Patent Publication Number: US-2007107776-A1

Title: Photovoltaic powered multimedia greeting cards and smart cards

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
      This application claims the benefits of and priority to U.S. patent application Ser. No. 10/057,394 filed on Jan. 25, 2002, to U.S. Provisional Patent Application Ser. No. 60/351,691 filed on Jan. 25, 2002, to U.S. Provisional Patent Application Ser. No. 60/368,832 filed on Mar. 29, 2002, to U.S. Provisional Patent Application Ser. No. 60/390,071 filed on Jun. 20, 2002, to U.S. Provisional Patent Application Ser. No. 60/396,173 filed on Jul. 16, 2002, and to U.S. Provisional Patent Application Ser. No. 60/400,289 filed on Jul. 31, 2002, all of which are owned by the assignee of the instant application and the disclosures of which are incorporated herein by reference in their entireties. 
    
    
     FIELD OF THE INVENTION  
      The invention relates generally to the field of display systems, and more particularly to display apparatus incorporating photovoltaic cells.  
     BACKGROUND OF THE INVENTION  
      Thin film solar cells composed of percolating networks of liquid electrolyte and dye-coated sintered titanium dioxide were developed by Dr. Michael Grätzel and coworkers at the Swiss Federal Institute of Technology. These photovoltaic devices fall within a general class of cells referred to as dye-sensitized solar cells (“DSSCs”). Conventionally, fabrication of DSSCs requires a high temperature sintering process (&gt;about 400° C.) to achieve sufficient interconnectivity between the nanoparticles and enhanced adhesion between the nanoparticles and a transparent substrate. Although the photovoltaic cells of Grätzel are fabricated from relatively inexpensive raw materials, the high temperature sintering technique used to make these cells limits the cell substrate to rigid transparent materials, such as glass, and consequently limits the manufacturing to batch processes and the applications to those tolerant of the rigid structure.  
      Presently, many applications would benefit from the availability of a flexible photovoltaic cell or module. For example, greeting cards that play music or produce other audiovisual outputs are presently powered by batteries. Generally, conventional batteries lack flexibility and thus present the risk of breakage or creation of shorts when mounted on flexible substrates. They also add thickness and weight to the greeting card. Similarly, so-called “smart cards” (i.e., electronic cards used for identification purposes or to perform various functions) presently employ rigid conventional batteries that add bulk and limit substrate options.  
     SUMMARY OF THE INVENTION  
      The invention, in one embodiment, addresses the deficiencies of the prior art by providing a flexible photovoltaic cell integrated into an active card such as a greeting card or smart card. The flexible photovoltaic cell may be fabricated separately and then integrated with additional electronics on the active card. Alternatively, the photovoltaic cell may be fabricated on the active card itself, constituting, if desired, part or all of its surface design.  
      In one aspect, the invention provides a card including an information module, which provides information (e.g., electronic data or an audiovisual response) to a user or to a card reader, and a flexible photovoltaic module that powers the information module. Both the information module and the flexible photovoltaic module are mounted on the card. In another aspect, the invention provides a method for forming such a card. In various embodiments, the information module includes an audio unit for playing a prerecorded message, digital communication elements for downloading digital information (e.g., audio information such as music) from an external source, and/or an interface for providing digital data to a receiver or reader. The information module may be flexible and both the information module and the photovoltaic module may be mounted to a single surface on the card. Alternatively, the information module and the photovoltaic module may be mounted to different surfaces of the card to increase photovoltaic conversion space.  
      The information presented by the card may include (or consist of) visual content. In one embodiment, the information module includes a display element and the downloaded digital information includes display information. The display information may be text, graphical images (e.g., color images), photographic information, handwritten characters, or video information. The display unit may include, for example, a polymer-dispersed liquid crystal (PDLC) display.  
      The card may also include a power source operably coupled to both the photovoltaic module and the information module. In this embodiment, the photovoltaic module charges the power source, and the power source energizes the information module. The photovoltaic module may include at least one photovoltaic cell having a photosensitized interconnected nanoparticle material and an electrolyte redox system, with both disposed between first and second flexible, significantly light transmitting substrates. The photovoltaic module may include at least one photovoltaic cell having a photosensitized nanomatrix layer and a charge carrier medium. The photovoltaic cell may have a spectral response profile overlapping a spectral output profile of either indoor ambient fluorescent lighting or incandescent lighting.  
      In embodiments wherein the information module includes a display element, the card may include a storage element for storing data that determines the appearance of the display element. The display element may be part of the information module, and the card may have a global positioning system powered by electricity generated by the photovoltaic module. In various embodiments, the card itself is formed from paper, a laminated material, or plastic. The card may, for example, have a thickness of less than about 500 micrometers, but in any case is preferably flexible  
      Other aspects and advantages of the invention will become apparent from the following drawings, detailed description, and claims, all of which illustrate the principles of the invention, by way of example only. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The foregoing and other objects, features, and advantages of the invention described above will be more fully understood from the following description of various illustrative embodiments, when read together with the accompanying drawings. In the drawings, like reference characters generally refer to the same parts throughout the different views. The drawings are not necessarily to scale, and emphasis instead is generally placed upon illustrating the principles of the invention.  
       FIG. 1  depicts an exemplary chemical structure of an illustrative embodiment of a polylinker for nanoparticles of an oxide of metal M, in accordance with the invention;  
       FIG. 2  depicts another exemplary chemical structure of an illustrative embodiment of a polylinker, according to the invention, for nanoparticles of an oxide of metal M;  
       FIG. 3A  shows an exemplary chemical structure for an interconnected nanoparticle film with a polylinker, according to an illustrative embodiment of the invention;  
       FIG. 3B  shows the interconnected nanoparticle film of  FIG. 3A  attached to a substrate oxide layer, according to an illustrative embodiment of the invention;  
       FIG. 4  depicts the chemical structure of poly(n-butyl titanate);  
       FIG. 5A  shows the chemical structure of a titanium dioxide nanoparticle film interconnected with poly(n-butyl titanate), according to the invention;  
       FIG. 5B  shows the interconnected titanium dioxide nanoparticle film of  FIG. 5A  attached to a substrate oxide layer, according to an illustrative embodiment of the invention;  
       FIG. 6  is a cross-sectional view of a flexible photovoltaic cell, according to an illustrative embodiment of the invention;  
       FIG. 7  depicts an illustrative embodiment of a continuous manufacturing process that may be used to form the flexible photovoltaic cell shown in  FIG. 6 ;  
       FIG. 8  depicts a current-voltage curve for an exemplary solar cell, according to the invention;  
       FIG. 9  shows a current-voltage curve for an exemplary solar cell, according to an illustrative embodiment of the invention;  
       FIG. 10  shows current-voltage curves for two additional exemplary solar cells, according to an illustrative embodiment of the invention;  
       FIG. 11  depicts the chemical structure of gelation induced by a complexing reaction of Li +  ions with complexable poly(4-vinyl pyridine) compounds, in accordance with an illustrative embodiment of the invention;  
       FIG. 12  shows the chemical structure of a lithium ion complexing with polyethylene oxide segments, according to another illustrative embodiment of the invention;  
       FIGS. 13A-13C  depict chemical structures for exemplary co-sensitizers, according to illustrative embodiments of the invention;  
       FIGS. 14A-14B  depict additional exemplary chemical structures of co-sensitizers, according to illustrative embodiments of the invention;  
       FIG. 15  shows a graph of the absorbance of the 455 nm cut-off filter (GC455) used to characterize photovoltaic cells according to the invention;  
       FIG. 16  shows a graph of the absorbance of diphenylaminobenzoic acid;  
       FIG. 17  depicts an illustrative embodiment of the coating of a semiconductor primer layer coating, according to the invention;  
       FIGS. 18A and 18B  show side elevation views of interconnected photovoltaic modules in unflexed and flexed states, respectively, according to an illustrative embodiment of the invention;  
       FIG. 19  depicts a cross-sectional view of a photovoltaic cell formed using rigid substrates, according to an illustrative embodiment of the invention;  
       FIGS. 20A and 20B  show the scoring of the base material of a photovoltaic cell or module, according to the invention;  
       FIG. 21  depicts a base material suitable for forming a photovoltaic cell or module using wire interconnects, according to the invention;  
       FIGS. 22A and 22B  show an exemplary photovoltaic module formed using wire interconnects, according to the invention;  
       FIG. 23  shows a photovoltaic module formed using a metal foil coated with a photosensitized nanoparticle material, according to the invention;  
       FIG. 24  depicts an exemplary photovoltaic module capable of receiving radiation through two surfaces, according to the invention.  
       FIG. 25  depicts a metal wire coated with a metal having a low melting temperature, according to the invention;  
       FIG. 26  shows an exemplary photovoltaic module with a wire interconnect including a metal coated wire, according to the invention;  
       FIGS. 27A and 27B  depict exemplary display devices incorporating flexible photovoltaic cells, according to an illustrative embodiment of the invention;  
       FIG. 28  depicts an exemplary display device incorporating a flexible photovoltaic module, according to the invention;  
       FIGS. 29A and 29B  show perspective views of an exemplary retail shelf display, according to an illustrative embodiment of the invention;  
       FIG. 29C  shows a cross-sectional view of the retail shelf-edge display depicted in  FIG. 29B ;  
       FIG. 30  shows a display device incorporating various external elements according to a illustrative embodiment of the invention;  
       FIG. 31  shows a perspective view of a display element connected to a plurality of photovoltaic cells, according to an illustrative embodiment of the invention;  
       FIG. 32  shows an exemplary photovoltaic-powered multimedia greeting card, according to the invention;  
       FIG. 33  depicts an exemplary photovoltaic-powered greeting card with an integrated display unit, according to the invention;  
       FIG. 34  depicts an exemplary photovoltaic-powered greeting card capable of playing videos or messages, according to the invention;  
       FIG. 35  shows an exemplary photovoltaic-powered greeting card with colored photovoltaic modules, according to the invention;  
       FIG. 36  shows an exemplary photovoltaic-powered smart music card, according to the invention;  
       FIGS. 37A and 37B  depict front and back views of an exemplary photovoltaic-powered smart music card, according to an illustrative embodiment of the invention;  
       FIG. 38  shows an exemplary photovoltaic-powered video card, according to the invention; and  
       FIG. 39  depicts an exemplary photovoltaic-powered smart card having a global positioning system, according to an illustrative embodiment of the invention. 
    
    
     DESCRIPTION OF THE ILLUSTRATIVE EMBODIMENTS  
      A. Low Temperature Interconnection of Nanoparticles  
      As discussed in the summary above, the invention, in one embodiment, provides a polymeric linking agent (hereinafter a “polylinker”) that enables the fabrication of thin film solar cells at relatively low “sintering” temperatures (&lt;about 300° C.). Although the term “sintering” conventionally refers to high temperature (&gt;about 400° C.) processes, as used herein, the term “sintering” is not temperature specific, but instead refers generally to the process of interconnecting nanoparticles at any suitable temperature. In one illustrative embodiment, the invention provides a method for using polylinkers to interconnect nanoparticles in a thin film solar cells. According to another illustrative embodiment, the relatively low temperature sintering process enables the manufacture of such photovoltaic cells using flexible polymer substrates. By employing flexible substrates, the invention also enables a continuous roll-to-roll or web manufacturing process to be employed.  
       FIGS. 1 and 2  schematically depict chemical structures of illustrative polylinkers, according to the invention. The particular polylinker structures depicted are for use with nanoparticles of the formula M x O y  where M may be, for example, titanium (Ti), zirconium (Zr), tungsten (W), niobium (Nb), lanthanum (La), tantalum (Ta), terbium (Tb), or tin (Sn) and x and y are integers greater than zero. According to the illustrative embodiment of  FIG. 1 , the polylinker  100  includes a backbone structure  102 , which is similar in structure to the metal oxide nanoparticles, and (OR) i  reactive groups, where R may be, for example, acetate, an alkyl, alkene, alkyne, aromatic, or acyl group; or a hydrogen atom and i is an integer greater than zero. Suitable alkyl groups include, but are not limited to, ethyl, propyl, butyl, and pentyl groups. Suitable alkenes include, but are not limited to, ethene, propene, butene, and pentene. Suitable alkynes include, but are not limited to, ethyne, propyne, butyne, and pentyne. Suitable aromatic group include, but are not limited to, phenyl, benzyl, and phenol. Suitable acyl groups include, but are not limited to, acetyl and benzoyl. In addition, a halogen including, for example, chlorine, bromine, and iodine may be substituted for the (OR) i  reactive groups.  
      Referring to  FIG. 2 , the polylinker  110  has a branched backbone structure that includes two -M-O-M-O-M-O— backbone structures, which include (OR) i  reactive groups and (OR) i+1  reactive groups, where R may be, for example, one of the atoms, molecules, or compounds listed above and i is an integer greater than zero. The two backbone structures have similar structures to the metal oxide nanoparticles. Collectively, the structure depicted in  FIG. 2  can be represented by -M(OR) i —O-(M(OR) i —O) n -M(OR) i+1 , where i and n are integers greater than zero.  
       FIG. 3A  depicts schematically the chemical structure  300  resulting from interconnecting the M x O y  nanoparticles  302  with a polylinker  304 . In various embodiments, the polylinker  304  has the chemical structure of the polylinkers  100  and  110  depicted in  FIGS. 1 and 2 , respectively. According to the illustrative embodiment, the nanoparticles  302  are interconnected by contacting the nanoparticles  302  with a polylinker  304  at or below room temperature or at elevated temperatures that are less than about 300° C. Preferably, the polylinker  304  is dispersed in a solvent to facilitate contact with the nanoparticles  302 . Suitable solvents include, but are not limited to, various alcohols, chlorohydrocarbons (e.g., chloroform), ketones, cyclic and linear chain ether derivatives, and aromatic solvents among others. It is believed that the reaction between surface hydroxyl groups of the nanoparticles  302  with alkoxy groups on the polymer chain of the polylinker  304  leads to bridging (or linking) the many nanoparticles  302  together through highly stable covalent links, and as a result, to interconnecting the nanoparticles  302 . It also is believed that since the polylinker  304  is a polymeric material with a chemical structure similar to that of the nanoparticles  302 , even a few binding (or linking) sites between the nanoparticles  302  and the polylinker  304  leads to a highly interconnected nanoparticle film with a combination of electrical and mechanical properties superior to those of a non-sintered or non-interconnected nanoparticle film. The electrical properties include, for example, electron and/or hole conducting properties that facilitate the transfer of electrons or holes from one nanoparticle to another through, for example, π-conjugation. The mechanical properties include, for example, improved flexibility.  
      Still referring to  FIG. 3A , at low concentrations of the polylinker  304 , a single polylinker  304  polymer can link many nanoparticles  302  forming a cross-linked nanoparticle network. However, by increasing the concentration of the polylinker  304  polymer, more polylinker  304  molecules may be attached to the surface of the nanoparticles  302  forming polymer-coated nanoparticles  300 . Such polymer-coated nanoparticles  300  may be processed as thin films due to the flexibility of the polymer. It is believed that the electronic properties of the polymer-coated nanoparticles are not affected to a significant extent due to the similar electronic and structural properties between the polylinker polymer and the nanoparticles.  
       FIG. 3B  depicts the chemical structure  306  of an illustrative embodiment of the interconnected nanoparticle film  300  from  FIG. 3A  formed on a flexible substrate  308  that includes an oxide layer coating  310 , which is an electrical conductor. In particular, the polylinkers may be used to facilitate the formation of such nanoparticle films  300  on flexible, significantly light transmitting substrates  308 . As used herein, the term “significantly light transmitting substrate” refers to a substrate that transmits at least about 60% of the visible light incident on the substrate in a wavelength range of operation. Examples of flexible substrates  308  include polyethylene terephthalates (PETs), polyimides, polyethylene naphthalates (PENs), polymeric hydrocarbons, cellulosics, combinations thereof, and the like. PET and PEN substrates may be coated with one or more electrical conducting, oxide layer coatings  310  of, for example, indium tin oxide (ITO), a fluorine-doped tin oxide, tin oxide, zinc oxide, and the like.  
      According to one preferred embodiment, by using the illustrative polylinkers, the methods of the invention interconnect nanoparticles  302  at temperatures significantly below 400° C., and preferably below about 300° C. Operating in such a temperature range enables the use of the flexible substrates  308 , which would otherwise be destructively deformed by conventional high temperature sintering methods. In one illustrative embodiment, the exemplary structure  306  is formed by interconnecting the nanoparticles  302  using a polylinker  304  on a substrate  308  at temperatures below about 300° C. In another embodiment, the nanoparticles  302  are interconnected using a polylinker  304  at temperatures below about 100° C. In still another embodiment, the nanoparticles  302  are interconnected using a polylinker  304  at about room temperature and room pressure, from about 18 to about 22° C. and about 760 mm Hg, respectively.  
      In embodiments where the nanoparticles are deposited on a substrate, the reactive groups of the polylinker bind with the substrate, substrate coating and/or substrate oxide layers. The reactive groups may bind to the substrate, substrate coating and/or substrate oxide layers by, for example, covalent, ionic and/or hydrogen bonding. It is believed that reactions between the reactive groups of the polylinker with oxide layers on the substrate result in connecting nanoparticles to the substrate via the polylinker.  
      According to various embodiments of the invention, metal oxide nanoparticles are interconnected by contacting the nanoparticles with a suitable polylinker dispersed in a suitable solvent at or below room temperature or at elevated temperatures below about 300° C. The nanoparticles may be contacted with a polylinker solution in many ways. For example, a nanoparticle film may be formed on a substrate and then dipped into a polylinker solution. A nanoparticle film may be formed on a substrate and the polylinker solution sprayed on the film. The polylinker and nanoparticles may be dispersed together in a solution and the solution deposited on a substrate. To prepare nanoparticle dispersions, techniques such as, for example, microfluidizing, attritting, and ball milling may be used. Further, a polylinker solution may be deposited on a substrate and a nanoparticle film deposited on the polylinker.  
      In embodiments where the polylinker and nanoparticles are dispersed together in a solution, the resultant polylinker-nanoparticle solution may be used to form an interconnected nanoparticle film on a substrate in a single step. In various versions of this embodiment, the viscosity of the polylinker-nanoparticle solution may be selected to facilitate film deposition using printing techniques such as, for example, screen-printing and gravure-printing techniques. In embodiments where a polylinker solution is deposited on a substrate and a nanoparticle film deposited on the polylinker, the concentration of the polylinker can be adjusted to achieve a desired adhesive thickness. In addition, excess solvent may be removed from the deposited polylinker solution prior to deposition of the nanoparticle film.  
      The invention is not limited to interconnection of nanoparticles of a material of formula M x O y . Suitable nanoparticle materials include, but are not limited to, sulfides, selenides, tellurides, and oxides of titanium, zirconium, lanthanum, niobium, tin, tantalum, terbium, and tungsten, and combinations thereof. For example, TiO 2 , SrTiO 3 , CaTiO 3 , ZrO 2 , WO 3 , La 2 O 3 , Nb 2 O 5 , SnO 2 , sodium titanate, and potassium niobate are suitable nanoparticle materials.  
      The polylinker may contain more than one type of reactive group. For example, the illustrative embodiments of  FIGS. 1-3B  depict one type of reactive group OR. However, the polylinker may include several types of reactive groups, e.g., OR, OR′, OR″, etc.; where R, R′ and R″ are one or more of a hydrogen, alkyl, alkene, alkyne, aromatic, or acyl group or where one or more of OR, OR′, and OR″ are a halide. For example, the polylinker may include polymer units of formulas such as, —[O-M(OR) i (OR′) j —]—, and —[O-M(OR) i (OR′) j (OR″) k —]—, where i, j and k are integers greater than zero.  
       FIG. 4  depicts the chemical structure of a representative polylinker, poly(n-butyl titanate)  400  for use with titanium dioxide (TiO 2 ) nanoparticles. Suitable solvents for poly(n-butyl titanate)  400  include, but are not limited to, various alcohols, chlorohydrocarbons (e.g., chloroform), ketones, cyclic and linear chain ether derivatives, and aromatic solvents among others. Preferably, the solvent is n-butanol. The poly(n-butyl titanate) polylinker  400  contains a branched —Ti—O—Ti—O—Ti—O— backbone structure with butoxy (OBu) reactive groups.  
       FIG. 5A  depicts the chemical structure of a nanoparticle film  500 , which is constructed from titanium dioxide nanoparticles  502  interconnected by poly(n-butyl titanate) polylinker molecules  504 . It is believed that the reaction between surface hydroxyl groups of the TiO 2  nanoparticles  502  with butoxy groups  506  (or other alkoxy groups) of the polylinker  504  leads to the bridging (or linking) of many nanoparticles  502  together through highly stable covalent links, and as a result, interconnecting the nanoparticles  502 . Furthermore, it is believed that since the polylinker  504  is a polymeric material with a chemical structure similar to that of TiO 2 , even a few binding (or linking) sites between nanoparticles  502  and polylinker  504  will lead to a highly interconnected nanoparticle film  500 , with electronic and mechanical properties superior to those of a non-sintered or non-interconnected nanoparticle film.  
       FIG. 5B  depicts the chemical structure  508  of the nanoparticle film  500  from  FIG. 5A  formed on a substrate  510 , which includes an electrically-conducting oxide layer coating  512 , by applying the polylinker solution to the substrate  510  and then depositing the nanoparticles  502  on the polylinker  504 . In the illustrative example using titanium dioxide nanoparticles  502 , a polylinker solution including poly(n-butyl titanate)  504  is dissolved in n-butanol and applied to the substrate  510 . The concentration of the polylinker  504  can be adjusted to achieve a desired adhesive thickness for the polylinker solution. A titanium dioxide nanoparticulate film  500  is then deposited on the polylinker coated substrate  510 . Reaction between the surface hydroxyl groups of the TiO 2  nanoparticles with reactive butoxy groups  506  (or other alkoxy groups) of poly(n-butyl titanate)  504  results in interconnecting the nanoparticles  502 , as well as connecting nanoparticles  502  with the oxide layers  512  on the substrate  510 .  
       FIG. 6  depicts a flexible photovoltaic cell  600 , in accordance with the invention, that includes a photosensitized interconnected nanoparticle material  603  and a charge carrier material  606  disposed between a first flexible, significantly light transmitting substrate  609  and a second flexible, significantly light transmitting substrate  612 . In one embodiment, the flexible photovoltaic cell further includes a catalytic media layer  615  disposed between the first substrate  609  and second substrate  612 . Preferably, the photovoltaic cell  600  also includes an electrical conductor  618  deposited on one or both of the substrates  609  and  612 . The methods of nanoparticle interconnection provided herein enable construction of the flexible photovoltaic cell  600  at temperatures and heating times compatible with such substrates  609  and  612 .  
      The flexible, significantly light transmitting substrates  609  and  612  of the photovoltaic cell  600  preferably include polymeric materials. Suitable substrate materials include, but are not limited to, PET, polyimide, PEN, polymeric hydrocarbons, cellulosics, or combinations thereof. Further, the substrates  609  and  612  may include materials that facilitate the fabrication of photovoltaic cells by a continuous manufacturing process such as, for example, a roll-to-roll or web process. The substrate  609  and  612  may be colored or colorless. Preferably, the substrates  609  and  612  are clear and transparent. The substrates  609  and  612  may have one or more substantially planar surfaces or may be substantially non-planar. For example, a non-planar substrate may have a curved or stepped surface (e.g., to form a Fresnel lens) or be otherwise patterned.  
      According to the illustrative embodiment, an electrical conductor  618  is deposited on one or both of the substrates  609  and  612 . Preferably, the electrical conductor  618  is a significantly light transmitting material such as, for example, ITO, a fluorine-doped tin oxide, tin oxide, zinc oxide, or the like. In one illustrative embodiment, the electrical conductor  618  is deposited as a layer between about 100 nm and about 500 nm thick. In another illustrative embodiment, the electrical conductor  618  is between about 150 nm and about 300 nm thick. According to a further feature of the illustrative embodiment, a wire or lead line may be connected to the electrical conductor  618  to electrically connect the photovoltaic cell  600  to an external load.  
      The photosensitized interconnected nanoparticle material  603  may include one or more types of metal oxide nanoparticles, as described in detail above. In one embodiment, the photosensitized interconnected nanoparticle material  603  includes nanoparticles with an average size of between about 2 nm and about 100 nm. In another embodiment, the photosensitized nanoparticle material  603  includes nanoparticles with an average size of between about 10 nm and about 40 nm. Preferably, the nanoparticles are titanium dioxide particles having an average particle size of about 20 nm.  
      A wide variety of photosensitizing agents may be applied to and/or associated with the nanoparticles to produce the photosensitized interconnected nanoparticle material  603 . The photosensitizing agent facilitates conversion of incident visible light into electricity to produce the desired photovoltaic effect. It is believed that the photosensitizing agent absorbs incident light resulting in the excitation of electrons in the photosensitizing agent. The energy of the excited electrons is then transferred from the excitation levels of the photosensitizing agent into a conduction band of the interconnected nanoparticles  603 . This electron transfer results in an effective separation of charge and the desired photovoltaic effect. Accordingly, the electrons in the conduction band of the interconnected nanoparticles are made available to drive an external load electrically connected to the photovoltaic cell. In one illustrative embodiment, the photosensitizing agent is sorbed (e.g., chemisorbed and/or physisorbed) on the interconnected nanoparticles  603 . The photosensitizing agent may be sorbed on the surfaces of the interconnected nanoparticles  603 , throughout the interconnected nanoparticles  603 , or both. The photosensitizing agent is selected, for example, based on its ability to absorb photons in a wavelength range of operation, its ability to produce free electrons (or electron holes) in a conduction band of the interconnected nanoparticles  603 , and its effectiveness in complexing with or sorbing to the interconnected nanoparticles  603 . Suitable photosensitizing agents may include, for example, dyes that include functional groups, such as carboxyl and/or hydroxyl groups, that can chelate to the nanoparticles, e.g., to Ti(IV) sites on a TiO 2  surface. Examples of suitable dyes include, but are not limited to, anthocyanins, porphyrins, phthalocyanines, merocyanines, cyanines, squarates, eosins, and metal-containing dyes such as, for example, cis-bis(isothiocyanato)bis(2,2′-bipyridyl-4,4′-dicarboxylato)-ruthenium(II) (“N3 dye”); tris(isothiocyanato)-ruthenium(II)-2,2′:6′,2″-terpyridine-4,4′,4″-tricarboxylic acid; cis-bis(isothiocyanato)bis(2,2′-bipyridyl-4,4′-dicarboxylato)-ruthenium(II)bis-tetrabutylammonium; cis-bis(isocyanato) (2,2′-bipyridyl-4,4′ dicarboxylato)ruthenium(II); and tris(2,2′-bipyridyl-4,4′-dicarboxylato)ruthenium(II) dichloride, all of which are available from Solaronix.  
      The charge carrier material  606  portion of the photovoltaic cells may form a layer in the photovoltaic cell, be interspersed with the material that forms the photosensitized interconnected nanoparticle material  603 , or be a combination of both. The charge carrier material  606  may be any material that facilitates the transfer of electrical charge from a ground potential or a current source to the interconnected nanoparticles  603  (and/or a photosensitizing agent associated therewith). A general class of suitable charge carrier materials can include, but are not limited to solvent based liquid electrolytes, polyelectrolytes, polymeric electrolytes, solid electrolytes, n-type and p-type transporting materials (e.g., conducting polymers), and gel electrolytes, which are described in more detail below.  
      Other choices for the charge carrier material  606  are possible. For example, the electrolyte composition may include a lithium salt that has the formula LiX, where X is an iodide, bromide, chloride, perchlorate, thiocyanate, trifluoromethyl sulfonate, or hexafluorophosphate. In one embodiment, the charge carrier material  606  includes a redox system. Suitable redox systems may include organic and/or inorganic redox systems. Examples of such systems include, but are not limited to, cerium(III)sulfate/cerium(IV), sodium bromide/bromine, lithium iodide/iodine, Fe 2+ /Fe 3+ , Co 2+ /Co 3+ , and viologens. Furthermore, an electrolyte solution may have the formula M i X j , where i and j are ≧1. X is an anion, and M is selected from the group consisting of Li, Cu, Ba, Zn, Ni, lanthanides, Co, Ca, Al, and Mg. Suitable anions include, but are not limited to, chloride, perchlorate, thiocyanate, trifluoromethyl sulfonate, and hexafluorophosphate.  
      In some illustrative embodiments the charge carrier material  606  includes a polymeric electrolyte. In one version, the polymeric electrolyte includes poly(vinyl imidazolium halide) and lithium iodide. In another version, the polymeric electrolyte includes poly(vinyl pyridinium salts). In still another embodiment, the charge carrier material  606  includes a solid electrolyte. In one version, the solid electrolyte includes lithium iodide and pyridinium iodide. In another version, the solid electrolyte includes substituted imidazolium iodide.  
      According to some illustrative embodiments, the charge carrier material  606  includes various types of polymeric polyelectrolytes. In one version, the polyelectrolyte includes between about 5% and about 100% (e.g., 5-60%, 5-40%, or 5-20%) by weight of a polymer, e.g., an ion-conducting polymer, about 5% to about 95%, e.g., about 35-95%, 60-95%, or 80-95%, by weight of a plasticizer and about 0.05 M to about 10 M of a redox electrolyte, e.g., about 0.05 M to about 10 M, e.g., 0.05-2 M, 0.05-1 M, or 0.05-0.5 M, of organic or inorganic iodides, and about 0.01 M to about 1 M, e.g., 0.05-5 M, 0.05-2 M, or 0.05-1 M, of iodine. The ion-conducting polymer may include, for example, polyethylene oxide (PEO), polyacrylonitrile (PAN), polymethylmethacrylate (acrylic) (PMMA), polyethers, and polyphenols. Examples of suitable plasticizers include, but are not limited to, ethyl carbonate, propylene carbonate, mixtures of carbonates, organic phosphates, butyrolactone, and dialkylphthalates.  
      Preferably, the flexible photovoltaic cell  600  also includes a catalytic media layer  615  disposed between the substrates  609  and  612 . According to the illustrative embodiment, the catalytic media layer  615  is in electrical contact with the charge carrier material  606 . The catalytic media  615  may include, for example, ruthenium, osmium, cobalt, rhodium, iridium, nickel, activated carbon, palladium, platinum, or hole transporting polymers (e.g., poly(3,4-ethylene dioxythiophene and polyaniline). Preferably, the catalytic media  615  further includes titanium, or some other suitable metal, to facilitate adhesion of the catalytic media to a substrate and/or substrate coating. Preferably, the titanium is deposited in regions or a layer about 10 Å thick. In one embodiment, the catalytic media  615  includes a platinum layer between about 13 Å and about 35 Å thick. In another embodiment, the catalytic media  615  includes a platinum layer between about 15 Å and about 50 Å thick. In another embodiment, the catalytic media  615  includes a platinum layer between about 50 Å and about 800 Å thick. Preferably, the catalytic media  615  includes a platinum layer about 25 Å thick.  
      In another aspect, the invention also provides methods of forming a layer of interconnected metal oxide nanoparticles on a substrate using a continuous manufacturing process, such as, for example, a roll-to-roll or web process. These methods may be used, for example, to produce DSSCs. The current processes for producing DSSCs in large numbers, for example using a continuous and cost effective assembly line process, are extremely difficult at best. The difficulties associated with a continuous assembly process for a DSSC may arise from the cell support or substrate, which is generally rigid and typically includes thermally resistant materials such as glass and metal. The primary reason for this is related to the high temperature sintering process for producing fused nanocrystals (typically about 400-500° C.). Rigid substrate materials, by their very nature, generally do not lend themselves to a continuous process for manufacture, but rather to a more expensive batch process.  
       FIG. 7  depicts an illustrative embodiment of a continuous manufacturing process  700  that may be used to form the photovoltaic cell shown in  FIG. 6 . According to the illustrative embodiment, an interconnected nanoparticle film is formed on an advancing substrate sheet  705 , which may be continuously advanced, periodically advanced, and/or irregularly advanced during a manufacturing run using rollers  708 . In this illustrative embodiment, the electrical conductor material  710 , which serves as the basis for one electrode of a photovoltaic cell, is deposited on the advancing substrate  705 . In various embodiments, the electrical conductor material  710  may be deposited on a target region of the substrate  705  by thermal evaporation or low temperature sputtering. In addition, the electrical conductor material  710  may be deposited, for example, by vacuum deposition.  
      According to the illustrative embodiment shown in  FIG. 7 , the photosensitized nanoparticle material  715  is then deposited. As described above, the photosensitized nanoparticle material  715  may be formed by applying a solution having a polylinker and metal oxide nanoparticles onto the advancing substrate sheet  705 . The polylinker-nanoparticle solution may be applied by any suitable technique including, but not limited to, dip tanks, extrusion coating, spray coating, screen printing, and gravure printing. In other illustrative embodiments, the polylinker solution and metal oxide nanoparticles are separately applied to the advancing substrate sheet  705  to form the photosensitized nanoparticle material  715 . In one illustrative embodiment, the polylinker solution is applied to the advancing substrate  705  and the metal oxide nanoparticles (preferably dispersed in a solvent) are disposed on the polylinker. In another illustrative embodiment, the metal oxide nanoparticles (preferably dispersed in a solvent) are applied to the advancing substrate  705  and the polylinker solution is applied to the nanoparticles to form the photosensitized nanoparticle material  715 . As described above with regard to  FIG. 6 , a wide variety of photosensitizing agents may be applied to and/or associated with the nanoparticles to produce the photosensitized nanoparticle material  715 .  
      After deposition of the photosensitized nanomatrix material  715 , the substrate sheet  705  may proceed to further processing stations depending on the ultimate product desired. According to this illustrative embodiment, the charge carrier material  720 , which facilitates the transfer of electrical charge from a ground potential or a current source to the photosensitized nanoparticle material  715 , is deposited. The charge carrier material  720  may be applied by, for example, spray coating, roller coating, knife coating, or blade coating. The charge carrier media  720  may be prepared by forming a solution having an ion-conducting polymer, a plasticizer, and a mixture of iodides and iodine. The polymer provides mechanical and/or dimensional stability; the plasticizer helps the gel/liquid phase transition temperature; and the iodides and iodine act as redox electrolytes.  
      Still referring to  FIG. 7 , the catalytic media layer  725 , which facilitates the transfer of electrons ejected by the photoexcited molecules within the photovoltaic cell, is then deposited. Subsequently, a second electrical conductor layer  730  is deposited. The second electrical conductor layer  730  serves as the basis for a second electrode of the photovoltaic cell. A second, flexible substrate  735  is then unwound and applied to the advancing sheet  705  to complete the photovoltaic cell using the continuous manufacturing process  700 . An ultrasonic slitting device  740  may be employed to simultaneously cut and seal a leading and trailing edge of a plurality of photovoltaic cells, thus forming photovoltaic modules. The ultrasonic slitting procedure is discussed in more detail below.  
      Further illustrative examples of the invention in the context of a DSSC including titanium dioxide nanoparticles are provided below. The following examples are illustrative and not intended to be limiting. Accordingly, it is to be understood that the invention may be applied to a wide range of nanoparticles including, but not limited to, SrTiO 3 , CaTiO 3 , ZrO 2 , WO 3 , La 2 O 3 , Nb 2 O 5 , sodium titanate, and potassium niobate nanoparticles. In addition, it should be realized that the invention is generally applicable to formation of interconnected nanoparticles for a wide variety of applications in addition to DSSC, such as, for example, metal oxide and semiconductor coatings.  
     EXAMPLE 1  
     Dip-Coating Application of Polylinker  
      In this illustrative example, a DSSC was formed as follows. A titanium dioxide nanoparticle film was coated on a SnO 2 :F coated glass slide. The polylinker solution was a 1% (by weight) solution of the poly(n-butyl titanate) in n-butanol. In this embodiment, the concentration of the polylinker in the solvent was preferably less than 5% by weight. To interconnect the particles, the nanoparticle film coated slide was dipped in the polylinker solution for 15 minutes and then heated at 150° C. for 30 minutes. The polylinker treated TiO 2  film was then photosensitized with a 3×10 4  N3 dye solution for 1 hour. The polylinker treated TiO 2  film coated slide was then fabricated into a 0.6 cm 2  photovoltaic cell by sandwiching a triiodide based liquid redox electrolyte between the TiO 2  film coated slide a platinum coated SnO 2 :F glass slide using 2 mil SURLYN 1702 hot melt adhesive available from DuPont. The platinum coating was approximately 60 nm thick. The cell exhibited a solar conversion efficiency of as high as 3.33% at AM 1.5 solar simulator conditions (i.e., irradiation with light having an intensity of 1000 W/m 2 ). The completed solar cells exhibited an average solar conversion efficiency (“η”) of 3.02%; an average open circuit voltage (“V oc ”) of 0.66 V; an average short circuit current (“I sc ”) of 8.71 mA/cm 2 , and an average fill factor of 0.49 (0.48 to 0.52).  FIG. 8  depicts a graph  800  that shows the current-voltage curve  802  for the dip-coated photovoltaic cell.  
     EXAMPLE 2  
     Polylinker-Nanoparticle Solution Application  
      In this illustrative example, a 5.0 mL suspension of titanium dioxide (P25, which is a titania that includes approximately 80% anatase and 20% rutile crystalline TiO 2  nanoparticles and which is available from Degussa-Huls) in n-butanol was added to 0.25 g of poly(n-butyl titanate) in 1 mL of n-butanol. In this embodiment, the concentration of the polylinker in the polylinker-nanoparticle solution was preferably less than about 50% by weight. The viscosity of the suspension changed from milk-like to toothpaste-like with no apparent particle separation. The paste was spread on a patterned SnO 2 :F coated glass slide using a Gardner knife with a 60 μm thick tape determining the thickness of wet film thickness. The coatings were dried at room temperature forming the films. The air-dried films were subsequently heat treated at 150° C. for 30 minutes to remove solvent, and sensitized overnight with a 3×10 −4  M N3 dye solution in ethanol. The sensitized photoelectrodes were cut into desired sizes and sandwiched between a platinum (60 nm thick) coated SnO 2 :F coated glass slide and a tri-iodide based liquid electrolyte. The completed solar cells exhibited an average η of 2.9% (2.57% to 3.38%) for six cells at AM 1.5 conditions. The average V oc  was 0.68 V (0.66 to 0.71 V); the average I sc  was 8.55 mA/cm 2  (7.45 to 10.4 mA/cm 2 ); and the average fill factor was 0.49 (0.48 to 0.52).  FIG. 9  depicts a graph  900  showing the current-voltage curve  902  for the photovoltaic cell formed from the polylinker-nanoparticle solution.  
     EXAMPLE 3  
     DSSC Cells Formed without Polylinker  
      In this illustrative example, an aqueous titanium dioxide suspension (P25) containing about 37.5% solid content was prepared using a microfluidizer and was spin coated on a fluorinated SnO 2  conducting electrode (15 Ω/cm 2 ) that was itself coated onto a coated glass slide. The titanium dioxide coated slides were air dried for about 15 minutes and heat treated at 150° C. for 15 minutes. The slides were removed from the oven, cooled to about 80° C., and dipped into 3×10 −4  M N3 dye solution in ethanol for about 1 hour. The sensitized titanium dioxide photoelectrodes were removed from dye solution rinsed with ethanol and dried over a slide warmer at 40° C. The sensitized photoelectrodes were cut into small pieces (0.7 cm×0.5-1 cm active area) and sandwiched between platinum coated SnO 2 :F-transparent conducting glass slides. A liquid electrolyte containing 1 M LiI, 0.05 M iodine, and 1 M t-butyl pyridine in 3-methoxybutyronitrile was applied between the photoelectrode and platinized conducting electrode through capillary action. Thus constructed photocells exhibited an average solar conversion efficiency of about 3.83% at AM 1.5 conditions. The η at AM 1.5 conditions and the photovoltaic characteristics I sc , V oc,  voltage at maximum power output (“V m ”), and current at maximum power output (“I m ”) of these cells are listed in Table 1 under column A.  FIG. 10  depicts a graph  1000  showing the current-voltage curve  1002  for the photovoltaic cell formed without the polylinker.  
                                       TABLE 1                                       B   C   D   E           A   0.1% polymer   0.4% polymer   1% polymer   2% polymer           Untreated   soln.   soln.   soln.   soln.                                                            η (%)   Avg = 3.83   Avg. = 4.30   Avg = 4.55   Avg = 4.15   Avg = 4.15           (3.37-4.15)   (4.15-4.55)   (4.4-4.82)   (3.48-4.46)   (3.7-4.58)       I sc     Avg = 10.08   Avg = 10.96   Avg = 10.60   Avg = 11.00   Avg = 11.24       (mA/cm 2 )   (8.88-10.86)   (10.44-11.5)   (9.79-11.12)   (10.7-11.28)   (10.82-11.51)       V oc  (V)   Avg = 0.65   Avg = 0.66   Avg = 0.71   Avg = 0.7   Avg = 0.69           (0.65-0.66)   (0.6-0.7)   (0.69-0.74)   (0.69-0.71)   (0.68-0.71)       V m  (V)   Avg = 0.454   Avg = 0.46   Avg = 0.50   Avg = 0.45   Avg = 0.44           (0.43-0.49)   (0.43-0.477)   (0.47-0.53)   (0.4-0.47)   (0.42-0.46)       I m     Avg = 8.4   Avg = 9.36   Avg = 9.08   Avg = 9.14   Avg = 9.28       (mA/cm 2 )   (7.5-8.96)   (8.75-9.71)   (8.31-9.57)   (8.70-9.55)   (8.66-9.97)                  
 
     EXAMPLE 4  
     DSSC Cells Formed with Various Concentrations of Polylinker Solution  
      In this illustrative example, a P25 suspension containing about 37.5% solid content was prepared using a microfluidizer and was spin coated on fluorinated SnO 2  conducting electrode (15 Ω/cm 2 ) coated glass slide. The titanium dioxide coated slides were air dried for about 15 minutes and heat treated at 150° C. for 15 minutes. The titanium dioxide coated conducting glass slide were dipped into a polylinker solution including poly(n-butyl titanate) in n-butanol for 5 minutes in order to carry out interconnection (polylinking) of nanoparticles. The polylinker solutions used were 0.1 wt % poly(n-butyl titanate), 0.4 wt % poly(n-butyl titanate), 1 wt % poly(n-butyl titanate), and 2 wt % poly(n-butyl titanate). After 5 minutes, the slides were removed from the polylinker solution, air dried for about 15 minutes and heat treated in an oven at 150° C. for 15 minutes to remove solvent. The slides were removed from the oven, cooled to about 80° C., and dipped into 3×10 −4  M N3 dye solution in ethanol for about 1 hour. The sensitized titanium dioxide photoelectrodes were removed from dye solution, rinsed with ethanol, and dried over a slide warmer at 40° C. The sensitized photoelectrodes were cut into small pieces (0.7 cm×0.5-1 cm active area) and sandwiched between platinum coated SnO 2 :F-transparent conducting glass slides. A liquid electrolyte containing 1 M LiI, 0.05 M iodine, and 1 M t-butyl pyridine in 3-methoxybutyronitrile was applied between the photoelectrode and platinized conducting electrode through capillary action. The η at AM 1.5 conditions and the photovoltaic characteristics I sc , V oc , V m , and I m  of the constructed cells are listed in Table 1 for the 0.1 wt % solution under column B, for the 0.4 wt % solution under column C, for the 1 wt % solution under column D, and for the 2 wt % solution under column E.  FIG. 10  depicts the current-voltage curve  1008  for the photovoltaic cell formed with the polylinker.  
     EXAMPLE 5  
     Modifier Solutions  
      In this illustrative example, titanium dioxide coated transparent conducting oxide coated glass slides were prepared by spin coating process as described in Example 4. The titanium oxide coated conducting glass slides were treated with polylinker solution including a 0.01 M poly(n-butyl titanate) solution in n-butanol for 5 minutes to interconnect the nanoparticles. The slides were air dried for about 5 minutes after removing from the polylinker solution. The slides were later dipped into a modifier solution for about 1 minute. The modifier solutions used were 1:1 water/ethanol mixture, 1 M solution of t-butyl pyridine in 1:1 water/ethanol mixture, 0.05 M HCl solution in 1:1 water/ethanol mixture. One of the slides was treated with steam from humidifier for 15 seconds. The slides were air dried for 15 minutes and heat-treated at 150° C. for 15 minutes to remove solvent and then sensitized with a 3×10 −4  M N3 dye solution for 1 hour. The sensitized photoelectrodes were sandwiched between platinized SnO 2 :F coated glass slides and studied for photovoltaic characteristics using a liquid electrolyte containing 1 M LiI, 0.05 M iodine, and 1 M t-butyl pyridine in 3-methoxybutyronitrile. Acid seems to help in increasing the photoconductivity and efficiency of these photocells. The η at AM 1.5 conditions and the photovoltaic characteristics of the cells of this example are listed in Table 2 as follows: slides not dipped into a modifier solution and not treated with polylinker solution (column A); slides not dipped into a modifier, but treated with polylinker solution (column B); slides were first treated with polylinker solution and then dipped in 1:1 water/ethanol mixture (column C); slides were first treated with polylinker solution and then dipped in 1 M solution of t-butyl pyridine in 1:1 water/ethanol mixture (column D); slides were first treated with polylinker solution and then dipped in 0.05 M HCl solution in 1:1 water/ethanol mixture (column E); and slides were first treated with polylinker solution and then treated with steam from humidifier (column F).  
                                           TABLE 2                                               D   E                   B   C   Treated   Treated   F               Treated   Treated   with 1M t-   with 0.05M   Steam from           A   with 0.01M   with 1:1   BuPy/1:1   HCl/1:1   Humidifier           Untreated   TiBut   EtOH/H 2 O   EtOH/H 2 O   EtOH/H 2 O   for 15 sec.                                                                η (%)   Avg = 3.92   Avg = 4.41   Avg = 4.11   Avg = 4.34   Avg = 4.67   Avg = 4.41           (3.75-4.15)   (4.12-4.74)   (4.06-4.15)   (4.27-4.38)   (4.61-4.73)   (4.38-4.45)       V oc  (V)   Avg = 0.66   Avg = 0.66   Avg = 0.65   Avg = 0.65   Avg = 0.66   Avg = 0.66           (0.66-0.67)   (0.65-0.66)   (0.64-0.65)   (0.64-0.66)   (0.65-0.66)   (0.66-0.67)       I sc     Avg = 9.97   Avg = 12.57   Avg = 11.85   Avg = 11.85   Avg = 12.51   Avg = 11.63       (mA/cm 2 )   (9.48-10.56)   (11.7-13.22)   (11.21-12.49)   (11.21-12.49)   (12.15-12.87)   (11.25-12.01)       V m  (V)   Avg = 0.468   Avg = 0.434   Avg = 0.44   Avg = 0.45   Avg = 0.457   Avg = 0.45           (0.46-0.48)   (0.4-0.457)   (0.43-0.45)   (0.44-0.456)   (0.453-0.46)   (0.44-0.46)       I m     Avg = 8.36   Avg = 10.08   Avg = 9.27   Avg = 9.52   Avg = 10.23   Avg = 9.67       (mA/cm 2 )   (7.85-8.89)   (9.57-10.37)   (9.01-9.53)   (9.22-9.75)   (10.17-10.29)   (9.38-9.96)                  
 
     EXAMPLE 6  
     Post-Interconnection Heating to 150° C.  
      In this illustrative example, a titanium-dioxide-coated, transparent-conducting-oxide-coated glass slide was prepared by a spin coating process as described in Example 4. The slide was dipped into 0.01 M poly(n-butyl titanate) in n-butanol for 30 seconds and was air-dried for 15 minutes. The slide was later heat treated at 150° C. for 10 minutes in an oven. The heat-treated titanium oxide layer was sensitized with N3 dye solution for 1 hour, washed with ethanol, and warmed on a slide warmer at 40° C. for 10 minutes. The sensitized photoelectrodes were cut into 0.7 cm×0.7 cm active area photocells and were sandwiched between platinized conducting electrodes. A liquid electrolyte containing 1 M LiI, 0.05 M iodine, and 1 M t-butyl pyridine in 3-methoxybutyronitrile was applied between the photoelectrode and platinized conducting electrode through capillary action. The photocells exhibited an average η of 3.88% (3.83, 3.9 and 3.92), an average V oc  of 0.73 V (0.73, 0.74 and 0.73 V), and an average I sc  of 9.6 mA/cm 2  (9.88, 9.65 and 9.26), all at AM 1.5 conditions.  
     EXAMPLE 7  
     Post-Interconnection Heating to 70° C.  
      In this illustrative example, a titanium-dioxide-coated, transparent-conducting-oxide-coated glass slide was prepared by a spin coating process as described in Example 4. The slide was dipped into 0.01 M poly(n-butyl titanate) in n-butanol for 30 seconds and was air-dried for 15 minutes. The slide was later heat treated at 70° C. for 10 minutes in an oven. The heat-treated titanium oxide layer was sensitized with N3 dye solution for 1 hour, washed with ethanol, and warmed on a slide warmer at 40° C. for 10 minutes. The sensitized photoelectrodes were cut into 0.7 cm×0.7 cm active area photocells and were sandwiched between platinized conducting electrodes. A liquid electrolyte containing 1 M LiI, 0.05 M iodine, and 1 M t-butyl pyridine in 3-methoxybutyronitrile was applied between the photoelectrode and platinized conducting electrode through capillary action. The photocells exhibited an average ηof 3.62% (3.55, 3.73 and 3.58), an average V oc  of 0.75 V (0.74, 0.74 and 0.76 V), and average I sc  of 7.96 mA/cm 2  (7.69, 8.22 and 7.97), all at AM 1.5 conditions.  
     EXAMPLE 8  
     Formation on a Flexible, Transparent Substrate  
      In this illustrative example, a PET substrate about 200 μm thick and about 5 inches by 8 feet square was coated with ITO and loaded onto a loop coater. An 18.0 mL suspension of titanium dioxide (P25 with 25% solid content) in n-butanol and 0.5 g of poly(n-butyl titanate) in 10 mL of n-butanol were in-line blended and coated onto the ITO coated PET sheet. After deposition, the coating was heated at about 50° C. for about 1 minute. The interconnected nanoparticle layer was then dye-sensitized by coating with a 3×10 −4  M solution of N3 dye in ethanol.  
      B. Gel Electrolytes for DSSCs  
      According to further illustrative embodiments, the invention provides electrolyte compositions that include multi-complexable molecules (i.e., molecules containing 2 or more ligands capable of complexing) and redox electrolyte solutions, which are gelled using metal ions, such as lithium ions. The multi-complexable compounds are typically organic compounds capable of complexing with a metal ion at a plurality of sites. The electrolyte composition can be a reversible redox species that may be liquid by itself or solid components dissolved in a non-redox active solvent, which serves as a solvent for the redox species and does not participate in reduction-oxidation reaction cycle. Examples include common organic solvents and molten salts that do not contain redox active ions. Examples of redox species include, for example, iodide/triiodide, Fe 2+ /Fe 3+ , Co 2+ /Co 3+ , and viologens, among others. The redox components are dissolved in non-aqueous solvents, which include all molten salts. Iodide based molten salts, e.g., methylpropylimidazolium iodide, methylbutylimidazolium iodide, methylhexylimidazolium iodide, etc., are themselves redox active and can be used as redox active liquids by themselves or diluted with non-redox active materials like common organic solvents or molten salts that do not undergo oxidation-reduction reaction cycles. Multi-dendate inorganic ligands may also be a source of gelling compounds.  
       FIG. 11  depicts an illustrative embodiment of an electrolyte gelled using metal ions. Lithium ions are shown complexed with poly(4-vinyl pyridine). The lithium ions and the organic compounds, in this instance poly(4-vinyl pyridine) molecules capable of complexing at a plurality of sites with the lithium ions, can be used to gel a suitable electrolyte solution. An electrolyte composition prepared in accordance with the invention may include small amounts of water, molten iodide salts, an organic polymer, and other suitable compound gels upon the addition of a metal ion such as lithium. Gelled electrolytes may be incorporated into individual flexible photovoltaic cells, traditional solar cells, photovoltaic fibers, interconnected photovoltaic modules, and other suitable devices. The dotted lines shown in  FIG. 11  represent the type of bonding that occurs in a photovoltaic gel electrolyte when the constituent electrolyte solution and organic compounds gel after the introduction of a suitable metal ion.  
      A non-exhaustive list of organic compounds that are capable of complexing with the metal ion at a plurality of sites, and which are suitable for use in the invention, include various polymers, starburst/dendrimeric molecules, and other molecules containing multiple functional groups, e.g., urethanes, esters, ethylene/propylene oxide/imines segments, pyridines, pyrimidines, N-oxides, imidazoles, oxazoles, triazoles, bipyridines, quinolines, polyamines, polyamides, ureas, β-diketones, and β-hydroxy ketones.  
      More generally, the multi-complexable molecules employed in various embodiments may be polymeric or small organic molecules that possess two or more ligand or ligating groups capable of forming complexes. Ligating groups are functional groups that contain at least one donor atom rich in electron density, e.g., oxygen, nitrogen, sulfur, or phosphorous, among others and form monodentate or multidentate complexes with an appropriate metal ion. The ligating groups may be present in non-polymeric or polymeric material either in a side chain or part of the backbone, or as part of a dendrimer or starburst molecule. Examples of monodentate ligands include, for example, ethyleneoxy, alkyl-oxy groups, pyridine, and alkyl-imine compounds, among others. Examples of bi- and multidentate ligands include bipyridines, polypyridines, urethane groups, carboxylate groups, and amides.  
      According to various embodiments of the invention, dye-sensitized photovoltaic cells having a gel electrolyte  1100  including lithium ions are fabricated at or below room temperature or at elevated temperatures below about 300° C. The temperature may be below about 100° C., and preferably, the gelling of the electrolyte solution is performed at room temperature and at standard pressure. In various illustrative embodiments, the viscosity of the electrolyte solution may be adjusted to facilitate gel electrolyte deposition using printing techniques such as, for example, screen-printing and gravure-printing techniques. The complexing of lithium ions with various ligands can be broken at higher temperatures, thereby permitting the gel electrolyte compositions to be easily processed during DSSC based photovoltaic module fabrication. Other metal ions may also be used to form thermally reversible or irreversible gels. Examples of suitable metal ions include: Li + , Cu 2+ , Ba 2+ , Zn 2+ , Ni 2+ , Ln 3+  (or other lanthanides), Co 2+ , Ca 2+ , Al 3+ , Mg 2+ , and any metal ion that complexes with a ligand.  
       FIG. 12  depicts a gel electrolyte  1200  formed by the complexing of an organic polymer, polyethylene oxide (PEO), by lithium ions. The PEO polymer segments are shown as being complexed about the lithium ions and crosslinked with each other. In another embodiment, the metal ion complexed with various polymer chains can be incorporated into a reversible redox electrolyte species to promote gelation. The gel electrolyte composition that results from the combination is suitable for use in various photovoltaic cell embodiments such as photovoltaic fibers, photovoltaic cells, and electrically interconnected photovoltaic modules.  
      Referring back to  FIG. 6 , the charge carrier material  606  can include an electrolyte composition having an organic compound capable of complexing with a metal ion at a plurality of sites; a metal ion such as lithium; and an electrolyte solution. These materials can be combined to produce a gelled electrolyte composition suitable for use in the charge carrier material  606  layer. In one embodiment, the charge carrier material  606  includes a redox system. Suitable redox systems may include organic and/or inorganic redox systems. Examples of such systems include, but are not limited to, cerium(III) sulfate/cerium(IV), sodium bromide/bromine, lithium iodide/iodine, Fe 2+ /Fe 3+ , Co 2+ /Co 3+ , and viologens.  
      Further illustrative examples of the invention in the context of a DSSC having a gel electrolyte composition are provided below. The photoelectrodes used in the following illustrative examples were prepared according to the following procedure. An aqueous, titania suspension (P25, which was prepared using a suspension preparation technique with total solid content in the range of 30-37%) was spun cast on SnO 2 :F coated glass slides (15 Ω/cm 2 ). The typical thickness of the titanium oxide coatings was around 8 μm. The coated slides were air dried at room temperature and sintered at 450° C. for 30 minutes. After cooling the slides to about 80° C., the slides were immersed into 3×10 −4  M N3 dye solution in ethanol for 1 hour. The slides were removed and rinsed with ethanol and dried over slide a warmer at 40° C. for about 10 minutes. The slides were cut into about 0.7 cm×0.7 cm square active area cells. The prepared gels were applied onto photoelectrodes using a glass rod and were sandwiched between platinum-coated, SnO 2 :F coated, conducting glass slides. The cell performance was measured at AM 1.5 solar simulator conditions (i.e., irradiation with light having an intensity of 1000 W/m 2 ).  
     EXAMPLE 9  
     Effect of Lithium Iodide in Standard Ionic Liquid Based Electrolyte Composition  
      In this illustrative example, the standard, ionic, liquid-based redox electrolyte composition that was used contained a mixture containing 99% (by weight) imidazolium iodide based ionic liquid and 1% water (by weight), combined with 0.25 M iodine and 0.3 M methylbenzimidazole. In various experimental trials, electrolyte solutions with at least a 0.10 M iodine concentration exhibit the best solar conversion efficiency. In a standard composition, butylmethylimidazolium iodide (MeBuImI) was used as the ionic liquid. Photovoltage decreased with increases in iodine concentration, while photoconductivity and conversion efficiency increased at least up to 0.25 M iodine concentration. Adding lithium iodide to the standard composition enhanced the photovoltaic characteristics V oc  and I sc  and the η. Therefore, in addition to lithium&#39;s use as a gelling agent, it may serve to improve overall photovoltaic efficiency. Table 3 summarizes the effect of LiI on photovoltaic characteristics.  
                                       TABLE 3                                       Standard +   Standard +   Standard +   Standard +           Standard   1 wt % LiI   2 wt % LiI   3 wt % LiI   5 wt % LiI                                                            η (%)   2.9%   3.57   3.75   3.70   3.93       V oc  (V)   0.59   0.61   0.6   0.6   0.61       I sc  (mA/cm 2 )   10.08   11.4   11.75   11.79   12.62       V m  (V)   0.39   0.4   0.39   0.4   0.39       Im   7.44   9.02   9.64   9.0   10.23       (mA/cm 2 )                  
 
      The fill factor (“FF”) is referenced below and can be calculated from the ratio of the solar conversion efficiency to the product of the open circuit voltage and the short circuit current, i.e., FF=η/[V oc *I sc ].  
     EXAMPLE 10  
     The Effect of Cations on the Enhancement in Photovoltaic Characteristics  
      In order to ascertain whether the enhancement in photovoltaic characteristics was due to the presence of lithium or iodide, controlled experimental trials using various iodides in conjunction with cations including lithium, potassium, cesium and tetrapropylammonium iodide were conducted. The iodide concentration was fixed at 376 μmols/gram of standard electrolyte composition. The standard composition used was a mixture containing 99% MeBuImI and 1% water, combined with 0.25 M iodine and 0.3 M methylbenzimidazole. 376 μmols of various iodide salts per gram of standard electrolyte composition were dissolved in the electrolyte. The complete dissolution of LiI was observed. The other salts took a long time to dissolve and did not dissolve completely over the course of the experimental trial. DSSC-based photovoltaic cells were fabricated using prepared electrolytes containing various cations. Table 4 shows the effect of the various cations on the photovoltaic characteristics. It is apparent from the second column of Table 4 that Li +  ion shows enhanced photovoltaic characteristics compared to the standard formula, while the other cations do not appear to contribute to the enhancement of the photovoltaic characteristics.  
                                       TABLE 4                                       Standard +   Standard +   Standard +   Standard +           Standard   LiI   NPR 4 I   KI   CsI                                                            η (%)   3.23   4.39   2.69   3.29   3.23       V oc  (V)   0.58   0.65   0.55   0.58   0.6       I sc  (mA/cm 2 )   10.96   12.03   9.8   9.91   10.14       V m  (V)   0.36   0.44   0.36   0.4   0.4       I m  (mA/cm 2 )   8.96   9.86   7.49   8.25   8.32                  
 
     EXAMPLE 11  
     Effect of Ionic Liquid Type  
      In one aspect of the invention, MeBuImI-based electrolyte compositions have been found to perform slightly better than MePrImI based electrolytes. In addition, experimental results demonstrate that a 1/1 blend of MeBuImI and MePrImI exhibit better performance than MeBuImI, as shown in Table 5.  
                           TABLE 5                                   376 μmoles of LiI               per 1 gram of   376 μmoles of LiI per 1 gram of           MeBuImI based standard   MeBuImI/MePrImI based           electrolyte composition.   standard electrolyte composition.                                                η (%)   3.64   3.99       V oc  (V)   0.63   0.63       I sc  (mA/cm 2 )   11.05   11.23       V m  (V)   0.42   0.42       I m  (mA/cm 2 )   8.69   9.57                  
 
     EXAMPLE 12  
     Using Li-Induced Gelling in Composition A Instead of a Dibromocompound  
      In this illustrative example, a Composition A was prepared by dissolving 0.09 M of iodine in a mixed solvent consisting of 99.5% by weight of 1-methyl-3-propyl imidazolium iodide and 0.5% by weight of water. Then, 0.2 g of poly(4-vinylpyridine) (“P4VP”), a nitrogen-containing compound, was dissolved in 10 g of the Composition A Further, 0.2 g of 1,6-dibromohexane, an organic bromide, was dissolved in the resultant Composition A solution, so as to obtain an electrolyte composition, which was a precursor to a gel electrolyte.  
      Gelling occurred quickly when 5 wt % of lithium iodide (376 μmols of lithium salt per gram of standard electrolyte composition) was used as the gelling agent in an electrolyte composition containing (i) 2 wt % P4VP and (ii) a mixture containing 99.5% MePrImI and 0.5% water. The gel did not flow when a vial containing the Li-induced gel was tilted upside down. One approach using a dibromo compound produced a phase-segregated electrolyte with cross-linked regions suspended in a liquid, which flows (even after gelling at 100° C. for 30 minutes). A comparison of the photovoltaic characteristics of Composition A, with and without LiI, is presented in the following Tables 6 and 7. The results demonstrate that functional gels suitable for DSSC-based photovoltaic cell fabrication can be obtained using lithium ions, while also improving the photovoltaic characteristics.  
                               TABLE 6                                   Composition   Composition   MeBuImI based           A with   A with 2   electrolyte + 2 wt. %           dibromohexane   wt. % P4VP   P4VP + 5 wt. % LiI                                                    η (%)   2.6   3.04   3.92       V oc  (V)   0.59   0.58   0.65       I sc  (mA/cm 2 )   9.73   10.0   11.45       V m  (V)   0.38   0.38   0.42       I m  (mA/cm 2 )   6.82   8.04   9.27                  
 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 7 
               
               
                   
                   
               
               
                   
                   
               
               
                   
                 (a) Composition A 
                   
               
               
                   
                 where MePrImI:water 
                 (b) Same 
               
               
                   
                 is 99.5:0.5 and with 2% 
                 composition as (a), but 
               
               
                   
                 P4VP and 0.09 M Iodine 
                 with 5 wt % of LiI 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                 Physical 
                 Reddish fluid; flows well 
                 Non-scattering Gel; does 
               
               
                 Properties 
                   
                 not flow; can be thinned by 
               
               
                   
                   
                 applying force using a glass rod. 
               
               
                 Efficiency 
                 2.53% 
                 3.63% 
               
               
                 V oc   
                 0.55 V 
                 0.62 V 
               
               
                 I sc   
                 9.82 mA/cm 2   
                 12.29 mA/cm 2   
               
               
                 V m   
                 0.343 V 
                 0.378 V 
               
               
                 FF 
                 0.47 
                 0.47 
               
               
                   
               
            
           
         
       
     
     EXAMPLE 13  
     Effect of Anions of Lithium Salts on the Efficiency and Photovoltage of DSSCs  
      Experiments were performed to study the effect of counter ions on lithium, given lithium&#39;s apparent role in enhancing the overall efficiency of DSSCs. 376 μmols of LiI, LiBr, and LiCl were used per gram of the electrolyte composition containing MePrImI, 1% water, 0.25 M iodine and 0.3 M methylbenzimidazole in order to study the photovoltaic characteristics of the cells. The photovoltaic characteristics of cells containing these electrolytes are presented in Table 8.  
                               TABLE 8                                   Electrolyte   Electrolyte   Electrolyte           composition   composition with   composition           with LiI   LiBr   with LiCl                                                    Efficiency   3.26%   3.64%   3.71%       V oc     0.59 V   0.62 V   0.65 V       I sc     10.98 mA/cm 2     11.96 mA/cm 2     11.55 mA/cm 2         V m     0.385 V   0.4 V   0.40 V       FF   0.5   0.49   0.49                  
 
     EXAMPLE 14  
     Passivation and Improved Efficiency and Photovoltage of DSSCs  
      In the field of photovoltaic cells, the term passivation refers to the process of reducing electron transfer to species within the electrolyte of a solar cell. Passivation typically includes treating a nanoparticle layer by immersion in a solution of t-butylpyridine in methoxypropionitrile or other suitable compound. After the nanomatrix layer, such as a titania sponge, of a photovoltaic cell has been treated with a dye, regions in the nanomatrix layer where the dye has failed to adsorb may exist. A passivation process is typically performed on a DSSC to prevent the reversible electron transfer reaction from terminating as result of reducing agents existing at the undyed regions. The typical passivation process does not appear to be necessary when ionic liquid compositions containing various lithium salts and/or other alkali metal salts are used in the DSSCs. A photovoltage greater than 0.65 V was achieved using a chloride salt of lithium without a passivation process.  
      In this illustrative example, a DSSC was passivated by immersing it in a solution containing 10 wt % of t-butylpyridine in methoxypropionitrile for 15 minutes. After passivation, the DSSC was dried on a slide warmer maintained at 40° C. for about 10 minutes. Electrolyte compositions containing MePrImI, 1% water, 0.3 M methylbenzimidazole, and 0.25 M iodine were gelled using 376 μmoles of LiI, LiBr, and LiCl per gram of standard electrolyte composition used during this study. Adding a t-butylpyridine-based passivation agent to the electrolyte enhanced the DSSC&#39;s photovoltage, but decreased the efficiency of the DSSC by decreasing the photoconductivity. Table 9 summarizes the effects of passivation on photovoltaic characteristics of electrolytes containing various lithium halides.  
                               TABLE 9                                           Electrolyte           Electrolyte   Electrolyte   gelled with           gelled with LiI   gelled with LiBr   LiCl                                                    Efficiency   3.5%   3.65%   3.85%       V oc     0.61 V   0.63 V   0.65 V       I sc     10.96 mA/cm 2     11.94 mA/cm 2     11.75 mA/cm 2         V m     0.395 V   0.4 V   0.405 V       FF   0.52   0.49   0.5                  
 
     EXAMPLE 15  
     Lithium &#39;s Role in Gelling the Electrolyte Compositions Containing Polyvinylpyridine and the Effect of Other Alkali Metal Ions on Gelability  
      Lithium cation appears to have a unique effect in gelling ionic liquid composition containing complexable polymers, e.g., P4VP, in as small an amount as 2 wt %. Other alkali metal ions such as sodium, potassium, and cesium were used to carry out gelling experiments. Alkali metal salts such as lithium iodide, sodium chloride, potassium iodide, cesium iodide were added to portions of electrolyte composition containing propylmethylimidazolium iodide (MePrImI), 1% water, 0.25 M iodine, and 0.3 M methylbenzimidazole. Only compositions containing lithium iodide gelled under the experimental conditions used. The remaining three compositions containing sodium, potassium, and cesium did not gel at the experimental conditions used. Divalent metal ions, such as calcium, magnesium, and zinc, or trivalent metals, such as aluminum or other transition metal ions, are other potential gelling salts.  
     EXAMPLE 16  
     Effect of Iodine and Lithium Concentration on Ionic Liquid Electrolyte Gels  
      In this illustrative example, gels were prepared by adding lithium salts to an electrolyte composition containing MeBuImI, iodine, and 2 wt % P4VP. The photovoltaic characteristics of the gels were tested using high-temperature sintered, N3 dye sensitized titanium-oxide photoelectrodes and platinized SnO 2 :F coated glass slides. Both LiI and LiCl gelled the ionic liquid-based compositions that contained small amounts (2% was sufficient) of complexable polymers like P4VP. In compositions lacking methylbenzimidazole, the lithium did not effect the photovoltage. 5 wt % corresponds to a composition including about 376 μmoles of lithium salt per gram of ionic liquid and a mixture of 99 wt % butylmethylimidazolium iodide, 1 wt % water, 0.3 M methyl benzimidazole, and 0.25 M iodine. Therefore, 1 wt % corresponds to a 376/5≅75 μmoles of lithium salt per gram of ionic liquid composition. The photovoltaic characteristics are summarized in Table 10.  
                               TABLE 10                           5% LiI   2.5% LiI   5% LiCl   2.5% LiCl                  0.05 M Iodine   η = 1.6%   η = 1.23%   η = 0.64%   η = 1.19%           V oc  = 0.6 V   V oc  = 0.59 V   V oc  = 0.59 V   V oc  = 0.58 V           I sc  = 4.89 mA   I sc  = 4.21 mA   I sc  = 2.95 mA   I sc  = 3.87 mA           FF = 0.54   FF = 0.495   FF = 0.36   FF = 0.53           V m  = 0.445 V   V m  = 0.415 V   V m  = 0.4 V   V m  = 0.426 V        0.1 M Iodine   η = 1.22%   η = 1.29%   η = 2.83%   η = 2.06%           V oc  = 0.48 V   V oc  = 0.56 V   V oc  = 0.57   V oc  = 0.58           I sc  = 6.46 mA   I sc  = 5.12 mA   I sc  = 9.04 mA   I sc  = 7.14 mA           FF = 0.39   FF = 0.45   FF = 0.55   FF = 0.5           V m  = 0.349 V   V m  = 0.386 V   V m  = 0.422 V   V m  = 0.42 V       0.25 M Iodine   η = 2.58%   η = 3.06%   η = 3.4%   η = 2.6%           V oc  = 0.55 V   V oc  = 0.55 V   V oc  = 0.56 V   V oc  = 0.56 V           I sc  = 11.49 mA   I sc  = 10.78 mA   I sc  = 11.32 mA   I sc  = 10.18 mA           FF = 0.41   FF = 0.52   FF = 0.54   FF = 0.46           V m  = 0.338 V   V m  = 0.36 V   V m  = 0.369 V   V m  = 0.364 V                  
 
     EXAMPLE 17  
     Effect of Polymer Concentration on Gelability and Photovoltaic Characteristics of Redox Electrolyte Gels  
      In this illustrative example, polymer concentration was varied to study its effect on gel viscosity and photovoltaic characteristics. The electrolyte composition used for this study was a mixture containing 99% MeBuImI, 1% water, 0.25 M iodine, 0.6 M LiI, and 0.3 M methylbenzimidazole. The concentration of the polymer, P4VP was varied from 1% to 5%. The electrolyte composition with 1% P4VP did flow slowly when the vial containing the gel was tilted down. The gels with 2%, 3%, and 5% did not flow. The gel with 5% P4VP appeared much more solid when compared to the 2% P4VP preparation. Table 11 summarizes the photovoltaic characteristics of the gels containing the various P4VP contents that were studied.  
      The results show that the photovoltaic characteristics do not vary with the increases in viscosity achieved by increasing the P4VP content. Therefore, the viscosity of the gel can be adjusted without causing degradation to the photovoltaic characteristics. Methylbenzimidazole may be necessary to achieve high η. Increasing the iodine concentration up to 0.25 M also increased the efficiency. Beyond 0.25 M, the photovoltage decreased drastically, reducing the overall efficiency. Other metal ions or cations like cesium, sodium, potassium or tetraalkylammonium ions were not found to contribute to the efficiency enhancement and did not cause gelling of the electrolyte solutions. Furthermore, chloride anion was found to enhance the efficiency along with lithium, by improving the photovoltage without causing decreased photoconductivity in compositions containing methylbenzimidazole.  
                               TABLE 11                       Photovoltaic                       Characteristics   1% P4VP   2% P4VP   3% P4VP   5% P4VP                                                    η (%)   3.23   3.48   3.09   3.19       I sc  (mA/cm 2 )   10.74   10.42   12.03   10.9       V oc  (V)   0.59   0.59   0.6   0.61       V m  (V)   0.39   0.4   0.38   0.40       I m  (mA/cm 2 )   8.27   8.69   8.07   8.03       FF   0.51   0.57   0.43   0.48                  
 
 C. Co-Sensitizers 
 
      According to one illustrative embodiment, the photosensitizing agent described above includes a first sensitizing dye and second electron donor species, the “co-sensitizer.” The first sensitizing dye and the co-sensitizer may be added together or separately to form the photosensitized interconnected nanoparticle material  603  shown in  FIG. 6 . As mentioned above with respect to  FIG. 6 , the sensitizing dye facilitates conversion of incident visible light into electricity to produce the desired photovoltaic effect. In one illustrative embodiment, the co-sensitizer donates electrons to an acceptor to form stable cation radicals, which improves the efficiency of charge transfer from the sensitizing dye to the semiconductor oxide nanoparticle material and reduces back electron transfer to the sensitizing dye or co-sensitizer. The co-sensitizer preferably includes (1) conjugation of the free electron pair on a nitrogen atom with the hybridized orbitals of the aromatic rings to which the nitrogen atom is bonded and, subsequent to electron transfer, the resulting resonance stabilization of the cation radicals by these hybridized orbitals; and (2) a coordinating group, such as a carboxy or a phosphate, the function of which is to anchor the co-sensitizer to the semiconductor oxide. Examples of suitable co-sensitizers include, but are not limited to, aromatic amines (e.g., such as triphenylamine and its derivatives), carbazoles, and other fused-ring analogues.  
      Once again referring back to  FIG. 6 , the co-sensitizer is electronically coupled to the conduction band of the photosensitized interconnected nanoparticle material  603 . Suitable coordinating groups include, but are not limited to, carboxylate groups, phosphates groups, or chelating groups, such as, for example, oximes or alpha keto enolates.  
      Tables 12-18 below present results showing the increase in photovoltaic cell efficiency when co-sensitizers are co-adsorbed along with sensitizing dyes on the surface of high temperature sintered or low temperature interconnected titania. In Tables 12-18, characterization was conducted using AM 1.5 solar simulator conditions (i.e., irradiation with light having an intensity of 1000 W/m 2 ). A liquid electrolyte including 1 M LiI, 1 M t-butylpyridine, 0.5 M I 2  in 3-methoxypropanitrile was employed. The data shown in the tables indicates an enhancement of one or more operating cell parameters for both low-temperature-interconnected (Tables 15, 17 and 18) and high-temperature-sintered (Tables 12, 13, 14 and 16) titania nanoparticles. The solar cells characteristics listed include η, V oc , I sc , FF, V m , and I m . The ratios of sensitizer to co-sensitizer are based on the concentrations of photosensitizing agents in the sensitizing solution.  
      In particular, it was discovered that aromatic amines enhance cell performance of dye sensitized titania solar cells if the concentration of the co-sensitizer is below about 50 mol % of the dye concentration. An example of the general molecular structure of the preferred aromatic amines is shown in  FIGS. 13 and 14 . Preferably, the concentration of the co-sensitizer is in the range of about 1 mol % to about 20 mol %, and more preferably in the range of about 1 mol % to about 5 mol %.  
       FIG. 13A  depicts a chemical structure  1300  that may serve as a co-sensitizer. The molecule  1300  adsorbs to the surface of a nanoparticle layer via its coordinating group or chelating group, A. A may be a carboxylic acid group or derivative thereof, a phosphate group, an oxime or an alpha ketoenolate, as described above.  FIG. 13B  depicts a specific embodiment  1310  of the structure  1300 , namely DPABA (diphenylaminobenzoic acid), where A =COOH.  FIG. 13C  depicts another specific amine  1320  referred to as DEAPA (N′,N-diphenylaminophenylpropionic acid), with A as the carboxy derivative COOH.  
       FIG. 14A  shows a chemical structure  1330  that may serve as either a co-sensitizer, or a sensitizing dye. The molecule does not absorb radiation above 500 nm, and adsorbs to a surface of the nanoparticle layer via its coordinating or chelating groups, A. A may be a carboxylic acid group or derivative thereof, a phosphate group, an oxime or an alpha ketoenolate. R 1  and R 2  may each be a phenyl, alkyl, substituted phenyl, or benzyl group. Preferably, the alkyl may contain between 1 and 10 carbons.  FIG. 14B  depicts a specific embodiment  1340  of the structure  1330 , namely DPACA (2,6 bis(4-benzoicacid)-4-(4-N,N-diphenylamino)phenylpyridine carboxylic acid), where R 1  and R 2  are phenyl and A is COOH.  
      DPACA  1340  may be synthesized as follows. 1.49 g (9.08 mmol) of 4-acetylbenzoic acid, 1.69 g (6.18 mmol) of 4-N,N-diphenylbenzaldehyde, and 5.8 g (75.2 mmol) of ammonium acetate were added to 60 ml of acetic acid in a 100 ml round bottom flask equipped with a condenser and stirring bar. The solution was heated to reflux with stirring under nitrogen for 5 hours. The reaction was cooled to room temperature and poured into 150 ml of water, which was extracted with 150 ml of dichloromethane. The dichloromethane was separated and evaporated with a rotary evaporator, resulting in a yellow oil. The oil was then eluted on a silica gel column with 4% methanol/dichloromethane to give the product, an orange solid. The solid was washed with methanol and vacuum dried to give 0.920 g of 2,6 bis(4-benzoicacid)-4-(4-N,N-diphenylamino)phenylpyridine (DPACA). The melting point was 199°-200° C., the λ max  was 421 nm, and the molar extinction coefficient, E was 39,200 L mole −1  cm −1 . The structure was confirmed by NMR spectroscopy.  
      Table 12 shows the results for high-temperature-sintered titania; photosensitized by overnight soaking in solutions of 1 mM N3 dye and three concentrations of DPABA. Table 12 also shows that the average η is greatest for the preferred 20/1 (dye/co-sensitizer) ratio.  
                       TABLE 12                                      I-V CHARACTERIZATION                                                             Cell                                   General       area       I m         I sc         conditions   Conditions   cm 2     V oc  V   mA/cm 2     V m  V   mA/cm 2     FF   η %   σ                                                             Adsorption   1 mM   0.44   0.62   6.69   0.44   8.38   0.56   2.91           Temp.   N3/EtOH,   0.52   0.64   6.81   0.43   8.59   0.54   2.94       RT° C.   Overnight       Solvent of Dye   CONTROL   0.54   0.63   6.95   0.41   8.72   0.52   2.84       EtOH   Average   0.50   0.63   6.82   0.43   8.56   0.54   2.90   0.05       Dye Concen.   1 mM N3,   0.50   0.64   7.70   0.45   9.31   0.58   3.43       N3, DPABA   0.05 mM   0.53   0.64   7.40   0.45   9.30   0.56   3.31           DPABA in       Sintering   EtOH for   0.50   0.64   7.70   0.45   9.38   0.57   3.44       Temp   Overnight;       450° C., 30   20/1       minutes   Average   0.51   0.64   7.60   0.45   9.33   0.57   3.39   0.07       Thickness of   1 mM N3, 1   0.53   0.63   7.21   0.41   8.58   0.55   2.96       Film   mM DPABA   0.50   0.63   6.75   0.44   8.23   0.57   2.97       TiO 2 , ˜10 μm   in EtOH for           Overnight   0.42   0.63   7.11   0.44   8.67   0.57   3.13           1/1           Average   0.48   0.63   7.02   0.43   8.49   0.56   3.02   0.10       Electrolyte   1 mM N3, 10   0.33   0.58   4.95   0.42   6.02   0.60   2.08           mM DPABA   0.52   0.60   5.51   0.42   6.67   0.58   2.31           in EtOH for       AM 1.5D, l 1   Overnight;   0.49   0.60   5.53   0.42   6.72   0.58   2.32       Sun   1/10       Film   Average   0.45   0.59   5.33   0.42   6.47   0.58   2.24   0.14       pretreatment                  
 
      Table 13 shows the results of using a cut-off filter (third and fourth entries) while irradiating a cell to test its I-V characteristics. Table 13 also shows that the efficiency of the cell still improves when DPABA is present, indicating that its effect when no filter is present is not simply due to absorption of UV light by DPABA followed by charge injection.  FIG. 15  shows a plot  1350  of the absorbance versus wavelength for the cut-off filter used to characterize the photovoltaic cells, according to an illustrative embodiment of the invention.  FIG. 16  shows a plot  1360  of the absorbance versus wavelength for DPABA, which absorbs below 400 nm. Because the absorbance of the cut-off filter is large, little light reaches the absorption bands of DPABA.  
                       TABLE 13                                      I-V CHARACTERIZATION                                                     Cell area       I m         I sc                     Conditions   cm 2     V oc  V   mA/cm 2     V m  V   mA/cm 2     FF   η %   σ                                                         1 mM N3 in   0.49   0.70   8.62   0.46   11.02   0.51   3.97           EtOH   0.49   0.70   8.13   0.45   10.20   0.51   3.66       Overnight   0.49   0.73   7.93   0.51   9.69   0.57   4.04       control       Average   0.49   0.71   8.23   0.47   10.30   0.53   3.89   0.20       1 mM N3   0.49   0.71   9.05   0.46   11.53   0.51   4.16       0.05 mM   0.49   0.71   9.24   0.46   11.56   0.52   4.25       DPABA in   0.49   0.71   9.39   0.46   11.50   0.53   4.32       EtOH, 20/1       Overnight       Average   0.49   0.71   9.23   0.46   11.53   0.52   4.24   0.08       1 mM N3 in   0.49   0.69   6.35   0.47   7.83   0.55   4.26   455 nm cut       EtOH   0.49   0.69   6.05   0.46   7.44   0.54   3.98   off filter       Overnight   0.49   0.72   5.74   0.52   6.94   0.60   4.27   used,       control                               70 mW/cm 2         Average   0.49   0.70   6.05   0.48   7.40   0.56   4.17   0.17       1 mM N3   0.49   0.70   6.73   0.47   8.21   0.55   4.52   455 nm cut       0.05 mM   0.49   0.70   6.74   0.47   8.19   0.55   4.53   off filter       DPABA in   0.49   0.70   6.74   0.49   8.25   0.57   4.72   used,       EtOH, 20/1                               70 mW/cm 2         Overnight       Average   0.49   0.70   6.74   0.48   8.22   0.56   4.59   0.11                  
 
      Table 14 shows that the addition of triphenylamine itself (i.e., no titania complexing groups such as carboxy) does not significantly enhance efficiency under the stated conditions.  
                       TABLE 14                                      I-V CHARACTERIZATION                                                     Cell       I m                                 area       mA/       I sc         Conditions   cm 2     V oc  V   cm 2     V m  V   mA/cm 2     FF   η %   σ                                                         0.5 mM N3   0.49   0.70   7.96   0.45   9.82   0.52   3.58           in EtOH,   0.49   0.71   8.09   0.48   9.58   0.57   3.88       Overnight   0.49   0.70   7.47   0.48   8.83   0.58   3.59       Average   0.49   0.70   7.84   0.47   9.41   0.56   3.68   0.17       0.5 mM N3,   0.49   0.69   7.44   0.45   9.21   0.53   3.35       0.025 mM   0.49   0.69   7.61   0.47   9.75   0.53   3.58       TPA in   0.49   0.69   6.98   0.45   8.56   0.53   3.14       EtOH       Overnight       20/1       Average   0.49   0.69   7.34   0.46   9.17   0.53   3.36   0.22       0.5 mM N3,   0.49   0.68   4.62   0.44   5.66   0.53   2.03       2.0 mM   0.49   0.66   4.18   0.45   5.38   0.53   1.88       TPA in   0.49   0.66   4.51   0.45   5.82   0.53   2.03       EtOH       Overnight       ¼       Average   0.49   0.67   4.44   0.45   5.62   0.53   1.98   0.09                  
 
      Table 15 shows that the effect is present using low temperature interconnected titania and that the 20/1 (dye/co-sensitizer) ratio is preferred.  
                       TABLE 15                                      I-V CHARACTERIZATION                                                     Cell       I m                                 area       mA/       I sc         Conditions   cm 2     V oc  V   cm 2     V m  V   mA/cm 2     FF   η %   σ                                                         0.5 nM   0.49   0.73   8.32   0.50   10.56   0.54   4.16           N3/EtOH,   0.51   0.72   8.13   0.49   10.30   0.54   3.98       overnight,   0.50   0.72   8.56   0.47   10.65   0.52   4.02       control       Average   0.50   0.72   8.34   0.49   10.50   0.53   4.06   0.09       0.5 mM N3,   0.49   0.73   8.55   0.51   10.48   0.57   4.36       0.0125 mM   0.53   0.72   8.53   0.50   11.00   0.54   4.27       DPABA in   0.49   0.74   8.08   0.54   10.96   0.54   4.36       EtOH, 40/1,       overnight       Average   0.50   0.73   8.39   0.52   10.81   0.55   4.33   0.06       0.5 nM N3   0.49   0.73   9.07   0.49   11.31   0.54   4.44       0.017 mM   0.49   0.75   8.64   0.52   10.97   0.55   4.49       DPABA in   0.52   0.73   8.19   0.52   10.88   0.54   4.26       EtOH, 30/1,       overnight       Average   0.50   0.74   8.63   0.51   11.05   0.54   4.40   0.12       0.5 mM N3,   0.50   0.75   8.57   0.52   11.56   0.51   4.46       0.025 mM   0.49   0.74   8.88   0.52   11.45   0.54   4.62       DPABA in   0.53   0.74   9.01   0.51   12.08   0.51   4.60       EtOH, 20/1,       overnight       Average   0.51   0.74   8.82   0.52   11.70   0.52   4.56   0.09       0.5 mM N3,   0.49   0.72   8.85   0.48   10.78   0.55   4.25       0.5 mM   0.51   0.74   8.62   0.47   10.37   0.53   4.05       DPABA in   0.50   0.75   8.38   0.49   10.02   0.55   4.11       EtOH, 1/1,       overnight       Average   0.50   0.74   8.62   0.48   10.39   0.54   4.14   0.10       0.5 mM N3,   0.49   0.68   7.56   0.44   9.09   0.54   3.33       5 mM   0.51   0.69   7.62   0.46   9.34   0.54   3.51       DPABA in   0.49   0.67   7.25   0.45   8.84   0.55   3.26       EtOH, 1/10,       overnight       Average   0.50   0.68   7.48   0.45   9.09   0.54   3.36   0.13                  
 
      Table 16 shows results for high-temperature-sintered titania sensitized with a high concentration of N3 dye while maintaining a 20/1 ratio of dye to co-sensitizer. Entries 1 and 2 show the increase in cell performance due to co-sensitizer. Entry 3 shows the effect of DPABA alone as a sensitizer, demonstrating that this material acts as a sensitizer by itself when irradiated with the full solar spectrum, which includes low-intensity UV radiation.  
                       TABLE 16                                      I-V CHARACTERIZATION                                                     General       Cell area       I m         I sc                     Conditions   Conditions   cm 2     V oc  V   mA/cm 2     V m  V   mA/cm 2     FF   η %   σ                                                             Adsorption   8 mM   0.49   0.68   8.51   0.44   10.07   0.55   3.74           Temp.   N3/aprotic   0.49   0.67   8.28   0.44   9.75   0.56   3.64       RT ° C.   polar solvent,       Solvent of Dye   1 hour   0.49   0.68   9.16   0.42   10.80   0.52   3.85       Aprotic polar   CONTROL       solvent   average   0.49   0.68   8.65   0.43   10.21   0.54   3.74   0.10           8 mM N3, 0.4 mM   0.49   0.68   9.52   0.44   11.18   0.55   4.19           DPABA   0.49   0.68   9.96   0.44   11.59   0.56   4.38           in aprotic polar   0.49   0.65   9.81   0.42   12.13   0.52   4.12           solvent,           20/1           1 hour           average   0.49   0.67   9.76   0.43   11.63   0.54   4.23   0.14           5 mM DPABA   0.49   0.55   1.02   0.42   1.22   0.64   0.43           in aprotic polar   0.49   0.55   0.94   0.41   1.13   0.62   0.39           solvent   0.49   0.58   0.89   0.44   1.07   0.63   0.39           Overnight               0.49   0.56   0.95   0.42   1.14   0.63   0.40   0.02                  
 
      Table 17 shows results for low-temperature-interconnected titania. Entry 5 shows the affect of DPACA alone as a sensitizer, demonstrating that this material acts as a sensitizer by itself when irradiated with the full solar spectrum, which includes low-intensity UV radiation.  
                       TABLE 17                                      I-V CHARACTERIZATION                                                     Cell       I m                                 area       mA/       I sc         Conditions   cm 2     V oc  V   cm 2     V m  V   mA/cm 2     FF   η %   σ                                                         0.5 mM   0.51   0.73   8.40   0.50   10.84   0.53   4.20           N3/EtOH,   0.53   0.72   8.13   0.49   10.30   0.54   3.98       overnight,   0.50   0.72   8.77   0.47   10.87   0.53   4.12       control       average   0.51   0.72   8.43   0.49   10.67   0.53   4.10   0.11       0.5 mM N3,   0.49   0.73   8.10   0.51   10.39   0.54   4.13       0.01 mM   0.50   0.74   7.95   0.50   10.01   0.54   3.98       DPACA in   0.49   0.72   8.10   0.50   9.85   0.57   4.05       EtOH, 50/1,       overnight       average   0.49   0.73   8.05   0.50   10.08   0.55   4.05   0.08       0.5 mM N3,   0.49   0.74   8.38   0.50   10.48   0.54   4.19       0.02 mM   0.52   0.73   8.18   0.48   9.74   0.55   3.93       DPACA in   0.49   0.76   8.08   0.54   9.45   0.61   4.36       EtOH, 25/1,       overnight       average   0.50   0.74   8.21   0.51   9.89   0.57   4.16   0.22       0.5 mM N3,   0.49   0.73   9.07   0.46   11.31   0.51   4.17       0.5 mM   0.49   0.75   7.41   0.53   9.24   0.57   3.93       DPACA in   0.52   0.76   7.93   0.52   9.12   0.59   4.12       EtOH, 1/1,       overnight       average   0.50   0.75   8.14   0.50   9.89   0.56   4.07   0.13       0.5 mM N3,   0.56   0.73   6.36   0.49   7.59   0.56   3.12       5.0 mM   0.52   0.73   6.63   0.49   7.84   0.57   3.25       DPACA in   0.50   0.72   6.53   0.49   7.59   0.59   3.20       EtOH, 1/10,       overnight       average   0.53   0.73   6.51   0.49   7.67   0.57   3.19   0.07       5.0 mM   0.43   0.65   3.12   0.49   3.77   0.62   1.53       DPACA in   0.45   0.65   2.93   0.49   3.51   0.63   1.44       EtOH,   0.49   0.66   2.83   0.49   3.40   0.62   1.39       overnight       average   0.46   0.65   2.96   0.49   3.56   0.62   1.45   0.07                  
 
      Table 18 shows results for low-temperature-interconnected titania. Entry 6 shows the affect of DEAPA alone as a sensitizer, demonstrating that this material acts as a sensitizer by itself when irradiated with the full solar spectrum, which includes low-intensity UV radiation.  
                       TABLE 18                                      I-V CHARACTERIZATION                                                     General       Cell area       I m         I sc                     conditions   Conditions   cm 2     V oc  V   mA/cm 2     V m  V   mA/cm 2     FF   η %   σ                                                             Adsorption   0.5 mM   0.51   0.72   8.67   0.49   10.60   0.56   4.25           Temp.   N3/EtOH,   0.49   0.75   8.15   0.47   10.50   0.49   3.83       RT ° C.   overnight,       Solvent of   control   0.49   0.74   8.74   0.44   10.63   0.49   3.85       Dye   average   0.50   0.74   8.52   0.47   10.58   0.51   3.97   0.24       EtOH       Dye Concen.   0.5 mM N3,   0.49   0.70   8.68   0.44   11.00   0.50   3.82       N3, DEAPA   0.01 mM   0.52   0.71   8.57   0.45   11.11   0.49   3.86       Sintering   DEAPA in   0.50   0.72   8.40   0.45   10.61   0.49   3.78       Temp   EtOH, 50/1,       120° C., 10   overnight       minutes   average   0.50   0.71   8.55   0.45   10.91   0.49   3.82   0.04       Thickness of   0.5 mM N3,   0.51   0.74   8.90   0.44   10.92   0.48   3.92       Film   0.02 mM   0.53   0.73   8.76   0.44   10.51   0.50   3.85       TiO 2 , ˜7 μm   DEAPA in       Liquid   EtOH, 25/1,   0.49   0.73   8.40   0.45   10.21   0.51   3.78           overnight           average   0.51   0.73   8.69   0.44   10.55   0.50   3.85   0.07           0.5 mM N3,   0.49   0.71   8.94   0.43   10.78   0.50   3.84       Electrolyte   0.5 mM   0.51   0.71   8.83   0.44   10.37   0.53   3.89       AM 1.5D, 1   DEAPA in   0.50   0.70   8.18   0.42   9.71   0.51   3.44       Sun   EtOH, 1/1,           overnight       Film   average   0.50   0.71   8.65   0.43   10.29   0.51   3.72   0.25       pretreatment   0.5 mM N3,   0.52   0.60   0.88   0.45   1.08   0.61   0.40           5.0 mM   0.49   0.59   0.71   0.44   0.85   0.62   0.31           DEAPA in   0.49   0.59   0.75   0.44   0.91   0.61   0.33           EtOH, 1/10,           overnight           average   0.50   0.59   0.78   0.44   0.95   0.62   0.35   0.04           5.0 mM   0.49   0.54   0.41   0.42   0.49   0.65   0.17           DEAPA in   0.49   0.54   0.35   0.39   0.46   0.55   0.14           CHCl3,   0.51   0.52   0.45   0.40   0.52   0.67   0.18           overnight           average   0.50   0.53   0.40   0.40   0.49   0.62   0.16   0.02                  
 
 D. Semiconductor Oxide Formulations 
 
      In a further illustrative embodiment, the invention provides semiconductor oxide formulations for use with DSSCs formed using a low temperature semiconductor oxide nanoparticle interconnection, as described above. The semiconductor oxide formulations may be coated at room temperature and, upon drying at temperatures between about 50° C. and about 150° C., yield mechanically stable semiconductor nanoparticle films with good adhesion to the transparent conducting oxide (TCO) coated plastic substrates. In one embodiment, the nanoparticle semiconductor of the photosensitized interconnected nanoparticle material  603  is formed from a dispersion of commercially available TiO 2  nanoparticles in water, a polymer binder, with or without acetic acid. The polymer binders used include, but are not limited to, polyvinylpyrrolidone (PVP), polyethylene oxide (PEO), hydroxyethyl cellulose (HOEC), hydroxypropyl cellulose, polyvinyl alcohol (PVA) and other water-soluble polymers. The ratio of semiconductor oxide particles, e.g., TiO 2 , to polymer can be between about 100:0.1 to 100:20 by weight, and preferably is between about 100:1 to 100:10 by weight. The presence of acetic acid in the formulation helps to improve the adhesion of the coating to the TCO coated substrate. However, acetic acid is not essential to this aspect of the invention and semiconductor oxide dispersions without acetic acid perform satisfactorily. In another embodiment, the TiO 2  nanoparticles are dispersed in an organic solvent, such as, e.g., isopropyl alcohol, with polymeric binders such as, e.g., PVP, butvar, ethylcellulose, etc.  
      In another illustrative embodiment, the mechanical integrity of the semiconductor oxide coatings and the photovoltaic performance of the dye sensitized cells based on these coatings can be further improved by using a crosslinking agent to interconnect the semiconductor nanoparticles. The polylinkers described above may be used for this purpose. These crosslinking agents can be applied, e.g., in the titania coating formulation directly or in a step subsequent to drying the titania coating as a solution in an organic solvent such as ethanol, isopropanol or butanol. For example, subsequent heating of the films to temperatures in the range of about 70° C. to about 140° C. leads to the formation of TiO 2  bridges between the TiO 2  nanoparticles. Preferably, the concentration of the polylinker in this example ranges from about 0.01 to about 20 weight % based on titania.  
      E. Semiconductor Primer Layer Coatings  
      In another illustrative embodiment, the invention provides semiconductor oxide materials and methods of coating semiconductor oxide nanoparticle layers on a base material to form DSSCs.  FIG. 17  depicts an illustrative embodiment  1370  of the coating process, according to the invention. In this illustrative embodiment, a base material  1376  is coated with a first primer layer  1382  of a semiconductor oxide, and then a suspension of nanoparticles  1388  of the semiconductor oxide is coated over the primer layer  1382 . The primer layer  1382  may include a vacuum-coated semiconductor oxide film (e.g., a TiO 2  film). Alternatively, the primer layer  1382  may include a thin coating with fine particles of a semiconductor oxide (e.g. TiO 2 , SnO 2 ). The primer layer  1382  may also include a thin layer of a polylinker or precursor solution, one example of which is the Ti (IV) butoxide polymer  400  shown in  FIG. 4  above. According to one illustrative embodiment of the invention, the base material  1376  is the first flexible, significantly light transmitting substrate  609  referred to in  FIG. 6 . Additionally, the base material  1376  is a transparent, conducting, plastic substrate. According to this illustrative embodiment, the suspension of nanoparticles  1388  is the photosensitized interconnected nanoparticle material  603  of  FIG. 6 . Numerous semiconducting metal oxides, including SnO 2 , TiO 2 , Ta 2 O 5 , Nb 2 O 5 , and ZnO, among others, in the form of thin films, fine particles, or precursor solutions may be used as primer layer coatings using vacuum coating, spin coating, blade coating or other coating methods.  
      The primer layer  1382  improves the adhesion of nano-structured semiconductor oxide films, like layer  1388 , to the base material  1376 . Enhancements in the performance of DSSCs with such primer layers have been observed and will be described below. The enhancement arises from an increase in the adhesion between the semiconductor oxide nanoparticles (or photoelectrodes) and the transparent conducting oxide coated plastic substrates, as well as from higher shunt resistance.  
      Examples of various illustrative embodiments of this aspect of the invention, in the context of a DSSC including a titanium dioxide nanoparticle layer, are as follows.  
     EXAMPLE 18  
     Vacuum Coated TiO 2  as Prime Layers for Nanoparticle TiO 2  Photoelectrodes  
      In this illustrative example, thin TiO 2  films with thicknesses ranging from 2.5 nm to 100 nm were sputter-coated under vacuum on an ITO layer coated on a polyester (here, PET) substrate. A water based TiO 2  (P25, with an average particle size of 21 nm) slurry was spin-coated on both the ITO/PET with sputter-coated thin TiO 2  and on the plain ITO/PET (i.e., the portion without sputter-coated thin TiO 2 ). The coated films were soaked in poly [Ti(OBu) 4 ] solution in butanol and then heat treated at 120° C. for 2 minutes. The low-temperature reactively interconnected films were placed into an aprotic, polar solvent-based N3 dye solution (8 mM) for 2 minutes. Photovoltaic cells were made with platinum (Pt) counter-electrodes, an I − /I 3   −  liquid electrolyte, 2 mil SURLYN, and copper conducting tapes. I-V characterization measurements were performed with a solar simulator.  
      Adhesion of nanostructured TiO 2  films from the P25 slurry coated on the ITO/PET with sputter-coated, thin TiO 2  was superior to films on the plain ITO/PET. Better photovoltaic performance was also observed from the PV cells prepared on the ITO/PET with sputter-coated, thin TiO 2  as compared to those on the plain ITO/PET. Improvement on the fill-factor was achieved as well. A FF as high as 0.67 was measured for the photovoltaic cells made on the ITO/PETs with sputter-coated, thin TiO 2 . For the photovoltaic cells made on the plain ITO/PET, the FF observed was not greater than 0.60. Higher photovoltaic conversion efficiencies (about 17% higher than the photoelectrodes made from the plain ITO/PET) were measured for the photoelectrodes prepared on the ITO/PET with thin sputter-coated TiO 2 . Improvement in shunt resistance was also observed for the photovoltaic cells made on the ITO/PET with thin sputter-coated TiO 2 .  
     EXAMPLE 19  
     Fine Particles of TiO 2  as Primer Layer for TiO 2  Suspensions  
      In this illustrative example, fine particles of TiO 2 , small enough such that they would stick in the valleys between spikes of ITO on the PET substrate, were prepared by hydrolyzing titanium (IV) isopropoxide. The fine particles were then spin coated at 800 rpm onto the ITO layer. A 37% TiO 2  (P25) suspension of approximately 21 nm average particle size was then spin coated at 800 rpm onto the fine particle layer. The coated TiO 2  was low temperature interconnected by dipping in 0.01 molar Ti (IV) butoxide polymer in butanol for 15 minutes followed drying on a slide warmer at 50° C. before heating at 120° C. for 2 minutes. The interconnected coating was dyed with N3 dye by dipping into an 8 mM aprotic polar solvent solution for 2 minutes, then rinsed with ethanol and dried on a slide warmer at 50° C. for 2 minutes. Control coatings were prepared in the same way, except without the fine particle prime coat. The cells&#39; performance characteristics were measured using a solar simulator. Results for test and control are listed below in Table 19. Fine particles of tin oxide as primer coating for TiO 2  suspensions yielded similar improvements.  
                                   TABLE 19                                   V oc     I sc     η   FF                                                                Control   0.64   4.86   1.67%   0.54           Invention   0.66   6.27   2.36%   0.57                      
 
     EXAMPLE 20  
     Titanium (IV) Butoxide Polymer in Butanol (Precursor Solution) as Primer Layer for TiO 2    
      In another test, titanium (IV) butoxide polymer in butanol at 0.01 molar was spin coated on an ITO/PET plastic base at 800 rpm. A 43% TiO 2  (P25) suspension of approximately 21 nm average particle size was spin coated at 800 rpm. The coated TiO 2  was interconnected at low temperature by dipping in 0.01 M titanium (IV) butoxide polymer in butanol for 15 minutes and then drying on a slide warmer at 50° C. before heating at 120° C. for 2 minutes. The sintered coating was dyed with N3 dye by dipping into an 8 mM aprotic, polar solvent solution for 2 minutes, then rinsed with ethanol and dried on a slide warmer at 50° C. for 2 minutes. Control coatings were prepared in the same way only without the primer layer coating. The I-V properties of the cells were measured with a solar simulator. Results for test and control are listed below in Table 20.  
                                   TABLE 20                                   V oc     I sc     η   FF                                                                Control   0.66   7.17   2.62%   0.56           Invention   0.70   8.11   3.38%   0.59                      
 
 F. Photovoltaic Cell Interconnection 
 
       FIG. 18A  depicts an illustrative embodiment of an interconnected photovoltaic module  1400  that includes a plurality of photovoltaic cells  1402   a ,  1402   b , and  1402   c  (collectively “ 1402 ”). Three photovoltaic cells  1402  are shown for illustrative purposes only; the photovoltaic module  1400  is not restricted to that number.  FIG. 18B  shows the photovoltaic module  1400  in a flexed state. The photovoltaic module  1400  is suited for display arrangements where illumination may be incident on either the top  1404  or bottom  1406  surfaces. Referring to  FIGS. 18A and 18B , the plurality of photovoltaic cells  1402  are disposed between a first electrical connection layer  1408  and a second electrical connection layer  1410 . Preferably, the photovoltaic cells and electrical connection layers  1408  and  1410  are in turn disposed between a first substrate  1414  and a second substrate  1420 . The photovoltaic cells  1402  include a photosensitized nanomatrix layer  1425  and a charge carrier medium  1430 . Preferably, the photovoltaic cells also include a catalytic medium  1435 . The electrical connection layers  1408  and  1410  include conductive regions  1440  and/or insulative regions  1445 . In one illustrative embodiment, the electrical connection layers  1408  and  1410  also include linking conductive regions  1450 . It is preferable in practice to minimize the width of the insulative regions  1445  and/or linking conductor regions  1450  relative to the conductive regions  1440 .  
      The conductive  1440  and insulative  1445  regions of the electrical connection layers  1408  and  1410  may include transparent materials, such as, for example, ITO, a fluorine-doped tin oxide, tin oxide, zinc oxide, and the like. In one illustrative embodiment, the conductive regions  1440  of the electrical connection layers  1408  and  1410  range in thickness between about 100 nm and about 400 nm. In another embodiment, the conductive regions  1440  are between about 150 nm and about 300 nm thick. According to an additional feature of the illustrative embodiment, a wire or lead line is connected to one or more conductive regions  1440  to electrically connect the photovoltaic module  1400  to an external load.  
      Preferably, the photovoltaic module  1400  is significantly light-transmitting. This is accomplished in some embodiments by forming all of the constituent layers and regions of the photovoltaic module  1400  from significantly light-transmitting materials. In other illustrative embodiments, the photovoltaic module  1400  is formed to be significantly light-transmitting even though some of the individual regions of the photovoltaic module&#39;s  1400  constituent layers are not significantly light-transmitting. Furthermore, the conductive regions  1440  of at least one of the electrical connection layers  1408  and  1410  may be significantly light-transmitting.  
      The electrical connection layers  1408  and  1410  may include a linking conductor region  1450  deposited between two or more of the conductive regions  1440 . Preferably, the linking conductor region  1450  includes a conductive material, such as, for example, copper, silver, gold, platinum, nickel, palladium, iron, and alloys thereof. Other suitable conductive materials include, but are not limited to, ITO and conductive polymers such as polyaniline and aniline. In one illustrative embodiment, one or more of the linking conductor regions  1450  are substantially transparent. Alternatively, one or more of the linking conductive regions  1450  are opaque.  
      According to one feature, the photosensitized nanomatrix layer  1425  includes photosensitized nanoparticles, e.g., metal oxide nanoparticles, as discussed above. In one embodiment, the photosensitized nanomatrix layer  1425  includes nanoparticles with an average size of between about 2 and about 400 nm. More particularly, the nanoparticles have an average size of between about 10 nm and about 40 nm. In a preferred embodiment, the nanoparticles are titanium dioxide particles having an average particle size of about 20 nm.  
      In another illustrative embodiment, the photosensitized nanomatrix layer  1425  includes a heterojunction composite material. Examples of suitable heterojunction composite materials are fullerenes (e.g., C 60 ), fullerene particles, and carbon nanotubes. The heterojunction composite material may be dispersed in polythiophene or some other hole transport material. The fullerene particles may have an average size of between about 100 nm and about 400 nm. Other examples of suitable heterojunction composite materials are composites that include conjugated polymers, such as polythiophene and polyquinoline, and composites of conjugated polymers, such as polyphenylene vinylene, in conjunction with non-polymeric materials.  
      The charge carrier medium  1430  is preferably a polymeric and relatively viscous, e.g., jelly-like, material. In other embodiments, the charge carrier medium  1430  is a liquid. As a result, the charge carrier medium  1430  can be applied using various techniques suitable for applying gels and liquids known to those skilled in the art. These techniques include, for example, spray coating, roller coating, knife coating, and blade coating. The charge carrier medium  1430  may be prepared by forming a solution having an ion-conducting polymer, a plasticizer, and a mixture of iodides and iodine. The polymer provides mechanical and/or dimensional stability; the plasticizer helps the gel/liquid phase transaction temperature, and the iodides and iodine act as redox electrolytes.  
      In one illustrative embodiment, the charge carrier medium  1430  includes about 10% of polyethylene oxide, about 90% of 1:1 by weight mixture of ethyl carbonate: propylene carbonate, about 0.05 M iodine, and about 0.5 M lithium tetramethylammonium iodide. This polymeric electrolyte typically has a relatively long shelf life (e.g., up to a few years), experiences minimal phase segregation during processing or while the photovoltaic module is in use, and may be used without processing, such as melting, prior to deposition.  
      G. Rigid Substrates  
      According to another illustrative embodiment, the invention provides photovoltaic cells or modules with substantially rigid substrates or a combination of a substantially rigid substrate and a flexible substrate. For example, the low-temperature (&lt;about 300° C.) sintering technique discussed above using polylinkers may be used to construct DSSCs utilizing substantially rigid substrates such as tempered glass plates. Alternatively, flexible photovoltaic cells, also made using the-low temperature sintering technique, may be disposed upon or incorporated between one or more substantially rigid substrates. That is, flexible and substantially rigid substrates can be combined with the other DSSC constituent elements described above to form a flexible or substantially rigid DSSC. One suitable substantially rigid substrate is the tempered glass product SOLARTEX (available from Ajiya Safety Glass, Johor, Malaysia). Generally, substantially rigid substrates are materials which possess physical durability characteristics similar to SOLARTEX. Materials that only lose their durability and other structurally desirable characteristics when exposed to temperatures outside the range of the low-temperature sintering processes described above are also suitable for use as substantially rigid substrates.  
      Tempered glass is significantly more resistant to thermal and mechanical stresses and strains than untempered glass. The application of dye-sensitized titania photovoltaic cells to glass plates requires sintering temperatures in excess of 400° C. using conventional methods. If the glass substrate includes tempered glass, the high temperatures required for sintering remove the temper from the glass plate and convert it to ordinary glass. This results in a substrate which can be shattered much more easily upon application of either a thermal or a mechanical stress. Given the breath of photovoltaic applications, substrate strength and structural integrity can be an important feature of a photovoltaic cell or module. Specifically, DSSC longevity and durability can be enhanced through the inclusion of substantially rigid substrates in the low-temperature sintering context. The low temperature range at which DSSCs can be fabricated allows tempered glass and other substrates to be used in various embodiments of the invention.  
      Referring back to  FIG. 6 , two rigid substrates may be substituted for the substrates  609  and  612  to form the photovoltaic cell  600 , as described above. An anti-reflective coating may be applied to the rigid substrates to increase their ability to transmit radiation. Alternatively, tempered glass may be coated with a suspension of titania in the presence of a polylinker. The sintering is preferably accomplished at temperatures below about 150° C. This low-temperature range facilitates use of a broad class of substantially rigid substrates that would otherwise undergo physical and chemical changes at higher temperatures, which would diminish the intrinsic material strength of many substrates.  
      After sequentially adding sensitizing dye and electrolyte to the sintered titania, a second tempered glass plate, bearing a counter electrode (e.g., a very thin coating of platinum), is laminated to the first plate, thus forming a complete photovoltaic cell or module. A photovoltaic cell  600  may be formed using one rigid and one flexible substrate as well, according to the invention. In various embodiments, the substantially rigid substrate(s) is/are about 3 mm or less thick. In other embodiments, the substantially rigid substrate(s) may range in thickness from about 3 mm to about 0.5 inches in thickness.  
       FIG. 13  depicts another illustrative embodiment in which the photovoltaic cell  600  from  FIG. 6  is sandwiched between two substantially rigid substrates  1500  and  1504  to form a rigid photovoltaic cell  1508 . The remaining layers are described in detail above with reference to  FIG. 6 . The overall durability of the photovoltaic cell  600  shown in  FIG. 6  is increased using this methodology. For example, the combination of a high speed, low cost, roll-to-roll manufacturing process with the shock resistance and gas barrier properties of tempered glass creates a unique photovoltaic system, suitable for outdoor applications. In a further illustrative embodiment, a suspension of titania is coated on a plastic substrate and sintered at low temperature by means of heating (&lt;about 150° C.) in the presence of a polylinker, according to the invention. Sensitizing dye and electrolyte are added sequentially by standard coating techniques, and a counter electrode including a very thin coating of platinum on a coating of inert metal on a flexible plastic substrate is laminated to the first electrode, thereby creating a photovoltaic cell or module. The flexible cell or module is then laminated between two tempered glass plates or one tempered glass top plate (defined as the plate facing the light source) and a second support plate of another material.  
      Another advantage of the plastic-based PV module/tempered glass combination manifests itself in a lamination step. Typically, lamination of crystalline silicon cells and tempered glass is accomplished by applying vacuum while pressing the two materials together. In the illustrative embodiments described above, the lamination of the flexible photovoltaic cells or module to the tempered glass top plate may be performed at atmospheric pressure and room temperature (or elevated temperature) by applying an adhesive onto the glass plate with a laminating roller, without the requirement of vacuum. Thermal setting or photocuring of the adhesive is accomplished during or after the lamination step. Thus, substantially rigid substrates can provide both structural and manufacturing enhancements to existing DSSCs.  
      H. Methods for Scoring for Fabricating DSSCs  
      In another illustrative embodiment, the invention provides methods that reduce the likelihood of cell contamination from debris when fabricating flexible photovoltaic cells and modules using a continuous manufacturing process, such as the one depicted in  FIG. 7 .  FIG. 20A  depicts a base material  1510  for a photovoltaic cell or module, where the base material  1510  is formed as a sheet or strip of material that includes a polymeric substrate  1515  and a conductive layer  1520 . A heated stylus  1530  scores the polymeric substrate  1515  and the conductive layer  1520  by parting the conductive layer  1520  and, to some depth, melting the polymeric material of the substrate  1515 . Scoring interrupts the electrical continuity of the otherwise continuous conductive layer  1520 , and to a certain extent, encapsulates any debris from the conductive layer  1520  in the wake of the heated stylus  1530  when the polymeric substrate  1515  cools. Coating and entrapping conductive debris that may be in the melt of the polymeric substrate  1515  prevents these debris from interfering with the performance of the photovoltaic cell or module. The conductive layer  1520  may be semiconductive.  
      According to one illustrative embodiment, the heated stylus  1530  (which may be a soldering iron tip or a heated razor blade) is held in position by a fixed support system. For example, referring to  FIGS. 7 and 20 A, the heated stylus  1530  may be positioned to contact the advancing substrate  705  prior to the deposition of the photosensitized nanoparticle material  715 . Preferably, by monitoring the temperature and geometry of the heated stylus  1530 , the distance from the heated stylus  1530  to the advancing substrate  705 , the pressure of the heated stylus  1530  on the advancing substrate  705 , and the rate at which the advancing substrate  705  advances, the width and depth of the score mark  1540 , for example as depicted in  FIG. 20B , is determined.  
      Suitable polymeric substrate materials  1515 , as described above, include, for example, PET, polyacrylonaphthalates, polyimides, PEN, polymeric hydrocarbons, cellulosics, and combinations thereof. Conductive layer  1520  materials include, but are not limited to, ITO and conductive polymers such as polyaniline and aniline, as described above. In various illustrative embodiments, the conductive layers  1520  are between about 0.5 microns and about 5 microns thick. Preferably, the conductive layers  1520  are between about 0.5 micron and about 1 micron thick. In addition, the conductive layers  1520  scored using this method may include a photosensitized nanomatrix layer, according to the invention.  
      I. Wire Interconnects for Fabricating DSSCs  
      In another illustrative embodiment, the invention provides methods of interconnecting photovoltaic cells (e.g., in parallel, in series, or combinations of both) to form photovoltaic modules using wire interconnects as part of a continuous manufacturing process, such as the one shown in  FIG. 7 .  FIG. 21  depicts a base material  1550  suitable for forming a photovoltaic cell or module using wire interconnects. The base material  1550  includes a polymeric substrate  1555  and a conductive layer  1560 , both in accordance with the invention. A portion of the base material  1550  is coated with a photoconductive layer  1565  (e.g., a photosensitized interconnected nanoparticle layer). A strip of hot melt adhesive  1570  is tacked to the conductive layer  1560 , and a wire  1575  is placed over the strip of hot melt adhesive  1570 .  
       FIGS. 22A and 22B  depict another illustrative embodiment of a photovoltaic module formed using wire interconnects. A first base material  1600  includes a polymeric substrate  1610  and a conductive layer  1620 , as described above. A heated stylus, as described above, forms a score mark  1630  in the base material  1600 , so that a plurality of discrete photovoltaic cells may be formed on a single base material  1600 . The methods and structures described in detail above with respect to  FIGS. 7 and 18  are used to complete the photovoltaic cells on the base material  1600 . A strip of hot melt adhesive  1640  is then tacked to the conductive layer  1620 , and a wire  1650  is placed over the strip of hot melt adhesive  1640 .  
      Referring to  FIG. 22B , the photovoltaic module  1660  is completed by placing a second base material  1670 , including a polymeric substrate  1610 , a conductive layer  1620 , a score mark  1630 , and a strip of hot melt adhesive  1640 , on top of the first base material. The photovoltaic module  1660  is laminated together under heat and pressure to complete the structure. Under appropriate process conditions, the wire  1640  forces its way through the molten adhesive  1630 , thus achieving electrical contact with both conductive layers  1620 . After cooling, the adhesive  1630  solidifies, thereby trapping the wire  1650  in place. This results in a strong and conductive bond.  
      In one illustrative example, the thermal laminating step was performed with an aluminum strip machined to an appropriate size to match the size of the adhesive strips. The aligned sheets were placed in a thermal laminator and pressed together for 10 seconds at moderate pressure. The photovoltaic module  1660  was then moved to align the next adhesive strip with the press, and the process was repeated for each adhesive strip until the photovoltaic module  1660  was finished. The same adhesive and laminating procedure (without the wire interconnect) was used for the side seals of the photovoltaic module.  
      The embodiment illustrated in  FIGS. 22A and 22B  has been used to construct a working three stripe photovoltaic module. In one illustrative example, stripes of N3-dyed titania nanoparticles were coated onto an ITO coated polyester substrate. Score marks were made in the ITO to break the electrical continuity between the cells. Score marks were also made in the appropriate places in the conductive coating of a platinum/titanium counter electrode on the complimentary polyester substrate. The adhesive used was THERMO-BOND 615 adhesive (available from the 3M Corporation) in 2.5 mm thickness. THERMO-BOND 615 adhesive is a polyester-based hot melt adhesive supplied as a coated film on release paper. Strips of THERMO-BOND adhesive 615 were pretacked onto a platinum sheet by a brief cycle in a hot press at 120° C. The release paper was removed, and one strand of 3 mm 316 stainless steel wire was lightly pressed into the exposed adhesive strips between the cells.  
      The wire  1575  or  1650  is an electrical conductor, and preferably a metal electrical conductor. Suitable wire materials include, but are not limited to, copper, silver, gold, platinum, nickel, palladium, iron, and alloys thereof. In another illustrative embodiment, more than one wire is used for each interconnection. Further, although the wire is illustrated in  FIGS. 22A and 22B  as having a substantially circular cross-section, the cross-sectional shape of the wire is not limited to a substantially circular cross-section. Other suitable cross-sectional shapes of the wire include, those that are substantially square, rectangular, elliptical, triangular, trapezoidal, polygonal, arcuate, and even irregular shapes. In addition, the wire may be a single stranded wire or a multi-stranded wire (e.g., a plurality of wires twisted together).  
      The hot melt adhesive  1570  and  1640  is not limited to THERMO-BOND adhesive 615. Other hot melt adhesives (e.g., BYNEL adhesive available from DuPont) that are thermally activated may be applied as a strip or film. The wire  1575  and  1650  may be coated with the adhesive  1570  and  1640 , applied under the adhesive  1570  and  1640 , over the adhesive  1570  and  1640 , or pre-embedded in a strip of adhesive  1570  and  1640 . In addition, curing adhesives (e.g., epoxies) may be used. Preferably, the interconnection region is held in the desired final configuration during the cure cycle when curing adhesives are used.  
      Wire interconnects are typically quite tolerant with respect to alignment. Furthermore, the wire providing the interconnection is protected by a nonconductive adhesive. As a result, if a minor misalignment occurs and the adhesive bridges are scored, an electrical short will not result. Wire interconnects also facilitate the reduction of gaps between cells, which typically leads to higher overall photovoltaic module efficiency.  
      J. DSSCs with Metal Foil  
      According to another illustrative embodiment, a flexible photovoltaic module is formed using conventional methods (i.e., temperatures&gt;about 400° C.) by first forming the interconnected nanoparticle layer on a flexible, metal foil, which is then laminated to a flexible substrate. The nanoparticle layer may be interconnected at low temperatures (&lt;about 300° C.) as well. In either temperature range, a continuous manufacturing process, such as a roll-to-roll process, may be used.  FIG. 23  depicts a photovoltaic module  1700  including a metal foil  1704 . More particularly, the metal foil  1704  is laminated to a first flexible substrate  1708  using a first adhesive  1712 . Score marks  1716  formed in the metal foil  1704  using a heated stylus serve to break the electrical continuity of the metal foil  1704 , thus enabling a plurality of photovoltaic cells to be formed on a single substrate. A photosensitized nanoparticle material  1720 , according to the invention, is disposed on the metal foil  1704 . According to the illustrative embodiment, the photovoltaic module  1700  includes wire interconnects  1724  embedded in a second adhesive layer  1728  to form an electrical connection between the metal foil  1704  and a platinum-coated  1732  counter electrode  1733 . The photovoltaic module  1700  also includes a charge carrier material  1736  and a conductive layer  1740  coated on a second substrate  1744 . The metal foil  1704  may be any suitable conductive metal such as, for example, titanium, copper, silver, gold, platinum, nickel, palladium, iron, and alloys thereof. Suitable materials for the adhesive layer  1728  include BYNEL and THERMO-BOND, which are described above. Thermoplastic and thermoset may also eb used as the adhesive layer. According to one illustrative embodiment, the counter electrode  1733  includes a patterned-semiconductor, an adhesive layer, and a wire interconnect all formed on a flexible, transparent substrate.  
      According to the illustrative embodiment and using methods described above, a nanoparticle layer is coated on the metal foil  1704  in a continuous, intermittent, or patterned format. The nanoparticle layer is sintered at either low or high temperatures. The metal foil  1704  is then scored to break the electrical continuity. The separated metal foil  1704  strips are thereupon laminated, according to the invention, to the non-conductive substrate  1708 . A sensitizing dye and/or co-sensitizer may be applied to the nanoparticle layer to complete the photosensitized nanoparticle material  1720 . A charge carrier material  1736  is coated over the photosensitized nanoparticle material  1720 , and an adhesive layer  1728  and wire interconnect  1724  are added to couple the metal foil  1704  to the counter electrode  1733 . A second flexible substrate coated with a conductive layer is laminated to complete the photovoltaic module.  
      In a preferred embodiment, the metal foil  1704  is titanium, the photosensitized nanoparticle material  1720  includes sintered titania, the conductive layer  1740  is ITO, and the second substrate  1744  is PET. The photovoltaic module may be constructed using a continuous manufacturing process, such as the one depicted in  FIG. 7 . Such a process (e.g., a roll-to-roll process) includes the step of (1) coating a titania dispersion continuously, intermittently, or in a patterned format (e.g., to discrete portions) on the metal foil; (2) in line high or low temperature sintering of the titania coated metal foil; (3) in line sensitization of the titania coating; (4) slitting (e.g., by an ultrasonic slitting technique described in more detail below) the metal foil into strips; (5) separating the strips, or ribbons, to a finite spacing by using a sequentially positioned series of guide roller or by simply conveying the slit strips over a contoured roll that provides lateral spreading and separation of the strips at a finite distance; and (6) laminating the strips to a first flexible, substrate. An electrolyte, counter electrode, and second substrate including a conductive layer may be laminated to the metal-foil-coated first substrate to complete the photovoltaic cell or module.  
       FIG. 24  depicts, according to another illustrative embodiment, a metal-foil photovoltaic module  1800  that includes two photovoltaic modules  1804  and  1808  sandwiched together, thus permitting radiation  1812  to be received through two surfaces. The photovoltaic module  1800  may be formed using a continuous manufacturing process. According to the illustrative embodiment, a metal foil  1816  includes a photosensitized nanoparticle material  1820  disposed on two surfaces. The metal foil  1816  is scored as indicated at  1824  to disrupt the electrical continuity, and an electrolyte  1828 , an adhesive layer  1832 , a wire interconnect  1836 , and a platinum-coated  1840  counter electrode  1841  are all disposed between the metal foil  1816  and two flexible substrates  1844  and  1848 , thus completing the two photovoltaic modules. According to one illustrative embodiment, a conductive layer  1852  is disposed on the flexible substrates  1844  and  1848 . In a preferred embodiment, the metal foil  1816  is titanium, the photosensitized nanoparticle material  1820  includes sintered titania, the conductive layer  1852  is ITO, and the two flexible substrates  1844  and  1848  are PET.  
      K. Ultrasonic Welding  
      According to another illustrative embodiment, an ultrasonic slitting device is used to slit the leading and the trailing edge of a flexible photovoltaic module as it advances during a continuous manufacturing process, such as the one depicted in  FIG. 7 . For example, the slitting device  740  may be translated across the advancing sheet of photovoltaic cells or modules after the second flexible substrate  735  is applied to the advancing substrate sheet  705 . In one illustrative embodiment, the photovoltaic cell or modules include a metal foil coated with a photosensitized nanomatrix material.  
      The ultrasonic slitting device  740  may be a Branson Model FS-90. Preferably, the FS-90 is set to an amplitude of 50%, a pressure of 50-60 PSI, a speed of 5-10, and an anvil specification of 90°. The device  740  creates friction that melts the photovoltaic cell or module, thus simultaneously cutting and sealing the material. This prevents the electrolyte from leaking and precludes the atmosphere from penetrating the photovoltaic cell or module. Furthermore, the process interrupts conductive layers coated on the substrate.  
      L. Wire Interconnects with Metal Coatings  
      In another illustrative embodiment, the invention provides a method of coating a wire interconnect with a metal or metal alloy with a low melting temperature. The metal or metal alloy melts during the lamination process, providing a more uniform contact between the two joined substrates. The improved contact enhances both the mechanical adhesion and the electrical connection.  FIG. 25  depicts a cross-section of a wire  1901  coated with a surrounding layer of a metal  1902  with a low melting temperature. The metal  1902  may be a metal alloy. According to one illustrative embodiment, the coated wire is substituted for the uncoated wire shown in  FIGS. 21, 22 , or  23 .  
       FIG. 26  depicts exemplary photovoltaic module  2000  including a metal foil  2004 . More particularly, the metal foil  2004  is laminated to a first flexible substrate  2008  using a first adhesive  2012 . Score marks  2016  formed in the metal foil  2004  using a heated stylus to break the electrical continuity of the metal foil  2004 , thus enabling a plurality of photovoltaic cells to be formed on a single substrate. A photosensitized nanoparticle material  2020 , according to the invention, is disposed on the metal foil  2004 . According to the illustrative embodiment, the photovoltaic module  2000  includes wire interconnects  2024  embedded in a second adhesive layer  2028  to form an electrical connection between the metal foil  2004  and a counter electrode  2033  coated with platinum  2032 . The wire  2024  is coated with a low melting temperature metal  2034 . During lamination, the metal  2034  melts and flows, thus forming a more uniform contact with the metal foil  2004  and the counter electrode  2033 . The photovoltaic module  2000  also includes a charge carrier material  2036  and a conductive layer  2040  coated on a second substrate  2044 .  
      The wire  2024 , as describe above, may be copper, titanium, stainless steel, or other suitable wire. In a preferred embodiment, the wire is a 3 mil copper wire, and is coated with a eutectic mixture of bismuth-tin. The melting temperature of bismuth-tin is 138° C. A thin sheet of BYNEL adhesive (ethylene maleic anhydride) is disposed between the metal foil and the counter electrode. The melting temperature of the adhesive is 126° C. The adhesive and coated metal wire are then thermally laminated (hot bar laminator at a temperature of about 150° C.) between two PET substrates with ITO conductive layers. Suitable metals have melting temperatures in the range of about 50° C. to about 250° C. Preferably, the melting temperatures are in the range of about 75° C. to about 180° C., and the adhesive melts at a lower temperature than the metal coating.  
      The general principles of the above-described photovoltaic cells and modules enable various combinations of photovoltaic cells, photovoltaic modules, or photovoltaic arrays that are suitable for incorporation into audio and visual display apparatus. The display apparatus may be used in a wide range of commercial and industrial applications where the flexible nature of the devices is desired. The display apparatus may be produced in any desired shape and installed in a wide range of geometrically shaped holders. Illustrative examples of the invention in the context of applications involving flexible electronic displays, such as retail labels, greeting cards, and smart cards, are provided below.  
     EXAMPLE 21  
     Retail Displays with Integrated Photovoltaic Devices  
       FIG. 27A , in one illustrative example, depicts a flexible display apparatus  2100  in accordance with the principles of the invention. The illustrative display apparatus  2100  includes a flexible base  2112  upon which a display element  2114  and a photovoltaic cell  2116  are disposed. The photovoltaic cell  2116  is the source of energy for the display element  2114 .  FIG. 27B  shows the display apparatus  2100  in a flexed state, thus illustrating the flexible nature of the base  2112 , the display element  2114 , and the photovoltaic cell  2116 .  
       FIG. 28  depicts a photovoltaic module  2204  that includes four photovoltaic cells  2208   a ,  2208   b ,  2208   c , and  2208   d  (collectively “ 2208 ”). The photovoltaic cells  2208  are fabricated on a flexible substrate  2212 . In the illustrative embodiment, the substrate  2212  also serves as the base material for the display element  2214 . Alternatively, the photovoltaic cells and the display element  2214  are fabricated on separate base materials, which are then fused together. The flexible nature of the display apparatus  2200  facilitates secure insertion into a wide variety of holders.  
      Referring to  FIGS. 27A-28 , the photovoltaic cell  2116  and the photovoltaic module  2204  may power their respective display elements  2114  and  2214  directly, or be used to charge a power source or capacitive element that in turn powers the display elements  2114  and  2214 . The operational parameters of the photovoltaic cells  2116  and  2208  may further expand the utility of the apparatus  2100  and  2200 . More specifically, photovoltaic cells that are photovoltaically responsive to indoor incandescent and fluorescent lighting may be manufactured. Therefore, the display apparatus  2100  and  2200  may be incorporated into indoor signage and display functions, such as shelf-edge retail labels.  
       FIGS. 29A and 29B  depict shelf-edge retail labels  2300  and  2304 , each placed within the edge of a shelf  2308  or  2312 .  FIG. 29A  shows a flexible display apparatus including a flexible display  2320  in electrical communication with a photovoltaic module  2324 .  FIG. 29B  shows a rigid display  2328  mounted on a flexible base  2332  connected to a photovoltaic module  2336 . The rigid display  2328  may, for example, be in direct electrical communication with the photovoltaic module  2336  or alternatively, in serial electrical communication via the flexible base  2332 . In either illustrative embodiment, an individual photovoltaic cell may be substituted for the photovoltaic module  2324  or  2336 .  FIG. 29C  depicts a cross-sectional view of the shelf-edge label  2304  shown in  FIG. 29B  along an x-y axis. The flexible display apparatus  2316  and the flexible base material  2332  are held in the grooves  2340  of the edges of the shelves  2308  and  2312  by flexure and release. The shelf-edge retail labels  2300  and  2304  may incorporate an additional element that would draw the attention of a consumer, such as a flashing light or speaker producing an audible sound. In addition, the displays  2320  and  2328  may include information regarding, for example, the price or bar code of an item.  
       FIG. 30  shows an illustrative embodiment of a flexible display apparatus  2400  including various external elements that control and regulate the display apparatus  2400 . An addressable processor  2404  for controlling a display element  2408 , according to the invention, is connected to a photovoltaic cell or module  2412 . The addressable processor  2404  may be in electrical communication with the display element  2408 . The processor may be any processor (e.g., a microprocessor or a microcontroller) suitable for generating outputs in response to data inputs. According to the illustrative embodiment, a computer network interface  2416 , which facilitates remote, network-based control of the processor  2404 , is in electrical communication with the photovoltaic cell or module  2412 . The network interface  2416  may operate wirelessly. In various embodiments, the wireless network interface  2416  utilizes a wireless protocol such as IEEE 802.11b, and may communicate using electromagnetic radiation, including, for example, optical, infrared, or radio-frequency transmissions.  
      According to the illustrative embodiment, display data, which determines the appearance of the display element  2408 , is stored in a data storage element  2420 . For example, the contents of a display buffer, which may draw data from the storage element  2420 , determines the instantaneous appearance of the display element  2408 . Accordingly, the addressable processor  2404  may be programmed to cause the content of the display element  2408  to change over time by continuously overwriting the contents of the display buffer with data from the storage element  2420 . The addressable processor  2404  may also receive data from a handheld programming unit that, for example, allows a clerk to specify price information to be displayed. Similarly, price data may be sent wirelessly and received by the network interface  2416 .  
      Different types of display elements can be employed in various applications of the invention. Electrophoretic displays, liquid crystal displays (LCD), light emitting diode (LED) displays, cathode ray tubes (CRTs), and/or any other suitable non-permanent, electronically configurable display may be utilized to form all or part of the display apparatus  2100 ,  2200 ,  2320 ,  2328 , or  2400 , according to the invention. The power required to drive the display, the need for flexibility, the available light conditions, and the space available for arrays of photovoltaic cells and modules are some parameters governing the selection of a particular display apparatus.  
      The spectral response profile of the photovoltaic cells or modules  2116 ,  2204 ,  2324 ,  2336 , and  2412  is preferably selected to maximize overlap with the spectral output profile of indoor ambient fluorescent and incandescent lighting. The light level in a room is described by the amount of light that strikes particular surfaces of interest, i.e., the ambient illuminance. Specifically, illuminance is the measurement of how bright a point source of light appears to the eye. The lux is the ISU unit of illuminance and is defined in terms of lumens per square meter (1 m/m 2 ). Various illuminance levels are shown below in Table 21. The ambient illuminance in an office is from 50-180 lux, while the illuminance in a grocery store can range between 1,000 and 2000 lux. Preferably, the DSSCs operate at wavelengths of light in the mid-visible range. The choice of dye used in a given DSSC can be tailored such that the photovoltaic response of the dye is complementary to the peak output wavelength of a given indoor lighting source. In other embodiments, amorphous silicon based photovoltaic cells disposed on flexible substrates can be used in conjunction with display elements. Amorphous silicon-based photovoltaic cells suitable for use in the invention can either be flexible or rigid in various embodiments.  
               TABLE 21                       Various Illuminances                                                Normal room   about 100-200 lux           Brightly lit office or grocery store   about 1,000-2,000 lux           Cloudy, January noon in Chicago   about 4,000 lux           Spring sunrise   about 10,000 lux           Summer early afternoon   about 100,000 lux                      
 
     EXAMPLE 22  
     Retail Displays Powered by Remote Photovoltaic Devices  
      The photovoltaic cells need not be integral with the display.  FIG. 31  depicts an illustrative example of a retail display  2500  powered by a remote photovoltaic device. The display apparatus  2500  includes a display element  2504 , according to the invention, disposed upon the edge of a shelf  2508 . The shelf  2508  is attached to a frame  2512 , which is capped by a curved base  2516 . According to the illustrative example, a plurality of photovoltaic cells  2520  are disposed on the curved base  2516 , and the contour of the curved base  2516  facilitates multidirectional exposure to incident light from ambient incandescent or fluorescent light.  
      The plurality of photovoltaic cells  2520  are in electrical communication with the display element  2504 , and any additional shelf-edge displays, via electrical cables  2524  and  2528 . The large number of photovoltaic cells  2520  that are accommodated by this design not only provides power to numerous displays, but also is advantageous when indoor light levels are reduced and the presence of numerous photovoltaic cells are necessary to capture the available ambient light.  
      Although the base  2516  is shown in a contoured configuration, the base  2516  may be flexible or rigid, while ranging in shape from substantially flat to any suitable contoured geometry. For example, a substantially flat base may be fixed by the frame in a configuration perpendicular to the shelf, thereby enabling photovoltaic cells disposed on both the front and back surfaces of the flat base to receive ambient light simultaneously. Furthermore, multiple substantially flat bases may be arranged at the top of a shelving system such that the flat bases are parallel to each other, increasing the available surface area for capturing ambient light.  
     EXAMPLE 23  
     Photovoltaically Powered Multimedia Greeting Card  
      In this illustrative example,  FIG. 32  depicts a greeting card  2600  including first  2610  and second  2620  page with a low-weight and flexible photovoltaic module  2625  attached (e.g., by means of a suitable adhesive) to the inner surface of the second page  2620 . of the greeting card  2600 . The photovoltaic module  2625  is in electrical communication with an electronic unit  2630  via first and second conducting wires  2640  and  2650  (e.g., conducting tape). In this illustrative example, the greeting card  2600  includes a printed message  2670  on the inner surface of the second page  2620 . The electronic unit  2630  may be an audio-reproduction module having, for example, speakers and music. Preferably, the electronic unit  2630  is lightweight and flexible. Opening the greeting card  2600  causes the photovoltaic module  2625  to be exposed to light, which generates electricity that powers the electronic unit  2630 , thereby playing the stored music or other audio. The electronic unit  2630  need not be attached to the inner surface of the second page  2620  of the greeting card. For example, the electronic unit  2630  may be disposed between two individual pages adhered together to form the second page  2620 . The pages themselves may be paper, cardboard, card stock, or other suitable material. The pages may be laminated.  
     EXAMPLE 24  
     Photovoltaic Greeting Cards with Customer-Recordable Music or Voice Messages  
      The electronic unit  2630  may include a small recording device, such as an integrated circuit capable of storing a few seconds or minutes of music or a voice message (e.g., in MP3 format) and a microphone. In this illustrative example, a customizable music or voice message plays when the greeting card  2600  is opened. The music or voice may be native, downloaded from a computer or other electronic unit, or recorded by the card&#39;s sender. The recording circuit may be replaceable.  
     EXAMPLE 25  
     Photovoltaic Greeting Cards with Integrated Display Unit  
       FIG. 33  depicts a greeting card  2700  including a visual display  2710  (e.g., an electrochromic display or a polymer-dispersed liquid crystal (PDLC) display) in electrical communication with an electronic driving unit  2720  via first and second conducting wires  2730  and  2740 . In the illustrated example, the greeting card  2700  includes a photovoltaic module  2750  in electrical communication with the electronic driving unit  2720  via third and fourth conducting wires  2760  and  2770 . The greeting card  2700  may also include a printed message. Because of the electrochromic nature of the electrochromic display or the capability of the PDLC display to transition from translucent to transparent, diverse images such as handwritten messages, hand-drawn pictures, or even photos may be placed behind the display. As the visual display  2710  becomes transparent, the images are revealed.  
     EXAMPLE 26  
     Photovoltaic Greeting Cards Capable of Playing Videos or Messages  
       FIG. 34  depicts a greeting card  2800  including a reflective liquid crystal display  2810 , or other advanced thin-film display unit, in electrical communication with an electronic driving unit  2820  via first and second conducting wires  2830  and  2840 . In the illustrated example, the greeting card  2800  includes a photovoltaic module  2850  in electrical communication with the electronic driving unit  2820  via third and fourth conducting wires  2860  and  2870 . The greeting card  2800  is capable of playing video (e.g., in MPEG format) that may be native or downloaded from a computer or another electronic unit. The greeting card  2800  also may include a speaker and microphone, as described above.  
     EXAMPLE 27  
     Photovoltaic Material Applied to Surface of a Greeting Card as a Design  
       FIG. 35  depicts a greeting card  2900  including a photovoltaic module  2910  that has, for example, four different dyes incorporated into four different portions  2920   a ,  2920   b ,  2960   c , and  2920   d  (collectively “ 2920 ”) of the card. The portions  2920  may define a design or picture on the greeting card  2900 . In the illustrated example, the photovoltaic module  2910  is in electrical communication with an electronic driving unit  2930  via first and second conducting wires  2940  and  2950 . The greeting card  2900  may include a printed or visual message  2960 , and the electronic driving unit  2930  may include a speaker or microphone, as described above. The dyes provide different colors for different portions of photovoltaic module  2910 . The dyes may be printed with any suitable printing technique (e.g., laser-patterning or inkjet printing).  
     EXAMPLE 28  
     Photovoltaically Powered Smart Music Card  
       FIG. 36  depicts a smart card  3000  including a light-weight and flexible photovoltaic module  3010  in electrical communication with an electronic unit  3020  via first and second conducting wires  3030  and  3040 . In this illustrative example, the electronic unit  3020  includes a speaker and pre-loaded audio content (e.g., in MP3 format). Exposing the smart card  3000  to light causes the stored music or voice to be played. The electronic unit  3020  may include a microphone to record audio content, and a storage unit to retain the content, as described above. The audio content may be native or downloaded from a computer or other electronic unit.  
     EXAMPLE 29  
     Photovoltaically Powered Smart Music Card with Enhanced Electrical Capacity  
       FIGS. 37A and 37B  show a smart card  3100  having a front surface  3110  and a back surface  3120 . A photovoltaic module  3130  on the front surface  3110  is in electrical communication with an electronic unit  3140  formed on the back surface  3120 . An advantage of this illustrative example is that the photovoltaic module  3130  is capable of generating more electricity than, for example, the photovoltaic module  3010  because of its larger surface area.  
     EXAMPLE 30  
     Photovoltaically Powered Smart Video Card  
       FIG. 38  depicts a smart card  3200  including a photovoltaic module  3210  in electrical communication with a display unit  3220  via first and second conducting wires  3230  and  3240 . In this illustrative example, the display unit  3220  includes conventional electronic storage and driver circuitry as described above. The smart card  3200  is capable of playing video (e.g., in MPEG format), and the video or voice may be native or downloaded from a computer or other electronic unit, as described in more detail above.  
     EXAMPLE 31  
     Photovoltaic Powered Smart Card with Global Positioning System (GPS) Capability  
       FIG. 39  shows a smart card  3300  including a photovoltaic module  3310  in electrical communication with a display unit  3320  via first and second conducting wires  3330  and  3340 . In this illustrative example, the display unit  3320  includes electronic storage and driving circuitry, as described above, as well as a GPS unit. Upon exposure to light, the smart card  3300  shows navigational maps and/or the global position of the user via the display unit  3320 . In particular, the display unit  3320  may include user-selectable features that activate and receive information from the GSP unit, and cause its display in accordance with the user&#39;s feature selection  
      While the invention has been particularly shown and described with reference to specific illustrative embodiments, it should be understood that various changes in form and detail may be made without departing from the spirit and scope of the invention as defined by the appended claims. By way of example, displays of the type described above may be employed with and/or used to power at least portions of devices such as, clocks, watches, personal digital assistants, telephones mobile or otherwise, televisions, handheld, laptop and/or desktop computers and/or handheld video games. Accordingly, the invention is to be defined not by the preceding illustrative description, but instead by the spirit and scope of the following claims. Also by way of example, any of the disclosed features may be combined with any of the other disclosed features to form a photovoltaic cell or module.