Polym. Bull. DOI 10.1007/s00289-015-1541-y ORIGINAL PAPER
An alternating copolymer of fluorene donor and quinoxaline acceptor versus a terpolymer consisting of fluorene, quinoxaline and benzothiadiazole building units: synthesis and characterization Desta Gedefaw1,2 • Zaifei Ma3 • Endale Mulugeta4 • Yang Zhao3 • Fengling Zhang3 • Mats R. Andersson1,2 Wendimagegn Mammo4
•
Received: 11 December 2014 / Revised: 15 October 2015 / Accepted: 22 October 2015 Ó Springer-Verlag Berlin Heidelberg 2015
Abstract An alternating polyfluorene copolymer based on fluorene donor and quinoxaline acceptor (P1) and an alternating terpolymer (P2) with fluorene (50 %) donor and quinoxaline (25 %) and benzothiadiazole (25 %) acceptor units were designed and synthesized for use as photoactive materials in solar cells. The presence of benzothiadiazole unit in P2 increased the optical absorption coverage in the range of 350–600 nm, which is an interesting property and a big potential for achieving improved photovoltaic performances with judicious optimization of the devices. Solar cells were fabricated from 1:4 blends of polymers-PCBM[70] using o-dichlorobenzene (o-DCB) as processing solvent, and P1 showed a power conversion efficiency (PCE) of 3.18 %, with a shortcircuit current density (JSC) of 7.78 mA/cm2, an open-circuit voltage (VOC) of 0.82 V, and a fill factor (FF) of 50 % while P2 showed an overall PCE of 2.14 % with corresponding JSC of 5.97 mA/cm2, VOC of 0.84 V and FF of 42 %. In general, P2 gave lower JSC and FF presumably due to the fine domain sizes of the polymer–PCBM[70] blend as seen from the atomic force microscopy (AFM) image which might have affected the charge carrier transport.
Electronic supplementary material The online version of this article (doi:10.1007/s00289-015-1541-y) contains supplementary material, which is available to authorized users. & Desta Gedefaw
[email protected] & Wendimagegn Mammo
[email protected] 1
Department of Chemical and Biological Engineering, Polymer Technology, Chalmers University of Technology, 412 96 Go¨teborg, Sweden
2
Future Industries Institute, University of South Australia, Mawson Lakes, SA 5095, Australia
3
Department of Physics, Chemistry and Biology, Linko¨ping University, 58183 Linko¨ping, Sweden
4
Department of Chemistry, Addis Ababa University, P.O. Box 1176, Addis Ababa, Ethiopia
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Graphical abstract Alternating (P1) and ternary (P2) conjugated polymers were designed, synthesized and used for fabrication of photovoltaic devices. 70 P1:PCBM[70] P1:PCBM[60] P2:PCBM[70] P2:PCBM[60]
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EQE (%)
50 40 30 20 10 0 400
500
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700
Wavelength (nm)
Keywords Polyfluorenes Suzuki coupling reaction Fluorene Quinoxaline Benzothiadiazole
Introduction In recent years, polymer solar cells (PSCs) have gained attention because of their advantages in producing low-cost and lightweight devices on flexible substrates for sustainable energy production [1–3]. The most widely utilized device structure in PSCs consists of nanometer-scale thick film from a blend of conjugated polymer (donor) and a derivative of fullerene (acceptor) sandwiched between transparent indium tin oxide (ITO) (anode) and a metal electrode (cathode) [4]. Increasing the power conversion efficiency (PCE) of PSCs is one of the targets of the research in recent times. The open-circuit voltage (VOC), short-circuit current density (JSC) and fill factor (FF) of the device need to be increased to achieve higher PCE. One of the strategies to realize high PCE is to design and synthesize conjugated polymers with desirable properties. For instance, the polymer is required to have (1) optical absorption over a broader region to extract more photons; (2) deeper HOMO level to get high VOC (as VOC is estimated by the difference between the HOMO of the polymer and the LUMO of the acceptor) [5]; (3) high and balanced hole and electron mobility in the polymer–acceptor blend [6, 7]. Currently, the PCE of labscale single- or multiple-junction bulk heterojunction (BHJ) solar cells has reached *7–10 % by the synergic effort of the development of a wide array of novel active materials, device optimization work and optimization of the device architecture, signifying a bright future of PSCs in commercial applications [8–15]. Incorporating electron-rich (donor) and electron-deficient (acceptor) moieties alternatively in the backbone of a conjugated polymer has been implemented in the past as a design strategy for the synthesis of a number of successful polymers for photovoltaics [16–18]. It is believed that the library of promising polymers can further be increased by the synthesis of terpolymers (made from three monomers)
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using the already developed monomeric units. In fact, terpolymers are currently becoming popular as they are simple to synthesize and at the same time an effective way to influence polymer properties by simply changing the feed ratios of the reacting monomers without resorting to the synthesis of complex and highly elaborated monomers. For instance, the optical absorption of alternating copolymers can be successfully manipulated by incorporating a certain amount of a third monomer in the backbone to give a terpolymer [19–26]. Here, we report the synthesis and characteristic study of two easily accessible polymers P1 (an alternating polymer containing fluorene donor and quinoxaline acceptor units) and P2 (a terpolymer consisting of fluorene (donor) and two acceptor units (quinoxaline and benzothiadiazole)). The effect of introduction of benzothiadiazole in the backbone of P2 on the optical, electrical and device properties was studied and compared with the parent polymer (P1), which lacks the benzothiadiazole unit.
Experimental General Tetrahydrofuran (THF) was dried over Na/benzophenone and freshly distilled prior to use. Other reagents and solvents from commercial sources were used as received without further purification. 1H and 13C NMR spectra of compounds were recorded at ambient temperature on Bruker Avance 400 NMR spectrometers. Size-exclusion chromatography (SEC) was performed on Waters Alliance GPCV2000 with a refractive index detector, with columns: Waters StyragelÒ HT 6E 9 1, Waters StyragelÒ HMW 6E 9 2. The eluent was 1,2,4-trichlorobenzene, the working temperature was 135 °C, and the resolution time was 2 h. The concentration of the samples was 0.5 mg/mL, which was filtered (filter: 0.45 lm) prior to the analysis. The relative molecular masses were calculated by calibration relative to polystyrene standards. Melting point was measured using METTLER TOLEDO FP82 Hot Stage with an FP90 processor interfaced with a Leica Galen III microscope. Thermogravimetric analysis (TGA) measurements were done on a Perkin Elmer TGA7 Thermo Graphic Analyzer, temperature range 30–600 °C, heating rate 10 °C/min. Synthesis of monomers and polymers Synthesis of 1,2-bis(4-octyloxyphenyl)ethane-1,2-dione (2) A Grignard reagent was prepared by dropwise addition of 1-bromo-4-octylbenzene (1) (8.0 g, 0.03 mol) in THF (10 mL) to a suspension of magnesium (0.85 g, 0.03 mol) in THF (18 mL) and refluxed for 2 h. In a separate flask, a solution of LiBr (5.17 g, 59.4 mmol) in THF (20 mL) was added to a stirred suspension of CuBr (4.28 g, 29.7 mmol) in THF (20 mL). The mixture was stirred until it become homogenous and was then cooled to a temperature of -50 °C. The Grignard reagent was added to the flask containing the mixture of LiBr/CuBr suspension and stirred.
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Oxalyl chloride (1.89 g, 14.88 mmol) was added to the reaction mixture in one portion and after 30 min of string the reaction was quenched with saturated NH4Cl. The organic layer was separated, washed with saturated NH4Cl, dried over anhydrous Na2SO4 and the solvent was removed to afford a pale yellow product which was purified by silica gel column chromatography using pet. ether:ethyl acetate (4.5:0.5) as an eluent to afford compound 2 as a pale yellow solid (3.01 g, 23 %). 1 H NMR (CDCl3, 400 MHz): d 7.94 (d, J = 8.0 Hz, 4H), 6.96 (d, J = 8.0 Hz, 4H), 4.04 (t, 4H), 1.82 (q, 4H), 1.47–1.2 7(m, 20H), 0.95 (t, 6H); 13C NMR (CDCl3, 100 MHz): d 193.6, 164.5, 132.4, 126.1, 114.7, 68.5, 31.8, 29.0, 25.9, 22.6, 14.1. Synthesis of 2,3-bis(4-octyloxyphenyl)-5,8-dithiophen-2-yl-quinoxaline (4) A mixture of 4,7-di(thiophen-2-yl)benzo[c] [1, 2, 5] thiadiazole (3) (1.0 g, 3.33 mmol) and zinc dust (2.62 g, 39.94 mmol) in acetic acid (90 mL) was stirred at 80 °C under nitrogen atmosphere for 5 h until the reaction mixture turned white. The insoluble material was separated by suction filtration. To the filtrate was added 1,2-bis(4-octyloxyphenyl)ethane-1,2-dione (2) (1.55 g, 3.23 mmol) and the mixture was heated at 60 °C under nitrogen atmosphere. After 5 h, the reaction mixture was cooled to room temperature, and the precipitate was collected by suction filtration and washed with water and methanol and was dried in a vacuum oven overnight to afford 4 (1.68 g, 71.1 %). 1 H NMR (CDCl3, 400 MHz,): d 8.12 (s, 2H), 7.87 (d, J = 4 Hz, 2H), 7.74 (d, J = 8.4 Hz, 4H), 7.53 (d, J = 5.2 Hz, 2H), 7.20 (t, J = 3.6, 4.4 Hz, 2H), 6.92 (d, J = 8.4 Hz, 4H), 4.02 (t, 4H), 1.83(q, 4H), 1.34–1.49 (m), 0.93 (t, 6H); 13C NMR (CDCl3, 100 MHz): d 159.9, 151.1, 138.8, 136.8, 131.9, 131.1, 131.0, 128.7, 128.6, 126.5, 126.2, 114.2, 68.1, 31.8, 31.0, 29.3, 29.1, 26.1, 22.6, 14.1 Synthesis of 5,8-bis(5-bromothiophen-2-yl)-2,3-bis(4-(octyloxy)phenyl)quinoxaline (5) Compound 4 (1.24 g, 1.76 mmol) was dissolved in THF (47.13 mL) and NBS (0.63 g, 3.53 mmol) was added and the mixture was stirred overnight in the dark under nitrogen atmosphere. The reaction was quenched by adding water and was extracted with CH2Cl2. The combined CH2Cl2 extract was washed with distilled water and brine, dried over anhydrous Na2SO4 and the solvent was removed by rotary evaporation. The resulting product was further purified by silica gel column chromatography using toluene:hexane (1.5:3.5) solvent mixture as an eluent to afford 5 (1.2 g, 78.9 %). 1 H NMR (CDCl3, 400 MHz): d 7.93 (s, 2H), 7.66 (d, J = 8 Hz, 4H), 7.50 (d, J = 4 Hz, 2H), 7.10 (d, J = 4 Hz, 2H), 6.93 (d, J = 8 Hz, 4H), 4.04 (t, 4H), 1.85 (q, 4H), 1.28–1.51 (m, 20H), 0.94 (t, 6H); 13C NMR (CDCl3, 100 MHz): d 160.0, 151.1, 138.8, 136.8, 131.9, 131.1, 131.0, 128.7, 128.6, 126.5, 126.2, 114.2, 68.1, 31.8, 29.3, 29.2, 29.1, 26.1, 22.7, 14.2.
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Synthesis of 9,9-bis(2-(2-methoxyethoxy)ethyl)-9H-fluorene (7) Fluorene (6) (8 g, 48.1 mmol) was dissolved in dry THF (120 mL) and cooled to -78 °C and 2.5 M solution of n-BuLi (20 mL, 50 mmol) in hexane was added and the mixture was stirred at the same temperature for 30 min. 1-Chloro-2-(2methoxyethoxy)ethane (6.8 g, 49.1 mmol) was then added and the mixture was stirred for 1 h at -78 °C. The cooling bath was removed and stirring continued at room temperature for 30 min. The cooling bath was placed back and the reaction mixture was cooled to -78 °C for 1 h and 15 min to which 2.5 M n-BuLi (24 mL, 60 mmol) was added and stirred for 45 min. Then, 1-chloro-2-(2-methoxyethoxy)ethane (7.2 g, 52 mmol) was added and stirred at -78 °C for 30 min and the cooler was turned off and the mixture was allowed to warm to room temperature gradually overnight. The reaction mixture was then quenched by adding water and the organic phase was separated and the aqueous solution was extracted with diethyl ether. The organic extracts were combined, washed with distilled water and dried over anhydrous Na2SO4. The solvent was removed on a rotary evaporator and the oily crude product was passed through a column of silica gel using chloroform and chloroform–methanol (4.9:0.1) as eluent and the desired product (7) (5.78 g, 32.4 %) was obtained in pure form. 1 H NMR (CDCl3, 400 MHz): d 7.71 (2 H, dd, J = 1.2, J = 8 Hz), 7.45 (2 H, dd, J = 6.8, 1.2 Hz), 7.38 (4 H, m), 3.32 (10 H, m), 3.21 (4 H, t), 2.77 (4 H, t), 2.46 (4 H, t). Synthesis of 2,7-dibromo-9,9-bis(2-(2-methoxyethoxy)ethyl)-9H-fluorene (8) 9,9-Bis(2-(2-methoxyethoxy)ethyl)-9H-fluorene (7) (5.78 g, 15.6 mmol) was dissolved in DMF (80 mL) and Br2 (11.8 g, 73.75 mmol) dissolved in DMF (5 mL) was added drop-by-drop from a pressure-equalizing dropping funnel over 30 min. The reaction mixture was stirred overnight and was quenched with 10 % aqueous Na2S2O35H2O solution and then extracted with diethyl ether. The ether extract was washed with distilled water, dried over anhydrous Na2SO4 and the solvent was removed to yield an oily material (11.8 g). The crude product was purified by passing it through a silica gel column using chloroform and chloroform:methanol (4.9:0.1) as eluent to afford 2,7-dibromo-9,9-bis(2-(2-methoxyethoxy)ethyl)-9Hfluorene (8) (8.0 g, 97 %). 1 H NMR (CDCl3, 400 MHz): d 7.56 (2 H, d, J = 1.6 Hz), 7.52 (2 H, s), 7.49 (2 H, d, J = 1.6 Hz), 3.32 (4 H, t), 3.3 (6 H, s), 3.2 (4 H, t), 2.8 (4 H, t), 2.39 (4 H, t); 13 C NMR (CDCl3, 400 MHz): d 150.8, 138.4, 130.7, 126.7, 121.7, 121.2, 71.7, 67.0, 66.8, 59.1, 51.8, 39.5. Synthesis of 2,20 -(9,9-bis(2-(2-methoxyethoxy)ethyl)-9H-fluorene-2,7diyl)bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolane) (9) 2,7-Dibromo-9,9-bis(2-(2-methoxyethoxy)ethyl)-9H-fluorene (8) (8.0 g, 16.0 mmol) was dissolved in dry THF (250 mL) under nitrogen atmosphere and cooled to -78 °C. 2.5 M n-BuLi (15.8 mL, 39.5 mmol) was added to the cooled mixture.
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Then, 2-isopropyl-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (8 g, 32.02 mmol) was added and the mixture was stirred for 30 min. The cooler was then turned off and stirring continued overnight at room temperature. The reaction mixture was quenched with water and extracted with diethyl ether. The organic phase was dried over anhydrous Na2SO4 and the solvent was removed to afford an oily material. This crude material was dissolved in hot petroleum ether and when cooled a white solid precipitated. The white solid was collected by suction filtration and dried in vacuum oven to afford compound 9 (2.67 g, 28 %). mp = 147.9–151.1 °C. 1 H NMR (CDCl3, 400 MHz): d 7.87 (2 H, s), 7.83 (2 H, d, J = 7.2 Hz), 7.73 (2 H, d, J = 7.6 Hz), 3.32 (4 H, t), 3.28 (6 H, s), 3.2 (4 H, t), 2.69 (4 H, t), 2.5 (4 H, t), 1.41 (24 H, s); 13C NMR (CDCl3, 400 MHz): d 148.5, 143.1, 134.0, 129.4, 119.5, 83.9, 71.7, 69.9, 66.9, 59.0, 51.0, 39.5, 25.0. Synthesis of poly[5-(5-(9,9-bis[2-(2-methoxyethoxy)ethyl]-9H-fluoren-2-yl)thiophen-2-yl)-2,3-bis(4-octyloxyphenyl)-8-thiophen-2-yl-quinoxaline] (P1) 5,8-Bis(5-bromothiophen-2-yl)-2,3-bis(4-(octyloxy)phenyl)quinoxaline (5) (0.1 g, 0.12 mmol) was mixed with 2,20 -(9,9-bis(2-(2-methoxyethoxy)ethyl)-9H-fluorene2,7-diyl)bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolane) (9) (0.17 g, 0.12 mmol), tetrakis(triphenylphosphine)palladium(0) (13 mg) and toluene (10 mL) and was refluxed for 10 min under nitrogen atmosphere. Tetraethylammonium hydroxide in water 20 % (w/w) (0.45 mL, 0.63 mmol) was added with a syringe and the mixture was refluxed for 3 h. Bromobenzene (0.158 mL, 1.69 mmol) and phenylboronic acid (0.18 g, 1.5 mmol) were added and heated to end-cap the polymer at 1 h interval. The reaction mixture was allowed to cool to room temperature and was precipitated by slowly adding the mixture into methanol, and collected by filtration. The resulting solid was dissolved in chloroform, and was washed with ammonia and water. The chloroform solution was concentrated to a small volume and the polymer was reprecipitated from methanol and collected by suction filtration. The dark redcolored solid was Soxhlet extracted first with diethyl ether and then with chloroform. The chloroform portion was concentrated to a small volume and the polymer was reprecipitated from methanol, filtered and dried to afford P1 (0.117 g, 97 %). 1 H NMR (CDCl3, 400 MHz): d 8.19 (2H), 7.98 (2H), 7.86–7.14 (8H), 7.52 (2H), 7.01–6.92 (4H), 4.07 (4H), 3.3–3.2 (11H), 2.83 (3H), 2.57 (3H), 1.87–1.84 (4H), 1.55–1.3 (18H), 0.95–0.83 (6H). Synthesis of poly[5-(5-(7-(7-(9,9-bis[2-(2-methoxyethoxy)ethyl]-9H-fluoren-2yl)benzo [1, 2, 5] thiadiazol-4-yl)-9,9-bis[2-(2-methoxy-ethoxy)ethyl]-9H-fluoren-2yl)-thiophen-2-yl)-2,3-bis(4-octyloxy-phenyl)-8-thiophen-2-yl-quinoxaline] (P2) 5,8-Bis(5-bromothiophen-2-yl)-2,3-bis(4-(octyloxy)phenyl)quinoxaline (5) (0.1 g, 0.12 mmol), 2,20 -(9,9-bis(2-(2-methoxyethoxy)ethyl)-9H-fluorene-2,7-diyl)bis(4,4,5, 5-tetramethyl-1,3,2-dioxaborolane) (9) (0.145 g, 0.23 mmol), 4,7-dibromobenzo[c] [1, 2, 5] thiadiazole (10) (0.034 g, 0.12 mmol) and tetrakis(triphenylphosphine)palladium(0) (0.013 g) were dissolved in toluene (10 mL) and refluxed for 10 min under
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nitrogen atmosphere. Tetraethylammonium hydroxide in water (20 % (w/w)) (0.35 mL, 0.49 mmol) was added with a syringe and the mixture was further refluxed for 3 h. Bromobenzene (0.1 mL, 0.96 mmol) and phenylboronic acid (0.19 g, 1.55 mmol) were added in 1 h interval to end-cap the polymer. The reaction mixture was allowed to cool to room temperature and the polymer was precipitated by slowly adding the mixture into methanol, stirred and collected by suction filtration. The resulting solid was dissolved in chloroform, and was washed with ammonia and water. The chloroform solution was concentrated to a small volume and the polymer was reprecipitated from methanol, filtered and dried. The red-colored solid was Soxhlet extracted first with diethyl ether and then with chloroform. The chloroform portion was concentrated to a small volume and the polymer was reprecipitated from methanol, filtered and dried to afford P2 (0.123 g, 67.2 %). 1 H NMR (CDCl3, 400 MHz): d 8.19 (6H), 7.98(4H), 7.86–7.74 (8H), 7.51 (2H), 7.01–6.92 (4H), 4.07–3.99 (4H), 3.32–3.22 (20H), 2.93 (7H), 2.57 (6H), 1.87–1.84 (4H), 1.55–1.26 (18H), 0.95–0.88 (6H) Physical properties and electrochemical characterization Square-wave voltammetric measurements were carried out on a CH-Instruments 650A Electrochemical Workstation. A three-electrode setup consisting of platinum wires, both as working electrode and counter electrode, and a Ag/Ag? quasi reference electrode were used. A 0.1 M solution of tetrabutylammonium hexafluorophosphate (Bu4NPF6) in anhydrous acetonitrile was used as supporting electrolyte. The polymers were deposited onto the working electrodes from chloroform solutions. The electrolyte was bubbled with nitrogen gas prior to each experiment. During the scans, nitrogen gas was flushed over the electrolyte surface. After each experiment, the system was calibrated by measuring the ferrocene/ferrocenium (Fc/Fc?) redox peak. The HOMO and LUMO energy levels of the polymers and electron acceptors were calculated from the peak values of the third scans by setting the oxidative peak potential of Fc/Fc? vs the normal-hydrogen electrode (NHE) to 0.630 V and the NHE vs the vacuum level to 4.5 V [27]. The optical absorption spectra of the samples were recorded using a Perkin Elmer Lambda 950 spectrophotometer from dilute solutions of the samples in chloroform with concentration of 1 9 10-5 mol/L and by spin-coating on glass substrates. Device fabrication and characterization Polymer solar cells were fabricated with ITO glass as an anode, LiF/Al as a cathode and the blend film of the polymer/PCBM between the two electrodes as the active layer. The ITO glass was pre-cleaned and modified by a conductive layer of PEDOT–PSS (Baytron P VP Al 4038). The PEDOT–PSS film was annealed at 120 °C for 10 min to remove water. The solutions of conjugated polymer and PCBM[70] (1:4 w/w) were spin-coated on top of the PEDOT–PSS using o-DCB solvent. Finally, the cathode, consisting of LiF (0.6 nm) and Al (80 nm), was deposited in vacuum. The thickness of the active layer was measured using a surface profiler, Dektak 6 M. The sizes of the diodes were defined by a mask to be
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approximately 4–6 mm2 when depositing Al. EQEs were calculated from the photocurrents at short-circuit conditions. The currents were recorded by a Keithley 485 picoammeter under illumination of monochromatic light through the anodic side of the devices. Current–voltage characteristics were recorded using a Keithley 2400 Source Meter under illumination of AM 1.5. The light intensity of the solar simulator (Model SS-50A, Photo Emission Tech. Inc.) was 100 mW/cm2. The surface morphology of the active layers was imaged by AFM, using a Dimension 3100 system (Digital Instruments/Veeco) operating in tapping mode. Silicon cantilevers (NSG10) with a force constant of 5.5–22.5 N/m, a resonance frequency of 190–355 kHz, and a tip curvature radius of 10 nm were used.
Results and discussion The structures of the two polymers described in this study are shown in Fig. 1 and the synthetic routes towards the monomers and the polymers are depicted in Schemes 1, 2 and 3. The synthesis of quinoxaline-based monomer 5 commenced by preparing 1,2bis(3-(octyloxy)phenyl)ethane-1,2-dione (2) as depicted in Scheme 1 following literature procedure [28, 29]. Thus, 1-bromo-4-(octyloxy)benzene (1) was converted to the corresponding Grignard reagent by treatment with Mg in THF and this reagent was added to a mixture of LiBr and CuBr. Oxalyl chloride was then added to this mixture to afford 2. Reduction of compound 3 with zinc powder in acetic acid at 80 °C and subsequent condensation of the resulting diamine with 2 gave 2,3bis(4-octyloxyphenyl)-5,8-dithiophen-2-yl-quinoxaline (4) in 71.7 % yield. Compound 4 was brominated using N-bromosuccinimide under nitrogen atmosphere and 5,8-bis(5-bromothiophen-2-yl)-2,3-bis(4-(octyloxy)phenyl)quinoxaline (5) was obtained in 78.9 % yield after purification by silica gel column chromatography. The fluorene co-monomer used for the synthesis of the two polymers was 2,20 (9,9-bis(2-(2-methoxyethoxy)ethyl)-9H-fluorene-2,7-diyl)bis(4,4,5,5-tetramethyl-
O
O
N S
N
N
S n
O P1
Fig. 1 Chemical structures of P1 and P2
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O
O
N
O O
S
S N
n
N
O O
O P2
O
O
m
O
O
S
O
O
Polym. Bull.
Scheme 1 Synthesis of 5,8-bis(5-bromothiophen-2-yl)-2,3-bis(4-(octyloxy)phenyl)quinoxaline (5)
1,3,2-dioxaborolane) (9), which was synthesized as shown in Scheme 2 following a previously published procedure [30]. Thus, alkylation of fluorene (6) twice at position 9 with 1-chloro-2-(2-methoxyethoxy)ethane afforded 9,9-bis(2-(2-methoxyethoxy)ethyl)-9H-fluorene (7). Bromination of 7 with molecular bromine gave 2,7dibromo-9,9-bis(2-(2-methoxyethoxy)ethyl)-9H-fluorene (8) which was subsequently converted to 2,20 -(9,9-bis(2-(2-methoxyethoxy)ethyl)-9H-fluorene-2,7-diyl)bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolane) (9) by treatment with n-butyllithium and 2-isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (Scheme 2). The synthesis of polymers P1 and P2 is shown in Scheme 3. Thus, Suzuki coupling reaction between compounds 9 and 5 using tetrakis(triphenylphosphine)palladium(0) as a catalyst and tetraethylammonium hydroxide as base gave P1. The second polymerization reaction was done between 5 (25 %), 4,7-dibromobenzo[c] [1, 2, 5] thiadiazole (10) (25 %) and 9 (50 %) to yield P2. The low molecular weight oligomers were separated by Soxhlet extraction with diethyl ether and the higher molecular weight materials were extracted with chloroform and precipitated from methanol as described in the Experimental section. Both P1 and P2 were obtained as red powders. The CHCl3 solutions of both polymers had deep
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Scheme 2 Synthesis of 2,20 -(9,9-bis(2-(2-methoxyethoxy)ethyl)-9H-fluorene-2,7-diyl)bis(4,4,5,5tetramethyl-1,3,2-dioxaborolane) (9)
Scheme 3 Synthesis of P1 and P2
red colors. The number (MN) and weight (MW) average molecular weights of the polymers, determined by size-exclusion chromatography (SEC) versus polystyrene standards, were found to be comparable and are summarized in Table 1. Optical and electrochemical properties The UV–Vis absorption spectra of P1 and P2 in chloroform solutions and as thin films were studied (Fig. 2) and the optical parameters extracted from the spectra are summarized in Table 1. For P1, the absorption maxima (kmax) were observed at
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k
MW
32.8
31.6
MN
11.5
11.8
P1
P2
380, 466
392, 519
max (nm)
Solution
Polymer
Mol. wt (kg mol-1)
3.6
3.9
3.9
4.3
Absorption coefficient (104 L mol-1 cm-1)
600
617
konset (nm)
Table 1 Summary of molecular weights, optical data and decomposition temperatures of P1 and P2
(ev)
2.06
2.00
Eg
Film (nm)
483
393, 529
kmax
630
630
konset
(nm)
(ev)
1.97
1.97
Eg
412.5
426.0
Td (°C)
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Fig. 2 UV–Vis spectra of P1 and P2 in solution and as thin films. The red (broken lines) and purple (solid line) correspond to the absorption coefficient spectra of 1 9 10-5 mol/L chloroform solutions of P1 and P2, respectively, and the arrow shows towards the absorption coefficient axis. The blue (P1) (broken lines) and green (P2) (solid line) represent normalized absorption spectra in thin films (color figure online)
392 and 519 nm with corresponding molar absorption coefficients of 4.3 9 104 and 3.9 9 104 L mol-1 cm-1, respectively, for the polymer solutions in chloroform. The absorption maxima in thin films appeared at 395 and 529 nm. In contrast, polymer P2 showed broad absorption profile between *380 and *500 nm in solution with absorption coefficients in the range of 3.9 9 104 and 3.6 9 104 L mol-1 cm-1. Similarly, a thin film of P2 showed a broad absorption spectrum. While the short wavelength bands in both polymers are assigned to localized p–p* transitions, the longer wavelength peaks are attributed to intramolecular charge transfer between the donor units (fluorene and thiophene) and acceptor moieties (quinoxaline and benzothiadiazole). The presence of the benzothiadiazole unit in P2 renders the polymer to have a broader absorption that extends from around 300 nm to beyond 550 nm with no noticeable valley unlike P1. Such an extended absorption behavior in the visible region in P2 is a clear advantage to harvest more photons which could translate to high power conversion efficiency in optimized devices. The optical bandgap calculated from the onset of absorption of P1 in CHCl3 solution was 2.00 eV and the corresponding value of P1 in film was found to be 1.97 eV. The optical bandgaps for P2 were calculated to be 2.06 eV (in CHCl3 solution) and 1.97 eV (in thin film). The absorption onsets of both polymers showed red-shifts in thin films as compared to the spectra recorded in CHCl3 solutions which could arise from higher degree of inter polymer chain interactions in the solid state [31, 32]. The electrochemical behaviors of P1 and P2 were studied by square-wave voltammetry (Fig. 3). The HOMO and LUMO levels were determined from the oxidation and reduction peaks to be -5.7 and -3.3 eV for both polymers with a corresponding bandgap of 2.4 eV. The deep HOMO level of the polymers could be an advantage to extract higher VOC as this device parameter is proportional to the energy difference between the HOMO of the polymer and the LUMO of the acceptor [5]. Moreover, the measured HOMO energies are in an ideal range to ensure good air stability of the polymers (the oxidation threshold from air being
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Fig. 4 TGA plot of the polymers with a heating rate of 10 °C/min under inert atmosphere
Weight percentage loss (%)
Fig. 3 Square-wave voltammograms of P1 and P2
110% 100% 90% P1 P2
80% 70% 60% 50
150
250
350
450
Temprature (oC)
around -5.2 eV) [33–35]. The electrochemical bandgap was found to be higher than the optical bandgap which is also consistent with other studies [36–38]. The thermal stabilities of the polymers were investigated using thermogravimetric analysis (TGA) and both polymers revealed excellent stability towards the effect of thermal stress under inert atmosphere (Fig. 4). However, P1 was found to be slightly more stable than P2 with decomposition temperature (Td, 5 % weight loss) occurring at 426 °C. On the other hand, the Td of P2 was 412.5 °C. Obviously, both polymers possess sufficient thermal stability for photovoltaic applications. Photovoltaic properties The photovoltaic performances of P1 and P2 were evaluated by fabricating BHJ solar cell devices with a standard device structure of glass/ITO/PEDOT:PSS/ Polymer:PCBM/LiF/Al. Blends of the polymers and PCBM[60] or PCBM[70] in 1:4 weight ratio were processed with o-DCB. The device properties are summarized in Table 2. The best-performing solar cell device, which was fabricated from a blend of P1 and PCBM[70] in 1:4 ratio processed with o-DCB, had a JSC of 7.78 mA/cm2, VOC of 0.82 V and a FF of 50 % resulting in a PCE of *3.2 %. The use of PCBM[60] as an acceptor together with P1 in a blend ratio of 1:4 gave a lower JSC of 4.43 mA/ cm2 even though the VOC (0.81 V) and FF (54 %) are comparable to the devices
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Polym. Bull. Table 2 Summary of the photovoltaic properties of the P1 and P2 together with PCBM[60] or PCBM[70] in 1:4 blend ratio using o-DCB as processing solvent Polymer:acceptor
JSC (mA/cm2)
FF
VOC (V)
PCE (%)
Thickness (nm)
P1:PCBM[60]
4.43
0.54
0.81
1.93
65
P1:PCBM[70]
7.78
0.50
0.82
3.18
70
P2:PCBM[60]
3.23
0.42
0.85
1.16
60
P2:PCBM[70]
5.97
0.42
0.84
2.14
60
70
Fig. 5 EQEs of the devices prepared P1 and P2 using PCBM[60] and PCBM[70] as acceptors
P1:PCBM[70] P1:PCBM[60] P2:PCBM[70] P2:PCBM[60]
60
EQE (%)
50 40 30 20 10 0 400
500
600
700
Wavelength (nm)
fabricated with PCBM[70] as acceptor. As a result of the lower JSC, a PCE of 1.93 % was achieved from the P1:PCBM[60] device which shows the advantage of the use of PCBM[70] in charge carrier generation [39–41]. The JSC data is also consistent with the EQE curves shown in Fig. 5. The devices fabricated from the P1:PCBM[70] blend gave a more pronounced intensity and also a wider charge carrier generation region encompassing the visible region than the EQE curve obtained from the P1:PCBM[60] device. When it comes to P2, it was possible to achieve a *2.1 % PCE from a device prepared together with PCBM[70] in 1:4 blend ratio with corresponding Jsc of 6.00 mA/cm2, Voc of 0.84 V and FF of 42 %. In this case, the use of PCBM[70] was found to be an advantage to generate a higher Jsc as compared to the use of PCBM[60]. The EQE curves of the devices fabricated with PCBM[70] as an acceptor were also found to be intensified and charge generation was possible over a wider region when compared to the EQE of the devices prepared with PCBM[60] as acceptor (Fig. 5). When the two polymers are compared, devices fabricated from P2 gave lower PCE, despite higher VOC, mainly due to lower Jsc and FF. AFM images of the best performing devices are shown in Fig. 6. The AFM image of the P2:PCBM[70]-based device shows a very finely dispersed polymer/fullerene blend and smaller domain morphology. The lack of favorable percolation paths could be the most likely reason for the lower Jsc and lower FF of the devices prepared from P2. The fine blend mixture can also encourage charge recombination losses that affect the JSC and FF. The higher miscibility of P2 with
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P1:PCBM[70]
2×2 µm
P1:PCBM[60]
2×2 µm
P2:PCBM[70]
2×2 µm
P2:PCBM[60]
2×2 µm
Fig. 6 AFM images of 1:4 blends of P1 and P2 with PCBM[70] and PCBM[60] with o-DCB as the processing solvent
PCBM[70] could be due to the larger amount of oxygenation that increases its compatibility with PCBM[70] [42]. Moreover, the random terpolymer chain could have a tendency to accommodate more PCBM[70] resulting in reduced phase separation [43]. On the other hand, the relatively higher JSC and FF obtained from P1:PCBM[70] blend could be ascribed to a favorable morphology with interconnected phases that would assist in charge carrier transport and collection.
Conclusions In this work, two medium-bandgap polymers (P1 and P2) based on fluorene, quinoxaline and benzothiadiazole were synthesized by the Suzuki coupling reaction. P1 was constructed from a fluorene donor and a DAD segment made from thiophene and quinoxaline acceptor. In P2, 25 % of benzothiadiazole was introduced to the polymer main chain structure on top of the main structure found in P1 to give a terpolymer. P2 showed an optical absorption that extends between 300 and 600 nm with no apparent valley unlike P1 which possesses a valley at ca 440 nm, implying that the introduction of benzothiadiazole is a synthetically simple alternative for improving optical absorption without the use of complicated reactions. Acknowledgments D. Gedefaw and W. Mammo acknowledge financial support from the International Science Programme (ISP), Uppsala University, Sweden. Compliance with ethical standards Conflict of interest The authors declare that they have no conflict of interest.
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