Abstract:
Methods of preparing electrodes, as well as related devices, components, systems, and methods, are disclosed.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
   This application claims priority to U.S. Provisional Application Ser. No. 60/693,829, filed Jun. 24, 2005, the contents of which are hereby incorporated by reference. 

   TECHNICAL FIELD 
   This disclosure relates to methods of preparing electrodes, as well as related devices, components, systems, and methods. 
   BACKGROUND 
   Photovoltaic cells are commonly used to transfer energy in the form of light into energy in the form of electricity. A typical photovoltaic cell includes a photoactive material disposed between two electrodes. Generally, light passes through one or both of the electrodes to interact with the photoactive material. As a result, the ability of one or both of the electrodes to transmit light (e.g., light at one or more wavelengths absorbed by a photoactive material) can limit the overall efficiency of a photovoltaic cell. In many photovoltaic cells, a film of semiconductive material (e.g., indium tin oxide) is used to form the electrode(s) through which light passes because, although the semiconductive material may have a lower electrical conductivity than electrically conductive materials, the semiconductive material can transmit more light than many electrically conductive materials. 
   SUMMARY 
   In one aspect, the invention features a method that includes contacting a die with a first layer, which supports a metal layer, so that at least a portion of the metal layer is transferred to a second layer to form a photovoltaic cell electrode. 
   In another aspect, the invention features a method that includes forming an electrode of a photovoltaic cell using stamping. 
   In still another aspect, the invention features a method that includes forming an electrode of a multilayer device using stamping. 
   Embodiments can include one or more of the following aspects. 
   The method can further include heating the die to at least about 100° C. (e.g., at least about 150° C., at least about 200° C., at least about 250° C., or at least about 300° C.). 
   The contact step can include applying a pressure of at least about 100 psi (e.g., at least about 1,000 psi or at least about 5,000 psi) to the die. 
   The method can further include disposing a release layer between the metal layer and the first layer. In some embodiments, the release layer includes a material selected from the group consisting of polyesters (e.g., aliphatic polyesters) or polyethylenes (e.g., low molecular weight polyethylenes). 
   The metal layer can include aluminum, iron, gold, silver, copper, nickel, palladium, platinum, titanium, or an alloy thereof. 
   The electrode can be a mesh electrode. 
   The mesh electrode can include a plurality of open regions, at least some of the open regions have a rectangular, square, circular, semicircular, triangular, diamond, ellipse, trapezoid, or irregular shape. In some embodiments, at least some of the open regions have a waved, sloped, arched, pointed, striped, or checkered shape. 
   The second layer can be a substrate of a photovoltaic cell. In some embodiments, the method can further include applying a photoactive material to the substrate. In some embodiments, the photoactive material can include an electron donor material and an electron acceptor material. In other embodiments, the photoactive material can include a photosensitized interconnected nanoparticle material. 
   The electron acceptor material can include a material selected from the group consisting of fullerenes, inorganic nanoparticles, oxadiazoles, discotic liquid crystals, carbon nanorods, inorganic nanorods, polymers containing CN groups, polymers containing CF 3  groups, and combinations thereof. 
   The electron donor material can include a material selected from the group consisting of discotic liquid crystals, polythiophenes, polyphenylenes, polyphenylvinylenes, polysilanes, polythienylvinylenes, polyisothianaphthalenes, and combinations thereof. 
   The photosensitized interconnected nanoparticle material can include a material selected from the group consisting of selenides, sulfides, tellurides, titanium oxides, tungsten oxides, zinc oxides, zirconium oxides, and combinations thereof. 
   The die can include a surface having a mesh pattern. In some embodiments, at least a portion of the surface is planar or curved. The die can be attached a rotary tooling or a fix tooling. 
   The first or second layer can include a flexible substrate. In some embodiments, the first or second layer can include a polymer selected from the group consisting of polyethylene terephthalates, polyimides, polyethylene naphthalates, polymeric hydrocarbons, cellulosic polymers, polycarbonates, polyamides, polyethers, polyether ketones, and combinations thereof. 
   The multilayer device can include a liquid crystal display, a light emitting diode, or a photovoltaic module. The liquid crystal display can be backlit or reflective. 
   Embodiments can provide one or more of the following advantages. 
   The electrode can be highly conductive and/or highly transparent. 
   The method can be used in any suitable light emitting or absorbing device in which one surface of the device needs to be conductive and transparent. 
   The method can be used in a continuous process, such as a roll-to-roll process. 
   Other features and advantages will be apparent from the description, drawings and from the claims. 

   
     DESCRIPTION OF DRAWINGS 
       FIG. 1(   a ) is a top view of a substrate having a plurality of regions coated with a photoactive material. 
       FIG. 1(   b ) is a front surface view of a substrate coated with a metal layer. 
       FIG. 1(   c ) is a top view of a die having a mesh pattern on a surface. 
       FIG. 1(   d ) is a top view of a substrate of  FIG. 1(   a ) with a mesh electrode formed thereon. 
       FIG. 2  is a cross-sectional view of an organic photovoltaic cell 
       FIG. 3  is an elevational view of an embodiment of a mesh electrode; 
       FIG. 4  is a cross-sectional view of the mesh electrode of  FIG. 3 ; 
       FIG. 5  is a cross-sectional view of a portion of a mesh electrode; 
       FIG. 6  is a cross-sectional view of another organic photovoltaic cell; 
       FIG. 7  is a schematic of a system containing multiple photovoltaic cells electrically connected in series; and 
       FIG. 8  is a schematic of a system containing multiple photovoltaic cells electrically connected in parallel. 
       FIG. 9  is a cross-sectional view of a dye sensitized solar cell. 
   

   Like reference symbols in the various drawings indicate like elements. 
   DETAILED DESCRIPTION 
   In general, this disclosure relates to methods of preparing electrodes. 
   In some embodiments, a mesh electrode can be stamped on a layer by the following method. A die (e.g., a hot stamping die) having a pattern (e.g., a mesh pattern) machined into its surface can be brought into contact with the back surface of a first layer (e.g., a flexible substrate). The front surface of the first layer can be coated with a continuous metal layer. The front surface of the first layer can then be brought into contact with a second layer, which serves as a receiving layer. When a pressure is applied to the die, the metal layer on the front surface of the first layer transfers and adheres to the second layer. The pressure applied to the die can be at least about 100 psi (e.g., at least about 1,000 psi or at least about 5,000 psi). In some embodiments, the front surface of the first layer can be brought into contact with the second layer before the die contacts the back surface of the first layer. 
   In some embodiments, the die can be heated to a suitable temperature (e.g., at least about 100° C., at least about 150° C., at least about 200° C., at least about 250° C., or at least about 300° C.) to facilitate transfer of the metal layer from the front surface of the first layer to the second layer. 
   In some embodiments, a release layer can be included between the metal layer and the first layer to aid release of the metal layer. The release layer can include a material that liquefies below the temperature of the die during the stamping process. Examples of such materials include wax or a polymer with a low melting point (e.g., aliphatic polyesters or low molecular weight polyethylenes). 
   In some embodiments, the methods described above can be used to prepare electrodes of a multilayer device, such as a liquid crystal display, a light emitting diode, or a photovoltaic module. The photovoltaic module can include one or more photovoltaic cells, such as organic photovoltaic cells, dye sensitized solar cells (DSSCs), and/or tandem cells. 
   In some embodiments, the methods described above can be used to prepare electrodes of a photovoltaic cell. For example,  FIG. 1(   a ) shows a top view of a receiving substrate  110  containing a plurality of regions  111  coated with a photoactive material. Receiving substrate  110  can be used as a substrate in a photovoltaic cell.  FIG. 1(   b ) shows a front surface view of a transfer substrate  120  coated with a metal layer  122 .  FIG. 1(   c ) is a top view of a die  130  having a mesh pattern  133  on a surface. During the stamping process, die  130  can be brought into contact with the back surface of transfer substrate  120  and the front surface of the transfer substrate  120  can be brought into contact with receiving substrate  110 . When a pressure is applied to the die, metal layer  122  on transfer substrate  120  transfers and adheres to the receiving substrate  110 .  FIG. 1(   d ) shows a top view of a substrate  140  with a mesh electrode  144 . As shown in  FIG. 1(   d ), mesh electrode  144  is formed on substrate  140  such that the photoactive material are located in the open regions of mesh electrode  144 . 
   In some embodiments, the methods described above can be used to print an electrode on a substrate for use in an organic photovoltaic cell.  FIG. 2  shows a cross-sectional view of an organic photovoltaic cell  200  that includes a transparent substrate  210 , a mesh cathode  220 , a hole carrier layer  230 , a photoactive layer (containing an electron acceptor material and an electron donor material)  240 , a hole blocking layer  250 , an anode  260 , and a substrate  270 . 
     FIGS. 3 and 4  respectively show an elevational view and a cross-sectional of a mesh electrode. As shown in  FIGS. 3 and 4 , mesh cathode  220  includes solid regions  222  and open regions  224 . In general, regions  222  are formed of electrically conducting material so that mesh cathode  220  can allow light to pass therethrough via regions  224  and conduct electrons via regions  222 . 
   As shown in  FIGS. 3 and 4 , mesh cathode  220  includes solid regions  222  and open regions  224 . In general, regions  222  are formed of electrically conducting material so that mesh cathode  220  can allow light to pass therethrough via regions  224  and conduct electrons via regions  222 . 
   The area of mesh cathode  220  occupied by open regions  224  (the open area of mesh cathode  220 ) can be selected as desired. Generally, the open area of mesh cathode  220  is at least about 10% (e.g., at least about 20%, at least about 30%, at least about 40%, at least about 50%, at least about 60%, at least about 70%, at least about 80%) and/or at most about 99% (e.g., at most about 95%, at most about 90%, at most about 85%) of the total area of mesh cathode  220 . 
   Mesh cathode  220  can be prepared in various ways. In some embodiments, mesh electrode can be stamped onto a layer (e.g., a substrate) as described above. In some embodiments, mesh cathode  220  is a woven mesh formed by weaving wires of material that form solid regions  222 . The wires can be woven using, for example, a plain weave, a Dutch, weave, a twill weave, a Dutch twill weave, or combinations thereof. In certain embodiments, mesh cathode  220  is formed of a welded wire mesh. In some embodiments, mesh cathode  220  is an expanded mesh formed. An expanded metal mesh can be prepared, for example, by removing regions  224  (e.g., via laser removal, via chemical etching, via puncturing) from a sheet of material (e.g., an electrically conductive material, such as a metal), followed by stretching the sheet (e.g., stretching the sheet in two dimensions). In certain embodiments, mesh cathode  220  is a metal sheet formed by removing regions  224  (e.g., via laser removal, via chemical etching, via puncturing) without subsequently stretching the sheet. 
   In certain embodiments, solid regions  222  are formed entirely of an electrically conductive material (e.g., regions  222  are formed of a substantially homogeneous material that is electrically conductive). Examples of electrically conductive materials that can be used in regions  222  include electrically conductive metals, electrically conductive alloys and electrically conductive polymers. Exemplary electrically conductive metals include gold, silver, copper, aluminum, nickel, palladium, platinum and titanium. Exemplary electrically conductive alloys include stainless steel (e.g., 332 stainless steel, 316 stainless steel), alloys of gold, alloys of silver, alloys of copper, alloys of aluminum, alloys of nickel, alloys of palladium, alloys of platinum and alloys of titanium. Exemplary electrically conducting polymers include polythiophenes (e.g., poly(3,4-ethelynedioxythiophene) (PEDOT)), polyanilines (e.g., doped polyanilines), polypyrroles (e.g., doped polypyrroles). In some embodiments, combinations of electrically conductive materials are used. In some embodiments, solid regions  222  can have a resistivity less than about 3 ohm per square. 
   As shown in  FIG. 5 , in some embodiments, solid regions  222  are formed of a material  302  that is coated with a different material  304  (e.g., using metallization, using vapor deposition). In general, material  302  can be formed of any desired material (e.g., an electrically insulative material, an electrically conductive material, or a semiconductive material), and material  304  is an electrically conductive material. Examples of electrically insulative material from which material  302  can be formed include textiles, optical fiber materials, polymeric materials (e.g., a nylon) and natural materials (e.g., flax, cotton, wool, silk). Examples of electrically conductive materials from which material  302  can be formed include the electrically conductive materials disclosed above. Examples of semiconductive materials from which material  302  can be formed include indium tin oxide, fluorinated tin oxide, tin oxide, and zinc oxide. In some embodiments, material  302  is in the form of a fiber, and material  304  is an electrically conductive material that is coated on material  302 . In certain embodiments, material  302  is in the form of a mesh (see discussion above) that, after being formed into a mesh, is coated with material  304 . As an example, material  302  can be an expanded metal mesh, and material  304  can be PEDOT that is coated on the expanded metal mesh. 
   Generally, the maximum thickness of mesh cathode  220  (i.e., the maximum thickness of mesh cathode  220  in a direction substantially perpendicular to the surface of substrate  210  in contact with mesh cathode  220 ) should be less than the total thickness of hole carrier layer  230 . Typically, the maximum thickness of mesh cathode  220  is at least 0.1 micron (e.g., at least about 0.2 micron, at least about 0.3 micron, at least about 0.4 micron, at least about 0.5 micron, at least about 0.6 micron, at least about 0.7 micron, at least about 0.8 micron, at least about 0.9 micron, at least about one micron) and/or at most about 10 microns (e.g., at most about nine microns, at most about eight microns, at most about seven microns, at most about six microns, at most about five microns, at most about four microns, at most about three microns, at most about two microns). 
   While shown in  FIG. 3  as having a rectangular shape, open regions  224  can generally have any desired shape (e.g., square, circle, semicircle, triangle, diamond, ellipse, trapezoid, irregular shape). In some embodiments, different open regions  224  in mesh cathode  220  can have different shapes. 
   Although shown in  FIG. 4  as having square cross-sectional shape, solid regions  222  can generally have any desired shape (e.g., rectangle, circle, semicircle, triangle, diamond, ellipse, trapezoid, irregular shape). In some embodiments, different solid regions  222  in mesh cathode  220  can have different shapes. In embodiments where solid regions  222  have a circular cross-section, the cross-section can have a diameter in the range of about 5 microns to about 200 microns. In embodiments where solid regions  222  have a trapezoid cross-section, the cross-section can have a height in the range of about 0.1 micron to about 5 microns and a width in the range of about 5 microns to about 200 microns. 
   In some embodiments, mesh cathode  220  is flexible (e.g., sufficiently flexible to be incorporated in photovoltaic cell  200  using a continuous, roll-to-roll manufacturing process). In certain embodiments, mesh cathode  220  is semi-rigid or inflexible. In some embodiments, different regions of mesh cathode  220  can be flexible, semi-rigid or inflexible (e.g., one or more regions flexible and one or more different regions semi-rigid, one or more regions flexible and one or more different regions inflexible). 
   In general, mesh electrode  220  can be disposed on substrate  210 . In some embodiments, mesh electrode  220  can be partially embedded in substrate  210 . 
   Substrate  210  is generally formed of a transparent material. As referred to herein, a transparent material is a material which, at the thickness used in a photovoltaic cell  200 , transmits at least about 60% (e.g., at least about 70%, at least about 75%, at least about 80%, at least about 85%, at least about 90%, at least about 95%) of incident light at a wavelength or a range of wavelengths used during operation of the photovoltaic cell. Exemplary materials from which substrate  210  can be formed include polyethylene terephthalates, polyimides, polyethylene naphthalates, polymeric hydrocarbons, cellulosic polymers, polycarbonates, polyamides, polyethers, polyether ketones, and combinations thereof. In certain embodiments, the polymer can be a fluorinated polymer. In some embodiments, combinations of polymeric materials are used. In certain embodiments, different regions of substrate  210  can be formed of different materials. 
   In general, substrate  210  can be flexible, semi-rigid or rigid (e.g., glass). In some embodiments, substrate  210  has a flexural modulus of less than about 5,000 megaPascals. In certain embodiments, different regions of substrate  210  can be flexible, semi-rigid or inflexible (e.g., one or more regions flexible and one or more different regions semi-rigid, one or more regions flexible and one or more different regions inflexible). 
   Typically, substrate  210  is at least about one micron (e.g., at least about five microns, at least about 10 microns) thick and/or at most about 1,000 microns (e.g., at most about 500 microns thick, at most about 300 microns thick, at most about 200 microns thick, at most about 100 microns, at most about 50 microns) thick. 
   Generally, substrate  210  can be colored or non-colored. In some embodiments, one or more portions of substrate  210  is/are colored while one or more different portions of substrate  210  is/are non-colored. 
   Substrate  210  can have one planar surface (e.g., the surface on which light impinges), two planar surfaces (e.g., the surface on which light impinges and the opposite surface), or no planar surfaces. A non-planar surface of substrate  210  can, for example, be curved or stepped. In some embodiments, a non-planar surface of substrate  210  is patterned (e.g., having patterned steps to form a Fresnel lens, a lenticular lens or a lenticular prism). 
   Hole carrier layer  230  is generally formed of a material that, at the thickness used in photovoltaic cell  200 , transports holes to mesh cathode  220  and substantially blocks the transport of electrons to mesh cathode  220 . Examples of materials from which layer  230  can be formed include polythiophenes (e.g., PEDOT), polyanilines, polyvinylcarbazoles, polyphenylenes, polyphenylvinylenes, polysilanes, polythienylenevinylenes and/or polyisothianaphthanenes. In some embodiments, hole carrier layer  230  can include combinations of hole carrier materials. 
   In general, the distance between the upper surface of hole carrier layer  230  (i.e., the surface of hole carrier layer  230  in contact with active layer  240 ) and the upper surface of substrate  210  (i.e., the surface of substrate  210  in contact with mesh electrode  220 ) can be varied as desired. Typically, the distance between the upper surface of hole carrier layer  230  and the upper surface of mesh cathode  220  is at least 0.01 micron (e.g., at least about 0.05 micron, at least about 0.1 micron, at least about 0.2 micron, at least about 0.3 micron, at least about 0.5 micron) and/or at most about five microns (e.g., at most about three microns, at most about two microns, at most about one micron). In some embodiments, the distance between the upper surface of hole carrier layer  230  and the upper surface of mesh cathode  220  is from about 0.01 micron to about 0.5 micron. 
   Active layer  240  generally contains an electron acceptor material and an electron donor material. 
   Examples of electron acceptor materials include formed of fullerenes, oxadiazoles, carbon nanorods, discotic liquid crystals, inorganic nanoparticles (e.g., nanoparticles formed of zinc oxide, tungsten oxide, indium phosphide, cadmium selenide and/or lead sulphide), inorganic nanorods (e.g., nanorods formed of zinc oxide, tungsten oxide, indium phosphide, cadmium selenide and/or lead sulphide), or polymers containing moieties capable of accepting electrons or forming stable anions (e.g., polymers containing CN groups, polymers containing CF 3  groups). In some embodiments, the electron acceptor material is a substituted fullerene (e.g., C61-phenyl-butyric acid methyl ester; PCBM). In some embodiments, active layer  240  can include a combination of electron acceptor materials. 
   Examples of electron donor materials include discotic liquid crystals, polythiophenes, polyphenylenes, polyphenylvinylenes, polysilanes, polythienylvinylenes, polyisothianaphthalenes, and combinations thereof. In some embodiments, the electron donor material is poly(3-hexylthiophene). In certain embodiments, active layer  240  can include a combination of electron donor materials. 
   Generally, active layer  240  is sufficiently thick to be relatively efficient at absorbing photons impinging thereon to form corresponding electrons and holes, and sufficiently thin to be relatively efficient at transporting the holes and electrons to layers  230  and  250 , respectively. In certain embodiments, layer  240  is at least 0.05 micron (e.g., at least about 0.1 micron, at least about 0.2 micron, at least about 0.3 micron) thick and/or at most about one micron (e.g., at most about 0.5 micron, at most about 0.4 micron) thick. In some embodiments, layer  140  is from about 0.1 micron to about 0.2 micron thick. 
   Hole blocking layer  250  is generally formed of a material that, at the thickness used in photovoltaic cell  200 , transports electrons to anode  260  and substantially blocks the transport of holes to anode  260 . Examples of materials from which layer  250  can be formed include LiF and metal oxides (e.g., zinc oxide, titanium oxide). 
   Typically, hole blocking layer  250  is at least 0.02 micron (e.g., at least about 0.03 micron, at least about 0.04 micron, at least about 0.05 micron) thick and/or at most about 0.5 micron (e.g., at most about 0.4 micron, at most about 0.3 micron, at most about 0.2 micron, at most about 0.1 micron) thick. 
   Anode  260  is generally formed of an electrically conductive material, such as one or more of the electrically conductive materials noted above. In some embodiments, anode  260  is formed of a combination of electrically conductive materials. 
   In general, substrate  270  can be identical to substrate  220 . In some embodiments, substrate  270  can be different from substrate  220  (e.g., having a different shape or formed of a different material or a non-transparent material). 
     FIG. 6  shows a cross-sectional view of a photovoltaic cell  400  that includes an adhesive layer  410  between substrate  210  and hole carrier layer  230 . 
   Generally, any material capable of holding mesh cathode  230  in place can be used in adhesive layer  410 . In general, adhesive layer  410  is formed of a material that is transparent at the thickness used in photovoltaic cell  400 . Examples of adhesives include epoxies and urethanes. Examples of commercially available materials that can be used in adhesive layer  410  include Bynel™ adhesive (DuPont) and 615 adhesive (3M). In some embodiments, layer  410  can include a fluorinated adhesive. In certain embodiments, layer  410  contains an electrically conductive adhesive. An electrically conductive adhesive can be formed of, for example, an inherently electrically conductive polymer, such as the electrically conductive polymers disclosed above (e.g., PEDOT). An electrically conductive adhesive can be also formed of a polymer (e.g., a polymer that is not inherently electrically conductive) that contains one or more electrically conductive materials (e.g., electrically conductive particles). In some embodiments, layer  410  contains an inherently electrically conductive polymer that contains one or more electrically conductive materials. 
   In some embodiments, the thickness of layer  410  (i.e., the thickness of layer  410  in a direction substantially perpendicular to the surface of substrate  210  in contact with layer  410 ) is less thick than the maximum thickness of mesh cathode  220 . In some embodiments, the thickness of layer  410  is at most about 90% (e.g., at most about 80%, at most about 70%, at most about 60%, at most about 50%, at most about 40%, at most about 30%, at most about 20%) of the maximum thickness of mesh cathode  220 . In certain embodiments, however, the thickness of layer  410  is about the same as, or greater than, the maximum thickness of mesh cathode  220 . 
   In general, a photovoltaic cell having a mesh cathode can be manufactured as desired. 
   In some embodiments, a photovoltaic cell can be prepared as follows. Electrode  260  is formed on substrate  270  using conventional techniques, and hole-blocking layer  250  is formed on electrode  260  (e.g., using a vacuum deposition process or a solution coating process). Active layer  240  is formed on hole-blocking layer  250  (e.g., using a solution coating process, such as slot coating, spin coating or gravure coating). Hole carrier layer  230  is formed on active layer  240  (e.g., using a solution coating process, such as slot coating, spin coating or gravure coating). Mesh cathode  220  is partially disposed in hole carrier layer  230  (e.g., by a stamping method described above). Substrate  210  is then formed on mesh cathode  220  and hole carrier layer  230  using conventional methods. 
   In certain embodiments, a photovoltaic cell can be prepared as follows. Electrode  260  is formed on substrate  270  using conventional techniques, and hole-blocking layer  250  is formed on electrode  260  (e.g., using a vacuum deposition or a solution coating process). Active layer  240  is formed on hole-blocking layer  250  (e.g., using a solution coating process, such as slot coating, spin coating or gravure coating). Hole carrier layer  230  is formed on active layer  240  (e.g., using a solution coating process, such as slot coating, spin coating or gravure coating). Adhesive layer  410  is disposed on hole carrier layer  230  using conventional methods. Mesh cathode  220  is partially disposed in adhesive layer  410  and hole carrier layer  230  (e.g., by disposing mesh cathode  220  on the surface of adhesive layer  410 , and pressing mesh cathode  220 ). Substrate  210  is then formed on mesh cathode  220  and adhesive layer  410  using conventional methods. 
   While the foregoing processes involve partially disposing mesh cathode  220  in hole carrier layer  230 , in some embodiments, mesh cathode  220  is formed by printing the cathode material on the surface of hole carrier layer  230  or adhesive layer  410  to provide an electrode having the open structure shown in the figures. For example, mesh cathode  220  can be printed using stamping, dip coating, extrusion coating, spray coating, inkjet printing, screen printing, and gravure printing. The cathode material can be disposed in a paste which solidifies upon heating or radiation (e.g., UV radiation, visible radiation, IR radiation, electron beam radiation). The cathode material can be, for example, vacuum deposited in a mesh pattern through a screen or after deposition it may be patterned by photolithography. 
   Multiple photovoltaic cells can be electrically connected to form a photovoltaic system. As an example,  FIG. 7  is a schematic of a photovoltaic system  500  having a module  510  containing photovoltaic cells  520 . Cells  520  are electrically connected in series, and system  500  is electrically connected to a load. As another example,  FIG. 8  is a schematic of a photovoltaic system  600  having a module  610  that contains photovoltaic cells  620 . Cells  620  are electrically connected in parallel, and system  600  is electrically connected to a load. In some embodiments, some (e.g., all) of the photovoltaic cells in a photovoltaic system can have one or more common substrates. In certain embodiments, some photovoltaic cells in a photovoltaic system are electrically connected in series, and some of the photovoltaic cells in the photovoltaic system are electrically connected in parallel. 
   In some embodiments, photovoltaic systems containing a plurality of photovoltaic cells can be fabricated using continuous manufacturing processes, such as roll-to-roll or web processes. In some embodiments, a continuous manufacturing process includes: forming a group of photovoltaic cell portions on a first advancing substrate; disposing an electrically insulative material between at least two of the cell portions on the first substrate; embedding a wire in the electrically insulative material between at least two photovoltaic cell portions on the first substrate; forming a group of photovoltaic cell portion on a second advancing substrate; combining the first and second substrates and photovoltaic cell portions to form a plurality of photovoltaic cells, in which at least two photovoltaic cells are electrically connected in series by the wire. In some embodiments, the first and second substrates can be continuously advanced, periodically advanced, or irregularly advanced. 
   In some embodiments, the stamping methods described above can be used to print an electrode on a substrate for use in a DSSC.  FIG. 9  is a cross-sectional view of DSSC  700  that includes a substrate  710 , an electrode  720 , a catalyst layer  730 , a charge carrier layer  740 , a photoactive layer  750 , an electrode  760 , a substrate  770 , and an external load  780 . Examples of DSSCs are discussed in U.S. patent application Ser. Nos. 11/311,805 filed Dec. 19, 2005 and Ser. No. 11/269,956 filed on Nov. 9, 2005, the contents of which are hereby incorporated by reference. 
   In some embodiments, the stamping methods described above can be used to print an electrode on a substrate for use in a tandem cell. Examples of tandem photovoltaic cells are discussed in U.S. patent application Ser. No. 10/558,878 and U.S. Provisional Application Ser. Nos. 60/790,606, 60/792,635, 60/792,485, 60/793,442, 60/795,103, 60/797,881, and 60/798,258, the contents of which are hereby incorporated by reference. 
   While certain embodiments have been disclosed, other embodiments are also possible. 
   As one example, while cathodes formed of mesh have been described, in some embodiments a mesh anode can be used. This can be desirable, for example, when light transmitted by the anode is used. In certain embodiments, both a mesh cathode and a mesh anode are used. This can be desirable, for example, when light transmitted by both the cathode and the anode is used. 
   As another example, while embodiments have generally been described in which light that is transmitted via the cathode side of the cell is used, in certain embodiments light transmitted by the anode side of the cell is used (e.g., when a mesh anode is used). In some embodiments, light transmitted by both the cathode and anode sides of the cell is used (when a mesh cathode and a mesh anode are used). 
   As a further example, while electrodes (e.g., mesh electrodes, non-mesh electrodes) have been described as being formed of electrically conductive materials, in some embodiments a photovoltaic cell may include one or more electrodes (e.g., one or more mesh electrodes, one or more non-mesh electrodes) formed of a semiconductive material. Examples of semiconductive materials include indium tin oxide, fluorinated tin oxide, tin oxide, and zinc oxide. 
   As an additional example, in some embodiments, one or more semiconductive materials can be disposed in the open regions of a mesh electrode (e.g., in the open regions of a mesh cathode, in the open regions of a mesh anode, in the open regions of a mesh cathode and the open regions of a mesh anode). Examples of semiconductive materials include tin oxide, fluorinated tin oxide, tin oxide and zinc oxide. Other semiconductive materials, such as partially transparent semiconductive polymers, can also be disposed in the open regions of a mesh electrode. For example, a partially transparent polymer can be a polymer which, at the thickness used in a photovoltaic cell, transmits at least about 60% (e.g., at least about 70%, at least about 75%, at least about 80%, at least about 85%, at least about 90%, at least about 95%) of incident light at a wavelength or a range of wavelengths used during operation of the photovoltaic cell. Typically, the semiconductive material disposed in an open region of a mesh electrode is transparent at the thickness used in the photovoltaic cell. 
   As another example, in certain embodiments, a protective layer can be applied to one or both of the substrates. A protective layer can be used to, for example, keep contaminants (e.g., dirt, water, oxygen, chemicals) out of a photovoltaic cell and/or to ruggedize the cell. In certain embodiments, a protective layer can be formed of a polymer (e.g., a fluorinated polymer). 
   As a further example, while certain types of photovoltaic cells have been described that have one or more mesh electrodes, one or more mesh electrodes (mesh cathode, mesh anode, mesh cathode and mesh anode) can be used in other types of photovoltaic cells as well. Examples of such photovoltaic cells include photoactive cells with an active material formed of amorphous silicon, cadmium selenide, cadmium telluride, copper indium sulfide, and copper indium gallium arsenide. 
   As an additional example, while described as being formed of different materials, in some embodiments materials  302  and  304  are formed of the same material. 
   As another example, although shown in  FIG. 5  as being formed of one material coated on a different material, in some embodiments solid regions  222  can be formed of more than two coated materials (e.g., three coated materials, four coated materials, five coated materials, six coated materials). 
   Other embodiments are in the claims.