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Author's personal copy Synthetic Metals 162 (2012) 768–774

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Improvement of efficiency of polymer solar cell by incorporation of the planar shaped monomer in low band gap polymer Woong Shin a , Mi Young Jo a , Dae Sung You a , Yeon Sook Jeong b , Do Y. Yoon b , Jae-Wook Kang c , Jeong Ho Cho d , Gun Dae Lee e , Seong-Soo Hong f , Joo Hyun Kim a,∗ a

Department of Polymer Engineering, Pukyong National University, Yongdang-Dong, Nam-Gu, Busan 608-739, Republic of Korea Department of Chemistry, Seoul National University, Seoul 151-741, Republic of Korea Department of Material Processing, Korea Institute of Materials Science, Changwon 641-831, Republic of Korea d SKKU Advanced Institute of Nanotechnology (SAINT) and Center for Human Interface Nano Technology (HINT), Department of Chemical Engineering, Sungkyunkwan University, Suwon 440-746, Korea e Department of Industrial Chemistry, Pukyong National University, Yongdang-Dong, Nam-Gu, Busan 608-739, Republic of Korea f Department of Chemical Engineering, Pukyong National University, Yongdang-Dong, Nam-Gu, Busan 608-739, Republic of Korea b c

a r t i c l e

i n f o

Article history: Received 22 November 2011 Received in revised form 9 January 2012 Accepted 8 February 2012 Keywords: Polymer solar cell Morphology Field effect mobility Suzuki coupling

a b s t r a c t A new ␲-conjugated polymer (PFDTBT-BTBT) with planar core based on 4,7-dithiophen-2-ylbenzo(2,1,3)thiadiazole (BT), [1]benzothieno-[3,2-b][1]benzothiophene (BTBT) and 9,9-didecylfluorene was synthesized by the Suzuki coupling reaction. In order to improve the photovoltaic performance, we introduce planar and rigid shaped BTBT in the polymer backbone. We also synthesized alternating copolymer of 9,9-didecylfluorene and BT (PFDTBT) to compare with optical, electrochemical and photovoltaic properties. The HOMO and LUMO energy levels of PFDTBT-BTBT were −5.67 and −3.75 eV, respectively, which were very similar to those of PFDTBT (−5.65 and −3.75 eV). The power conversion efficiency (PCE) of the device with a structure of ITO/PEDOT:PSS/PFDTBT-BTBT:PCBM (1:3)/Al was 2.08%, which is higher than that of PFDTBT:PCBM (1:3) (1.66%). The root-mean-square (RMS) roughness of PFDTBT-BTBT:PCBM (1:3) blend was 0.820 nm, which was smaller than that of PFDTBT:PCBM (1:3) blend (1.75 nm). PFDTBTBTBT:PCBM showed smaller domain size and more well-penetrated morphology than PFDTBT:PCBM. The average field-effect hole mobility of the PFDTBT-BTBT was 6.2 × 10−4 cm2 /Vs, much larger than that of PFDTBT (2.3 × 10−4 cm2 /Vs). The results strongly support that polymer solar cell based on PFDTBT-BTBT shows better performance than that of the device based on PFDTBT. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Nowadays, ␲-conjugated polymer-based bulk hetero-junction (BHJ) polymer solar cells (PSCs) have been attracting considerable attention due to their unique advantages of low cost, light weight and applications in flexible large-area devices [1,2]. For improving the efficiency of PCSs, a lot of ␲conjugated polymers have been developed. As representative photoactive polymers, poly[2-methoxy-5-(3,7-dimethyloctyloxy)p-phenylenevinylene] (MDMO-PPV) [3] and region-regular poly(3-hexylthiophene) (P3HT) [4,5] were reported and which have led to power conversion efficiency (PCE) up to 2.0–5.0% when blended with [6,6]-phenyl-C61 -butyric acid methyl ester (PCBM). The derivatives of poly(p-phenylenevinylene)

∗ Corresponding author. Tel.: +82 51 629 6452; fax: +82 51 629 6429. E-mail address: [email protected] (J.H. Kim). 0379-6779/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2012.02.004

(PPV), poly(3-alkylthiophene) (P3AT) and copolymers of fluorene-thiophen-benzothiadiazole have been attracted due to their good optical and electrochemical properties, which have led to applications for PCSs, polymer light-emitting diodes (PLEDs) and field effect transistors (FETs) [6–12]. The PSCs made from P3HT and poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b]di-thiophene)-alt-4,7(2,1,3-benzothiadiazole)] (PCPDTBT) have high fill factor (FF) and short circuit current density (Jsc ), while PCE is still limited by low open-circuit voltage (Voc ) between 0.60 and 0.65 V [16]. Improvement of Voc is one of the methods to get high efficiency PSCs [14,15,17,18]. Recently, the PSC based alternating copolymer, poly((5,5-(4,7-dithiophen-2-yl-benzo2,1,3-thiadiazole))-alt-2,7-(9-(2 -ethylhexyl)-9-hexyl-fluorene), has demonstrated to get large Voc up to 1.04 V [19]. However, their PCE is still low due to the lower Jsc and FF than those of P3HT and PCPDTBT based PSCs. If the polymer with large Voc has high Jsc and FF as much as P3HT and PCPDTBT, very high PCE of the polymer based PSC can be expected. The Jsc and FF are crucial factors for

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improving the device efficiency and these are depend on charge mobility, morphology of blend and series resistance of the device. Incorporation of rigid and planar structured monomers into the polymer backbone would be one of the strategies to improve morphology of the polymer and PCBM blend. 2,7-Diiodo[1]benzothieno[3,2-b][1]benzothiophene (BTBT) is known as the core part of active materials for organic field-effect transistor [20]. The active materials based on BTBT have very strong intermolecular interactions in the thin film state and the field effect transistors based on BTBT have very high field-effect mobility. In order to improve the performances of the PSCs, we take advantage of structural feature of BTBT. In general, the solubility of the polymers with rigid and planar structured will be very poor so that we introduce small amount of BTBT in the polymer backbone. We synthesized poly((5,5-(4,7-dithiophen-2-yl-benzo2,1,3-thiadiazole))-co-2.7-(9,9-didecyl-9H-fluorene)-co-(2,7[1]benzothieno[3,2-b][1]benzothiophene)) (PFDTBT-BTBT), which has very small amount of BTBT unit in the polymer backbone. The high Voc value of PSC based on PFDTBT-BTBT is expected and the high Jsc and FF are expected as well. In this paper we investigated the optical, electrochemical, and photovoltaic properties of the polymer. 2. Experimental 2.1. Materials Tetrahydrofuran (THF) and diethyl ether were distilled over sodium/benzophenone. Methylene chloride (MC) was distilled over CaH2 . All other chemicals were purchased from Sigma–Aldrich Co, Tokyo Chemical Industry (TCI) or Alfa Aesar (A Johnson Matthey Company) and used as received unless otherwise described. 9,9-Didecyl-2,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxa-borolan-2yl)-fluorene, 2,7-diiodo-5,10-dithia-indeno[2,1-a]indene (BTBT) and 4,7-bis(5-bromo-2-thienyl)-2,1,3-benzothiadiazole were synthesized according to literature procedures [19–21]. 2.2. Synthesis of monomers and polymers 2.2.1. Synthesis of 2,7-dibromo-9,9-bis-decyl-9H-fluorene (1) A portion of 50% NaOH solution (20 mL) was added to a solution of 2,7-dibromofluorene (5.00 g, 15.4 mmol) in dimethylsulfoxide (DMSO, 20 mL). The reaction mixture was stirred for 30 min. A portion of n-decylbromide (7.50 g, 33.9 mmol) was added to the reaction mixture and then stirred for 24 h at room temperature. The reaction mixture was neutralized with 2 M HCl and extracted with 100 mL of n-hexane three times. The combined organic layer was washed with aqueous brine and dried over anhydrous magnesium sulfate (MgSO4 ) and the solvent was evaporated under reduced pressure. The crude liquid product was purified by the flash column chromatography using n-hexane to give colorless liquid (7.66 g, 82.3%). MS: [M+ ], m/z 604. 1 H NMR (400 MHz, CDCl3 , ppm): ı 7.49 (d, J = 8.8 Hz, 2H), 7.44 (s, 2H), 7.42 (d, J = 4.0 Hz, 2H), 1.91 (m, 4H), 1.27–1.04 (m, 28H), 0.85 (t, J = 7.0 Hz, 6H), 0.58 (m, 4H). 13 C NMR (100 MHz, CDCl3 , ppm): ı 152.51, 139.03, 130.12, 126.14, 121.46, 121.08, 55.65, 40.14, 31.85, 29.85, 29.52, 29.49, 29.25, 29.19, 23.61, 22.65, 14.11. Anal. Calcd. for C33 H48 Br2 : C, 65.56; H, 8.00; Br, 26.43. Found: C, 65.15; H, 8.09. 2.2.2. 2,7-bis(4,4,5,5-Tetramethyl-[1,3,2]dioxaborolane)-9,9dihexyl-9H-fluoren-2-yl (2) A mixture of compound 1 (3.00 g, 4.96 mmol), bis(pinacolato)diboron (3.76 g, 14.8 mmol), potassium acetate (2.92 g, 29.8 mmol) and Pd(dppf)Cl2 (0.243 g, 0.298 mmol) in 50 mL of DMF was stirred at 60 ◦ C for 24 h under the N2 atmosphere.

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The reaction mixture was cooled down to room temperature and 50 mL of water was added to the reaction mixture and then extracted with 50 mL of ethyl acetate three times. The combined organic layer was washed with brine and dried over anhydrous magnesium sulfate and then the solvent was evaporated under reduced pressure. The crude product was purified by the flash chromatography using methylene chloride (MC)/n-hexane (1:9, v/v). The product yield was 2.16 g (62.3%). MS: [M+ ], m/z 698. 1 H NMR (400 MHz, CDCl3 , ppm): ı 7.82 (d, J = 7.3 Hz, 2H), 7.75 (s, 2H), 7.73 (d, J = 7.4 Hz, 2H), 2.00 (m, 4H), 1.38 (s, 24H), 1.24–1.01 (m, 28H), 0.84 (t, J = 7.0 Hz, 6H), 0.55 (m, 4H). 13 C NMR (100 MHz, CDCl3 , ppm): ı 150.42, 143.87, 133.63, 128.86, 127.79, 119.34, 83.66, 55.13, 40.10, 31.84, 29.96, 29.55, 29.49, 29.24, 29.21, 24.90, 23.62, 22.62, 14.08. Anal. calcd. for C45 H72 B2 O4 : C, 77.36; H, 10.39; B, 3.09; O, 9.16. Found: C, 77.15; H, 10.42. 2.2.3. Synthesis of 4,7-di-thienyl-2,1,3-benzothiadiazole (3) A mixture of 2.50 g (8.50 mmol) of 4,7-dibromobenzo[1,2,5]thiadiazole, 3.93 g (18.7 mmol) of 4,4,5,5-tetramethyl-2-thiophen-2-yl-[1,3,2]dioxaborolane, 0.47 g (0.43 mmol) of Pd(PPh3 )4 and few drops of aliquat 336 in 30 mL of 2 M K2 CO3 and 30 mL toluene was refluxed for 24 h. The reaction mixture was cooled down to room temperature and extract with 100 mL of diethyl ether three times. The organic layer was washed with brine and then dried over anhydrous magnesium sulfate. The solvent was removed using a rotary evaporator. The crude solid product was purified by flash chromatography. The orange solid product yield was 1.15 g (45.0%). mp: 120.1 ◦ C. MS: [M+ ], m/z 300. 1 H NMR (400 MHz, CDCl , ppm): ı 8.12 (dd, J = 3.3 and J = 1.1 Hz, 3 1 2 2H), 7.89 (s, 2H), 7.46 (dd, J1 = 5.1 and J2 = 1.1 Hz, 2H), 7.22 (dd, J1 = 4.8 and J2 = 4 Hz, 2H). 13 C NMR (100 MHz, CDCl3 , ppm): ı 152.7. 139.4, 128.1, 127.6, 126.9, 126.1, 125.9. MS [M+ ], m/z; 300. Anal. calcd. for C14 H8 N2 S3 : C, 55.97; H, 2.68; N, 9.32; S, 32.02. Found: C, 56.09; H, 2.48; N, 9.85; S, 31.57. 2.2.4. Synthesis of 4,7-bis(5-bromo-2-thienyl)-2,1,3-benzothiadiazole (4) A portion of N-bromosuccinicimide (NBS, 1.78 g, 10.0 mmol)) was added to a solution of compound 3 (1.00 g, 3.33 mmol) in N,N -dimethylformamide (DMF, 50 mL). The reaction mixture was stirred for 3 h at room temperature. A portion of water (50 mL) was added to the reaction mixture. The precipitate was filtered and recrystallized with DMF. The dark red product yield was 1.10 g (72%). mp: 233.0 ◦ C. MS: (M+ , m/z); 458. 1 H NMR (400 MHz, CDCl3 , ppm): 7.81 (d, J = 4.1 Hz, 2H), 7.79 (s, 2H) 7.16 (d, J = 4.0 Hz, 2H). Anal. calcd. for C14 H6 Br2 N2 S3 : C, 36.70; H, 1.32; Br, 34.88; N, 6.11; S, 20.99. Found: C, 35.92; H, 1.551; N, 6.33; S, 21.32. 2.2.5. 2,7-Diiodo[1]benzothieno[3,2-b][1]benzothiophene (5) 2,7-Diiodo[1]benzothieno[3,2-b]benzothiophene was synthesized according to the reported procedure [13], which was started with 4,4 -dinitro-2,2 stilbenedisulfonate. There were five steps in total to synthesize this compound and the total yield was 27.2%. MS (FAB+ ): [M]+ , m/z 492. 1 H NMR (400 MHz, CDCl3 , ppm): ı 8.27 (d, 2H), 7.96 (dd, 2H), 7.64 (d, 2H). 2.2.6. Polymerization of PFDTBT The polymer was polymerized by the Suzuki coupling reaction between compounds 2 and 4. A mixture of compound 2 (0.229 g, 0.50 mmol), compound 4 (0.349 g, 0.50 mmol), tetrakis(triphenylphosphine) palladium(0) (4.2 mg, 3.65 mmol) and 8 ml of toluene was stirred for 10 min under N2 atmosphere at 110 ◦ C. Tetraethylammonium hydroxide solution (20 wt.% in water, 1.4 ml, 1.94 mmol) was added and stirred for 2.5 h. Bromobenzene (0.078 g, 0.50 mmol) was added and then after an hour phenylboronic ester (0.06 g, 0.50 mmol) was added. The reaction mixture

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Scheme 1. Synthetic routes to monomers and polymers.

was stirred for an additional hour. The reaction mixture was poured into methanol and the precipitated polymer was collected by filtration. The polymer was dissolved in chloroform and then aq. NH4 OH was added to a polymer solution. This solution was stirred for 12 h. After that, the organic phases was separated and washed with deionized water several times. The organic layer was dried over anhydrous magnesium sulfate and the organic solvent was evaporated using a rotary evaporator. The crude polymer was dissolved in 1 ml of THF and the solution was dropped into methanol and filtered to obtain dark violet polymer. The yield of the polymer was 57.5%. 1 H NMR (400 MHz, CDCl3 , ppm): 8.16–7.47 (br, 12H), 2.09 (br, 4H), 1.25–1.02 (br, 28H), 0.79 (br, 6H).

counter and working electrode and an Ag/Ag+ electrode was used as the reference electrode. Prior to each measurement, the cell was deoxygenated with nitrogen. The J–V measurements under 1.0 sun (100 mW cm−2 ) condition form a 150 W Xe lamp with an 1.5 G filter were performed using a KEITHLEY Model 2400 source-measure unit. A calibrated Si reference cell with a KG5 filter certified by National Institute of Advanced Industrial Science and Technology was used to confirm 1.0 sun condition. Current–voltage characteristics of transistors were measured using Keithley 2400 and 236 source/measure units at room temperature under vacuum conditions (∼10−5 Torr) in a dark environment. The thickness of films was measured by Alpha-Step IQ surface profiler (KLA-Tencor Co.).

2.2.7. Polymerization of PFDTBT-BTBT PFDTBT-BTBT was synthesized by the Suzuki coupling reaction between 0.5 mmol of compound 2, 0.45 mmol of compound 4 and 0.05 mmol of compound 5. The same reaction condition and procedure were used as in the polymerization of PFDTBT. The yield of the polymer was 54.3%. 1 H NMR (400 MHz, CDCl3 , ppm): 8.13–7.22 (br, 12H), 2.11 (br, 4H), 1.25–0.94 (br, 28H), 0.82–0.73 (br, 6H).

2.4. Fabrication of PSCs Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS, Baytron P) (diluted with 2-propanol 1:2, v/v) was spin-coated on pre-cleaned indium tin oxide (ITO) glass substrate (sheet resistance = 13 /sq.) which was pre-treated by UV/O3 for 120 s. Typical thickness of PEDOT:PSS layer was 40 nm. After being baked at 150 ◦ C for 10 min under the air, active layer was spin-cast

2.3. Measurements 1H

NMR and 13 C NMR spectra of compounds were recorded with a JEOL JNM ECP-400 spectrometer. The elemental and mass analyses of synthesized compounds were carried out on an Elementar Vario macro/micro elemental analyzer and a Shimadzu GC-MS QP-5050A spectrometer. Gel permeation chromatography (GPC) measurements were conducted by GPC system equipped with a Water 510 pump, a Rheodyne 6-port sample injection valve, a Waters Temperature Control Module, a Waters 410 differential RI detector and two Waters Styragel linear columns using polystyrene as standard and chloroform as eluent. UV–visible (UV–vis) and photoluminescence (PL) spectra of the polymers were recorded using a JASCO V-530 Spectrophotometer and a HITACHI F-4500, respectively. Atomic force microscope (AFM) images were taken on a Digital Instruments (U.S.A), Multi ModeTM SPM. AFM images were obtained by the tapping mode and a scan rate of 2 Hz. Cyclic voltammetry (CV) was performed by a EG&G 362 Scanning Potentiostat with a three electrode cell in a solution of 0.10 M tetrabutylammonium hexafluorophosphate (Bu4 NPF6 ) in freshly distilled MC at a scan rate of 100 mV/s. Pt coil and wire were used as the

Fig. 1. FT-IR spectrum of PFDTBT (solid) and PFDTBT-BTBT (dash-dot).

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Fig. 2. TGA thermogram of (a) PFDTBT and (b) PFDTBT-BTBT. Fig. 4. Cyclic voltammogram of (a) PFDTBT and (b) PFDTBT-BTBT.

from the blend solution of polymer/PCBM (15 mg of polymer and 45 mg of PCBM were dissolved in 1 mL of o-dichlorobenzene (ODCB) at 600 rpm for 120 s. The blend solution was stirred for 12 h at 60 ◦ C in the glove box. Prior to spin coating, the blend solution was filtered through 0.45-␮m membrane filter. Typical thickness of active layer was 200 nm. A 100 nm-thick layer of Al was vacuum-deposited as a cathode through shadow mask onto the top of active layer at 2 × 10−6 Torr. Typical active area of the devices was 0.12 cm2 . 2.5. Fabrication of organic field effect transistors Organic field-effect transistors (OFETs) based on PFDTBT and PFDTBT-BTBT were built on heavily doped n-type Si substrates, which are commonly used as gate electrodes. A thermally grown 300 nm thick SiO2 layer served as the gate dielectric. Prior to treating the silicon oxide surface, the wafer was cleaned in piranha solution for 30 min at 100 ◦ C and washed with copious amounts of distilled water. The capacitance of the SiO2 gate dielectric was ∼11 nF/cm2 . PFDTBT and PFDTBT-BTBT films were spin-coated from a 15 mg/mL of ODCB solution onto the SiO2 /Si substrates. The samples were dried in a vacuum chamber for 24 h. The 50 nm Au source/drain electrodes were thermally evaporated through

shadow masks. The length and width of channels were 100 and 800 ␮m, respectively. 3. Results and discussion 3.1. Material synthesis and characterization Scheme 1 shows the synthetic procedures for monomers and polymers, respectively. The chemical structures of the compounds in Scheme 1 were confirmed by 1 H NMR, 13 C NMR, MASS spectrum and elemental analysis. As shown in Fig. 1, the peak position and integration of PFDTBT-BTBT is almost identical to those of PFDTBT because the feed ratio of BTBT was just 0.05 mol%. In order to confirm the chemical structure of the copolymers, we measured the FT-IR spectra. Fig. 1 shows the FT-IR spectra of the polymers. The spectra are normalized with the absorption peak of C(sp3 )-H stretching vibration at 2928 cm−1 . The absorption peaks at 1577, 1567, 1377, and 1348 cm−1 are characteristic absorption band of 2,1,3-benzothiadiazole (BT). From the FT-IR spectrum, one can easily noticed that the absorption intensity of PFDTBT-BTBT at 1577, 1567, 1377, and 1348 cm−1 weaker than those of PFDTBT. Therefore, we can confirm that the amount of BT unit in PFDTBTBTBT is smaller than that of PFDTBT. The polymers were good soluble in organic solvents such as THF, toluene, chloroform and o-dichlorobenzene. The number average molecular weight (Mn ) of PFDTBT was 11,400 g/mol with a polydispersity index of 1.26. For PFDTBT-BTBT, Mn was 10,982 g/mol with a polydispersity index of 1.18. As shown in Fig. 2, PFDTBT and PFDTBT-BTBT were thermally stable up to 427 and 406 ◦ C, respectively. The point of 5%-weight loss in TGA thermogram of PFDTBT and PFDTBT-BTBT were 427 ◦ C and 406 ◦ C, respectively. 3.2. Optical and electrochemical properties

Fig. 3. UV–visible spectrum of (a) PFDTBT film and (b) PFDTBT-BTBT film (spectra are offset for clarity).

The polymer thin film for measuring the UV–vis and PL spectrum was prepared by spin-coating from the polymer solution with ODCB (10 mg/ml) at 1000 rpm for 120 s. Fig. 3 shows UV–vis and PL spectrum of PFDTBT and PFDTBT-BTBT thin film. The polymers showed broad absorption band from 300 to 700 nm. The absorption maximum of PFDTBT appeared at 556 nm. The band gap energy figured out from the absorption edge was 1.92 eV. For PFDTBT-BTBT, the absorption maximum wavelength and the band gap were 562 nm and 1.92 eV, respectively. The absorption maximum of PFDTBTBTBT was slightly red-shifted than those of PFDTBT. This is due to that BTBT has rigid and planar structure. BTBT unit in the polymer

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Fig. 5. Energy level diagram of PFDTBT and PFDTBT-BTBT.

Fig. 6. Current density–voltage curves of polymer solar cell based on (a) PFDTBT and (b) PFDTBT-BTBT under illumination (100 mA/cm2 ) of AM 1.5G condition.

Fig. 7. AFM topographic images of (a) PFDTBT:PCBM (1:3) blend and (b) PFDTBT-BTBT:PCBM (1:3) blend.

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Fig. 8. Transfer characteristics (drain current (ID )–gate voltage (VG )) of PFDTBT (square) and PFDTBT-BTBT (triangle) FETs.

backbone improves the inter-chain interaction so that the effective ␲-conjugation length increases compared with that of the polymer without BTBT [20,22]. In addition, the electron rich sulfur atom in BTBT also elongated ␲-conjugation length of PFDTBT-BTBT. The information about energy levels of ␲-conjugated polymer is very important to determine the open circuit voltage (Voc ) and the charge separation of PSCs. Cyclic voltammetry was used to investigate the redox behavior of polymers and assess the energy levels [23]. The energy levels of polymers were calculated from the measured onset potential of oxidation by assuming the energy level of ferroccene (Fc) as −4.8 eV. As shown in Fig. 4, PFDTBT and PFDTBT-BTBT show irreversible reduction process and reduction potential at −1.54 and −1.50 V vs. Fc/Fc+ , respectively. The reduction onset potential of PFDTBT appeared at −1.05 V vs. Fc/Fc+ , which is almost same as the reduction onset potential of PFDTBT-BTBT. The LUMO energy level of PFDTBT and PFDTBT-BTBT figured out from the reduction onset potential was −3.75 eV. The HOMO energy level of polymer was estimated from the LUMO energy level and the band gap energy because the oxidation process of the polymers is not observed in the range from 0 to 2.0 V vs. Fc/Fc+ . Fig. 5 shows the energy level data estimated from the UV–vis spectrum and cyclic voltammogram. The estimated HOMO energy level of PFDTBT and PFDTBT-BTBT were −5.65 and −5.67 eV, respectively. From the LUMO energy level data, we can expect that the photoinduced electron transfer process from PFDTBT-BTBT to PCBM and Voc of PSC based on PFDTBT-BTBT would be identical to the case of PFDTBT. 3.3. Photovoltaic and field effect transistor properties From the energy level data of the polymer, the polymers can be used as the donor materials in PSCs. PSCs were fabricated by using PFDTBT and PFDTBT-BTBT as donors and PCBM as acceptor. The current density–voltage characteristics of PSCs based on PFDTBT (ITO/PEDOT/PFDTBT:PCBM (1:3)/Al) and PFDTBT-BTBT (ITO/PEDOT/PFDTBT-BTBT:PCBM (1:3)/Al) measured under AM 1.5 G simulated illumination with an intensity of 100 mW/cm2 and shown in Fig. 6. The PSC based on PFDTBT showed a Voc of 0.91 V, a short circuit current (Jsc ) of 4.76 mA/cm2 , a fill factor of 38.8%, and a power conversion efficiency (PCE) of 1.66%. The device based on PFDTBT-BTBT gave a Voc of 0.89 V, a short circuit current (Jsc ) of 5.02 mA/cm2 , a fill factor of 46.6%, and a power conversion efficiency (PCE) of 2.08%. The Jsc and FF value of the device based on the polymer with BTBT showed better performances that those of the device based on improved that those of the device based on PFDTBT. This is presumably due to that the morphology of the polymer

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with BTBT would be improved. In order to understand the photovoltaic performances, we studied the surface morphology and field effect charge mobility of the blended film. The atomic force microscopic topography of PFDTBT:PCBM (1:3) and PFDTBT-BTBT:PCBM (1:3) blended film were presented in Fig. 7. The root-mean-square (RMS) roughness of PFDTBT:PCBM blend and PFDTFT-BTBT:PCBM blend were 1.75 and 0.820 nm, respectively. PFDTBT-BTBT showed smaller domain size and more well-penetrated morphology than that of PFDTBT:PCBM. This indicates that the reduced interface between PFDTBT and PCBM leads to lower photovoltaic performance. The Voc of the device based on PFDTBT:PCBM is slightly higher than that of the device based on PFDTBT-BTBT:PCBM. From the photovoltaic, the morphology and the field effect mobility data, we confirm that small amount of rigid and planar shape monomer in the polymer backbone improves the performances of the PSC. Fig. 8 shows the transfer curves of OFETs based on PFDTBT and PFDTBT-BTBT. Both devices were well-behaved p-type transistors. The transfer characteristics permitted calculation of the field-effect mobility in the saturation regime (VD = −60 V) using the relationship of ID = Ci W(VG − Vth )2 /2L, where W and L are the channel width and length, respectively, Ci the specific capacitance of the gate dielectric, and  the field-effect mobility. The average hole mobility of the PFDTBT-BTBT FETs was 6.2 × 10−4 cm2 /Vs, much larger than that of PFDTBT FETs (2.3 × 10−4 cm2 /Vs). Field-effect mobility data also strongly supports that PSC based on PFDTBTBTBT shows better performance than that of the device based on PFDTBT. 4. Conclusion We synthesized copolymers with fluorene, 4,7-di-thienyl-2,1,3benzothiadiazole (BT) and benzothieno[3,2-b][1]benzothiophene (BTBT) through the Suzuki coupling reaction. A new ␲-conjugated polymer with BTBT, PFDTBT-BTBT, showed similar optical and electrochemical properties with those of ␲-conjugated polymer without BTBT. However, photovoltaic properties of the device based on PFDTBT-BTBT were higher than those of PFDTBT. The RMS roughness of the PFDTBT-BTBT:PCBM blend was smaller than that of the blend of PFDTBT and PCBM. PFDTBT-BTBT:PCBM blend showed smaller domain size and more penetrated morphology than those of PFDTBT:PCBM. In addition, field-effect mobility of PFDTBT-BTBT was higher than that of PFDTBT. From the results, we confirm that the small amount of BTBT units in the polymer backbone can improve the photovoltaic performances. Acknowledgments This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2011-0006807) and Converging Research Center Program through the Ministry of Education, Science and Technology (2011K000588). References [1] J.Y. Kim, K. Kim, N.E. Coates, D. Moses, T.-Q. Nguyen, M. Dante, A.J. Heeger, Science 317 (2007) 222–225. [2] S. Gunes, H.S. Neugebauer, N.S. Sariciftci, Chem. Rev. 107 (2007) 1324–1338. [3] C.J. Brabec, N.S. Sariciftci, J.C. Hummelen, Adv. Funct. Mater. 11 (2001) 15–26. [4] G. Li, V. Shrotriya, J. Huang, Y. Yao, T. Moriarty, K. Emery, Y. Yang, Nat. Mater. 4 (2005) 864–868. [5] W. Ma, C. Yang, X. Gong, K. Lee, A.J. Heeger, Adv. Funct. Mater. 15 (2005) 1617–1622. [6] J.M. Halls, K. Pichler, R.H. Friend, S.C. Moratti, A.B. Holmes, Appl. Phys. Lett. 68 (1996) 3120–3122. [7] M. Granstrom, K. Petritsch, A.C. Arias, A. Lux, M.R. Andersson, R.H. Friend, Nature 395 (1998) 257–260. [8] L.H. Slooff, S.C. Veenstra, J.M. Kroon, D.J.D. Moet, J. Sweelssen, M.M. Koetse, Appl. Phys. Lett. 90 (2007), 143506-1–143506-3.

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