Abstract:
Polymers with low band gaps and high charge mobility, as well as related systems, methods and components are disclosed.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]     This application is a continuation-in-part of U.S. Utility application Ser. No. 11/375,643, filed Mar. 14, 2006, which claims priority to U.S. Provisional Application Ser. No. 60/699,123, filed Jul. 14, 2005, the contents of which are hereby incorporated by reference. 
     
    
     TECHNICAL FIELD  
       [0002]     This invention generally relates to the field of electron donor materials, as well as related photovoltaic cells.  
       BACKGROUND OF THE INVENTION  
       [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. 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 can have a lower electrical conductivity than electrically conductive materials, the semiconductive material can transmit more light than many electrically conductive materials.  
       SUMMARY  
       [0004]     An aspect of the invention relates to a new combination of monomers that produce polymers, wherein the polymers have properties suitable for use as charge carriers in the active layer of a photovoltaic cell.  
         [0005]     In one aspect, the invention features a class of co-polymers including at least two co-monomers, at least one of which is a cyclopentadithiophene.  
         [0006]     In another aspect, this invention features a photovoltaic cell including a first electrode, a second electrode, and a photoactive material disposed between the first and second electrodes. The photoactive material includes a polymer having a first comonomer repeat unit and a second comonomer repeat unit. The first comonomer repeat unit includes a cyclopentadithiophene moiety. The second comonomer repeat unit includes a thienothiophene moiety, a thienothiophene tetraoxide moiety, a dithienothiophene moiety, a dithienothiophene dioxide moiety, a dithienothiophene tetraoxide moiety, or a tetrahydroisoindole moiety.  
         [0007]     In another aspect, this invention features a polymer that includes a first comonomer repeat unit containing a cyclopentadithiophene moiety, and a second comonomer repeat unit containing a thienothiophene moiety, a thienothiophene tetraoxide moiety, a dithienothiophene moiety, a dithienothiophene dioxide moiety, a dithienothiophene tetraoxide moiety, or a tetrahydroisoindole moiety.  
         [0008]     In another aspect, this invention features a photovoltaic cell including a first electrode, a second electrode, and a photoactive material disposed between the first and second electrodes. The photoactive material includes 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 includes a cyclopentadithiophene moiety.  
         [0009]     In a further aspect, this invention features a polymer that includes a first comonomer repeat unit containing a cyclopentadithiophene moiety, and a second comonomer repeat unit containing a benzothiadiazole moiety, a thiadiazoloquinoxaline moiety, a cyclopentadithiophene dioxide moiety, a cyclopentadithiophene monoxide moiety, a benzoisothiazole moiety, a benzothiazole moiety, a thiophene dioxide moiety, a dihydropentalenothiophene dioxide moiety, a cyclopentadithiophene tetraoxide moiety, or a moiety containing at least three thiophene moieties.  
         [0010]     In yet a further aspect, this invention features a polymer that includes a first comonomer repeat unit and a second comonomer repeat unit different from the first comonomer repeat unit. The first comonomer repeat unit contains a cyclopentadithiophene moiety substituted with at least one substituent selected from the group consisting of hexyl, ethylhexyl, dimethyloctyl, C 1 -C 20  alkoxy, aryl, heteroaryl, C 3 -C 20  cycloalkyl, and C 3 -C 20  heterocycloalkyl.  
         [0011]     Embodiments can include one or more of the following features.  
         [0012]     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, aryl, heteroaryl, C 3 -C 20  cycloalkyl, C 3 -C 20  heterocycloalkyl, halo, CN, NO 2 , and 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 3 -C 20  heterocycloalkyl. Examples of C 1 -C 20  alkyl can be hexyl, 2-ethylhexyl, or 3,7-dimethyloctyl.  
         [0013]     The cyclopentadithiophene moiety can be substituted at 4-position.  
         [0014]     The first comonomer repeat unit can include a cyclopentadithiophene moiety of formula (I):  
                         
 
 In formula (I), each of R 1 , R 2 , R 3 , and R 4 , independently, is H, C 1 -C 20  alkyl, C 1 -C 20  alkoxy, aryl, heteroaryl, C 3 -C 20  cycloalkyl, C 3 -C 20  heterocycloalkyl, halo, CN, NO 2 , 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 3 -C 20  heterocycloalkyl. In some embodiments, at least one of R 1  and R 2 , independently, is hexyl, 2-ethylhexyl, or 3,7-dimethyloctyl. In certain embodiments, each of R 1  and R 2 , independently, is hexyl, 2-ethylhexyl, or 3,7-dimethyloctyl. In some embodiments, one of R 1  and R 2  is hexyl, ethylhexyl, dimethyloctyl, C 1 -C 20  alkoxy, aryl, heteroaryl, C 3 -C 20  cycloalkyl, or C 3 -C 20  heterocycloalkyl, the other of R 1  and R 2  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. 
 
         [0015]     The second comonomer repeat unit can include a benzothiadiazole moiety, a thiadiazoloquinoxaline moiety, a cyclopentadithiophene dioxide moiety, a cyclopentadithiophene monoxide moiety, a benzoisothiazole moiety, a benzothiazole moiety, a thiophene dioxide moiety, or a cyclopentadithiophene tetraoxide moiety. In some embodiments, the second comonomer repeat unit can include a 3,4-benzo-1,2,5-thiadiazole moiety.  
         [0016]     In some embodiments, the second comonomer repeat unit can include a benzothiadiazole moiety of formula (II), a thiadiazoloquinoxaline moiety of formula (III), a cyclopentadithiophene dioxide moiety of formula (IV), a cyclopentadithiophene monoxide moiety of formula (V), a benzoisothiazole moiety of formula (VI), a benzothiazole moiety of formula (VII), a thiophene dioxide moiety of formula (VIII), a cyclopentadithiophene dioxide moiety of formula (IX), or a cyclopentadithiophene tetraoxide moiety of formula (X):  
                         
                         
 
 in which each of R 5 , R 6 , and R 7 , 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 can include a benzothiadiazole moiety of formula (II). In certain embodiments, R 5  and R 6  is H. 
 
         [0017]     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.  
         [0018]     The second comonomer repeat unit can also include a thienothiophene, thienothiophene tetraoxide, dithienothiophene, dithienothiophene dioxide, dithienothiophene tetraoxide, or tetrahydroisoindole moiety, each of which is optionally 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, C 3 -C 20  heterocycloalkyl, halo, CN, NO 2 , and 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 3 -C 20  heterocycloalkyl.  
         [0019]     In some embodiments, the second comonomer repeat unit can include a thienothiophene moiety of formula (XI), a thienothiophene tetraoxide moiety of formula (XII), a dithienothiophene moiety of formula (XIII), a dithienothiophene dioxide moiety of formula (XIV), a dithienothiophene tetraoxide moiety of formula (XV), or a tetrahydroisoindole moiety of formula (XVI):  
                         
 
 in which 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, aryl, heteroaryl, C 3 -C 20  cycloalkyl, C 3 -C 20  heterocycloalkyl, halo, CN, NO 2 , or SO 2 R, in which R is C 1 -C 20  alkyl, C 1 -C 20  alkoxy, aryl, heteroaryl, C 3 -C 20  cycloalkyl, or C 3 -C 20  heterocycloalkyl; and R 7  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. 
 
         [0020]     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.  
         [0021]     The photovoltaic cell can be a tandem photovoltaic cell.  
         [0022]     The photoactive material can further include an electron acceptor material. In some embodiments, the electron acceptor material can be a fullerene (e.g., C61-phenylbutyric acid methyl ester, PCBM).  
         [0023]     The polymer and the electron acceptor material each can have a LUMO energy level. The LUMO energy level of the polymer can be at least about 0.2 eV (e.g., at least about 0.3 eV) less negative than the LUMO energy level of the electron acceptor material.  
         [0024]     Embodiments can provide one or more of the following advantages.  
         [0025]     In some embodiments, using a polymer containing a cyclopentadithiophene moiety can be advantageous because the cyclopentadithiophene moiety can contribute to a shift in the maximum absorption wavelength toward the red or near IR region of the electromagnetic spectrum. When such a polymer is incorporated into a photovoltaic cell, the current and efficiency of the cell can increase.  
         [0026]     In some embodiments, a polymer fullerene cell containing a polymer described above can have a band gap that is relatively ideal for its intended purposes.  
         [0027]     In some embodiments, a photovoltaic cell having high cell voltage can be created, whereby the HOMO level of the polymer is at least about 0.2 electron volts more negative relative to the LUMO or conduction band of an electron acceptor material.  
         [0028]     In some embodiments, a photovoltaic cell containing a polymer described above can have relatively fast and efficient transfer of an electron to an electron acceptor material, whereby the LUMO of the donor is at least about 0.2 electron volt (e.g., at least about 0.3 electron volt) less negative than the conduction band of the electron acceptor material.  
         [0029]     In some embodiments, a photovoltaic cell containing a polymer described above can have relatively fast charge separation, whereby the charge mobility of the positive charge, or hole, is relatively high and falls within the range of 10 −4  to 10 −1  cm 2 /Vs.  
         [0030]     In some embodiments, the polymer is soluble in an organic solvent and/or film forming.  
         [0031]     In some embodiments, the polymer is optically non-scattering.  
         [0032]     In some embodiments, the polymer can be used in organic field effect transistors and OLEDs.  
         [0033]     Other features and advantages of the invention will be apparent from the description, drawings, and claims. 
     
    
     DESCRIPTION OF DRAWING  
       [0034]      FIG. 1  is a cross-sectional view of an embodiment of a photovoltaic cell.  
         [0035]      FIG. 2  is a schematic of a system containing one electrode between two photoactive layers. 
     
    
       [0036]     Like reference symbols in the various drawings indicate like elements.  
       DETAILED DESCRIPTION  
       [0037]      FIG. 1  shows a cross-sectional view of a photovoltaic cell  100  that includes a substrate  110 , a cathode  120 , a hole carrier layer  130 , an active layer  140  (containing an electron acceptor material and an electron donor material), a hole blocking layer  150 , an anode  160 , and a substrate  170 .  
         [0038]     In general, during use, light impinges on the surface of substrate  110 , and passes through substrate  110 , cathode  120 , and hole carrier layer  130 . The light then interacts with active layer  140 , causing electrons to be transferred from the electron donor material (e.g., a polymer described above) to the electron acceptor material (e.g., PCBM). The electron acceptor material then transmits the electrons through hole blocking layer  150  to anode  160 , and the electron donor material transfers holes through hole carrier layer  130  to cathode  120 . Anode  160  and cathode  120  are in electrical connection via an external load so that electrons pass from anode  160 , through the load, and to cathode  120 .  
         [0039]     Electron acceptor materials of active layer  140  can include fullerenes. In some embodiments, active layer  140  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 . An example of substituted fullerenes is PCBM. Other components in photovoltaic cell are known in the art, such as those described in U.S. patent application Ser. No. 10/723,554, the contents of which are incorporated herein by references.  
         [0040]     Electron donor materials of active layer  140  can include polymers, such as co-polymers. A polymers mentioned herein include at least two identical or different monomer repeat units (e.g., at least 5 monomer repeat units, at least 10 monomer repeat units, at least 50 monomer repeat units, at least 100 monomer repeat units, or at least 500 monomer repeat units). A copolymer mentioned herein refers to a polymer that includes at least two co-monomers of differing structures.  
               TABLE 1                                                                                                                                                                    
 
         [0041]    
       
         
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                   
               
             
             
               
                   
               
               
                 
                   
                             
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 
                   
                             
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 
                   
                             
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 
                   
                             
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 
                   
                             
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 
                   
                             
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 
                   
                             
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 
                   
                             
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 
                   
                             
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 
                   
                             
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 
                   
                             
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 
                   
                             
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 
                   
                             
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 
                   
                             
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 
                   
                             
                     
                         
                         
                     
                   
                 
               
               
                   
               
             
          
         
       
     
         [0042]     Table 1 above lists examples of the first co-monomer, which can be an electron donor and can serve as a conjugative link. Table 2 above lists examples of the second co-monomer, which can be an electron withdrawing monomer.  
         [0043]     Referring to formulas listed in Tables 1 and 2 above, each of X and Y, independently, can be CH 2 , O, or S; each of R 1 , R 2 , R 3 , R 4 , R 5 , and R 6 , independently, can be H, C 1 -C 20  alkyl, C 1 -C 20  alkoxy, aryl (e.g., phenyl or substituted phenyl), heteroaryl, C 3 -C 20  cycloalkyl, C 3 -C 20  heterocycloalkyl, halo, CN, NO 2 , or SO 2 R; and R 7  can be H, C 1 -C 20  alkyl, C 1 -C 20  alkoxy, aryl (e.g., phenyl or substituted phenyl), heteroaryl, C 3 -C 20  cycloalkyl, or C 3 -C 20  heterocycloalkyl; in which R is C 1 -C 20  alkyl, C 1 -C 20  alkoxy, aryl, heteroaryl, C 3 -C 20  cycloalkyl, or C 3 -C 20  heterocycloalkyl. 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═C 2 H 4 . 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 moieties 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.  
         [0044]     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, aryloxy, 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.  
         [0045]     The copolymers described above can be prepared by methods known in the art. 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 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.  
         [0046]     Table 3 below lists two exemplary polymers described in the Summary section above. These polymers can have unique properties, which make them particularly suitable as charge carriers in the active layer of a photovoltaic cell. These polymers can be obtained by the methods described in Examples 4 and 7 below.  
               TABLE 3                                                                                                                          
 
         [0047]     Generally, one co-monomer in the polymers described in the Summary section above is a cyclopentadithiophene. An advantage of a co-polymer containing a cyclopentadithiophene moiety is that its absorption wavelength can shift toward the red and near IR portion (e.g., 650-800 nm) of the electromagnetic spectrum, which is not accessible by most other polymers. When such a co-polymer is incorporated into a photovoltaic cell, it enables the cell to absorb the light in this region of the spectrum, thereby increasing the current and efficiency of the cell.  
         [0048]     The polymers described above can be useful in solar power technology because the band gap is close to ideal for a photovoltaic cell (e.g., a polymer-fullerene cell). The HOMO level of the polymers can be positioned correctly relative to the LUMO of an electron acceptor (e.g., PCBM) in a photovoltaic cell (e.g., a polymer-fullerene cell), allowing for high cell voltage. The LUMO of the polymers can be positioned correctly relative to the conduction band of the electron acceptor in a photovoltaic cell, thereby creating efficient transfer of an electron to the electron acceptor. For example, using a polymer having a band gap of about 1.4-1.6 eV can significantly enhance cell voltage. Cell performance, specifically efficiency, cam benefit from both an increase in photocurrent and an increase in cell voltage, and can approach and even exceed 15% efficiency. The positive charge mobility of the polymers can be relatively high and approximately in the range of 10 −4  to 10 −1  cm 2 /Vs. In general, the relatively high positive charge mobility allows for relatively fast charge separation. The polymers can also be soluble in an organic solvent and/or film forming. Further, the polymers can be optically non-scattering.  
         [0049]     In some embodiments, the polymer described above can be used as an electron donor material in a system in which two photovoltaic cells share a common electrode. Such a system is also known as tandem photovoltaic cell. Examples of tandem photovoltaic cells are discussed in U.S. patent application Ser. No. 10/558,878, filed Nov. 29, 2005, the contents of which are hereby incorporated by reference.  
         [0050]     As an example,  FIG. 2  is a schematic of a tandem photovoltaic cell  200  having a substrate  210 , three electrodes  220 ,  240 , and  260 , and two photoactive layers  230  and  250 . Electrode  240  is shared between photoactive layers  230  and  250 , and is electrically connected with electrodes  220  and  260 . In general, electrodes  220 ,  240 , and  260  can be formed of an electrically conductive material, such as those described in U.S. patent application Ser. No. 10/723,554. In some embodiments, one or more (i.e., one, two, or three) electrodes  220 ,  240 , and  260  is a mesh electrode. In some embodiments, one or more electrodes  220 ,  240 , and  260  is formed of a semiconductive material. Examples of semiconductive materials include titanium oxides, indium tin oxides, fluorinated tin oxides, tin oxides, and zinc oxides. In certain embodiments, one or more (i.e., one, two, or three) electrodes  220 ,  240 , and  260  are formed of titanium dioxide. Titanium dioxide used to prepare an electrode can be in any suitable forms. For example, titanium dioxide can be in the form of interconnected nanoparticles. Examples of interconnected titanium dioxide nanoparticles are described, for example, in U.S. Pat. No. 7,022,910, the contents of which are incorporated herein by reference. In some embodiments, at least one (e.g., one, two, or three) of electrodes  220 ,  240 , and  260  is a transparent electrode. As referred to herein, a transparent electrode is formed of a material which, at the thickness used in a photovoltaic cell, transmits at least about 60% (e.g., at least about 70%, at least about 75%, at least about 80%, at least about 85%, at least about 90%, at least about 95%) of incident light at a wavelength or a range of wavelengths used during operation of the photovoltaic cell. In certain embodiments, both electrodes  220  and  260  are transparent electrodes.  
         [0051]     Each of photoactive layers  230  and  250  can contain at least one semiconductive material. In some embodiments, the semiconductive material in photoactive layer  230  has the same band gap as the semiconductive material in photoactive layer  250 . In certain embodiments, the semiconductive material in photoactive layer  230  has a band gap different from that of the semiconductive material in photoactive layer  250 . Without wishing to be bound by theory, it is believed that incident light not absorbed by one photoactive layer can be absorbed by the other photoactive layer, thereby maximizing the absorption of the incident light.  
         [0052]     In some embodiments, at least one of photoactive layers  230  and  250  can contain an electron acceptor material (e.g., PCBM) and an electron donor material (e.g., a copolymer described above). In general, suitable electron acceptor materials and electron donor materials can be those described above. In certain embodiments, each of photoactive layers  230  and  250  contains an electron acceptor material and an electron donor material.  
         [0053]     Substrate  210  can be formed of one or more suitable polymers, such as those described in U.S. patent application Ser. No. 10/723,554. In some embodiments, an additional substrate (not shown in  FIG. 2 ) can be disposed on electrode  260 .  
         [0054]     Photovoltaic cell  200  can further contain a hole carrier layer (not shown in  FIG. 2 ) and a hole blocking layer (not shown in  FIG. 2 ), such as those described in U.S. patent application Ser. No. 10/723,554.  
         [0055]     The following examples are illustrative and not intended to be limiting.  
       EXAMPLE 1  
     Synthesis of 4,4-Dihexyl-4H-cyclopenta[2,1-b;3,4-b′]dithiophene  
       [0056]    
       
                 
         
             
             
         
       
     
         [0057]     4H-Cyclopenta[2,1-b;3,4-b′]dithiophene was synthesized according to literature procedure illustrated in Coppo et al.,  Macromolecules  2003, 36, 2705-2711. All other starting materials were purchased from Sigma-Aldrich and used as received.  
         [0058]     4H-Cyclopenta[2,1-b;3,4-b′]dithiophene (1.5 g, 0.00843 mol) was dissolved in DMSO (50 mL). The solution was purged with nitrogen, and grounded KOH (1.89 g, 0.0337 mol) and sodium iodide (50 mg) were added, followed by hexyl bromide (3.02 g, 0.0169 mol). The reaction was stirred for 17 h under nitrogen at room temperature. Water was added and the reaction was extracted with t-butyl-methyl ether. The organic layer was separated and dried over magnesium sulfate. Solvent was removed under vacuum and the residue was purified by chromatography using hexanes as eluent.  
         [0059]     Fractions containing pure 4,4-dixeyl-4H-cyclopenta[2,1-b;3,4-b′]dithiophene product were combined and the solvents evaporated. The product was obtained as a colorless oil. Yield: 2.36 g (81%).  
       EXAMPLE 2  
     The Synthesis of 4,4-Dihexyl-2,6-bis-trimethylstannanyl-4H-cyclopenta[2,1-b;3,4-b′]dithiophene  
       [0060]    
       
                 
         
             
             
         
       
     
         [0061]     Starting material 4,4-dihexyl-4H-cyclopenta[2,1-b;3,4-b′]dithiophene (1.5 g, 0.00433 mol) was dissolved in dry THF (30 mL). The solution was cooled to −78° C. and butyl lithium (6.1 mL, 0.0130 mol) was added drop wise. The reaction was stirred at this temperature for 2 h and warmed to room temperature, stirred for 3 h. Again reaction was cooled to −78° C. and trimethyltin chloride (1 M in hexanes, 16.0 mL, 16.0 mmol) was added dropwise. The reaction was allowed to warm to rt and stirred for 17 h. Water was added and the reaction was extracted with toluene. The organic layer was washed with water and dried over sodium sulfate. Solvent was removed under vacuum and the residue was dissolved in toluene, and quickly passed through a plug of silica gel pretreated with triethyl amine. Solvent was removed and the residue dried under vacuum to afford 2.65 g of the bis(trimethyltin) monomer.  1 H NMR (CDCl 3 , 200 MHz): 6.97 (m, 2H), 1.84 (m, 4H), 1.20 (m, 16H), 0.88 (m, 6H), 0.42 (m, 18H).  
       EXAMPLE 3  
     The Synthesis of bis-(tributylstannyl)-4,4-dihexyl-cyclopenta[2,1-b:3,4-b′]dithiophene  
       [0062]    
       
                 
         
             
             
         
       
     
         [0063]     4,4-Dihexyl-4H-cyclopenta[2,1-b;3,4-b′]dithiophene (2.2 g, 0.0065 mol) was dissolved in dry THF (20 mL). The solution was cooled to −78° C. BuLi (7.62, 2.5 M in hexanes, 0.019 mol) was then added to the solution. The reaction mixture was allowed to warm to room temperature and was stirred for 5 hours. The mixture was then cooled again to −78° C. and Bu 3 SnCl (7.44 g, 0.0229 mol) was added. The reaction mixture was allowed to warm to room temperature and was stirred for another 48 hours. Water was then added and the mixture was extracted with dihicholomethane. Organic layer was collected, dried over anhydrous Na 2 SO 4 , and concentrated. The residue thus obtained was dissolved in hexane and quickly passed through a plug of silica gel pretreated with triethylamine. The solvent was removed and the residue was dried under vacuum to afford bis-(tributylstannyl)-4,4-dihexyl-cyclopenta[2,1-b:3,4-b′]dithiophene (5.7 g).  
       EXAMPLE 4  
     Polymerization of bis-(tributylstannyl)-4,4-dihexyl-cyclopenta[2,1-b:3,4-b′]dithiophene and 4,7-dibromo-2,1,3-benzothiadiazole  
       [0064]    
       
                 
         
             
             
         
       
     
         [0065]     Bis-(tributylstannyl)-4,4-dihexyl-cyclopenta[2,1-b:3,4-b′]dithiophene (0.775 g, 0.000816 mol) and 4,7-dibromo-2,1,3-benzothiadiazole (0.24 g, 0.000816 mol) were first dissolved in toluene. After the reaction was purged with nitrogen, palladium tretakistriphenylphosphine (15 mg, 0.0065 mmol) was added. The reaction mixture was heated at 100° C. for 24 hour. After the solvent was removed, the residue was washed with acetone and extracted in a Soxlet extractor for 8 hours to afford the product as an insoluble blue solid.  
       EXAMPLE 5  
     Synthesis of 4,4-Bis-(2-ethyl-hexyl)-4H-cyclopenta[2,1-b;3,4-b′]dithiophene  
       [0066]    
       
                 
         
             
             
         
       
     
         [0067]     4H-Cyclopenta[2,1-b;3,4-b′]dithiophene (1.5 g, 0.00843 mol) was dissolved in DMSO (50 mL). After the solution was purged with nitrogen, and grounded KOH (1.89 g, 0.0337 mol), sodium iodide (50 mg), and 2-ethylhexyl bromide (3.25 g, 0.0169 mol) were sequentially added. The reaction mixture was stirred overnight under nitrogen (c.a. 16 hours). Water was added and the reaction was extracted with t-butylmethyl ether. The organic layer was collected, dried over magnesium sulfate, and concentrated. The residue was purified by chromatography using hexanes as eluent. Fractions containing pure 4,4-Bis-(2-ethyl-hexyl)-4H-cyclopenta[2,1-b;3,4-b′]dithiophene product were combined and concentrated. The product was obtained as a colorless oil after drying under vacuum. Yield: 2.68 g (79%).  1 H NMR (CDCl 3 , 250 MHz): 7.13 (m, 2H), 6.94 (m, 2H), 1.88 (m, 4H), 0.94 (m, 16H), 0.78 (t, 6.4 Hz, 6H), 0.61 (t, 7.3 Hz, 6H).  
       EXAMPLE 6  
     Synthesis of 4,4-Bis-(2-ethyl-hexyl)-2,6-bis-trimethylstannanyl-4H-cyclopenta[2,1-b;3,4-b′]dithiophene  
       [0068]    
       
                 
         
             
             
         
       
     
         [0069]     Starting material 4,4-Bis-(2-ethyl-hexyl)-4H-cyclopenta[2,1-b;3,4-b′]dithiophene (1.5 g, 0.00372 mol) was dissolved in dry THF (20 mL). After the solution was cooled to −78° C., butyl lithium (5.21 mL, 0.0130 mol) was added dropwise. The reaction mixture was stirred at this temperature for 1 hour. It was then warmed to room temperature and stirred for another 3 hours. The mixture was again cooled to −78° C. and trimethyltin chloride (1 M in hexane, 15.6 mL, 15.6 mmol) was added dropwise. The reaction mixture was allowed to warm to room temperature and stirred overnight (c.a. 16 hours).  
         [0070]     Water was added and the reaction was extracted with toluene. The organic layer was washed with water, dried over sodium sulfate, and concentrated. The residue was dissolved in toluene, and quickly passed through a small plug of silica gel pretreated with triethylamine. The solvent was removed and the residue was dried under vacuum. 1.25 g of the product was obtained.  1 H NMR (CDCl 3 , 250 MHz): 6.96 (m, 2H), 1.85 (m, 4H), 1.29 (m, 2H), 0.92 (m, 16H), 0.78 (t, 6.8 Hz, 6H), 0.61 (t, 7.3 Hz, 6H), 0.38 (m, 18H).  
       EXAMPLE 7  
     Polymerization of Bis-(trimethylstannyl)-4,4-Di(2-ethylhexyl)-cyclopenta[2,1-b:3,4-b′]dithiophen and 4,7-dibromo-2,1,3-benzothiadiazole  
       [0071]    
       
                 
         
             
             
         
       
     
         [0072]     Bis-(trimethylstannyl)-4,4-di(2-ethylhexyl)-cyclopenta[2,1-b:3,4-b′]dithiophene (0.686 g, 0.000943 mol) and 4,7-dibromo-2,1,3-benzothiadiazole (0.269 g, 0.000915 mol) were dissolved in toluene (20 mL). After the reaction was purged with nitrogen, tris(dibenzylideneacetone)dipalladium(0) (25.1 mg, 0.0275 mmol) and triphenylphosphine (57.6 mg, 0.220 mmol) were added. The reaction was further purged with nitrogen for 10 minutes and heated to 120° C. under nitrogen for 24 hours. The solvent was removed under vacuum and the residue was dissolved in chloroform. After the mixture was poured into methanol (500 mL), the blue precipitate thus obtained was collected by filtration, washed with methanol, and dried. The precipitate was dissolved in chloroform (30 mL) under heating, and filtered through a 0.45 μm membrane. The solution was loaded on to recycling HPLC (2H+2.5H column on a Dychrome recycling HPLC, 5 cycles for each injection), in 3 mL portions for purification. Higher-molecular-weight fractions were combined to give 120 mg pure polymer (Mn=35 kDa).  
       EXAMPLE 8  
     Copolymerization of 4,4-Dihexyl-2,6-bis-trimethylstannanyl-4H-cyclopenta[2,1-b;3,4-b′]dithiophene, 4,4-Bis-(2-ethyl-hexyl)-2,6-bis-trimethylstannanyl-4H-cyclopenta[2,1-b;3,4-b′]dithiophene, and 4,7-Dibromo-benzo[1,2,5]thiadiazole  
       [0073]    
       
                 
         
             
             
         
       
     
         [0074]     4,4-Dihexyl-2,6-bis-trimethylstannanyl-4H-cyclopenta[2,1-b;3,4-b′]dithiophene (0.0863 g, 0.000128 mol), 4,4-bis-(2-ethyl-hexyl)-2,6-bis-trimethylstannanyl-4H-cyclopenta[2,1-b;3,4-b′]dithiophene (0.187 g, 0.000257 mol), and 4,7-Dibromo-benzo[1,2,5]thiadiazole (0.111 g, 0.000378 g) were dissolved in toluene (15 mL) and the solution was degassed and purged with N 2 . Tris(dibenzylideneacetone)dipalladium(0) (6.78 mg, 0.0074 mmol) and triphenylphosphine (15.5 mg, 0.0 593 mmol) were then added. The reaction was purged again with nitrogen for 30 minutes and heated at 120° C. under nitrogen. The solvent was then removed under vacuum. The residue was dissolved in chloroform and the solution was added into methanol. The precipitates were collected and extracted with hexane for 24 hours and then extracted with chloroform for 8 hours. The resultant blue solution was concentrated and added to methanol. The precipitates were collected to afford a first fraction of the polymer (70 mg). The remaining materials on the thimble was further extracted with chloroform for 20 hours. 20 mg additional polymer was collected.  
       EXAMPLE 9  
     Preparation of 4H-4,4-bis(2′-ethylhexyl)cyclopenta[2,1-b:3,4-b′]thiophene-2,6-bis(pinacolborate) ester  
       [0075]    
       
                 
         
             
             
         
       
     
         [0076]     100 mL oven dried Schlenk flask was charged with 1.097 g (2.72 mmol) of 4H-4,4-bis(2′-ethylhexyl)cyclopenta[2,1-b:3,4-b′]dithiophene. The flask was evacuated and purged with argon three times. To this flask was then added 20 mL of dry, distilled THF. The resulting solution was cooled to −78° C. and 4.35 mL (10.88 mmol, 4 equv.) of 2.5M BuLi was added dropwise. The reaction was stirred for 1 hour at −78° C. and then warmed to room temperature and stirred for an additional 3 hours. The solution was cooled again to −78° C. and 2.77 mL (13.6 mmol, 5 equiv.) of 2-isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane was added in one portion via syringe. The reaction was stirred at −78° C. for 1 hour and then allowed to warm to room temperature overnight. The solution was poured into water and extracted with 4×150 mL of methyl tert-butyl ether. The organic layers were combined and washed with 2×150 mL of brine, dried with anhydrous MgSO 4 , and filtered. The solvent was removed under vacuum to yield and orange oil, which was purified by column chromatography (5% EtOAc in hexanes) to yield a colorless, viscous oil, 1.34 g (75% yield).  
       EXAMPLE 10  
     Preparation of a Pentathienyl-cyclopentadithiophene Copolymer  
       [0077]    
       
                 
         
             
             
         
       
     
         [0078]     A 50 mL Schlenk flask was charged with 0.309 g (0.472 mmol) of 4H-4,4-bis(2′-ethylhexyl)cyclopenta[2,1-b:3,4-b′]dithiophene-2,6-bis(pinacolborate) ester prepared in Example 9, 0.367 g (0.510 mmol) of 5,5′-dibromo-3″,4″-dihexyl-a-pentathiophene (its synthesis was described in WO 2005/092947, which is incorporated herein by reference) 0.0013 g (0.00185 mmol) of PdCl 2 (PPh 3 ) 2 , and 0.057 g (0.142 mmol) of trioctylmethylammonium chloride (Aliquot 336, Aldrich, St. Louis, Mo.). The flask was fitted with a reflux condenser and the flask was evacuated and refilled with nitrogen three times. The solids were dissolved in 6 mL of toluene and then 0.88 mL of 2M Na 2 CO 3  were added via syringe. The reaction was then heated to 95° C. with stirring for 5 hours. Phenylboronic acid (0.031 g, 0.250 mmol) and 0.0016 g (0.00228 mmol) of PdCl 2 (PPh 3 ) 2  were dissolved in 1 mL of THF and added to the reaction mixture, and stirring was continued for 16 h at 95° C. The reaction mixture was diluted with toluene (50 mL) and the organic layer was separated and washed with warm water (3×50 mL). The solution was then treated with an aqueous solution of diethyldithiocarbarnic acid sodium salt trihydrate (7.5%, DDC, 5 mL) and heated at 80° C. overnight. The aqueous layer was separated and discarded and the organic layer was washed with warm water (3×50 mL) and the polymer precipitated into methanol (500 mL). The polymer was collected by filtration, washed with methanol (50 mL) and redissolved in hot toluene (200 mL). The hot polymer solution was passed through a tightly packed column of celite (1×8 cm), silica get (3×8 cm), and basic alumina (3×8 cm) (previously rinsed with 200 mL of hot toluene). The polymer solution was collected and the volume concentrated to approximately 50 mL. The polymer was precipitated into methanol (500 mL), washed with methanol (100 mL), acetone (100 mL) and again with methanol (100 mL). The polymer was then dried in vacuo overnight to yield a brick red material. Yield: 0.327 g.  
       EXAMPLE 11  
     Fabrication of Solar Cell  
       [0079]     The polymer solar cells were fabricated by doctor-blading a blend of the polymer prepared in Example 7 (PCPDTBT) and PC 61 BM or PC 71 BM (purchased from Nano-C, Westwood, Mass.) in a 1:3 w/w ratio sandwiched between a transparent anode and an evaporated metal cathode. The transparent anode was an indium tin oxide (ITO)-covered glass substrate (Merck, Whitehouse Station, N.J.) which was coated with a ˜60 nm thick PEDOT:PSS layer (Baytron PH from H.C. Starck) applied by doctorblading. The ITO-glass-substrate was cleaned by ultrasonification subsequently in acetone, isopropyl alcohol and deionized water. The cathode, a bilayer of a thin (1 nm) LiF layer covered with 80 nm Al, was prepared by thermal evaporation. PCPDTBT and PC 61 BM or PC 71 BM were dissolved together in o-dichlorobenzene (ODCB) to give an overall 40 mg/ml solution and was stirred overnight at 60-70° C. inside a glovebox. The active layer thickness, as determined by AFM, was between 150-250 nm. Device characterization was done under AM 1.5G irradiation (100 mW/cm 2 ) on an Oriel Xenon solar simulator with a well calibrated spectral mismatch of 0.98 jV-characteristics were recorded with a Keithley 2400. Active areas were in the range of 15 to 20 mm 2 . EQE was detected with a lock-in amplifier under monochromatic illumination. Calibration of the incident light was done with a monocrystalline silicon diode. Mobility measurements were done using an Agilent 4155C parameter analyzer. Absorption measurements were done inside the glovebox with an Avantes fiberoptic spectrometer or outside with a HP spectrometer.  
         [0080]     The interaction with PCBM and the photoinduced charge transfer was investigated by PL quenching. The PL of pristine PCPDTBT versus PCPDTBT/PCBM composites was measured at liquid N 2  temperatures in a cryostat, excitation was provided by an Ar laser at 488 nm.  
         [0081]     Electrochemical experiments were carried out on dropcast polymer films at room temperature in a glovebox. The supporting electrolyte was tetrabutylammonium-hexafluorophosphate (TBAPF 6 , electrochemical grade, Aldrich) ˜0.1 M in acetonitrile anhydrous (Aldrich). The working electrode (WE), as well as the counter electrode (CE), was a platinum foil. A silver wire coated with AgCl was used as a reference electrode (RE). After each measurement, the RE was calibrated with ferrocene (E 0 =400 mV vs. NHE) and the potential axis was corrected to NHE (using −4.75 eV for NHE 24,25 ) according to the difference of E 0  (ferrocene) and the measured E 1/2  (ferrocene). λ max  (CHCl 3 )=710 nm, λ band edge  (CHCl 3 )=780 nm, band gap (CHCl 3 )=1.59 eV, λ max  (film)=700-760 nm, λ band edge  (film)=855 nm, band gap (film)=1.45 eV, HOMO=−5.3 eV, −5.7 eV (electrochem), LUMO=−3.85 eV, −4.25 eV, μ + =2×10 −2  cm 2 /Vs (TOF), 1×10 −3  cm 2 /Vs (FET).  
         [0082]     Other embodiments are in the claims.