Synthesis and characterization of porphyrin-terthiophene and oligothiophene π-conjugated copolymers for polymer solar cells

European Polymer Journal 46 (2010) 1084–1092

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Synthesis and characterization of porphyrin-terthiophene and oligothiophene p-conjugated copolymers for polymer solar cells Na Xiang a, Yijiang Liu a, Weiping Zhou a, Hui Huang a, Xia Guo a, Zhuo Tan a, Bin Zhao a, Ping Shen a, Songting Tan a,b,* a

College of Chemistry and Key Laboratory of Advanced Functional Polymeric Materials, College of Hunan Province, and Key Laboratory of Environmentally Friendly Chemistry and Applications of Ministry of Education, Xiangtan University, Xiangtan 411105, PR China b Key Laboratory of Polymeric Materials and Application Technology of Hunan Province, Xiangtan University, Xiangtan 411105, PR China

a r t i c l e

i n f o

Article history: Received 16 November 2009 Received in revised form 8 January 2010 Accepted 19 January 2010 Available online 25 January 2010 Keywords: Conjugated polymer Polymer solar cells Porphyrin-terthiophene Synthesis

a b s t r a c t Two novel conjugated polymers with alternating main chain structures of zinc porphyrinterthiophene (P-PTT) and zinc porphyrin-oligothiophene (P-POT) were synthesized by Stille reaction. The effect of different lengths of thiophene chains on the thermal, optical, electrochemical, and photovoltaic properties of the two copolymers were investigated in detail. P-PTT exhibited higher onset decomposition temperature (392 °C) and glass-transition temperature (152 °C) than those of P-POT. The introduction of thiophene units in the meso-aryl positions of porphyrin resulted in the red shift and broader absorption spectrum compared with zinc porphyrin (PZn) monomer both in chloroform solutions and on thin solid films. The electrochemical properties indicated that the energy levels of the polymers were suitable for efficient charge transfer and separation at the interface between the polymer donor and PCBM acceptor. The bulk heterojunction solar cells based on P-PTT and PPOT showed power conversion efficiencies up to 0.32% and 0.18%, respectively. Ó 2010 Elsevier Ltd. All rights reserved.

1. Introduction Porphyrins and their derivatives have been intensively studied for many years because of their importance in the photochemistry and photo-biology processes. Porphyrins are tetrapyrrolic macrocycles and have a special structure with a big p-orbital on the carbon–nitrogen framework [1]. Under the influence of the large planar p-conjugated structure, porphyrin derivatives exhibit good thermal stability, strong two-photon absorption [2], efficient electron transfer [3], and interesting photo-electrochemical properties [4]. Therefore, porphyrins have been frequently employed in some fields such as biomimetic catalysis [5], chemical and biological sensors [6], organic

* Corresponding author. Address: Key Laboratory of Polymeric Materials and Application Technology of Hunan Province, Xiangtan University, Xiangtan 411105, PR China. Fax: +86 731 5829 2207. E-mail address: [email protected] (S. Tan). 0014-3057/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2010.01.015

light-emitting diodes [7], field effect transistors [8] and solar cells [9]. Recently, porphyrin derivatives have been studied as favorable photosensitizers in photovoltaic cells. For example, Campbell et al. synthesized one kind of darkgreen porphyrin sensitizers with the highest power conversion efficiency (PCE) of 7.1% in dye-sensitized solar cells (DSSCs) [10]. Our group also reported the thiophene-linked porphyrin dyes for DSSCs. The new dyes exhibited very high molar absorption coefficients and gave the relative high PCE of 5.14% [11]. In the field of polymer solar cells (PSCs), some D–A polymers with porphyrins as donor and fullerenes as acceptor have been reported and the research results showed that the polymers were also suitable for efficient electron transfer due to the fullerenes containing an extensively conjugated three-dimensional p system [12–14]. Owing to the p–p interaction between the porphyrin and fullerene, the interdigitating D–A film structures exhibited remarkable photocurrent generation

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[15,16]. However, when fullerenes directly connected to porphyrin, it is hard to form an efficient conjugation for the steric hindrance. Consequently, some researches have been devoted to synthesizing a series of porphyrin polymers with reduced steric hindrance and extended conjugation [17]. In addition, thiophene derivatives as one of the classical and excellent materials were incorporated into the porphyrin backbone applied in solar cells. Very recently, Hiroshi Imahori group studied the photophysical and photovoltaic properties of porphyrin-thiophene copolymer (PZnPT), and obtained a power conversion efficiency of 0.027% for PZnPT:PCBM devices [18]. The main reason of the low PCE may be ascribed to the narrow absorption of porphyrin polymers. In order to broaden the absorption spectrum of porphyrin, our group has synthesized a starshaped polythiophene with porphyrin core, which exhibited strong and broad absorption of the range from 350 to 650 nm. A PCE up to 0.61% was obtained for bulk heterojunction solar cells based on the polymer [19]. In this paper, we reported the synthesis of two novel soluble p-conjugated porphyrin copolymers P-PTT and PPOT (Scheme 1), which incorporated terthiophene and oligothiophene into the polymer main chain, respectively. Considering better light-harvesting characteristics and lower oxidation potential than thiophene [20], terthiophene was linked at the meso-carbon position of porphyrin to extend conjugation, enhance absorption and improve charge transport property. At the same time, the introduction of oligothiophene units may broaden the Soret and Qbands of porphyrin polymer and the long octyloxy chains at the meta-position of 5,15-phenyl groups can improve the solubility of the copolymer. The effects of different lengths of thiophene chains on the thermal, optical absorption, fluorescence, electrochemical properties and photovoltaic properties of the porphyrin-thiophene copolymers were investigated in detail.

toluene were distilled from sodium-benzophenone prior to use. All other reagents were used as received. 2.2. Characterization The 1H NMR and 13C NMR spectra were measured with Bruker AVANCE 400 spectrometer. Molecular mass was determined by matrix assisted laser desorption-ionization time-of-flight mass spectrometry (MALDI-TOF MS) using a Bruker Aupoflex-III mass spectrometer. Element analyses were measured by an Elementar Vario EL III element analyzer. FT-IR spectra were obtained on a Perkin–Elmer Spectra One spectrometer, using KBr pellets. UV–vis absorption spectra were measured on a Perkin–Elmer Lamada 25 spectrometer. The photoluminescence emission spectra were recorded with a Perkin–Elmer LS-50 luminescence spectrometer. The average molecular weights and polydispersity index (PDI) of the polymers were determined by Waters-1515 gel permeation chromatography (GPC) system with chloroform as eluent and polystyrene as standard. Thermogravimetric analysis (TGA) measurements were performed on a Netzsch TG 209 analyzer under nitrogen at a scan rate of 20 °C/min. Differential scanning calorimetry (DSC) analysis was made on a TA DSCQ10 instrument at a scan rate of 20 °C/min. Cyclic voltammetry (CV) measurements were done on a EG&G Princeton Applied Research Model 273 Electrochemical Workstation with Pt wires as both working and counter electrode, saturated calomel electrode (SCE) as reference electrode in a 0.1 M Bu4NPF6 acetonitrile solution. 2.3. Fabrication and characterization of polymer solar cells The PSCs were fabricated with the traditional sandwich structure (denoted as ITO/PEDOT:PSS/polymer:PCBM/Al). The fabrication process was as follows: a layer about 30 nm of poly(3,4-ethylenedioxythiophene)–poly(styrenesulfonate) (PEDOT:PSS) was spin-coated from an aqueous solution (Bayer AG) on a precleaned indium tin oxide (ITO)/glass substrate, and then dried at 150 °C for 30 min. After cooling to room temperature, the solutions containing both polymer and PCBM (in chloro-benzene at a concentration of 10 mg/mL were spin-coated onto the ITO/ PEDOT:PSS electrode, and then dried at 85 °C for 30 min

2. Experimental section 2.1. Materials All the chemical materials were purchased from Alfa Aesar and Shanghai Medical Company (China), THF and OC8H17

N

N Zn

N

OC8H17

N

N

S S

S

m

N Zn

N

N

OC8H17

OC8H17

P-PTT

P-POT Scheme 1. The chemical structures of P-PTT and P-POT.

S S

n

m

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in a nitrogen-filled glovebox. Finally, the aluminium (100 nm) electrode was evaporated onto the top of polymer film at 5  104 Pa. The effective area of one cell was 9 mm2. The current density–voltage characteristics (J–V) were measured by a computer controlled Keithley 2602 source measurement unit in dark and under AM 1.5 illumination conditions, 100 mW/cm2. All these measurements were carried out under ambient atmosphere at room temperature.

2.4. Synthesis of monomers and polymers 2.4.1. 4-(Octyloxy)benzaldehyde (1) In a 100 mL one-necked flask, 4-hydroxybenzaldehyde (2.4 g, 20 mmol), n-octylbromide (3.8 g, 20 mmol) and potassium carbonate (5.5 g, 80 mmol) in the solution of N,N-dimethylformamide (DMF) (50 mL) were heated to 100 °C for 8 h. After the mixture was stirred at room temperature for 30 min, the reaction was quenched by adding cooled water (100 mL). The mixture was extracted with chloroform, and the organic layer was washed with saturated brine for three times and dried over anhydrous MgSO4. After the solvent was removed by rotary evaporation, the residue was purified by column chromatography (silica gel, petroleum ether/acetic ether = 8/1) to afford a colorless liquid of compound 1 (4.2 g, 90%). 1H NMR (CDCl3, 400 MHz, ppm): 9.88 (s, 1H, –CHO), 7.83 (d, 2H, Ar-H), 6.98 (d, 2H, Ar-H), 4.03 (t, 2H, –CH2–), 1.83 (t, 2H, –CH2–), 1.43 (m, 10H, –CH2–), 0.89 (t, 3H, –CH3). 13C NMR (CDCl3, 100 MHz, ppm): 187.05, 163.45, 130.51, 128.53, 114.87, 70.14, 31.87, 29.75, 29.55, 29.43, 26.07, 13.96. Elem. Anal. for C15H22O2 Calc.: C, 76.88; H, 9.46. Found: C, 76.82; H, 9.49%. MALDI-TOF MS (C15H22O2) m/ z: calcd. for 234.16; found 234.20.

2.4.2. 5-(4-(Octyloxy)phenyl)-dipyrromethane (2) Pyrrole (40 mL, 570 mmol, 25 equiv.) and the 4-(octyloxy)benzaldehyde (5.4 g, 23 mmol, 1.0 equiv.) were added to a dry round-bottomed flask and degassed with a stream of Ar for 30 min trifluoroacetic acid (TFA) (0.10 mL, 2.3 mmol, 0.10 equiv.) was added, and the solution was stirred under Ar at room temperature for 30 min and then quenched with 0.1 M NaOH. Then ethyl acetate was added. The organic phase was washed with water and dried over anhydrous MgSO4. Then the solvent was removed under vacuum to afford an orange crude product and purified by column chromatography (silica gel, petroleum ether/ dichloromethane = 3/1) to gave a solid which was recrystallized twice from petroleum ether to obtain a pale green solid of compound 2 (4.3 g, 54%). 1H NMR (CDCl3, 400 MHz, ppm): 5.43 (s, 1H, C–H), 5.91 (m, 2H, pyrrolic-H), 6.16 (d, 2H, pyrrolic-H), 6.62 (m, 2H, pyrrolic-H), 6.80–6.85 (m, 2H, Ar-H), 7.06–7.12 (m, 2H, Ar-H), 3.92 (t, 2H, –CH2–), 1.78 (m, 2H, –CH2–), 1.44–1.28 (m, 10H, –CH2–), 0.89 (t, 3H, CH3–). 13C NMR (CDCl3, 100 MHz, ppm): 154.32, 130.47, 129.65, 129.33, 118.20, 114.33, 108.55, 108.13, 68.88, 45.32, 31.87, 29.69, 29.53, 29.40, 26.07, 22.82, 14.15.

2.4.3. 5,15-Bis(4-bromophenyl)-10,20-bis(4-(octyloxy) phenyl)porphyrin (3) In a 1000 mL three-necked flask, a solution of compound 2 (2.8 g, 8 mmol) and 4-bromobenzaldehyde (1.5 g, 8 mmol) in CH2Cl2 (800 mL) was bubbled with nitrogen for 30 min, and then TFA (0.22 mL, 5 mmol) was added. The mixture was stirred for 1 h at room temperature, and the p-chloranil (4.2 g, 16 mmol) was added. After the mixture was stirred at room temperature for an additional 1 h, the reaction was quenched by adding triethylamine (6 mL). The solvent was removed and the residue was purified by flash column chromatography on silica gel using dichloromethane as the eluent, and a purple solid was afforded (1.7 g, 40%). 1H NMR (CDCl3, 400 MHz, ppm): 8.90 (d, 4H, pyrrolic-H), 8.81 (d, 4H, pyrrolic-H), 8.10 (t, 8H, Ar-H), 7.90 (d, 4H, Ar-H), 7.29 (d, 4H, Ar-H), 4.27 (t, 4H, – OCH2–), 2.00 (t, 4H, –CH2–), 1.54 (m, 20H, –CH2–), 0.95 (t, 6H, CH3), –2.80 (s, 2H, N–H). 13C NMR (CDCl3, 100 MHz, ppm): 160.87, 156.70, 155.68, 138.85, 137.78, 136.55, 132.23, 131.61, 128.59, 127.46, 125.09, 124.23, 122.28, 119.94, 114.72, 103.18, 68.93, 31.88, 29.67, 29.45, 28.94, 25.84, 22.75, 14.42. Elem. Anal. for C60H60Br2N4O2 Calc.: C, 70.04; H, 5.88; N, 5.45. Found: C, 70.10; H, 5.83; N, 5.47%. MALDI-TOF MS (C60H60Br2N4O2) m/z: calcd. for 1028.31; found 1028.27. 2.4.4. 5,15-Bis(4-bromophenyl)-10,20-bis(4-(octyloxy) phenyl)porphyrin zinc (4) The solution of compound 3 (1.7 g, 1.6 mmol) in chloroform (200 mL) was added to the solution of Zn(OAc)2H2O (3.2 g, 0.37 mmol) in methanol (20 mL). The reaction mixture was refluxed for 1 h and the mixture was washed with water after cooling to room temperature. The organic layer was dried over anhydrous MgSO4 and concentrated. A purple-red solid of compound 4 was obtained (1.7 g, 97%). 1H NMR (CDCl3, 400 MHz, ppm): 8.99 (d, 4H, pyrrolic-H), 8.84 (d, 4H, pyrrolic-H), 8.10 (t, 8H, Ar-H), 7.92 (d, 4H, Ar-H), 7.29 (d, 4H, Ar-H), 4.25 (t, 4H, –OCH2–), 2.00 (t, 4H, – CH2–), 1.54 (m, 20H, –CH2), 0.95 (t, 6H, –CH3), –2.80 (s, 2H, N–H). 13C NMR (CDCl3, 100 MHz, ppm): 156.55, 139.10, 138.57, 131.65, 128.64, 127.07, 124.20, 122.29, 121.18, 115.42, 115.24, 114.37, 68.95, 31.64, 29.72, 29.12, 26.31, 22.80, 14.08. Elem. Anal. for C60H58Br2N4O2Zn Calc.: C, 65.97; H, 5.35; N, 5.13. Found: C, 65.94; H, 5.38; N, 5.11%. MALDI-TOF MS (C60H58Br2N4O2Zn) m/z: calcd. for 1092.22; found 1092.18. 2.4.5. 5,500 -Bis(tri-n-butylstannyl)-2,20 :50 ,200 -terthiophene (5) Under argon atmosphere, 2,20 :50 ,200 -terthiophene (2.5 g, 10 mmol) and 50 mL freshly distilled dry THF were added in a 100 mL three-necked flask. n-Butyllithium (8.8 mL, 2.5 M in hexane, 22 mmol) was added dropwise at 78 °C, and the solution was stirred for 1 h at 78 °C, then tributyltin chloride (6.1 mL, 22 mmol) was added. The mixture was allowed up to room temperature slowly and stirred for another 24 h. Finally, the mixture was poured into 100 mL of cooled water, and extracted with hexane. The organic layer was dried over anhydrous MgSO4. Removal of the solvent by rotary evaporation, the residue

N. Xiang et al. / European Polymer Journal 46 (2010) 1084–1092

was distilled under vacuum, a yellow–brown liquid stannylene derivative 5 (6.2 g, 75%) was obtained. 1H NMR (CDCl3, 400 MHz, ppm): 7.47 (d, 2H, Th-H), 7.44 (s, 2H, Th-H), 7.25 (d, 2H, Th-H), 1.81 (m, 12H, –CH2), 1.60 (m, 12H, –CH2), 1.33 (m, 12H, –CH2), 1.12 (m, 18H, –CH3). 13C NMR (CDCl3, 100 MHz, ppm): 138.10, 136.75, 128.55, 128.14, 127.65, 124.97, 28.72, 27.39, 13.82, 10.67. Elem. Anal. for C36H60S2Sn2 Calc.: C, 52.32; H, 7.32. Found: C, 52.37; H, 7.28%. MALDI-TOF MS (C36H60S2Sn2) m/z: calcd. for 826.19; found 826.95. 2.4.6. 2,5-Dibromo-3-hexyl-thiophene (6) In a 100 mL two-necked flask, 3-hexylthiophene (1.7 g, 10 mmol) and THF (50 mL) were added, then a solution of N-bromosuccinimide (3.9 g, 21 mmol) and 10 mL of THF was added dropwise at 0 °C. The reaction mixture was stirred for 8 h with the temperature maintained at 0 °C in the dark. The solvent was removed by rotary evaporation, and the residue was purified by silica gel column chromatography with petroleum ether as the eluent to yield a colorless liquid of compound 6 (2.1 g, 64%). 1H NMR (CDCl3, 400 MHz, ppm): 6.77 (s, 1H, Th-H), 2.50 (t, 2H, –CH2–), 1.53 (d, 2H, –CH2–), 1.29 (s, 6H, –CH2–), 0.88 (t, 3H, –CH3). 113C NMR (CDCl3, 100 MHz, ppm): 142.99, 130.95, 110.31, 107.93, 31.55, 29.52, 29.48, 28.77, 22.54, 14.02. Elem. Anal. for C10H14Br2S Calc.: C, 36.83; H, 4.33. Found: C, 36.79; H, 4.32%. MALDI-TOF MS (C10H14Br2S) m/z: calcd. for 325.92; found 326.03. 2.4.7. 2,5-Bis(tributylstannyl)thiophene (7) Under argon atmosphere, 2,5-dibromothiophene (3.6 g, 15 mmol) and 100 mL freshly distilled dry THF were added in a 250 mL three-necked flask. n-Butyllithium (16 mL, 2.5 M in hexane, 40 mmol) was added dropwise at 78 °C, and the solution was stirred for 1 h with the temperature maintained at 78 °C, then tributyltin chloride (16 mL, 60 mmol) was added in one portion. The mixture was allowed up to room temperature slowly and stirred for another 24 h. Finally, the mixture was poured into 100 mL of cooled water, and extracted with hexane. The organic layer was dried over anhydrous MgSO4. Removal of the solvent by rotary evaporation, the residue was distilled under vacuum, a yellow–brown liquid stannylene derivative 7 (8.4 g, 83%) was obtained. 1H NMR (CDCl3, 400 MHz, ppm): 7.39 (s, 1H, Th-H), 1.62–1.53 (m, 12H, –CH2–), 1.38–1.29 (m, 12H, –CH2–), 1.13–1.08 (m, 12H, –CH2–), 0.94–0.84 (m, 18H, –CH3). 13C NMR (CDCl3, 100 MHz, ppm): 128.34, 127.45, 28.75, 27.50, 13.82, 10.37. 2.4.8. Polymer P-PTT In the 50 mL three-necked flask, 5,15-bis(4-bromophenyl)-10,20-bis-(4-(octyloxy)-phenyl)porphyrin zinc (4) (0.22 g, 0.2 mmol), 5,500 -bis(tributylstannyl)-2,20 :50 ,200 -terthiophene (5) (0.16 g, 0.2 mmol) and freshly distilled toluene (10 mL) were added. The mixture bubbled with nitrogen for 30 min, and Pd(PPh3)4 (0.046 g, 0.04 mmol) was added. After stirring at 110 °C for 3 days, the mixture was cooled to room temperature, and the solvent was removed by rotary evaporation. Then the residue was poured into 100 mL methanol. The precipitate was filtered and washed with methanol, then subjected to Soxhlet

1087

extraction with methanol, hexane and chloroform, subsequently. The fraction that was extracted with chloroform was evaporated under reduced pressure, then precipitated in methanol, filtered and finally dried under vacuum to obtain a dark purple solid (0.09 g, 37%). 1H NMR (CDCl3, 400 MHz, ppm): 8.87–9.07(m, pyrrolic-H), 7.85–8.29 (m, Ar-H), 6.98–7.60 (m, Ph-H), 4.19–4.33 (t, –OCH2), 0.96– 2.10 (m, alkyl-H). FT-IR (KBr, cm1): 2921, 2850, 1604, 1487, 1337, 1243, 1172, 997, 792. Mn = 5400, Mw/Mn = 1.4. 2.4.9. Polymer P-POT In a 50 mL three-necked flask, 2,5-dibromo-3-hexylthiophene (6) (0.16 g, 0.50 mmol), 2,5-bis(tributylstannyl)thiophene (7) (0.40 g, 0.60 mmol), and freshly distilled toluene (10 mL) were added. The mixture was bubbled with nitrogen for 30 min, and Pd(PPh3)4 (56 mg, 0.050 mmol) was added. After stirring at 110 °C for 2 days, a solution of porphyrin monomer 4 (0.10 g, 0.10 mmol) and Pd(PPh3)4 in 15 mL toluene was dropwise slowly and stirred for another 4 days. The mixture was cooled to room temperature, the solvent was removed by rotary evaporation and the residue was poured into 100 mL methanol. The precipitate was filtered and washed with methanol. Then the solid was subjected to Soxhlet extraction with methanol, hexane and chloroform, subsequently. The fraction that was extracted by chloroform was evaporated under reduced pressure, then precipitated in methanol, filtered and finally dried under vacuum to obtain a dark purple solid (74 mg, 59%). 1H NMR (CDCl3, 400 MHz, ppm): 8.94–9.07 (m, pyrrolic-H), 7.82–8.31 (m, Ar-H), 6.81–7.23 (m, Ph-H), 4.25 (t, –OCH2), 0.96–2.89 (m, alkylH). FT-IR (KBr, cm1): 2962, 2920, 2850, 1609, 1503, 1497, 1461, 1251, 1167, 991, 795. Mn = 4100, Mw/Mn = 1.5. 3. Results and discussion 3.1. Synthesis and characterization The synthesis route of monomers and polymers is shown in Scheme 2. Firstly, 5-(4-(octyloxy)phenyl)-dipyrromethane (2) was synthesized by the TFA-catalyzed condensation of 4-(octyloxy)benzaldehyde (1) with pyrrole (25 equiv.) [21]. Then the compound (2) reacted with 4bromobenzaldehyde in the presence of TFA and was treated with Zn(OAc)2 to afford 5,15-bis(4-bromophenyl)10,20-bis(4-(octyloxy)phenyl) porphyrin zinc (4) in 40% yield [17]. The copolymers P-PTT and P-POT were prepared by palladium-catalyzed Stille coupling reaction. The two polymers can be dissolved in THF and CHCl3 easily, but they are relatively insoluble in DMF. The structures of polymers were characterized by 1H NMR spectroscopy and FT-IR. In the 1H NMR spectra of two polymers, the characteristic peaks at 9.12–8.84, 8.34–7.74 and 4.28– 0.90 ppm are assigned to the resonance of protons in porphyrin units. The signal peaks of terthiophene unit for PPTT and oligothiophene unit for P-POT appeared at 7.23– 6.81 and 7.60–6.98 ppm, respectively. From the 1H NMR spectrum of P-POT, we calculated that the ratio between porphyrin units and thiophene units is 1:11. In the FT-IR spectra of P-PTT and P-POT, the strong absorptions of the

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Scheme 2. Synthetic route of the monomers and polymers.

carbon–carbon of porphyrin and carbon–hydrogen of aromatic ring are observed at 997, 2921 cm1, respectively. The number-average molecular weights (Mn) of P-PTT and P-POT were 5400 and 4100, with polydispersity index of 1.4 and 1.5, respectively.

for P-PTT and P-POT, respectively. The glass-transition temperatures (Tg) of P-PTT and P-POT are observed to be 110

P-PTT P-POT

100

The thermal properties of the polymers have been investigated by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). The results are listed in Table 1. The TGA curves (Fig. 1) of polymers show that 5% weight loss temperatures (Td) are at 413 °C and 335 °C

Weight (%)

3.2. Thermal properties

90

80

70 Table 1 Polymerization copolymers.

results

Polymer

Mn [103]

P-PTT P-POT

and

thermal

properties

Mw [103]

PDI

of

the

porphyrin

60 5.4 4.1

7.6 6.2

1.4 1.5

Td [°C] 413 335

Tg [°C] 152 90

200

300

400

500

600

700

Temperature (°C) Fig. 1. TGA plots of the polymers with a heating rate of 20 °C/min.

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152 °C and 90 °C. The Td value of P-PTT is higher than that of P-POT, indicating that P-PTT has the better thermal stability. The result may be explained by the shorter thiophene chains and stronger rigidity of P-PTT compared with those of P-POT.

900

PL intensity (a.u.)

700

3.3. Optical properties The photophysical characteristics of the copolymers have been investigated by UV–vis absorption and photoluminescence (PL) spectra in dilute chloroform solution as well as in solid films. Fig. 2a shows the UV–vis absorption spectra of P-PTT, P-POT and PZn (compound 4) measured in CHCl3. PZn exhibits a sharp Soret band at 420 nm and two weak Q-bands at 549 and 589 nm as typical absorption peaks of zinc porphyrin compounds. As expected, the Soret bands of P-PTT and P-POT are broadened compared with PZn owing to the extended conjugation of the p systems through the thiophene chain. P-PTT exhibits an intense Soret absorption peak at 425 nm, and P-POT shows a sharp Soret absorption peak at 424 nm and a wide shoulder at 470 nm. The shoulder should be due to the absorption of conjugated oligothiophene chain. The UV–vis spectra of the solid films for P-PTT and PPOT are shown in Fig. 2b. The Soret bands of P-PTT and P-POT are located at 444 nm and 442 nm, which largely broaden and red-shift by 19 nm and 17 nm compared with those in solution, respectively. The broadened Soret band, stronger Q-bands and shoulder can be attributed to the p stacking of the polymers in the solid phase. The polymer P-PTT shows pronounced Q-bands based on porphyrin ring at 590–650 nm, which is beneficial to exploit sunlight because of the solar emission spectrum peaked at 600– 800 nm. However, the Q-bands for P-POT solution and film are not pronounced between 590 and 650 nm due to the low content (about 8% mole ratio) of porphyrin unit relative to long oligothiophene unit. Fig. 3 shows the photoluminescence (PL) spectra of the monomer PZn, polymers P-PTT and P-POT in chloroform solution and thin solid films. PZn shows two emission peaks at 600 nm and 650 nm. P-PTT exhibits the maximum

a

PZn P-PTT P-POT

0.8 0.6 0.4 0.2 0.0 300

P-POT

600 500 400 300 200 100 0 -100 450

500

550

600

650

700

750

Wavelength (nm) Fig. 3. PL spectra of P-PTT and P-POT in CHCl3 solution.

emission peak at 605 nm with shoulder peaks at 655 nm, and no emission peaks of terthiophene are detected. The results reveal that there is an effective energy transfer from the thiophene units to the porphyrin units in polymer P-PTT [22,23]. For P-POT, a stronger emission peak at 564 nm and weaker peak at 605 nm are attributed to oligothiophene units and the porphyrin units, respectively. It is noted that the emission peak of porphyrin units was suppressed by the intensive emission of oligothiophene units. The fluorescence quantum yields (UF) of P-PTT and P-POT in CHCl3 solution with TPP (UF = 0.11) as the reference standard (irradiated at 423 nm) were 0.37 and 0.55 for both P-PTT and P-POT. When the two polymers are blended with PCBM and spin-coated on the quartz plates, the fluorescence are hardly observed which indicates that the efficient charge transfer occurs from the polymers to the PCBM [24].

3.4. Electrochemical properties Cyclic voltammetry (CV) is often utilized to investigate the information on the charge injection and measure the

1.0

Normalized absorbance

Normalized absorbance

1.0

PZn P-PTT

800

b

P-PTT P-POT

0.8 0.6 0.4 0.2 0.0

400

500

Wavelength (nm)

600

350

400

450

500

550

Wavelength (nm)

Fig. 2. UV–vis absorption spectra of P-PTT and P-POT in CHCl3 solution (a) and thin films (b).

600

650

700

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N. Xiang et al. / European Polymer Journal 46 (2010) 1084–1092 Table 3 Photovoltaic performances of the PSCs.

P-PTT P-POT

-3

-2

-1

0

1

2

Fig. 4. Cyclic voltammograms for P-PTT and P-POT in CH3CN/0.1 M Bu4NPF6 at 50 mV/s.

Table 2 UV–vis, PL spectral data and electrochemical properties of the polymers. Polymer Solution kmax (nm)a Film kmax (nm)b HOMOc LUMOc Egd

c d

UV

PL

UV

(eV)

(eV)

425 424

605 564

444 442

5.10 5.08

3.25 1.85 3.34 1.74

(eV)

Measured in chloroform solution. Polymer cast from chloroform solution. Energy levels calculated from the cyclic voltammograms. Band gap estimated from HOMO and LUMO.

highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels of conjugated polymers. Fig. 4 exhibits the CV curves of P-PTT and P-POT films on Pt electrodes in acetonitrile containing 0.1 M tetrabutylammonium hexafluorophosphate (Bu4 NPF6) as supporting electrolyte, and the corresponding data are summarized in Table 2. P-PTT shows an irreversible oxidation peak and a quasi-reversible reduction peak. However, P-POT shows two irreversible oxidation peaks and a quasi-reversible reduction peak. The second oxidation peak of P-POT should be due to the oligothiophene units. P-PTT and P-POT have onset oxidation potentials

Jsc (mA/ cm2)

FF

PCE (%)

P-PTT

1:1 1:2 1:3 1:4

0.63 0.58 0.46 0.49

1.36 1.49 2.03 1.69

0.28 0.30 0.34 0.27

0.24 0.26 0.32 0.22

P-POT

1:1 1:2 1:3 1:4

0.32 0.50 0.46 0.48

1.60 1.12 1.70 1.07

0.27 0.27 0.23 0.25

0.14 0.15 0.18 0.13

3.5. Photovoltaic properties The bulk heterojunction solar cells of ITO/PEDOT:PSS/ polymer:PCBM/Ca/Al device structures with different weight ratios of P-PTT or P-POT to PCBM were fabricated to investigate the photovoltaic properties. The thicknesses of the photoactive layers for two polymers are measured to be around 120 nm. The open-circuit voltage (Voc), shortcircuit current density (Jsc), fill factor (FF), and the power 5

5

Current density mA/ cm2

Voc (V)

(Eox) at 0.70 and 0.68 V (vs. SCE) and onset reduction potentials (Ered) at 1.15 and 1.06 V (vs. SCE). The HOMO and LUMO energy levels of the polymers can be calculated from the onset oxidation and reduction potentials according to the equations: EHOMO = e(Eox + 4.4) (eV); ELUMO = e(Ered + 4.4) (eV) [25]. The energy levels of HOMO/ LUMO for P-PTT and P-POT are 5.10/3.25 eV and 5.08/3.34 eV, respectively. The HOMOs of the donors (P-PTT and P-POT) are higher by 1.10 eV and 1.12 eV than the HOMO level of the accepter (PCBM) (6.2 eV). Similarly, the LUMOs of P-PTT and P-POT are higher by 0.95 eV and 0.86 eV than the LUMO of PCBM (4.2 eV). Both the HOMO and LUMO levels of the donors (P-PTT and P-POT) are more 0.5 eV higher than that of the acceptor PCBM, indicating that the polymers blended with PCBM are suitable for efficient charge transfer and separation at the interface between the donor and acceptor [26,27]. The electrochemical band gap (Eg) of P-PTT (1.85 eV) is slight higher than that of P-POT (1.74 eV).

P-PTT Light Dark

4 3 2 1 0

V =0.46 oc Jsc=2.03 ff =0.34 PCE=0.32%

-1 -2 -3 -4

Current Density mA/ cm2

a b

Polymer/PCBM (w/w)

3

Potential (V)

P-PTT P-POT

Polymer

P-POT Light Dark

4 3 2 1 0 -1

Voc=0.46 Jsc=1.70 ff =0.23 PCE=0.18%

-2 -3 -4 -5

-5 -0.2

0.0

0.2

0.4

0.6

Voltage (v)

0.8

1.0

1.2

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

Voltage (V)

Fig. 5. J–V curves of the photovoltaic cells based on P-PTT and P-POT (polymers:PCBM, 1:3, w/w) under the illumination of AM 1.5, 100 mW/cm2.

N. Xiang et al. / European Polymer Journal 46 (2010) 1084–1092

14

IPCE %

polymers for the PSCs, and the photovoltaic performance may be further improved by optimization of the solar cells fabricated conditions.

P-PTT P-POT

12

1091

10

Acknowledgements

8

This work was supported by National Nature Science Foundation (50973092) and Nature Science Foundation (09JJ3020, 09JJ4005) of Hunan Province of China.

6 4

References

2 0 450

500

550

600

650

700

Wavelength (nm) Fig. 6. IPCE plots for the bulk heterojunction solar cells based on P-PTT and P-POT.

conversion efficiency (PCE) are showed in Table 3. When the polymers:PCBM weight ratio is 1:3, the PSCs device exhibited the best performance. Fig. 5 shows the J–V curves of the devices for P-PTT and P-POT with the polymers:PCBM weight ratio of 1:3. The parameters of the photovoltaic properties such as Voc, Jsc, FF and PCE based on P-PTT are 0.46 V, 2.03 mA/cm2, 0.34% and 0.32%, respectively. The device based on P-POT yields PCE of 0.18% (Voc = 0.46 V, Jsc = 1.70 mA/cm2 and FF = 0.23). The PCE of P-PTT is about two times larger than that of P-POT on the same fabricated condition. The different PCE values mainly result from the Jsc value. Fig. 6 displays photocurrent action spectra of the two polymers. Both spectra largely parallel the absorption spectra of the corresponding films (Fig. 2b), implying the involvement of the porphyrin units of the conjugated polymers for the photocurrent generation [15]. In accordance with the difference in Jsc values, the integrated IPCE value of P-PTT is larger than that of P-POT. As for P-PTT, better sunlight absorbing ability at the range of 590–650 nm lead to the higher IPCE value than that of P-POT. The lower PCE value of P-POT is likely caused by the low IPCE values and low electron transport efficiency of the long oligothiophene chain.

4. Conclusions Conjugated polymers with alternating main chain structures of zinc porphyrin-terthiophene (P-PPT) and zinc porphyrin-oligothiophene (P-POT) were synthesized by Stille reaction. The different lengths of thiophene derivatives at the meso-aryl of porphyrin in the main chain have significant influence on the thermal, optical, electrochemical and photovoltaic properties. The P-PTT exhibited better thermal stability, pronounced absorption at the range of 590–650 nm, and larger electrochemical band gap in comparison with P-POT. The photovoltaic device based on PPTT:PCBM (1:3, w/w) exhibits an open-circuit voltage of 0.46 V, a short-circuit current density of 2.03 mA/cm2, an overall power conversion efficiency of 0.32% in the PSC system. The PCE based on P-PTT shows good performance among the reported porphyrin-containing linear chain

[1] Gao BJ, Zhang Y, Wang R, Guo H. Preparation of two kinds of porphyrin-functionalized polymeric materials based on poly(glyceryl methacrylate). Polym Adv Technol 2008;19:1495–503. [2] Krebs FC, Spanggaard H. Antibatic photovoltaic response in zinc porphyrin-linked oligothiophenes. Solar Energy Mater Solar Cells 2005;88:363–75. [3] Regehly M, Wang T, Siggel U, Fuhrhop JH, Röder B. Electron transfer in oligothiophene-bridged bisporphyrins. J Phys Chem B 2009;113:2526–34. [4] Fox MA, Grant JV, Melamed D, Torimoto T, Liu CY, Bard AJ. Effect of structural variation on photocurrent efficiency in alkyl-substituted porphyrin solid-state thin layer photocells. Chem Mater 1998;10:176–7. [5] Sendhil K, Vijayan C, Kothiyal MP. Nonlinear optical properties of a porphyrin derivative incorporated in nafion polymer. Opt Mater 2005;27:1606–9. [6] Gong FC, Wu DX, Cao Z, He XC. A fluorescence enhancement-based sensor using glycosylated metalloporphyrin as a recognition element for levamisole assay. Biosens Bioelectron 2006;22:423–8. [7] Montes VA, Bolivar CP, Agarwal N, Shinar Jr J. Molecular-wire behavior of OLED materials: exciton dynamics in multichromophoric Alq3-oligofluorene-Pt(II) porphyrin triads. J Am Chem Soc 2006;128:12436–8. [8] Huang XB, Zhu CL, Zhang SM, Li WW, Guo YL, Zhan XW, et al. Porphyrin-dithienothiophene p-conjugated copolymers: synthesis and their applications in field-effect transistors and solar cells. Macromolecules 2008;41:6895–902. [9] Ma RM, Guo P, Cui HJ, Zhang XX, Mohammad KN, Grätzel M. Substituent effect on the meso-substituted porphyrins: theoretical screening of sensitizer candidates for dye-sensitized solar cells. J Phys Chem A 2009;113:10119–24. [10] Campbell WM, Jolley KW, Wagner P, Wagner K, Walsh PJ, Gordon KC, Mende LS, Nazeeruddin MK, Wang Q, Graltzel M, Officer DL. Highly efficient porphyrin sensitizers for dye-sensitized solar cells. J Phys Chem C 2007;111:11760–2. [11] Liu YJ, Xiang N, Feng XM, Shen P, Zhou WP, Weng C, et al. Thiophenelinked porphyrin derivatives for dye-sensitized solar cell. Chem Commun 2009;18:12499–501. [12] Fukuzumi S, Endo Y, Imahori H. A negative temperature dependence of the electron self-exchange rates of zinc porphyrin pi radical cations. J Am Chem Soc 2002;124:10974–5. [13] Otsubo T, Aso Y, Takimiya K. Functional oligothiophenes as advanced molecular electronic materials. J Mater Chem 2002;12:2565–75. [14] Choi MS, Aida T, Luo HX, Araki Y, Ito O. Fullerene-terminated dendritic multiporphyrin arrays: ‘‘dendrimer effects” on photoinduced charge separation. Angew Chem 2003;42:4060–3. [15] Hasobe T, Kashiwagi Y, Absalom MA, Sly J, Hosomizu K, Crossley MJ, et al. Supramolecular photovoltaic cells using porphyrin dendrimers and fullerenes. Adv Mater 2004;16:975–9. [16] Hasobe T, Imahori H, Kamat PV, Ahn TK, Kim SK, Kim D, et al. Photovoltaic cells using composite nanoclusters of porphyrins and fullerenes with gold nanoparticles. J Am Chem Soc 2005;127:1216–28. [17] Lu FS, Xiao SQ, Li YL, Liu HB, Li HM, Zhuang JP, et al. Synthesis and chemical properties of conjugated polyacetylenes having pendant fullerene and/or porphyrin units. Macromolecules 2004;37: 7444–50. [18] Umeyama T, Takamatsu T, Tezuka N, Matano Y, Araki Y, Wada T, et al. Synthesis and photophysical and photovoltaic properties of porphyrin-furan and-thiophene alternating copolymers. J Phys Chem C 2009;113:10798–806. [19] Liu YJ, Guo X, Xiang N, Zhao B, Huang H, Li H, Shen P, Tan ST. Synthesis and photovoltaic properties of polythiophene stars with porphyrin cores. J Mater Chem 2010;20:1140–6.

1092

N. Xiang et al. / European Polymer Journal 46 (2010) 1084–1092

[20] Chen J, Burrell AK, Campbell WM, Officer DL, Too CO, Wallace GG. Photoelectrochemical cells based on a novel porphyrin containing light harvesting conducting copolymer. Electrochim Acta 2004;49:329–37. [21] Littler BJ, Miller MA, Hung CH, Wagner RW, O’Shea DF, Boyle PD, et al. Refined synthesis of 5-substituted dipyrromethanes. J Org Chem 1999;64:1391–6. [22] Shen P, Sang GY, Lu JJ, Zhao B, Wan MX, Zou YP, et al. Effect of 3D p– p stacking on photovoltaic and electroluminescent properties in triphenylamine-containing poly(p-phenylene vinylene) derivatives. Macromolecules 2008;41:5716–22. [23] Choi MS, Aida T, Yamazaki T, Yamazaki I. A large dendritic multiporphyrin array as amimic of the bacterial light-harvesting antenna complex: molecular design of an efficient energy funnel for visible photons. Angew Chem 2001;113:3294–8.

[24] Kesti TJ, Tkachenko NV, Vehmanen V, Yamada H, Imahori H, Fukuzumi S, et al. Refined synthesis of 5-substituted dipyrromethanes. J Am Chem Soc 2002;124:8067–77. [25] Agrawal AK, Jenekhe SA. Electrochemical properties and electronic structures of conjugated polyquinolines and polyanthrazolines. Chem Mater 1996;8:579–89. [26] Chen CP, Chan SH, Chao TC, Ting C, Ko BT. Low-bandgap poly(thiophene-phenylene-thiophene) derivatives with broaden absorption spectra for use in high-performance bulkheterojunction polymer solar cells. J Am Chem Soc 2008;130:12828–33. [27] Halls JJM, Cornill J, Santos DAD, Silbey R, Hwang DH, Holmes AB, et al. Charge- and energy-transfer processes at polymer/polymer interfaces: a joint experimental and theoretical study. Phys Rev B 1999;60:5721–7.