Patent Publication Number: US-2007108539-A1

Title: Stable organic devices

Description:
CROSS REFERENCE TO RELATED APPLICATION  
      This application claims priority to U.S. Provisional Application Ser. No. 60/753,884, filed Dec. 23, 2005, and U.S. Provisional Application Ser. No. 60/699,124, filed Jul. 14, 2005, the contents of which are hereby incorporated by reference. 
    
    
     TECHNICAL FIELD  
      This disclosure relates to stable organic devices, as well as related components, systems, and methods.  
     BACKGROUND  
      Polymer photovoltaic cells can be used to convert solar energy to electrical energy. Such cells generally include a photoactive layer disposed between two electrodes that contains an electron donor material and an electron acceptor material. Generally, light passes through one or both of the electrodes to interact with the photoactive layer to convert solar energy to electrical energy.  
     SUMMARY  
      In one aspect, the invention features an article that includes first and second electrodes, a photoactive layer between the first and second electrodes, and a material disposed between the photoactive layer and at least one of the first and second electrodes. The material is different from the at least one of the first and second electrodes and includes a semiconductive metal oxide or a metal capable of forming a semiconductive metal oxide. The photoactive layer includes an electron acceptor material and an electron donor material. The article is a photovoltaic cell.  
      In another aspect, the invention features a device that includes first and second electrodes, an organic semiconductive layer between the first and second electrodes, and a material disposed between the semiconductive layer and at least one of the first and second electrodes. The material is different from the at least one of the first and second electrodes and includes a semiconductive metal oxide or a metal capable of forming a semiconductive metal oxide.  
      In another aspect, the invention features a method that includes forming the article or the device described above by a continuous process.  
      Embodiments can include one or more of the following aspects.  
      The material can include semiconductive metal oxides, such as titanium oxides, zinc oxides, tin oxides, tungsten oxides, copper oxides, chromium oxides, silver oxides, nickel oxides, gold oxides, or combinations thereof.  
      The material can include a metal capable of forming a semiconductive metal oxide, such as titanium, gold, silver, copper, chromium, tin, nickel, zinc, or tungsten, or combinations thereof.  
      The material can have a surface resistivity of at most about 1,000 Ohm/sq (e.g., at most about 10 Ohm/sq, at most about 0.1 Ohm/sq).  
      The material can form a layer having a thickness of at least about 0.1 nm or at most about 50 nm.  
      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. In some embodiments, the electron acceptor material can include substituted fullerenes.  
      The electron donor material can inclue a material selected from the group consisting of discotic liquid crystals, polythiophenes, polyphenylenes, polyphenylvinylenes, polysilanes, polythienylvinylenes, polyisothianaphthalenes, and combinations thereof. In some embodiments, the electron donor material can include poly(3-hexylthiophene).  
      At least one of the first and second electrodes can include a mesh electrode. In some embodiments, at least one of the first and second electrodes includes a metal.  
      The device can be an organic photovoltaic cell, an organic photodetector, an organic light-emitting diode, or an organic field-effect transistor.  
      The continuous process can be a roll-to-roll process.  
      Embodiments can provide one or more of the following advantages.  
      Electrodes in organic devices (e.g., organic photovoltaic cells) can be oxidized in the presence of water or oxygen, which leads to large contact resistivities. Without wishing to be bound by theory, it is believed that including a protecting layer containing semiconductive metal oxides or metals capable of forming such metal oxides between electrodes and semiconductive polymers used in organic devices can prevent oxidation or damage to the electrodes, thereby significantly enhancing the stability of the electrodes. Further, since the metal oxides are semiconductive, the protecting layer can minimize an increase in the contact resistivities, thereby maintaining the performance of the organic device.  
      Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims. 
    
    
     DESCRIPTION OF DRAWINGS  
       FIG. 1  is a cross-sectional view of an embodiment of a photovoltaic cell;  
       FIG. 2  is an elevational view of an embodiment of a mesh electrode;  
       FIG. 3  is a cross-sectional view of the mesh electrode of  FIG. 2 ;  
       FIG. 4  is a cross-sectional view of a portion of a mesh electrode;  
       FIG. 5  is a cross-sectional view of another embodiment of a photovoltaic cell;  
       FIG. 6  is a schematic of a system containing multiple photovoltaic cells electrically connected in series; and  
       FIG. 7  is a schematic of a system containing multiple photovoltaic cells electrically connected in parallel. 
    
    
      Like reference symbols in the various drawings indicate like elements.  
     DETAILED DESCRIPTION  
       FIG. 1  shows a cross-sectional view of a photovoltaic cell  100  that includes a transparent substrate  110 , a mesh cathode  120 , a protecting layer  125 , a hole carrier layer  130 , a photoactive layer (containing an electron acceptor material and an electron donor material)  140 , a hole blocking layer  150 , a protecting layer  155 , an anode  160 , and a substrate  170 . In general, during use, light impinges on the surface of substrate  110 , and passes through substrate  110 , the openings in cathode  120 , protecting layer  125 , and hole carrier layer  130 . The light then interacts with photoactive layer  140 , causing electrons to be transferred from the electron donor material in layer  140  to the electron acceptor material in layer  140 . The electron acceptor material then transmits the electrons through hole blocking layer  150  and protecting layer  155  to anode  160 , and the electron donor material transfers holes through hole carrier layer  130  and protecting layer  125  to mesh cathode  120 . Anode  160  and mesh cathode  120  are in electrical connection via an external load so that electrons pass from anode  160 , through the load, and to cathode  120 .  
      Protecting layers  125  and  155  can include a semiconductive metal oxide or a metal capable of forming a semiconductive metal oxide. Examples of the semiconductive metal oxides include titanium oxides, zinc oxides, tin oxides, tungsten oxides, copper oxides, chromium oxides, silver oxides, nickel oxides, gold oxides, or combinations thereof. Examples of the metals capable of forming semiconductive metal oxides include titanium, gold, silver, copper, chromium, tin, nickel, zinc, tungsten, or combinations thereof. Without wishing to be bound by theory, it is believed that protecting layers  125  and  155  can prevent oxidation or damage to electrodes  120  and  160  (e.g., oxidized by hole carrier layer  130  or hole blocking layer  150 ), thereby significantly enhancing the stability of the electrodes and the photovoltaic cell. In some embodiments, including a protecting layer in a photovoltaic cell can enhance the stability of the photovoltaic cell by a factor of  100  or more.  
      Each of protecting layers  125  and  155  can include either p-type or n-type semiconductive metal oxides or metals capable of forming either p-type or n-type semiconductive metal oxides. In some embodiments, protective layer  125  includes p-type semiconductive metal oxides (e.g., copper oxides) or metals capable of forming p-type semiconductive metal oxides. In some embodiments, protective layer  155  includes n-type semiconductive metal oxides (e.g., titanium oxides) or metals capable of forming n-type semiconductive metal oxides.  
      In some embodiments, protecting layers  125  and  155  can include metal oxides that are intrinsically semiconductive. In certain embodiments, protective layers  125  and  155  can include semiconductive metal oxides that are doped. In some embodiments, the semiconductive metal oxides can have a bandgap of at least about 2 eV (e.g., at least about 2.5 eV, at least about 3 eV, at least about 3.5 eV, at least about 4 eV).  
      In some embodiments, the semiconductive metal oxides can have an electron mobility of at least about 10 −6  cm 2 /Vs (e.g., at least about 10 −5  cm 2 /Vs, at least about 10 −4  cm 2 /Vs, at least about 10 −3  cm 2 /Vs).  
      In some embodiments, the semiconductive metal oxides can have a conductivity of at least about 10 −9  S/cm (e.g., at least about 10 −8  S/cm, at least about 10 −7  S/cm, at least about 10 −6  S/cm, at least about 10 ‘5  S/cm, at least about 10 −4  S/cm, at least about 10 −3  S/cm, at least about 10 −2  S/cm).  
      In some embodiments, the semiconductive metal oxides can have a conduction band between about 3.0 eV and about 5.0 eV (e.g., about 4.0 eV).  
      In some embodiments, each of protecting layers  125  and  155  can have a thickness at least about 0.1 nm (e.g., at least about 1 nm, at least about 5 nm) or at most about 50 nm (e.g., at least about 25 nm, at least about 10 nm). In some embodiments, each of protecting layers  125  and  155  can have a thickness at which the protecting layer has a 50% absorption at the UV/Vis/NIR region.  
      In some embodiments, each of protecting layers  125  and  155  can have a surface resistivity of at most about 1,000 Ohm/sq (e.g., at most about 100 Ohm/sq, at most about 10 Ohm/sq, at most about 1 Ohm/sq, at most about 0.1 Ohm/sq).  
      Protecting layer  125  can be formed of a material the same as or different from the material used to form protecting layer  155 . In some embodiments, photovoltaic cell  100  can include only one protecting layer.  
      While  FIG. 1  shows that protecting layer  125  is used to enhance the stability of mesh electrode  120  (e.g., by minimizing its oxidation), in some embodiments, it can also be used to enhance the stability of a non-mesh electrode (e.g., an ITO electrode).  
      Protecting layers  125  and  155  can be formed by methods known in the art. In some embodiments, when protecting layers  125  and  155  include a semiconductive metal oxide, they can be formed by vacuum deposition or solution deposition (e.g., from nanoparticle dispersions or from sol gel precursors). Examples of solution depositions have been described, for example, in WO 2004/112162, the contents of which are incorporated herein by reference. In some embodiments, when protecting layers  125  and  155  include a metal capable of forming a semiconductive metal oxide, they can be formed by vacuum deposition.  
      Turning to other components of photovoltaic cell  100 , as shown in  FIGS. 2 and 3 , mesh cathode  120  includes solid regions  122  and open regions  124 . In general, regions  122  are formed of an electrically conducting material so that mesh cathode  120  can allow light to pass therethrough via regions  124  and conduct electrons via regions  122 .  
      The area of mesh cathode  120  occupied by open regions  124  (the open area of mesh cathode  120 ) can be selected as desired. Generally, the open area of mesh cathode  120  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  120 .  
      Mesh cathode  120  can be prepared in various ways. In some embodiments, mesh cathode  120  is a woven mesh formed by weaving wires of material that form solid regions  122 . 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  120  is formed of a welded wire mesh. In some embodiments, mesh cathode  120  is an expanded mesh formed. An expanded metal mesh can be prepared, for example, by removing regions  124  (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  120  is a metal sheet formed by removing regions  124  (e.g., via laser removal, via chemical etching, via puncturing) without subsequently stretching the sheet.  
      In certain embodiments, solid regions  122  are formed entirely of an electrically conductive material (e.g., regions  122  are formed of a substantially homogeneous material that is electrically conductive). Examples of electrically conductive materials that can be used in regions  122  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  122  can have a resistivity less than about  3  ohm per square.  
      As shown in  FIG. 4 , in some embodiments, solid regions  122  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  120  (i.e., the maximum thickness of mesh cathode  120  in a direction substantially perpendicular to the surface of substrate  110  in contact with mesh cathode  120 ) should be less than the total thickness of hole carrier layer  130 . Typically, the maximum thickness of mesh cathode  120  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. 2  as having a rectangular shape, open regions  124  can generally have any desired shape (e.g., square, circle, semicircle, triangle, diamond, ellipse, trapezoid, irregular shape). In some embodiments, different open regions  124  in mesh cathode  120  can have different shapes.  
      Although shown in  FIG. 3  as having square cross-sectional shape, solid regions  122  can generally have any desired shape (e.g., rectangle, circle, semicircle, triangle, diamond, ellipse, trapezoid, irregular shape). In some embodiments, different solid regions  122  in mesh cathode  120  can have different shapes. In embodiments where solid regions  122  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  122  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  120  is flexible (e.g., sufficiently flexible to be incorporated in photovoltaic cell  100  using a continuous, roll-to-roll manufacturing process). In certain embodiments, mesh cathode  120  is semi-rigid or inflexible. In some embodiments, different regions of mesh cathode  120  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  120  can be disposed on substrate  110 . In some embodiments, mesh electrode  120  can be partially embedded in substrate  110 .  
      Substrate  110  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  100 , 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  110  can be formed include polyethylene terephthalates, polyimides, polyethylene naphthalates, polymeric hydrocarbons, cellulosic polymers, polycarbonates, polyamides, polyethers and polyether ketones. 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  10  can be formed of different materials.  
      In general, substrate  10  can be flexible, semi-rigid or rigid (e.g., glass). In some embodiments, substrate  10  has a flexural modulus of less than about 5,000 megaPascals. In certain embodiments, different regions of substrate  110  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  110  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  110  can be colored or non-colored. In some embodiments, one or more portions of substrate  110  is/are colored while one or more different portions of substrate  110  is/are non-colored.  
      Substrate  110  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  10  can, for example, be curved or stepped. In some embodiments, a non-planar surface of substrate  10  is patterned (e.g., having patterned steps to form a Fresnel lens, a lenticular lens or a lenticular prism).  
      Hole carrier layer  130  is generally formed of a material that, at the thickness used in photovoltaic cell  100 , transports holes to mesh cathode  120  and substantially blocks the transport of electrons to mesh cathode  120 . Examples of materials from which layer  130  can be formed include polythiophenes (e.g., PEDOT), polyanilines, polyvinylcarbazoles, polyphenylenes, polyphenylvinylenes, polysilanes, polythienylenevinylenes and/or polyisothianaphthanenes. In some embodiments, hole carrier layer  130  can include combinations of hole carrier materials.  
      In general, the distance between the upper surface of hole carrier layer  130  (i.e., the surface of hole carrier layer  130  in contact with photoactive layer  140 ) and the upper surface of substrate  110  (i.e., the surface of substrate  110  in contact with mesh electrode  120 ) can be varied as desired. Typically, the distance between the upper surface of hole carrier layer  130  and the upper surface of mesh cathode  120  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  130  and the upper surface of mesh cathode  120  is from about 0.01 micron to about 0.5 micron.  
      Photoactive layer  140  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., PCBM). In some embodiments, active layer  140  can include a combination of electron acceptor materials.  
      Examples of electron donor materials include discotic liquid crystals, polythiophenes, polyphenylenes, polyphenylvinylenes, polysilanes, polythienylvinylenes, and polyisothianaphthalenes. In some embodiments, the electron donor material is poly(3-hexylthiophene). In certain embodiments, photoactive layer  140  can include a combination of electron donor materials.  
      Generally, photoactive layer  140  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  130  and  150 , respectively. In certain embodiments, photoactive layer  140  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, photoactive layer  140  is from about 0.1 micron to about 0.2 micron thick.  
      Hole blocking layer  150  is general formed of a material that, at the thickness used in photovoltaic cell  100 , transports electrons to anode  160  and substantially blocks the transport of holes to anode  160 . Examples of materials from which layer  150  can be formed include LiF and metal oxides (e.g., zinc oxide, titanium oxide).  
      Typically, hole blocking layer  150  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  160  is generally formed of an electrically conductive material, such as one or more of the electrically conductive materials noted above. In some embodiments, anode  160  is formed of a combination of electrically conductive materials.  
      Substrate  170  can be formed of a transparent material or a non-transparent material. For example, in embodiments in which photovoltaic cell uses light that passes through anode  160  during use, substrate  170  is desirably formed of a transparent material.  
      Exemplary materials from which substrate  170  can be formed include polyethylene terephthalates, polyimides, polyethylene naphthalates, polymeric hydrocarbons, cellulosic polymers, polycarbonates, polyamides, polyethers and polyether ketones. 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  110  can be formed of different materials.  
      In general, substrate  170  can be flexible, semi-rigid or rigid. In some embodiments, substrate  170  has a flexural modulus of less than about 5,000 megaPascals. In certain embodiments, different regions of substrate  170  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). Generally, substrate  170  is substantially non-scattering.  
      Typically, substrate  170  is at least about one micron (e.g., at least about five microns, at least about 10 microns) thick and/or at most about 200 microns (e.g., at most about 100 microns, at most about 50 microns) thick.  
      Generally, substrate  170  can be colored or non-colored. In some embodiments, one or more portions of substrate  170  is/are colored while one or more different portions of substrate  170  is/are non-colored.  
      Substrate  170  can have one planar surface (e.g., the surface of substrate  170  on which light impinges in embodiments in which during use photovoltaic cell  100  uses light that passes through anode  160 ), two planar surfaces (e.g., the surface of substrate  170  on which light impinges in embodiments in which during use photovoltaic cell  100  uses light that passes through anode  160  and the opposite surface of substrate  170 ), or no planar surfaces. A non-planar surface of substrate  170  can, for example, be curved or stepped. In some embodiments, a non-planar surface of substrate  170  is patterned (e.g., having patterned steps to form a Fresnel lens, a lenticular lens or a lenticular prism).  
       FIG. 5  shows a cross-sectional view of a photovoltaic cell  400  that includes an adhesive layer  410  between substrate  110  and hole carrier layer  130 . In some embodiments, photovoltaic cell  400  can include a protecting layer between cathode  120  and hole carrier layer  130  (not shown in  FIG. 5 ) and/or a protecting layer between anode  160  and hole blocking layer  150  (not shown in  FIG. 5 ). The protecting layer can include a semiconductive metal oxide or a metal capable of forming such metal oxide.  
      Generally, any material capable of holding mesh cathode  130  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  110  in contact with layer  410 ) is less thick than the maximum thickness of mesh cathode  120 . 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  120 . In certain embodiments, however, the thickness of layer  410  is about the same as, or greater than, the maximum thickness of mesh cathode  130 .  
      In general, a photovoltaic cell can be manufactured as desired.  
      In some embodiments, photovoltaic cell  100  can be prepared as follows. Electrode  160  is formed on substrate  170  using conventional techniques, and protecting layer  155  and hole-blocking layer  150  are sequentially formed on electrode  160  (e.g., using a vacuum deposition process or a solution coating process). Photoactive layer  140  is then formed on hole-blocking layer  150  (e.g., using a solution coating process, such as slot coating, spin coating or gravure coating). Hole carrier layer  130  is formed on photoactive layer  140  (e.g., using a solution coating process, such as slot coating, spin coating or gravure coating). Protecting layer  125  is then formed on hole carrier layer  130  (e.g., using a vacuum deposition or a solution coating process). Mesh cathode  120  is disposed on protecting layer  125 . Substrate  110  is then formed on mesh cathode  120  and hole carrier layer  130  using conventional methods.  
      In certain embodiments, a photovoltaic cell can be prepared as follows. Electrode  160  is formed on substrate  170  using conventional techniques, and hole-blocking layer  150  is formed on electrode  160  (e.g., using a vacuum deposition or a solution coating process). Photoactive layer  140  is formed on hole-blocking layer  150  (e.g., using a solution coating process, such as slot coating, spin coating or gravure coating). Hole carrier layer  130  is formed on photoactive layer  140  (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  130  using conventional methods. Mesh cathode  120  is partially disposed in adhesive layer  410  and hole carrier layer  130  (e.g., by disposing mesh cathode  120  on the surface of adhesive layer  410 , and pressing mesh cathode  120 ). Substrate  110  is then formed on mesh cathode  120  and adhesive layer  410  using conventional methods. In some embodiments, a protecting layer can be formed on electrode  160  or hole carrier layer  130  (e.g., using a vacuum deposition or a solution coating process).  
      While the foregoing processes involve partially disposing mesh cathode  120  in hole carrier layer  130 , in some embodiments, mesh cathode  120  is formed by printing the cathode material on the surface of carrier layer  130  or adhesive layer  410  to provide an electrode having the open structure shown in the figures. For example, mesh cathode  120  can be printed using 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. 6  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. 7  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 protecting layers described above can be used to enhance the stability of the electrodes 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.  
      In some embodiments, photovoltaic cells can include layers in addition to those described above. For example, photovoltaic cells can include one or more barrier layers to minimize permeation of air and moisture into the photovoltaic cells. The barrier layer can be formed of a metal (e.g., aluminum) or a polymer (e.g., an organic-inorganic hybrid polymer such as ORMOCER). In some embodiments, a barrier layer can be disposed between an electrode and an adjacent substrate that supports the electrode. As another example, photovoltaic cells can include one or more dielectric layers. Without wishing to be bound by theory, it is believed that a dielectric layer can be used to control the electronic and/or optical properties of the interfaces of a photovoltaic cell. In some embodiments, a dielectric layer can include silicon oxide, silicon carbide, silicon nitrile, titanium oxide, zinc oxide, or magnesium fluoride.  
      While certain embodiments have been disclosed, other embodiments are also possible.  
      As another 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 an 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 selenide.  
      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. 4  as being formed of one material coated on a different material, in some embodiments solid regions  122  can be formed of more than two coated materials (e.g., three coated materials, four coated materials, five coated materials, six coated materials.  
      As a further example, while photovoltaic cells having one or more protecting layers have been described, one or more protecting layers can also be used in other organic devices (e.g., devices in which the electrodes can be oxidized). Examples of such organic devices include organic photodetectors, organic light-emitting diodes, or organic field-effect transistors.  
      The following example is illustrative and not intended to be limiting.  
     EXAMPLE  
      A photovoltaic cell having the following components was prepared: glass/ITO/˜50 nm PEDOT PH/&gt;1 μm blend from OCDB/10 nm Ti/70 nm Al. Ti was chosen as the protecting layer for the Al anode since it has the potential advantages that (1) Ti has a conduction band well matched to the LUMO of PCBM (i.e., about 4.3 eV) and (2) Ti forms an electrically conductive oxide with a conduction band that is well matched to the LUMO of PCBM.  
      The photovoltaic cell prepared above underwent a light soak test performed in a chamber with a UV filter. The photovoltaic cell did not include an encapsulation layer. The test results at time zero and after  16 -hour light soak are summarized in Table 1 below.  
                                   TABLE 1                           area       J sc     efficiency           Ti/Al electrode   (cm 2 )   V oc  (V)   (mA/cm 2 )   (%)   fill factor (%)                                                        time zero   0.172   0.56   8.229   2.18   47.3       after 16-hour   0.172   0.41   6.17   1.03   40.7       light soak                  
 
      The results showed the efficiency of the photovoltaic cell slightly deteriorated after 16-hour light soak. J-C curves of the photovoltaic cell were also plotted. The curves showed that a Ti layer can be used in combination with the Al electrode in a photovoltaic cell.  
      Other embodiments are in the claims.