Patent Publication Number: US-2017358766-A1

Title: Organic semiconductor photovoltaic devices and compositions with acceptor-donor-acceptor type polymer electron donors

Description:
GOVERNMENT RIGHTS 
     The United States Government has rights in this invention under Contract No. DE-AC36-08GO28308 between the United States Department of Energy and the Alliance for Sustainable Energy, LLC, the manager and operator of the National Renewable Energy Laboratory. 
    
    
     BACKGROUND 
     The state of the art in electron donors for organic photovoltaic (OPV) bulk heterojunction (BHJ) devices is the Donor-Acceptor copolymer electron donor. While early success in OPV BHJ devices was based upon the electron donor homopolymer poly (3-hexylthiophene) (P3HT), this material has a relatively large band gap (˜1.9 eV) that limits the wavelengths of light that can be absorbed, and thus limits the current the devices can produce. It was found that if the BHJ was instead fabricated with alternating copolymers consisting of a repeating sequence of an electron-rich moiety (referred to an as electron donor) and an electron poor moiety (referred to as an electron acceptor) the result was the occurrence of a so-called push-pull phenomenon within the BHJ that resulted in significantly lower band gaps as compared to a BHJ with homopolymer electron donors. Very low bandgap materials are potentially very attractive for semitransparent OPV devices for use in building-integrated photovoltaics (BIPV) applications. The limited absorption width of organic semiconductors has the potential for efficient near-infrared (IR) absorption while still allowing for high visible light transmission (VLT), unlike the case with inorganic semiconductors. In current OPV devices, utilization of Donor-Acceptor copolymers has been extremely successful as electron donors in BHJs that also utilize fullerenes for the absorber layer electron acceptor. However, there are limits in the materials properties accessible from Donor-Acceptor copolymers. In particular, it is difficult to obtain ultra-low bandgap (≦1 eV) materials using the conventional Donor-Acceptor copolymer paradigm, due to the limits of known donor and acceptor strengths, which ultimately limits the currents and open circuit voltages that can be realized from Donor-Acceptor based devices. 
     While small molecule OPV systems that deviate from the Donor-Acceptor copolymer paradigm exist and have been used with some success in OPV devices, they are limited to relatively moderate band gaps (for example, ˜1.5 eV) and exhibit difficulties in processing due to issues with solubility, low hole mobilities, and limited phase segregation often seen in such small molecule-based BHJ systems. 
     For the reasons stated above and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the specification, there is a need in the art for systems and methods that provide organic semiconductor photovoltaic devices with acceptor-donor-acceptor type polymer electron donors. 
     SUMMARY 
     The Embodiments of the present disclosure provide methods and systems for organic semiconductor photovoltaic devices with acceptor-donor-acceptor type polymer electron donors and will be understood by reading and studying the following specification. 
     Organic semiconductor photovoltaic devices and compositions with acceptor-donor-acceptor type polymer electron donors are provided. In one embodiment, a composition of matter comprises a copolymer material having an acceptor-donor-acceptor moiety repeat unit. 
    
    
     
       DRAWINGS 
       Embodiments of the present disclosure can be more easily understood and further advantages and uses thereof more readily apparent, when considered in view of the description of the preferred embodiments and the following figures in which: 
         FIG. 1  is a diagram of an organic photovoltaic device of one embodiment of the present disclosure; 
         FIG. 2  is a diagram illustrating a repeat unit of an electron donor copolymer of one embodiment of the present disclosure; 
         FIG. 2A  illustrates extrapolated estimates of HOMO and LUMO levels and shift in absorption gap for a hypothetical electron donor copolymer of one embodiment of the present disclosure; 
         FIG. 3  is a diagram illustrating a repeat unit of an electron donor copolymer of one embodiment of the present disclosure; 
         FIGS. 4A-4I  are diagrams illustrating various A-D-A repeat units for electron donor copolymers of alternate embodiments of the present disclosure; 
         FIG. 5  is a diagram illustrating a transparent photovoltaic device of one embodiment of the present disclosure; 
         FIG. 6  is a diagram illustrating an organic photovoltaic device of one embodiment of the present disclosure; and 
         FIG. 7  is a flow chart illustrating a method of one embodiment of the present disclosure. 
     
    
    
     In accordance with common practice, the various described features are not drawn to scale but are drawn to emphasize features relevant to embodiments of the present disclosure. Reference characters denote like elements throughout figures and text. 
     DETAILED DESCRIPTION 
     In the following detailed description, reference is made to the accompanying drawings that form a part hereof, and in which are shown specific illustrative examples in which embodiments may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the described embodiments, and it is to be understood that other embodiments may be utilized and that logical, mechanical and electrical changes may be made without departing from the scope of the embodiments of the present disclosure. The following detailed description is, therefore, not to be taken in a limiting sense. 
     Embodiments of the present disclosure provide for organic semiconductor devices and associated compositions of matter that incorporate an Acceptor-Donor-Acceptor copolymer repeat unit paradigm in the electron donor polymer material of an organic semiconductor absorber layer, such as for example, a bulk heterojunction (BHJ) absorber layer of a photovoltaic device. As the term is used herein and in the art of polymers, a repeat unit (which is sometimes also referred to as a repeating unit) is a part of a polymer whose repetition produces the chain or backbone of the polymer through the sequential linking of the repeat units together. That is, the polymer chain of repeat units can be expressed as a molecule chain defined by -[repeat unit] n - where n is greater than or equal to two. In particular with embodiments presented herein, the polymer comprises a molecule chain defined by -[Acceptor-Donor-Acceptor] n - where n is greater than or equal to two. Electron donor polymer materials having an Acceptor-Donor-Acceptor copolymer repeat unit allows for increased relative acceptor strength with respect to donor strength in the electron donor material component of the BHJ by incorporating two acceptors for each donor. This architecture creates a stronger push-pull affect than can be achieved using the conventional Donor-Acceptor copolymer paradigm. The novel Acceptor-Donor-Acceptor copolymer repeat unit architecture discussed herein provides access to different absorption spectra and energy levels than attainable with a Donor-Acceptor architecture, as is shown based on computations of electronic energy levels of polymers in Table 1. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 1 
               
             
            
               
                   
                   
               
               
                   
                 DA copolymer 
                 ADA copolymer 
               
            
           
           
               
               
               
               
               
               
               
            
               
                   
                   
                   
                 Absorption 
                   
                   
                 Absorption 
               
               
                 Donor Identity 
                 HOMO 
                 LUMO 
                 gap 
                 HOMO 
                 LUMO 
                 gap 
               
               
                   
               
               
                 ProDOT 
                 −4.26 
                 −2.95 
                 1.06 
                 −4.41 
                 −3.16 
                 0.97 
               
               
                 EDOT 
                 −4.26 
                 −2.99 
                 1.03 
                 −4.46 
                 −3.22 
                 0.96 
               
               
                 Triazole 
                 −4.78 
                 −3.01 
                 1.51 
                 −4.54 
                 −3.25 
                 1.10 
               
               
                 BDT 
                 −4.63 
                 −3.07 
                 1.32 
                 −4.62 
                 −3.32 
                 1.06 
               
               
                 CPDT 
                 −4.37 
                 −3.07 
                 1.04 
                 −4.49 
                 −3.25 
                 0.96 
               
               
                 DTS 
                 −4.39 
                 −3.01 
                 1.09 
                 −4.49 
                 −3.24 
                 0.98 
               
               
                 Fluorene 
                 −4.68 
                 −2.91 
                 1.65 
                 −4.57 
                 −3.27 
                 1.11 
               
               
                 Napthodithiophene 
                 −4.54 
                 −2.96 
                 1.27 
                 −4.58 
                 −3.32 
                 1.03 
               
               
                 Benzotrithiophene 
                 −4.44 
                 −3.02 
                 1.14 
                 −4.54 
                 −3.25 
                 1.02 
               
               
                   
               
            
           
         
       
     
     In Table 1, the calculated electronic properties of donor-acceptor repeat unit copolymers (DA) and acceptor-donor-acceptor repeat unit copolymers (ADA) are shown with the acceptor unit being diketo-pyrrolo-pyrrole (DPP) for all polymers. Properties are reported in units of electron-Volts (eV) and are obtained by extrapolating explicit calculations on the n=1 and n=2 cases to the polymer limit as described by Larsen (See, Larsen, R. E. (2016). Simple Extrapolation Method To Predict the Electronic Structure of Conjugated Polymers from Calculations on Oligomers.  J. Phys. Chem. C,  120 (18), pp 9650-9660, which is incorporated herein by reference in its entirety). The explicit calculations used density functional theory (DFT) on geometrically optimized structures to compute the HOMO and LUMO and used time-dependent DFT to compute the first excitation energy, here called the absorption gap. For all calculations the B3LYP exchange-correlation functional and a 6-31g(d) Pople-type Gaussian basis set were used. 
     Some embodiments of the present disclosure include organic photovoltaic (OPV) devices having a bulk heterojunction material absorber layer that includes an electron donor polymer comprising repeating sequences of an Acceptor-Donor-Acceptor copolymer repeat unit. The resulting OPV devices exhibit smaller bandgaps than previously achievable and also exhibit an enhanced responsiveness for tuning Highest Occupied Molecule Orbital (HOMO) and Lowest Unoccupied Molecule Orbital (LUMO) energy levels as well as tuning the absorption bandwidth of the absorber layer. 
       FIG. 1  is a diagram illustrating an example OPV device  100  of one embodiment of the present disclosure. As shown in  FIG. 1 , the layers of OPV device  100  may be deposited onto an underlying substrate layer  105 . In some alternate implementations of this embodiment, the substrate layer  105  may be optionally removed such that the final OPV device  100  may, or may not, include the presence of substrate layer  105 . The layers deposited onto substrate layer  105  which define OPV device  100  include a front contact layer  112 , an electron collection layer (ECL)  114 , an absorber layer  116 , a hole collection layer (HCL)  118  and a back contact layer  120 . In the particular embodiment shown in  FIG. 1 , absorber layer  116  operates as a photovoltaic absorber layer that generates electron and hole charges from absorbed photons, utilizing an electron donor polymer having a repeating sequence of an Acceptor-Donor-Acceptor copolymer repeat unit. 
     The purpose of the hole collection layer  118  is to function as a barrier to electrons attempting to migrate to the back contact layer  120  while at the same time allowing hole charges produced in the absorber layer  116  to flow into the back contact layer  120 . Similarly, the purpose of the electron collection layer  114  is to function as a barrier to holes attempting to migrate to the front contact layer  112  while allowing electrons produced in the absorber layer  116  to flow from into the front contact layer  112 . The resulting collection of opposing charges accumulating in the front contact layer  112  (negative electron charges) and back contact layer  120  (positive hole charges) manifests a voltage potential across the OPV device  100 . As would be appreciated by one of ordinary skill in the art of photovoltaics, at least one of either front contact layer  112  or the back contact layer  120  are transparent to a spectrum of photons that falls within the absorption band of the absorber layer  116  in order for such photons to reach the absorber layer  116  for the photovoltaic effect to occur. In some embodiments, device  100  is at least semitransparent to the visible light spectrum meaning that at least some light visible to human beings (generally considered to be light in the wavelength range of approximately 380 nm to 680 nm) completely penetrates through OPV device  100  without being absorbed. In such embodiments, both the front contact layer  112  and back contact layer  120  are transparent contact layers (TCLs). In some implementations, a TCL may comprise a transparent conducting oxide (TCO), transparent film or other material. 
     For embodiments where visible light transparency of OPV device  100  is not desired or otherwise not of concern, a metallization layer of silver or other opaque conducting material may be used to form one of the contact layers  112 ,  120 . In one embodiment, hole collection layer  118  comprises a thin film layer of a doped conjugated polymer such as, but not limited to PEDOT:PSS (poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)). In at least one such embodiment where the hole collection layer  118  comprises PEDOT:PSS, the same layer may also serve as the transparent back contact layer  120 . The electron collection layer  114  may comprise a transparent oxide such as but not limited to zinc oxide (ZnO) or a titanium dioxide (TiO 2 ). In other embodiments, other materials for hole collection layer  110 , election conduction layer  114 , front contact layer  112  and/or back contact layer  120  known to those of ordinary skill in the art for such uses may be utilized. 
     As mentioned above, in accordance with one embodiment of the present disclosure, the absorber layer  116  may comprise an electron acceptor material  130  and a copolymer electron donor material  132  that are blended into a bulk composite of the two materials to form a bulk-heterojunction (BHJ). The electron acceptor material  130  may comprise for example, fullerene or a fullerene derivative, or alternately a polymer or small molecule material known to those of skill in the art. The electron donor copolymer material  132  incorporates a repeat unit having an Acceptor-Donor-Acceptor pattern of co-polymerized monomers. That is, in the polymer chain of the copolymer electron donor material  132  there exists a repeating sequence of a repeat unit that comprises two acceptor moieties arranged on either side of a donor moiety. The Acceptor-Donor-Acceptor repeat unit results in a novel electrical structure within the copolymer material  132  in which two acceptor moieties are located adjacent to each other in the polymer sequence where repeat units are linked in sequence. It should be noted however that embodiments comprising a bilayer organic absorber layer  116  having the acceptor material  130  and donor material  132  as distinct layers are also contemplated as falling within the scope of embodiments of the present disclosure. 
     For example,  FIG. 2  illustrates at  200  a sequence of Acceptor-Donor-Acceptor repeat units  210  for an example electron donor copolymer material  132  of one embodiment of the present disclosure. The Acceptor-Donor-Acceptor repeat units  210  for this polymer chain each have a sequence that includes a first acceptor moiety  212 , bonded to a donor moiety  214 , which in turn is bonded to a second acceptor moiety  216 . As evident from  FIG. 2 , when such Acceptor-Donor-Acceptor repeat units  210  are synthesized together into a polymer molecule, neighboring repeat units  210  are linked together by adjacent acceptor moieties such as shown at  220 . That is, the neighboring Acceptor-Donor-Acceptor repeat units  210  result in a polymer chain that includes a doubling of acceptors on either side of the donor (i.e., . . . A-A-D-A-A . . . ) which is distinct from what is found in prior small molecule or polymer OPV systems. As such, it should be noted that the polymer chain in  FIG. 2  may also be alternately described as a sequence of Acceptor-Acceptor-Donor (AAD) or Donor-Acceptor-Acceptor (DAA) repeat units, both of which would refer to the same structure as an Acceptor-Donor-Acceptor (ADA) repeat unit. This structure of adjacent acceptor moieties results in a lower electron density within the copolymer material  132  component of the absorber layer  116  than previously achievable through Donor-Acceptor copolymer chains and therefore results in lower bandgaps within in the absorber layer  120 . The degree of polymerization (that is, the number of repeat units (n)) comprised by the material of the example electron donor copolymer will have a direct effect on the bandgap and viscosity of the resulting absorber layer. As the number of repeat units in the polymer chain increases, the bandgap narrows and the viscosity increases. Conversely, as the number of repeat units in the polymer chain decreases, the bandgap widens and the viscosity decreases. This is illustrated in  FIG. 2A , which shows for an acceptor-donor-acceptor repeat unit copolymer of a DPP moiety and a ProDOT moiety (such as shown in  FIG. 4A ) the expected shift in HOMO and LUMO levels (shown at  250 ) and the shift in absorption gap (shown at  252 ) as a function of the number of repeat units, based on an extrapolation procedure that utilizes explicit calculations or measurements for n=1 and n=2 to predict these properties as a function of n. Such as the previously mentioned extrapolation procedure described by Larsen ( 2016 ). For the acceptor-donor-acceptor repeat unit copolymer illustrated by  FIG. 2A , extrapolation predicts that that 90% of the total gap shift (going from n=1 to infinity) may be complete by n˜6, and 99% of the total gap shift may be complete by n˜21. 
     Although synthesizing semiconductor polymers having Acceptor-Donor-Acceptor repeat units  210  can be slightly more complex than synthesizing Donor-Acceptor repeat units, it should be appreciated that synthesizing Acceptor-Donor-Acceptor polymer chains having a desired repeat unit is within the skill of one of ordinary skill in the art who has studied this disclosure. The particular donor and acceptor moieties combined to synthesize electron donor copolymer material  132  can be selected from a wide range of different moieties. Although it is generally well established whether specific moieties are considered electron donors or electron acceptors, it should also be noted that some moieties may function as either a donor or acceptor depending on the relative electron density of the moiety it is paired with. 
       FIG. 3  illustrates at  300  an example of one general class of an electron donor copolymer material  132  that incorporates an Acceptor-Donor-Acceptor repeat unit  210  such as introduced by  FIG. 2 . In the particular embodiment of  FIG. 3 , the repeat units  310  of electron donor copolymer material  132  may comprise a donor moiety  314  selected from a range of known donor moieties, but specifically utilizes the acceptor moiety diketo-pyrrolo-pyrrole (DPP) for the first and second acceptor moieties  312  and  316 . Although DPP is a known ingredient for various industrial processes (for example, in making highly saturated color paints), the embodiment illustrated in  FIG. 3  discloses a novel application by incorporating the monomer as an electron acceptor moiety in electron donor copolymer material  300  having an Acceptor-Donor-Acceptor repeat unit  310 . The resulting electron donor copolymer material  300  may be expected to possess excellent stability, good solubility, and to have a strong tendency to aggregate leading to exceptional hole mobility. 
     Further an absorber layer  116  incorporating a DPP-Donor-DPP repeat unit electron donor copolymer material  132  may be expected to exhibit very desirable band gaps and energy levels for producing devices that have a significant percentage of photon absorption occurring outside of the visible light spectrum range. Such an absorber layer may therefore provide a desirable Visible Light Transmission (VLT) percentage for applications where transparency of the resulting device is desirable (such as window or natural lighting applications, for example). For example, through the appropriate selection of the donor moiety  314 , HOMO and LUMO energy levels and bandgaps may be adjusted with the objective to push the optical absorption band of the absorber layer  116  out to infrared (IR) or near IR frequencies. 
     In addition, embodiment incorporating DPP-Donor-DPP polymer systems may exhibit improved properties as compared to conventional Donor-DPP polymer systems, as the increased density of DPP moieties along the backbone of the polymer molecule increases the aggregation and consequent high hole mobilities, while retaining excellent solubility. This is in addition to the ability to access lower band gaps than with conventional Donor-DPP systems. 
     In alternate implementations, the donor moiety  314  of the Acceptor-Donor-Acceptor repeat unit  310  can include of any of a number of structures, including but not limited to: ethylenedioxythiophene (EDOT), propylenedioxythiophene (ProDOT) and derivatives thereof, benzodithiophene (BDT) derivatives, dithieneopyrrole (DTP) derivatives, dithienosilole (DTS) derivatives, cyclopentadithiophene (CPDT) derivatives, carbazole derivatives, benzotrithiophene, naphtodithiophene, and fluorene derivatives. 
       FIGS. 4A-4I  illustrate example acceptor-donor-acceptor repeat units that may be used in conjunction with any of the embodiments presented in the present disclosure. It should be appreciated by one skilled in the art that while the examples shown exhibit methyl substituents off of the constituent moieties, the substituents could consist of any of a number of different side-chains of different lengths and chemical composition.  FIG. 4A  illustrates an Acceptor-Donor-Acceptor repeat unit comprising DPP acceptor moieties (shown at  410 ) and a ProDOT donor moiety (shown at  420 ).  FIG. 4B  illustrates an Acceptor-Donor-Acceptor repeat unit comprising DPP acceptor moieties (shown at  410 ) and an EDOT donor moiety (shown at  421 ).  FIG. 4C  illustrates an Acceptor-Donor-Acceptor repeat unit comprising DPP acceptor moieties (shown at  410 ) and a Triazole donor moiety (shown at  422 ).  FIG. 4D  illustrates an Acceptor-Donor-Acceptor repeat unit comprising DPP acceptor moieties (shown at  410 ) and a BDT donor moiety (shown at  423 ).  FIG. 4E  illustrates an Acceptor-Donor-Acceptor repeat unit comprising DPP acceptor moieties (shown at  410 ) and a CPDT donor moiety (shown at  424 ).  FIG. 4F  illustrates an Acceptor-Donor-Acceptor repeat unit comprising DPP acceptor moieties (shown at  410 ) and a DTS donor moiety (shown at  425 ).  FIG. 4G  illustrates an Acceptor-Donor-Acceptor repeat unit comprising DPP acceptor moieties (shown at  410 ) and a fluorene donor moiety (shown at  426 ).  FIG. 4H  illustrates an Acceptor-Donor-Acceptor repeat unit comprising DPP acceptor moieties (shown at  410 ) and a naphtodithiophene donor moiety (shown at  427 ).  FIG. 4I  illustrates an Acceptor-Donor-Acceptor repeat unit comprising DPP acceptor moieties (shown at  410 ) and a benzotrithiophene donor moiety (shown at  428 ). 
       FIG. 5  is a diagram illustrating an example OPV window  500  embodiment of the present disclosure. In alternate implementations, OPV window  500  comprises part of a window unit used, for example in a building-integrated photovoltaics application that allows natural lighting into an interior space of a building. In other implementations, OPV window  500  comprises a window used for a vehicle. In still other implementations, OPV window  500  comprises part of a display screen or other transparent component of an electronics device. 
     OPV window  500  comprises a plurality of OPV cells  510  fabricated on a base window material substrate  505 . In alternate implementations, base window material substrate  505  may comprise a rigid semi-transparent material such as a glass window pane or a sheet of acrylic or acrylic glass, semi-transparent plastic or film material. Each of the OPV cells  510  include the same structure and operate as described above with respect to OPV device  100 . In particular, the OPV cells  510  include front and back contact layers  512  and  520  (both of which are realized as transparent contact layers), a hole collection layer  518 , an organic semiconductor absorber layer  516  and an electron collection layer  514 . In one implementation, the absorber layer  516  is a BHJ absorber layer that comprises an electron acceptor material blended with an electron donor polymer having an Acceptor-Donor-Acceptor copolymer repeat unit. As such, any of the alternate compositions applicable to the Acceptor-Donor-Acceptor copolymer repeat unit  210  of the electron donor copolymer  132  described herein are applicable to the electron donor copolymer material of absorber layer  516 . The electron acceptor material of absorber layer  516  may comprise for example, fullerene or a fullerene derivative, or alternately a polymer or small molecule material known to those of skill in the art, such as described for electron acceptor material  130  of OPV device  100  above. Each of the OPV cells  510  are electrically coupled by electrical interconnects  530 . The electrical interconnects  530  may provide for series interconnection of the OPV cells  510  (as illustrated in  FIG. 5 ) or alternately parallel interconnection of the OPV cells  510 . In one embodiment, the various device layers of the OPV cells  510  are deposited across the base window material substrate  505  and scribes are cut at least partially into the layers to form the electrically distinct OPV cells  510 . Subsequent layers of material may then be deposited to create the electrical interconnects  530  between the OPV cells  510 . 
     It should be appreciated that the designation of “front” and “back” with respect to the OPV cells  510  and contact layers  520 ,  512  is essentially arbitrary as light may enter the OPV cells  510  from either side to produce electricity. In non-transparent OPV devices, the top contact layer,  120 , is opaque and often reflective so as to reflect light back into the BHJ layer. By convention, light is expected to enter or exit the device from the opposing “front” side, which makes the substrate and adjacent contact layer the “front” side. As such, as the terms are used herein, the “front” side of an OPV device refers to layers between the BHJ layer and the substrate on which the layers were deposited, while the “back” side of an OPV device refers to those layers on the opposite side of the BHJ layer from the substrate, regardless as to whether or not the OPV device is semi-transparent. 
     In the embodiment of  FIG. 5 , when light  501  enters into the OPV cells  510  the absorber layers  516  generate electron and hole charges from absorbed photons. The positive and negative electrical charges are collection in the respective contact layers  520  and  512  and through the electrical interconnects  530  (which may couple the respective contact layers  520  and  512  in either a serial or parallel configuration) bring the charges to positive and negative electrodes  540  and  542  (which may be positioned, for example, at the edges of OPV window  500 ). The positive and negative electrodes  540  and  542  may in turn be coupled to one or more electronic devices in order to provide electrical power to the devices, and/or for storage of the energy generated by OPV window  500 . 
       FIG. 6  is a diagram of a general organic photovoltaic device  600  of one embodiment of the present disclosure. In some implementations, organic semiconductor device  600  may comprise an OPV device such as OPV device  100  or OPV window  500 . As shown in  FIG. 6 , the layers of organic semiconductor device  600  may be deposited onto an underlying substrate layer  605 . In some alternate implementations of this embodiment, the substrate layer  605  may be optionally removed such that the final organic semiconductor device  600  may, or may not, include the presence of substrate layer  605 . The layers deposited onto substrate layer  605  which define organic semiconductor device  600  include a first contact layer  612 , a first charge collection layer  614  (which may include either a hole collection layer (HCL) or an electron collection layer (ECL)), an absorber layer  616 , a second charge transport layer  618  (which may include either an electron collection layer (ECL) or a hole collection layer (HCL)), and a second contact layer  120 . It should be appreciated that when the first charge collection layer  614  comprises an HCL, then the second charge collection layer  618  comprises an ECL. Similarly, when the first charge collection layer  614  comprises an ECL, then the second charge collection layer  618  comprises an HCL. The second contact layer  620  may be implemented as a transparent contact layer to allow photons to either enter or exit device  600 . Where organic semiconductor device  600  is intended to be a semi-transparent device, then both the first and second contact layers  612 ,  620  are implemented as transparent contact layers to allow photons to either enter or exit device  600 . 
     In accordance with the present disclosure, the absorber layer  616  comprises an electron acceptor material  630  and a copolymer electron donor material  632 . In some embodiments, the acceptor material  630  and donor material  632  are blended into a bulk composite of the two materials to form a bulk-heterojunction (BHJ). In other embodiments, the absorber layer  616  comprises a bilayer organic absorber layer having the acceptor material  630  and donor material  632  as distinct layers. The electron acceptor material  630  may comprise for example, fullerene or a fullerene derivative, or alternately a polymer or small molecule material known to those of skill in the art. 
     The electron donor copolymer material  632  incorporates a repeat unit having an Acceptor-Donor-Acceptor pattern of co-polymerized monomers. In the polymer chain of the copolymer electron donor material  632  there exists a repeating sequence of a repeat unit that comprises two acceptor moieties arranged on either side of a donor moiety. The Acceptor-Donor-Acceptor repeat unit results in a novel electrical structure within the copolymer material  632  in which two acceptor moieties are located adjacent to each other in the polymer sequence where repeat units are linked in sequence. Because the electron donor copolymer material  632  incorporates the Acceptor-Donor-Acceptor repeat unit pattern as the Acceptor-Donor-Acceptor repeat unit  210  introduced by  FIG. 2 , any of the alternate compositions applicable to the Acceptor-Donor-Acceptor copolymer repeat unit  210  of the electron donor copolymer  132  described previously herein are applicable to the electron donor copolymer material of absorber layer  616 . In some embodiments, a plurality of organic semiconductor devices  600  may be coupled together in either series or parallel configurations such as through electrical interconnects in the manner shown in  FIG. 5 . 
       FIG. 7  is a flow chart illustrating a method  700  of one embodiment of the present disclosure. It should be understood that method  700  may be implemented in conjunction with any of the embodiments described above with respect to  FIGS. 1, 2, 3, 4A-4I, 5 and 6 . As such, elements of method  700  may be used in conjunction with, in combination with, or substituted for elements of those embodiments described above. Further, the functions, structures and other description of elements for such embodiments described above may apply to like named elements of method  700  and vice versa. 
     The method begins at  700  with synthesizing an electron donor copolymer having an acceptor-donor-acceptor repeat unit. That is, the electron donor copolymer comprises a repeating unit of co-polymerized monomers that incorporate an acceptor-donor-acceptor moiety pattern. This acceptor-donor-acceptor moiety pattern is illustrated by the Acceptor-Donor-Acceptor repeat units  210  of  FIG. 2 . The Acceptor-Donor-Acceptor repeat units  210  each have a sequence that includes a first acceptor moiety  212 , bonded to a donor moiety  214 , which in turn is bonded to a second acceptor moiety  216 . When such Acceptor-Donor-Acceptor repeat units are synthesized together into a polymer molecule, neighboring repeat units are linked together by adjacent acceptor moieties. This structure of adjacent acceptor moieties results in a lower electron density within the electron donor copolymer than previously achievable through Donor-Acceptor copolymer chains. It should be appreciated that synthesizing Acceptor-Donor-Acceptor polymer chains as a desired repeat unit is within the skill of one of ordinary skill in the art who has studied this disclosure. For example, in one embodiment, synthesizing Acceptor-Donor-Acceptor polymer chains may be accomplished by the homopolymerization of individual Acceptor-Donor-Acceptor monomers to form the chain of Acceptor-Donor-Acceptor repeat units. In another embodiment, synthesizing Acceptor-Donor-Acceptor polymer chains may be accomplished by co-polymerization of an Acceptor-Acceptor monomer with a Donor monomer to produce an Acceptor-Donor-Acceptor repeat unit copolymer. 
     The particular donor and acceptor moieties combined to synthesize the electron donor copolymer can be selected from a wide range of different moieties. Although it is generally well established whether specific moieties are considered electron donors or electron acceptors, it should also be noted that some moieties may function as either a donor or acceptor depending on the relative electron density of the moiety it is paired with. 
     One general class of an electron donor copolymer that may be synthesized at  710  is shown at  300  in  FIG. 3 . This particular electron donor copolymer material  300  incorporates an Acceptor-Donor-Acceptor repeat unit  310  comprises a donor moiety  314  selected from a range of known donor moieties, but specifically utilizes the acceptor moiety diketo-pyrrolo-pyrrole (DPP) for the first and second acceptor moieties  312  and  316 . Although DPP is a known ingredient for various industrial processes (for example, in making highly saturated color paints), applying DPP as part of synthesizing an electron donor copolymer having an acceptor-donor-acceptor repeat unit discloses a novel application. The resulting electron donor copolymer material  300  possesses excellent stability, good solubility, and a strong tendency to aggregate leading to exceptional hole mobility. 
     The method proceeds to  720  with combining the electron donor copolymer with an electron acceptor. In one embodiment, the electron donor copolymer is combined with an electron acceptor by layering one material on top of the other to produce a bilayer organic material layer. In other embodiments, the electron donor copolymer is combined with an electron acceptor by blending the electron donor copolymer with an electron acceptor into a bulk heterojunction material. The electron acceptor material may comprise for example, fullerene or a fullerene derivative, or alternately a polymer or small molecule material known to those of skill in the art. The formation of a bulk heterojunction material may be used to form an absorber layer of an OPV device that generates electron and hole charges from absorbed photons. A bilayer structure may alternately be used to form an absorber layer of an OPV device. Organic semiconductor devices incorporating the resulting bulk heterojunction material exhibit smaller bandgaps than previously achievable from donor-acceptor polymer architectures and also exhibit an enhanced responsiveness for tuning Highest Occupied Molecule Orbital (HOMO) and Lowest Unoccupied Molecule Orbital (LUMO) energy levels as well as tuning the absorption band width of the absorber layer. 
     In some embodiments, a bulk heterojunction material produced from steps  710  and  720  may be used to fabricate an absorber layer of a semi-transparent organic semiconductor device, such as but not limited to OPV window  500  described above. In that case, the bulk heterojunction material at  730  is deposited on a transparent material layer. For example, in producing a transparent device such as in  FIG. 5 , the bulk heterojunction material may be applied onto the transparent material of a transparent electron collection layer  514 , which itself was deposited on a transparent contact layer  512  and transparent substrate  505 . Subsequent device layers may then be deposited on the bulk heterojunction material and, electrical interconnects optionally fabricated, to complete the particular structure of the desired device, such as any illustrated or discussed with respect to the above figures. 
     Example Embodiments 
     Example 1 includes a composition of matter, the composition of matter comprising a copolymer material having an acceptor-donor-acceptor moiety repeat unit. 
     Example 2 includes the composition of matter of example 1, wherein: the acceptor moieties of the acceptor-donor-acceptor moiety repeat unit comprise a diketo-pyrrolo-pyrrole (DPP) monomer. 
     Example 3 includes the composition of matter of example 2, wherein the donor moiety of the acceptor-donor-acceptor moiety repeat unit comprises one of a group of monomers comprising: ethylenedioxythiophene (EDOT) and EDOT derivatives; propylenedioxythiophene (ProDOT) and ProDOT derivatives; benzodithiophene (BDT) and BDT derivatives; dithieneopyrrole (DTP) and DTP derivatives; dithieneosilole (DTS) and DTS derivatives; cyclopentadithiophene (CPDT) and CPDT derivatives; carbazole and carbazole derivatives; benzotrithiophene and benzotrithiophene derivatives; naphtodithiophene and naphtodithiophene derivatives; and fluorene and fluorene derivatives. 
     Example 4 includes the composition of matter of any of examples 1-3, wherein the copolymer material is further blended with an electron acceptor material into a bulk composite to form a bulk heterojunction material, wherein the copolymer material defines an electron donor material within the bulk heterojunction material. 
     Example 5 includes the composition of matter of example 4, wherein the electron acceptor material comprise one of a group of electron acceptor materials comprising: fullerene; a fullerene derivative; a polymer; and a small molecule material. 
     Example 6 includes an organic photovoltaic device, the device comprising: an organic semiconductor layer comprising a combination of an electron acceptor material with an electron donor copolymer material; wherein the electron donor copolymer material comprises a repeating sequence of a repeat unit having an acceptor-donor-acceptor moiety pattern. 
     Example 7 includes the device of example 6, wherein the organic semiconductor layer comprises a blend of the electron acceptor material with the electron donor copolymer material forming a bulk heterojunction. 
     Example 8 includes the device of example 6, wherein the organic semiconductor layer comprises a layering of the electron acceptor material and the electron donor copolymer material. 
     Example 9 includes the device of any of examples 6-8, wherein: the repeat unit acceptor moieties each comprise a diketo-pyrrolo-pyrrole (DPP) monomer. 
     Example 10 includes the device of examples 6-9, wherein the repeat unit donor moiety comprises one of a group of monomers comprising: ethylenedioxythiophene (EDOT) and EDOT derivatives; propylenedioxythiophene (ProDOT) and ProDOT derivatives; benzodithiophene (BDT) and BDT derivatives; dithieneopyrrole (DTP) and DTP derivatives; dithieneosilole (DTS) and DTS derivatives; cyclopentadithiophene (CPDT) and CPDT derivatives; carbazole and carbazole derivatives; benzotrithiophene and benzotrithiophene derivatives; naphtodithiophene and naphtodithiophene derivatives, and fluorene and fluorene derivatives. 
     Example 11 includes the device of any of examples 6-10, wherein the electron acceptor material comprise one of a group of electron acceptor materials comprising: fullerene; a fullerene derivative; a polymer; and a small molecule material. 
     Example 12 includes the device of any of examples 6-11, further comprising: a first contact layer; a second contact layer; a hole collection layer; an electron collection layer; and an absorber layer that comprises the bulk heterojunction layer, the absorber layer positioned between the hole collection layer and the electron collection layer. 
     Example 13 includes an organic photovoltaic device, the device comprising: a first contact layer; a second contact layer; a hole collection layer adjacent to the first transparent contact layer; an electron collection layer adjacent to the second transparent contact layer; and an absorber layer positioned between the hole collection layer and the electron collection layer, the absorber layer comprising an electron acceptor material and an electron acceptor polymer material, wherein the electron acceptor polymer material has an acceptor-donor-acceptor repeat unit. 
     Example 14 includes the device of examples 13, wherein at least one of the first contact layer or the second contact layer comprises a transparent contact layer. 
     Example 15 includes the device of any of examples 13-14, wherein both the first contact layer and the second contact layer comprises a transparent conducting layer; and wherein the absorber layer is at least semi-transparent to light having a wavelength in the visible light spectrum. 
     Example 16 includes the device of any of examples 13-15, wherein: the acceptor-donor-acceptor repeat unit comprises a diketo-pyrrolo-pyrrole (DPP) monomer acceptor moiety, 
     Example 17 includes the device of any of examples 13-16, wherein a donor moiety of the acceptor-donor-acceptor repeat unit comprises one of a group of monomers comprising: ethylenedioxythiophene (EDOT) and EDOT derivatives; propylenedioxythiophene (ProDOT) and ProDOT derivatives; benzodithiophene (BDT) and BDT derivatives; dithieneopyrrole (DTP) and DTP derivatives; dithieneosilole (DTS) and DTS derivatives; cyclopentadithiophene (CPDT) and CPDT derivatives; carbazole and carbazole derivatives; benzotrithiophene and benzotrithiophene derivatives; naphtodithiophene and naphtodithiophene derivatives; and fluorene and fluorene derivatives. 
     Example 18 includes the device of any of examples 13-17, wherein the electron acceptor material comprise one of a group of electron acceptor materials comprising: fullerene; a fullerene derivative; a polymer; and a small molecule material. 
     Example 19 includes a method for fabricating an organic semiconductor material, the method comprising: synthesizing an electron donor copolymer having an acceptor-donor-acceptor repeat unit; and combining the electron donor copolymer with an electron acceptor. 
     Example 20 includes the method of example 19, wherein combining the electron donor copolymer with the electron acceptor further comprises: layering the electron donor copolymer and the electron acceptor to produce a bilayer organic material layer. 
     Example 21 includes the method of example 19, wherein combining the electron donor copolymer with the electron acceptor further comprises: blending the electron donor copolymer with an electron acceptor into a bulk heterojunction material. 
     Example 22 includes the method of example 21, further comprising: depositing the bulk heterojunction material on at least one transparent material layer. 
     Example 23 includes the method of any of examples 19-22. wherein the acceptor moieties of the acceptor-donor-acceptor moiety repeat unit comprise a diketo-pyrrolo-pyrrole (DPP) monomer. 
     Example 24 includes the method of any of examples 19-23, wherein the donor moiety of the acceptor-donor-acceptor moiety repeat unit comprises one of a group of monomers comprising: ethylenedioxythiophene (EDOT) and EDOT derivatives; propylenedioxythiophene (ProDOT) and ProDOT derivatives; benzodithiophene (BDT) and BDT derivatives; dithieneopyrrole (DTP) and DTP derivatives; dithieneosilole (DTS) and DTS derivatives; cyclopentadithiophene (CPDT) and CPDT derivatives; carbazole and carbazole derivatives; benzotrithiophene and benzotrithiophene derivatives; naphtodithiophene and naphtodithiophene derivatives; and fluorene and fluorene derivatives. 
     Example 25 includes the method of any of examples 19-24, wherein the electron acceptor material comprise one of a group of electron acceptor materials comprising: fullerene; a fullerene derivative; a polymer; and a small molecule material. 
     Although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that any arrangement, which is calculated to achieve the same purpose, may be substituted for the specific embodiment shown. This application is intended to cover any adaptations or variations of the embodiments described herein. Therefore, it is manifestly intended that embodiments of the present disclosure be limited only by the claims and the equivalents thereof.