Technical Field

The invention is related to copolymers, method of synthesis thereof and solar cells where these copolymers are used as donors.

Prior Art

Due to its low cost, ease of fabrication, applicability on flexible substrates, good film-forming properties, high morphological stability, and light weight, many scientists have been studying organic thin-film photovoltaics (PV) over the past two decades. ^1-5 Polymer solar cells (PSCs), one of the most popular third-generation photovoltaic cells, provide lower energy requirements, and the most efficient bulk heterojunction (BHJ) polymer solar cell has recently achieved 18% power conversion efficiency (PCE) ⁶ . Nowadays, considerable efforts in materials engineering ^{7- 10} , morphology control ^11-13 and optimize the structure of the devices ^14-16 has been expended to achieve the abovementioned record.

The D-A copolymers are characterized by their narrow bandgap, a wide range of light absorption wavelengths, controllable energy level, and photon absorption characteristics. They have an alternating electron-rich unit (D) and an electron-deficient unit (A) ²⁰ . In 2010, after the promising power conversion efficiency (5.5%) obtained from the use of thieno [3,4-c] pyrrole-4, 6-dione (TPD) and benzodi thiophene (BDT) derivative polymers in solar cells, the interest in polymers containing TPD and BDT had increased considerably ²¹ .

Description of Figures

Figure 1: The energy level diagram of the fabricated P-HTBDT, P-FTDBT and P-FBDT based polymer solar cells.

Figure 2: J-V curves that summarize photovoltaic performance of P-HTBDT, P-FTBDT and P-FBDT

Figure 3: TEM images of a) P-HTBDT:PC71BM processed from o-dcb b) P-FTBDT:PC71BM processed from o-dcb c) P-FBDT:PC71BM processed from o-dcb d) P-HTBDT:PC71BM processed from o-dcb with 2% DIO e) P-FTBDT:PC71BM processed from o-dcb with 2% DIO f) ) P-FBDT:PC71BM processed from o-dcb with 6% DPE Figure 4: AFM images of a) P-HTBDT:PC71BM processed from o-dcb b) P-FTBDT:PC71BM processed from o-dcb c) P-FBDT:PC71BM processed from o-dcb d) P-HTBDT:PC71BM processed from o-dcb with 2% DIO e) P-FTBDT:PC71BM processed from o-dcb with 2% DIO f) P-FBDT:PC71BM processed from o-dcb with 6% DPE

Figure 5: The EQE curves of the solar cells

Figure 6: Cyclic Voltammograms of

(a) P-HTBDT

(b) P-FTBDT

(c) P-FBDT

Figure 7: Normalized UV-vis absorption spectra of

(a) P-HTBDT

(b) P-FTBDT

(c) P-FBDT

Detailed Description of the Invention

The random D-A copolymers containing thieno[3,4-c]pyrrole-4, 6-dione (TPD) and benzothiadiazole as acceptors and benzodithiophene as donors in the backbone of the polymers is disclosed by this invention. Because of its planar structure, which is beneficial for the electron delocalization, TPD can stabilize excited state energy; hence TPD-bearing conjugated polymers are expected to have high open-circuit voltage (Voc) values. Moreover, TPD has a strong electron-withdrawing characteristic, significantly lowering the lowest unoccupied molecular orbital (LUMO). ¹⁰ ’ ²²

Three random D-A copolymers containing thieno[3,4-c] pyrrole-4, 6-dione (TPD) derivative and benzodi thiophene (BDT) derivative named as P-HTBDT, P-FTBDT and P-FBDT are synthesized. Schemes for Synthesis is shown in Scheme 1-4. The effect of side chains on BDT and fluorination to benzothiadiazole on photovoltaic performances of fabricated solar cells was investigated. Moderate molecular weights have been obtained for all polymers from the highest P-FBDT Mn:59 kDa to the lowest P-HTBDT Mn:44 kDa.

Synthesis of BT-H is shown in Scheme 1(a) where benzothiadi azole was brominated in hydrobromic acid (37 %) with bromine under refluxed overnight. After cooling the reaction mixture to the room temperature excess bromine was treated with saturated sodium bisulfide solution. Clear/almost transparent mixture was filtered and washed with excessive amount of water then with diethyl ether and the final product was recrystallized in ethanol.

Synthesis of BT-F is shown in Scheme 1(b). 4-fluorobenzene-l,2-diamine was reacted with thionyl chloride in triethylamine to obtain the 5-fluorobenzo[c][l,2,5]thiadiazole core. 5- fluorobenzo[c][l,2,5]thiadiazole was brominated with molecular bromine in HBr solution at reflux temperature. After workup, obtained solid was recrystallized from ethanol to give the desired product 4,7-dibromo-5-fluorobenzo[c][l,2,5]thiadiazole.

Scheme 1

Synthesis of P-HTBDT

As it is shown in Scheme 2; 337 mg 0.378 mmol TBDT, 56 mg 0.189 mmol BT-H, 80 mg 0.189 mmol TPD were added in two way round bottom flask and toluene was added via syringe needle under inert atmosphere (N2) and solution was bubbled for 20 minutes. After that, palladium catalyst was added, and temperature was set to reflux for two days. Next, 5 mol % palladium catalyst added with end gapper stannylated thiophene 0.756 mmol and refluxed for 5 hours and then 1.512 mmol bromothiophene was added and refluxed overnight. Next day, polymerization reaction mixture was cooled down and solvent was evaporated to obtain dense liquid. This liquid was added into methanol drop wise to obtain solid polymer. Solid polymer was filtered and dried. Further purification was performed with Soxhlet extraction with methanol, acetone, hexane and polymer solution was obtained from chloroform portion. To this chloroform portion 35 mg Quadrasil was added and stirred at room temperature for 1 hour. After filtering the chloroform portion, evaporation of chloroform solvent was performed to obtain dense liquid which was later dropped wise added into a methanol gave the desired polymer PH- TBDT was obtained as dark blue/black solid (257 mg 89% yield). 1H NMR of the polymer did not give an informative spectrum, yet intense and broad aliphatic hydrogens were observed. GPC results gave Mwt: 76kDa Mn: 44 kDa and PDI: 1.73.

Scheme 2

Synthesis of P-FTBDT

Similar methodology was followed with 337 mg 0.378 mmol TBDT, 59 mg 0.189 mmol BT- F, 80 mg 0.189 mmol TPD as it is shown in Scheme 3. Desired polymer was obtained as dark blue/black solid (245 mg, 84 % yield). 1H NMR of the polymer did not give an informative spectrum yet intense and broad aliphatic hydrogens were observed. GPC results gave Mwt: 120 kDa Mn: 46 kDa and PDI: 2.5.

Scheme 3 Synthesis of P-FBDT

Similar methodology was followed with 286 mg 0.378 mmol BDT, 59 mg 0.189 mmol BT-F, 80 mg 0.189 mmol TPD asit is shown in Scheme 4. Desired polymer was obtained as dark blue/black solid (213mg, 88 % yield). 1H NMR of the polymer did not give an informative spectrum yet intense and broad aliphatic hydrogens were observed. GPC results gave Mwt: 220 kDa Mn: 59 kDa and PDI: 3.7.

Scheme 4

As shown in Figure 1, The Highest occupied Molecular orbital (HOMO) levels of the polymers were -5.57, —5.51, and -5.65 eV for P-HTBDT, P-FTBDT, and P-FBDT respectively, suggesting low-lying HOMO energy levels. The optimized weight ratios of the polymer to acceptor which is PC71BM (fullerene derivative) are determined to be 1 :2 for all polymers, and the maximum PCEs of the devices were 7.35%, 7.76%, and 9.21% for P-HTBDT, P-FTBDT, and P-FBDT, respectively, after optimizations with 1,8-diiodooctane (DIO) and 1-

Chloronaphthalene (CN). The morphologic and topographic investigations were carried out by the images from Transmission electron microscopy (TEM) and Atomic Force Microscopy (AFM). The best performing device was P-FBDT because of its deeper HOMO level, high molecular weight, and exhibits better morphology.

All bulk heterojunction Polymer solar cells, contain n-type acceptor and p-type conjugated polymer donor. For the invention, the acceptor is fullerene derivative PC71BM, and the donors were the P-HTBDT, P-FTBDT, and P-FBDT, random polymers. The PCE value depends on basically three parameters, which are open-circuit voltage (Voc), short circuit current density (./sc), and the fill factor (FF). The lower the HOMO level of the conjugated polymer, the higher the Voc value, and thus the PCE value. ^{17 18} To obtain a high Jsc value, the absorption region of the conjugated polymer should be as broad as possible, and/or its bandgap should be close to the optimum level. Due to the "internal" nature of efficient bulk heterojunction (BHJ), it is challenging to improve fill factor (FF) in BHJ devices continuously. Some intrinsic variables in BHJ, including randomly mixed morphology, imbalanced donor and acceptor mobility, and bimolecular recombination, are well recognized to influence FF. ¹⁹

By the invention, the effect of mono-fluorine substitution of benzothiadi azole is determined and compared with the di-fluorine atom attached to benzothiadiazole ²³ . The fluorine substitution is a very effective way to lower the HOMO and LUMO energy levels of the polymer, resulting in higher open-circuit voltage, Voc ²⁴ . Second, the effect of replacing the alkoxy group with alkylthienyl on the performance of bulk heterojunction (BHJ) is discussed. The introduction of 2-alkylthienyl as the conjugated side group in benzo[l,2-b:4,5- b']dithiophene (BDT) units alkylthienyl substituted BDT (TBDT) units are obtained. TBDT been widely employed to design innovative photovoltaic polymers, enhancing power conversion efficiencies (PCEs) to new levels in the field of PSCs.

Cyclic voltammetry technique was employed via three electrode systems to observe redox characteristics of conjugated polymers. Platinum wire, silver wire and polymer coated indium tin oxide (ITO) coated glass were chosen as counter electrode (CE), reference electrode (RE) and working electrode, respectively. For preparation of working electrode, polymers were dissolved in chloroform (1 mg/ml) and coated on ITO surface via spray gun. The electrodes were immersed in 0.1 M TBAPFe/ACN electrolyte solution and their cyclic voltammograms were recorded at 100 mV s ^-1 scan rate by using Gamry 600 potentiostat. Polymer coated indium tin oxide (ITO) coated glass were also used as thin film in optical characterization which were carried by Varian Cary 5,000 UV-Visible spectrophotometer.

Device Fabrication

In a typical polymer solar cell, the photoactive blend layer, comprising a conjugated polymer donor and a molecular acceptor is sandwiched between an indium tin oxide (ITO) electrode (anode) and a metal electrode (cathode). The polymer donor serves as the main solar light absorber and as the hole transporting phase, whereas the fullerene derivative acceptor molecule acts as electron transporting phase.

PSCs of the invention are fabricated with the device architecture of

ITO/PEDOT:PSS/Donor: PCviBM/LiF/Al.

Etched Indium Tin Oxide (ITO) coated glasses were ultrasonicated with Hellmanex, distilled water, acetone, and water, respectively, for 15 mins. Then oxygen plasma was applied to regulate the work function and clean from the organic impurities. After cleaning processes, poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) was coated and annealed at 135 °C for 15 mins. The optimized weight concentration was 24 mg/ml for polymer P-FBDT, P-FTBDT, and P-HTBDT. 1,2-di chlorobenzene (o-dcb) was chosen as solvent. Lithium fluoride (LiF) and Aluminum (Al) were deposited under low pressure at the top of the device. The I-V characterizations of generated PSCs were performed with Keithley 2400 under simulated AM 1.5 G solar irradiation (100 mW cm" ² ) between -0.2 V and 1.0 V. The external quantum efficiency (EQE) is measured using a grating monochromator setup. The energy level diagram was given in Figure 5.

Some critical parameters such as band gap, HOMO and LUMO energy levels for organic solar cell applications were calculated by using cyclic voltammogram and following equations.

Eg ^el = - (H0M0-LUM0),

HOMO = -(4.75 + E ^ox onset) and

LUMO = -(4.75 + E ^red onset).

As shown in Figure 6, all polymers exhibited p-type and n-type doping behavior (ambipolar character). When electrochemical properties of polymers were compared as shown in Table 1, P-FTBDT has higher oxidation potential with respect to P-HTBDT. Same behavior was valid for mono fluorinated P-FBDT which has lower oxidation potential from two fluorinated PF given in prior art ²³ . Scheme for synthesis of PF is given below.

Scheme 5 (prior art)

These observations were resulted from different electron densities because electron withdrawing nature of fluorine atom causes to lower electron densities and lowering oxidation potential. HOMO levels of P-FTBDT and P-HTBDT were -5.51 and -5.57 eV, respectively. On the other hand, HOMO levels of mono fluorinated P-FBDT and two fluorinated PF were -5.65 and -5.78 eV, respectively. According to Table 1 among the three synthesized novel materials, P-FBDT possesses deeper HOMO level which leads to higher V _oc value of fabricated solar cells.

Table 1: Summary of electrochemical studies of P-HTBDT, P-FBDT and P-FTBDT

Table 2: Summary of optical studies of P-HTBDT, P-FBDT and P-FTBDT

Besides HOMO and LUMO energy levels, the absorption behavior of polymers in the UV-Vis region is also critical information for organic solar cell applications. From UV-Vis absorption spectrum (Figure 7), maximum absorption wavelengths ( max), onset of maximum absorption wavelengths (Xmax ^onset ), and optical band gaps (E _g ^op ) the polymers were given in Table 2. The Xmax values were determined as 635 nm, 640 nm and 605 nm for P-HTBDT, P-FBDT and P- FTBDT, accordingly. This value was 703 nm for PF. Moreover, optical band gaps were 1.67, 1.71, 1.77, 1.79 eV for P-HTBDT, P-FBDT and P-FTBDT and PF, respectively. Increasing band gap with fluorine substituent was compatible with literature studies. When compared to solution spectrum, thin film spectrum showed red-shift which was resulted from aggregation in thin film.

Photovoltaic Properties

Bulk heterojunction PSCs were fabricated with a conventional device architecture based on ITO/PEDOT: PSS/Active Layer/LiF/Al. Details of device fabrication are emphasized in the experimental part. Donor-acceptor ratio, active layer thickness, solvent additives (CN, DIO, and DPE), and thermal annealing were performed during the device fabrication process to obtain the optimum morphology. The best working solvent was 1,2-di chlorobenzene (o-dcb) for all devices. Photovoltaic performances of the devices completed by using three polymers as donors were given in table 3. able 3: Photovoltaic properties of fabricated solar cells

Among all polymers, P-FBDT based PSC with the 6% (diphenyl ether) DPE solvent addition has reached the best PCE of 9.21% together with 60% (fill factor) FF and 15.4 mA.cm' ² circuit current density (./sc). The increased PCE can be attributed from the improved polymer morphology, probably because of the high solubility and high molecular weight of P-FBDT. Although DPE was tested for P-HTBDT and P-FTDBT based PSCs, the addition of 2% 1,8- diiodooctane (DIO) improved the morphology of these devices more effectively. While the highest PCE achieved without using any solvent additives for P-HTBDT was 5.51%, the PCE value has reached 7.35% at the use of 2% DIO. On the other hand, while the highest efficiency obtained without using DIO for P-FTDBT was 6.72%, this value increased to 7.76% at the use of DIO. Treatment with the addition of DPE and DIO enhanced the device performance up to 25% due to a simultaneous increase in all the photovoltaic parameters. As is well known, DIO dissolves ([6,6]-Phenyl-C71 -butyric acid methyl ester) PCBM aggregates selectively in the bulk heterojunction (BHJ) film, allowing PCBM molecules to be intercalated into the polymer domains. Moreover, DIO as an additive allows for a slower crystallization process during spincoating, resulting in improved morphology due to improved intermolecular ordering and phase separation. DPE works as a theta solvent for photovoltaic polymers, assisting in the formation of optimal bulk-heterojunction film morphologies and reducing bimolecular charge recombination. Morphological analyzes will be processed with TEM images in the next section.

Morphological Studies

For morphological and topographical examinations of active layers of P-HTBDT, P-FTBDT, and P-FBDT based (organic solar cells) OSCs, transmission electron microscopy (TEM) and atomic force microscopy (AFM) were utilized.

Figures 3(a) and 3 (b) show TEM images of the optimized active layer of P-HTBDT and P- FTBDT, respectively, when the additive 1,8-diiodooctane DIO is not used. The dark areas correspond to PCBM-rich regions, whereas the bright regions correspond to polymer-rich areas. These images have PCBM aggregations when viewed at the 50 nm scale. Diiodooctane preferentially dissolves PCBM aggregates, according to the literature. The films created with the addition of 2% DIO, as shown in Figures 3(d)and 3(e), underwent a drastic alteration due to this additive addition. The interpenetrating bicontinuous network was seen after DIO was added, and the interpenetrated network shape allows superior exciton separation and charge transport, resulting in greater Jsc. Optimized P-FBDT's film is shown in Figure 3(c) without the solvent addition of DPE. DPE is a well-known theta solvent. The enthalpy of mixing is equal to zero, making the solution perfect. A considerably more homogenous distribution between the donor and acceptor was observed when DPE was utilized. Ultimately, the polymer donor materials formed the ideal nano-scaled morphology with the acceptor in the blend, which in shown in Figure 3(f).

The Atomic Force Microscopy (AFM) images of the films are shown in Fig 4. The root mean square values of the films are located at the left bottom of each image. P-FBDT : PCBM with 6% DPE has the highest RMS value of 2.28 in Fig 4f, explaining why film P-FBDT based OSCs have the highest Jsc. Increased surface roughness in the active layer may increase the surface area of the device, internal reflection and light collecting, enhancing device efficiency. The addition of the solvent additive causes the roughness values to increase or decrease, and there is no clear association between the roughness value and PCE. For all three polymers, the thickness of the film increased with the addition of an additive without any significant change in the roughness value. Thickening the film generates more significant absorption, resulting in a higher Jsc value. It is not unexpected that a thicker film is formed because the donor-acceptor blend is viscous due to the additives.

The EQE measurement was performed to verify the value of the current density on the I-V curve and is shown in Figure 5. The maximum short circuit current density value was obtained from P-FBDT based solar cells and the integration of the EQE curves which are specified in the parentheses agrees with the Jsc value in Table 1. The best working polymer was chosen as P-FBDT because of its highest molecular weight, deepest HOMO level and forming the best morphology when it used as a donor in the solar cell, throughout the study.

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