominal voltage output of 0.5 volts, coupled in series the voltage output of the solar panel  310  is approximately 1.0 volt. The size of the aforementioned Plastecs&#39; solar cell is 4″×4″ and can be trimmed to a smaller size if desired. Thus, if each solar cell  312 ,  314  is trimmed to a size of approximately 3.8 inches×2.5 inches and appropriately positioned, the entire active display area subject to incident radiation will utilized to generate electrical energy from the solar cell assembly. 
     Ambient light incident on the outer surface  22   b  of the first substrate  22  of the display  20  passes through the first substrate  22 , the liquid crystal material  40  and the second substrate  24  where it impinges upon the light absorbing solar panel assembly  310 . Upon being exposed to incident light, the solar cells  312 ,  314  each generate output power whose magnitude is proportional to the intensity of incident light on the respective cells. 
     Preferably, a thin layer of index matching optical material  50  (FIG. 2) is applied between the solar panel assembly  310  and the back substrate  24  to provide suitable optical coupling and to reduce reflections at the substrate/solar cell interface. One suitable index matching optical material is glycerol, a high viscosity liquid with a refractive index near that of glass available from Aldrich Chemical Company, 1001 West Saint Paul Avenue, Milwaukee, Wis. 53233. Another suitable index matching optical material is optical adhesive such as Norland NOA 65 or NOA 72 manufactured by Norland Products, Inc., 695 Joyce Kilmer Avenue, New Brunswick, N.J. 08902. Yet another suitable index matching optical material is optical gel such as OC-440 or OCK-451 manufactured by Nye Lubricants, 12 Howland Road, Fairhaven, Mass. 02719. 
     A suitable energy storage device  340  is a nickel-cadmium (NiCd) rechargeable battery pack comprising two 1.2 volt NiCd battery cells connected in series and having an output voltage of 2.4 volts. Suitable NiCd batteries may be purchased from a number of battery vendors including Panasonic (e.g., Panasonic part no. P-25AAA). Other suitable rechargeable energy storage devices include capacitors and other types of rechargeable battery packs such a nickel-metal hydroxide (Ni—MH) battery pack comprising two Ni—MH rechargeable batteries (e.g., Panasonic part no. HHR-65TA with nominal 2.4 volt output) or lithium power cells such as lithium dioxide (Li—SO 2  having nominal 3.0 volt output). 
     If the voltage output of the solar cell assembly is increased the required capacitance will, of course, decrease exponentially in accordance with the formula E=½ CV 2 . 
     Assuming the output voltage of the energy storage device  340  is 2.4 volts, the output of the solar cell assembly  300  must be stepped up to 2.4 volts or more to fully charge the energy storage device  340 . Given that the output of the solar panel is 1.0 volt (0.5 volt from each of the solar cells  312 ,  314 ), a DC/DC voltage converter must be used to step up the voltage from 1.0 volts to 2.4 volts. As such, the solar panel assembly  310  is electrically coupled though a DC/DC voltage converter  320  and a diode  330  to the rechargeable energy storage device  340 . An electrical conductor  350  (seen in FIG. 1) electrically couples power generated by the solar panel assembly  310  to the voltage converter  320 . 
     The DC/DC converter  320  boosts the solar panel output voltage (1.0 volts) to a voltage greater than the output voltage rechargeable energy source  340 , e.g., 2.4 volts. A suitable DC/DC converter is Maxim product no. MAX866 available from Maxim Electronics, 120 San Gabriel Drive, Sunnyvale Calif. 94086. The MAX866 is a 3.3V/5V or adjustable output, single cell DC/DC converter that steps up DC voltage from a minimum of 0.8 volts to 3.3 volts or 5 volts (pin selectable) utilizing a 330 μH inductor, a 47 μF and a 0.22 μF capacitors. Another suitable DC/DC converter is the MAX1642 also sold by Maxim Electronics which steps up DC voltage from a range of 0.7 volts to 1.65 volts to a range of 2.0 volts to 5.2 volts output utilizing an inductor and two capacitors. An alternative to using a voltage converter to step up the solar panel output voltage is to electrically couple in series the requisite number of solar cells in series to generate the desired output voltage which exceed the energy storage device output voltage. For example if each solar cell has an output of 0.5 volts, coupling five in series would result in an output voltage of 2.5 volts, sufficient to charge the 2.4 volt energy source  340 . The disadvantage of this method is that five solar cells have to be purchased, trimmed as required and electrically coupled. 
     The diode  330  (FIG. 6) restricts the flow of current from the DC/DC voltage converter  320  to the energy storage device  340  and prevents current from the energy storage device  340  from flowing to the solar panel assembly  310  when the display  20  is exposed to no or low ambient illumination. It should be noted that depending on the nature of the solar cells selected, the diode  330  may not be required since certain types of solar cells perform the functionality of the diode as a result of the specific internal silicon structure of the solar cell material. 
     The energy storage device  340  is required since the solar cell assembly  300  is typically not capable of supplying enough energy to allow the display driver circuitry  200  to continuously update the display image. Furthermore, even if the display  20  does not require continuous updating, the energy storage device  340  permits the display  20  to be operated during periods of darkness. It should be understood, of course, that the display  20  may be driven directly from the electrical power generated by the solar cell assembly  300  if continuous updating and low light operation is not a requirement. 
     With regard to the selection of a solar cell, it should be understood that the spectral response of different solar cell materials differ. The spectral response or sensitivity of a solar cell material can be viewed as a plot of the efficiency of the solar cell material in converting incident radiation to electric energy as a function of radiation wavelength. A typical solar cell material spectral response is schematically illustrated at  500  in FIG.  7 . Generally, the spectral response of a solar cell material is a approximately a bell shaped curve or normal distribution as can be seen in FIG.  7 . The graph indicates that for this particular single crystalline solar cell material, the greatest spectral responsiveness and, therefore, the greatest efficiency in conversion of electromagnetic energy into electrical energy, occurs at an incident radiation wavelength labeled X in FIG. 7 of about 0.6 μm. Since different solar cell materials will have different values of X, the greatest spectral responsiveness wavelength, if it can be determined in advance the lighting condition or conditions which a display will generally be used in, it would be possible to select the most efficient solar cell material for the expected lighting conditions under which the display will be used. For example, in an florescent light environment, the emitted radiation centers around 0.45 μm. For a display used exclusively in such an environment, it would be best to use a solar cell material with an X=0.45 μm. For a display used in an outdoor sunlight environment, the solar cell material should have a greater X value since the radiation wavelengths of sunlight center around a value higher than 0.45 μm. 
     Second Preferred Embodiment 
     In the second embodiment of the display denoted as  20 ′ in FIGS. 3 &amp; 4, a thin infrared (IR) transmissive layer  400 ′ is coated on the outwardly facing surface  24   b ′ (FIG. 3) of the second or back substrate  24 ′. Two solar cells  312 ′,  314 ′ of a solar panel assembly  310 ′ of the solar cell assembly  300 ′ are positioned adjacent the second substrate outwardly facing surface  24   b ′. Since the IR transmissive layer  400 ′ is dark in color to provide contrast for non-reflecting image pixels versus reflecting image pixels, the solar cells  312 ′,  314 ′ of the solar panel assembly  310 ′ may be any color. A suitable IR transmissive layer or coating  400 ′ is 40E black ink manufactured by Excelsior Marking Products, Inc., 4524 Hudson Drive, Stow, Ohio 44224. The IR coating  400 ′ may be flow coated, dipped or sprayed onto the second substrate outwardly facing surface  24   b ′. Spraying of the IR coating  400 ′ may be accomplished using a Preval power spray unit manufactured by Preval Sprayer Division, Precision Valve Corporation, P.O. Box 309, Yonkers, N.Y. 10702. 
     Other than the utilization of the IR transmissive layer  400 ′, the operation of the display  20 ′ and the operation and advantages of the solar cell assembly of the display  20 ′ are identical to that described in the first embodiment (i.e., the display  20 ). The presence of the IR transmissive layer  400 ′ reduces solar energy recovery by the solar energy cell  310 ′ by about 25-98% depending on the wavelength of light. FIG. 8 illustrates this concept. The transmitivity of radiation through the IR coating layer  400 ′ exhibits a generally step type function labeled as  600  in FIG. 8, the exact functional relationship is dependent on the particular characteristics of the IR ink selected as the coating. For this particular IR transmissive layer  400 ′ it can be seen that for radiation having a wavelength below 0.7 μm., a very small percentage of the incident radiation is transmitted through the layer  400 ′ while at wavelengths above 0.8 μm. about 80-85% of the incident radiation is transmitted through the layer  400 ′. It is clear that a solar cell material having a higher spectral response radiation wavelength center value (X) is very desirable so that the intersection region (labeled  700  in FIG. 8) between the IR transmitivity function and the solar cell material spectral response curve  500  is as great as possible. 
     Thus, the second embodiment of the display  20 ′ is less efficient from the perspective of utilization of solar energy compared to the display  20  (first embodiment). However, those skilled in the art will realize, in view of this disclosure, that other IR transmissive materials having the ability to transmit a greater proportion of the near IR radiation (radiation having a wavelength of 0.75 μm. to 1.5 μm.) incident thereon, as well as solar cells having a greater efficiency in converting radiation to electrical energy in this radiation wavelength region, may be used in accordance with the present invention. Preferably, a thin layer of index matching optical material  50 ′ (FIG. 4) is applied between the IR coating  400 ′ and the solar panel assembly  310 ′ to provide suitable optical coupling and to reduce reflections at the substrate/solar cell interface. 
     Third Preferred Embodiment 
     It should be appreciated that if solar cell or cells of the solar cell assembly and a substrate or base material are integrated into a single unit such a unit may advantageously function as both the solar panel assembly and the second or back substrate of the display thereby eliminating the need for a back substrate separate from the solar panel assembly. A third preferred embodiment of the display  20 ″ of the present invention employs such a construction and is shown in FIG.  9 . The display  20 ″ includes a solar cell assembly  300 ″ that includes a solar panel assembly  310 ″ that also functions as the back substrate. The solar panel assembly  310 ″ includes a solar cell comprising a layer of photovoltaic material  312 ″ applied to an outwardly facing surface  24   b ″ of the base material  24 ″ comprising the solar cell. The photovoltaic material  312 ″ may be silicon or other types of material such as organic solar cell material. The base material  24 ″ may be comprised of either a plastic or glass base material. The solar cell assembly  300 ″ also includes a suitable electrode array  37 ″ applied to an inwardly facing surface  24   a ″ of the base material  24 ″. Preferably, the electrode array  37 ″ consists of an array of parallel indium tin oxide (ITO) traces comprising a set of row electrode segments  37 ″ coated on the base material inwardly facing surface  24   a ″, as described in the first two embodiments. Alternatively, if it is desired that the display  20 ″ be configured as an active matrix display, a matrix of silicon transistors would be applied to the inwardly facing surface of the base material  24 ″ to construct an actively driven display, as is well known in the art of fabricating active and passive matrix displays. It should be appreciated that the same active matrix structure applied to the back substrate could be utilized in all four embodiments if an active matrix display is desired. 
     In one preferred method of fabricating the solar cell assembly  300 ″ of the third embodiment, the solar panel assembly  310 ″ consists of photovoltaic material  312 ″ applied to the outwardly facing surface  24   b ″ of the glass or plastic base material  24 ″ such that the photovoltaic material  312 ″ is originally fabricated upon the base material. Suitable thin film solar cells may purchased from Photon Technologies, Inc., P.O. Box 790, Severna Park Md. 21146. If glass is selected as the base material for the base material  24 ″, a suitable thin film solar cell would be the ASE 3.0 V, 27 mA amorphous, thin film solar cell. If plastic is selected as the base material  24 ″, a suitable thin film solar cell would be a 3.0 V, 40-50 mA thin film solar cell. In the event that the solar panel assembly  310 ″ is not black or dark colored when the inwardly facing surface  24   a ″ is viewed by a viewer of the display  20 ″, an IR transmissive layer or coating (not shown) may be applied to the inwardly facing surface  24   a ″ of the solar panel assembly  310 ″ to provide a dark color background and thereby improving display contrast. Of course, such an IR transmissive layer has the disadvantage of filtering or blocking radiation in the visible spectrum thereby preventing such radiation from reaching the silicon of the solar cell. As was described in the second embodiment, this filtering decreases the radiation energy available for conversion to electrical energy by the solar cell. 
     Fourth Preferred Embodiment 
     In a fourth preferred embodiment, the display  20 ′″ is a stacked cholesteric liquid crystal display including a solar cell assembly  300 ′″ constructed in accordance with either the first, second or third embodiments described above. As can be seen in FIG. 10, the display consists of three layers of liquid crystal material  40   a ′″,  40   b ′″,  40   c ′″ and associated substrates  22 ′″,  23   a ′″,  23   b ′″,  24 ′″. The layer of liquid crystal material  40   a ′″ is bounded by the substrates  22 ′″,  23   a ′″. The layer of liquid crystal material  40   b ′″ is bounded by the substrates  23   a ′″,  23   b ′″. The layer of liquid crystal material  40   c ′″ is bounded by the substrate  23   b ′″ and a solar panel assembly  310 ′″ of the solar cell assembly  300 ′″. The solar panel assembly  310 ′″ includes a base material  24 ″ which functions as the back substrate. The solar panel assembly  310 ′″ includes a solar cell comprising a layer of photovoltaic material  312 ′″ applied to an outwardly facing surface  24   b ′″ of the base material  24 ′″ comprising the solar cell. The photovoltaic material  312 ′″ may be silicon or other types of material such as organic solar cell material. The base material  24 ′″ may be comprised of either a plastic or glass base material. 
     Alternatively, the solar cell assembly  300 ′″ of the display′″ may comprise either of the structures of the solar cell assemblies  300  or  300 ′ described with respect to the first and second embodiments of the display of the present invention. The solar cell assembly  300 ′″ also includes a suitable electrode array  37   c ′″ applied to an inwardly facing surface  24   a ′″ of the base material  24 ′″. Preferably, the electrode array  37   c ′″ consists of an array of parallel indium tin oxide (ITO) traces comprising a set of row electrode segments coated on the base material inwardly facing surface  24   a ′″, as described in the first three embodiments. A vertical electrode array  35   a ′″ is applied to an inwardly facing surface  22   a ′″ of the front substrate  22 ′″. A horizontal electrode array  37   a ′″ is applied to one side of the substrate  23   a ′″ facing the front substrate  22 ′″ and a vertical electrode array  35   b ′″ is applied to the opposite side of the substrate  23   a ′″ facing the substrate  23   b ′″. A horizontal electrode array  37   b ′″ is applied to one side of the substrate  23   b ′″ facing the substrate  23   a ′″ and a vertical electrode array  35   c ′″ is applied to the opposite side of the substrate  23   b ′″ facing the back substrate  24 ′″. 
     Alternatively, if it is desired that the display  20 ′″ be configured as an active matrix display, a matrix of silicon transistors would be applied to the frontwardly or forwardly facing surfaces of substrate  23   a ′″,  23   b ′″,  24 ′″ (namely  24   a ′″) to construct an actively driven display, as is well known in the art of fabricating active and passive matrix displays. The method of fabricating the solar cell assembly  300 ′″ may be any of the methods described in the third embodiment. 
     The stacked cholesteric liquid crystal display  20 ′″ is a color display, the cholesteric liquid crystal material layers  40   a ′″,  40   b ′″,  40   c ′″ are appropriately selected to have respective pitch lengths to reflect wavelengths of light in the red, green, and blue portions of the visible spectrum respectively. Thus, color of an image pixel visible to a viewer of the display  20 ′″ results from an additive color mix of the colors reflected by the three aligned pixels of the first second and third liquid crystal material layers that together comprise the image pixel seen by a viewer of the display  20 ′″. U.S. patent application Ser. No. 08/823,329, filed on Mar. 22, 1997 and entitled “Display Device Reflecting Visible and Infrared Radiation,” Ser. No. 09/330,104, filed on Jun. 10, 1999 and entitled “Stacked Color Liquid Crystal Display Device,” and Ser. No. 09/329,587, filed on Jun. 10, 1999 and entitled “Stacked Color Liquid Crystal Display Device,” respectively disclose a stacked liquid crystal display providing for a multi-color display. Application Ser. Nos. 08/823,329, 09/330,104, and 09/329,587 are assigned to the assignee of the present invention and each is incorporated herein in its respective entirety by reference. 
     Even if a color display is not required, the display  20 ′″ may be configured in a stacked display, for example, as a double stacked display (two layers of liquid crystal material). Such a double stacked configuration has the advantage of providing enhanced brightness for image pixels in the reflective state. A double stacked display is disclosed in U.S. application Ser. No. 09/244,731, filed Feb. 5, 1999 and entitled “Stacked Bistable Cholesteric Liquid Crystal Display Utilizing Single Set of Drive Electronics.” Application Ser. No. 09/244,731 is assigned to the assignee of the present invention and is incorporated herein in its entirety by reference. 
     While the invention has been described herein in it currently preferred embodiment or embodiments, those skilled in the art will recognize that other modifications may be made without departing from the invention and it is intended to claim all modifications and variations as fall within the scope of the invention.