Abstract:
Improved electro-optic devices are provided which may be in the configuration of variable transmittance windows, variable transmittance eyeglasses, variable transmittance light filters and displays and other devices wherein the transmittance of light therethrough automatically varies as a function of light impinging thereon. The electro-optic devices include a self-erasing electro-optic medium, and the transmittance of light through such medium varies as a function of electrical signals applied thereto through the agency of at least one photovoltaic cell, enclosed within the electro-optic device, and obviating the necessity of providing external drive voltage or external bleeder resistors or external wiring. In addition, a method and apparatus are provided for making such electro-optic devices.

Description:
This application is a continuation-in-part of application Ser. No. 09/012,957, U.S. Pat. No. 6,045,643, filed Jan. 26, 1998, which is a division of application Ser. No. 08/616,698, filed Mar. 15, 1996, issued as U.S. Pat. No. 5,805,330, dated Sep. 8, 1998. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to electro-optic devices and, more particularly, to electro-optic devices having enclosed therein at least one photovoltaic device. 
     Heretofore, devices of reversibly variable transmittance to electromagnetic radiation have been proposed for such applications as the variable transmittance element in variable transmittance light-filters, variable transmittance eyeglasses, variable reflectance mirrors; and display devices which employ such light-filters or mirrors in conveying information. These variable transmittance light filters have included windows. Among such devices are those where the transmittance is varied by thermochromic, photochromic, or electro-optic (e.g., liquid crystal, dipolar suspension, electrophoretic, electrochromic, etc.) means and where the variable transmittance characteristic affects electromagnetic radiation that is at least partly in the visible spectrum (wavelengths from about 3800 Å to about 7600 Å). Typically, proposed control schemes for variable transmittance windows either allow the windows to be power controlled window-by-window with a person determining when the window should darken or have all windows controlled by a central computerized power source such that the window is darkened when the sun shines on them or on a sensor placed on a particular side of a building. 
     Devices of reversibly variable transmittance to electromagnetic radiation, wherein the transmittance is altered by electrochromic means are described, for example, by Chang, “Electrochromic and Electrochemichromic Materials and Phenomena,” in Non-emissive Electrooptic Displays, A. Kmetz and K. von Willisen, eds. Plenum Press, New York, N.Y., pp. 155-196 (1976) and in various parts of Eletrochromism, P. M. S. Monk, R. J. Mortimer, D. R. Rosseinsky, VCH Publishers, Inc., New York, N.Y. (1995). Numerous electrochromic devices are known in the art. See, e.g., Manos, U.S. Pat. No. 3,451,741; Bredfeldt et al., U.S. Pat. No. 4,090,358; Clecak et al., U.S. Pat. No. 4,139,276; Kissa et al., U.S. Pat. No. 3,453,038; Rogers, U.S. Pat. Nos. 3,652,149, 3,774,988 and 3,873,185; and Jones et al., U.S. Pat. Nos. 3,282,157, 3,282,158, 3,282,160 and 3,283,656. 
     In addition to these devices there are commercially available electro-optic devices and associated circuitry, such as those disclosed in U.S. Pat. No. 4,902,108, entitled “Single-Compartment, Self-Erasing, Solution-Phase Electro-optic Devices Solutions for Use Therein, and Uses Thereof”, issued Feb. 20, 1990 to H. J. Byker; Canadian Patent No. 1,300,945, entitled “Automatic Rearview Mirror System for Automotive Vehicles”, issued May 5, 1992 to J. H. Bechtel et al.; U.S. Pat. No. 5,128,799, entitled “Variable Reflectance Motor Vehicle Mirror”, issued Jul. 7, 1992 to H. J. Byker; U.S. Pat. No. 5,202,787, entitled “Electro-Optic Device”, issued Apr. 13, 1993 to H. J. Byker et al.; U.S. Pat. No. 5,204,778, entitled “Control System For Automatic Rearview Mirrors”, issued Apr. 20, 1993 to J. H. Bechtel; U.S. Pat. No. 5,278,693, entitled “Tinted Solution-Phase Electrochromic Mirrors”, issued Jan. 11, 1994 to D. A. Theiste et al.; U.S. Pat. No. 5,280,380, entitled “UV-Stabilized Compositions and Methods”, issued Jan. 18, 1994 to H. J. Byker; U.S. Pat. No. 5,282,077, entitled “Variable Reflectance Mirror”, issued Jan. 25, 1994 to H. J. Byker; U.S. Pat. No. 5,282,077, entitled “Variable Reflectance Mirror”, issued Jan. 25, 1994 to H. J. Byker; U.S. Pat. No. 5,294,376, entitled “Bipyridinium Salt Solutions”, issued Mar. 15, 1994 to H. J. Byker; U.S. Pat. No. 5,336,448, entitled “Electrochromic Devices with Bipyridinium Salt Solutions”, issued Aug. 9, 1994 to H. J. Byker; U.S. Pat. No. 5,434,407, entitled “Automatic Rearview Mirror Incorporating Light Pipe”, issued Jan. 18, 1995 to F. T. Bauer et al.; U.S. Pat. No. 5,448,397, entitled “Outside Automatic Rearview Mirror for Automotive Vehicles”, issued Sep. 5, 1995 to W. L. Tonar; and U.S. Pat. No. 5,451,822, entitled “Electronic Control System”, issued Sep. 19, 1995 to J. H. Bechtel et al. Each of these patents is commonly assigned with the present invention and the disclosures of each, including the references contained therein, are hereby incorporated herein in their entirety by reference. 
     Photoelectrochromism is discussed generally in pages 192-197 of Electrochromism, P. M. S. Monk, R. J. Mortimer, D. R. Rosseinsky, VCH Publishers, Inc., New York, N.Y. (1995). Specifically, section 12.2.3, entitled “Cells Containing Photovoltaic Materials”, discusses how a photovoltaic material produces a potential when illuminated and where the photovoltaic material has an internal rectifying field which provides a driving force for the electrons. This section goes on to describe that the voltage created by the photovoltaic material is insufficient, by itself, to darken the electrochromic material. Therefore the electrochromic cell incorporating a photovoltaic material needs an external bias applied which is supplemented by the small photovoltaic-voltage to cause electron transfer to proceed, i.e., have the electrochromic material darken. 
     Heretofore, various other electrochromic devices have been devised wherein the transmission of light therethrough or reflected thereby automatically varies as a function of light impinging thereon. For example: U.S. Pat. No. 5,377,037, entitled “Electrochromic-Photovoltaic Film for Light-Sensitive control of Optical Transmittance” to H. M. Branz et al. teaches a variable transmittance optical component which includes a solar cell-type photovoltaic device. The photovoltaic material is deposited over the entire surface of a transparent electrically conductive layer section. The photovoltaic material includes a p-type hydrogenated silicon carbide section, an undoped hydrogenated silicon carbide section, and phosphorous-doped hydrogenated silicon carbide section. A standard solid-state electrochromic multilayer structure is then deposited over the layer of photovoltaic material such that the light traveling through the optical transmitter must travel through the photovoltaic material and through the electrochromic material. The photovoltaic material will absorb some portion of the light and will also create sufficient current to darken the electrochromic material. Solid-state electrochromic devices with good memory, once darkened, will not clear or bleach quickly without an external method of closing the electrochemical circuit, i.e., the device will not clear in a reasonable time even though the “darkening potential” is removed. The device taught by Branz et al. attempts to overcome this significant limitation by connecting a bleeder resistor to the two transparent conductive electrode layers to provide the electric potential and circuit across the device (to slowly bleach the device). In operation, the photovoltaic device produces a DC current which is applied between the transparent conductive layers and across the bleeder resistor. However, it takes a light source with the intensity of 1-2 suns to produce a transmission drop of only 10 percent, in approximately 12-13 minutes. Thus, incorporating a bleeder resistor complicates the circuitry required for the window system and also draws some power that otherwise could be used in darkening. 
     U.S. Pat. No. 5,348,653, entitled “Stand-Alone Photovoltaic (PV) Powered Electrochromic Window” to D. K. Benson et al. teaches a variable transmittance double pane window including a five-layer solid state electrochromic portion, an array of photovoltaic cells with a n-type conductivity region on the front side of a p-type silicon substrate, and an external switch-containing circuit. The photovoltaic cells are deposited directly on the glass and not on the transparent electrode. The photovoltaic cells and the battery circuit are connected in parallel to the electrochromic portion of the device. This allows selective activation of the electrochromic portion to either a substantially opaque state or a substantially transparent state by switching the external switch-containing circuit between having the photovoltaic devices drive the device to a dark state, or to a transparent state or having the battery device drive the device to a transparent state when the conditions are such that the incident sunlight is not sufficient for the photovoltaic array to produce the required energy. Again, solid-state electrochromic devices with good memory, once darkened, will not clear in a reasonable amount of time absent some method of closing the circuit, typically by applying a bleaching potential. 
     U.S. Pat. No. 5,457,564, entitled “Complementary Surface Confined Polymer Electrochromic Materials, Systems, and Methods of Fabrication Therefore” to Leventis et al. teaches an electrochromic device having polypyrrole-prussian blue composite material on the oxidatively coloring electrode and a heteroaromatic substance with at least one quaternized nitrogen on the reductively coloring electrode. Preferably, either the oxidative or reductive polymer is electro-deposited onto a metallic oxide to increase the cycle life of the device to an acceptable level. Leventis et al. also teaches using an external photovoltaic cell to generate power to darken the electrochromic device. The photovoltaic cells operate as forward biased diodes and allow current to flow in the opposite or “reverse” direction. Further, Leventis et al. places the photovoltaic cells behind the electrochromic device such that the light which drives them must first travel through the electrochromic material. As the degree of colorization of the device increases, the intensity of light impinging on the photovoltaic cells decreases and the output from the photovoltaic cells decreases, creating a limit of how much light the device can block. 
     When retrofitting electro-optic devices in the configuration of windows it is disadvantageous to have to run wires to each window to supply the external bias. Furthermore, even when installing electrochromic windows into a new building it would be easier and less expensive if no wires were needed to supply an external bias or no external circuits were necessary to help control colorization or bleaching of the window. Consequently, it is desirable to provide an improved electro-optic device having an improved photovoltaic drive mechanism. 
     OBJECTS OF THE INVENTION 
     Accordingly, a primary object of the present invention is to provide an improved electro-optic device having a discrete photovoltaic device integrally combined with the electro-optic device where no external drive voltage is needed, no bleaching circuit is required, and no external wiring is necessary. 
     Another object of the present invention is to overcome disadvantages in prior electro-optic devices of the indicated character and to provide improved electro-optic devices wherein the transmittance of light therethrough automatically varies as a function of light impinging thereon. 
     Another object of the present invention is to provide improved electro-optic devices which may be in the configuration of windows and eyeglasses which darken and clear uniformly in an aesthetically pleasing manner. 
     Another object of the present invention is to provide improved windows and eyeglasses which incorporate improved means for adjusting the amount of light that is transmitted therethrough to a desired and comfortable level. 
     Another object of the present invention is to provide improved self-erasing electro-optic devices that are economical to manufacture, durable, efficient and reliable in operation. 
     Another object of the present invention is to provide improved electro-optic devices wherein excellent speed of light transmissive change, good uniformity of light change across the entire surface area thereof, and continually variable light transmissive characteristics are obtained throughout the range of light transmittance of the devices. 
     SUMMARY OF THE INVENTION 
     The above and other objects, which will become apparent from the specification as a whole, including the drawings, are accomplished in accordance with one embodiment of the present invention by enclosing within an electro-optic device a discrete photovoltaic assembly which is capable of driving the electro-optic medium. The electro-optic device has front and back spaced-apart glass elements sealably bonded together defining a chamber filled with an electro-optic material. The front glass element has a transparent conductive layer on the face confronting the rear glass element and the rear glass element has a transparent conductive layer on the face confronting the front glass element. The seal is generally disposed some small distance from the perimeter of three edges of both glass elements and some greater distance in from the remaining (fourth) edge. The photovoltaic assembly is placed between the two glass elements on the outer perimeter along this fourth edge with the photon-absorbing (active) side of all the photovoltaic cells within the photovoltaic assembly facing in one direction. Alternately, the photovoltaic assembly or an array of assemblies may be placed in a sealed off region or regions any place within the window area and may even be in the form of a decorative design, such as a diamond, circle, and the like, and may assist in providing and maintaining the spacing between the transparent conductor-coated glass elements. The photovoltaic assembly is electrically connected to the two transparent conductive layers and when light impinges on the photovoltaic cell an electrical potential is generated which darkens the electro-optic material in proportion to the amount or intensity of impinging light. By choosing the relative area of the photovoltaic assembly to produce the correct current for the electro-optically active window area, the amount of darkening of the electro-optic portion can be directly and accurately controlled without the need for any circuit, wires or shorting resistors. 
     In addition, an apparatus for making an electro-optic window having two members capable of securing and holding two glass elements in a spaced-apart and parallel relationship is provided. The glass elements may be secured by vacuum-applying members or simple clips. The glass elements may be held in a spaced-apart and parallel relationship by a hydraulic mechanism or by simple spacers placed between the securing members. 
     In accordance with another embodiment of the present invention, electrochromic eyeglasses are provided wherein the transmission of light therethrough automatically varies as a function of light impinging thereon. Eyeglasses embodying the present invention include left and right lenses which are integrally mechanically and electrically connected together as a unitary structure. The lenses have front and rear spaced glass or plastic lens elements with a chamber disposed therebetween, the front and rear lens elements being transparent. One side of the front element confronting the rear element includes transparent electrically conductive means, and one side of the rear element confronting the front element also includes transparent electrically conductive means. The chamber disposed between the front and rear elements contains an electrochromic reversibly variable transmittance medium in contact with the transparent electrically conductive means on the front and rear elements. A photovoltaic cell is provided for applying electrical potential to the electrochromic medium to cause variations in the light transmittance of the electrochromic medium, the photovoltaic cell being disposed between the right and left lens portions of the eyeglasses. The photovoltaic cell is electrically connected to the two transparent electrically conductive layers so that when light impinges on the photovoltaic cell an electrical potential is generated which causes the electrochromic material to darken in proportion to the amount or intensity of light impinging thereon. By controlling the relative area of the photovoltaic assembly to produce the desired electrical current for the electro-optically active lens area, the amount of darkening of the electrochromic material may be directly and accurately controlled without the need for external electrical wiring, batteries or bleeder resistors. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The subject matter which is regarded as the invention is particularly pointed out and distinctly claimed in the concluding portion of the specification. The invention, together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawings, where like numerals represent like components, in which: 
     FIG. 1 is an exploded perspective view of a photovoltaic-powered electro-optic device in the configuration of a window embodying the present invention; 
     FIG. 2 is a cross-sectional view of the electro-optic device of FIG. 1 taken along the lines  2 — 2 ; 
     FIG. 3 is a perspective view of the electro-optic device of FIG. 1 in a frame assembly; 
     FIG. 4 is a cross-sectional view of an apparatus used in the assembly of an electro-optic device in the con figuration of a window; 
     FIG. 5 is a prospective view of an electro-optic device in the configuration of a pair of electrochromic eyeglasses embodying the present invention; 
     FIG. 6 is a simplified, enlarged cross-sectional view of the eyeglasses illustrated in FIG. 5, taken on the line  6 — 6  of FIG. 5; 
     FIG. 7 is a perspective view of another embodiment of the invention; and 
     FIG. 8 is a simplified, enlarged cross-sectional view of the electro-optic device illustrated in FIG. 7, taken on the line  8 — 8  thereof. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 is an exploded perspective view of a photovoltaic-powered electro-optic device  10  having a first transparent element  12  with a front face  12   a  and a rear face  12   b , and a rear element  14  having a front face  14   a  and a rear face  14   b . By electro-optic device we mean variable transmittance light-filters, such as, for example, variable transmittance windows; variable transmittance eyeglasses; variable reflectance mirrors; and display devices which employ such light-filters or mirrors in conveying information. Although the following description discusses, inter alia, electrochromic windows, as one embodiment of the present invention, it will be understood by those skilled in the art that the present invention may be utilized in any of the above-referenced electro-optic devices. Further, since some of the layers of the electro-optic window  10  are very thin, the scale has been distorted for pictorial clarity. Front transparent element  12  and rear transparent element  14  may be any one of a number of materials which are transparent in at least part of the visible region of the electromagnetic spectrum and have sufficient strength to be able to operate in the conditions, e.g., varying temperatures and potential impact from hail and other flying objects such as birds, commonly experienced by a window. For example, elements  12  and  14  may comprise various types of glass, including soda lime float glass, polymers or plastic sheet materials, and the like, with glass being preferred. Elements  12  and  14  may possess UV barrier properties to help protect the electro-optic material  28 . The thicknesses of elements  12  and  14  are well known in the art and typically range from about 1 millimeter to about 6 millimeters. 
     A layer of a transparent electrically conductive material  16  is deposited on the rear face  12   b  of the first element  12  to act as an electrode, and another layer of a transparent electrically conductive material  18  is deposited on the front face  14   a  of the second element  14 . Both transparent conductive materials  16  and  18  generally cover the entire surface onto which they are deposited, i.e.,  12   b  and  14   a , respectively. The layers of transparent conductive materials  16  and  18  may be the same or different and may be any material which adheres well to front element  12  and rear element  14 , is resistant to adverse interaction with any materials within the electro-optic window that elements  12  and  14  come into contact, is resistant to adverse interaction with the atmosphere, has minimal diffuse or specular reflectance, high light transmission, and good electrical conductance. Layers of transparent conductive material ( 16  and  18 ) may be fluorine doped tin oxide, tin doped indium oxide (ITO), thin metal layers, ITO/metal/ITO (IMI) as disclosed in “Transparent Conductive Multilayer-Systems for FPD Applications”, by J. Stollenwerk, B. Ocker, K. H. Kretschmer, 1995 Display Manufacturing Technology Conference Digest of Technical Papers,  SID,  p 111, and the materials described in above-referenced U.S. Pat. No. 5,202,787, such as TEC 20 or TEC 15, available from Pilkington, Libbey Owens-Ford Co. of Toledo, Ohio. Generally, the conductance of the layers of transparent conductive materials ( 16  and  18 ) will depend on their thickness and composition if ITO or fluorine doped tin oxide is used. The thickness of either layer may range from about 500 Å to about 5000 Å, and is preferably from about 1400 Å to about 3500 Å. IMI, on the other hand, may have superior conductivity compared with the other materials, but is generally more difficult and expensive to manufacture and therefore may be useful in applications where high conductance is desirable. The thickness of the various layers in the IMI structure may vary but generally the thickness of the first ITO layer ranges from about 150 Å to about 1000 Å, the metal ranges from about 10 Å to about 250 Å and the second layer of ITO ranges from about 150 Å to about 1000 Å. The metals for the intermediate layer may be silver, gold, and the like. There may be additional layers of metal and ITO if desired, e.g., IMIMI. Also, an optional layer or layers of an anti-iridescent, an antireflection and/or a color suppression material or materials (not shown) may be deposited between transparent conductive material  16  and front glass rear face  12   b  and/or between transparent conductive material  18  and rear glass front face  14   a  to suppress or filter out any unwanted portion of the electromagnetic spectrum. 
     As is shown in FIG. 2, front glass element  12  is sealably bonded to rear glass element  14  in a spaced-apart and parallel relationship by a sealably bonding member  20  disposed between and adhered to transparent conductive layers  16  and  18 . Sealably bonding member  20  is generally disposed some distance D 1  in from the outer perimeter of three edges of both second face  12   b  and third face  14   a  and is disposed some distance D 2  in from the fourth edge. Sealably bonding member  20  may be any material which is capable of adhesively bonding the layers of transparent conductive material  16  and  18 , while, after curing, being capable of maintaining a generally constant distance therebetween. Sealably bonding member  20  should also preferably not be permeable to water or oxygen in any significant degree, and should be generally inert to an electro-optic material  28  disposed in chamber  24  (both of which are described in more detail hereinbelow). Sealably bonding member  20  generally comprises a strip or gasket of a polymeric material, such as rubbers, urethanes, acrylates, epoxies and the like, with epoxies being presently preferred. 
     Chamber  24 , defined by transparent conductive material  16  (disposed on front element  12 ), transparent conductive material  18  (disposed on back element  14 ), and an inner circumferential wall  26  of sealing member  20 , is filled with an electro-optic medium  28 . In this embodiment of the invention the electro-optic medium  28  may be a wide variety of materials capable of changing properties such that light traveling therethrough is attenuated (e.g., liquid crystal, polymer dispersed liquid crystal (PDLC), dipolar suspension, electrophoretic, and electrochromic, etc.). Electro-optic devices incorporating a PDLC medium generally required higher voltages and further require a number of cells placed in series to obtain adequate light attenuation. The presently preferred electro-optic media are electrochromic, which may be further broken down into the subcategories of solution-phase, surface-confined, electro-deposition, or combinations thereof. In this embodiment of the invention the electrochromic media disclosed and claimed in above-referenced U.S. Pat. Nos. 4,902,108; 5,128,799, 5,278,693; 5,280,380; 5,282,077; 5,294,376; 5,336,448, are the presently preferred electrochromic media whether they are in a liquid solution-phase or free-standing gel-type solution-phase. However, in this embodiment of the invention, the most preferred electrochromic media are free-standing gel-type, such as those described in co-pending U.S. Pat. No. 5,679,283, entitled “Electrochromic Layer and Devices Comprising Same” to W. L. Tonar et al., or in co-filed U.S. Pat. No. 5,888,431 entitled “Electrochromic Layer and Devices Comprising Same” filed on or about Mar. 15, 1996 to W. L. Tonar et al all of which are hereby incorporated herein in their entirety by reference. The electrochromic medium  28  is inserted into chamber  24  through a sealable fill port or ports (not shown) through well known techniques such as injection, vacuum backfilling and the like. If a gel is used, it is filled as a liquid and gelled in accordance with the teachings of either of these U.S. Patents. 
     In accordance with the present invention, at least one discrete photovoltaic assembly  22  is enclosed within (or placed between) the two layers of transparent conductive material ( 16  and  18 ) such that the photon absorbing side  22   ia-via  of each individual photovoltaic cell  22   i-vi  is facing one direction, i.e., out the front face  12   a  of front element  12 . Although shown in FIG. 1 as two sets of photovoltaic cells, those skilled in the art will realize that photovoltaic assembly  22  may be one or more sets of cells and may even be a single cell. An important aspect of the present invention is placing photovoltaic assembly  22  between the two layers of conductive materials ( 16  and  18 ). This allows the glass elements ( 12  and  14 ) to protect the assembly  22  from damage and, since the photovoltaic assembly  22  is in direct contact with the transparent conductive materials, the need for any external wiring or circuitry is eliminated. The unexpected benefits of this simple design should not be overlooked. Since no clips or other electrical connection mechanisms are needed to connect the conductive materials to an external circuit there are no concerns with the contact stability between the conductive materials and the clips. Furthermore, no separate housing need be constructed for the photovoltaic cells which simplifies the design and decreases the overall costs of the window  10 . Finally, as will become more clear by the discussion hereinbelow, the window embodying the present invention has a cell spacing which uniquely matches commercially available photovoltaic cells. This allows the window design to be simpler in that no wires need to be run to interconnect the photovoltaic cells and the transparent conductive materials ( 16  and  18 ). 
     As those skilled in the art will understand, the design of photovoltaic assembly  22  may vary in the placement of the photovoltaic cells  22   i-vi,  in the composition of the photovoltaic cells and in the number and arrangement of the electrical connection of the photovoltaic cells. For example, some photovoltaic cells can be produced in sufficient size to allow only two cells electrically connected in series. Furthermore it may be possible to produce a single photovoltaic cell which produces the voltage and current necessary to drive the electrochromic window  10 . Finally, the only restriction on the photovoltaic assembly  22  is that it be placed between the two layers of transparent conductive materials ( 16  and  18 ). In fact, it is possible to place the assembly  22  in the center of the chamber  24 , provided the photovoltaic cells  22   i-vi  are not shorted by the electrochromic material  28 . This may be accomplished by having a seal disposed around the assembly  22  to ensure there is no contact with the electrochromic media  28 . In this case, connection to the bus bars ( 36  and  40 ) may be made by conductive strips brought into the center of the window  10 . Each discrete photovoltaic device in assembly  22  may have a distinct shape and each device may be arranged in such a way that the entire assembly  22  has a particular design such as diamond, circle, or other known or unique configuration. Alternatively, an array of photovoltaic cells may be distributed throughout the window area, with no bus bars, such that each photovoltaic cell produced enough potential to drive a portion of the electrochromic media  28  around it, and the density of the photovoltaic cells are such that the entire electrochromic material  28  is capable of being darkened. Since the cell spacing of the window is comparable to the thickness of a typical photovoltaic cell, the photovoltaic cells may be optionally used to provide and maintain spacing between the glass elements ( 12  and  14 ). An electrochromic window made in accordance with the present invention has a cell spacing which ranges from about 100 microns to about 5000 microns and, more typically has a cell spacing from about 300 microns to about 3000 microns. 
     Photovoltaic devices or solar cells are well known and may comprise a wide variety of p-n junction and Schottky barrier devices comprising materials such as, but not limited to, polycrystalline-, amorphous- and single crystal-structures of silicon, gallium arsenide, gallium phosphide, indium phosphide and indium antimonide, as well as amorphous cadmium sulfide, cadmium selenide, copper indium selenide, copper indium selenide/cadmium sulfide, and the like. The amorphous structures can be made into thin films which can be easily bonded onto a layer of transparent conductive material and, therefore, allow a plurality of photovoltaic cells to be electrically connected in series (discussed in detail hereinbelow). This makes manufacturing of the overall electrochromic window  10  easier and less costly. The presently preferred photovoltaic cells are single crystal and polycrystalline silicon cells. 
     One important aspect to selecting the size and structure of the photovoltaic assembly  22  is to ensure that the voltage and current output of assembly  22  matches the voltage and current necessary to darken and accurately control the amount of darkening of the electro-optic window  10 . The presently preferred electro-optic device is a self-erasing electrochromic window. In such a system the intensity of electromagnetic radiation is modulated or attenuated by passing through electrochromic media  28  which is in contact with transparent conductive materials  16  and  18 . Typically the media  28  includes at least one anodic compound and at least one cathodic compound. The anodic compound is electrochemically oxidized and the cathodic compound is electrochemically reduced when a DC electrical potential difference is impressed across the electrochromic media  28 . The self-erasing property of the present invention means that, after a potential difference between the electrodes of a device is decreased or eliminated, the transmittance of the solution in the device will increase spontaneously, without need of reversal of the polarity of the electrodes or a bleeder resistor or an external switch, to a value characteristic of the new potential difference. The self-erasing feature is provided by the spontaneous, apparently diffusion-limited, reactions of oxidized anodic compound with reduced cathodic compound to yield anodic compound and cathodic compound in their respective zero-potential equilibrium states. The electrochromic device  10  may be a hybrid between a surface confined electrochromic material on one transparent electrode and a solution phase electrochromic material. In a hybrid system, diffusion from the other transparent electrode to the surface confined electrochromic material provides the self-erasing feature when power is removed or decreased. 
     This is especially important for a photovoltaic-powered device of the present invention. As the sun rises and begins to impinge on the window (and the photovoltaic device), the photovoltaic device generates a current which travels to the two layers of the transparent conductive materials ( 16  and  18 ) and a certain electrical potential (P 1 ) is impressed across (and darkens) the electrochromic media  28 . When the potential is sufficient for current to flow through an all solution phase electrochromic media  28 , the anodic material is continually being oxidized and the cathodic material is being reduced to replace the anodic and cathodic compounds which diffuse away from the transparent conductive layers and spontaneously react to form non-colored species in the bulk of media  28 . As the sun continues to rise, more light hits the window and more power is generated by the photovoltaic assembly  22  and impressed on the window, the electrochromic media  28  darkens further. As the sun begins to set, less light hits the window and less power is generated by the photovoltaic assembly  22  and impressed on the window. The transmittance of the electrochromic media  28  spontaneously increases to a new level because the number of species being electrochemically colored is less than before. No other system allows for this simple and accurate auto-adjustment without complicated circuitry. 
     Although this surprising feature has significant advantages, it is important to ensure that the photovoltaic cells are chosen such that the output matches the requirements of the electrochromic window  10 . Generally speaking, the photovoltaic cells should make up less than about 10 percent of the total area of the electrochromic window  10 , whether placed along the edge or in the center of the window  10 . For example, an electrochromic window  10  made with the electrochromic materials disclosed and claimed in U.S. Pat. No. 5,679,283, entitled “Electrochromic Layer and Devices Comprising Same” to W. L. Tonar et al., needs a voltage range of about 0.4 volts to about 1.2 volts and a current range from about 500 microamps per squared centimeter to about 1 microamp per squared centimeter. More typically the current ranges from about 100 microamps per squared centimeter to about 1 microamp per squared centimeter. Photovoltaic devices are well known in the art and their voltage and current output can be adjusted simply by adjusting the size of the device and/or by electrically connecting one or more devices in series. It is possible to have a single photovoltaic device produce from about 0 to about 2.0 volts when exposed to radiant energy ranging from about 0 watts per square centimeter to about 1,000 watts per square centimeter. Therefore, given the window&#39;s  10  specified output, simple experimentation will lead one skilled in the art to match the photovoltaic assembly with what is required for the window  10 . For example, for a dipolar suspension device operating at 20 volts approximately 30-40 silicon photovoltaic cells could be connected in series to supply the required voltage. For a polymer dispersed liquid crystal device operating at 100 volts approximately 150 to 200 photovoltaic cells in series would be needed. 
     Photovoltaic assembly  22  is made up of at least one photovoltaic cell if silicon is used with an electrochromic device made in accordance with the teachings of above-referenced U.S. Pat. No. 5,679,283. Assembly  22  is preferably made up of one or more sets of photovoltaic cells electrically connected in series. FIG. 1 shows six photovoltaic cells  22   i  through  22   vi  set up in two sets of three cells, i.e.,  22   i-iii  electrically connected together in series with  22   iv-vi.  Thus each set of three cells produces a voltage of approximately 0 to about 0.6 volts depending on the brightness of the sun and, when electrically connected in series, produce a voltage of about 0 to about 1.2 volts. As those skilled in the art will realize, it is possible to combine more sets in series to produce higher voltages. Such a setup allows gray-scale control in that the level of visible light transmittance of electrochromic material  28  in chamber  24  is continuously variable from a transmittance value of typically about 80 percent to a transmittance of typically about 4 percent. This variable transmission is controlled by the amount of light impinging on the photovoltaic assembly  22  and therefore the power output from photovoltaic assembly  22  relative to the power requirements of the window  10  to achieve a given level of darkening. This control of the level of darkening is automatic if the area and efficiency of the photovoltaic assembly  22  is scaled to the area and power requirements of the window  10 . However, if desired the transmittance may be more narrow to provide some tint at all times, for example, the range may be from about 50 percent to about 10 percent transmittance. In addition, in some climates it may also be desirable to provide for a covering for the photovoltaic area if there was a desire to prevent the window from darkening. This would be particularly useful in climates which experience large temperature variations. For warm weather conditions, the window could be allowed to darken as a result of the solar illumination. In cold weather, the photovoltaic area might be covered to prevent the window from coloring which will allow maximum sunlight to enter the building to provide light and heat. 
     Referring specifically to FIG. 1, in operation, light impinges on the top surfaces  22   ia-via  of cells  22   i-vi.  The light impinging on cell surfaces  22   ia-iiia  provides a certain voltage output which depends on the composition and size of the photovoltaic cells  22   i-vi,  current draw of the window  10  and the intensity of the impinging light. The current path leads out the bottom surfaces  22   ib-iiib  of photovoltaic cells  22   i-iii  and travels to the top surfaces  22   iva-via  of photovoltaic cells  22   iv-vi  through bus bar  30 , via interconnect  32  and bus bar  34 . Bus bars  30  and  34  may be made of any material which will ensure that photovoltaic cells  22   i-iii  are conductivity adhered to the layer of transparent conductive material  18 , and cells  22   iv-vi  are consecutively adhered to the layer of transparent conductive material  16 , respectively. The bus bars  30  and  34  may comprise a layer of deposited metal, silver paint, a conductive frit, a spring clip, or a combination thereof. The presently preferred material for bus bars  30  and  34  is 112-15 which is a silver ink made by Creative Materials Inc., in Tynsboro, Mass. Interconnect  32  ensures a current path between the bus bar  30  and bus bar  34  and may be any conductive material, such as a strip or roll of copper, a silver epoxy, or other materials well known in the art for use as conductive interconnects. Assuming cells  22   i-iii  and  22   iv-vi  each produce approximately 0.6 volts, then approximately 1.2 volts are applied between the layers of transparent conductive materials ( 16  and  18 ) through bus bar  36 . Channel  38  extends through transparent material  18  and optionally through a small portion of second glass element  14  such that none of the current transmitted through bus bar  36  to material  18  is allowed to reach cells  22   i-iii  without passing though electro-optic material  28  in chamber  24 . Channel  38  also ensures that the current produced by cells  22   i-iii  must travel through via interconnect  32 , through cells  22   iv-vi  and bus bar  36  before contacting that portion of material  18  in contact with electrochromic media  28 . The potential difference between transparent materials  16  and  18  causes the electro-active species within electrochromic material  28  to be either reduced or oxidized thereby allowing current flow through medium  28 . As a result, the window  10  darkens, i.e., attenuates, the light traveling therethrough. Transparent coating  16  is in electrical contact with bus bar  40  which carries the current back to the top surfaces  22   ia-iiia  of cells  22   i-iii  to complete the electrical circuit. Channel  42  extends through transparent coating  16  and optionally through a small portion of first glass element  12  such that none of the voltage transmitted through bus bar  40  to cells  22   i-iii  is allowed to reach cells  22   iv-vi  without being transmitted though cells  22   i-iii.  Bus bars  36  and  40  may also comprise a layer of deposited metal, silver paint, a conductive frit, a spring clip, or the presently preferred silver ink. Bus bars  36  and  40  are disposed along the outer periphery of sealing member  20  and, as shown in FIG. 2, are not in electrical contact with one another other than through assembly  22  and electrochromic media  28 . Other methods of achieving series/parallel connections of the photovoltaic cell assembly  22  are known in the art, but the above is presently preferred. 
     Referring to FIG. 3, electro-optic devices in the configuration of windows embodying the present invention may include a frame  44  which extends around the entire periphery of electro-optic window  10 . The frame  44  conceals and protects the peripheral edge portions of sealing member  16  and both the front and rear glass elements ( 12  and  14 , respectively). A wide variety of frame designs are well known in the art of window manufacturing. Frame  44  has an opening  46  allowing photovoltaic cells  22   i-vi  a clear view of the sunlight which is impinging on the window  10 . 
     The following examples are intended to describe certain embodiments of the present invention and should not be interpreted in any manner as limiting the scope of the invention as set forth in the accompanying claims. 
     EXAMPLE 1 
     A self-erasing, solution-phase electrochromic window may be produced having an active electrochromic area approximately 25 cm×30 cm with a cell spacing of approximately 890 microns. The window will have concentrations of anodic and cathodic electrochromic materials of about 6 mM each and approximately 5% by volume of polymethylmethacrylate dissolved in propylene carbonate as a solvent. The anodic material is 5,10-dihydro-5,10-dimethylphenazine and the cathodic material is 1,1′-di(phenylpropyl)-4,4′-dipyridinium difluoroborate. Such a window will require approximately 1 volt to reduce the transmission of the window from above 70% to below 10% and 20 mA of current to maintain a steady-state darkened condition. 
     For two sets of p-n junction silicon photovoltaic devices connected in series, a solar illumination of 500 W/m2 will produce a current output of 2.5 mA/cm 2  of photovoltaic cell area at 1.0 volt. A minimum photovoltaic area of 8 cm 2 will be required to produce the 20 mA necessary to darken the electrochromic window in this example. 
     EXAMPLE 2 
     Propylene carbonate was added to a reaction flask and deoxygenated with dry nitrogen. 20 weight percent of monomers were added to the reaction flask in the molar ratio of 1 part 2-hydroxyethyl methacrylate (HEMA) to 5 parts methyl methacrylate (MMA). The MMA was purified by through distillation at atmospheric pressure using a short neck distillation column. The HEMA was purified by distillation at approximately 3 mm-Hg using a short neck distillation column. The reaction flask was heated to 70 degrees Celsius and a free radical initiator (V-601{Dimethyl 2,2′-Azobis(2-methylpropionate)}(Dimethyl 2,2′-azobisisobutyrate) was added. When the viscosity of the resulting solution increased noticeably, more propylene carbonate (at reaction temperature) was added to the flask. The reaction was continued until completion with the resulting weight percent of the pre-polymer being 10 percent. The pre-polymer solution was diluted to 5% by weight with propylene carbonate. 30 millimolar of Tinuvin p was added to and dissolved in the pre-polymer solution. 3 millimolar of 1,1′-di(3-phenyl(n-propyl)-4,4′bipyridinium and 3 millimolar 5,10-dihydro-5,10-dimethylphenazine were added to the pre-polymer solution. This solution was degassed by vacuum and flushed with nitrogen. Sufficient toluene diisocyanate crosslinker was added to crosslink approximately 60 percent of the theoretical hydroxyl positions. This solution was thoroughly mixed to ensure that the crosslinker was evenly distributed throughout the fluid. 
     A part was assembled which was large enough to allow observation of its performance over long periods of time in a use similar to that proposed for an electrochromic window. Two glass elements coated with a fluorine doped tin oxide transparent conductive coating were cut to the desired size. An epoxy seal material was dispensed on one of the glass elements, then both glass elements were placed on half inch glass vacuum platens. The platens were then held apart with metal spacers and placed in a near vertical position. The platen assembly was then placed in an oven to cure the epoxy. This assembly had the dimensions of about 100 cm by 140 cm, and had an interpane space of about 1.5 millimeters. The solution was introduced into the device. 
     The electrochromic polymer solution had reached its gel point within 3 days of adding the crosslinking agent. Crosslinking may either be continued at room temperature or may be optionally accelerated by placing the part in a warm oven, e.g., 70 degrees Celsius. 
     After the electrochromic solution was cured, its function was tested by applying 1.0 volts. Transmission of these parts in the bleached state is 78%. In the fully darkened state the transmission is 5%. The transmission went from 78 to 5 in around 20 minutes and colors from the edges to the center. Brush marks and streaks are apparent especially at transition levels in transmission. The marks and streaks became more apparent over time. The window requires approximately 60 mA to maintain steady-state transmission of the window at about 5% for visible light. 
     For two sets of p-n junction silicon photovoltaic devices connected in series with a solar illumination of 500 W/m2 with a current output of 2.5 mA/cm2 at 1.0 volts, a minimum photovoltaic area of 24 cm2 would be required to produce the 60 mA necessary to maintain the darkened condition of the electrochromic window in this example. 
     Liquid crystal devices and some types of electrochromic devices, e.g., solution phase-, gel- and hybrid-types, require uniform spacing between the two glass elements ( 12  and  14 ). This uniform spacing is needed to ensure even and uniform darkening as well as to minimize any double imaging problems. Also, as stated above, the cell spacing of the electrochromic window  10  of the present invention are surprisingly matched to the thickness of commercially available photovoltaic cells. Although it is common practice in the electro-optic art, in some instances utilizing the photovoltaic cells, or other objects such as glass beads and the like between the glass elements (usually in the seal), as an internal cell spacer is not the presently preferred method for ensuring uniform cell thickness. The reason for this is that these spacers are either fragile or rigid, both of which have associated manufacturing problems. When the seal material  20  of the present invention cures it shrinks. As the spacing between the glass elements  12  and  14  increases, so does the amount of seal material which must be used to fill seal the space. As the thickness of the seal material  20  increases so does the amount it shrinks during cure. Therefore, if the photovoltaic cells are used by themselves as a spacer element they will be damaged as the glass elements  12  and  14  move toward one another as the seal material  20  shrinks during cure. If glass beads are used, high stress will develop between the beads and the glass elements  12  and  14  as the seal material  20  tries to shrink. When the cell spacing exceed approximately 300 microns, this stress may be high enough to break the glass elements ( 12  and/or  14 ). Further, this higher stress in the seal makes the seal less durable to thermal and mechanical stresses experienced when the device is exposed to variable environmental conditions. 
     One method for reducing this stress is to use a flexible seal material, however, flexible seals have their own operational deficiencies in that they do not have adequate oxygen- and moisture-barrier properties. If oxygen or moisture are able to permeate the seal they can degrade the electrochromic system and cause the window to not function properly. Thus, although rigid seals are preferred due to their barrier properties an improved method is needed to bond the assembly together (prior to filling the chamber  24  with electrochromic medium  28 ). 
     In accordance with another embodiment of the present invention, a spacerless electrochromic window  10  is provided, as is the method for producing such a device. A key aspect of this embodiment is ensuring that the glass elements  12  and  14  are held in a spaced-apart and parallel relationship while the seal material  20  cures. The substrates can be held by a variety of methods, including edge clamps, edge clips and vacuum chucking, with a vacuum chucking system being preferred. Typically a near vertical cure is preferred to reduce the amount of sag the substrates experienced as a result of gravitational pull, however, with a vacuum chucking system, since both glass elements are held to a planar vacuum substrate, the glass elements may be bonded in a horizontal orientation. Referring to FIG. 4, two vacuum-applying members  50  and  52  are provided to contact glass elements  12  and  14  on the sides which are most remote from each other (i.e., the sides that do not confront one another) and apply a vacuum to each glass element. The sealably bonding material  20  is disposed along the periphery of one of the transparent conductive coatings, e.g., material  18 , and the second glass element (i.e., transparent coating  16 ) is brought into a spaced-apart and parallel relationship with the first glass element such that the circumferential edges of each glass element are substantially aligned. Finally, sufficient heat or UV light is applied to uniformly and completely cure seal material  20 . The vacuum-applying members can be held in the spaced apart relationship by a simple spacer  54  such that, as the seal member  20  cures and contracts, the glass elements  12  and  14  may pull away from the vacuum-applying member and reduce the stress in the seal member  20 . In a more complicated system, the two vacuum-applying members  50  and  52  can be held in a spaced-apart and parallel relationships by a hydraulics system (not shown) well known in the art. Optionally, the vacuum-applying members can have embedded heaters, or the entire assembly can be heated with infra-red radiation, a convection heating oven or other methods well known in the art. It is important, however, that the seal member  20  be heated or, when a UV curing epoxy is utilized have radiation applied, in a uniform manner to prevent uneven curing which can produce inconsistencies in the spacing of the substrates. 
     Referring to FIGS. 5 and 6, there is illustrated therein another embodiment of the invention in the configuration of a pair of eyeglasses, generally designated  110 , embodying the present invention. In general, in eyeglasses embodying the present invention, the eyeglasses are comprised of a frame  112  and conventional eyeglass temples  114  and  116  pivotally connected to the frame  112  in a conventional manner. The frame  112  carries the optical glass or plastic elements which will be described hereinafter in greater detail. In this embodiment of the invention the eyeglasses  110  include a thin layer  118  of an electrochromic chemical solution disposed between two glass or plastic lens elements  120  and  122 . When the electrochromic chemical solution  118  is electrically energized, it darkens and begins to absorb light. The higher the voltage, the darker the eyeglasses become. When the electrical voltage is decreased to zero, the eyeglasses return to their clear state. Given the proximity to the human eye and the high risk of impact/breakage, the use of plastic substrates in any sunglass application is preferred. In addition, the use of aqueous rather than organic solvents in electrochromic sunglasses would be favored from the standpoint of substrate compatibility. Numerous electrochromic materials can be utilized for aqueous-based electrochromic systems, including but not limited to bipyridinium salts (especially the halide, nitrate, and triflate salts), Fe(aq) 3+ / 2+ , Eu(aq) 3+ / 2+ , [Ru(NH 3 ) 6 ] 3+ / 2+ , [Fe(CN) 6 ] 3− / 4− , various water-soluble ferrocene derivatives, and conductive polymers such as polyaniline, polythiophene and their derivatives. Aqueous-based electrochromic media may optionally include additives such as electrolyte salts, UV stabilizers, antioxidants, thickeners, or the like. Examples of suitable additives include ethylene glycol, polyvinyl alcohol, and polystyrenesulfonate. It should be understood, however, that if desired, the electrochromic components of the eyeglasses embodying the present invention may be of the types disclosed in U.S. Pat. No. 4,902,108, issued Feb. 20, 1990, for Single-Compartment, Self-Erasing, Solution-Phase Electrochromic Devices, Solutions for Use Therein, and Uses Thereof, and assigned to the assignee of the present invention. 
     The pair of eyeglasses  110  embodying the present invention is depicted in simplified cross-section in FIG.  6 . Since some of the layers of the eyeglasses are very thin, the scale has been distorted for pictorial clarity. As shown in FIG. 6, the eyeglasses  110  include a sealed chamber  124 . The front element  120  has a transparent electrically conductive layer  126  thereon, and the rear element  122  has a transparent electrically conductive layer  128  thereon. The chamber  124  is thus defined by the transparent electrically conductive layer  126 , an edge seal  130 , and the transparent electrically conductive layer  128 . The chemical solution  118  having the desired electrochromic properties fills the chamber  124 . A photovoltaic cell  132  is provided which is disposed in the chamber  124 , the terminals  134  and  136  of the photovoltaic cell  132  being electrically connected to the conductive layers  126  and  128 , respectively. The photovoltaic cell  132  and its terminals  134  and  136  are surrounded by a tubular member  138  which functions to insulate the photovoltaic cell and its terminals from the electrochromic solution in the chamber  124  to prevent short circuiting of the terminals  134  and  136  by the electrochromic solution. Of course, other means of electrically insulating the photovoltaic cell and its terminals from the electrochromic solution may be utilized. 
     As shown in FIGS. 5 and 6, the front and rear elements  120  and  122  each include a right lens portion  140  and a left lens portion  142  integrally joined by a bridge portion  144  whereby each of the lens elements is in the form of a continuous unitary structure. Also as shown in FIG. 6, the electrical terminals  134  and  136  of the photovoltaic cell  132  are electrically connected to the bridge portion  144  of each of the conductive layers  126  and  128  so that the entire conductive layer  126  is electrically connected to the terminal  136  of the photovoltaic cell  132  while the entire conductive layer  128  on the rear element  122  is electrically connected to the terminal  134  of the photovoltaic cell  132 . 
     The active area of the exposed front face of the photovoltaic cell may be matched with the area of the electrochromic solution, the ratio of the relative area of the photovoltaic cell with respect to the area of the electrochromic solution being set whereby a predetermined light level provides enough electrical power to dim the electrochromic solution to a desired comfortable level. 
     Light rays enter through the front lens element  120 , the transparent electrically conductive layer  126  and the electrochromic layer.  118  before being transmitted through the other conductive layer  128  and the rear lens element  122 . Thus, the entering light rays are attenuated in proportion to the degree to which the electrochromic solution  118  is light absorbing. When the electrochromic solution is highly light absorbing, the intensity of the entering light rays reaching the eyes of the wearer is diminished. Thus the basic structural elements of the electrochromic assembly include two electrode-bearing lens elements  120  and  122 , a seal  130  which spaces apart and holds the lens elements in substantially parallel relationship in an assembled pair of eyeglasses  110 , and which surrounds a chamber  124  which in assembled eyeglasses is defined by the electrode layers  126  and  128  on the electrode-bearing lenses as well as the circumferential inside walls of the spacing and sealing layer  130 . The volume of the chamber  124  may be filled with any of the solutions disclosed in U.S. Pat. No. 4,902,108 which have reversibly variable transmittance in the operation of the eyeglasses, the solution in the chamber being in contact with both electrode layers  126  and  128  during operation of the eyeglasses  110 . 
     As illustrated in the drawings, the frame  112  surrounds the electrochromic assembly in a circumferential manner, the frame  112  including flange portions  139  and  140  integrally joined by a web portion  141 . The frame  112  conceals the edge portions of the front and rear lens elements and the sealing member  130 . The frame  112  thus can extend around the entire circumference of the electrochromic eyeglass assembly. It should be understood that if the seal is pleasing in appearance, it is not necessary to contain it. 
     Referring in greater detail to the drawings, the photovoltaic powered electrochromic eyeglasses  110  embodying the present invention includes the front transparent lens element  120  having a front face  144  and a rear face  146 , and the rear lens element  122  having a front face  148  and a rear face  150 . The front transparent lens element  120  and the rear transparent lens element  122  may be formed of any one of a number of materials which are transparent in the visible region of the light spectrum and have sufficient strength to withstand the forces exerted thereon that may vary as a result of varying temperatures and/or impact forces conventionally exerted on eyeglasses. The front and rear lens elements  120  and  122  may be formed of various types of polymers or plastic sheet materials and the like. By way of example, the lens elements may be formed of polyolefins such as Cyclic Olefin Copolymers, like Topas, available from Hoechst of Frankfurt, Germany, or polycarbonate such as CR-39 from PPG of Pittsburgh, Pa., or acrylics such as Lucite from Dow Chemical of Midland, Mich., or polyester such as mylar available from DuPont of Wilmington, Del., or commercially available clear polyvinyl chloride or polystyrene polymer. It will be understood that, if desired, the front and rear elements  120  and  122  may be formed of suitable glass and may possess ultraviolet barrier properties to protect the electrochromic material. The thicknesses of the front and rear lens elements  120  and  122  may typically range from about 1 millimeter to about 6 millimeters. 
     As previously mentioned, a layer  126  of transparent electrically conductive material is deposited on the rear face  146  of the front lens element  120  to act as an electrode, and another layer  128  of transparent electrically conductive material is deposited on the front face  148  of the rear lens element  122 . Both layers of the transparent conductive materials cover the entire surfaces of the right and left lens sections  140  and  142  and the bridge section  144  on which they are deposited. The layers of transparent conductive materials may be the same or different and may be of any material which adheres satisfactorily to the front lens element  120  and the rear lens element  122 , is resistant to adverse interaction with any materials within the electrochromic eyeglasses that the front and rear elements may come in contact with, is resistant to adverse interaction with the atmosphere, has minimal diffused or specular reflectance, high light transmission, and good electrical conductance. The layers  126  and  128  of transparent conductive material may be fluorine doped tin oxide, tin doped indium oxide (ITO), thin metal layers, ITO/metal/ITO (IMI), or other transparent conductive materials. The conductive layers may be undercoated with hard coat materials such as SiO 2  or other layers that would, for example, retard oxygen or moisture or other gas permeation and enhance adhesion to the plastic such as with thin layers of chromium metal. In general, the conductance of the layers  126  and  128  of transparent conductive materials will depend on their thickness and composition if ITO or fluorine doped tin oxide is used. The thickness of either transparent conductive layer may range from about 50 Å about 5000 Å, it being understood that if it is a transparent metal oxide, thicknesses may range from about 250 Å to 3500 Å. Transparent metal layers are typically thinner from about 10 Å to 300 Å. IMI, on the other hand, may have superior conductivity compared with the other materials, but is generally more difficult and expensive to manufacture and therefore may be useful where high conductance is desirable. The thickness of the various layers in the IMI structure may vary but generally the thickness of the first ITO layer ranges from about 150 Å to about 1000 Å, the metal ranges from about 10 Å to about 250 Å and the second layer of ITO ranges from about 150 Å to about 1000 Å. The metal for the intermediate layer may be silver, gold, rhodium, platinum, aluminum and the like. Also there may be additional layers of metal and ITO if desired, such as IMI. Moreover, an optional layer or layers of an anti-iridescent, and anti-reflection and/or a color suppression material or materials may be deposited between the transparent conductive material  126  and the front lens rear face  146  and/or between the transparent conductive material  128  and the rear lens front face  148  so as to suppress or filter out any unwanted portions of the light spectrum. Also, if desired, conventional anti-scratch material may be applied to the exposed surfaces  144  and  150  of the lens elements or a thin layer of chrome can be added to promote adhesion to the substrate. 
     As show in FIG. 6, the front lens element is sealably bonded to the rear glass element in a spaced apart and parallel relationship by the bonding seal  130  disposed between and adhered to the transparent conductive layers  126  and  128 . The bonding seal  130  is generally disposed around the entire periphery of the front and rear lens elements, and the bonding seal may be formed of any suitable material which is capable of adhesively bonding the layers  126  and  128  of transparent conductive material, while, after adhering, being capable of maintaining a generally constant distance therebetween. The seal  130  should also not be permeable to water or oxygen to any significant degree, and should be generally inert to the electrochromic material disposed in the chamber  124 . The seal  130  may comprise a strip or gasket of polymeric material, such as rubber, urethane, acrylate, epoxies and the like. 
     The chamber  124  defined by the transparent conductive material  126  disposed on the front element  120  and the transparent conductive material  128  disposed on the rear lens element  122 , and the inner circumferential wall of the seal  130 , is filled with the electrochromic medium  118  previously described. The electrochromic medium is capable of changing properties such that light traveling therethrough is attenuated when voltage is applied to the electrochromic medium. The electrochromic medium  118  may be inserted into the chamber  124  through a sealable fill port through well known techniques such as by injection, vacuum back filling and the like. 
     In accordance with the present invention, at least one photovoltaic cell  132  is disposed between the two layers of transparent conductive material  126  and  128  such that the photon absorbing side of the photovoltaic cell is facing toward the forward surface  144  of the front lens element  120 . In addition, the photovoltaic cell  132  is disposed in the bridge area  144  of the front and rear lens elements between the left and right lens areas  140  and  142  so that the photovoltaic cell terminals  134  and  136  are in electrical contact with the transparent electrically conductive layers  126  and ( 128  which cover the right and left lens areas  140  and  142  and the bridge area  144  disposed therebetween as previously described. As shown in FIGS. 5 and 6, the photovoltaic assembly is preferably a single cell, although it will be understood that two or more cells may be connected in series if so desired. An important aspect of the present invention resides in placing the photovoltaic cell  132  between the two layers  126  and  128  of the electrically conductive materials which cover the right and left lens areas  140  and  142  and the bridge area  144  therebetween. With such a construction, the front and rear lens elements  120  and  122  protect the photovoltaic assembly, including the photovoltaic cell  132  and its terminals, from damage. Since the photovoltaic cell terminals are in direct contact with the transparent conductive materials, the need for any external wiring or circuitry is eliminated. Moreover, the need for batteries or other sources of electrical potential is also eliminated. Furthermore, electrical connection devices, such as spring clips, are not needed to connect the conductive materials to external circuitry thereby simplifying the design and reducing the cost of the electrochromic eyeglasses. However, if desired the transparent conductive layer could be supplemented by a conductive bus such as a metallic film layer around the perimeter or a wire conductor in the sealing area to help distribute electrical current and aid in uniform coloration or darkening. In the embodiment of the invention illustrated in FIGS. 5 and 6, a single photovoltaic cell is provided which produces the voltage and current necessary to drive the electrochromic material, the photovoltaic assembly being placed between the two layers of transparent electrically conductive material. In order to prevent the photovoltaic cell from being short circuited by the electrochromic material, the tubular member  138  is provided which is disposed around the photovoltaic cell  132  to ensure that there is no direct contact with the electrochromic media. Since the distance between the conductive layers  126  and  128  defining the chamber  124  is greater than the thickness of a typical photovoltaic cell, the photovoltaic cell may be easily accommodated in the chamber  124  in the area of the bridge portion  144  between the right and left lens portions  140  and  142  of the eyeglasses  110 . 
     The photovoltaic cells are well known and may comprise a wide variety of p-n junctions and Schottky barrier devices comprising materials such as, but not limited to, polycrystalline-, amorphous- and single crystal-structures of silicon, gallium arsenide, gallium phosphide, indium phosphide and indium antimonide, as well as amorphous cadmium sulfide, cadmium selenite, copper indium selenite, copper indium selenite/cadmium sulfide, and the like. The amorphous structures may be made into thin films which can be easily bonded onto a layer of transparent conductive material and, therefore, can be easily accommodated within the electrochromic chamber. It is preferred that the photovoltaic cells be single crystal and polychrystalline silicon cells. 
     An important aspect in the selection of the size and structure of the photovoltaic assembly in this embodiment of the invention is to ensure that the voltage and current output of the photovoltaic assembly match the voltage and current necessary to darken and accurately control the amount of darkening of the electrochromic eyeglasses. In this embodiment of the invention, it is preferred that the electrochromic media be self-erasing. In such a system the intensity of the light is modulated or attenuated by passing through the electrochromic media which is in contact with the transparent electrically conductive materials  126  and  128 . Typically the electrochromic media  118  includes at least one anodic compound and at least one cathodic compound. The anodic compound is electrochemically oxidized and the cathodic compound is electrochemically reduced when a DC electrical potential difference is impressed across the electrochromic media. The self-erasing property of the present invention means that, after a potential difference between the electrodes  126  and  128  is decreased or eliminated, the transmittance of the solution  118  in the chamber  124  will increase spontaneously, without the need of reversal of the polarity of the electrodes and without the need of a bleeder resistor or an external switch, to a value characteristic of the new potential difference. The self-erasing feature is provided by the spontaneous, apparently diffusion-limited, reactions of oxidized anodic compounds with reduced cathodic compounds to yield anodic compounds and cathode compounds in their respective zero-potential equilibrium states. It should be understood that with the solution phase electrochromic media, the self-erasing electrochromic solution performs a double function, i.e. performs the function of coloring and also acts as a bleeder resistor which constantly dissipates electrical energy thereby obviating the need for a separate bleeder resistor as used in the above mentioned U.S. Pat. No. 5,377,037. 
     In photovoltaic powered eyeglasses embodying the present invention, as light impinges on the photovoltaic cell  132 , the photovoltaic cell generates an electrical current which travels to the two layers of the transparent conductive materials, and an electrical potential is impressed across and darkens the electrochromic media. When the potential is sufficient for current to flow through the solution-phase electrochromic media, the anodic material is continually being oxidized and the cathodic material is being reduced to replace the anodic and cathodic compounds which diffuse away from the transparent conductive layers and spontaneously react to form non-colored species in the bulk of the electrochromic media. As additional light impinges upon the photovoltaic cell  132 , more power is generated by the photovoltaic assembly and impressed on the eyeglasses, and the electrochromic material darkens further. When less light impinges upon the photovoltaic cell and less power is generated by the photovoltaic cell the transmittance of the electrochromic media spontaneously increases to a new level because the number of species being electrochemically colored is less than before. It will be understood that such accurate automatic adjustment is obtained without complicated circuitry. As illustrated in FIGS. 7 and 8 of the drawings, and as will be described hereinafter in greater detail, a mechanical shutter may also be provided to control the amount of light impinging upon the photovoltaic cell, thereby enabling the user to adjust the darkening of the electrochromic material to an individually comfortable level. 
     It should be understood that the photovoltaic cell  132  is chosen whereby the output matches the requirements of the electrochromic media. Photovoltaic cells are well known in the art and their voltage and electrical current output can be adjusted simply by adjusting the size of the photovoltaic cell and/or by electrically connecting one or more cells in series. It should be understood that a single photovoltaic cell can produce from about 0 to about 2.0 volts when exposed to light energy ranging from about 0 watts per square centimeter to about 1,000 watts per square centimeter. Therefore, given the eyeglasses&#39; specified output, simple experimentation will enable one skilled in the art to match the photovoltaic cell with the requirements for the eyeglasses. It should be understood that the present invention permits gray-scale control in that the level of visible light transmittance of the electrochromic material in the chamber is continuously variable from a transmittance value of approximately 80% to a transmittance value of approximately 4%. This variable transmission is controlled by the amount of light impinging on the photovoltaic assembly and therefore the power output from the photovoltaic assembly relative to the power requirements of the eyeglasses is controlled to achieve a predetermined level of darkening. This control of the level of darkening is automatic if the area and efficiency of the photovoltaic assembly is scaled to the area and power requirements of the eyeglasses. If desired, the transmittance may be adjusted to provide some tint at all times, for example, the range may be from about 50% to about 10% transmittance. In addition, as previously mentioned, it may also be desirable to provide for a mechanical shutter or other covering for the photovoltaic area if there is a desire to adjust the darkening or a desire to prevent the eyeglasses from darkening. 
     Referring again to the drawings, in operation, light impinges on the forward facing surface of the photovoltaic cell  132 . The light impinging on such cell surface provides a certain voltage output which depends on the composition and size of the photovoltaic cell, the current draw of the eyeglasses, and the intensity of the impinging light. The photovoltaic cell is electrically connected to the transparent electrically conductive layers whereby the voltage generated by the photovoltaic cell is applied between the layers of transparent conductive material. The potential difference between the transparent materials causes the electro-active species within the electrochromic material to be either reduced or oxidized thereby allowing current flow through the electrochromic medium. As a result, the eyeglasses darken, i.e., attenuate the light traveling therethrough. 
     Another embodiment of the invention is illustrated in FIGS. 7 and 8 of the drawings. This embodiment of the invention is comprised of a pair of eyeglasses  210  which include a frame  212  having conventional eyeglass temples  214  and  216  pivotally connected thereto. In this embodiment of the invention, the eyeglasses  210  include the electrochromic chemical solution  118  disposed between the lens elements  120  and  122 , as previously described. This embodiment of the invention also includes the photovoltaic cell  132  and the associated terminals  134  and  136  electrically connected to the conductive layers  126  and  128  and electrically insulated by the tubular member  138  from the electrochromic solution as previously described. 
     As shown in FIGS. 7 and 8, in this embodiment of the invention the front and rear elements  120  and  122  each include a right lens section  140  and a left lens section  142  integrally joined by a bridge section  144  whereby each of the lens elements is in the form of a continuous unitary structure as previously described, the electrical terminals  134  and  136  of the photovoltaic cell being electrically connected to the bridge section  144  of each of the conductive layers  126  and  128  whereby the entire conductive layer  126  is electrically connected to the terminal  136  of the photovoltaic cell  132  while the entire conductive layer  128  on the rear element  122  is electrically connected to the terminal  134  of the photovoltaic cell  132 . 
     As shown in FIGS. 7 and 8 of the drawings, in this embodiment of the invention mechanical shutter means is provided which may be manually adjusted to control the amount of light striking the front face of the photovoltaic cell whereby the degree to which the electrochromic solution  118  is light absorbing may be controlled. Thus, when the front face of the photovoltaic cell  132  is fully exposed, the electrochromic solution will be highly light absorbing and the intensity of the light rays reaching the eyes of the wearer will be diminished. On the other hand, when the front face of the photovoltaic cell  132  is partially or fully blocked by the mechanical shutter means, the electrochromic solution will absorb less light, and the intensity of the light rays reaching the eyes of the wearer will be increased. 
     As shown in FIGS. 7 and 8, in this embodiment of the invention, the frame  212  is provided with integral spaced angle portions  251  and  253  that define opposed channels  252  and  254  in which the opposite side edge portions of a manually moveable opaque slide  256  are disposed for frictionally inhibited movement up and down as viewed in FIG.  7 . The slide  256  includes an opaque flat plate portion  258  the opposite sides of which are disposed in the channels  252  and  254  for sliding movement, and the slide  256  may be provided with a transversely extending ledge  260  to facilitate manual movement of the slide in the channels  252  and  254  by the wearer of the eyeglasses. It should be understood that the slide has a relatively tight fit in the channels  252  and  254  whereby the slide will remain in the selected adjusted position in the channels  252  and  254 . It should also be understood that other means, such as conventional detents, may be provided to hold the slide in the desired position within the channels  252  and  254 . 
     In the operation of this embodiment of the invention the amount of light impinging on the forward facing surface of the photovoltaic cell  132  may be manually controlled by the wearer of the eyeglasses. The amount of light impinging on such cell surface determines the output voltage of the photovoltaic cell with the result that the eyeglasses attenuate the light traveling therethrough as a function of the active area of the exposed front face of the photovoltaic cell. Thus the wearer of the eyeglasses may manually adjust the slide  256  to vary the amount of light impinging on the active face of the photovoltaic cell thereby controlling the degree to which the electrochromic solution attenuates the light reaching the eyes of the wearer of the eyeglasses. 
     While the invention has been described in detail herein in accordance with certain preferred embodiments thereof, many modifications and changes therein may be effected by those skilled in the art. Accordingly, it is our intent to be limited only by the scope of the appending claims and not by way of the details and instrumentalities describing the embodiments shown herein.