Abstract:
In one embodiment of the present disclosure, a series of conjugated polymers used, among other things, as polymer solar cell or polymer photovoltaic device active layer materials, is provided. In one embodiment, the conjugated polymers have the general structure and formula shown in (I), wherein: R1 and R2 are independently selected from proton, halogens, alkyls, aryls and substituted aryls; Ar is selected from the group consisting of monocyclic, bicyclic and polycyclic arylene, or monocyclic, bicyclic and polycyclic heteroarylene. In another embodiment, the conjugated photovoltaic polymers are comprised of repeated units having the general structure of formula (II), wherein, R1, R2, R3, R4, R5, and R6 are independently selected from proton, alkyls, halogens, aryls, substituted aryls, and other kinds of substituents. Synthesis methods of several polymers of the present disclosure are provided, and absorption spectra and electrochemical cyclic voltammetry data of some polymers, and also the photovoltaic properties of the polymers in this present disclosure are also provided.

Description:
BACKGROUND 
     The present disclosure relates generally to conjugated polymers. More specifically, the present disclosure relates to a class of conjugated photovoltaic polymers, which among other things, are useful as active layer materials in polymer solar cell or polymer photovoltaic devices and the like. 
     SUMMARY 
     Conjugated polymers are polymers containing {tilde over (π)}-electron conjugated units along the main chain. These polymers can be used as the active layer material of some kinds of photon-electronic devices, such as polymer light emission devices, polymer solar cells, polymer field effect transistors, and the like. As a polymer solar cell material, conjugated polymers should ideally possess certain properties. These properties are high mobility, good harvest of sunlight, good processability, and proper molecular energy level. Some conjugated polymers have proved to be good solar cell materials. For example, some derivatives of poly(p-phenylene vinylene), such as MEH-PPV and MDMO-PPV, and some derivatives of poly(3-alky-thiophene), such as P3HT and P3OT, and some conjugated polymers with heterocyclic aromatic rings, such as poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b′]-dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (PCPDTBT), have successfully been used as photo-active layer materials. 
     Poly[4,8-dialkyl-benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl-alt-2,3-substituted-thieno[3,4-b]thiophene-4,6-diyl] is a class polymeric materials in the application of polymer solar cells. These polymers have shown excellent photovoltaic properties. When the substituent as 2-position of the thieno[3,4-b]thiophene was selected from ester, amide, cyano, alkyl, polyfluoroalkyl, polychloroalkyl, aryl, or heteroaryl, the polymers will be a class of potential materials in polymer solar cells. 
     In another example, the band gap and molecular energy of benzo[1,2-b:4,5-b′]dithiophene-based polymers can be tuned effectively by copolymerizing with different conjugated units, such as ethylenedioxy-thiophene-2,5-diyl, thieno[3,4-b]pyrazine-2,5-diyl, benzo[c][1,2,5]selenadiazole-4,7-diyl, and benzo[c][1,2,5]thiadiazole-4,7-diyl, and the substituted benzo[1,2-b:4,5-b′]dithiophene can be copolymerized with different conjugated units, including thieno[3,4-b]thiophene, benzo[c][1,2,5]thiadiazole, and other kinds of heteroarylene. From this, it can be concluded that the molecular energy, band gap, and hence photovoltaic properties of poly[benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl-alt-thieno[3,4-b]thiophene-4,6-diyl]-based conjugated polymers can be tuned by changing the substituent&#39;s. However, it is hard to say that what kind of substituent will have positive effects on photovoltaic properties of poly[benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl-alt-thieno[3,4-b]thiophene-4,6-diyl]-based conjugated polymers. 
     In order to improve the photovoltaic properties of thieno[3,4-b]thiophene-4,6-diyl]-based conjugated polymers, we investigated different substituent&#39;s on 2-position of the thieno[3,4-b]thiophene units of the polymer and found that the thieno[3,4-b]thiophene-4,6-diyl]-based conjugated polymers with carbonyl groups on 2-position units exhibited excellent photovoltaic properties. 
     The present disclosure provides a family of conjugated polymers useful in photovoltaic devices including polymer photodetector devices and polymer solar cell devices, and the general structure of such polymers is shown in formula (I). 
     
       
                 
         
             
             
         
      
     
     wherein: R1 and R2 are independently selected from a proton, halogens, alkyls, substituted alkyls, amino, N-substituted amino groups, aryls and substituted aryls; Ar is selected from the group consisting of ethenylene, or ethynylene, or monocyclic, bicyclic and polycyclic arylenes, or monocyclic, bicyclic and polycyclic heteroarylenes, or unit being comprised of two or more compounds choosing from ethenylene, ethynylene, or monocyclic, bicyclic and polycyclic arylene, or monocyclic, bicyclic and polycyclic heteroarylenes. 
     It is another object of the present disclosure to provide polymer solar cell devices containing one or several polymers of the present disclosure as photovoltaic material. 
     In one embodiment, the conjugated photovoltaic polymers are comprised of repeated units having the general structure of formula (II) 
     
       
                 
         
             
             
         
      
     
     wherein: R1 is selected from proton, alkyls, substituted alkyls, aryls, substituted aryls; R2, R3, R4, R5 and R6 are selected independently from proton, alkyls, substituted alkyls, alkoxyls, substituted alkoxyls, halogens, aryls, substituted aryls. 
     In another embodiment, R5 and R6 are protons; R3, R4 and R1 are selected independently from alkyls, substituted alkyls, alkoxyls, substituted alkoxyls, aryls, substituted aryls. 
     In yet another embodiment, photovoltaic devices are provided that contain a polymer of the present disclosure as the photovoltaic material. In yet other embodiments, it is desirable to incorporate fullerene or its derivatives or other additives into the polymer for use in photovoltaic devices. These devices include photodetector devices and polymer solar cell devices. In yet another embodiment, a polymer solar cell device prepared with a polymer of the present disclosure is a bulk-heterojunction photovoltaic device. 
     Carbonyl group substituted polymers can be readily prepared. Results indicated that carbonyl groups of the thieno[3,4-b]thiophene-4,6-diyl]-based conjugated polymers improve photovoltaic properties, especially in obtaining higher open circuit voltage, and as a result, a power conversion efficiency of 7.7% with an open circuit of 0.78V and a short circuit current of 15 mA has been achieved by using one of the 2-carbonyl-thieno[3,4-b]thiophene-4,6-diyl] contained conjugated polymers as the electron donor material in the active layer of polymer solar cells. 
    
    
     
       DRAWINGS 
       The above-mentioned features and objects of the present disclosure will become more apparent with reference to the following description taken in conjunction with the accompanying drawings wherein like reference numerals denote like elements and in which: 
         FIG. 1  is an I-V curve of a polymer solar cell in accordance with the present disclosure. with a structure of ITO/PEDOT:PSS/poly[4,8-bis(2-ethylhexoxy)-benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl-alt-2-(2-ethylhexanoyl)thieno[3,4-b]thiophene-4,6-diyl] (PBDTTT)/PCBM (1:1.5 wt/wt)/Ca/Al. 
         FIG. 2  is another I-V curve of a polymer solar cell with a structure of ITO/PEDOT:PSS/poly[4,8-bis(2-ethylhexoxy)-benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl-alt-2-(octanoyl)-3-fluoro-thieno[3,4-b]thiophene-4,6-diyl] (PBDTTT-F)/PCBM (1:1.5 wt/wt)/Ca/Al. 
     
    
    
     DETAILED DESCRIPTION 
     Definitions and Nomenclature 
     Unless otherwise indicated, the present disclosure is not limited to specific starting materials, regents or reaction conditions, as such may vary. 
     The term “alkyl” as used herein refers to a branched or unbranched saturated hydrocarbon group typically although not necessarily containing 1 to 24 carbon atoms, such as methyl, ethyl, n-propyl, isopropyl, n-octyl, isooctyl, decyl, and the like, as well as cycloalkyl groups such as cyclopentyl, cyclohexyl and the like. 
     The term “heteroarylene” as used herein refers to a hydrocarbon arylene in which one or more carbon atoms are replaced with a “heteroatom” other than carbon, e.g., nitrogen, oxygen, sulfur, silicon, selenium, phosphorus. 
     The term “N-containing heteroarylene” as used herein refers to a heteroarylene in which one or more “heteroatom” defined above are nitrogen atoms. 
     The term “substituted” as in “substituted alkyl”, “substituted arylene”, “substituted heteroarylene”, and the like, is meant that in the arylene or heteroarylene, or other moiety, at least one hydrogen atom bound to a carbon (or other) atom is replaced with one or more non-hydrogen substituents. Such substituents include, but not limited to, functional groups such as halogen, oxygen, hydroxyl, alkylthio, alkoxy, aryloxy, alkylcarbonyl, acyloxy, nitro, cyano, and the like. 
     The polymers of the present disclosure are comprised of repeated units having the general structure of formula (I) 
     
       
                 
         
             
             
         
      
     
     wherein: A1, A2, R1 and R2 are independently selected from proton, alkyl groups with up to 18 C atoms, alkoxy groups with up to 18 C atoms, cyano, nitro, aryls and substituted aryls. 
     Ar is selected from the group consisting of ethenylene, or ethynylene, or monocyclic, bicyclic and polycyclic arylene, or monocyclic, bicyclic and polycyclic heteroarylene, or may contain one to five, typically one to three such groups, either fused or linked. 
     In a polymer formula (I) are comprised of repeated units having the structure of formula (II) 
     
       
                 
         
             
             
         
      
     
     wherein: R1 is selected from proton, alkyls, substituted alkyls, aryls, substituted aryls; R2, R3, R4, R5 and R6 are selected independently from proton, alkyls, substituted alkyls, alkoxyls, substituted alkoxyls, halogens, aryls, substituted aryls. 
     In a specific polymer structure of formula (II), R5 and R6 are protons; R3, R4 and R1 are selected independently from alkyls, substituted alkyls, alkoxyls, substituted alkoxyls, aryls, substituted aryls; R2 is selected from proton, halogens, cyano. 
     In another embodiment polymers of formula (I) are comprised of repeated units having the structure of formula (III) 
     
       
                 
         
             
             
         
      
     
     wherein: R1 is selected from proton, alkyls, substituted alkyls, aryls, substituted aryls; R2, R3, R4, R5 and R6 are selected independently from proton, alkyls, substituted alkyls, alkoxyls, substituted alkoxyls, halogens, aryls, substituted aryls. 
     In a specific polymer of formula (III), R3 and R6 are protons; R4, R5 and R1 are selected independently from alkyls, substituted alkyls, alkoxyls, substituted alkoxyls, aryls, substituted aryls; R2 is selected from proton, halogen atom, cyano. 
     Typically, the number average molecular weight of the polymers is in the range of approximately 1000 to 1,000,000, with ideal polymers having a number average molecular weight in the range of about 5000 to 500,000, and some ideal polymers having a number average molecular weight in the range of approximately 20,000 to 200,000. It will be appreciated that molecular weight can be varied to optimize polymer properties and the inventions of the present disclosure cover all molecular weights. For example, lower molecular weight can ensure solubility, while a higher molecular weight can ensure good film-forming properties. 
     Semiconductive compositions may be prepared that comprise a polymer of the present disclosure optionally combined with an admixer, typically a compound selected such that charge and/or energy transfer takes place between the admixer and the polymer when a excitation source including light or voltage is applied across the composition. For example, the admixer can be fullerene such as: C 60 , C 70 , or C 80 , or some substituted fullerene compounds such as PCBM ([6,6]-phenyl C 61  butyric acid methyl ester) and PCBB ([6,6]-phenyl C 61  butyric acid butyl ester). 
     The polymers of the present disclosure are particularly useful as photovoltaic materials in photovoltaic devices such as photodetector devices, solar cell devices, and the like. Photovoltaic devices, including solar cell devices, are generally comprised of laminates of a suitable photovoltaic material between a hole-collecting electrode layer and an electron-collecting layer. Additional layers, elements or a substrate may or may not be present. 
     In practice, the present disclosure may employ conventional techniques of organic chemistry and polymer chemistry. In the following examples, efforts have been made to ensure accuracy with respect to numbers used, including amounts, temperature, reaction time, and the like, but some experimental error and deviation should be accounted for. Temperature used in the following examples is in degrees Celsius, and the pressure is at or near atmospheric pressure. All solvents were purchased as HPLC grade, and all reaction were routinely conducted under an inert atmosphere of argon. All reagents were obtained commercially unless otherwise indicated. 
     EXAMPLE 1 
     Synthesis of poly[4,8-bis(2-ethylhexoxy)-benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl-alt-2-(2-ethylhexanoyl)thieno[3,4-b]thiophene-4,6-diyl] (PBDTTT) 
     Synthesis procedure of one of the monomers of the polymer PBTTTT is shown in the following scheme. 
     
       
                 
         
             
             
         
      
     
     Step 1 
     Thiophene (1 mol, 84 g), 2-2-ethylhexanoyl chloride (1 mol, 162.5 g) and methylene chloride (500 ml) were put in one flask. Aluminum chloride powder (1 mol, 133.5 g) was added in small portions, during the addition, the reactant was kept below 0 degrees C. After completing the addition, the reactant was warm up slowly to ambient temperature and stirred for 30 min. Then, the reactant was poured into 1 Kg of cracked ice. The mixture was extracted by ethyl ether for several times, and the organic layers were combined and the volatile was removed under reduced pressure. The residual oil was purified by distillation and gave compound C1 as colorless oil (Yield 85-90%). 
     Step 2 
     Compound C1 (1.5 mol, 105 g) was mixed with methyl chloride methyl ether in a flask. Under ice-water bath, Stannum tetrachloride was added dropwisely. Then, the reactant was stirred at 0 degrees C. for two hours. Successively, the reactant was poured into ice and extracted by ethyl ether for several times and the organic layers were combined and the volatile was removed under reduced pressure. The residual oil was purified by silica gel column using hexane: ethyl acetate (10:1) as eluent and gave compound C2 as brown oil. Although there are unknown impurities, the purity of this compound is good enough to next step. (Yield 50%) 
     Step 3 
     Compound C2 (6.14 g, 20 mmol) and methanol (120 ml) was mixed in a flask and heated to reflux slightly. Then, a solution of 60% sodium sulfide (2.6 g, 20 mmol) and 60 ml methanol was added into the flask dropwisely. The reactant was stirred for 1 hour and methanol was removed. The residue was purified by silica gel column using hexane: ethyl acetate (8:1) as eluent and gave compound C3 as yellow oil. (Yield 60%) 
     Step 4 
     All of the compound C3 from last step was dissolved into 70 ml ethyl acetate and a solution of MCPBA (1.72 g, 10 mmol) in 40 ml ethyl acetate was added under −40 degrees C. very slowly. After the addition, the cooling bath was removed and the reactant was stirred under ambient temperature for 8 hours, and then the ethyl acetate was removed by reduced pressure and 30 ml of acetic anhydride was added. The reactant was heated to reflux for 1 hour. After removed acetate anhydride under vacuum, the residue was purified by silica gel column using hexane: methylene chloride (1:1) as eluent and gave compound C4 as yellow oil. 
     Step 5 
     Compound C4 (2.66 g, 10 mmol) was dissolved into 30 ml DMF under protect of argon. NBS (4.45 g, 25 mmol) was added in one portion. After 20 min of stirring, the reactant was poured into a cold solution of sodium thiosulfate and stirred for several minutes. Then, the mixture was extracted by ethyl ether and purified by silica gel column using hexane as eluent and gave compound C5 as pale yellow oil. (Yield 70%) 
     Synthesis of PBDTTT by Stille Coupling Reaction 
     2,6-bis(trimethyltin)-4,8-bis(2-ethylhexoxy)-benzo[1,2-b;3,4-b]-dithiophene (1 mmol, compound C5 (1 mmol), 20 ml of toluene and 2 ml of DMF were put into a two-necked flask with oil bath. The solution was flushed with argon for 10 minutes, then 30 mg of Pd(PPh 3 ) 4  were added into the flask. The solution was flushed again for 20 minutes. The oil bath was heated to 110 degrees C. carefully, and the reactant was stirred for 16 hours at this temperature under argon atmosphere. Then, the reactant was cooled to room temperature and the polymer was precipitated by addition of 100 ml methanol, and filtered through a Soxhlet thimble, which was then subjected to Soxhlet extraction with methanol, hexane, and chloroform. The polymer was recovered as a solid sample from the chloroform fraction by rotary evaporation. The solid was dried under vacuum for 1 day to get the final product. 
     EXAMPLE 2 
     Synthesis of poly[4,8-bis(2-ethylhexoxy)-benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl-alt-2-octanoyl-3-fluoro-thieno[3,4-b]thiophene-4,6-diyl] (PBDTTT-F) 
     Synthesis procedure of one of the monomers of the polymer PBTTTT-F is described as follows. 
     Step 1 
     3-Fluoro-4,6-dihydrothieno[3,4-b]thiophene-2-carboxylic acid prepared through the procedure described above (0.1 mmol, 20.4 g), copper power (5 g) and quinoline 50 ml mixed and heated to 200 degrees C. for one hour. The reactant was cooled down and filtered by suction. The filter residue was washed by hexanes several times and the filtrate was collected and washed by 300 ml of dilute hydrochloride acid 3 times. After removal the volatile solvent under vacuum, the residue oil was purified by silica gel column using petroleum ether as eluent. The compound D1 was obtained as colorless oil. 
     Step 2 
     Compound D1 (0.05 mol, 7.1 g), octanoyl chloride (0.05 mol, 8.1 g) and methylene chloride (50 ml) were put in one flask. Aluminum chloride powder (0.05 mol, 6.7 g) was added in small portions. During the addition, the reactant was kept below 0 degrees C. After completing the addition, the reactant was heated slowly to ambient temperature and stirred for 30 min. Then, the reactant was poured into 100 g of cracked ice. The mixture was extracted by ethyl ether several times, the organic layers were combined and the volatile was removed under reduced pressure. The residual oil was purified by silica gel column using petroleum ether:ethyl acetate (30:1, V/V) as eluent and gave compound D2 as colorless oil (Yield 85%). 
     Step 3 
     Compound D2 (10 mol, 1.94 g) from last step was dissolved into 70 ml ethyl acetate and a solution of MCPBA (1.72 g, 10 mmol) in 40 ml ethyl acetate was added under −40 degrees C. very slowly. After the addition, the cooling bath was removed and the reactant was stirred under ambient temperature for 8 hours. Then the ethyl acetate was removed by reduced pressure and 30 ml of acetic anhydride was added. The reactant was heated to reflux for 1 hour. After removal of the acetate anhydride under vacuum, the residue was purified by silica gel column using hexane: methylene chloride (1:1) as eluent and gave compound D3 as yellow oil. 
     Step 4 
     Compound D3 (2.66 g, 10 mmol) was dissolved into 30 ml DMF under protection of argon. NBS (4.45 g, 25 mmol) was added in one portion. After 20 min of stirring, the reactant was poured into a cold solution of sodium thiosulfate and stirred for several minutes. Then, the mixture was extracted by ethyl ether and purified by silica gel column using hexane as eluent and gave compound D4 as light orange oil. (Yield 70%) 
     Synthesis of PBDTTT-F by Stille Coupling Reaction 
     2,6-bis(trimethyltin)-4,8-bis(2-ethylhexoxy)-benzo[1,2-b;3,4-b]-dithiophene (1 mmol), compound D4 (1 mmol), 20 ml of toluene and 2 ml of DMF were put into a two-necked flask with oil bath. The solution was flushed with argon for 10 minutes, then 30 mg of Pd(PPh3)4 were added to the flask. The solution was flushed again for 20 minutes. The oil bath was heated to 110 degrees C. carefully, and the reactant was stirred for 16 hours at this temperature under argon atmosphere. Then, the reactant was cooled to room temperature and the polymer was precipitated by addition of 100 ml methanol, and filtered through a Soxhlet thimble, which was then subjected to Soxhlet extraction with methanol, hexane, and chloroform. The polymer was recovered as a solid sample from the chloroform fraction by rotary evaporation. The solid was dried under vacuum for 1 day to achieve the final product. 
     EXAMPLE 3 
     Polymer Solar Cell Devices Using PBDTTT as Electron Donor Material 
     PBDTTT (30 mg) was dissolved in chlorobenezene to make 20 mg ml −1  solution, followed by blending with PCBM in 50 wt. %. 
     Polymer solar cell were fabricated on a transparent, indium-tin oxide (ITO) coated glass substrate. A thin layer of a conducting polymer, poly(styrenesulfonate) doped poly(3,4-ethylenedioxy-thiophene) (PEDOT:PSS), was spin-coated onto the ITO surface for a better interface. The thickness of the PEDOT:PSS layer was about 30 nm, measured with Dektek profilometer. Then, a thin layer was spin-coated using the solution prepared above. Then, thin layers of calcium and aluminum were evaporated successively at pressure around 10 −4  Pa. Testing was performed in a N 2  filled glove box under AM 1.5 G irradiation (100 mW cm −2 ) using a Xenon lamp solar simulator calibrated with a silicon diode (with KG5 visible filter) calibrated in National Renewable Energy Laboratory (NREL). 
     The I-V curve of the polymer solar cell device was shown in  FIG. 1 . 
     EXAMPLE 4 
     Polymer Solar Cell Devices Using PBDTTT-F as Electron Donor Material 
     The same procedure as described in EXAMPLE 3 was used by using PBDTTT-F instead of PBDTTT as electron donor material in the polymer solar cell device. 
     The I-V curve of the polymer solar cell device is shown in  FIG. 2 . 
     While the apparatus and method have been described in terms of what are presently considered to be the most practical and preferred embodiments, it is to be understood that the disclosure need not be limited to the disclosed embodiments. It is intended to cover various modifications and similar arrangements included within the spirit and scope of the claims, the scope of which should be accorded the broadest interpretation so as to encompass all such modifications and similar structures. The present disclosure includes any and all embodiments of the following claims.