Abstract:
Photovoltaic cells containing a novel electrode, as well as related systems, methods, modules, and components, are disclosed.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    This application is a continuation of co-pending international patent application PCT/US2011/031362, filed on Apr. 6, 2011, and claims benefit of U.S. provisional patent application Ser. No. 61/321,361, filed on Apr. 6, 2010. The entire disclosures of both applications are incorporated herein by reference. 
     
    
     TECHNICAL FIELD 
       [0002]    This disclosure relates to photovoltaic cells having a novel electrode, as well as related systems, methods, modules, 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, thereby generating charge carriers (i.e., electrons and holes). As a result, the ability of the electrodes to transmit light and transport charge carriers can limit the overall efficiency of a photovoltaic cell. 
       SUMMARY 
       [0004]    This disclosure is based on unexpected findings that the electrode disclosed herein (e.g., a bottom electrode) can exhibit both improved light transmittance and improved electrical conductivity compared to a conventional electrode (e.g., an indium tin oxide (ITO) electrode or an ITO/metal/ITO electrode). As a result, the electrode disclosed herein can be used to make photovoltaic cells or modules with improved transparency and efficiency. 
         [0005]    In one aspect, this disclosure features an article that includes a first electrode, a second electrode, and a photoactive material disposed between the first and second electrodes. The first electrode includes first, second, and third layers, in which the first layer includes a grid and an electrically insulating material, the second layer is between the first and third layers and includes an electrically conductive material, and the third layer is between the second layer and the photoactive layer, and includes a hole blocking material and an electrically conductive metal oxide. The article is configured as a photovoltaic cell. 
         [0006]    In another aspect, this disclosure features an article that includes a first electrode containing a first layer, a second electrode, and a photoactive material disposed between the first and second electrodes. The first layer includes a grid and an electrically insulating material, and has a root mean squared surface roughness of at most about 20 nm. The article is configured as a photovoltaic cell. 
         [0007]    Embodiments can include one or more of the following optional features. 
         [0008]    In some embodiments, the grid can include a metal or an alloy. For example, the grid can include gold, silver, copper, aluminum, nickel, palladium, platinum, titanium, or an alloy thereof. 
         [0009]    In some embodiments, the electrically insulating material can include an acrylate polymer, a urethane polymer, an epoxy polymer, or a combination thereof. 
         [0010]    In some embodiments, the electrically conductive material can include metal nanoparticles or an electrically conductive polymer. The metal nanoparticles can have a shape of a sphere, tube, rod, or needle. The electrically conductive polymer can include a polythiophene (e.g., doped polythiophene). 
         [0011]    In some embodiments, the hole blocking material includes a polyamine (e.g., a polyethylenimine). In certain embodiments, at least some molecules of the hole blocking material are cross-linked by a cross-linking agent, such as an epoxy-containing compound (e.g., glycerol triglycidyl ether or glycerol diglycidyl ether). 
         [0012]    In some embodiments, the electrically conductive metal oxide can include indium tin oxide. 
         [0013]    In some embodiments, the first layer can have a root mean squared surface roughness of at most about 20 nm (e.g., at most about 5 nm). 
         [0014]    In some embodiments, the first layer can have a maximum peak height of at most about 200 nm. 
         [0015]    In some embodiments, the first electrode can have a light transmittance of at least about 70% during use of the article. 
         [0016]    In some embodiments, the first electrode can have a sheet resistance of at most about 10 ohm/square. 
         [0017]    In some embodiments, the electrically insulating material has a surface contacting the second layer, the grid has a surface contacting the second layer, and the surface of the electrically insulating material is substantially at the same level as the surface of the grid. 
         [0018]    In some embodiments, the first substrate has a surface with a root mean squared surface roughness of at most about 20 nm before the grid is disposed on the first substrate. In certain embodiments, the grid is disposed on the surface. 
         [0019]    In still another aspect, this disclosure features a method that includes disposing a grid on a first substrate to form an article, in which the grid contacts the first substrate; laminating the article with a curable material and a second substrate such that the grid and the curable material are disposed between the first and second substrates; curing the curable material; and removing the first substrate to form an electrode layer. 
         [0020]    In some embodiments, the curable material can include an acrylate, a urethane, an epoxide, or a combination thereof. In certain embodiments, the curable material can be cured by UV irradiation, heating, or a combination thereof. 
         [0021]    In some embodiments, the method can further include disposing an electrically conductive material on the electrode layer. In certain embodiments, the method can further include disposing a mixture on the electrically conductive material, in which the mixture can include a hole blocking material and an electrically conductive metal oxide. In certain embodiments, the method can further include disposing a photoactive layer and an electrode on the mixture to form a photovoltaic cell. 
         [0022]    Embodiments can include one or more of the following advantages. 
         [0023]    Without wishing to be bound by theory, it is unexpectedly discovered that an electrode including a layer containing a grid and an electrically insulating material, a layer containing an electrically conductive material, and a layer containing a hole blocking material or an electrically conductive metal oxide can exhibit both improved light transmittance and improved electrical conductivity compared to a conventional electrode (e.g., an indium tin oxide (ITO) electrode or an ITO/metal/ITO electrode). 
         [0024]    Without wishing to be bound by theory, it is believed that including a smooth layer containing a grid and an electrically insulating material having a relatively low roughness can prevent or minimize short circuit of a photovoltaic cell (e.g., caused by a large peak on the grid) and can significantly reduce the thickness of the layers on top of this layer required to form a photovoltaic cell without causing short circuit. 
         [0025]    Other features and advantages of the invention will be apparent from the description, drawings, and claims. 
     
    
     
       DESCRIPTION OF DRAWINGS 
         [0026]      FIG. 1  is a cross-sectional view of an embodiment of a photovoltaic cell; 
           [0027]      FIGS. 2(   a )- 2 ( c ) illustrate an exemplary process of making the electrode shown in  FIG. 1 ; 
           [0028]      FIG. 3  is a schematic of a system containing multiple photovoltaic cells electrically connected in series; and 
           [0029]      FIG. 4  is a schematic of a system containing multiple photovoltaic cells electrically connected in parallel. 
       
    
    
       [0030]    Like reference symbols in the various drawings indicate like elements. 
       DETAILED DESCRIPTION 
       [0031]      FIG. 1  shows a cross-sectional view of a photovoltaic cell  100  that includes a substrate  110 , an electrode  120  (e.g., a bottom electrode), a photoactive layer  130  (e.g., containing an electron acceptor material and an electron donor material), an optional hole carrier layer  140 , an electrode  150  (e.g., a top electrode), and a substrate  160 . Electrode  120  includes a first layer  121  containing a grid  122  and an electrically insulating material  124 , a second layer  123 , and a third layer  125 . Electrodes  120  and  150  are electrically connected to an external load  170 . 
         [0032]    In general, first layer  121  includes grid  122 , which can be formed of an electrically conductive material. Exemplary electrically conductive materials include electrically conductive metals (e.g., alloys), electrically conductive polymers, and electrically conductive metal oxides. 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., doped poly(3,4-ethylenedioxythiophene) (doped PEDOT)), polyanilines (e.g., doped polyanilines), and polypyrroles (e.g., doped polypyrroles). Exemplary electrically conducting metal oxides include indium tin oxide (ITO), tin oxide (e.g., fluorinated tin oxide), titanium oxide, zinc oxide, tungsten oxide, molybdenum oxide, copper oxide, strontium copper oxide, and strontium titanium oxide. In some embodiments, combinations of electrically conductive materials are used. 
         [0033]    In some embodiments, the electrically conductive material can be in the form of nanoparticles (e.g., metal nanoparticles such as silver nanoparticles). 
         [0034]    In some embodiments, grid  122  is in the form of a mesh (e.g., when the space between the grid lines is relatively small). Examples of grid electrodes (e.g., mesh electrodes) are described in commonly owned co-pending U.S. Patent Application Publication Nos. 2004-0187911 and 2006-0090791. 
         [0035]    In general, the dimensions of grid  122  are adjusted such that first layer  121  has sufficiently large electrical conductivity and sufficiently high transparency. In some embodiments, grid  122  has a thickness of at least about 200 nm (e.g., at least about 400 nm) and/or at most about 900 nm (e.g., at most about 800 nm), and a width of at least about 10 microns (e.g., at least about 20 microns) and/or at most 100 microns (e.g., at most about 50 microns). If the dimensions of grid  122  are too large, first layer  121  may not have sufficient transparency. If the dimensions of grid  122  are too small, first layer  121  may not have sufficient electrical conductivity. 
         [0036]    First layer  121  generally also includes an electrically insulating material  124  in the space formed by grid  122 , which functions as a leveling layer but does not cover the surface of grid  122  contacting second layer  123 . Exemplary electrically insulating polymers include acrylate polymers, urethane polymers, epoxy polymers, and combinations thereof. 
         [0037]    In some embodiments, electrically insulating material  124  can have a thickness greater than that of grid  122 . In such embodiments, as shown in  FIG. 1 , the surface of grid  122  contacting layer  123  and the surface of electrically insulating material  124  contacting layer  123  are substantially at the same level, while grid  122  is separated from substrate  110  by electrically insulating material  124 . In certain embodiments, electrically insulating material  124  has the same thickness as that of grid  122  (not shown in  FIG. 1 ). In some embodiments, electrically insulating material  124  can have a thickness of at least about 200 nm (e.g., at least about 1 micron, at least about 10 microns, or at least about 40 microns) and/or at most about 80 microns (e.g., at most about 60 microns, at most 40 microns, at most about 20 microns, at most about 10 microns, at most about 1 micron, or at most about 800 nm). In some embodiments, electrically insulating material  124  has a thickness of between 12 microns and 50 microns (e.g., between 12 microns and 25 microns). 
         [0038]    Electrically insulating material  124  is generally transparent. As used herein, a transparent material refers to a material that, at the thickness used in a photovoltaic cell, transmits at least about 90% (e.g., at least about 95%, at least about 98%, or at least about 99%) 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. 
         [0039]    In general, the surface of electrically insulating material  124  contacting second layer  123  is substantially at the same level as the surface of grid  122  contacting second layer  123 . As used herein, the term “substantially at the same level” refers to a surface level difference at most about 500 nm. For example, the surface of grid  122  can be at most 250 nm (e.g., at most about 200 nm, at most about 100 nm, at most about 50 nm, at most about 20 nm, or at most about 10 nm) higher or lower than the surface of electrically insulating material  124 . 
         [0040]    In general, the surface of first layer  121  contacting second layer  123 , i.e., surface  126 , can be relatively smooth (i.e., having a relatively low roughness). For example, surface  126  can have a root mean squared surface roughness RMS of at most about 20 nm (e.g., at most about 15 nm, at most about 10 nm, at most about 5 nm, at most about 1 nm, or at most about 0.5 nm) and/or at least about 0.1 nm (e.g., at least about 0.5 nm or at least about 1 nm). As used herein, the root mean squared surface roughness is measured by Atomic Force Microscopy. As another example, surface  126  can have a maximum peak height of at most about 200 nm (e.g., at most about 150 nm, at most about 100 nm, at most about 50 nm, at most about 20 nm, or at most about 10 nm) and/or at least about 1 nm (e.g., at least about 5 nm, at least about 10 nm, or at least about 15 nm). As used herein, the term “maximum peak height” refers to the distance from the tip of the highest peak on a surface to the level that is the average of the peaks and valleys on the surface. The maximum peak height used herein is measured by Atomic Force Microscopy. Without wishing to be bound by theory, it is believed that forming a relatively smooth surface  126  can prevent or minimize short circuit of photovoltaic cell  100  (e.g., caused by a large peak on grid  122 ) and can significantly reduce the thickness of the layers on top of first layer  121  required to form a photovoltaic cell without causing short circuit. 
         [0041]      FIGS. 2(   a )- 2 ( c ) illustrate an exemplary process of making first layer  121 . As shown in  FIG. 2(   a ), a grid  222  can first be coated on a smooth surface of a first substrate  201  (also known as a release substrate) to form an article. For example, an ink containing metal nanoparticles (e.g., silver nanoparticles) dispersed in a solvent (e.g., an alcohol) can be coated on a smooth surface of first substrate  201  to form metal grid  222 . 
         [0042]    In some embodiments, the surface on first substrate  201  can be relatively smooth (i.e., having a relatively low roughness) before it is coated with grid  222 . For example, the surface can have a root mean squared surface roughness RMS of at most about 20 nm (e.g., at most about 15 nm, at most about 10 nm, at most about 5 nm, at most about 1 nm, or at most about 0.5 nm) and/or at least about 0.1 nm (e.g., at least about 0.5 nm or at least about 1 nm). As another example, the surface of first layer  121  contacting second layer  123  can have a maximum peak height of at most about 200 nm (e.g., at most about 150 nm, at most about 100 nm, at most about 50 nm, at most about 20 nm, or at most about 10 nm) and/or at least about 1 nm (e.g., at least about 5 nm, at least about 10 nm, or at least about 15 nm). Without wishing to be bound by theory, it is believed that coating a grid onto a relatively smooth surface on first substrate  201  can result in a smooth surface  126  on first layer  121  shown in  FIG. 1  as surface  126  mimics the surface on first substrate  201  on which grid  222  is coated. 
         [0043]    In some embodiments, first substrate  201  is formed from a polymer, e.g., polyethylene terephthalates, polyimides, polyethylene naphthalates, polymeric hydrocarbons, cellulosic polymers, polycarbonates, polyamides, polyethers, and polyether ketones. In certain embodiments, first substrate  201  is formed of a polyethylene terephthalate (PET). 
         [0044]    After the article containing first substrate  201  and grid  222  is formed, a curable material  204  can be added onto first substrate  201  to fill the space between the grid lines formed by grid  222 . The curable material can be cured (i.e., polymerized and/or cross-linked) by heat, UV irradiation, or a combination thereof, and can be a monomer or an oligomer (e.g., a pre-polymer containing less than 10 repeating units). Exemplary curable materials include acrylates (e.g., urethane acrylate oligomers) and epoxides (e.g., cyclic aliphatic epoxides). Examples of suitable epoxide includes 3,4-epoxycyclohexylmethyl-3,4-epoxycyclohexanecarboxylate and bis((3,4-epoxycyclohexyl)methyl)adipate. When an acrylate is used as a curable material, a urethane acrylate oligomer can be used as a component to form a cured material. 
         [0045]    In some embodiments, a photoinitiator and/or a sensitizer (i.e., a compound that absorbs light at a wavelength used in the curing process) can be used in combination of curable material  140  to form a cured material (e.g., an epoxy). Examples of suitable photoinitators include iodonium salts (e.g., bis(4-tert-butylphenyl)iodonium hexaflouroantimonate). An example of a sensitizer is isopropylthioxanthone. 
         [0046]    In general, the layer formed by curable material  204  can have a thickness the same as or larger than that of grid  222 . When curable material  204  has a thickness larger than grid  222 , curable material  204  covers grid  222  so that grid  222  is embedded in curable material after curing. In such embodiments, since grid  222  is already coated on first substrate  201 , its surface roughness is not affected by curable material  204  after first substrate  201  is peeled off. 
         [0047]    After curable material  204  is disposed onto first substrate  201 , a second substrate  210  (also known as the receiving substrate) can be brought into contact with the article thus formed such that curable material  204  and grid  222  are disposed between first substrate  201  and second substrate  210 . In some embodiments, second substrate  210  can be brought into contact with first substrate  201  before curable material  204  is disposed on first substrate  201 . In such embodiments, curable material  204  can be disposed in the gap formed by first substrate  201  and second substrate  210 . Second substrate  210  can be formed of the same material (e.g., PET) as that of first substrate  201 . In some embodiment, second substrate  210  does not include a smooth surface. 
         [0048]    As shown in  FIG. 2(   b ), the article formed above can then be passed through a pair of nip rollers to laminate first substrate  201  and second substrate  210  together. Subsequently, curable material  204  can be cured by heating (e.g., at a temperature of at least about 40° C., at least about 60° C., at least about 80° C., or at least about 100° C.) or UV irradiation (e.g., having a wavelength ranging from about 200 nm to about 350 nm). In some embodiments, curable material  204  is adjusted such that it provides a sufficient adhesion between first substrate  201  and second substrate  210  so as to adhere these two substrates together during the lamination process. However, the cured material generally does not have an adhesion so high that it sticks to first substrate  201  when first substrate  201  is peeled off from the article formed above. In general, the cured material thus formed is a transparent electrically insulating material, such as urethane acrylates or cyclic aliphatic epoxies. 
         [0049]    In some embodiments, a primer layer (e.g., containing one or more polyesters) can be coated on second substrate  210  to ensure that the cured material adheres to second substrate  210 . For example, the primer layer can be formed by coating a blend of L7063/VITEL 2700/BOSCODUR 16 obtained from Bostik, Inc. (Middleton, Mass.) at a dry weight ratio of 1.67:1.0:0.054 on second substrate  210 . As shown in  FIG. 2(   c ), after the curable material is cured, first substrate  201  can be peeled off from the article formed above to form a layer containing grid  222  and a cured material (i.e., corresponding to first layer  121  shown in  FIG. 1)  and substrate  210  (i.e., corresponding to substrate  110  shown in  FIG. 1 ). The exposed surface of grid  222  and the cured material after first substrate  201  is peeled off forms surface  126  shown in  FIG. 1 . 
         [0050]    Without wishing to be bound by theory, an advantage of the above process is that it can form an electrode layer having a relatively smooth surface (e.g., having a root mean squared surface roughness of at most about 20 nm or having a maximum peak height of at most about 200 nm) as the surface of the electrode layer mimics the smooth surface of first substrate  201 . Further, without wishing to be bound by theory, another advantage of the above process is that it can form an electrode layer in which the surface of grid  222  and the surface of the electrically insulating material formed by curable material  204  are substantially at the same level. 
         [0051]    In some embodiments, when electrode  120  is a top electrode, the roughness of surface  126  may not be critical in preventing or minimizing short circuit of photovoltaic cells. In such embodiments, first layer  121  can be prepared by a process (e.g., by using a simple solution coating or inkjet printing method) different from that described above. 
         [0052]    Turning to other components in electrode  120 , second layer  123  generally includes an electrically conductive material, such as electrically conductive polymers, electrically conductive metals (e.g., alloys), or electrically conductive metal oxides. Exemplary electrically conductive polymers include polythiophenes (e.g., PEDOT), polyanilines, polycarbazoles, polyvinylcarbazoles, polyphenylenes, polyphenylvinylenes, polysilanes, polythienylenevinylenes, polyisothianaphthanenes, and copolymers thereof. In some embodiments, second layer  123  can include a dopant used in combination with one of the just-mentioned electrically conductive polymers. Examples of dopants include poly(styrene-sulfonate)s, polymeric sulfonic acids, or fluorinated polymers (e.g., fluorinated ion exchange polymers). 
         [0053]    Exemplary electrically conductive metals include gold, silver, copper, aluminum, nickel, palladium, platinum, titanium, and an alloy thereof. 
         [0054]    Exemplary electrically conductive metal oxides include ITO, tin oxide (e.g., fluorinated tin oxide), titanium oxide, zinc oxide, copper oxide, strontium copper oxide, and strontium titanium oxide. The metal oxides can be either undoped or doped with a dopant. Examples of dopants for metal oxides include salts or acids of fluoride, chloride, bromide, and iodide. 
         [0055]    In some embodiments, the electrically conductive materials that can be used to form second layer  123  include carbon allotropes (e.g., carbon nanotubes). The carbon allotropes can be embedded in a polymer binder. 
         [0056]    In some embodiments, the electrically conductive materials that can be used to form second layer  123  can be in the form of nanoparticles, such as metal nanoparticles. The nanoparticles can have any suitable shape, such as a spherical, cylindrical, rod-like, or needle-like shape. 
         [0057]    In some embodiments, second layer  123  can include combinations of electrically conductive materials described above. 
         [0058]    In some embodiments, second layer  123  can be formed of the same hole carrier material (e.g., a polythiophene) as that used to form hole carrier layer  140 . In such embodiments, without wishing to be bound by theory, it is believed that the material used in second layer  123  functions as an electrically conductive material that can transport charge carriers (e.g., electrons) from the hole blocking material used in third layer  125  to grid  122 . 
         [0059]    In general, the thickness of second layer  123  can be varied as desired. Typically, the thickness of second layer  123  is at least about 100 nm (e.g., at least about 150 nm, at least about 200 nm, at least about 250 nm, or at least about 300 nm) and/or at most about 450 nm (e.g., at most about 400 nm, at most about 350 nm, at most about 300 nm, at most about 250 nm, at most about 200 nm, or at most about 150 nm). Without wishing to be bound by theory, it is believed that, as surface  126  on first layer  121  has a relatively low roughness, second layer  123  do not need have a large thickness to cover the peaks on surface  126  to prevent short circuit and therefore can be made relatively thin to minimize absorption loss of incident light while still providing sufficient electrical conductivity to transport charge carriers to grid  122 . 
         [0060]    In some embodiments, second layer  123  can have a thickness of between about 100 nm and about 200 nm. Without wishing to be bound by theory, it is believed that including second layer  123  having the above thickness can provide sufficient electrical conductivity to electrode  120  while still maintain the transparency of this electrode. Further, without wishing to be bound by theory, it is believed that including second layer  123  having the above thickness in a bottom electrode can result in a photovoltaic cell with significantly higher current density than a photovoltaic cell including an ITO/metal/ITO bottom electrode. 
         [0061]    Without wishing to be bound by theory, it is believed that including second layer  123  in electrode  120  can facilitate transport of charge carriers (e.g., electrons) from the space between the grid lines formed by grid  122  to grid  122 , thereby improving the efficiency of photovoltaic cell  100 . 
         [0062]    In some embodiments, when second layer  123  contains electrically conductive metal (e.g., metal nanoparticles), the metal can be applied to first substrate  201  shown in  FIG. 2(   a ) before grid  222  is disposed on first substrate  201 . In such embodiments, after first substrate  201  is peeled off as shown in  FIG. 2(   c ), second layer  123  is formed as a smooth layer on top of the layer containing grid  222  and a cured material. Without wishing to be bound by theory, it is believed that this process can significantly minimize the possibility that the metal nanoparticles causes short circuit in photovoltaic cell  100  as compared to applying the metal nanoparticles directly onto a smooth layer containing grid  222  and a cured material formed in the process shown in  FIGS. 2(   a )- 2 ( c ). 
         [0063]    Third layer  125  generally includes a hole blocking material and an electrically conductive metal oxide (e.g., ITO). The hole blocking material is generally a material that, at the thickness used in photovoltaic cell  100 , transports electrons to grid  122  and substantially blocks the transport of holes to grid  122 . Exemplary hole blocking materials include polyamines (e.g., polyethylenimines). As used herein, the term “polyamine” refers to a polymer having two or more amino groups, including primary amino groups, secondary amino groups, and tertiary amino groups. Other examples of the hole blocking material have been described in, e.g., commonly-owned co-pending U.S. Application Publication No. 2008-0264488. 
         [0064]    Without wishing to be bound by theory, it is believed that when photovoltaic cell  100  includes a hole blocking material, the hole blocking material can facilitate the formation of ohmic contact between photoactive layer  130  and electrode  120  without being exposed to UV light, thereby reducing damage to photovoltaic cell  100  resulted from UV exposure. 
         [0065]    In some embodiments, third layer  125  can further include a cross-linking agent. Exemplary cross-linking agents include epoxy-containing compounds, such as glycerol triglycidyl ether and glycerol diglycidyl ether (e.g., 1,3-diglycidyl glyceryl ether). In certain embodiments, at least some of the molecules of the hole blocking material can be cross-linked via the cross-linking agent. 
         [0066]    Exemplary electrically conductive metal oxide that can be used in third layer  125  include ITO, tin oxide (e.g., fluorinated tin oxide), titanium oxide, zinc oxide, copper oxide, strontium copper oxide, and strontium titanium oxide. In some embodiments, the electrically conductive metal oxide can be in the form of nanoparticles (e.g., ITO nanoparticles). 
         [0067]    In some embodiments, when hole carrier layer is present in photovoltaic cell  100 , third layer  125  can have a work function different from hole carrier layer  140 , thereby creating an electric field to facilitate separation of charge carriers (i.e., electrons or holes) and transfer charge carriers from photoactive layer  130  to electrodes  120  and  150  during use of photovoltaic cell  100 . Without wishing to be bound by theory, it is believed that, due to this work function difference, one can include the material used in hole carrier layer  140  into second layer  123  to serve as an electrically conductive material to facilitate transport of charge carriers (e.g., electrons) from the space between the grid lines formed by grid  122  to grid  122 . Without wishing to be bound by theory, the above configuration would not be feasible in the absence of third layer  125  as there would not electric field in photovoltaic cell  100  to facilitate separation and transfer of charge carriers. Further, without wishing to be bound by theory, due to the work function difference between third layer  125  and hole carrier layer  140 , one can use the same material (e.g., a metal such as silver) to form electrode  150  and grid  122  for the same reason set forth above. In addition, without wishing to be bound by theory, it is believed that including an electrically conductive metal oxide can significantly increase the electrical conductivity of third layer  125  (as the hole blocking material in layer  125  is generally electrically insulating) and improve the conductivity of electrode  120 . 
         [0068]    In some embodiments, the electrically conductive metal oxide, the hole blocking material, and the cross-linking agent in third layer  125  can have a weight ratio of at least about 1:1:1 (e.g., at least about 2:1:1, at least about 3:1:1, or at least about 4:1:1) and/or at most about 8:1:1 (e.g., at most about 7:1:1, at most about 6:1:1, or at most about 5:1:1). Without wishing to be bound by theory, the inventors unexpected discovered that lowering the level of the electrically conductive metal oxide (e.g., to about 3:1:1 or less) can significantly reducing shunting of photovoltaic cell  100 . On the other hand, without wishing to be bound by theory, it is believed that photovoltaic cell  100  having a level of the electrically conductive metal oxide that is too low (e.g., less than to about 1:1:1) can significantly lower the fill factor. 
         [0069]    In some embodiments, third layer  125  has a thickness of at most about 150 nm (e.g., at most about 120 nm, at most about 100 nm, at most about 50 nm, or at most about 10 nm) or at least about 20 nm (e.g., at least about 30 nm, at least about 40 nm, at least about 50 nm). In some embodiments, third layer  125  has a thickness between about 40 nm and about 100 nm (e.g., about 50, 60, or 75 nm). 
         [0070]    In some embodiments, electrode  120  is a bottom electrode in photovoltaic cell  100 . As used herein, the term “bottom electrode” refers to an electrode onto which other layers (e.g., a photoactive layer and the counter electrode) are applied to form a photovoltaic cell. In such embodiments, electrode  150  is a top electrode in photovoltaic cell  100 . 
         [0071]    In general, electrode  120  is transparent. For example, electrode  120  can transmit at least about 70% (e.g., at least about 75%, at least about 80%, at least about 85%, at least about 90%, or at least about 95%) 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. Without wishing to be bound by theory, it is believed that an advantage of electrode  120  is that it possesses a flatter transmission curve compared to rounded transmission curves of a conventional ITO or IMI electrode, and therefore is generally more transparent in a wide range of light wavelength (e.g., from about 350 nm to about 1,000 nm) than such a conventional electrode. In some embodiments, electrode  120  can transmit between 72% and 90% (e.g., between 75% and 85%) of incident light. 
         [0072]    In general, electrode  120  is electrically conductive. For example, electrode  120  can have a sheet resistance of at most about 10 ohm/square (e.g., at most about 8 ohm/square, at most about 6 ohm/square, at most about 4 ohm/square, at most about 2 ohm/square, at most about 1 ohm/square) and/or at least about 0.01 ohm/square (e.g., at least about 0.05 ohm/square, at least about 0.1 ohm/square, or at least about 0.5 ohm/square). In some embodiments, electrode  120  can have a sheet resistance between about 4 ohm/square and about 6 ohm/square (e.g., 5 ohm/square). 
         [0073]    Without wishing to be bound by theory, it is unexpectedly discovered that including all of first layer  121 , second layer  123 , and third layer  125  described above in electrode  120  can result in an electrode that exhibits both improved light transmittance and improved electrical conductivity compared to a conventional electrode (e.g., an indium tin oxide (ITO) electrode or an ITO/metal/ITO electrode). As a result, a photovoltaic cell containing such an electrode can have improved efficiency and cell performance. 
         [0074]    Turning to other components in photovoltaic cell  100 , substrate  110  is generally formed of a transparent material. 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  110  can be formed of different materials. 
         [0075]    In general, substrate  110  can be flexible, semi-rigid or rigid (e.g., glass). In some embodiments, substrate  110  has a flexural modulus of less than about 5,000 megaPascals (e.g., less than about 1,000 megaPascals or less than about 500 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). 
         [0076]    Typically, substrate  110  is at least about one micron (e.g., at least about five microns or at least about 10 microns) thick and/or at most about 1,000 microns (e.g., at most about 500 microns, at most about 300 microns, at most about 200 microns, at most about 100 microns, or at most about 50 microns) thick. 
         [0077]    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. 
         [0078]    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  110  can, for example, be curved or stepped. In some embodiments, a non-planar surface of substrate  110  is patterned (e.g., having patterned steps to form a Fresnel lens, a lenticular lens or a lenticular prism). 
         [0079]    Photoactive layer  130  is generally formed of an electron acceptor material (e.g., an organic electron acceptor material) and an electron donor material (e.g., an organic electron donor material). 
         [0080]    In some embodiments, the electron donor material can include a photoactive polymer (e.g., a conjugated polymer) containing one or more of the following monomer repeat units: a benzodithiophene moiety, a cyclopentadithiazole moiety, a benzothiadiazole moiety, a thiadiazoloquinoxaline moiety, a benzoisothiazole moiety, a benzothiazole moiety, a dithienopyrrole moiety, a dibenzosilole moiety, a thienothiophene moiety, a carbazole moiety, a dithienothiophene moiety, a tetrahydroisoindole moiety, a fluorene moiety, a silole moiety, a cyclopentadithiophene moiety, a thiazole moiety, a selenophene moiety, a thiazolothiazole moiety, a naphthothiadiazole moiety, a thienopyrazine moiety, a silacyclopentadithiophene moiety, a thiophene moiety, an oxazole moiety, an imidazole moiety, a pyrimidine moiety, a benzoxazole moiety, a benzimidazole moiety, a quinoxaline moiety, a pyridopyrazine moiety, a pyrazinopyridazine moiety, a pyrazinoquinoxaline moiety, a thiadiazolopyridine moiety, a thiadiazolopyridazine moiety, a benzooxadiazole moiety, an oxadiazolopyridine moiety, an oxadiazolopyridazine moiety, a benzoselenadiazole moiety, a benzobisoxazole moiety, a thienothiadiazole moiety, a thienopyrroledione moiety, or a tetrazine moiety. The moieties mentioned above can be substituted by one or more halo (e.g., F, Cl, Br, or I), C 1 -C 24  alkyl, C 1 -C 24  alkoxy, aryl, heteroaryl, C 3 -C 24  cycloalkyl, C 3 -C 24  heterocycloalkyl, COR, COOR, CO—N(RR′), CN, or SO 3 R, in which each of R and R′, independently, is H, C 1 -C 24  alkyl, aryl, heteroaryl, C 3 -C 24  cycloalkyl, or C 3 -C 24  heterocycloalkyl. The photoactive polymer can be a homopolymer or a copolymer containing two or more of the above monomer repeat units. 
         [0081]    In some embodiments, electron acceptor materials of photoactive layer  130  can include fullerenes. In some embodiments, photoactive layer  130  can include one or more unsubstituted fullerenes and/or one or more substituted fullerenes. Examples of unsubstituted fullerenes include C 60 , C 70 , C 76 , C 78 , C 82 , C 84 , and C 92 . Examples of substituted fullerenes include fullerene substituted with phenyl-butyric acid methyl esters (PCBMs, such as C61-PCBM and C71-PCBM) or fullerenes substituted with C 1 -C 20  alkoxy optionally further substituted with C 1 -C 20  alkoxy and/or halo (e.g., (OCH 2 CH 2 ) 20 CH 3  or OCH 2 CF 2 OCF 2 CF 2 OCF 3 ). Without wishing to be bound by theory, it is believed that fullerenes substituted with long-chain alkoxy groups (e.g., oligomeric ethylene oxides) or fluorinated alkoxy groups have improved solubility in organic solvents and can form a photoactive layer with improved morphology. Other examples of fullerenes have been described in, e.g., commonly-owned co-pending U.S. Application Publication No. 2005-0279399. In some embodiments, the electron acceptor material can include one or more of the photoactive polymers described in the preceding paragraph. In certain embodiments, a combination of electron acceptor materials (e.g., a fullerene and a photoactive polymer described above) can be used in photoactive layer  130 . 
         [0082]    Other examples of electron donor materials and electron accepting materials have been described in, e.g., commonly-owned U.S. Pat. Nos. 7,772,485, and 7,781,673, U.S. Application Publication Nos. 2007-0017571, 2007-0020526, 2008-0087324, 2008-0121281, and 2010-0032018, and PCT Application No PCT/US2011/020227. 
         [0083]    In general, the thickness of photoactive layer  130  should be sufficiently large to absorb incident light to generate electrons and holes, and sufficiently small to allow transport of the generated electrons or holes to a neighboring layer. Typically, the thickness of photoactive layer  130  is at least about 50 nm (e.g., at least about 75 nm, at least about 100 nm, at least about 125 nm, at least about 150 nm, or at least about 200 nm) and/or at most about 300 nm (e.g., at most about 250 nm, at most about 200 nm, or at most about 150 nm). In some embodiments, the thickness of photoactive layer  130  is between about 210 nm and about 230 nm (e.g., about 220 nm). 
         [0084]    Optionally, photovoltaic cell  100  can include a hole carrier layer  140 . Hole carrier layer  140  is generally formed of a material that, at the thickness used in photovoltaic cell  100 , transports holes to electrode  150  and substantially blocks the transport of electrons to electrode  150 . Examples of materials from which layer  140  can be formed include polythiophenes (e.g., PEDOT), polyanilines, polycarbazoles, polyvinylcarbazoles, polyphenylenes, polyphenylvinylenes, polysilanes, polythienylenevinylenes, polyisothianaphthanenes, and copolymers thereof. In some embodiments, hole carrier layer  140  can include a dopant used in combination with one of the just-mentioned materials. Examples of dopants include poly(styrene-sulfonate)s, polymeric sulfonic acids, or fluorinated polymers (e.g., fluorinated ion exchange polymers). 
         [0085]    In some embodiments, the materials that can be used to form hole carrier layer  140  include metal oxides, such as tungsten oxide or molybdenum oxide. The metal oxides can be either undoped or doped with a dopant. Examples of dopants for metal oxides include salts or acids of fluoride, chloride, bromide, and iodide. 
         [0086]    In some embodiments, hole carrier layer  140  can include combinations of hole carrier materials described above. 
         [0087]    In general, the thickness of hole carrier layer  140  (i.e., the distance between the surface of hole carrier layer  140  in contact with photoactive layer  130  and the surface of electrode  150  in contact with hole carrier layer  140 ) can be varied as desired. Typically, the thickness of hole carrier layer  140  is at least about 100 nm (e.g., at least about 150 nm, at least about 200 nm, at least about 250 nm, at least about 300 nm) and/or at most about 500 nm (e.g., at most about 450 nm, at most about 400 nm, at most about 350 nm, at most about 300 nm, at most about 250 nm, or at most about 200 nm). In some embodiments, the thickness of hole carrier layer  140  is from about 180 nm to about 220 nm. 
         [0088]    Electrode  150  is generally formed of an electrically conductive material, such as one or more of the electrically conductive materials described above with respect to grid  122 . In some embodiments, electrode  150  is formed of a combination of electrically conductive materials. In certain embodiments, electrode  150  can be formed of a mesh or grid electrode. In other embodiments, electrode  150  can be formed from metal nanoparticles (e.g., silver nanorods) dispersed in a polymer matrix, such as that disclosed in commonly-owned co-pending U.S. Application Publication No 2008-0236657. 
         [0089]    In general, substrate  160  can serve as a barrier layer and can be made from materials that sufficiently prevent diffusion of oxygen or moisture in the environment into photovoltaic cell  100 . In some embodiments, substrate  160  has a multilayer structure and includes a composite containing at least two (e.g., at least three) polymer layers (e.g., each containing a polyester such as a polyethylene terephthalate or a polyethylene naphthalate) and at least two ceramic layers (e.g., each containing silica or alumina) between the at least two polymer layers. In such embodiments, substrate  160  can have three polymer layers and two ceramic layers in the following order: a polymer layer, a ceramic layer, a polymer layer, a ceramic layer, and a polymer layer. 
         [0090]    In some embodiments, when electrode  150  is made from a transparent material, substrate  160  can also be made from a transparent material (e.g., the multilayer structure described in the preceding paragraph). In certain embodiments, when electrode  150  is not transparent, barrier layer  190  can include a metal foil. Without wishing to be bound by theory, it is believed that a metal foil is generally a better barrier material than a composite material formed from polymers and ceramics. On the other hand, as a metal foil is generally less transparent than a composite material formed from polymers and ceramics, it is typically used only on the side of photovoltaic cell  100  that is not transparent. 
         [0091]    In some embodiments, photovoltaic cell  100  can include a buffer layer between electrode  150  and substrate  160  to prevent delamination of substrate  160  and to increase the useable life of photovoltaic cell  100 . Examples of such a buffer layer have been described in, for example, commonly-owned co-pending PCT/US2011/027722. 
         [0092]    In general, the methods of preparing layers  123 ,  125 ,  130 ,  140 , and  150  in photovoltaic cell  100  described above can vary as desired. In some embodiments, such a layer can be prepared by a liquid-based coating process. In certain embodiments, such a layer can be prepared by a gas phase-based coating process, such as chemical or physical vapor deposition processes. 
         [0093]    The term “liquid-based coating process” mentioned herein refers to a process that uses a liquid-based coating composition. Examples of the liquid-based coating composition include solutions, dispersions, or suspensions. The liquid-based coating process can be carried out by using at least one of the following processes: solution coating, ink jet printing, spin coating, dip coating, knife coating, bar coating, spray coating, roller coating, slot coating, gravure coating, flexographic printing, or screen printing. Examples of liquid-based coating processes have been described in, for example, commonly-owned co-pending U.S. Application Publication No. 2008-0006324. 
         [0094]    In some embodiments, when a layer includes inorganic semiconductor nanoparticles, the liquid-based coating process can be carried out by (1) mixing the nanoparticles with a solvent (e.g., an aqueous solvent or an anhydrous alcohol) to form a dispersion, (2) coating the dispersion onto a substrate, and (3) drying the coated dispersion. For example, third layer  125  can be prepared by mixing metal oxide nanoparticles (e.g., ITO nanoparticles) with a solution containing a polyamine (e.g., a polyethylenimine) and a solvent to form a dispersion, coating the dispersion on second layer  123 , and drying (e.g., by heating) the coated dispersion. The solution containing the polyamine can further include a cross-linking agent (e.g., glycerol diglycidyl ether), which can cross-link the polyamine during the drying process. In certain embodiments, a liquid-based coating process for preparing a layer containing inorganic metal oxide nanoparticles can be carried out by (1) dispersing a precursor (e.g., a titanium salt) in a suitable solvent (e.g., an anhydrous alcohol) to form a dispersion, (2) coating the dispersion on a substrate, (3) hydrolyzing the dispersion to form an inorganic semiconductor nanoparticles layer (e.g., a titanium oxide nanoparticles layer), and (4) drying the inorganic semiconductor material layer. In certain embodiments, the liquid-based coating process can be carried out by a sol-gel process (e.g., by forming metal oxide nanoparticles as a sol-gel in a dispersion before coating the dispersion on a substrate). 
         [0095]    In general, the liquid-based coating process used to prepare a layer containing an organic semiconductor material can be the same as or different from that used to prepare a layer containing an inorganic semiconductor material. In some embodiments, to prepare a layer includes an organic semiconductor material, the liquid-based coating process can be carried out by mixing the organic semiconductor material with a solvent (e.g., an organic solvent) to form a solution or a dispersion, coating the solution or dispersion on a substrate, and drying the coated solution or dispersion. For example, second layers  123 , photoactive layer  130 , and hole carrier layer  140  can be prepared by this method. 
         [0096]    In some embodiments, electrode  150  can be prepared either by a gas phase-based coating process (e.g., applying a metal layer onto optional hole carrier layer  140  by using a chemical vapor deposition process) or a liquid-based coating process (e.g., applying a metal mesh layer onto optional hole carrier layer  140  by an inkjet printing process). 
         [0097]    In some embodiments, substrate  160  can be attached to electrode  150  by methods known in the art (e.g., by using an adhesive). 
         [0098]    In some embodiments, photovoltaic cell  100  can be prepared in a continuous manufacturing process, such as a roll-to-roll process, thereby significantly reducing the manufacturing cost. Examples of roll-to-roll processes have been described in, for example, commonly-owned co-pending U.S. Application Publication No. 2005-0263179. 
         [0099]    In general, during use, incident light can impinge on the surface of substrate  110 , and passes through substrate  110  and electrode  120 . The light then interacts with photoactive layer  130 , causing electrons to be transferred from the electron donor material (e.g., a conjugated polymer) to the electron acceptor material (e.g., a substituted fullerene). The electron acceptor material then transmits the electrons through third layer  125  and second layer  123  to grid  122  in electrode  120 , and the electron donor material transfers holes through hole carrier layer  140  to electrode  150 . Electrodes  120  and  150  are in electrical connection via an external load  170  so that electrons pass from electrode  120  through the load to electrode  150 . When substrate  160  and electrode  150  are transparent, incident light can also pass through these two layers to interact with photoactive layer  130  to generate electrons and holes. 
         [0100]    While certain embodiments have been disclosed, other embodiments are also possible. 
         [0101]    In some embodiments, photovoltaic cell  100  can be a tandem photovoltaic cell, such as those described in commonly-owned co-pending U.S. Application Publication Nos. 2007-0246094, 2007-0181179, 2007-0272296, and 2009-0211633. 
         [0102]    In some embodiments, photovoltaic cell  100  includes a cathode as a bottom electrode and an anode as a top electrode. In some embodiments, photovoltaic cell  100  can include an anode as a bottom electrode and a cathode as a top electrode. 
         [0103]    In some embodiments, photovoltaic cell  100  can include the layers shown in  FIG. 1  in a reverse order. In other words, photovoltaic cell  100  can include these layers from the bottom to the top in the following sequence: a substrate  160 , an electrode  150 , an optional hole carrier layer  140 , a photoactive layer  130 , an electrode  120  containing a third layer  125 , a second layer  123 , and a first layer  121 , and a substrate  110 . In such embodiments, electrode  150  is a bottom electrode and electrode  120  is a top electrode. In such embodiments, without wishing to be bound by theory, the surface on first layer  121  contacting second layer  123  can have a surface roughness larger than that of surface  126  shown in  FIG. 1  as the roughness of this surface is not critical in preventing or minimizing short circuit of photovoltaic cell  100 . 
         [0104]    In some embodiments, multiple photovoltaic cells can be electrically connected to form a photovoltaic system. As an example,  FIG. 3  is a schematic of a photovoltaic system  300  having a module  310  containing a plurality of photovoltaic cells  320 . Cells  320  are electrically connected in series, and system  300  is electrically connected to a load  330 . As another example,  FIG. 4  is a schematic of a photovoltaic system  400  having a module  410  that contains a plurality of photovoltaic cells  420 . Cells  420  are electrically connected in parallel, and system  400  is electrically connected to a load  430 . In some embodiments, some (e.g., all) of the photovoltaic cells in a photovoltaic system can be disposed on 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. 
         [0105]    While organic photovoltaic cells have been described, electrode  120  described herein can also be used in other photovoltaic cells. Examples of such photovoltaic cells include dye sensitized photovoltaic cells and inorganic photoactive cells with a photoactive material formed of amorphous silicon, cadmium selenide, cadmium telluride, copper indium selenide, and copper indium gallium selenide. In some embodiments, electrode  120  can be used in a hybrid photovoltaic cell. 
         [0106]    While photovoltaic cells have been described above, in some embodiments, electrode  120  described herein can be used in other devices and systems. For example, electrode  120  can be used in suitable organic semiconductive devices, such as field effect transistors, photodetectors (e.g., IR detectors), photovoltaic detectors, imaging devices (e.g., RGB imaging devices for cameras or medical imaging systems), light emitting diodes (LEDs) (e.g., organic LEDs (OLEDs) or IR or near IR LEDs), lasing devices, conversion layers (e.g., layers that convert visible emission into IR emission), amplifiers and emitters for telecommunication (e.g., dopants for fibers), storage elements (e.g., holographic storage elements), and electrochromic devices (e.g., electrochromic displays). 
         [0107]    The contents of all publications cited herein (e.g., patents, patent application publications, and articles) are hereby incorporated by reference in their entirety. 
         [0108]    The following examples are illustrative and not intended to be limiting. 
       EXAMPLE 1 
     Method of Forming a Smooth Electrode Layer 
       [0109]    Two monomers, SYNASIA S-21 (3,4-epoxycyclohexylmethyl-3,4-epoxycyclohexanecarboxylate, 220 g) and SYNASIA S-28 (bis((3,4-epoxycyclohexyl)methyl)adipate, 200 g) obtained from Synasia Inc. (Metuchen, N.J.) were added to a 500 ml wide mouth amber glass bottle and mixed by using an overhead stainless steel stirrer. Under subdued light, a solid photoinitiator bis(4-tert-butylphenyl)iodonium hexafluoroantimonate (4.4 g) obtained from Hampford Research (Stratford, Conn.) and a sensitizer isopropylthioxanthone (2.2 g) obtained from Lambson Ltd. (West Yorkshire, UK) were added sequentially with continued stirring. The bottle was loosely sealed around the stirrer shaft with aluminum foil and the stirring was continued for 4 hours at room temperature, at which time all of the solids had dissolved. After the stirrer was removed, the bottle was sealed with a Teflon lined screw cap. The solution thus obtained was used in the next step as the filler solution. 
         [0110]    An ink containing silver nanoparticles having an average diameter of about 10-30 nm dispersed in an aqueous solution was printed on a smooth surface of a first PET substrate (i.e., a release substrate) to form a metal grid. The article thus formed was then brought into contact with a second PET substrate (i.e., a receiving substrate) with a primer layer having a thickness of about 0.5 micron. The primer layer was formed by coating a blend of L7063/VITEL 2700/BOSCODUR 16 obtained from Bostik, Inc. (Middleton, Mass.) at a dry weight ratio of 1.67:1.0:0.054 to the second PET substrate. The filler solution prepared above was then disposed into the gap between the two PET 7″ wide substrates at a flow rate of 0.8 ml/minute. The article thus formed was then passed through a pair of nip rollers to laminate the two PET substrates together at a web speed of 2 fpm. The nip rollers had a nip pressure of 10 psi, an unwind tension of 3.0 psi, and a rewind tension of 3.0 psi. The filler solution was then cured by exposing the laminate to the light from 2 Phoseon LED lamps (390 nm peak emission, 1.1 mJ/cm2 delivered by each) positioned on each side of the laminate. The article thus formed was post cured by heating at 60° C. for 20 hours in roll format or by heating at 85° C. for 30 minutes to obtain optimal cure. The release substrate was then peeled off to form a silver grid electrode embedded in a hardened electrically insulating polymer layer on the receiving substrate. 
         [0111]    The smoothness of the embedded grid was measured by using an Ambios XP-1 surface profilometer. In surface scans of 5 mm sections of the grid that crossed 9 individual lines, the root mean square surface roughness RMS was measured to be about 5 nm. 
       EXAMPLE 2 
     Photovoltaic Cells Containing Different Amounts of Electrically Conductive Polymer in the Middle Layer in Bottom Electrode 
       [0112]    Photovoltaic cell (1) in this Example was prepared as follows: The electrode layer prepared in Example 1 was coated with a layer containing ITO (an electrically conductive oxide), polyethylenimine (PEI; a hole blocking material) and diglycidyl glycerol either (DEG, a cross-linking agent) by using a dispersion containing these three compounds to form a bottom electrode. The ratio of the weight of ITO and the total weight of PEI and DEG was about 8:1. The article thus formed was subsequently coated with a photoactive layer containing a conjugated polymer and PCBM, a hole carrier layer containing a PEDOT:PSS, and a silver top electrode. Photovoltaic cells (2) and (3) were prepared in a manner similar to that of photovoltaic cells (1) except that PEDOT layers that were 20% and 40% by weight of the bottom electrode were respectively coated onto the electrode layer prepared in Example 1 before the ITO/PEI-DEG layer was coated. The PEDOT layers were prepared using ORGACON HBS5 obtained from Agfa. Photovoltaic cells (4)-(6) were prepared in a manner similar to Photovoltaic cells (1)-(3), respectively, except that the electrode layer prepared in Example 1 was replaced with an ITO/metal/ITO (IMI) electrode. 
         [0113]    The properties of photovoltaic cells (1)-(6) were measured under 1.5 AM conditions. The results are summarized in Table 1 below. 
         [0000]    
       
         
               
               
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                   
                   
                 PEDOT 
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                   
                   
                 level in 
               
               
                   
                 ITO/PEI- 
                 bottom 
                 V oc   
                 J sc   
                 V max   
                 J max   
                 Eff 
                   
                 R series   
               
               
                 Cell 
                 DEG: %/% 
                 electrode 
                 (V) 
                 (mA/cm 2 ) 
                 (V) 
                 (mA/cm 2 ) 
                 (%) 
                 FF 
                 (Ω/Sq) 
                 Yield 
               
               
                   
               
             
             
               
                 (1) 
                 8/1 
                 Control 
                 0.534 
                 8.65 
                 0.396 
                 6.83 
                 2.70 
                 58.50 
                 10.74 
                 12 of 12 
               
               
                 (2) 
                 8/1 
                 20% 
                 0.532 
                 9.29 
                 0.387 
                 7.31 
                 2.83 
                 57.27 
                 11.07 
                 11 of 12 
               
               
                 (3) 
                 8/1 
                 40% 
                 0.541 
                 8.50 
                 0.372 
                 6.25 
                 2.33 
                 50.58 
                 15.03 
                 10 of 12 
               
               
                 (4) 
                 PEI-DEG 
                 Control 
                 0.521 
                 9.93 
                 0.356 
                 7.57 
                 2.70 
                 52.16 
                 14.36 
                  9 of 12 
               
               
                   
                 only 
               
               
                 (5) 
                 PEI-DEG 
                 20% 
                 0.533 
                 9.58 
                 0.336 
                 6.51 
                 2.19 
                 42.90 
                 20.17 
                  7 of 12 
               
               
                   
                 only 
               
               
                 (6) 
                 PEI-DEG 
                 40% 
                 0.526 
                 8.93 
                 0.333 
                 5.81 
                 1.93 
                 41.10 
                 21.87 
                 3 of 8 
               
               
                   
                 only 
               
               
                   
               
             
          
         
       
     
         [0114]    The results showed that photovoltaic cells (1)-(3) all possessed bottom electrodes with significantly lower sheet resistance than the bottom electrode in photovoltaic cells (4)-(6). Further, photovoltaic cells (1)-(3) all exhibited significantly better fill factors and production yields than those of photovoltaic cells (4)-(6), respectively. In addition, a comparison between cells (3) and (4) showed that including a certain amount of PEDOT in the bottom electrode lowered cell efficiency when the bottom electrode included IMI. Unexpectedly, a comparison between cells (1) and (2) showed that including a certain amount of PEDOT in the bottom electrode increased cell efficiency when the bottom electrode included silver grid. 
       EXAMPLE 3 
     Photovoltaic Cells Containing Different Amounts of Electrically Conductive Metal Oxide in the Hole Blocking Material in Bottom Electrode 
       [0115]    Photovoltaic cells (7)-(12) were prepared in a manner similar to photovoltaic cell (1) described in Example 2 above except that they including the following weight ratio between ITO, PEI, and DEG: (7) 1:1:1, (8) 2:1:1, (9) 3:1:1, (10) 4:1:1, (11) 5:1:1, and (12) 3:1:1. 
         [0116]    The properties of photovoltaic cells (7)-(12) were measured under 1.5 AM conditions. The results are summarized in Table 2 below. 
         [0000]    
       
         
               
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                   
                   
                 Cell 
                   
                 J sc   
                 V max   
                 J max   
                 % 
                   
                 Rs 
                   
               
               
                 Cell 
                 ITO/PEI/DEG 
                 Area 
                 V oc  (V) 
                 (mA/cm 2 ) 
                 (V) 
                 (mA/cm 2 ) 
                 Eff 
                 FF 
                 (Ω/Sq) 
                 Yield 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                  (7) 
                 1/1/1 
                 0.69 
                 0.511 
                 10.18 
                 0.32 
                 6.96 
                 2.26 
                 43.33 
                 19.62 
                 12 of 12 
               
               
                  (8) 
                 2/1/1 
                 0.75 
                 0.503 
                 11.14 
                 0.35 
                 8.37 
                 2.92 
                 52.03 
                 11.63 
                 12 of 12 
               
               
                  (9) 
                 3/1/1 
                 0.67 
                 0.502 
                 11.72 
                 0.36 
                 8.87 
                 3.20 
                 54.43 
                 9.39 
                 8 of 8 
               
               
                 (10) 
                 4/1/1 
                 0.72 
                 0.500 
                 11.26 
                 0.36 
                 8.62 
                 3.07 
                 54.48 
                 9.54 
                 12 of 12 
               
               
                 (11) 
                 5/1/1 
                 0.78 
                 0.505 
                 11.55 
                 0.36 
                 8.97 
                 3.22 
                 55.25 
                 9.49 
                  8 of 12 
               
               
                 (12) 
                 8/1/1 
                 0.70 
                 0.515 
                 10.68 
                 0.37 
                 8.16 
                 3.05 
                 55.53 
                 10.05 
                 8 of 8 
               
               
                   
               
             
          
         
       
     
         [0117]    The results showed that photovoltaic cells containing from the ITO/PEI/DEG at a weight ratio of from about 3/1/1 to about 5/1/1 exhibited improved cell efficiencies. 
         [0118]    Other embodiments are in the claims.