Introduction of Perylene Units for Enhanced Interchain Interaction in

Article pubs.acs.org/Macromolecules

Introduction of Perylene Units for Enhanced Interchain Interaction in Conjugated Polymers for Organic Photovoltaic Devices Ji-Hoon Kim,† Hee Un Kim,† Dongbo Mi,† Sung-Ho Jin,‡ Won Suk Shin,∥ Sung Cheol Yoon,∥ In-Nam Kang,§ and Do-Hoon Hwang*,† †

Department of Chemistry, and Chemistry Institute for Functional Materials, Pusan National University, Busan 609-735, Korea Department of Chemistry Education and Interdisciplinary Program of Advanced Information and Display Materials, Pusan National University, Busan 609-735, Korea ∥ Korea Research Institute of Chemical Technology, 100 Jang-dong, Yuseong-gu, Daejeon 305-343, Korea § Department of Chemistry, The Catholic University of Korea, Bucheon 420-743, Korea ‡

ABSTRACT: A series of semiconducting copolymers, poly[2,7-(9,9′-dioctylfluorene)-alt-5,5′-(4′,7′-di-2-thienyl-2′,1′,3′benzothiadiazole)] (PFDTBT), poly[2,2′-(9,9-dioctyl-9H-fluorene-2,7-diyl)dithiophene-alt-5,5′-(4′,7′-di-2-thienyl-2′,1′,3′benzothiadiazole)] (PFD2TBT), and their ter-polymers containing perylene units were synthesized using Suzuki coupling polymerization. The perylene units were introduced to improve the charge-transport ability by enhancing the π−π interaction between polymer chains. The resulting polymers were characterized by 1H NMR, elemental analysis, DSC, and TGA. The synthesized polymers were soluble in common organic solvents, and formed smooth and uniform spin-coated thin films. All of the polymers studied were found to exhibit good thermal stability, losing less than 5% of their weight upon heating to approximately 350 °C. Perylene- containing polymers showed higher field-effect mobilities than the corresponding PFDTBT or PFD2TBT polymers because of the enhanced π−π interaction between polymer chains upon the introduction of perylene units. Bulk heterojunction solar cells were fabricated with configuration of ITO/PEDOT:PSS/polymer:PC71BM/TiOx/Al. The devices using the perylene-containing polymers showed higher short-circuit currents, and fill factors than the corresponding PFDTBT or PFD2TBT devices. One of the fabricated devices using a perylene-containing copolymer showed a maximum power conversion efficiency of 3.16%, with a short circuit current density of 9.61 mA/cm2, open circuit voltage of 0.81 V, and fill factor of 41%.

1. INTRODUCTION Extensive research has been carried out during the past decade on bulk heterojunction solar cells that utilize organic semiconducting materials.1 The main advantages of organic photovoltaic devices (OPVs) are their lightweight, flexible nature, and their cost-effective manufacture by various printing technologies. The photoconversion process in OPVs is accomplished by efficient light absorption by conjugated polymers and accepting materials, charge separation at the interface of the donor polymer and the accepting materials, and subsequent charge collection at the electrodes. Matching the absorption of the conjugated polymer to the solar spectrum determines the ultimate performance of the solar cell. Among the variety of conjugated polymers that have been studied, poly(p-phenylenevinylene) (PPV),2 poly(alkylfluorene) (PF), and polythiophene (PT)3 derivatives are typical p-type electron donor materials. However, the performance of photovoltaic cells with these polymers is limited by their relatively large band gap energies, which are not yet optimized with respect to the solar spectrum. Therefore, the design and synthesis of new low-band gap polymers become very important to better harvest the solar spectrum, especially in the red and near-infrared ranges, which © 2012 American Chemical Society

can lead to possible enhancements in the photocurrents of OPVs.4 The most widely used approach for obtaining low-band gap structures is to utilize internal charge transfer (ICT) from an electron-donating to an electron-accepting moiety.5−7 Some interesting structures using the ICT approach have been reported. For example, Brabec et al. reported a low-band gap polymer, poly[2,6-(4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1b;3,4-b′]-dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (PCPDTBT), which showed absorption in the range 380−900 nm in the AM 1.5 G spectrum, good hole mobility (0.01−0.1 cm2 V−1 s−1) and a high power conversion efficiency (PCE) of 3.2% with PC71BM as the electron acceptor.8,9 A 9.9dialkylfluorene unit has previously been used as a building block for constructing donor−acceptor-type conjugated polymers in OPVs such as poly[2,7-(9,9′-dialkylfluorene)-alt-5,5′(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PFDTBT). PFDTBT showed a maximum PCE of 2.2%, and PFDTBT derivatives containing branched alkyl chains (BisDMOPFDTBT) showed higher PCEs.10,11 Received: December 15, 2011 Revised: February 18, 2012 Published: February 24, 2012 2367

dx.doi.org/10.1021/ma202712r | Macromolecules 2012, 45, 2367−2376

Macromolecules

Article

Scheme 1. Synthetic Routes of PFDTBT and the Perylene Terpolymers

Scheme 2. Synthetic Routes of PFD2TBT and the Perylene Terpolymers

molecular recognition.19 In organic photovoltaic devices, π−π interactions between low-band gap donor polymers are important for charge transportation. Enhanced π−π interactions in p-type donor polymers could improve their chargetransport ability, and thus increase short-circuit currents in solar cells.

Some molecules and polymers that have flat structures such as fluorene,12−16 anthracene,17 and pyrene18,19 have been reported to form excimers or aggregates by strong interchain π−π interactions. Excimer formation could affect the emission properties of molecules or polymers, therefore, the excimer or exciplex formation has been used as a fluorescence probe for 2368


Macromolecules

Article

the illumination of simulated solar light with 100 mW/cm2 (AM 1.5 G) by an Oriel 1000 W solar simulator. Electric data were recorded using a Keithley 236 source-measure unit, with all characterizations carried out in an ambient environment. The illumination intensity was calibrated by a standard Si photodiode detector (PV Measurements . Inc.) which had been calibrated at the NREL. The incident photon-tocurrent conversion efficiency (IPCE) was measured as a function of the wavelength in the range 360−800 nm by using a halogen lamp as the light source, where the calibration was performed using a silicon reference photodiode. Measurements were carried out after masking the fabricated device except the active cell area. All the characterization steps were carried out under ambient air conditions. 2.5. Synthesis of Monomers and Polymers. 2,7-Dibromo-9,9dioctyl-9H-fluorene and 2,2′-(9,9-dioctyl-9H-fluorene-2,7-diyl)bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolane) were synthesized according to a previous report.20 Dibromo-9,9-dioctyl-9H-fluorene. 1H NMR (CDCl3, ppm): δ 7.52 (d, 1H). 7.50 (s, 1H), 7.44−7.47 (m, 3H), 7.43 (d, 1H), 1.99−1.95 (m, 4H), 1.31−1.29 (m, 24H), 0.98 (t, 6H). 13C NMR (CDCl3, ppm): δ 151.1, 141.8, 134.1, 130.5, 127.4, 125.1, 50.8, 44.3, 33.1, 30.2, 29.9, 29.1, 22.4, 14.8. 2,2′-(9,9-Dioctyl-9H-fluorene-2,7-diyl)bis(4,4,5,5-tetramethyl1,3,2-dioxaborolane) (1). 1H NMR (CDCl3, ppm): δ 7.77 (d, 2H), 7.72 (d, 2H), 7.69 (s, 2H), 1.99−1.95 (m, 4H), 1.37 (s, 24H), 1.09− 1.21 (m, 20H), 0.78 (t, 6H), 0.53 (m, 4H).13C NMR (CDCl3, ppm): δ 150.71, 144.14, 133.87, 130.90, 129.16, 119.58, 83.93, 55.40, 40.30, 32.00, 30.15, 29.41, 29.36, 25.16, 23.83, 22.80, 14.27. Anal. Calcd for C41H64B2O4:C, 76.64; H, 10.04; B, 3.36; O, 9.96. Found: C, 76.61; H, 10.01; B, 3.32; O, 9.89. Synthesis of 4,7-Dibromobenzo[c][1,2,5]thiadiazole. A solution of bromine (35.21 g, 22.32 mmol) in 100 mL of HBr (48%) was slowly added dropwise to a solution of 10 g (73.44 mmol) of benzothiadiazole. After refluxing for 6 h, an orange solid precipitated was formed. The mixture was allowed to cool to room temperature, and a saturated solution of NaHSO3 was then added to neutralize the residual Br2. The mixture was filtered and exhaustively washed with water. The solid was then washed once with cold diethyl ether and purified by flash chromatography to give 20.51 g (95%) of compound 3. 1H NMR (CDCl3, ppm): δ 7.73 (s, 2H). 13C NMR (CDCl3, ppm): δ 152.6, 132.1. 113.6. Synthesis of 4,7-Di-2′-thienyl-2,1,3-benzothiadiazole. Tripropyl(thiophen-2-yl)stannane (1.5 g, 11.7 mmol) was added to a stirred solution of 4,7-dibromo-2,1,3-benzothiadiazole (1.32 g, 4.5 mmol), and bis(triphenylphosphine)palladium(II) dichloride (95 mg, 0.09 mmol) in THF (50 mL). The mixture was refluxed overnight. The resulting mixture was extracted with ethyl acetate and brine. The organic layer was separated, dried with anhydrous MgSO4 and the resulting solution was then concentrated by evaporation. A red solid (product yield 85%) was obtained by recrystallizing the crude product in methanol. 1H NMR (CDCl3, ppm): δ 7.84 (s, 2H), 7.43 (d, 2H), 7.21 (t, 2H), 6.61 (d, 2H) 13C NMR (CDCl3, ppm): δ 155.1, 142.8, 135.2, 129.1, 128.0, 127.9, 127.1. Synthesis of 4,7-Di-2′-(5′-bromo)-thienyl-2,1,3-benzothiadiazole (2). N-Bromosuccinimide (2.05 g, 11.5 mmol) was added to a stirred solution of 4,7-di-2′-thienyl-2,1,3-benzothiadiazole (1.44 g, 4.8 mmol) in acetic acid (25 mL) in darkness. The mixture was stirred at room temperature for 36 h, and 2 M HCl was added to this mixture. The product was extracted with chloroform and the organic phase was successively washed with 10% sodium bisulfate, 2 M HCl and water before being dried over MgSO4. The solvent was removed to afford a dark-red crystalline product (1.30 g, 59%) after recrystallization from chloroform. 1H NMR (CDCl3, ppm): δ 7.80 (dd, 4H), 7.13 (d, 2H) 13 C NMR (CDCl3, ppm): δ 154.2, 140.8, 131.1, 129.6, 129.1, 128.0, 111.6. Anal. Calcd for C14H6Br2N2S3: C, 36.70; H, 1.32; Br, 34.88; N, 6.11;S, 20.99. Found: C, 36.68; H, 1.28; Br, 34.78; N, 6.08; S, 20.88. Synthesis of 3,9-Dibromoperylene (3). Bromine (8.81 g, 7.92 mmol) was added in darkness to a stirred solution of perylene (5 g, 19.81 mmol) in carbon tetrachloride (200 mL). The mixture was stirred at 110 °C for 12 h and 2 M HCl was added to the mixture. The product was extracted with chloroform and the organic phase was

In this study we report on a new approach to improve PCE of OPVs by enhancing the interchain π−π interaction. To do this we introduced flat perylene units into the conjugated polymers. We synthesized poly[2,7-(9,9′-dioctylfluorene)-alt5,5′-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PFDTBT), poly[2,2′-(9,9-dioctylfluorene-2,7-diyl)dithiophene-alt-5,5′(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PFD2TBT), and their ter-polymers with perylene units. Optical, electrical and photovoltaic properties of the polymers were investigated. Synthetic routes and polymer structures are shown in Schemes 1 and 2.

2. EXPERIMENT 2.1. Materials. Fluorene, HBr, 2,1,3-benzothiadiazole, n-octyl bromide, bromine, perylene, diethyl ether, N-bromosuccinimide, tripropyl(thiophen-2-yl)stannane, CCl4, 2-isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane, tetrakis(triphenylphosphine)palladium were all purchased from Aldrich. [6,6]-Phenyl C71-butyric acid methyl ester (PC71BM) was purchased from Nano-C. All chemicals were used without further purification. 2.2. Measurements. 1H and 13C NMR spectra were recorded using a Bruker AM-400 spectrometer and absorption spectra were measured using a JASCO JP/V-570 spectrometer. The molecular weights of the polymers were determined by gel permeation chromatography (GPC) analysis relative to a polystyrene standard using a Waters high-pressure GPC assembly model M590. Thermal analyses were carried out on a Mettler Toledo TGA/SDTA 851e, DSC 822 e analyzer under a N2 atmosphere with a heating and cooling rate of 10 °C/min. Cyclic voltammetry (CV) measurements were performed on an AUTOLAB/PGSTAT12 at room temperature with a three-electrode cell in a solution of TBABF4 (0.10 M) in acetonitrile and with a scan rate of 50 mV/s. Polymer films were prepared by dipping platinum working electrodes into the polymer solutions and then drying in air. A platinum wire was used as a counter electrode and a Ag/Ag+ electrode was used as a reference electrode. 2.3. Fabrication of Organic Thin-Film Transistor Devices. Organic thin-film transistor (OTFT) were fabricated using a bottomcontact geometry device (channel length L = 12 μm, and width, W = 120 μm). The source and drain contacts consisted of gold (100 nm), and the dielectric was silicon oxide (SiO2) with a thickness of 300 nm. The SiO2 surface was cleaned, dried, and pretreated with a solution of 10.0 mM octyltrichlorosilane (OTS-8) in toluene at room temperature for 2 h under a N2 atmosphere to produce nonpolar and smooth surfaces onto which the polymers could be spin-coated. The polymers were dissolved to a concentration of 0.5 wt % in chlorobenzene. Films of the organic semiconductors were spin-coated at 1000 rpm for 50 s to a thickness of 60 nm. All device fabrication procedures and measurements were carried out in air at room temperature. 2.4. Fabrication of Photovoltaic Devices. Composite solutions (1:5 wt % ratio) of polymers and PC71BM were prepared using 1,2dichlorobenzene as a solvent. The concentration was adequately controlled in the range 1.0−2.0 wt %. For the measurement of the optical absorption spectra of PCBM, the solutions were spun on precleaned UV-grade silica substrates. The polymer photovoltaic devices were fabricated with a typical sandwich structure of ITO/ PEDOT:PSS/active layer/LiF or TiOx/Al. The ITO coated glass substrates were cleaned through a routine cleaning procedure, including sonication in detergent followed by distilled water, acetone, and 2-propanol. A 45 nm thick layer of PEDOT:PSS (Baytron P) was spin-coated onto a cleaned ITO substrate after exposing the ITO surface to ozone for 10 min. The PEDOT:PSS layer was then baked on a hot plate at 140 °C for 10 min. The active layer was spin-coated from the predissolved composite solution after filtering through a 0.45 μm (PTFE) syringe filter. The device structure was completed by depositing 0.7 nm of LiF and a 100 nm Al cathode (to act as the top electrode) onto the polymer active layer under vacuum (3 × 10−6 Torr) in a thermal evaporator. Current density−voltage (J−V) characteristics of all polymer photovoltaic cells were measured under 2369


Macromolecules

Article

(CDCl3, ppm): δ 8.12−7.44 (12H), 1.96 (4H), 1.35−0.67 (30H). Found: C, 70.98; H, 6.17; N, 4.86; S, 20.16. Poly[2,7-(9,9′-dioctylfluorene)-co-5,5′-(4′,7′-di-2-thienyl-2′,1′,3′benzothiadiazole)-co-3″,9″-perylene] (PFDTBT_5Per). 2,2′-(9,9-Dioctyl-9H-fluorene-2,7-diyl)bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolane) (0.5 g, 0.78 mmol) was mixed with 4,7-di-2′-(5′-bromo)thienyl2,1,3-benzothiadiazole (0.32 g, 0.691 mmol, 0.9 equiv), 3,6dibromoperylene(0.032 g, 0.078 mmol, 0.1 equiv), tetrakis(triphenylphosphine)palladium (3.0 mg, 2.6 μmol), and toluene (10 mL) for this polymerization. 1H NMR (CDCl3, ppm): δ 8.22−7.44 (∼22H), 1.96 (4H), 1.35−0.67 (∼30H). Found: C, 75.47; H, 6.78; N, 3.41; S, 14.99. Poly[2,7-(9,9′-dioctylfluorene)-co-5,5′-(4′,7′-di-2-thienyl-2′,1′,3′benzothiadiazole)-co-3″,9″-perylene] (PFDTBT_10Per). 2,2′-(9,9-Dioctyl-9H-fluorene-2,7-diyl)bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolane) (0.5 g, 0.78 mmol) was mixed with 4,7-di-2′-(5′-bromo)thienyl2,1,3-benzothiadiazole (0.28 g, 0.62 mmol, 0.8 equiv), 3,6dibromoperylene(0.06 g, 0.14 mmol, 0.2 equiv), tetrakis(triphenylphosphine)palladium (3.0 mg, 2.6 μmol) and toluene (10 mL) for this polymerization. 1H NMR (CDCl3, ppm): δ 8.22−7.44 (∼22H), 1.96 (4H), 1.35−0.67 (∼30H). Found: C, 76.29; H, 6.38; N, 3.78; S, 16.19. Poly[2,7-(9,9′-dioctylfluorene)-co-5,5′-(4′,7′-di-2-thienyl-2′,1′,3′benzothiadiazole)-co-3″,9″-perylene] (PFDTBT_20Per). 2,2′-(9,9-Dioctyl-9H-fluorene-2,7-diyl)bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolane) (0.5 g, 0.78 mmol) was mixed with 4,7-di-2′-(5′-bromo)thienyl2,1,3-benzothiadiazole (0.24 g, 0.35 mmol, 0.6 equiv), 3,6dibromoperylene(0.09 g, 0.31 mmol, 0.4 equiv), tetrakis(triphenylphosphine)palladium (3.0 mg, 2.6 μmol), and toluene (10 mL) for this polymerization. 1H NMR (CDCl3, ppm): δ 8.22−7.44 (∼22H), 1.96 (4H), 1.35−0.67 (∼30H). Found: C, 77.28; H, 5.98; N, 3.31; S, 13.65. Poly[2,2′-(9,9-dioctylfluorene-2,7-diyl)dithiophene-alt-5,5′-(4′,7′di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PFD2TBT). 2,2′-(5,5′-(9,9Dioctyl-9H-fluorene-2,7-diyl)bis(thiophene-5,2-diyl))bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolane) (0.5 g, 0.61 mmol) was mixed with 4,7-di2′-(5′-bromo)thienyl-2,1,3-benzothiadiazole (0.34 g, 0.74 mmol, 1.2 equiv) tetrakis(triphenylphosphine)palladium (3.0 mg, 2.6 μmol), and toluene (10 mL) for this polymerization. 1H NMR (CDCl3, ppm): δ 8.18−7.44 (∼16H), 1.96 (4H), 1.35−0.67 (∼30H). Found: C, 66.82; H, 5.08; N, 3.30; S, 22.76. Poly(2,2′-(9,9-dioctyl-9H-fluorene-2,7-diyl)dithiophene-co-5,5′(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)-co-3″,9″-(perylene)) (PFD2TBT_5Per). 2,2′-(5,5′-(9,9-Dioctyl-9H-fluorene-2,7-diyl)bis(thiophene-5,2-diyl))bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolane) (0.5 g, 0.61 mmol) was mixed with 4,7-di-2′-(5′-bromo)thienyl-2,1,3benzothiadiazole (0.25 g, 0.54 mmol, 0.9 equiv), 3,6-dibromoperylene (0.025 g, 0.06 mmol, 0.1 equiv), tetrakis(triphenylphosphine) palladium (3.0 mg, 2.6 μmol), and toluene (10 mL) for this polymerization. 1H NMR (CDCl3, ppm): δ 8.21−7.44 (∼26H), 1.96 (4H), 1.35−0.67 (∼30H). Found: C, 68.65; H, 5.46; N, 3.01; S, 23.12. Poly[2,2′-(9,9-dioctylfluorene-2,7-diyl)dithiophene-co-5,5′-(4′,7′di-2-thienyl-2′,1′,3′-benzothiadiazole)-co-3″,9″-perylene] (PFD2TBT_10Per). 2,2′-(5,5′-(9,9-Dioctyl-9H-fluorene-2,7-diyl)bis(thiophene-5,2-diyl))bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolane) (0.5 g, 0.61 mmol) was mixed with 4,7-di-2′-(5′-bromo)-thienyl-2,1,3benzothiadiazole (0.22 g, 0.49 mmol, 0.8 equiv), 3,6-dibromoperylene (0.05 g, 0.12 mmol, 0.2 equiv), tetrakis(triphenylphosphine)palladium (3.0 mg, 2.6 μmol), and toluene (10 mL) for this polymerization. 1H NMR (CDCl3, ppm): δ 8.21−7.44 (∼26H), 1.96 (4H), 1.35−0.67 (∼30H). Found: C, 73.40; H, 5.94; N, 2.41; S, 20.91.

successively washed with 10% sodium bisulfate, 2 M HCl and water before being dried over MgSO4. The solvent was removed to afford dark-red crystals (2.81 g, 63%) that were recrystallized from hot hexane. 1H NMR (CDCl3, ppm): δ 8.21 (dd, 2H), 7.83 (dd, 2H), 7.80(dd, 2H), 7.40−7.37(m, 4H) 13C NMR (CDCl3, ppm): δ 118.0, 121.3, 124.1, 125.9, 126.9, 127.1, 128.2, 128.9, 130.1, 135.8. Anal. Calcd for C20H10Br2: C, 58.55; H, 2.42; Br, 38.94. Found: C, 58.30; H, 2.30; Br, 38.66. Synthesis of 2,2′-(9,9-Dioctyl-9H-fluorene-2,7-diyl)dithiophene (4). Compound 1 (7.3 g, 13.31 mmol),5-(4,4,5,5-tetramethyl-1,3,2dioxaborolan-2-yl)thiophen-2-ylium (6.23 g, 29.28 mmol) and 50 mL of toluene were placed in two neck flasks under a N2 atmosphere. Then 32 mL of a 2 M K2CO3 solution was injected, and Pd(PPh3)4 (0.46 g, 0.39 mmol) and Aliquat 336 (0.53 g, 1.33 mmol) were added to the solution. The solution was refluxed at 90 °C for 2 days. The reaction mixture was them cooled to room temperature. The reaction mixture was extracted with methylene chloride and brine and the solvent was evaporated. The mixture was purified by column chromatography using hexane to give a liquid product yield was of 75%. 1H NMR (CDCl3, ppm): δ 7.65 (d, 2H), 7.60 (d, 2H), 7.55 (s, 2H), 7.37 (d, 2H), 7.28 (d, 2H), 7.09 (t, 2H), 2.01 (t, 4H), 1.20 (m, 20H), 0.77 (t, 6H), 0.67 (m, 4H). 13C NMR (CDCl3, ppm): δ 149.1, 141.0, 140.7, 133.5, 131.2, 129.0, 128.8, 128.2, 127.3, 125.1, 53.8, 44.1, 32.1, 30.0, 29.5, 29.3, 24.3, 22.5, 13.9. Synthesis of 2,2′-(5,5′-(9,9-Dioctyl-9H-fluorene-2,7-diyl)bis(thiophene-5,2-diyl))bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolane) (5). A n-BuLi solution in hexanes (11.26 mL, 2.0 M) was added at −70 °C to a solution of compound 4 (5 g, 9.01 mmol) in dry THF (150 mL). The mixture was stirred for 30 min at −70 °C, and 2-(4,4,5,5tetramethyl-1,3,2-dioxaborolan-2-yloxy)propan-1-ylium (5.52 mL, 27.03 mmol) was then added at −70 °C. This mixture was allowed to reach room temperature overnight. The resulting mixture was extracted with methylene chloride and brine and then dried with anhydrous MgSO4. After filtering, the solvent was evaporated, to provide a product yield of 56%. 1H NMR (CDCl3, ppm): δ 7.43−7.63 (m, 10H), 1.96 (t, 4H), 1.35 (s, 24H), 1.20 (m, 20H), 0.77 (t, 6H), 0.67 (m, 4H). 13C NMR (CDCl3, ppm): δ 152.14, 151.93, 140.68, 138.30, 133.24, 128.20, 125.19, 124.42, 120.73, 120.31, 84.30, 55.37, 40.52, 31.89, 30.11, 29.34, 29.28, 24.92, 23.87, 22.71, 14.20. Anal. Calcd for C49H68B2O4S2: C,72.94; H,8.50; B,2.68; O,7.93;S,7.95. Found: C,72.89; H,8.42; B,2.55; O,7.89; S,7.81. General Polymerization Procedure. Three copolymers were synthesized by Suzuki coupling polymerization.21,22 The 2,2′-(9,9dioctyl-9H-fluorene-2,7-diyl)bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolane), 2,2′-(5,5′-(9,9-dioctyl-9H-fluorene-2,7-diyl)bis(thiophene-5,2diyl))bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolane), and 4,7-di-2′-(5′bromo)-thienyl-2,1,3-benzothiadiazole, 3,9-dibromoperylene monomers were synthesized according to previous reports. The reaction mixture of 2 M K2CO3 (8 mL), tetrakis(triphenylphosphine)palladium (0.06 g), and Aliquat 336 (0.07 g) in 15 mL of anhydrous toluene was stirred at 90 °C for 3 days. Following this an excess amount of 1bromobenzene and phenylboronic acid (the end-capper) was dissolved in 1 mL of anhydrous toluene and stirred continuously for 12 h. The reaction mixture was cooled to approximately 50 °C and 200 mL of methanol was added slowly with vigorous stirring of the reaction mixture. The polymer fibers were collected by filtration and reprecipitation from methanol and acetone. The polymers were then purified further by washing for 2 days in a Soxhlet apparatus, with acetone used to remove oligomers and catalyst residues. Column chromatography with a chloroform solution was then performed on the polymer. The reprecipitation procedure in chloroform/methanol was then repeated several times. The resulting polymers were soluble in common organic solvents. Poly[2,7-(9,9′-dioctylfluorene)-alt-5,5′-(4′,7′-di-2-thienyl-2′,1′,3′benzothiadiazole)] (PFDTBT). 2,2′-(9,9-Dioctyl-9H-fluorene-2,7-diyl)bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolane) (0.5 g, 0.78 mmol) was mixed with 4,7-di-2′-(5′-bromo)thienyl-2,1,3-benzothiadiazole (0.40 g, 0.87 mmol, 1.1 equiv), tetrakis(triphenylphosphine)palladium (3.0 mg, 2.6 μmol), and toluene (10 mL) for this polymerization. 1H NMR

3. RESULTS AND DISCUSSION 3.1. Synthesis and Characterization of Polymers. The synthesis of the polymers was carried out using palladiumcatalyzed Suzuki coupling reactions between dibromoaryl and diborolanylaryl monomers (Schemes 1 and 2). The synthesized polymers were soluble in common organic solvents such as chloroform, chlorobenzene, and toluene, and the polymer 2370


Macromolecules

Article

main chain.21 Peak and edge positions were almost equivalent for PFDTBT, PFDTBT_5Per and PFDTBT_10Per, but the maximum absorption peak for PFDTBT_20Per moved to a shorter wavelength region. This might be due to a decrease in the internal charge- transfer (ICT) interaction cause by a decreased content of electron-withdrawing DTBT monomer in the copolymer. PFD2TBT and the ter-polymer solutions also exhibited two absorption bands at around 410 and 520 nm. The absorption maxima and edges of the polymer solutions showed little dependence upon perylene contents. The UV−visible absorption spectra of the synthesized polymers in a chloroform solution are shown in Figure 2. The polymer thin films showed a similar absorption trend; however, the films showed increasingly larger redshifts in absorption than those for the solution as shown in Figure 3. A strong interchain interaction between polymer chains in the solid state would result in the red-shifting of the absorption peak and the onset. Optical band-gaps of the polymers were obtained from the absorption onsets of the polymer films. The measured optical band gaps of the copolymers were in the range 1.95−1.82 eV. 3.4. Electrochemical Properties. Electrochemical cyclic voltammetry (CV) was employed to investigate the redox behavior of the polymers and to determine their HOMO and LUMO energy levels.25 CV was performed under an Ar atmosphere in a solution of TBABF4 (0.10 M) in acetonitrile at a scan rate of 50 mV/s and room temperature. A platinum plate was used as a working electrode. A platinum wire was used as the counter electrode, and a Ag/AgNO3 electrode was used as the reference electrode. The HOMO energy level was determined by using oxidation onsets (Eox) of the polymer films.26 The LUMO energy levels of the polymers were determined by combining the HOMO energy levels obtained from CV and the optical bandgap energies obtained from the edges of absorption.26 To obtain the oxidation potentials of the polymer films, the reference electrode was calibrated using a ferrocene/ferrocenium (Fc/Fc+) salt couple, whose redox potential is assumed to have an absolute energy level of −4.80 eV in a vacuum.26 The potential of this external standard determined under the same conditions was 0.4 eV. As a result, the HOMO energy values were calculated using the equation EHOMO = −(Eoxonset + 4.4) eV, where Eoxonset is the onset oxidation potential vs Ag/Ag+. The HOMO energy levels of PFDTBT, PFD2TBT, and the ter-polymers were determined to lie in the range from −5.58 to −5.41 eV. Optical band gap energies show little change with by changing perylene unit ratios, but the HOMO energy levels of the polymers were moved slightly toward higher positions with the introduction of perylene units, as illustrated in Figure 4. This arises from the result of a decrease in the contents of high electron affinity DTBT units as the number of perylene units in the polymers increased.27 The UV−visible absorption spectra, optical band-gaps, and HOMO/LUMO energy levels of the polymers are summarized in Table 2. 3.5. TFT Characteristics of the Polymer Thin Films. To measure the field effect mobility of the charge carriers, OTFT were fabricated on a silicon wafer using a bottom contact geometry (channel length L = 12 μm, and width, W = 120 μm) under nitrogen. The TFT devices of the polymers exhibited typical p-channel transistor characteristics. Figure 5 shows the transfer curves of the devices fabricated using the polymers as the active layer. The field-effect mobility was calculated in the saturation regime using the following equation: Ids = (W/

solutions formed uniform thin films after spin-coating. The number-average molecular weights (Mn) of the synthesized polymers were determined by GPC using polystyrene as a standard. PFDTBT showed a relatively low molecular weight but other synthesized polymers including PFD2TBT gave number-average molecular weights higher than 10 000 g/mol with polydispersity index (PDI = Mw/Mn) ranging from 1.86 to 3.49. The actual compositions of the polymers were determined by elemental analysis using the sulfur contents as the standard. The actual compositions of the copolymers were not the same as the feed ratios but we found that the differences were not significant. 3.2. Thermal Properties. The thermal properties of the polymers were investigated by using thermo-gravimetric analysis (TGA) and a differential scanning calorimeter (DSC). PFDTBT-based polymers showed better thermal stability than PFD2TBT-based polymers. Moreover, PFDTBT-based polymers exhibited good thermal stability, losing less than 5% of their weight upon heating to temperatures greater than 400 °C. The thermally induced phase-transition behavior of the polymers was investigated by using a DSC under a N2 atmosphere. The glass transition temperatures (Tg) of the polymers were in the range 105−161 °C and were found to be dependent on the polymer structure. PFD2TBT-based polymers showed higher glass transition temperatures than PFDTBT-based polymers, and as the perylene contents in the copolymers increased, the polymer gave a higher glass transition temperature. No synthesized polymer showed any crystalline temperature (Tc), except PFDTBT_20Per. The value of Tc for PFDTBT_20Per was 191 °C. TGA thermograms of the polymers and DSC curve of PFDTBT_20Per are shown in Figure 1. Molecular weights and thermal properties of the synthesized polymers are summarized in Table 1.

Figure 1. TGA thermograms of polymers and DSC curve of PFDTBT_20Per.

3.3. Optical Properties. The UV−visible absorption spectra of the synthesized polymers were recorded in both chloroform solution and as a solid thin film. PFDTBT and the ter-polymer solutions exhibited two distinct absorption bands, where the first absorption peak is at around 387 nm, corresponding to the π−π* transition of the conjugated polymer backbone,23,24 and the second absorption band in the longer wavelength region at around 530 nm, can be attributed to the strong internal charge-transfer (ICT) interaction between donating and accepting groups in the 2371


Macromolecules

Article

Table 1. Average Molecular Weights and Thermal Properties of the Synthesized Polymers ratio (perylene mol %) polymer

polymer yield (%)

Mna (g/mol)

Mwa (g/mol)

PDIa

Tdb (°C)

Tgc (°C)

feed ratio

actual ratiod

PFDTBT PFDTBT_5Per PFDTBT_10Per PFDTBT_20Per PFD2TBT PFD2TBT_5Per PFD2TBT_10Per

72 78 69 58 71 73 65

4500 14 000 12 000 11 000 16 000 12 000 11 000

7900 50 000 32 000 33 000 36 000 39 000 30 000

1.8 3.5 2.7 3.0 2.3 3.3 2.7

434 408 430 431 300 332 392

105 110 124 128 154 158 161

− 5.0 10.0 20.0 − 5.0 10.0

− 2.8 11.4 17.3 − 6.1 13.3

a

Mn, Mw, and PDI of the polymers were determined by gel permeation chromatography using polystyrene standards in CHCl3. bTemperature at 5% weight loss by a heating rate of 10 °C/min under nitrogen. cDetermined by DSC at a heating rate of 10 °C/min under nitrogen. dCalculated by elemental analysis through the amount of sulfur contained in copolymers as a standard.

PFDTBT and PFD2TBT plays an important role in increasing the carrier mobility, possibly due to the enhanced interchain π−π interaction. The characteristics of the TFTs using the polymers as the active layers are summarized in Table 3. 3.6. Photovoltaic Properties. Photovoltaic devices were fabricated using PFDTBT, PFD2TBT and the ter-polymers as p-type electron donors and by using [6,6]-phenyl C71-butyric acid methyl ester (PC71BM) as an n-type acceptor. Two different types of photovoltaic devices were fabricated and characterized, with the device structures of ITO/PEDOT:PSS/ polymer:PC 71 BM/LiF/Al and ITO/PEDOT:PSS/polymer:PC71BM/TiOx/Al shown in Figure 6. The solutionprocessable titanium suboxide (TiOx) layer was employed as an optical spacer30,31 and as a hole blocker27 between the bulk heterojunction(BHJ) layer and the top metal electrode. Figure 7 shows the current−voltage (J−V) characteristics of the ITO/PEDOT:PSS/polymer:PC71BM/LiF/Al devices. It can be observed that the open circuit voltages of cells fabricated using PFDTBT and its ter-polymers range from 0.70 to 0.77 V. The device fabricated using PFDTBT showed higher open circuit voltages than those of perylene-containing copolymers. This coincides with the relative HOMO energy levels of the polymers. As the perylene content in the copolymers increased, the amount of DTBT contents decreased, and so the HOMO of the polymers moved to higher energy levels. PFDTBT has the deepest HOMO energy level, thus it showed the highest open circuit voltage (0.77 V). It is also noteworthy that, the short circuit current of the devices increased with an increase in the number of perylene units in the polymer, except for the case of PFDTBT_20Per. Even though PFDTBT_20Per showed the highest hole mobility among the PFDTBT-based copolymers, it is UV−visible absorption spectrum showed greater blue-shifting than PFDTBT, as shown in Figures 2 and 3, and it showed the widest band gap energy among all of the polymers considered herein. This may be why the short circuit current of the device fabricated using PFDTBT_20Per is lower than that of PFDTBT, despite it higher mobility. The device fabricated using PFDTBT-10Per showed the best photovoltaic performance among the PFDTBT-based polymers (Voc = 0.74 V, Jsc = 7.19 mA/cm2, PCE = 2.16%). For the photovoltaic devices fabricated using PFD2TBT-based polymers, the shortcircuit currents of the devices also increased as the perylene content in the copolymers increased; however, the PCEs of the devices using PFD2TBT-based polymers were lower than those of PFDTBT-based polymers because of lower open-circuit voltages and short-circuit currents. The device fabricated using PFD2TBT_10Per showed the highest PCE (1.51%) among the PFD2TBT-based polymers. The characteristics of the ITO/

Figure 2. UV−visible absorption spectra of the polymers in chloroform solution.

Figure 3. UV−visible absorption spectra of the polymer thin films.

2L)μCi(Vgs − Vth)2, where Ids is the drain-source current in the saturated region, W and L are the channel width and length, respectively, μ is the field-effect mobility, Ci is the capacitance per unit area of the insulating layer, and Vgs and Vth are the gate and threshold voltages, respectively.28,29 Interestingly, the fieldeffect mobility increased with an increase in the number of perylene units in the corresponding parent polymers. The fieldeffect mobility for PFDTBT, PFDTBT-5Per, PFDTBT-10Per, and PFDTBT-20Per was determined to be 1.0 × 10−4, 1.2 × 10−4, 5.3 × 10−4, and 1.2 × 10−3 cm2/(V s), respectively. Similarly, the measured field-effect mobility of PFD2TBT, PFD2TBT-5Per, and PFD2TBT-10Per was 2.7 × 10−6, 1.1 × 10−5, and 2.1 × 10−5 cm2/(V s), respectively. This result suggests that the introduction of the flat perylene structure to 2372


Macromolecules

Article

Figure 4. Band diagram for PC71BM and the synthesized polymers.

Table 2. Optical and Electrochemical Properties of the Synthesized Polymers λmax, abs (nm) polymers PFDTBT PFDTBT_5Per PFDTBT_10Per PFDTBT_20Per PFD2TBT PFD2TBT_5Per PFD2TBT_10Per

solutiona 387, 385, 387, 387, 412, 406, 409,

531 534 537 487 518 524 527

λonset (nm) filmb

387, 387, 390, 385, 422, 416, 420,

filmb

optical Egopt (eV)c

HOMO (eV)

LUMO (eV)

652 650 645 642 675 679 682

1.90 1.90 1.92 1.93 1.83 1.82 1.81

−5.58 −5.56 −5.55 −5.51 −5.43 −5.41 −5.41

−3.68 −3.66 −3.63 −3.58 −3.60 −3.59 −3.60

543 545 558 522 547 550 560

1 × 10−5 M in anhydrous chloroform. bPolymer film on a quartz plate by spin-casting from a solution in chloroform at 1500 rpm for 30 s. Calculated from the absorption band edge of the copolymer films.

a c

The morphologies of the polymers and PC71BM blend films were observed by tapping-mode atomic force microscopy (AFM). Figure 10 shows the AFM images of the polymer:PC71BM (1:5 wt %) films, which were taken after removal of the Al electrode and after device fabrication. The Al electrode was removed by dipping the devices for 1 min in a mixed acid solution containing of 16 mL of concentrated phosphoric acid, 1 mL of concentrated hydrochloric acid, 1 mL of acetic acid, and 2 mL of D2O. The blend films showed almost equivalently measured smooth surfaces (according to their root-mean-square (rms) roughness) with small degrees of phase separation. The rms roughness of the active layers were 0.49 nm for PFDTBT, 0.44 nm for PFDTBT-10Per, 0.51 nm for PFD2TBT, and 0.50 nm for PFD2TBT-10Per, respectively. This result suggests that the morphologies of all the active films are very similar, so that such a property will not affect the Jsc of the photovoltaic devices. Therefore, the improvement of PCE and Jsc in the fabricated photovoltaic devices is derived from the enhanced π−π interaction between polymers that results from the introduction of perylene units.

PEDOT:PSS/polymer:PC71BM/LiF/Al devices are summarized in Table 4. Figure 8 shows that the J−V characteristics of ITO/ PEDOT:PSS/polymer:PC71BM/TiOx/Al devices provide a similar trend to those of ITO/PEDOT:PSS/polymer:PC71BM/LiF/Al devices but their PCEs increased mainly due to increased short circuit currents. The increase in the short circuit current originates from the introduction of the thin titanium suboxide (TiOx) layer (ca. ∼10 nm). The titanium suboxide layer could block holes to the aluminum electrode, thus it could increase the short circuit current of the photovoltaic device. The device fabricated using PFDTBT10Per showed the highest PCE of 3.16% with an open circuit voltage of 0.81 V, a short circuit current of 9.61 mA/cm2 and the highest fill factor (0.41) among the fabricated devices. The characteristics of the ITO/PEDOT:PSS/polymer:PC71BM/ TiOx/Al devices are summarized in Table 5. Figure 9 shows the incident photon-to-current conversion efficiency (IPCE) spectra of the ITO/PEDOT:PSS/PFDTBT10Per + PC71BM/LiF and TiOx/Al devices. The absorption onset corresponding to the photon-to-current conversion efficiency was about 750 nm for these devices, and the absorption onset values corresponding to the maximum conversion efficiency values were 427 and 560 nm.

4. CONCLUSIONS We introduced perylene units into PFDTBT and PFD2TBT to enhance the interchain π−π interaction. The optical and electrochemical properties of the synthesized copolymers 2373


Macromolecules

Article

Figure 5. Transfer characteristics of OTFTs fabricated using the copolymers as the active layer at a constant source-drain voltage of −80 V. (a) PFDTBT, (b) PFDTBT-10Per, (c) PFDTBT-20Per, (d) PFD2TBT, and (e) PFD2TBT-10Per.

Table 3. Summary of the characteristics of OTFTs fabricated using the polymers as the active layers polymer PFDTBT PFDTBT-5Per PFDTBT-10Per PFDTBT-20Per PFD2TBT PFD2TBT-5Per PFDTBT-10Per

Mobility (cm2/(V s)) 1.0 1.2 5.3 1.2 2.7 1.1 2.1

× × × × × × ×

10−4 10−4 10−4 10−3 10−6 10−5 10−5

Ion/Ioff 1 1 1 2 6 1 5

× × × × × × ×

104 104 105 105 102 103 103

Figure 6. Structure of photovoltaic devices fabricated using the polymers.

showed were slight modification upon perylene addition. Interestingly, the field-effect mobility of the TFTs, using the 2374


Macromolecules

Article

Figure 7. J−V characteristics of the photovoltaic devices with ITO/ PEDOT:PSS/polymer+PC71BM (1:5)/LiF/Al configuration. Figure 9. Incident photon-to-current efficiency (IPCE) spectra of the two different structured devices using PFDTBT-10Per.

Table 4. Summary of the Characteristics of Photovoltaic Devices with ITO/PEDOT:PSS/Polymer + PC71BM (1:5)/ LiF/Al Configuration polymer

Voc (V)

Jsc (mA/cm2)

FF (%)

PCE (%)

PFDTBT PFDTBT_5Per PFDTBT_10Per PFDTBT_20Per PFD2TBT PFD2TBT_5Per PFD2TBT_10Per

0.77 0.72 0.74 0.70 0.65 0.67 0.64

5.92 6.13 7.19 6.23 5.02 5.54 5.95

0.34 0.38 0.40 0.38 0.31 0.36 0.39

1.56 1.69 2.16 1.67 1.03 1.35 1.51

Figure 10. AFM topography images (5 × 5 μm) of active composite films of (a) PFDTBT:PC71BM(1:5), (b) PFDTBT-10Per:PC71BM(1:5), (c) PFD2TBT:PC 7 1 BM(1:5), and (d) PFD2TBT10Per:PC71BM(1:5).

Figure 8. J−V characteristics of the photovoltaic devices with ITO/ PEDOT:PSS/polymer + PC71BM(1:5)/TiOx/Al configuration.

electron donor, also increased as the perylene content in the copolymers increased, which is consistent with the field-effect mobility results of the polymers. We conclude that the introduction of perylene to the conjugated polymer chain enhances the π−π interaction between polymer chains and improves its charge-transport ability, leading to increases short circuit currents in the photovoltaic devices.

Table 5. Summary of the Characteristics of Photovoltaic Devices with ITO/PEDOT:PSS/Polymer + PC71BM(1:5)/ TiOx/Al Configuration polymer

Voc (V)

Jsc (mA/cm2)

FF (%)

PCE (%)

PFDTBT PFDTBT_5Per PFDTBT_10Per PFD2TBT PFD2TBT_5Per PFD2TBT_10Per

0.79 0.76 0.81 0.64 0.72 0.76

8.40 9.09 9.61 8.52 9.46 9.68

0.35 0.38 0.41 0.35 0.36 0.38

2.35 2.64 3.16 1.92 2.46 2.82

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

polymers as the active layer increased with an increasing perylene content in the copolymers. The short circuit currents of the photovoltaic devices fabricated using the polymers as an

Notes

The authors declare no competing financial interest. 2375


Macromolecules

■

Article

(31) Park, S. H.; Roy, A.; Beaupre, S.; Cho, S.; Coates, N.; Moon, J. S.; Moses, D.; Leclerc, M.; Lee, K.; Heeger, A. J. Nat. Photonics 2009, 3, 297.

ACKNOWLEDGMENTS This research was financially supported by the Ministry of Knowledge Economy (MKE) under the New & Renewable Energy Program through a KETEP grant (No. 20103020010050 and No. 2011T100200034), and by the National Science Foundation (NRF) grant funded by the Korean government (MEST) (No. 2011-0011122) through GCRC-SOP (Grant 2011-0030668).

■

REFERENCES

(1) Sariciftci, N. S.; Braun, D.; Zhang, C.; Srdanov, V. I.; Heeger, A. J.; Stucky, G.; Wudl, F. Appl. Phys. Lett. 1993, 62, 585. (2) Greenham, N. C.; Moratti, S. C.; Bradley, D. D. C.; Friend, R. H.; Holmes, A. B. Nature 1993, 365, 628. (3) Dang, T. T. M.; Park, S −J.; Park, J.−W.; Chung, D.−S.; Park, C. E.; Kim, Y.−H.; Kwon, S.−K. J. Polym. Sci., Part A: Polym. Chem. 2007, 45, 5277. (4) Peng, Q.; Liu, X.; Su, D.; Fu, G.; Xu, J.; Dai, L. Adv. Mater. 2011, 23, 4554. (5) Zhang, M.; Fan, H.; Guo, X.; He, Y.; Zhang, Z.−G.; Min, J.; Zhang, J.; Zhao, G.; Zhan, X.; Li, Y. Macromolecules 2010, 43, 8714. (6) Huo, L.; Guo, X.; Zhang, S.; Li, Y.; Hou., J. Macromolecules 2011, 44, 4035. (7) Li, H.; Tam, T. L.; Lam, Y. M.; Mhaisalkar, S. G.; Grimsdale, A. C. Org. Lett. 2011, 13, 46. (8) Mühlbacher, D.; Scharber, M.; Morana, M.; Zhu, Z.; Waller, D.; Gaudiana, R.; Brabec, C. Adv. Mater. 2006, 18, 2884. (9) Zhu, Z.; Waller, D.; Gaudiana, R.; Morana, M.; Mühlbacher, D.; Scharber, M.; Brabec, C. Macromolecules 2007, 40, 1981. (10) Svensson, M.; Zhang, F.; Veenstra, S. C.; Verhees, W. J. H.; Hummelen, J. C.; Kroon, J. M.; Inganas, O.; Andersson, M. R. Adv. Mater. 2003, 15, 988. (11) Chen, M,-H.; Hou, J.; Hong, Z.; Yang, G.; Sista, S.; Chen, L,-M.; Yang, Y. Adv. Mater. 2009, 21, 4238. (12) Zeng, W. L.; Yu, S. J.; Chua, W. H. Macromolecules 2002, 35, 6907. (13) Yu, W.-L.; Pei, J.; Huang, W.; Heeger, A. J. Adv. Mater. 2000, 12, 828. (14) Lee, J.-H.; Hwang, D.-H. Chem. Commun. 2003, 22, 2836. (15) Grimsdale, A. C. Curr. Org. Chem. 2010, 14, 2196. (16) Ferenczi, T. A. M.; Sims, M.; Bradley, D. D. C. J. Phys.: Condens. Matter. 2008, 20, 045220. (17) Landis, C. A.; Parkin, S. R.; Anthony, J. E. Jpn. J. Appl. Phys. 2005, 44, 3921. (18) Pandey, M. D.; Mishra, A. K.; Chandrasekhar, V.; Verma, S. Inorg. Chem. 2010, 49, 2020. (19) Seo, Y. J.; Lee, I. J.; Yi, J. W.; Kim, B. H. Chem. Commun. 2007, 2817. (20) Park, M,-J.; Lee, J.; Jung, I.-H.; Park, J,-H.; Hwang, D.-H.; Shim, H,-K. Marcromolecules 2008, 41, 9643. (21) Ranger, M.; Rondeau, D.; Leclerc, M. Marcromolecules 1997, 30, 7686. (22) Miyaura, N.; Suzuki, A. Chem. Rev. 1995, 95, 2457. (23) Ranger, M.; Leclerc, M. Macromolecules 1999, 32, 3306. (24) Huang, F.; Chen, K.-S.; Yip, H.-L.; Hau, S. K.; Acton, O.; Zhang, Y.; Luo, J.; Jen, A. K. Y. J. Am. Chem. Soc. 2009, 131, 13886. (25) Zhang, Z.-G.; Zhang, K.-L.; Liu, G.; Zhu, C.-X.; Neoh, K.-G.; Kang, E.−T. Macromolecules 2009, 42, 3104. (26) Leriche, P.; Frere, P.; Cravino, A.; Aleveque, O.; Roncali, J. J. Org. Chem. 2007, 72, 8332. (27) Li, Y.; Xu, B.; Li, H.; Cheng, W.; Xue, L.; Chen, F.; Lu, H.; Tian, W. J. Phys. Chem. C. 2011, 115, 2386. (28) Shaw, J. M.; Seidler, P. F. IBM J. Res. Dev. 2001, 45, 3. (29) Dimitrakopoulos, C. D.; Mascaro, D. J. IBM J. Res. Dev. 2001, 45, 11. (30) Kim, J. Y.; Kim, S. H.; Lee, H.-H.; Lee, K.; Ma, W.; Gong, X. Adv. Mater. 2006, 18, 572. 2376


Introduction of Perylene Units for Enhanced Interchain Interaction in

Recommend Documents