Synthesis and photovoltaic properties of novel alternating phenylenevinylene or fluorenevinylene copolymers containing perylene bisimide

Reactive & Functional Polymers 70 (2010) 426–432

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Synthesis and photovoltaic properties of novel alternating phenylenevinylene or fluorenevinylene copolymers containing perylene bisimide John A. Mikroyannidis a,*, Kai Yin Cheung b, Man Kin Fung b, Aleksandra B. Djurišic´ b a b

Chemical Technology Laboratory, Department of Chemistry, University of Patras, GR-26500 Patras, Greece Department of Physics, The University of Hong Kong, Pokfulam Road, Hong Kong, China

a r t i c l e

i n f o

Article history: Received 22 February 2010 Received in revised form 7 April 2010 Accepted 10 April 2010 Available online 24 April 2010 Keywords: Conjugated polymers Perylene bisimide Phenylenevinylene Fluorenevinylene Photophysics Polymer solar cells Synthesis

a b s t r a c t Two novel alternating phenylenevinylene copolymers P6 and P12 as well as one fluorenevinylene copolymer F connected at the 1,7 bay positions with perylene bisimide were synthesized by Heck coupling. They were characterized by GPC, FT-IR, 1H NMR, TGA, TMA, UV–vis, cyclic voltammetry and photoluminescence (PL) emission spectra. The copolymers were soluble in common organic solvents and thermally stable up to 300 °C. Their glass transition temperatures were 48–60 °C. The long wavelength absorption maximum was located at 510–542 nm with optical band gaps of 2.0 eV. The PL emission maximum of P6 and P12 was red-shifted relative to F. The photovoltaic performance of P6, P12 and F was also investigated. Ó 2010 Elsevier Ltd. All rights reserved.

1. Introduction In recent years, conjugated polymers and organic materials have attracted much attention for their applications in light-emitting diodes (LEDs) and plastic solar cells [1–3]. Poly(p-phenylenevinylene) (PPV) and its derivatives are one of the most promising classes of conjugated polymers for LEDs due to their high luminescence and easy modification of the chemical structure [4–8]. Donor–acceptor conjugated complexes or oligomers have attracted intense scientific investigation due to their unique optical and electrical properties with potential applications in many fields such as molecular electronics, optoelectronics, and artificial photosynthetic systems [9]. Perylene bisimides are potential candidates as electron-accepting materials in organic photovoltaic solar cells [10]. They represent a class of highly thermostable n-type semiconductors with relatively high electron affinity and excellent transport property. More and more attention has been focused on the modification of perylene bisimide structures through high-yield synthetic routes to improve their chemical and physical properties [11]. These molecules and their derivatives have been used not only as building blocks for * Corresponding author. Tel.: +30 2610 997115; fax: +30 2610 997118. E-mail addresses: [email protected], [email protected] (J.A. Mikroyannidis). 1381-5148/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.reactfunctpolym.2010.04.005

electronics and optoelectronics devices such as organic LEDs [11f,11g,12], light-harvesting arrays [13], photovoltaic cells [14] but also for studying the photoinduced energy and electron-transfer process [15]. Both PPV and perylene bisimide chromophores have been applied in bulk-heterojunction-like solar cell configurations as donor and as acceptor materials, respectively [15b,16]. Recently, a new class of donor–acceptor polymers consisting of alternating oligo(p-phenylenevinylene) and perylene bisimide connected via saturated spacers was synthesized [14g]. Moreover, two copolymers containing perylene bisimide and PPV and triphenylamine segments were synthesized and used for photovoltaic cells [17]. Furthermore, two n-type conjugated polymers based on perylene bisimide were synthesized by Stille coupling reaction [18]. Finally, an alternating phenylenevinylene copolymer bearing perylene bisimide moieties along the backbone was synthesized in our laboratory and used as electron acceptor in photovoltaic cells [19]. In this investigation we describe the synthesis, characterization, photophysics and photovoltaic properties of three novel conjugated alternating copolymers based on perylene bisimide. Specifically, two phenylenevinylene copolymers P6 and P12 with hexyloxy and dodecyloxy side groups, respectively, and one fluorenevinylene copolymer F were successfully synthesized by Heck coupling. These donor–acceptor copolymers showed that charge transfer from the excited state takes place.

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2. Experimental

2.4. Preparation of copolymers

2.1. Characterization methods

The preparation of P6 is given as a typical example for the preparation of copolymers. A flask was charged with a mixture of 2 (0.1298 g, 0.182 mmol), 3a (0.0602 g, 0.182 mmol), Pd(OAc)2 (0.0017 g, 0.007 mmol), P(otolyl)3 (0.0128 g, 0.042 mmol), DMF (8 mL) and triethylamine (3 mL). The flask was degassed and purged with N2. The mixture was heated at 90 °C for 24 h under N2. Then, it was filtered and the filtrate was poured into methanol. The purple precipitate was filtered and washed with methanol. The crude product was purified by dissolving in THF and precipitating into methanol (0.14 g, 87%). FT-IR (KBr, cm1): 2926, 2852, 1694, 1590, 1450, 1420, 1336, 1258, 1206, 1188, 1033, 984, 896, 810, 750, 568. 1 H NMR (CDCl3, ppm) (Fig. 2 top): 8.73–7.64 (broad, 6H, aromatic ‘‘i”); 7.15–7.00 (m, 4H, olefinic ‘‘f”); 6.92 (m, 2H, aromatic ‘‘e”); 3.96 (m, 4H, aliphatic ‘‘d”); 2.47 (s, 2H, aliphatic ‘‘h”); 1.91 (m, 4H, aliphatic ‘‘c”); 1.78, 1.48 (m, 20H, aliphatic ‘‘g”); 1.35 (m, 12H, aliphatic ‘‘b”); 0.90 (t, J = 7.2 Hz, 6H, aliphatic ‘‘a”). Anal. Calcd. for (C58H60N2O6)n: C, 79.06; H, 6.86; N, 3.18. Found: C, 78.12; H, 6.94; N, 3.25. Copolymer P12 was similarly prepared in 80% yield from the reaction of 2 with 3b. FT-IR (KBr, cm1) (Fig. 1): 2928, 2852, 1694, 1654, 1590, 1456, 1420, 1338, 1258, 1202, 1132, 1036, 985, 896, 810, 752, 568. 1 H NMR (CDCl3, ppm): 8.74–7.66 (broad, 6H, aromatic of perylene bisimide); 7.14–7.00 (m, 4H, olefinic); 6.93 (m, 2H, aromatic ortho to OC12H25); 3.95 (m, 4H, aliphatic OCH2(CH2)10CH3); 2.48 (s, 2H, aliphatic of cyclohexyl rings close to the nitrogen); 1.79, 1.48 (m, 20H, other aliphatic of cyclohexyl rings); 1.71 (m, 4H, aliphatic

IR spectra were recorded on a Perkin-Elmer 16PC FT-IR spectrometer with KBr pellets. 1H NMR (400 MHz) spectra were obtained using a Brucker spectrometer. Chemical shifts (d values) are given in parts per million with tetramethylsilane as an internal standard. UV–vis spectra were recorded on a Beckman DU-640 spectrometer with spectrograde THF. The PL spectra were obtained with a Perkin-Elmer LS45 luminescence spectrometer. The PL spectra were recorded with the corresponding excitation maximum as the excitation wavelength. TGA was performed on a DuPont 990 thermal analyzer system. Ground samples of about 10 mg each were examined by TGA and the weight loss comparisons were made between comparable specimens. Dynamic TGA measurements were made at a heating rate of 20 °C/min in atmospheres of N2 at a flow rate of 60 cm3/min. Thermomechanical analysis (TMA) was recorded on a DuPont 943 TMA using a loaded penetration probe at a scan rate of 20 °C/min in N2 with a flow rate of 60 cm3/min. The TMA experiments were conducted at least in duplicate to ensure the accuracy of the results. The TMA specimens were pellets of 10 mm diameter and 1 mm thickness prepared by pressing powder of sample for 3 min under 8 kp/cm2 at ambient temperature. The Tg is assigned by the first inflection point in the TMA curve and it was obtained from the onset temperature of this transition during the second heating. Elemental analyses were carried out with a Carlo Erba model EA1108 analyzer. 2.2. Reagents and solvents N,N-Dimethylformamide (DMF) and tetrahydrofuran (THF) were dried by distillation over CaH2. Triethylamine was purified by distillation over KOH. All other reagents and solvents were commercially purchased and were used as supplied. 2.3. Preparation of monomers 2.3.1. 1,7-Dibromo-3,4,9,10-perylenetetracarboxylic dianhydride (1) Compound 1 was prepared by bromination of 3,4,9,10-perylenetetracarboxylic dianhydride by means of bromine and a catalytic amount of iodine in sulfuric acid according to the literature [20]. The 1,7-dibromo derivative was the predominant isomer which was obtained as reaction product. 2.3.2. 1,7-Dibromo-N,N00 -dicyclohexyl-3,4,9,10-perylenetetracarboxylic dianhydride (2) Compound 2 was prepared from the reaction of 1 with excess of cyclohexylamine (mol ratio 1:3) in N-methyl-2-pyrrolidone (NMP) in the presence of glacial acetic acid [20]. 2.3.3. 1,4-Divinyl-2,5-bis(hexyloxy)-benzene (3a) and 1,4-divinyl-2,5bis(dodecyloxy)-benzene (3b) Compounds 3a and 3b were synthesized by Stille coupling reaction [21] of 1,4-dibromo-2,5-bis(hexyloxy)-benzene and 1,4dibromo-2,5-bis(dodecyloxy)-benzene, respectively, with tributylvinyltin [22]. 2.3.4. 9,9-Dihexyl-2,7-divinylfluorene (4) Compounds 4 was synthesized by Stille coupling reaction [21] of 9,9-dihexyl-2,7-dibromofluorene with tributylvinyltin in the presence of PdCl2(PPh3)2 as catalyst and a few crystals of 2,6-ditert-butylphenol as polymerization inhibitor utilizing toluene as reaction medium. The synthesis and characterization of 4 has been described in our previous publication [23].

Fig. 1. FT-IR spectra of monomers 1 (top), 2 (middle) and copolymer P12 (bottom).

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OCH2CH2(CH2)9CH3); 1.30 (m, 36H, aliphatic OCH2CH2(CH2)9CH3); 0.93 (t, J = 7.2 Hz, 6H, aliphatic OCH2(CH2)10CH3). Anal. Calcd. for (C70H84N2O6)n: C, 80.11; H, 8.07; N, 2.67. Found: C, 79.56; H, 8.12; N, 2.56. Copolymer F was similarly prepared in 79% yield from the reaction of 2 with 4. FT-IR (KBr, cm1): 2926, 2854, 1696, 1654, 1592, 1458, 1338, 1258, 1188, 986, 896, 812, 752, 568. 1 H NMR (CDCl3, ppm) (Fig. 2 bottom): 8.80–8.25 (broad, 6H, aromatic ‘‘g”); 7.62 (m, 2H, aromatic ‘‘e”); 7.37 (m, 4H, aromatic ‘‘d”); 7.15–7.07 (m, 4H, olefinic ‘‘f”); 2.48 (s, 2H, aliphatic ‘‘i”); 2.10 (m, 4H, aliphatic ‘‘c”); 1.95, 1.36 (m, 20H, aliphatic ‘‘h”); 1.03 (m, 16H, aliphatic ‘‘b”); 0.76 (t, J = 7.2 Hz, 6H, aliphatic ‘‘a”). Anal. Calcd. for (C65H64N2O4)n: C, 83.30; H, 6.88; N, 2.99. Found: C, 82.56; H, 6.95; N, 3.10. 2.5. Electrochemical characterization Cyclic voltammetry was used to measure the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) levels of copolymers. The HOMO and LUMO were calculated using the half-wave potentials determined from electrochemical measurements. The experiments were performed by coating the copolymer films on the ITO/glass as working electrode with a Ag/AgCl wire as the reference electrode, at a scan rate of 50 mV s1. The solvent in all measurements was acetonitrile, and the supporting electrolyte was 0.1 M TBABF4. 2.6. Solar cell fabrication and characterization Device structure of polymer solar cell (PSCs) was indium tin oxide (ITO)/poly(3,4-ethylene-dioxythiophene):poly(styrene sulfo-

P6

F

Fig. 2. 1H NMR spectra in CDCl3 solution of copolymers P6 (top) and F (bottom). The solvent peak is denoted by an asterisk.

nate) (PEDOT:PSS)/polymer:(phenyl-C61-butyric acid methyl ester) PCBM (1:5) or polymer:poly(3-hexylthiophene) (P3HT) (1:1)/ Al. PCBM and P3HT were purchased from American Dyes. PEDOT:PSS (Baytron VPAI 4083) was purchased from H.C. Starck. The patterned ITO glass substrates (10 X per square) were cleaned by sonication in toluene, acetone, ethanol and deionized water. Then the cleaned substrates were dried in the vacuum oven at 100 °C and exposed to UV ozone for 300s before spin-coating. PEDOT:PSS aqueous solution (passed through the 0.45 lm filter) was spincoated on patterned ITO substrates at 5000 r.p.m. for 3 min. The substrates were then baked in a nitrogen atmosphere at 150 °C for 15 min. A 20 mg/ml chlorobenzene solution of (P6, P12 and F):PCBM (1:5) and (P6, P12 and F):P3HT (1:1) were then spincoated at 1000 r.p.m. for 2 min. The substrates were dried at room temperature in low vacuum (vacuum oven) for 1 h, and then stored in high vacuum (105 to 106 Torr) overnight. The substrates were transferred to a thermal evaporator where Al electrode (100 nm) was then evaporated through a shadow mask. The effective active area of the devices is 3.14 mm2. The measurement of the prepared polymer solar cells was performed in air. I–V curves were measured in the dark and under illumination from a solar simulator (Oriel 66002 solar light simulator) with an AM 1.5 (AM = air mass) spectral filter. The light intensity (100 mW cm2) was measured by a Molectron Power Max 500D laser power meter. The power conversion efficiency was determined from the J–V curve and the electrical measurement was performed using a Keithley 2400 sourcemeter.

3. Results and discussion 3.1. Synthesis and characterization Scheme 1 outlines the synthesis of monomers 1 [20] and 2 [20]. The alternating copolymers P6, P12 and F were synthesized by Heck coupling [24] of dibromide 2 with divinyls 3a, 3b [22] and 4 [23], respectively (Scheme 2). Copolymers P6 and P12 are PPV derivatives with hexyloxy and dodecyloxy side groups, respectively, while F is fluorenevinylene derivative. The copolymers have the donor–acceptor (D–A) architecture with the electron-rich 2,5dialkoxybenzene or 9,9-dialkylfluorene as D unit and the electron-deficient perylene bisimide as A unit. Even thought perylene bisimide is a polynuclear segment of low solubility, the copolymers were very soluble in THF, dichloromethane, chloroform, toluene and other common organic solvents. The alkoxy or alkyl side chains and the cyclohexyl rings which were attached to the perylene bisimide enhanced the solubility of copolymers. The preparation yields were 79–87% and the number-average molecular weights (Mn) which were determined by GPC were 7500–10,300 with polydispersity of 1.5–1.8 (Table 1). The copolymers derived from the Heck reaction present usually low degree of polymerization [25]. Two related copolymers containing perylene bisimide and PPV and triphenylamine moieties [17] which have been synthesized by means of Wittig reaction showed Mn of 6800. In the present case the bulky and rigid dibromide 2 with the two bromines at the 1,7 bay positions of perylene bisimide may present low reactivity towards the divinyls. The synthesis of copolymers could be monitored by FT-IR and 1 H NMR spectroscopy. Fig. 1 presents the IR spectra of monomers 1 and 2 and copolymer P12. Monomer 1 showed characteristic absorption bands at 1772, 1736 and 1038 cm1 associated with the anhydride structure, while 2 and P12 lacked of these absorptions. Monomer 2 and copolymer P12 displayed common absorptions at 2928, 2852 (C–H stretching of aliphatic moieties) and 1694, 1654 cm1 (imide structure). Copolymer P12, as compared to 2, exhibited additional absorptions at 1202 and 985 cm1 as-

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Scheme 1. Synthesis of monomers 1 and 2.

Scheme 2. Synthesis of copolymers P6, P12 and F.

Table 1 Molecular weights, thermal and optical properties of copolymers. Copolymer

P6

P12

F

Mna Mw/Mna Tdb (°C) Ycc (%) Tgd ka,maxe in solution (nm) kf,maxf in solution (nm) ka,maxe in thin film (nm)

10,300 1.8 375 51 55 422, 510, 542 564 427, 527 2.02 eV (614 nm)h

9100 1.7 367 39 48 400, 512, 541 556 405, 522 1.99 eV (623 nm)h

7500 1.5 378 57 60 382, 513, 542 530 386, 520 2.00 eV (620 nm)h

5.75 3.75 2.00

5.76 3.78 1.98

5.77 3.76 2.01

g Eopt g

HOMO (eV) LUMO (eV) i Eel g (eV)

The PL emission spectra were recorded at 490 nm excitation wavelength. a Molecular weights determined by GPC using polystyrene standard. b Decomposition temperature corresponding to 5% weight loss in N2 determined by TGA. c Char yield at 800 °C in N2 determined by TGA. d Glass transition temperature determined by TMA. e ka,max: The absorption maxima from the UV–vis spectra in THF solution or in thin film. f kf,max: The PL maxima in THF solution. g Eopt g : The optical band gap calculated from the onset of thin film absorption spectrum. h Numbers in parentheses indicate the onset of thin film absorption spectrum. i Eel g : Electrochemical band gap determined from cyclic voltammetry.

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signed to the ether bond of the dodecyloxy chains and the out-ofplane bending mode of the trans-vinylene groups, respectively. Fig. 2 depicts the 1H NMR spectra of copolymers P6 and F. Copolymer P6 showed an upfield broad resonance at 8.73– 7.64 ppm assigned to the six perylene bisimide protons labeled ‘‘i”. The olefinic protons ‘‘f” and the aromatic protons ‘‘e” gave signals at 7.15–7.00 and 6.92 ppm, respectively. The cyclohexyl protons ‘‘g” gave multiplets at 1.78 and 1.48 ppm, while the cyclohexyl protons ‘‘h” gave a signal at 2.47 ppm. Finally, the aliphatic protons ‘‘a”, ‘‘b”, ‘‘c” and ‘‘d” of the hexyloxy chains resonated at 0.90, 1.35, 1.91 and 3.96 ppm, respectively. Similarly, F showed a broad signal at 8.80–8.25 ppm from the perylene bisimide protons ‘‘g”. The fluorene protons ‘‘e” and ‘‘d” and the olefinic protons ‘‘f” resonated at 7.62, 7.37 and 7.15–7.07 ppm, respectively. The cyclohexyl protons ‘‘h” and ‘‘i” gave similar resonances with those of cyclohexyl protons of P6. Finally, the aliphatic protons ‘‘a”, ‘‘b” and ‘‘c” of the hexyl chains resonated at 0.76, 1.03 and 2.10 ppm, respectively. It is noteworthy that the small doublets between 5 and 6 ppm appeared in both 1H NMR spectra are assigned to the protons CH2@CHA of the vinyl terminal groups. The thermal properties of copolymers were studied by TGA and TMA (Fig. 3). The decomposition temperature (Td), the char yield (Yc) at 800 °C in N2 by TGA as well as the glass transition temperature (Tg) by TMA are listed in Table 1. The copolymers were stable up to about 300 °C and gave Yc of 39–57%. Their Tg ranged from 48 to 60 °C. On the basis of the Td and Yc values, the thermal stability of copolymers was of the order F > P6 > P12. The Tg values followed also the same trend. This suggests that the 9,9-dihexylfluorene moiety was more thermally stable and rigid than the 2,5-dihexyloxy- and didodecyloxy-benzene unit. P6, as compared to P12, displayed higher thermal stability and rigidity because carried shorter alkoxy side chains. The Tg values of the present copolymers are lower than that (103 °C) of another related perylene-containing PPV [17], since the latter carried octyl instead of cyclohexylamine groups and an additional phenylene ring along the backbone. A Tg value of 65 °C has been reported for 2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinlylene (MEH-PPV) [26].

3.2. Photophysical properties

Fig. 4. Normalized UV–vis absorption spectra of copolymers in 105 M THF solution (a) and thin film (b).

Figs. 4 and 5 present the normalized UV–vis absorption spectra of copolymers in both dilute (105 M) THF solution and thin films. The latter were prepared from THF solutions by spin-coating on quartz substrate. Table 1 summarizes the photophysical characteristics of copolymers.

Fig. 3. TGA thermograms of copolymers in N2. The inset shows the TMA traces of copolymers. Conditions: N2 flow, 60 cm3/min; heating rate, 20 °C/min.

Fig. 5. Normalized PL emission spectra of polymers in 105 M THF solution. The PL emission spectra were recorded by excitation at 490 nm.

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The absorption spectra of copolymer solutions (Fig. 4) showed similar curves with three maxima (ka,max) at about 400, 510 and 540 nm. The short wavelength absorption (400 nm) is associated with the perylene segment, while the long wavelength absorptions (510, 540 nm) correspond to the p–p* transitions of the conjugated main chain. The copolymer thin films displayed two distinct ka,max around 400 and 520 nm. The absorption edges of copolymer thin films were bathochromic shifted in comparison with those of the corresponding polymer solutions, as a result of the interchain interactions in the polymer films. The visible absorption band of copolymers covers a broad wavelength range approximately from 300 to 600 nm which bodes well for their photovoltaic properties. The absorption onset in thin film of copolymers P6, P12 and F were 614 and 623 and 620 nm, respectively, from which the optical band gaps ðEopt g Þ of the copolymers were determined to be 2.02, 1.99 and 2.00 eV, respectively. The Eopt g of the present copolymers is value of slightly lower than that of MEH-PPV [26] (2.10 eV). A Eopt g 2.13 eV has been reported for donor–acceptor type thiophene–perylene–thiophene polymer [27]. Finally, p-conjugated polymers composed of phenothiazine, bithiophene, and perylene moieties of which were prepared by the Suzuki coupling reaction had Eopt g 2.26–2.33 eV [28]. Band gaps of copolymers were also estimated from cyclic voltammetry measurements. From the onset values of oxidation potential (Eonset,ox) and reduction potential (Eonset,red), the HOMO and LUMO as well as the electrochemical band gaps (Eel g ) of the copolymers were calculated from to the following equations:

Fig. 6. J–V curves of the best polymer solar cells with polymer (P6, P12 and F):PCBM (1:5) and (P6, P12 and F):P3HT (1:1) active layers under simulated AM 1.5 solar irradiation.

Table 2 Solar cell performance of polymer blend devices (P6, P12 and F):PCBM (1:5) and (P6, P12 and F):P3HT (1:1). Voc (V)

EHOMO ¼ ðEox þ 4:72Þ eV ELUMO ¼ ðEred þ 4:72Þ eV; where the unit of potential is V versus Ag/AgCl. Obtained results are shown in Table 1. The electrochemically estimated band gaps of copolymers were 1.98–2.01 eV and they were in agreement with the optically estimated band gaps. The copolymer solutions emitted yellow-orange light with maximum (kf,max) at 530–564 nm when they photoexcited at 490 nm (Fig. 5). P6 and P12 had comparable kf,max which supports that the length of the alkoxy side chains did not influence their emission considerably. In contrast, the kf,max of P6 and P12 was significantly red-shifted by 34 and 26 nm, respectively, relative to F because of the presence of the electron-donating alkoxy side chains that usually causes a bathochromic shift. The emission spectra of copolymer thin films were not recorded because they were not fluorescent to a detectable extent. This feature could be attributed to aggregate quenching because of strong intramolecular interactions in solid state.

3.3. Photovoltaic behavior Photovoltaic devices were fabricated using pure polymers as well as polymer blends (P6, P12 and F):PCBM (1:5) and (P6, P12 and F):P3HT (1:1) as active layers. Pure polymers in all cases exhibited very poor photovoltaic performance. Since blends containing perylene derivatives both with PCBM [29] and P3HT [30] have been reported, we have tried both of these materials. Fig. 6 shows the J– V curve of the PSCs under AM 1.5 illumination (100 mW cm2), and Table 2 lists the photovoltaic performance parameters. Better results have been obtained for (P6, P12 and F):PCBM (1:5) compared to (P6, P12 and F):P3HT (1:1). The best result was obtained for F:PCBM 1:5 blends, resulting in open circuit voltage Voc = 0.34 V, short circuit current density Jsc = 0.15 mA cm2, fill factor FF = 0.24, and efficiency g = 0.012%. An efficiency value of 0.07 has been reported for a related perylene-containing PPV [17].

a b c d

a

Jsc (mAcm2)

b

FF

c

g (%)d

1:5 PCBM P6 P12 F

0.30 0.35 0.34

0.10 0.11 0.15

0.26 0.23 0.24

0.008 0.009 0.012

1:1 P3HT P6 P12 F

0.38 0.35 0.34

0.009 0.013 0.007

0.20 0.17 0.20

0.0007 0.0008 0.0005

Voc: Open circuit voltage. Jsc: Short circuit current. FF: Fill factor. g: Efficiency.

All the devices exhibited relatively low fill factor and short circuit current density, so that resulting photovoltaic efficiency is low in spite of good absorption coverage of the visible part of the solar spectrum. However, the resistance of the cells was rather large, which indicates either inferior mobility of the materials or unfavorable phase separation leading to poor charge transport and large recombination losses, and thus low values of Jsc and FF. The efficiency could possibly be improved by increasing the degree of polymerization to improve film quality and charge transport, as well as by modifying the chemical structure of the polymer to further enhance charge transport and induce favorable phase separation in the blends. In some cases, efficiencies of the order 1% have been obtained for blends of polymers containing perylene diimide and a polythiophene derivative [31], which indicates that by tailoring the molecular structures and blend compositions efficiencies of solar cells based on this class of compounds can be further increased. 4. Conclusions Two phenylenevinylene copolymers P6 and P12 as well as one fluorenevinylene copolymer F were synthesized by means of the Heck reaction. The introduction of the dialkoxyphenylene or dialkylfluorene moieties into the bay positions of the perylene bisimide and the cyclohexyl rings attached to the perylene bisimide

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