Molecular Engineering of Efficient Organic Sensitizers Incorporating

Samsung Advanced Institute of Technology. , §. Korea National University of ..... Anal. calcd for C60H61NO2S3: C, 77.96; H, 6.65. Found: C, 77.53; H,...
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14646

J. Phys. Chem. C 2010, 114, 14646–14653

Molecular Engineering of Efficient Organic Sensitizers Incorporating a Binary π-Conjugated Linker Unit for Dye-Sensitized Solar Cells Sanghyun Paek,† Hyunbong Choi,† Hyeju Choi,† Chi-Woo Lee,† Moon-sung Kang,‡ Kihyung Song,§ Mohammad K. Nazeeruddin,| and Jaejung Ko*,† Department of New Material Chemistry, Korea UniVersity, Jochiwon, Chungnam 339-7000, Korea, Energy & EnVironmet Lab, Samsung AdVanced Institute of Technology, Yongin 446-712, Korea, Department of Chemistry, Korea National UniVersity of Education Cheongwon, Chungbuk 363-791, Korea, and LPI, Institut des Sciences et Inge´nierie Chimiques Faculte´ des Sciences de Base E´cole Polytechnique Fe´de´rale de Lausanne, 1015 Lausanne, Switzerland ReceiVed: May 11, 2010; ReVised Manuscript ReceiVed: June 26, 2010

Six organic sensitizers containing 3,4-ethylenedioxythiophene and thienothiophene in the bridged group are designed and synthesized, which under standard global AM 1.5 solar conditions, the device using JK-184 gave a conversion efficiency of 8.70%. The photovoltaic performance data are quite sensitive to the structural modification of sensitizer. The hexyl substituent of the hexyloxy group on the sensitizer was shown to improve the efficiency and stability. The device based on the JK-184 with an ionic liguid electolyte exhibits an excellent stability after 1000 h of light soaking at 60 °C. Introduction The current global warming and depletion of fossil fuels have created an urgent need to develop new renewable energy sources. Dye-sensitized solar cells (DSSCs) represent one of the most promising candidates due to their low cost and high efficiency.1 In these cells, the sensitizer is one of the key components for DSSCs. Over the last 19 years, ruthenium sensitizers have achieved remarkable efficiencies over 11% under standard air mass 1.5 sunlight.2 However, there are still many problems to be addressed in terms of precious ruthenium metal and their synthetic high cost. In this regard, organic sensitizers might be considered as an alternative to ruthenium sensitizers. Recently, the DSSC performance based on organic sensitizers has been remarkably improved and reached impressive efficiencies in the range of 8-9%.3 However, the efficiency is still low as compared with that of the ruthenium sensitizer. One of factors for the low efficiency of an organic DSSC is the formation of dye aggregate on the semiconductor surface.4 Another disadvantage of organic sensitizers is the sharp and narrow bands in the visible region. Thus, the molecular engineering of the organic sensitizers for achieving the high photovoltaic performance has been focused on the development of efficient panchromatic sensitizers. Organic sensitizers with donor and acceptor segments are bridged by a π-conjugation unit where amine derivatives act as the electron donor, while a 2-cyanoacrylic acid or rhodanine unit acts as the electron acceptor.5 The π-conjugation systems include the introduction of a variety of functional moieties as a linker, such as bithiophene,6 phenylene vinylene,7 dithienosilole,8 benzothiadizole,9 squaraine,10 and dithieno[3,2-b;2′,3′-d]thiophene.11 Recently, a binary spacer of a combination of 3,4-ethylenedioxythiophene and thienothiophene achieved a high conversion * To whom correspondence should be addressed. Tel: 82 41 860 1337. Fax: 82 41 867 5396. E-mail: [email protected]. † Korea University. ‡ Samsung Advanced Institute of Technology. § Korea National University of Education Cheongwon. | LPI.

efficiency.11 Our strategy for obtaining high photovoltaic performance from organic DSSCs is based on the structural modification of the dye because small structural changes of dyes result in significant changes for the interfacial recombination and absorption behavior of the dyes on TiO2 surfaces. A successful molecular engineering was achieved by incorporating long alkyl groups into the spacer group and triphenylamine derivatives, which not only increased the photoconversion efficiency but also minimized charge recombination. As part of our efforts to investigate the structural modification of spacer group that can suppress aggregation as well as broaden the UV spectrum, we adapted a binary π-conjugated spacer. In this paper, we report six new organic sensitizers (JK-178, JK-179, JK-184, JK-185, JK-200, and JK-201, see Figure 1) containing three kinds of bis-dimethylfluorenyl amino groups as electron donors and cyanoacrylic acid as an electron acceptor bridged by an alkyl-substituted binary spacer. We also describe the effect of bridged structural modifications on the photovoltaic parameters. Experimental Section General Methods. All reactions were carried out under an argon atmosphere. Solvents were distilled from appropriate reagents. All reagents were purchased from Sigma-Aldrich, TCI, and Acros Organics. 1H NMR and 13C NMR spectra were recorded on a Varian Mercury 300 spectrometer. Elemental analyses were performed with a Carlo Erba Instruments CHNS-O EA 1108 analyzer. Mass spectra were recorded on a JEOL JMS-SX102A instrument. The absorption and photoluminescence spectra were recorded on a Perkin-Elmer Lambda 2S UV-visible spectrometer and a Perkin LS fluorescence spectrometer, respectively. Cyclic voltammogram was carried out with a BAS 100B (Bioanalytical System, Inc.). A three electrode system was used and consisted of a gold disk, a working electrode, and a platinum wire electrode. The redox potential of dyes on TiO2 was measured in CH3CN with a scan rate at 50 mV s-1 (vs Fc/Fc+). Fabrication of DSSC. FTO glass plates (Pilkington TEC Glass-TEC 8, Solar 2.3 mm thickness) were cleaned in a

10.1021/jp104310r  2010 American Chemical Society Published on Web 08/11/2010

Organic Sensitizers for Dye-Sensitized Solar Cells

J. Phys. Chem. C, Vol. 114, No. 34, 2010 14647

Figure 1. Structures of the dyes of JK-178, JK-179, JK-184, JK-185, JK-200, and JK-201.

detergent solution using an ultrasonic bath for 30 min and rinsed with water and ethanol. The FTO glass plates were immersed in 40 mM TiCl4 (aqueous) at 70 °C for 30 min and washed with water and ethanol. A transparent nanocrystalline layer on the FTO glass plate was prepared by doctor blade printing TiO2 paste (Solaronix, Ti-Nanoxide T/SP) and then dried for 2 h at 25 °C. The TiO2 electrodes were gradually heated under an air flow at 325 °C for 5 min, at 375 °C for 5 min, at 450 °C for 15 min, and at 500 °C for 15 min. The thickness of the transparent layer was measured by using an Alpha-step 250 surface profilometer (Tencor Instruments, San Jose, CA), and a paste for the scattering layer containing 400 nm sized anatase particles (CCIC, PST-400C) was deposited by doctor blade printing and then dried for 2 h at 25 °C. The TiO2 electrodes were gradually heated under an air flow at 325 °C for 5 min, at 375 °C for 5 min, at 450 °C for 15 min, and at 500 °C for 15 min. The TiO2 electrodes were treated again by TiCl4 at 70 °C for 30 min and sintered at 500 °C for 30 min. The TiO2 electrodes were immersed into the JK-178, JK-179, JK-184, JK-185, JK-200, and JK-201 [0.3 mM in EtOH/THF (2/1) containing 10 mM 3a,7a-dihydroxy-5b-cholic acid (Cheno)] and kept at room temperature for overnight. The FTO plate for counter electrodes was cleaned with an ultrasonic bath in H2O, acetone, and aqueous 0.1 M HCl, subsequently. Counter electrodes were prepared by coating them with a drop of H2PtCl6 solution (2 mg of Pt in 1 mL of ethanol) on a FTO plate and heating at 400 °C for 15 min. The dye-adsorbed TiO2 electrode and Pt counter electrode were assembled into a sealed sandwich type cell by heating at 80 °C with a hot-melt ionomer film (Surlyn) as a spacer between the electrodes. A drop of electrolyte solution was placed on the drilled hole in the counter electrode of the assembled cell and was driven into the cell via vacuum backfilling. Two electrolytes were used for device evaluation. Electrolyte A: 0.6 M 1,2-dimethyl-3-propylimidazolium iodie, 0.05 M iodine, 0.1 M LiI, and 0.5 M tert-butylpyridine in

acetonitrile. Electrolyte B: 0.2 M iodine, 0.5 M NMBI, and 0.1 M GuNCS in PMII/EMINCS (13:7). Finally, the hole was sealed using additional Surlyn and a cover glass (0.1 mm thickness). Characterization of DSSC. The cells were measured using 1000 W xenon light source, whose power of an AM 1.5 Oriel solar simulator was calibrated by using KG5 filtered Si reference solar cell. The incident photon-to-current conversion efficiency (IPCE) spectra for the cells were measured on an IPCE measuring system (PV Measurements). Electron Transport Measurements. The electron diffusion coefficient (De) and lifetimes (τe) in TiO2 photoelectrode were measured by the stepped light-induced transient measurements of photocurrent and voltage (SLIM-PCV).16-19 The transients were induced by a stepwise change in the laser intensity. A diode laser (λ ) 635 nm) as a light source was modulated using a function generator. The initial laser intensity was a constant 90 mW cm-2 and was attenuated up to approximately 10 mW cm-2 using a ND filter, which was positioned at the front side of the fabricated samples (TiO2 film thickness ) ca. 10 µm; active area ) 0.04 cm2). The photocurrent and photovoltage transients were monitored using a digital oscilloscope through an amplifier. The De value was obtained by a time constant (τc) determined by fitting a decay of the photocurrent transient with exp(-t/τc) and the TiO2 film thickness (ω) using the equation De ) ω2/(2.77τc).16 The τe value was also determined by fitting a decay of photovoltage transient with exp(-t/τe).16 All experiments were carried out at room temperature. N-(4-(2,3-Dihydrothieno[3,4-b][1,4]dioxin-5-yl)phenyl)-N(9,9-dimethyl-9H-fluoren-2-yl)-9,9-dimethyl-9H-fluoren-2amine (1). Under nitrogen atmosphere, a mixture of N-(4bromophenyl)-N-(9,9-dimethyl-9H-fluoren-2-yl)-9,9-dimethyl9H-fluoren-2-amine (0.3 g, 0.54 mmol), 2-(2,3-dihydrothieno[3,4-b][1,4]dioxin-5-yl)-4,4,5,5-tetramethyl-1,3,2-dioxabor -olane (0.17 g, 0.64 mmol), Pd(PPh3)4 (0.062 g, 0.054 mmol), K2CO3 (0.74 g, 5.4 mmol), and degassed water (10 mL) in dry

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THF (40 mL) was refluxed overnight. After it was cooled to room temperature, the solution was extracted with dichloromethane and dried over MgSO4. The solvent was removed in vacuo. The pure product 1 was obtained by silica gel chromatography (eluent, dichloromethane:hexane ) 1:4); mp 115 °C. IR (THF) νmax: 2958, 2923, 2863, 1603, 1581, 1513, 1448, 1364, 1319, 1276, 1170, 1071, 923, 828. 1H NMR (CDCl3): δ 7.66-7.58 (m, 6H), 7.40 (d, 2H, 3J ) 7.2 Hz), 7.35-7.26 (m, 6H), 7.19 (d, 2H, 3J ) 8.4 Hz), 7.10 (d, 2H, 3J ) 8.1 Hz), 6.27 (s, 1H), 4.28 (dd, 4H), 1.42 (s, 12H). 13C{1H} NMR (CDCl3): δ 154.98, 153.48, 147.16, 146.35, 142.20, 138.98, 137.51, 134.09, 127.48, 127.02, 126.85, 126.48, 124.02, 123.14, 122.53, 120.62, 119.44, 118.55, 117.47, 96.92, 64.92, 64.63, 47.01, 27.27. MS: m/z 618 [M+]. Anal. calcd for C42H35NO2S: C, 81.65; H, 5.71. Found: C, 81.95; H, 5.41. N-(4-(7-Bromo-2,3-dihydrothieno[3,4-b][1,4]dioxin-5-yl)phenyl)-N-(9,9-dimethyl-9H-fluoren-2-yl)-9,9-dimethyl-9H-fluoren-2-amine (4). To a solution of 1 (0.3 g, 0.48 mmol) in CHCl3 (30 mL) at 0 °C was added N-bromosuccinimide (NBS) (0.088 g, 0.5 mmol) over 30 min as a small potion. The mixture was stirred at the same temperature for 1 h. After the solvent evaporated, the residue was extracted by dichloromethane (MC) and washed by H2O. The organic layer was separated and dried over MgSO4. The solvent was removed in vacuo. The pure product 4 was obtained by silica gel chromatography (eluent, dichloromethane:hexane ) 4:1); mp 131 °C. IR (THF) νmax: 2956, 2925, 2861, 1606, 1518, 1444, 1374, 1309, 1266, 1180, 1075, 921, 828. 1H NMR (CDCl3): δ 7.68-7.60 (m, 6H), 7.40 (d, 2H, 3J ) 7.2 Hz), 7.30-7.26 (m, 6H), 7.19 (d, 2H, 3J ) 8.4 Hz), 7.10 (d, 2H, 3J ) 8.1 Hz), 4.28 (s, 4H), 1.42 (s, 12H). 13 C{1H} NMR (CDCl3): δ 154.78, 153.58, 147.26, 146.55, 142.10, 138.88, 137.61, 134.19, 127.58, 127.12, 126.87, 126.48, 124.02, 123.14, 122.53, 120.62, 119.44, 118.55, 117.47, 96.92, 64.92, 64.63, 47.01, 27.27. MS: m/z 697 [M+]. Anal. calcd for C42H34BrNO2S: C, 72.41; H, 4.92. Found: C, 72.64; H, 5.01. N-(4-(7-(3,6-Dihexylthieno[3,2-b]thiophen-2-yl)-2,3-dihydrothieno[3,4-b][1,4]dioxin-5-yl)pheny-l)-N-(9,9-dimethyl9H-fluoren-2-yl)-9,9-dimethyl-9H-fluoren-2-amine (8). Under nitrogen atmosphere, a mixture of 4 (0.3 g, 0.43 mmol), 2-(3,6dihexylthieno[3,2-b]thiophen-2-yl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (0.22 g, 0.51 mmol), Pd(PPh3)4 (0.05 g, 0.043 mmol), K2CO3 (0.59 g, 4.3 mmol), and degassed water (10 mL) in dry THF (40 mL) was refluxed overnight. After it was cooled to room temperature, the solution was extracted with dichloromethane and dried over MgSO4. The solvent was removed in vacuo. The pure product 8 was obtained by silica gel chromatography (eluent, dichloromethane:hexane ) 1:3); mp 157 °C. IR (THF) νmax: 2956, 2925, 2857, 1603, 1501, 1486, 1447, 1361, 1320, 1276, 1088, 916, 828. 1H NMR (CDCl3): δ 7.66-7.58 (m, 6H), 7.38 (d, 2H, 3J ) 6.9 Hz), 7.34-7.24 (m, 6H), 7.19 (d, 2H, 3J ) 8.4 Hz), 7.10 (d, 2H, 3J ) 8.4 Hz), 6.94 (s, 1H), 4.34 (s, 4H), 2.86 (t, 2H), 2.69 (t, 2H), 1.76 (m, 4H), 1.41 (s, 12H), 1.31 (m, 12H), 0.88 (t, 6H). 13C{1H} NMR (CDCl3): δ 155.00, 153.47, 147.08, 146,47, 139.34, 138.96, 138.78, 137.82, 137.28, 136.89, 135.37, 134.17, 127.91, 126.92, 126.51, 123.78, 123.22, 122.34, 120.80, 120.49, 119.57, 118.91, 117.14, 114.12, 114.87, 109.66, 65.15, 64.89, 47.01, 31.83, 31.45, 30.98, 28.94, 27.35, 27.18, 22.86, 14.44. MS: m/z 924 [M+]. Anal. calcd for C60H61NO2S3: C, 77.96; H, 6.65. Found: C, 77.53; H, 6.91. 7-(Hexyloxy)-N-(7-(hexyloxy)-9,9-dimethyl-9H-fluoren-2yl)-9,9-dimethyl-N-(4-(7-(thieno[3,2b]thiophen-2-yl)-2,3-dihydrothieno[3,4-b][1,4]dioxin-5-yl)phenyl)-9H-fluoren-2amine (11). Compound 11 was synthesized by a procedure similar to 7 except that 6 (0.35 g, 0.39 mmol) was used in place

Paek et al. of 4; mp 143 °C. IR (THF) νmax: 2955, 2927, 2859, 1605, 1581, 1500, 1461, 1436, 1361, 1274, 1196, 1028, 813. 1H NMR (CDCl3): δ 7.61 (d, 2H, 3J ) 8.7 Hz), 7.53 (d, 2H, 3J ) 8.1 Hz), 7.50 (d, 2H, 3J ) 8.1 Hz), 7.43 (s, 2H), 7.31 (d, 1H, 3J ) 5.4 Hz), 7.21 (s, 3H), 7.20 (d, 1H, 3J ) 5.4 Hz), 7.16 (d, 2H, 3 J ) 8.4 Hz), 6.86 (s, 2H), 6.85 (d, 2H, 3J ) 9.6 Hz), 4.39 (dd, 4H), 4.01 (t, 4H), 1.82 (m, 4H), 1.48 (m, 12H), 1.39 (s, 12H), 0.92 (t, 6H). 13C{1H} NMR (CDCl3): δ 158.63, 155.39, 154.54, 146.76, 146.04, 139.66, 138.36, 137.68, 137.51, 136.90, 134.33, 132.34, 131.74, 127.82, 127.43, 126.95, 126.80, 126.27, 123.92, 123.40, 123.30, 119.70, 118.86, 115.47, 112.93, 109.41, 68.42, 65.04, 64.85, 46.99, 31.85, 31.41, 31.19, 31.16, 30.98, 29.60, 29.47, 29.12, 27.39, 26.02, 25.87, 14.30, 14.12. MS: m/z 956 [M+]. Anal. calcd for C60H61NO4S3: C, 75.36; H, 6.43. Found: C, 75.42; H, 6.54. 5-(7-(4-(Bis(9,9-dimethyl-9H-fluoren-2-yl)amino)phenyl)2,3-dihydrothieno[3,4-b][1,4] dioxin-5-yl)-3,6-dihexylthieno[3,2b]thiophene-2-carbaldehyde (14). To N,N-dimethylformamide (5 mL) was added POCl3 (0.12 mL) dropwise with ice-bath cooling and stirring. This solution was added 7 (0.15 g, 0.16 mmol) in chloroform (20 mL), and the solution was stirred for 6 h at room temperature. The mixture was poured onto ice water, basified (saturated aqueous K2CO3 solution), extracted with dichloromethane, and dried over MgSO4. The solvent was removed in vacuo. The pure product 13 was obtained by silica gel chromatography (eluent EA: HX ) 1:10); mp 155 °C. IR (THF) νmax: 2958, 2930, 2865, 1657, 1604, 1459, 1440, 1361, 1319, 1232, 1087, 862. 1H NMR (CDCl3): δ 10.06 (s, 1H), 7.66-7.59 (m, 6H), 7.54 (s, 2H), 7.47-7.10 (m, 10H), 4.48 (m, 4H), 3.08 (t, 2H), 2.92 (t, 2H), 1.80 (m, 4H), 1.42 (s, 12H), 1.31 (m, 12H), 0.89 (t, 6H). 13C{1H} NMR (CDCl3): δ 182.14, 155.09, 153.52, 153.24, 146.99, 146.69, 145.60, 142.60, 139.70, 139.24, 138.93, 138.72, 137.67, 134.41, 133.29, 127.59, 128.07, 126.60, 125.91, 123.67, 122.56, 120.69, 120.29, 119.50, 118.81, 108.72, 107.29, 65.02, 64.73, 46.94, 31.77, 30.65, 29.41, 28.84, 28.39, 27.48, 27.28, 14.36. MS: m/z 952 [M+]. Anal. calcd for C61H61NO3S3: C, 76.93; H, 6.46. Found: C, 76.88; H, 6.38. 5-(7-(4-(Bis(7-(hexyloxy)-9,9-dimethyl-9H-fluoren-2-yl)amino)phenyl)-2,3-dihydro-thieno[3,4-b][1,4]dioxin-5-yl)thieno[3,2b]thiophene-2-carbaldehyde (17). The product was synthesized according to the procedure as described above for synthesis of 14; mp 136 °C. IR (THF) νmax: 2957, 2930, 2861, 1654, 1604, 1577, 1460, 1433, 1362, 1232, 1087, 1055, 862. 1H NMR (CDCl3): δ 9.9 (s, 1H), 7.84 (s, 1H), 7.62 (d, 2H, 3J ) 8.7 Hz), 7.54 (d, 2H, 3J ) 8.4 Hz), 7.50 (d, 2H, 3J ) 8.4 Hz), 7.45 (s, 1H), 7.21 (s, 2H), 7.16 (d, 2H, 3J ) 8.7 Hz), 7.09 (d, 2H, 3J ) 8.4 Hz), 6.94 (s, 2H), 6.87 (d, 2H, 3J ) 8.4 Hz), 4.38 (dd, 4H), 4.01 (t, 4H), 1.82 (m, 4H), 1.48 (m, 12H), 1.40 (s, 12H), 0.92 (t, 6H). 13C{1H} NMR (CDCl3): δ 183.01, 158.88, 155.42, 154.77, 146.34, 146.12, 139.97, 138.54, 137.96, 137.42, 136.71, 134.28, 132.76, 131.63, 127.22, 126.91, 126.31, 125.89, 123.56, 123.23, 123.11, 120.17, 119.89, 118.75, 115.23, 112.67, 109.23, 68.24, 65.23, 64.85, 47.12, 31.95, 31.12, 30.77, 29.53, 27.21, 26.23, 14.54. MS: m/z 984 [M+]. Anal. calcd for C61H61NO5S3: C, 74.43; H, 6.25. Found: C, 74.48; H, 6.39. (E)-3-(5-(7-(4-(Bis(7-hexyl-9,9-dimethyl-9H-fluoren-2-yl)amino)phenyl)-2,3-dihydro-thieno[3,4-b][1,4]dioxin-5-yl)-3,6-dihexylthieno[3,2-b]thiophen-2-yl)-2-cyanoacrylic Acid (JK178). A mixture of 16 (0.15 g, 0.15 mmol) and cyanoacetic acid (0.02 g, 0.25 mmol) was added to CHCl3 (30 mL) and piperidine (0.02 g, 0.23 mmol). The solution was refluxed for 6 h. After it was cooled to room temperature, the solution was extracted with dichloromethane and dried over MgSO4. The solvent was removed in vacuo. The pure product JK-178 was

Organic Sensitizers for Dye-Sensitized Solar Cells

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SCHEME 1: Schematic Diagram for the Synthesis of JK-178, JK-179, JK-184, JK-185, JK-200, and JK-201

obtained by silica gel chromatography (eluent MC:MeOH ) 9:1); mp 173 °C. IR (THF) νmax: 2960, 2927, 2863, 2205, 1604, 1465, 1439, 1381, 1310, 1250, 1088, 861. 1H NMR (CDCl3): δ 8.20 (s, 1H), 7.70 (d, 2H, 3J ) 8.4 Hz), 7.62 (d, 4H, 3J ) 7.5 Hz), 7.31-7.25 (m, 4H), 7.14-7.08 (m, 4H), 7.00 (d, 2H, 3J ) 7.5 Hz), 4.41 (m, 4H), 2.86 (m, 4H), 2.69 (m, 4H), 1.97 (m, 8H), 1.60 (s, 12H), 1.29 (m, 24H), 0.85 (t, 12H). MS: m/z 1187 [M+]. Anal. calcd for C76H86N2O4S3: C, 76.86; H, 7.30. Found: C, 76.88; H, 7.21. (E)-3-(5-(7-(4-(Bis(7-(hexyloxy)-9,9-dimethyl-9H-fluoren2-yl)amino)phenyl)-2,3-dihy-drothieno[3,4-b][1,4]dioxin-5yl)-3,6-dihexylthieno[3,2-b]thiophen-2-yl)-2-cyanoacrylic Acid (JK-184). JK-184 was synthesized by a procedure to JK-178 except that 18 (0.15 g, 0.15 mmol) was used in place of 16; mp 170 °C. IR (THF) νmax: 2954, 2927, 2859, 2209, 1605, 1581, 1461, 1435, 1384, 1306, 1197, 1087, 1027, 810. 1H NMR (CDCl3): δ 8.20 (s, 1H), 7.64-7.59 (m, 6H), 7.22 (s, 2H), 7.08 (s, 2H), 7.07 (d, 2H, 3J ) 9.9 Hz), 6.98 (d, 2H, 3J ) 8.4 Hz), 6.86 (d, 2H, 3J ) 8.4 Hz), 4.40 (m, 4H), 3.99 (t, 4H), 2.85 (m, 4H), 1.72 (m, 8H), 1.48 (m, 24H), 1.35 (s, 12H), 0.92 (t, 12H). MS: m/z 1219 [M+]. Anal. calcd for C76H86N2O6S3: C, 74.84; H, 7.11. Found: C, 74.62; H, 7.23. (E)-3-(5-(7-(4-(Bis(7-(hexyloxy)-9,9-dimethyl-9H-fluoren2-yl)amino)phenyl)-2,3-dihydrothieno[3,4-b][1,4]dioxin-5yl)thieno[3,2-b]thiophen-2-yl)-2-cyanoacrylic Acid (JK-185). JK-185 was synthesized by a procedure to JK-178 except that 17 (0.15 g, 0.13 mmol) was used in place of 16; mp 176 °C. IR (THF) νmax: 2956, 2940, 2873, 2213, 1605, 1463, 1441, 1364, 1275, 1058, 861. 1H NMR (CDCl3): δ 8.14 (s, 1H), 7.95 (s, 1H), 7.64-7.60 (m, 7H), 7.22 (s, 2H), 7.08 (s, 2H), 7.07 (d, 2H, 3J ) 9.3 Hz), 6.98 (d, 2H, 3J ) 7.8 Hz), 6.87 (d, 2H, 3J ) 8.4 Hz), 4.39 (dd, 4H), 3.99 (t, 4H), 1.71 (m, 4H), 1.43 (m, 12H), 1.34 (s, 12H), 0.89 (t, 6H). MS: m/z 1051 [M+]. Anal. calcd for C64H62N2O6S3: C, 73.11; H, 5.94. Found: C, 73.24; H, 5.91. (E)-3-(5-(7-(4-(Bis(9,9-dimethyl-9H-fluoren-2-yl)amino)phenyl)-2,3-dihydrothieno[3,4-b][1,4]dioxin-5-yl)-3,6-dihexylthieno[3,2-b]thiophen-2-yl)-2-cyanoacrylic Acid (JK-200). JK200 was synthesized by a procedure to JK-178 except that 14

(0.15 g, 0.15 mmol) was used in place of 16; mp 182 °C. IR (THF) νmax: 2957, 2930, 2869, 2209, 1603, 1460, 1445, 1361, 1306, 1087, 827. 1H NMR (CDCl3): δ 8.19 (s, 1H), 7.76-7.73 (m, 4H), 7.63 (d, 2H, 3J ) 8.4 Hz), 7.50 (d, 2H, 3J ) 6.9 Hz), 7.31-7.26 (m, 6H), 7.12 (d, 2H, 3J ) 8.7 Hz), 7.02 (d, 2H, 3 J ) 10.2 Hz), 4.40 (s, 4H), 2.85 (m, 4H), 1.65 (m, 4H), 1.36 (s, 12H), 1.27 (m, 12H), 0.83 (t, 6H). MS: m/z 1019 [M+]. Anal. calcd for C64H62N2O4S3: C, 75.41; H, 6.13. Found: C, 75.88; H, 6.21. Results and Discussion Scheme 1 illustrates the synthetic protocol of organic sensitizers JK-178, JK-179, JK-184, JK-185, JK-200, and JK-201. The Suzuki coupling reaction12 of 4-[bis(9,9-dimethylfluoren-2yl)amino]-4-bromophenyl derivatives with 1.2 equiv of (3,4ethylene-dioxythiophen-5-yl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane afforded compounds 1-3. The bromothiophene derivatives 4-6 were synthesized by bromination of 1-3 with NBS in CHCl3. The attachment of thieno[3,2-b]thiophene-2-yl unit to 4-6 was achieved by the Suzuki coupling reaction of 4-6 with 2-(thieno[3,2-b]thiophene-2-yl)borolane derivatives. Carbaldehydes 13-18 were synthesized from 7 to 12 by using a Vilsmeier-Hack reaction.13 The Knoevenagel condensation of the aldehydes 13-18 with cyanoacetic acid in the presence of piperidine in CH3CN yielded the six organic sensitizers. The molecular structure of JK derivatives was confirmed by various spectroscopic methods such as NMR, mass spectra, and elemental analysis. Details of some compound preparation are provided in the Supporting Information. The absorption and emission spectra of six sensitizers measured in THF/EtOH (1/2) are shown in Figure 2 or Figure S1 in the Supporting Information and are listed in Table 1. The absorption spectrum of JK-184 shows two absorption bands at 466 (ε ) 45000 dm3 mol-1 cm-1) and 370 nm (ε ) 69600 dm3 mol-1 cm-1), which are assigned as the π-π* transitions of the conjugated system. Under similar conditions, the JK-185 sensitizer with a unsubstituted thieno [3,2-b] group on the spacer unit exhibits two absorption bands at 484 (ε ) 47800 dm3 mol-1

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Figure 2. Absorption (left scale) and emission (right scale) spectra of JK-178 (black solid line), JK-184 (red solid line), JK-185 (blue solid line), and JK-200 (green solid line) in THF/EtOH (1/2) solution and absorption spectra of JK-178 (black dash line), JK-184 (red dash line), JK-185 (blue dash line), and JK-200 (green dash line) adsorbed on TiO2 film.

cm-1) and 369 nm (ε ) 46300 dm3 mol-1 cm-1). The peak at 484 nm in JK-185 is significantly red-shifted relative to that of JK-184. This observation can be readily interpreted from molecular modeling studies of two sensitizers. The ground state structure of JK-185 possesses a 18.5° twist between the phenylamino unit and the EDOT unit. The dihedral angle of EDOT and thienothiophene is 0.7° (Figure 3). In the case of the JK-184 sensitizer, the dihedral angles between the phenylamino and EDOT unit and the next two thienyl units are 14.8 and 44.2°, respectively, giving a more twisted configuration than that of JK-185. A significant red shift of JK-185 relative to JK-184 derives from more delocalization over an entire conjugated system in JK-185. Under the same conditions, the JK-179 sensitizer that contains a hexyl unit on the fluorenyl unit exhibits a red-shifted absorption band at 476 nm (ε ) 38400 dm3 mol-1 cm-1) relative to the JK-178 sensitizer that shows at 455 nm, probably due to the coplanarity of spacer group. A similar phenomena was observed in the JK-200 and JK-201 sensitizers. In addition, the JK-184 and JK-185 sensitizers incorporating hexyloxy unit exhibit a red-shifted absorption relative to its counterparts (JK178 and JK-179) containing a hexyl unit. Adsorption of six sensitizers on TiO2 film resulted in a red-shift as compared to solution absorption maxima and broaden the absorption spectra up to 700 nm. Such broadening and red-shifted absorption have been reported in other organic sensitizers on TiO2 electrode due to the J aggregation.14 We also observed that the six sensitizers exhibited strong luminescence maxima at 630-650 nm when

Figure 3. Optimized structures calculated by TD-DFT using the B3LYP functional and the 3-21G* basis set for JK-184 and JK-185.

they are excited with their π-π* bands in an air-equilibrated solution at 298 K. The cyclic voltammetry at the six sensitizers was measured in MeCN with 0.1 M tetrabutylammonium hexafluorophosphate at a scan rate 50 mV s-1 (see Table 1) and TiO2 films anchored with the sensitizers were used as working electrodes. The six organic sensitizers show quasi-reversible couples, at 0.95-1.03 V vs NHE (Table 1), providing the thermodynamic driving force for efficient dye regeneration. The reduction potentials of the six sensitizers were calculated from the oxidation potentials, and the E0-0 was determined from the intersection of absorption and emission spectra. The exited state oxidation potentials (Eox*) of the sensitizers (JK-184, -1.26 V; JK-185, -1.24 V; JK178, -1.24 V; and JK-179, -1.22 V vs NHE) are much more negative than the conduction band of TiO2 at approximately -0.5 V vs NHE, providing enough driving force for electron injection.15 To scrutinize the geometrical and photophysical properties, molecular orbital calculations of JK-178, JK-179, JK-184, JK185, JK-200, and JK-200 were carried out using the TD-DFT and B3LYP/3-21G* program (Figure 4). The calculation shows that the highest occupied molecular orbital (HOMO) of JK184 and JK-185 is populated over the fluorenylaminophenyl and EDOT moieties because the torsion angles between the phenyl attached to the nitrogen atom and the EDOT unit in JK-184 and JK-185 are 14.8 and 18.5°, respectively, leading to an effective π-conjugation. The lowest unoccupied molecular orbital (LUMO) of JK-184 is delocalized over the cyanoacrylic and EDOT unit through the thienothiophene, with a sizable contribution on the far end cyanoacrylic unit. As light excitation is related to the vectorial electron transfer from the HOMO to the LUMO for efficient electron flow, the calculation results of HOMO-LUMO of JK-184 indicate that its excitation moves

TABLE 1: Optical, Redox, and DSSC Performance Parameters of Dyes dye JK-200 JK-201 JK-178 JK-179 JK-184 JK-185

λabsa (nm) (ε/M-1 cm-1) 368 371 371 370 370 369

(38, (25, (44, (38, (69, (46,

100), 700), 900), 300), 600), 300),

450 481 455 476 466 484

(23, (25, (32, (38, (45, (47,

100) 000) 100) 400) 100) 800)

Eredoxb (V)

E0-0c (V)

ELUMOd (V)

Jsc (mA cm-2)

Voc (V)

FF

ηe (%)

0.95 1.00 1.03 1.02 0.99 0.97

2.28 2.23 2.27 2.24 2.25 2.21

-1.33 -1.23 -1.24 -1.22 -1.26 -1.24

13.02 13.81 16.34 15.22 17.49 15.94

0.57 0.59 0.64 0.58 0.70 0.66

0.72 0.68 0.74 0.70 0.70 0.67

5.34 5.65 7.83 6.21 8.70 7.04

a Absorption spectra were measured in THF. b Eox is the oxidation potential. The redox potential of dyes on TiO2 was measured in CH3CN with 0.1 M (n-C4H9)4NPF6 with a scan rate of 50 mV s-1 (vs Fc/Fc+). c E0-0 is the voltage of intersection point between absorption and emission spectra. E0-0 was determined from the intersection of absorption and emission spectra in THF. d ELUMO was calculated by Eox - E0-0. e Performances of DSSCs were measured with 0.175 cm2 working area. Electrolyte: 0.6 M DMPImI, 0.05 I2, and 0.1 M LiI in acetonitrile.

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Figure 6. Evolution of solar cell parameters with JK-184 (9) and JK185 (2) during visible light soaking (AM 1.5 G, 100 mW cm-2) at 60 °C. A 420 nm cutoff filter was placed on the cell surface during illumination. Electrolyte B: 0.2 M iodine, 0.5 M NMBI, and 0.1 M GuNCS in PMII/EMINCS (13:7). Figure 4. Isodensity surface plots of the HOMO and LUMO of different dyes (i.e., JK-178, JK-184, JK-185, and JK-200).

Figure 5. J-V curve and IPCE spectra of JK-178 (block solid line), JK-184 (red solid line), JK-185 (blue solid line), and JK-200 (green solid line). The dark current bias potential relationship is shown as dotted curves.

the electron distribution from the phenylamino unit to the cyaniacrylic unit. Therefore, the photoinduced electron transfer from the dye to TiO2 electrode can efficiently occur by the HOMO-LUMO transition. On the other hand, the LUMO of JK-185 having no hexyl group on the thienothiophene is delocalized over the cyanoacrylic unit; thienothiophene and EDOT have an equal population because the dihedral angle between the EDOT and the thienothiophene is small (0.7°), reflecting an entire conjugation through the EDOT cyanoacrylic unit. A relatively low efficiency in JK-185 relative to JK-184 can be explained as an insufficient driving force of vertorial electron flow from the HOMO to the LUMO. A similar pattern was observed in the JK-178 and JK-179. The incident monochromatic photon-to-current conversion efficiency (IPCE) with a sandwich type cell based on JK-178, JK-184, JK-185, and JK-200 using the redox electrolyte consisting of 0.6 M DMPImI, 0.05 M I2, 0.1 M LiI, and 0.5 M TBP in acetonitrile is shown in the insert of Figure 5. The onset of IPCE for the device of JK-184 was ca. 800 nm. IPCE values higher than 85% were observed in the range of 440-620 nm

with a maximum value of 91% at 525 nm for the device based on JK-184. The action spectrum of a DSSC with JK-185 exceeds 70% in the visible spectral region from 450 to 640 nm, reaching its maximum of 85% at 498 nm. The JK-184-sensitized device generates the highest conversion efficiency among six sensitizers (see Table 1), which may be due to its broad and intense photocurrent action spectrum. The action spectrum of JK-184 is red-shifted by 20 nm as compared to that of JK-185 in spite of the fact that the absorption spectrum of JK-185 is red-shifted by 13 nm relative to the JK-184. Similar phenomena were observed in the JK-178 and JK-179. Under standard global AM 1.5 solar conditions, the JK-184-sensitized cell gave a short circuit photocurrent density (Jsc) of 17.49 mA cm-2, an open circuit voltage (Voc) of 0.70 V, and a fill factor of 0.70, corresponding to an overall conversion efficiency η of 8.70% (Figure 5). Under the same conditions, the JK-185-sensitized cell gave a Jsc of 15.94 mA cm-2, a Voc of 0.66 V, and a fill factor of 0.67, corresponding to η of 7.04%. The open circuit voltage and short circuit photocurrent for the device based on JK-184 are significantly higher than those of JK-185. Of particular importance is the 40 mV increase in Voc of the JK-184-based cell relative to the JK-185-based cell. This improved Voc value is attributed to suppression of dark current because of the blocking effect of the hexyl group substituted at the thienotheiophene unit. The hexyl groups can lead to an effective separation of the charges, which aids the retardation of charge recombination. The minimization of interfacial charge recombination losses in the device is evident from the dark current data of the cells (Figure 5). Because photostability is a vital parameter for sustained cell operation, we studied cell performance changes using an ionic liquid electrolyte. Figure 6 shows the stability data of the device employing JK-184. The cell showed an excellent long-term stability, and the initial efficiency of 7.22% slightly decreased to 6.69% after the 1000 h light soaking test at 60 °C. The longterm stability of the device is impressive because only few organic sensitizers passed the light-soaking test for 1000 h while retaining an efficiency over 6.5% using an ionic liquid electrolyte. After 1000 h of light soaking, the Voc decreased by 57 mV. However, the short circuit photocurrent remained almost constant. JK-185 gave a rather low initial conversion efficiency of 6.52%, which kept 92% of the initial performance after the

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Figure 7. Electron diffusion coefficients (A) and lifetimes (B) in the photoelectrodes adsorbing different dyes (i.e., JK-178, JK-184, JK185, and JK-200).

1000 h light soaking test. The enhanced stability of JK-184 relative to JK-185 can be attributed to the substituted hexyl groups on thienothiophene by preventing the dark current. Figure 7 shows the electron diffusion coefficient (Dc) and lifetimes (τc) of the DSSCs employing JK-178, JK-184, JK185, and JK-200 displayed as a fuction of Jsc and Voc. No significant differences among the Dc values were seen at the identical short circuit current conditions. The result demonstrates that the Dc values are hardly affected by structural modifications in the sensitizers. On the other hand, the τc values show a big gap among the dyes, giving the increasing order of JK-184 > JK-185 > JK-178 > JK-201. The different τc values might be caused by the different dye sizes and structures. The longer electron lifetime of JK-184 may be obtained because the hexyl groups on both thienothiophene and fluorenyl unit block the I3- or cations approaching the TiO2 surface. Therefore, the decreased electronic coupling between the surface and the hole results in a long-live charge-separated pair, which leads to an increased open circuit photovoltage. The result of the electron lifetime in JK-184 is consistent with that of Voc shown in Table 1. Figure 8 shows the ac impedance spectra measured under illumination and dark conditions. Upon illumination of 100 mW cm-2 under open circuit conditions, the radius of the intermediate frequency semicircle in the Nyquist plot decreased in the order of JK-185 (23.36 Ω) > JK-184 (14.21 Ω), indicating the improved charge generation and transport. This result is in accord with that of short circuit photocurrent shown in Table 1. In the dark under forward bias (-0.67 V), the radius of the intermediate frequency semicircle showed the increasing order of JK-185 (56.48 Ω) < JK-184 (140.1 Ω), in accord with the trends of the Voc and τc values. Conclusion In conclusion, we designed and synthesized six efficient organic sensitizers containing a binary π-conjugated bridging

Figure 8. Electrochemical impedance spectra measured the illumination (1 sun) and the dark for the cells with different dye adsorption conditions [i.e., JK-184 (red) and JK-185 (blue)].

unit. The device based on the six sensitizers using a volatile electrolyte gave an overall conversion efficiency of 5.34-8.70%. The photovoltaic performance was shown to be quite sensitive to the substituent on the sensitizers. Some general performance trends are clearly discerned as follows: (i) The power conversion efficiencies increased in the order of hexyloxy-substituted fluorenyl > hexyl-substituted fluorenyl > none-substituted fluorenyl based devices. (ii) The hexyl-substituted thienothiophenebased device outperforms that based on none-substituted thienothiophene in the bridged unit. The JK-184-sensitized solar cell with an ionic liquid electrolyte showed an excellent stability under light soaking at 60 °C for 1000 h. The high efficiency and excellent stability may be derived from the introduction of hydrophobic hexyl or hexyloxy groups on the fluoren-2-yl or thienothiophene ligand. We believe that the development of highly efficient organic sensitizers having an excellent stability is possible through structural modifications, and work on these is now in progress. Acknowledgment. This work was supported by the WCU (the Ministry of Education and Science) program (no. R31-2008000-10035-0) and Basic Science Research program through the National Research Foundation of Korea (NRF) grant funded from the Ministry of Education, Science and Technology (MEST) of Korea for the Center for Next Generation Dyesensitized Solar Cells (no. 2010-0001842). Supporting Information Available: Synthetic details and additional data for UV, PL, CV, DFT calculations, performance,

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