Patent Publication Number: US-2015075602-A1

Title: Photovoltaic cell with graphene-ferroelectric electrode

Description:
CLAIM OF PRIORITY 
     This Application claims the benefit of priority of U.S. Provisional Patent Application Ser. No. 61/614,655, entitled “Photovoltaic Cell Based on Graphene Ferroelectric Interface and the Method of Fabrication Thereof,” filed on Mar. 23, 2012, and which is incorporated by reference herein. 
    
    
     FIELD 
     The present disclosure relates to photovoltaic cells, and in particular relates to a photovoltaic cell with at least one graphene-ferroelectric electrode, and methods of forming the photovoltaic cell. 
     BACKGROUND 
     Photovoltaic cells (or solar cells) are devices that convert light into electricity. A typical photovoltaic cell includes a material that can absorb light and generate charge carriers in the form of electrons and holes. Conductive contacts are used to support an electric potential that causes the separation of the charge carriers to create a photocurrent. 
     Most photovoltaic cells utilize a semiconductor material, such as silicon. While semiconductor-based photovoltaic cells are relatively efficient, they are also expensive. Attention has thus been directed to employing organic photopolymers in place of the semiconductor material. While less efficient, the organic photopolymers are much less expensive, and can also be used to make flexible photovoltaic cells. The organic photopolymer is configured in a matrix that includes electron donor and electron acceptor materials. Thus, while the organic polymer matrix does not include a p/n junction per se like a true semiconductor, the matrix includes interfaces that allow for the dissociation of excitons in a manner similar to a semiconductor-based p/n junction. In this sense, organic polymers act as pseudo-semiconductors. 
     As with photovoltaic cells based on conventional semiconductors, photovoltaic cells based on organic polymers suffer from reduced conversion efficiency due to the recombination of electrons and holes (i.e., exciton recombination), which reduces the amount of electricity produced. 
     This charge-carrier recombination is the main cause of energy loss in organic polymer photovoltaic cells and limits their efficiency. It has been shown that in such photovoltaic cells, more than 50% of the energy loss is due to this non-radiative recombination process. Moreover, organic-based photovoltaic cells typically employ rigid and fragile electrodes like ITO/Ag/Al, which limit their applications in many sectors, including flexible devices. 
     SUMMARY 
     An aspect of the disclosure is a photovoltaic cell device for generating a photocurrent when irradiated with light. The device includes an active layer having top and bottom surfaces and that generates charge carriers when irradiated with the light. The device also includes top and bottom electrodes respectively interfaced with the top and bottom layers of the active layer. The top electrode comprises a first graphene layer and a first polarized ferroelectric layer. The first polarized ferroelectric layer defines an internal electric field that extends into the active layer and that facilitates the generation of the photocurrent. 
     Another aspect of the disclosure is the photovoltaic cell device as described above, wherein the first graphene layer includes either two-dimensional (2D) graphene or three-dimensional (3D) graphene. 
     Another aspect of the disclosure is the photovoltaic cell device as described above, wherein the first polarized ferroelectric layer comprises a ferroelectric polymer. 
     Another aspect of the disclosure is the photovoltaic cell device as described above, wherein the ferroelectric polymer comprises P(VDF-TrFE). 
     Another aspect of the disclosure is the photovoltaic cell device as described above, wherein the active layer comprises one of: silicon, an organic semiconducting polymer, dye-sensitized molecules, gallium arsenide, cadmium telluride, and copper indium gallium selenide. 
     Another aspect of the disclosure is the photovoltaic cell device as described above, wherein the organic semiconducting polymer is P3HT:PC 70 BM. 
     Another aspect of the disclosure is the photovoltaic cell device as described above, wherein the bottom electrode comprises either a metal electrode or a second graphene layer and a second polarized ferroelectric layer. 
     Another aspect of the disclosure is the photovoltaic cell device as described above, wherein the first graphene layer resides between the first polarized ferroelectric layer and the active layer. 
     Another aspect of the disclosure is the photovoltaic cell device as described above, wherein the top electrode further includes a conductive layer on the first polarized ferroelectric layer. 
     Another aspect of the disclosure is the photovoltaic cell device as described above, wherein the first graphene layer comprises doped graphene. 
     Another aspect of the disclosure is the photovoltaic cell device as described above, wherein the first graphene layer has a select work function that is defined by the first polarized ferroelectric layer. 
     Another aspect of the disclosure is the photovoltaic cell device as described above, wherein the charge carriers are subject to an amount of charge-carrier recombination, and wherein the internal electric field reduces the amount of charge carrier recombination. 
     Another aspect of the disclosure a photovoltaic cell device capable of generating a photocurrent. The device includes an active layer comprising an organic semiconducting polymer layer having top and bottom surfaces. The device also includes a top electrode interfaced with the top surface of the active layer. The top electrode comprises a graphene layer and a ferroelectric layer that includes a polarized ferroelectric polymer that generates an internal electric field that extends into the active layer. The active layer generates charge carriers in response to being irradiated with light through the top electrode. The internal electric field reduces an amount of charge-carrier recombination as compared to that in the absence of the internal electric field and serves to generate the photocurrent. 
     Another aspect of the disclosure is the photovoltaic cell device as described above, wherein the polarized ferroelectric polymer comprises P(VDF-TrFE) and wherein the organic semiconducting polymer layer comprises P3HT:PC 70 BM. 
     Another aspect of the disclosure is the photovoltaic cell device as described above, wherein the graphene layer comprises between one sheet and forty sheets of one-atom-thickness graphene. 
     Another aspect of the disclosure is the photovoltaic cell device as described above, wherein the graphene layer has a select work function that is defined by the polarized ferroelectric polymer of the ferroelectric layer. 
     Another aspect of the disclosure is a method of generating a photocurrent in a photovoltaic cell having an active layer sandwiched by first and second electrodes. The method includes: illuminating the active layer through the first electrode to generate electrons and holes in the active layer, wherein the first electrode includes a first graphene layer and a first polarized ferroelectric layer; using the first polarized ferroelectric layer, forming a first internal electric field that extends into the active layer; and generating a photocurrent by the first internal electric field causing the electrons and holes to move to opposite ones of the first and second electrodes. 
     Another aspect of the disclosure is the method described above, wherein the second electrode comprises a second graphene layer and a second polarized ferroelectric layer, and further comprising: the second polarized ferroelectric layer forming a second internal electric field that extends into the active layer, thereby further contributing to the moving of the electrons and holes to opposite ones of the first and second electrodes. 
     Another aspect of the disclosure is the photovoltaic cell device as described above, wherein the active layer comprises one of: silicon, an organic semiconducting polymer, dye-sensitized molecules, gallium arsenide, cadmium telluride, and copper indium gallium selenide. 
     Another aspect of the disclosure is the photovoltaic cell device as described above, wherein the first graphene layer has a select work function that is defined by the first polarized ferroelectric layer. 
     The photovoltaic cells disclosed herein are cost-effective, have high conversion efficiency, can be made flexible, and can be scaled to small and large sizes. The photovoltaic cells disclosed herein have industrial applicability for providing power to a wide range of electrically powered devices such as mobile phones, smart phones, portable computers, cameras, watches, and the like. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings are included to provide a further understanding, and are incorporated in and constitute a part of this specification. The drawings illustrate one or more embodiment(s), and together with the Detailed Description serve to explain principles and operation of the various embodiments. As such, the disclosure will become more fully understood from the following Detailed Description, taken in conjunction with the accompanying Figures, in which: 
         FIGS. 1A and 1B  show main example embodiments of the photovoltaic cell disclosed herein; 
         FIG. 2A  shows an example photovoltaic cell that includes top and bottom graphene-ferroelectric electrodes, wherein the graphene layers resides immediately adjacent to the active layer; 
         FIG. 2B  shows an example photovoltaic cell similar to that of  FIG. 2A , but wherein the order of the graphene layer and the ferroelectric layer is reversed in each of the graphene-ferroelectric electrodes; 
         FIGS. 2C and 2D  show example photovoltaic cells similar to that of  FIGS. 2A and 2B , but wherein the bottom graphene-ferroelectric electrode is replaced by a conventional electrode; 
         FIGS. 2E and 2F  show example photovoltaic cells similar to that of  FIGS. 2C and 2D , but wherein the top graphene ferroelectric electrode includes a conductive layer, which in the example of  FIG. 2F  is a second graphene layer; and 
         FIGS. 3A and 3B  show example photovoltaic cells similar to those shown in  FIGS. 2A ,  2 B and  FIGS. 2C ,  2 D, respectively, wherein the cell include protective substrates that sandwich the top and bottom electrodes. 
     
    
    
     DETAILED DESCRIPTION 
     Reference is now made in detail to various embodiments of the disclosure, examples of which are illustrated in the accompanying drawings. Whenever possible, the same or like reference numbers and symbols are used throughout the drawings to refer to the same or like parts. The drawings are not necessarily to scale, and one skilled in the art will recognize where the drawings have been simplified to illustrate the key aspects of the disclosure. 
     The claims as set forth below are incorporated into and constitute part of this Detailed Description. 
     General Photovoltaic Cell Embodiments 
       FIGS. 1A and 1B  are schematic diagrams that respectively illustrate two main example embodiments of a photovoltaic cell device (“photovoltaic cell”)  10  according to the disclosure. The photovoltaic cell  10  includes an active layer  20  made of one or more materials and that generates photo-induced electrons e and holes h when irradiated with light  50 . Active layer  20  has a top surface  22 T and a bottom surface  22 B. In an example, active layer  20  comprises an organic semiconducting polymer photovoltaic matrix, such as a polymeric blend of P3HT:PC 70 BM. In another example, active layer  20  comprises silicon. In yet other examples, active layer  20  comprises dye-sensitized molecules, gallium arsenide, cadmium telluride, or copper indium gallium selenide. 
     Photovoltaic cell  10  also includes top and bottom graphene-ferroelectric electrodes  30 , respectively denoted as  30 T and  30 B. Top and bottom (or “first and second”) graphene-ferroelectric electrodes  30 T and  30 B are respectively interfaced with the top and bottom surfaces  22 T and  22 B of active layer  20 . 
       FIG. 1A  illustrates an embodiment of photovoltaic cell  10  wherein the photovoltaic cell is electrically connected to a device  40  being powered by photovoltaic cell  10 . In this embodiment, device  40  represents a load having an effective load resistance R L . 
       FIG. 1B  illustrates an embodiment wherein photovoltaic cell  10  is electrically connected to a voltage source  42 . Such an arrangement can be used when photovoltaic cell  10  is used as a photodetector. Voltage source  42  generates an external electric field E E . 
     Graphene-ferroelectric electrodes  30 T and  30 B each give rise to (i.e., define) an internal electric field E I  for the reasons discussed below. The direction of internal electric field E I  can be selected based on the desired direction of the photocurrent i pc . 
     Light  50  is shown as being incident on photovoltaic cell  10  at top graphene-ferroelectric electrode  30 T, which is substantially transparent to the incident light. Light  50  thus passes through top graphene-ferroelectric electrode  30 T to active layer  20 , which in response generates excitons, e.g., pairs of electrons e and holes h that are bound by the Coulomb force. A fraction of the excitons will separate into electrons e and holes h, with the internal electric field E I  causing the holes h to move toward top graphene-ferroelectric  30 T and the electrons e to move toward bottom graphene-ferroelectric  30 B. This gives rise to the aforementioned photocurrent i pc , which can be used to operate device  40 . 
     A fraction of the excitons will also undergo recombination and thus will not contribute to the photocurrent i pc . The mitigation of this phenomenon is explained in greater detail below in connection with the advantages of utilizing one or more graphene-ferroelectric electrodes  30  in photovoltaic cell  10 . 
     Example Graphene-Ferroelectric Electrode Embodiments 
       FIG. 2A  is a schematic cross-sectional view of an example photovoltaic cell  10  that includes top and bottom graphene-ferroelectric electrodes  30 T and  30 B. Top graphene-ferroelectric electrode  30 T comprises a graphene layer  32  interfaced with the top surface  22 T of active layer  20 , and a polarized ferroelectric layer  34  interfaced with the graphene layer on the side opposite the active layer. Likewise, bottom graphene-ferroelectric electrode  30 B comprises a graphene layer  32  interfaced with the bottom surface  22 B of active layer  20 , and a polarized ferroelectric layer  34  interfaced with the graphene layer on the side opposite the active layer. In an example, graphene-ferroelectric electrode  30  consists of graphene layer  32  and polarized ferroelectric layer  34 . 
     Graphene layer  32  includes graphene  33  in or more of its available forms, as discussed in greater detail below. Graphene  33  is shown in  FIG. 2A  as a single sheet (i.e., a one-atomic-layer sheet) by way of example. The internal electric fields E I  contributed by top and bottom graphene-ferroelectric electrodes  30 T and  30 B are also shown by way of example as being oriented towards the top electrode. 
       FIG. 2B  shows an example photovoltaic cell  10  similar to that of  FIG. 2A , but wherein the order of graphene layer  32  and the polarized ferroelectric layer  34  is reversed in each of the top and bottom graphene-ferroelectric electrodes  30 T and  30 B so that the polarized ferroelectric layers are interfaced with active layer  20  and the graphene layers reside on the polarized ferroelectric layers on the side opposite the active layer. 
       FIG. 2C  shows an example photovoltaic cell  10  similar to that of  FIG. 2A , but wherein the bottom graphene-ferroelectric electrode  30 B is replaced with a conventional bottom electrode  31 . Because light  50  is incident upon photovoltaic cell  10  through the top graphene-ferroelectric  30 T, the conventional bottom electrode  31  can be opaque. 
       FIG. 2D  is similar to  FIG. 2C , but with order of the graphene layer  32  and the polarized ferroelectric layer  34  in the top graphene-ferroelectric electrode reversed so that the polarized ferroelectric layer is interfaced with active layer  20 . 
       FIG. 2E  is similar to  FIG. 2C , but with a conductive layer  36  formed on polarized ferroelectric layer  34 . Conductive layer  36  can be fixed or can be removable. If conductive layer  36  is fixed, it can be made of a substantially transparent material, such as ITO, carbon nanotubes, nanowires, thin films of gold, silver, copper or other metal conductors, etc. Conductive layer  36  is used to polarize the ferroelectric layer  34  by allowing for an electric field to be applied between the conductive layer and graphene layer  32 . This electric field can be established by connecting voltage source  42  to conductive layer  36  and graphene layer  32 . As noted below, in an example this electric field need only applied for a time sufficient to polarize ferroelectric layer  34 , since this layer is capable of remaining polarized in the absence of the electric field. The benefits of having a polarized ferroelectric layer  34  are discussed below. 
       FIG. 2F  is similar to  FIG. 2E , except that the conductive layer  36  is formed by a second graphene layer  32 . This is possible because graphene is electrically conductive. 
     The Graphene Layer 
     In the various example embodiments of photovoltaic cell  10 , graphene layer  32  can have a number of different configurations based on the various available forms for graphene  33 . In one example, graphene layer  32  can comprise one or more 2D graphene sheets, while in another example the graphene layer can comprise 3D graphene foam (also called corrugated graphene). 
     In an example embodiment, graphene layer  32  includes between one and forty layers of graphene  33 , with a single layer (i.e., a one-atomic-layer sheet) being about 0.4 nm in thickness (see close-up view of  FIG. 2A ). In another example, graphene layer  32  when made up of 2D graphene  33  can be up to 15 nm thick. In another example, graphene layer  32  can have from one to four layers of graphene  33 . Each single layer of graphene  33  reduces the transparency in the visible (optical) wavelength band by about 2.3%. 
     In an example embodiment where graphene layer  32  is made up of 3D graphene foam, there can be some mixing between the graphene foam and the ferroelectric material that makes up ferroelectric layer  34 . In an example embodiment, graphene layer  32  can have a minimum thickness of about  15  nm when graphene  33  comprises 3D graphene foam. 
     The work function of graphene can be varied from its nominal value of 4.5 eV by doping. The doping can be accomplished by using atoms (sometimes called “hetero-doping”), by using molecules (sometimes called “chemical modification”) or by using an electric field (sometimes called “electric field tuning”). Thus, in an example, graphene layer  32  comprises graphene  33  that is doped using one or more of these doping mechanisms. 
     In addition, since graphene is substantially transparent to visible, near-UV and mid-UV wavelengths of light, an example graphene layer  32  is configured to be substantially transparent to these wavelengths of light. 
     In an example embodiment, the one or more graphene layers (sheets)  33  in graphene layer  32  can be grown as a single film by a chemical vapor deposition (CVD) method. In another example, the one or more graphene layers  33  are formed by a controlled stacking process using a stacking solution that allows for small graphene platelets to be formed into a continuous graphene film. 
     In another example embodiment, graphene  33  in graphene layer  32  can comprise the aforementioned 3D graphene foam, which is particularly useful in forming highly conductive electrodes. Graphene foam typically has lower transparency (i.e., has greater opacity) than 2D graphene sheets, i.e., its absorption is greater than the 2.3% associated with a single graphene sheet. 
     Because graphene is impermeable to the diffusion of atoms and molecules therethrough, graphene layer  32  serves as an impermeable layer for photovoltaic cell  10 . This reduces the degradation of photovoltaic cell  10  and in particular active layer  20 . For example, graphene  33  in graphene layer  32  can prevent metal (e.g., from electrical contacts, not shown) or can prevent a gas (e.g., oxygen from the ambient atmosphere) from reaching the underlying layers, e.g., polarized ferroelectric layer  34  (in some configurations) and active layer  20 . This is in contrast to conventional photovoltaic cell interfaces, where the interdiffusion of metal and atoms and molecules over time reduces the photovoltaic cell efficiency. In the specific case of organic-based photovoltaic cells, the organic active layer can be rendered completely nonfunctional if exposed to air. Consequently, the undesirable interdiffusion of atoms and molecules into the organic active region limits the lifespan of the organic photovoltaic cell to days to up to about a year for the best devices. 
     The Polarized Ferroelectric Layer 
     The polarized ferroelectric layer  34  can be organic or inorganic. The polarized ferroelectric layer  34  is in a polarized state so that can give rise to the aforementioned internal electric field E. The polarized ferroelectric layer  34  can be in a polarized state either by virtue of its inherent crystalline order, or by being placed into a polarized state by subjecting the layer to an electric field. The polarized ferroelectric layer  34  can be polarized either prior to being incorporated into photovoltaic cell  10 . 
     The polarized ferroelectric layer  34  can also be polarized during the process used to form the ferroelectric layer as part of forming graphene-ferroelectric electrode  30 . The polarized ferroelectric layer  34  can also be polarized after the full photovoltaic cell  10  is created. Once polarized ferroelectric layer  34  is put into its polarized state by an electric field, the electric field need not be maintained. In an example, the polarizing electric field can be periodically applied when needed to re-establish the polarization of polarized ferroelectric layer  34 . 
     An aspect of the disclosure includes tuning the degree of polarization of polarized ferroelectric layer  34  in order to vary the work function for graphene layer  32 , and in an example provide (e.g., define) a select value for the work function. This is an example of the aforementioned electric-field-based graphene doping. The polarized ferroelectric layer  34  in the graphene-ferroelectric electrode  30  can be used to dope graphene layer  32  with opposite charge carriers (i.e., electrons e and holes h). The opposite doping induces a difference in the work function of graphene layer  32 , thereby introducing an electric field on top of the internal electric field E I  from the ferroelectric layer polarization. It is estimated that this can increases the conversion efficiency of photovoltaic cell  10  by 10% to 20%. 
     In one experiment conducted by the inventors, polarized ferroelectric layer  34  was constituted by a ferroelectric polymer P(VDF-TrFE). This ferroelectric polymer was polarized and the graphene-ferroelectric electrode  30  was formed. A change in the graphene work function of up to +/−0.7 eV was measured relative to the nominal graphene work function of 4.5 eV. Depending on the exact ferroelectric material making up polarized ferroelectric layer  34 , the graphene work function can be tuned over an even greater range. Thus, in an example, the graphene work function can be defined by the polarized ferroelectric layer  34  to have a select value other than its nominal value. 
     In another example embodiment, the graphene work function is changed by changing the composition of the polarized ferroelectric layer  34  to change the amount of polarization this layer can have. For example, where polarized ferroelectric layer  34  comprises a ferroelectric copolymer, the copolymer ratio can be changed. Thus, in an example where polarized ferroelectric layer  34  comprises the copolymer P(VDF-TrFE), the ratio of PVDF to TrFE can be changed to change the maximum polarization of the copolymer, which in turn affects the amount of change in the graphene work function. In another example where polarized ferroelectric layer  34  comprises the inorganic ferroelectric ceramic material PZT (i.e., lead zirconate titanate or (Pb[Zr(x)Ti(1−x)]O3)), changing the ratio of Zr to Ti in the ferroelectric crystal, changes the maximum polarization of the material. 
     By matching the work function of the top graphene-ferroelectric electrode  30 T to that of the interface between this electrode and active layer  20 , the conversion efficiency of photovoltaic cell  10  can be optimized. 
     The internal electric field E I  defined by polarized ferroelectric layer  34  can also serve to mitigate the adverse effects on conversion efficiency caused by charge-carrier recombination. The internal electric field E I  from polarized ferroelectric layer  34  extends into active layer  20  and so can be felt by the charge carriers residing therein. This internal electric field serves to accelerate the charge carriers to their respective electrodes, e.g., holes h to top graphene-ferroelectric  30 T and electrons e to bottom graphene-ferroelectric  30 B or  31 . 
     The faster the charge carriers can reach their respective electrodes, the less time they remain within active medium  20 , which reduces the rate of charge-carrier recombination. The smaller the rate of charge-carrier recombination, the greater the conversion efficiency of photovoltaic cell  10 . The use of top and bottom graphene-ferroelectric electrodes  30 T and  30 B contributes two (i.e., first and second) internal electric fields E I , such as shown in  FIGS. 2A and 2B . This increases the photocurrent and provides a greater reduction in the charger-carrier combination as compared to using a single graphene-ferroelectric electrode. 
     A typical polarized ferroelectric layer  34  can give rise to an internal electric field E I  of about 50V μm −1 . This is nearly ten times larger than that achievable by the use of conventional electrodes. This translates into an improvement in efficiency of photovoltaic cell  10  over conventional organic photovoltaic cells, e.g., from 1% to 2% without layers to 4% to 5% with layers. These enhanced efficiencies are 10% to 20% higher than those achieved by other methods, such as conventional morphology and electrode work-function optimization. 
     In the example embodiment of photovoltaic cell  10  where polarized ferroelectric layer  34  is spaced apart from active layer  20  by graphene layer  32  (see, e.g.,  FIG. 2A ), an example range of thickness for the polarized ferroelectric layer is between 1 nm and about 100 microns. Other thicknesses can be employed as needed, and this range is only exemplary. The same thickness range can be employed when the polarized ferroelectric layer  34  is sandwiched between graphene layer  32  and active layer  20 , noting that thickness on the smaller end of this range may be preferred for enhanced performance of photovoltaic cell  10 . 
     Fabrication Methods 
     Photovoltaic cell  10  can be fabricated in a number of different ways. In one example, 2D or 3D graphene  33  for graphene layer  32  is formed by CVD on a metal substrate or a corrugated metal mesh (e.g., copper) that catalyzes its growth. In another example, the aforementioned controlled stacking process is employed to form a graphene film. 
     An ultra-thin (e.g., 1 nm to 2 nm) layer of ferroelectric polymer (e.g. PVDF-TrFE) is deposited on the graphene layer  32  as the polarized ferroelectric layer  34 , thereby forming graphene-ferroelectric electrode  30 . The graphene-ferroelectric electrode  30  (i.e., the PVDF-TrFE integrated graphene structure) can then be transferred onto transparent and flexible substrate  38 , such as a PET substrate. The resulting structure can be cut to form two graphene-ferroelectric electrodes  30  that can be used as the top and bottom electrodes  30 T and  30 B. 
     Next, a thin (e.g., about 100-150 nm thick) active layer  20  of organic semiconducting polymer (e.g. P3HT:PC 70 BM) is deposited on one of the graphene-ferroelectric electrodes  30 , say the bottom electrode  30 B. Then, the remaining graphene-ferroelectric electrode  30  is interfaced with the active layer  20  supported by bottom electrode  30 B so that the organic semiconducting polymer active layer is sandwiched between the two ferroelectric polymer layers of the top and bottom electrodes  30 T and  30 B. The resulting photovoltaic cell  10  is shown in  FIG. 3A  and includes the two flexible, thin substrates  38 , which serve to protect the device. 
     At any time along the way in the above process, ferroelectric layer  34  can be polarized by subjecting it to an electric field, such as the external electric field E E  as shown in  FIG. 1B . 
     In a variation of the above fabrication method, a conventional bottom electrode  31  can be employed in forming the embodiment of photovoltaic cell  10 .  FIG. 3B  is similar to  FIG. 3A  and to  FIGS. 2C and 2D , and shows an example embodiment of photovoltaic cell  10  formed in such a manner, wherein the bottom electrode is a conventional electrode  31 , which is shown as being covered by protective substrate  38 . 
     In another example, the graphene-ferroelectric electrodes  30  formed as described above are transferred onto a corresponding flexible substrate  38 , e.g., a PET substrate. A thin active layer  20  in the form of an organic semiconducting polymer matrix is then sandwiched between the two electrode/substrate structures. A potential is then applied across the graphene-ferroelectric electrodes  30 T and  30 B to polarize (pole) the ferroelectric polymer in (polarized) ferroelectric layers  34 . This obviates the need to provide an uninterrupted external electric field. 
     As discussed above, photovoltaic cell  10  can optionally include the aforementioned voltage source  42  (see e.g.,  FIG. 1B ) to periodically provide an external electric field E E  to re-polarize the ferroelectric layer(s)  34 . In other words, voltage source  42  can provide an interrupted external electric field E E  that only needs to be applied occasionally, in contrast to certain conventional photovoltaic cells that require the application of an uninterrupted electric field. 
     Additional Advantages 
     Conventional organic photovoltaic cells use ITO/silver/aluminum electrodes that encase the organic active layer. Upon photo-illumination, the organic active layer generates pairs of electrons and holes. An uninterrupted (i.e., a continuously applied) external electric field is applied to separate the electron-hole pairs to generate the photo-current. This external electric field can be provided by a voltage source configured to establish an electrical potential between the top and bottom electrodes. The external electric field can also be established by the different layers having different work functions. 
     An advantage of photovoltaic cell  10  is that it does not require an external electric field E E  or an internal electric field E I  generated by a difference in work functions to facilitate the separation of the charge carriers. Instead, the internal electric field(s) E I  from one or more polarized ferroelectric layers  34  in the one or more graphene-ferroelectric electrodes  30  serves this function. The ability of photovoltaic cell  10  to function without the need for such electric fields provides greater flexibility in how the photovoltaic cell  10  can be deployed and used in a host of applications. 
     In an example, polarized ferroelectric layer  34  is substantially transparent to near-UV and mid-UV wavelengths. An example material for such a transparent polarized ferroelectric layer  34  is the aforementioned ferroelectric polymer, P(VDF-TrFE). It is noted that ITO is substantially opaque at near-UV and mid-UV wavelengths. Thus, when transparent polarized ferroelectric layer  34  is combined with transparent graphene layer  32 , the composite graphene-ferroelectric electrode  30  is also substantially transparent to light  50  over this UV-wavelength range. Thus, graphene-ferroelectric electrode  30  can be used in place of ITO for photovoltaic cells that need to be operational at UV wavelengths. 
     A conventional organic photovoltaic cell has a high series resistance at the interfaces between the organic and inorganic layers. The use of ITO as the transparent electrode gives rise to this high series resistance. A metal is often used for the other electrode, so that this second interface also has a high series resistance. In contrast, photovoltaic cell  10  as disclosed herein utilizes one or more graphene-ferroelectric electrodes  30 , with each having graphene layer  32 . As graphene is an organic material, the series resistance at the interfaces between the active layer  20  and the graphene-ferroelectric electrodes  30  is significantly reduced as compared to the conventional configurations, thereby resulting in a much higher conversion efficiency. If polarized ferroelectric layer  34  resides between active layer  20  and graphene layer  32 , the use of an organic ferroelectric layer (e.g., an organic polymer) will provide a relatively low series resistance. 
     Graphene-ferroelectric electrode  30  can also be flexible, so that when used with a flexible active layer  20  (e.g., an organic thin film), photovoltaic cell  10  can be flexible. Moreover, the flexible photovoltaic cell  10  is expected to have a greater conversion efficiency than conventional flexible organic photovoltaic cells. An example flexible graphene-ferroelectric electrode  30  employs a ferroelectric polymer film for polarized ferroelectric layer  34 .