Abstract:
Tandem photovoltaic cells having a recombination layer, as well as related systems, methods, and components, are disclosed.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     Pursuant to 35 U.S.C. § 119(e), this application claims priority to U.S. Provisional Application Ser. No. 60/752,608, filed Dec. 21, 2005, U.S. Provisional Application Ser. No. 60/790,606, filed Apr. 11, 2006, U.S. Provisional Application Ser. No. 60/792,485, filed Apr. 17, 2006, U.S. Provisional Application Ser. No. 60/792,635, filed Apr. 17, 2006, U.S. Provisional Application Ser. No. 60/793,442, filed Apr. 20, 2006, U.S. Provisional Application Ser. No. 60/795,103, filed Apr. 26, 2006, U.S. Provisional Application Ser. No. 60/797,881, filed May 5, 2006, and U.S. Provisional Application Ser. No. 60/798,258, filed May 5, 2006, the contents of which are hereby incorporated by reference. 
     
    
     TECHNICAL FIELD  
       [0002]     The invention relates to tandem photovoltaic cells having a recombination layer, as well as related systems, methods, and components.  
       BACKGROUND  
       [0003]     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 to generate electricity. 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.  
         [0004]     There is an increasing interest in the development of photovoltaic technology due primarily to a desire to reduce consumption of and dependency on fossil fuel-based energy sources. Photovoltaic technology is also viewed by many as being an environmentally friendly energy technology. However, for photovoltaic technology to be a commercially feasible energy technology, the material and manufacturing costs of a photovoltaic system (a system that uses one or more photovoltaic cells to convert light to electrical energy) should be recoverable over some reasonable time frame. But, in some instances the costs (e.g., due to materials and/or manufacture) associated with practically designed photovoltaic systems have restricted their availability and use.  
       SUMMARY  
       [0005]     The invention relates to tandem photovoltaic cells having a recombination layer, as well as related systems, methods, and components.  
         [0006]     In one aspect, this invention features a system that includes first and second electrodes, a recombination layer between the first and second electrodes, a first photoactive layer between the first electrode and the recombination layer, and a second photoactive layer between the second electrode and the recombination layer. The recombination layer includes a semiconductor material. The system is configured as a photovoltaic system.  
         [0007]     In another aspect, this invention features a system that include first and second electrodes, first and second photoactive layers between the first and second electrodes, and a third layer between the first and second photoactive layers. The first photoactive layer includes a first semiconductor material and the second photoactive layer includes a second semiconductor material. The third layer includes a third semiconductor material different from the first or second semiconductor material. The system is configured as a photovoltaic system.  
         [0008]     In another aspect, this invention features a system that includes first and second electrodes, first and second photoactive layers between the first and second electrodes, a third layer including an n-type semiconductor material, and a fourth layer include an p-type semiconductor material. The first photoactive layer is between the first electrode and the third layer, which is between the first and second photoactive layers. The second photoactive layer is between the second electrode and the fourth layer, which is between the second photoactive layer and the third layer. The system is configured as a photovoltaic system.  
         [0009]     In another aspect, this invention features a system that includes first and second electrodes, a recombination layer between the first and second electrodes, a first photoactive layer between the first electrode and the recombination layer, and a second photoactive layer between the second electrode and the recombination layer. At least one of the first and second electrodes includes a mesh electrode. The recombination layer includes a semiconductor material. The system is configured as a photovoltaic system.  
         [0010]     In still another aspect, this invention features a method that includes preparing a photovoltaic system having a recombination layer by a roll-to-roll process. Embodiments can include one or more of the following features. In some embodiments, the semiconductor material in the recombination layer includes a p-type semiconductor material and an n-type semiconductor material.  
         [0011]     In some embodiments, the p-type semiconductor material includes a polymer selected from the group consisting of polythiophenes (e.g., poly(3,4-ethylene dioxythiophene) (PEDOT)), polyanilines, polyvinylcarbazoles, polyphenylenes, polyphenylvinylenes, polysilanes, polythienylenevinylenes, polyisothianaphthanenes, polycyclopentadithiophenes, polysilacyclopentadithiophenes, polycyclopentadithiazoles, polythiazolothiazoles, polythiazoles, polybenzothiadiazoles, poly(thiophene oxide)s, poly(cyclopentadithiophene oxide)s, polythiadiazoloquinoxalines, polybenzoisothiazoles, polybenzothiazoles, polythienothiophenes, poly(thienothiophene oxide)s, polydithienothiophenes, poly(dithienothiophene oxide)s, polytetrahydroisoindoles, and copolymers thereof.  
         [0012]     In some embodiments, the p-type semiconductor material includes a metal oxide. For example, the metal oxide can include an oxide selected from the group consisting of copper oxides, strontium copper oxides, and strontium titanium oxides. In certain embodiments, the p-type semiconductor material includes a p-doped metal oxide (e.g., p-doped zinc oxides or p-doped titanium oxides).  
         [0013]     In some embodiments, the n-type semiconductor material includes a metal oxide. For example, the metal oxide can include an oxide selected from the group consisting of titanium oxides, zinc oxides, tungsten oxides, molybdenum oxides, and combinations thereof. In other embodiments, the n-type semiconductor material includes 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.  
         [0014]     In some embodiments, the p-type and n-type semiconductor materials are blended into one layer.  
         [0015]     In some embodiments, the recombination layer includes two layers, one layer including the p-type semiconductor material and the other layer including the n-type semiconductor material.  
         [0016]     In some embodiments, the first or second photoactive layer includes an electron donor material and an electron acceptor material.  
         [0017]     In some embodiments, the electron donor material includes a polymer selected from the group consisting of polythiophenes, polyanilines, polyvinylcarbazoles, polyphenylenes, polyphenylvinylenes, polysilanes, polythienylenevinylenes, polyisothianaphthanenes, polycyclopentadithiophenes, polysilacyclopentadithiophenes, polycyclopentadithiazoles, polythiazolothiazoles, polythiazoles, polybenzothiadiazoles, poly(thiophene oxide)s, poly(cyclopentadithiophene oxide)s, polythiadiazoloquinoxaline, polybenzoisothiazole, polybenzothiazole, polythienothiophene, poly(thienothiophene oxide), polydithienothiophene, poly(dithienothiophene oxide)s, polytetrahydroisoindoles, and copolymers thereof. For example, the electron donor material can include a polymer selected from the group consisting of polythiophenes (e.g., poly(3-hexylthiophene) (P3HT)), polycyclopentadithiophenes (e.g., poly(cyclopentadithiophene-co-benzothiadiazole)), and copolymers thereof.  
         [0018]     In some embodiments, the electron acceptor material includes 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. For example, the electron acceptor material can include a substituted fullerene (e.g., C61-phenyl-butyric acid methyl ester (PCBM)).  
         [0019]     In some embodiments, the first photoactive layer has a first band gap and the second photoactive layer has a second band gap different from the first band gap.  
         [0020]     In some embodiments, the system further includes a hole carrier layer between the first photoactive layer and the first electrode. The hole carrier layer can include a polymer selected from the group consisting of polythiophenes, polyanilines, polyvinylcarbazoles, polyphenylenes, polyphenylvinylenes, polysilanes, polythienylenevinylenes, polyisothianaphthanenes, and copolymers thereof.  
         [0021]     In some embodiments, the system further includes a hole blocking layer between the second photoactive layer and the second electrode. The hole blocking layer can include a material selected from the group consisting of LiF, metal oxides, and combinations thereof.  
         [0022]     In some embodiments, the system includes a tandem photovoltaic cell.  
         [0023]     In some embodiments, the method further includes disposing the recombination layer onto a photoactive layer. The disposing can include disposing a first layer containing a first semiconductor material onto the photoactive layer and disposing a second layer containing a second semiconductor material different from the first semiconductor onto the first layer. In some embodiments, one of the first and second semiconductor materials is an n-type semiconductor material and the other of the first and second semiconductor materials is a p-type semiconductor material.  
         [0024]     In some embodiments, the recombination layer is disposed on the photoactive layer using at least one process selected from the group consisting of solution coating, ink jet printing, spin coating, dip coating, knife coating, bar coating, spray coating, roller coating, slot coating, gravure coating, and screen printing.  
         [0025]     Embodiments can provide one or more of the following advantages.  
         [0026]     In some embodiments, the recombination layer can be prepared by using a solution process that can be readily used in a continuous roll-to-roll process. Such a process can significantly reduce the cost of preparing a photovoltaic cell.  
         [0027]     In some embodiments, the photoactive layer can include a low band gap electron donor material, such as a polymer having an absorption wavelength at the red and near IR regions (e.g., 650-800 nm) of the electromagnetic spectrum, which is not accessible by most other conventional polymers. When such a polymer is incorporated into a photovoltaic cell together with a conventional polymer, it enables the cell to absorb the light in this region of the spectrum, thereby increasing the current and efficiency of the cell.  
         [0028]     In some embodiments, the first and second photoactive layers have different band gaps. Thus, light not absorbed by one photoactive layer can be absorbed by another photoactive layer, thereby increasing the efficiency of the photovoltaic cell.  
         [0029]     The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features and advantages of the invention will be apparent from the description and drawings, and from the claims. 
     
    
     DESCRIPTION OF DRAWINGS  
       [0030]      FIG. 1  is a cross-sectional view of an embodiment of a tandem photovoltaic cell.  
         [0031]      FIG. 2  is an elevational view of an embodiment of a mesh electrode.  
         [0032]      FIG. 3  is a cross-sectional view of the mesh electrode of  FIG. 2 .  
         [0033]      FIG. 4  is a cross-sectional view of another embodiment of a tandem photovoltaic cell. 
     
    
       [0034]     Like reference symbols in the various drawings indicate like elements.  
       DETAILED DESCRIPTION  
       [0035]      FIG. 1  shows a tandem photovoltaic cell  100  having a cathode  110 , a hole carrier layer  120 , a photoactive layer  130 , a recombination layer  140 , a photoactive layer  150 , a hole blocking layer  160 , an anode  170 , and an external load  180  connected to photovoltaic cell  100  via cathode  110  and anode  170 .  
         [0036]     In general, a recombination layer refers to a layer in a tandem cell where the electrons generated from a first cell recombine with the holes generated from a second cell. Recombination layer  140  typically includes a p-type semiconductor material and an n-type semiconductor material. In general, n-type semiconductor materials selectively transport electrons and p-type semiconductor materials selectively transport holes. As a result, electrons generated from the first cell recombine with holes generated from the second cell at the interface of the n-type and p-type semiconductor materials.  
         [0037]     In some embodiments, the p-type semiconductor material includes a polymer and/or a metal oxide. Examples p-type semiconductor polymers include polythiophenes (e.g., poly(3,4-ethylene dioxythiophene) (PEDOT)), polyanilines, polyvinylcarbazoles, polyphenylenes, polyphenylvinylenes, polysilanes, polythienylenevinylenes, polyisothianaphthanenes, polycyclopentadithiophenes, polysilacyclopentadithiophenes, polycyclopentadithiazoles, polythiazolothiazoles, polythiazoles, polybenzothiadiazoles, poly(thiophene oxide)s, poly(cyclopentadithiophene oxide)s, polythiadiazoloquinoxaline, polybenzoisothiazole, polybenzothiazole, polythienothiophene, poly(thienothiophene oxide), polydithienothiophene, poly(dithienothiophene oxide)s, polytetrahydroisoindoles, and copolymers thereof. The metal oxide can be an intrinsic p-type semiconductor (e.g., copper oxides, strontium copper oxides, or strontium titanium oxides) or a metal oxide that forms a p-type semiconductor after doping with a dopant (e.g., p-doped zinc oxides or p-doped titanium oxides). Examples of dopants includes salts or acids of fluoride, chloride, bromide, and iodide. In some embodiments, the metal oxide can be used in the form of nanoparticles.  
         [0038]     In some embodiments, the n-type semiconductor material includes a metal oxide, such as titanium oxides, zinc oxides, tungsten oxides, molybdenum oxides, and combinations thereof. The metal oxide can be used in the form of nanoparticles. In other embodiments, the n-type semiconductor material includes 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.  
         [0039]     In some embodiments, the p-type and n-type semiconductor materials are blended into one layer. In certain embodiments, the recombination layer includes two layers, one layer including the p-type semiconductor material and the other layer including the n-type semiconductor material.  
         [0040]     In some embodiments, recombination layer  140  includes at least about 30 wt % (e.g., at least about 40 wt % or at least about 50 wt %) and/or at most about 70 wt % (e.g., at most about 60 wt % or at most about 50 wt %) of the p-type semiconductor material. In some embodiments, recombination layer  140  includes at least about 30 wt % (e.g., at least about 40 wt % or at least about 50 wt %) and/or at most about 70 wt % (e.g., at most about 60 wt % or at most about 50 wt %) of the n-type semiconductor material.  
         [0041]     Recombination layer  140  generally has a sufficient thickness so that the layers underneath are protected from any solvent applied onto recombination layer  140 . In some embodiments, recombination layer  140  can have a thickness at least about 10 nm (e.g., at least about 20 nm, at least about 50 nm, or at least about 100 nm) and/or at most about 500 nm (e.g., at most about 200 nm, at most about 150 nm, or at most about 100 nm).  
         [0042]     In general, recombination layer  140  is substantially transparent. For example, at the thickness used in a tandem photovoltaic cell  100 , recombination layer  140  can transmit at least about 70% (e.g., at least about 75%, at least about 80%, at least about 85%, or at least about 90%) of incident light at a wavelength or a range of wavelengths (e.g., from about 350 nm to about 1,000 nm) used during operation of the photovoltaic cell.  
         [0043]     Recombination layer  140  generally has a sufficiently low resistivity. In some embodiments, recombination layer  140  has a resistivity of at most about 1×10 5  ohm/square, (e.g., at most about 2×10 5  ohm/square, at most about 5×10 5  ohm/square, or at most about 1×10 6  ohm/square).  
         [0044]     Without wishing to be bound by theory, it is believed that recombination layer  140  can be considered as a common electrode between two sub-cells (one including cathode  110 , hole carrier layer  120 , and photoactive layer  130 , and the other include photoactive layer  150 , hole blocking layer  160 , and anode  170 ) in photovoltaic cells  100 .  
         [0045]     Cathode  110  is generally formed of an electrically conductive material. Examples of electrically conductive materials 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., PEDOT), polyanilines (e.g., doped polyanilines), polypyrroles (e.g., doped polypyrroles). In some embodiments, combinations of electrically conductive materials are used.  
         [0046]     In some embodiments, cathode  110  can include a mesh electrode. Examples of mesh electrodes are described in commonly owned co-pending U.S. Patent Application Publication Nos. 20040187911 and 20060090791, the contents of which are hereby incorporated by reference.  
         [0047]      FIGS. 2 and 3  shows a mesh cathode  110  that includes solid regions  112  and open regions  114 . In general, regions  112  are formed of electrically conducting material so that mesh cathode  110  can allow light to pass therethrough via regions  114  and conduct electrons via regions  112 .  
         [0048]     The area of mesh cathode  110  occupied by open regions  114  (the open area of mesh cathode  110 ) can be selected as desired. Generally, the open area of mesh cathode  110  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  110 .  
         [0049]     Mesh cathode  110  can be prepared in various ways. In some embodiments, mesh cathode  110  is a woven mesh formed by weaving wires of material that form solid regions  112 . 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  110  is formed of a welded wire mesh. In some embodiments, mesh cathode  110  is an expanded mesh formed. An expanded metal mesh can be prepared, for example, by removing regions  114  (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  110  is a metal sheet formed by removing regions  114  (e.g., via laser removal, via chemical etching, via puncturing) without subsequently stretching the sheet.  
         [0050]     In certain embodiments, solid regions  112  are formed entirely of an electrically conductive material (e.g., regions  112  are formed of a substantially homogeneous material that is electrically conductive), such as those described above. In some embodiments, solid regions  112  can have a resistivity less than about 3 ohm per square.  
         [0051]     In some embodiments, solid regions  112  are formed of a first material that is coated with a second material different from the first material (e.g., using metallization, using vapor deposition). In general, the first material can be formed of any desired material (e.g., an electrically insulative material, an electrically conductive material, or a semiconductive material), and the second material is an electrically conductive material. Examples of electrically insulative material from which the first material 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 the first material can be formed include the electrically conductive materials disclosed above. Examples of semiconductive materials from which the first material can be formed include indium tin oxide, fluorinated tin oxide, tin oxide and zinc oxide. In some embodiments, the first material is in the form of a fiber, and the second material is an electrically conductive material that is coated on the first material. In certain embodiments, the first material is in the form of a mesh (see discussion above) that, after being formed into a mesh, is coated with the second material. As an example, the first material can be an expanded metal mesh, and the second material can be PEDOT that is coated on the expanded metal mesh.  
         [0052]     Generally, the maximum thickness of mesh cathode  110  should be less than the total thickness of hole carrier layer  120 . Typically, the maximum thickness of mesh cathode  110  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).  
         [0053]     While shown in  FIG. 2  as having a rectangular shape, open regions  114  can generally have any desired shape (e.g., square, circle, semicircle, triangle, diamond, ellipse, trapezoid, irregular shape). In some embodiments, different open regions  114  in mesh cathode  110  can have different shapes.  
         [0054]     Although shown in  FIG. 3  as having square cross-sectional shape, solid regions  112  can generally have any desired shape (e.g., rectangle, circle, semicircle, triangle, diamond, ellipse, trapezoid, irregular shape). In some embodiments, different solid regions  112  in mesh cathode  110  can have different shapes. In embodiments where solid regions  112  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  112  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.  
         [0055]     In some embodiments, mesh cathode  110  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  110  is semi-rigid or inflexible. In some embodiments, different regions of mesh cathode  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).  
         [0056]     In general, mesh electrode  110  can be disposed on a substrate. In some embodiments, mesh electrode  110  can be partially embedded in the substrate.  
         [0057]     Hole carrier layer  120  is generally formed of a material that, at the thickness used in photovoltaic cell  100 , transports holes to cathode  110  and substantially blocks the transport of electrons to cathode  110 . Examples of materials from which layer  120  can be formed include polythiophenes (e.g., PEDOT), polyanilines, polyvinylcarbazoles, polyphenylenes, polyphenylvinylenes, polysilanes, polythienylenevinylenes, polyisothianaphthanenes, and copolymers thereof. In some embodiments, hole carrier layer  120  can include combinations of hole carrier materials.  
         [0058]     In general, the thickness of hole carrier layer  120  (i.e., the distance between the surface of hole carrier layer  120  in contact with first photoactive layer  130  and the surface of cathode  110  in contact with hole carrier layer  120 ) can be varied as desired. Typically, the thickness of hole carrier layer  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, or 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, or at most about one micron). In some embodiments, the thickness of hole carrier layer  120  is from about 0.01 micron to about 0.5 micron.  
         [0059]     Each of photoactive layers  130  and  150  generally contains an electron acceptor material and an electron donor material.  
         [0060]     Examples of electron acceptor materials include 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, a combination of electron acceptor materials can be used in photoactive layer  130  or  150 .  
         [0061]     Examples of electron donor materials include conjugated polymers, such as polythiophenes, polyanilines, polyvinylcarbazoles, polyphenylenes, polyphenylvinylenes, polysilanes, polythienylenevinylenes, polyisothianaphthanenes, polycyclopentadithiophenes, polysilacyclopentadithiophenes, polycyclopentadithiazoles, polythiazolothiazoles, polythiazoles, polybenzothiadiazoles, poly(thiophene oxide)s, poly(cyclopentadithiophene oxide)s, polythiadiazoloquinoxalines, polybenzoisothiazoles, polybenzothiazoles, polythienothiophenes, poly(thienothiophene oxide)s, polydithienothiophenes, poly(dithienothiophene oxide)s, polytetrahydroisoindoles, and copolymers thereof. In some embodiments, the electron donor material can be polythiophenes (e.g., poly(3-hexylthiophene)), polycyclopentadithiophenes, and copolymers thereof. In certain embodiments, a combination of electron donor materials can be used in photoactive layer  130  or  150 .  
         [0062]     In some embodiments, the electron donor materials or the electron acceptor materials can include a polymer having a first comonomer repeat unit and a second comonomer repeat unit different from the first comonomer repeat unit. The first comonomer repeat unit can include a cyclopentadithiophene moiety, a silacyclopentadithiophene moiety, a cyclopentadithiazole moiety, a thiazolothiazole moiety, a thiazole moiety, a benzothiadiazole moiety, a thiophene oxide moiety, a cyclopentadithiophene oxide moiety, a polythiadiazoloquinoxaline moiety, a benzoisothiazole moiety, a benzothiazole moiety, a thienothiophene moiety, a thienothiophene oxide moiety, a dithienothiophene moiety, a dithienothiophene oxide moiety, or a tetrahydroisoindoles moiety.  
         [0063]     In some embodiments, the first comonomer repeat unit includes a cyclopentadithiophene moiety. In some embodiments, the cyclopentadithiophene moiety is substituted with at least one substituent selected from the group consisting of C 1 -C 20  alkyl, C 1 -C 20  alkoxy, C 3 -C 20  cycloalkyl, C 1 -C 20  heterocycloalkyl, aryl, heteroaryl, halo, CN, OR, C(O)R, C(O)OR, and SO 2 R; R being H, C 1 -C 20  alkyl, C 1 -C 20  alkoxy, aryl, heteroaryl, C 3 -C 20  cycloalkyl, or C 1 -C 20  heterocycloalkyl. For example, the cyclopentadithiophene moiety can be substituted with hexyl, 2-ethylhexyl, or 3,7-dimethyloctyl. In certain embodiments, the cyclopentadithiophene moiety is substituted at 4-position. In some embodiments, the first comonomer repeat unit can include a cyclopentadithiophene moiety of formula (1):  
                         
 
 In formula (1), each of H, C 1 -C 20  alkyl, C 1 -C 20  alkoxy, C 3 -C 20  cycloalkyl, C 1 -C 20  heterocycloalkyl, aryl, heteroaryl, halo, CN, OR, C(O)R, C(O)OR, or SO 2 R; R being H, C 1 -C 20  alkyl, C 1 -C 20  alkoxy, aryl, heteroaryl, C 3 -C 20  cycloalkyl, or C 1 -C 20  heterocycloalkyl. For example, each of R 1  and R 2 , independently, can be hexyl, 2-ethylhexyl, or 3,7-dimethyloctyl. 
 
         [0064]     An alkyl can be saturated or unsaturated and branch or straight chained. A C 1 -C 20  alkyl contains 1 to 20 carbon atoms (e.g., one, two, three, four, five, six, seven, eight, nine, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, and 20 carbon atoms). Examples of alkyl moieties include —CH 3 , —CH 2 —, —CH 2 ═CH 2 —, —CH 2 —CH═CH 2 , and branched —C 3 H 7 . An alkoxy can be branch or straight chained and saturated or unsaturated. An C 1 -C 20  alkoxy contains an oxygen radical and 1 to 20 carbon atoms (e.g., one, two , three, four, five, six, seven, eight, nine, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, and 20 carbon atoms). Examples of alkoxy moieties include —OCH 3  and —OCH═CH—CH 3 . A cycloalkyl can be either saturated or unsaturated. A C 3 -C 20  cycloalkyl contains 3 to 20 carbon atoms (e.g., three, four, five, six, seven, eight, nine, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, and 20 carbon atoms). Examples of cycloalkyl moieities include cyclohexyl and cyclohexen-3-yl. A heterocycloalkyl can also be either saturated or unsaturated. A C 3 -C 20  heterocycloalkyl contains at least one ring heteroatom (e.g., O, N, and S) and 3 to 20 carbon atoms (e.g., three, four, five, six, seven, eight, nine, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, and 20 carbon atoms). Examples of heterocycloalkyl moieties include 4-tetrahydropyranyl and 4-pyranyl. An aryl can contain one or more aromatic rings. Examples of aryl moieties include phenyl, phenylene, naphthyl, naphthylene, pyrenyl, anthryl, and phenanthryl. A heteroaryl can contain one or more aromatic rings, at least one of which contains at least one ring heteroatom (e.g., O, N, and S). Examples of heteroaryl moieties include furyl, furylene, fluorenyl, pyrrolyl, thienyl, oxazolyl, imidazolyl, thiazolyl, pyridyl, pyrimidinyl, quinazolinyl, quinolyl, isoquinolyl, and indolyl.  
         [0065]     Alkyl, alkoxy, cycloalkyl, heterocycloalkyl, aryl, and heteroaryl mentioned herein include both substituted and unsubstituted moieties, unless specified otherwise. Examples of substituents on cycloalkyl, heterocycloalkyl, aryl, and heteroaryl include C 1 -C 20  alkyl, C 3 -C 20  cycloalkyl, C 1 -C 20  alkoxy, aryl, aryloky, heteroaryl, heteroaryloxy, amino, C 1 -C 10  alkylamino, C 1 -C 20  dialkylamino, arylamino, diarylamino, hydroxyl, halogen, thio, C 1 -C 10  alkylthio, arylthio, C 1 -C 10  alkylsulfonyl, arylsulfonyl, cyano, nitro, acyl, acyloxy, carboxyl, and carboxylic ester. Examples of substituents on alkyl include all of the above-recited substituents except C 1 -C 20  alkyl. Cycloalkyl, heterocycloalkyl, aryl, and heteroaryl also include fused groups.  
         [0066]     The second comonomer repeat unit can include a benzothiadiazole moiety, a thiadiazoloquinoxaline moiety, a cyclopentadithiophene oxide moiety, a benzoisothiazole moiety, a benzothiazole moiety, a thiophene oxide moiety, a thienothiophene moiety, a thienothiophene oxide moiety, a dithienothiophene moiety, a dithienothiophene oxide moiety, a tetrahydroisoindole moiety, a fluorene moiety, a silole moiety, a cyclopentadithiophene moiety, a fluorenone moiety, a thiazole moiety, a selenophene moiety, a thiazolothiazole moiety, a cyclopentadithiazole moiety, a naphthothiadiazole moiety, a thienopyrazine moiety, a silacyclopentadithiophene moiety, an oxazole moiety, an imidazole moiety, a pyrimidine moiety, a benzoxazole moiety, or a benzimidazole moiety. In some embodiments, the second comonomer repeat unit is a 3,4-benzo-1,2,5-thiadiazole moiety.  
         [0067]     In some embodiments, the second comonomer repeat unit can include a benzothiadiazole moiety of formula (2), a thiadiazoloquinoxaline moiety of formula (3), a cyclopentadithiophene dioxide moiety of formula (4), a cyclopentadithiophene monoxide moiety of formula (5), a benzoisothiazole moiety of formula (6), a benzothiazole moiety of formula (7), a thiophene dioxide moiety of formula (8), a cyclopentadithiophene dioxide moiety of formula (9), a cyclopentadithiophene tetraoxide moiety of formula (10), a thienothiophene moiety of formula (11), a thienothiophene tetraoxide moiety of formula (12), a dithienothiophene moiety of formula (13), a dithienothiophene dioxide moiety of formula (14), a dithienothiophene tetraoxide moiety of formula (15), a tetrahydroisoindole moiety of formula (16), a thienothiophene dioxide moiety of formula (17), a dithienothiophene dioxide moiety of formula (18), a fluorene moiety of formula (19), a silole moiety of formula (20), a cyclopentadithiophene moiety of formula (21), a fluorenone moiety of formula (22), a thiazole moiety of formula (23), a selenophene moiety of formula (24), a thiazolothiazole moiety of formula (25), a cyclopentadithiazole moiety of formula (26), a naphthothiadiazole moiety of formula (27), a thienopyrazine moiety of formula (28), a silacyclopentadithiophene moiety of formula (29), an oxazole moiety of formula (30), an imidazole moiety of formula (31), a pyrimidine moiety of formula (32), a benzoxazole moiety of formula (33), or a benzimidazole moiety of formula (34):  
                         
                         
                         
                         
                         
                         
                         
                         
 
 In the above formulas, each of X and Y, independently, is CH 2 , O, or S; each of R 5  and R 6 , independently, is H, C 1 -C 20  alkyl, C 1 -C 20  alkoxy, C 3 -C 20  cycloalkyl, C-C 20  heterocycloalkyl, aryl, heteroaryl, halo, CN, OR, C(O)R, C(O)OR, or SO 2 R, in which R is H, C 1 -C 20  alkyl, C 1 -C 20  alkoxy, aryl, heteroaryl, C 3 -C 20  cycloalkyl, or C 1 -C 20  heterocycloalkyl; and each of R 7  and R 8 , independently, is H, C 1 -C 20  alkyl, C 1 -C 20  alkoxy, aryl, heteroaryl, C 3 -C 20  cycloalkyl, or C 3 -C 20  heterocycloalkyl. In some embodiments, the second comonomer repeat unit includes a benzothiadiazole moiety of formula (2), in which each of R 5  and R 6  is H. 
 
         [0068]     The second comonomer repeat unit can include at least three thiophene moieties. In some embodiments, at least one of the thiophene moieties is substituted with at least one substituent selected from the group consisting of C 1 -C 20  alkyl, C 1 -C 20  alkoxy, aryl, heteroaryl, C 3 -C 20  cycloalkyl, and C 3 -C 20  heterocycloalkyl. In certain embodiments, the second comonomer repeat unit includes five thiophene moieties.  
         [0069]     The polymer can further include a third comonomer repeat unit that contains a thiophene moiety or a fluorene moiety. In some embodiments, the thiophene or fluorene moiety is substituted with at least one substituent selected from the group consisting of C 1 -C 20  alkyl, C 1 -C 20  alkoxy, aryl, heteroaryl, C 3 -C 20  cycloalkyl, and C 3 -C 20  heterocycloalkyl.  
         [0070]     In some embodiments, the polymer can be formed by any combination of the first, second, and third comonomer repeat units. In certain embodiments, the polymer can be a homopolymer containing any of the first, second, and third comonomer repeat units. In some embodiments, the polymer can be  
                         
 
 in which n can be an integer greater than 1. 
 
         [0071]     The monomers for preparing the polymers mentioned herein may contain a non-aromatic double bond and one or more asymmetric centers. Thus, they can occur as racemates and racemic mixtures, single enantiomers, individual diastereomers, diastereomeric mixtures, and cis- or trans-isomeric forms. All such isomeric forms are contemplated.  
         [0072]     The polymers described above can be prepared by methods known in the art, such as those described in commonly owned co-pending U.S. application Ser. No 11/601,374, the contents of which are hereby incorporated by reference. For example, a copolymer can be prepared by a cross-coupling reaction between one or more comonomers containing two alkylstannyl groups and one or more comonomers containing two halo groups in the presence of a transition metal catalyst. As another example, a copolymer can be prepared by a cross-coupling reaction between one or more comonomers containing two borate groups and one or more comonomers containing two halo groups in the presence of a transition metal catalyst. The comonomers can be prepared by the methods described herein or by the methods know in the art, such as those described in U.S. patent application Ser. No. 11/486,536, Coppo et al.,  Macromolecules  2003, 36, 2705-2711 and Kurt et al.,  J. Heterocycl. Chem.  1970, 6, 629, the contents of which are hereby incorporated by reference.  
         [0073]     Without wishing to be bound by theory, it is believed that an advantage of the polymers described above is that their absorption wavelengths shift toward the red and near IR regions (e.g., 650-800 nm) of the electromagnetic spectrum, which is not accessible by most other conventional polymers. When such a polymer is incorporated into a photovoltaic cell together with a conventional polymer, it enables the cell to absorb the light in this region of the spectrum, thereby increasing the current and efficiency of the cell.  
         [0074]     In some embodiments, photoactive layer  130  has a first band gap and photoactive layer  150  has a second band gap different from the first band gap. In such embodiments, light not absorbed by one photoactive layer can be absorbed by another photoactive layer, thereby increasing the efficiency of photovoltaic cell  100 .  
         [0075]     Generally, photoactive layer  130  or  150  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. In certain embodiments, photoactive layer  130  or  150  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  130  or  150  is from about 0.1 micron to about 0.2 micron thick.  
         [0076]     In general, photoactive layer  130  or  150  can be formed by using a suitable process, such as solution coating, ink jet printing, spin coating, dip coating, knife coating, bar coating, spray coating, roller coating, slot coating, gravure coating, or screen printing.  
         [0077]     Hole blocking layer  160  is generally formed of a material that, at the thickness used in photovoltaic cell  100 , transports electrons to anode  170  and substantially blocks the transport of holes to anode  170 . Examples of materials from which hole blocking  10  layer  160  can be formed include LiF and metal oxides (e.g., zinc oxide, titanium oxide).  
         [0078]     Typically, hole blocking layer  160  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.  
         [0079]     Anode  170  is generally formed of an electrically conductive material, such as one or more of the electrically conductive materials noted above. In some embodiments, anode  170  is formed of a combination of electrically conductive materials. In certain embodiments, anode  170  can be formed of a mesh electrode.  
         [0080]     Without wishing to be bound by theory, it is believed that tandem photovoltaic cell  100  achieves the highest efficiency when photoactive layers  130  and  150  generate substantially the same amount of current.  
         [0081]      FIG. 4  shows a tandem photovoltaic cell  400  having a cathode  410 , a hole carrier layer  420 , a photoactive layer  430 , a recombination layer  440 , a photoactive layer  450 , a hole blocking layer  460 , an anode  470 , and an external load  480  connected to photovoltaic cell  400  via cathode  410  and anode  470 . Recombination layer  440  includes a layer  442  that contains an an-type semiconductor material and a layer  444  that contains a p-type semiconductor material. In some embodiments, recombination layer  440  can include a layer of mixed n-type and p-type semiconductor material at the interface of layer  442  and layer  444 .  
         [0082]     In some embodiments, a two-layer recombination layer can be prepared by applying a layer of an n-type semiconductor material and a layer of a p-type semiconductor material separately. For example, when titanium oxide nanoparticles are used as an n-type semiconductor material, a layer of titanium oxide nanoparticles can be formed by (1) dispersing a precursor (e.g., a titanium salt) in a solvent (e.g., an anhydrous alcohol) to form a dispersion, (2) coating the dispersion on a photoactive layer, (3) hydrolyzing the dispersion to form a titanium oxide layer, and (4) drying the titanium oxide layer. As another example, when a polymer (e.g., PEDOT) is used a p-type semiconductor, a polymer layer can be formed by first dissolving the polymer in a solvent (e.g., an anhydrous alcohol) to form a solution and then coating the solution on a photoactive layer. In some embodiments, a one-layer recombination layer can be prepared by applying a blend of an n-type semiconductor material and a p-type semiconductor material on photoactive layer. For example, an n-type semiconductor and a p-type semiconductor can be first dispersed and/or dissolved in a solvent together to form a dispersion or solution and then coated the dispersion or solution on a photoactive layer to form a recombination layer. The coating process mentioned above can be achieved by using at least one process selected from the group consisting of solution coating, ink jet printing, spin coating, dip coating, knife coating, bar coating, spray coating, roller coating, slot coating, gravure coating, and screen printing.  
         [0083]     Without wishing to bound by theory, it is believed that the solution process described above can be readily used in a continuous manufacturing process, such as a roll-to-roll process, thereby significantly reducing the cost of preparing a photovoltaic cell. Examples of roll-to-roll processes have been described in, for example, U.S. Application Publication No. 2005-0263179.  
         [0084]     The following examples are illustrative and not intended to be limiting.  
       EXAMPLE 1  
       [0085]     A tandem photovoltaic cell having the structure of ITO/TiO 2 /P3HT:PCBM/ PEDOT/TiO2/P3HT:PCBM/PEDOT/Ag was prepared as follows. A substrate with ITO (having a resistivity of 13 ohm/square) was cleaned sequentially with acetone and isopropanol for 10 minutes in an ultrasonic bath at room temperature. Tetra-n-butyl-titanate (TYZOR; E. I. du Pont de Nemours and Company, Wilmington, Del.) diluted 1:199 in anhydrous isopropanol was applied onto the ITO via doctor-blading (40 mm/s; 600 μm slot at 40° C.) and hydrolyzed by distilled water. The coating thus obtained was dried for 10 minutes to give a titanium oxide layer having a thickness of 10±5 nm. A solution of poly-(3-hexylthiophen) (P3HT): C61-phenyl-butyric acid methyl ester (PCBM) in ortho-xylene (1.5 mg:1.2 mg:100 μl) was then applied onto the titanium oxide layer via doctor-blading (7.5 mm/s; 600 μm slot at 65° C.) to give a P3HT:PCBM layer having a thickness of 100±10 nm. A solution of PEDOT in isopropanol (1 ml:5 ml) was subsequently coated on the P3HT:PCBM layer via doctor-blading (2×5 mm/s; 150 μm slot at 85° C.) to give in a PEDOT layer of 30±10 nm. After the device thus obtained was baked for 10 minutes at 140° C. in nitrogen atmosphere, tetra-n-butyl-titanate diluted 1:199 in anhydrous isopropanol was applied onto the PEDOT layer via doctor-blading (40 mm/s; 600 μm slot at 40° C.). The coating was hydrolyzed and dried for 10 minutes to give a second titanium oxide layer of 10±5 nm. The PEDOT layer and the second titanium oxide layer obtained above constituted as the recombination layer in the final tandem photovoltaic cell. A solution of P3HT:PCBM in ortho-xylene (1.5 mg:1.2 mg:100 μl) was then applied onto the second titanium oxide layer via doctor-blading (65 mm/s; 600 μm slot at 65° C.) to give a second P3HT:PCBM layer having a thickness of 300±30 nm. Subsequently, a solution of PEDOT in isopropanol (1 ml:5 ml) was applied onto the second P3HT:PCBM layer via doctor-blading (2×5 mm/s; 150 μm slot at 85° C.) to give a second PEDOT layer having a thickness of 30±10 nm. After the device thus obtained was baked for 20 minutes at 140° C. in nitrogen atmosphere, a 100 nm layer of silver was applied onto the second PEDOT layer via thermal evaporation (0.05-0.5 nm/s at 3×10 −6  mbar) to give a tandem photovoltaic cell.  
         [0086]     A single photovoltaic cell having the structure of ITO/TiO 2 /P3HT:PCBM/ PEDOT/Ag was also prepared. The titanium oxide layer, the P3HT:PCBM layer, the PEDOT layer, and the silver layer were prepared using the same methods described in the preceding paragraph.  
         [0087]     The tandem photovoltaic cell and single cell were tested for their properties. The open circuit voltage of both cells were measured at zero current using a Source Measurement Unit (SMU) Keithley 2400 when the device was illuminated by a solar simulator (Oriel) at 1 kW/m 2  Air Mass 1.5 global. The results show that the open circuit voltage of the tandem photovoltaic cell was 1.025 V, twice as much as that of a single photovoltaic cell having the structure of ITO/TiO 2 /P3HT:PCBM/PEDOT/Ag.  
         [0088]     Other embodiments are in the claims.