Dyes and Pigments 91 (2011) 192e198 Contents lists available at ScienceDirect Dyes and Pigments journal homepage: www.elsevier.com/locate/dyepig Organic dyes incorporating low-band-gap chromophores based on p-extended benzothiadiazole for dye-sensitized solar cells Dong Hyun Lee a, Myung Jun Lee a, Hae Min Song a, Bok Joo Song a, Kang Deuk Seo a, Mariachiara Pastore b, Chiara Anselmi b, Simona Fantacci b, c, Filippo De Angelis b, Mohammad K. Nazeeruddin d, Michael Gräetzel d, Hwan Kyu Kim a, * a Department of Advanced Materials Chemistry & Center for Advanced Photovoltaic Materials (ITRC), Korea University, ChungNam 339-700, Republic of Korea Istituto CNR di Scienze e Tecnologie Molecolari c/o Dipartimento di Chimica, Università di Perugia, via Elce di Sotto 8, I-06123 Perugia, Italy Italian Institute of Technology (IIT), Center for Biomolecular Nanotechnologies, Via Barsanti 73010, Arnesano, Lecce, Italy d Laboratory for Photonics and Interfaces, Institute of Chemical Sciences and Engineering, Ecole Polytechnique Federale de Lausanne, 1015 Lausanne, Switzerland b c a r t i c l e i n f o a b s t r a c t Article history: Received 1 February 2011 Received in revised form 9 March 2011 Accepted 9 March 2011 Available online 21 March 2011 A series of new p-conjugated organic dyes (HKK-BTZ1, HKK-BTZ2, HKK-BTZ3 and HKK-BTZ4), comprising triphenylamine (TPA) moieties as the electron donor and benzothiadiazole moieties as the electron acceptor/anchoring groups, was synthesized for the use in dye-sensitized solar cells (DSSCs). TPA units are bridged to benzothiadiazole with single(S), double(D) and triple bonds(T) in different derivatives. And HKK-BTZ1 was modified by introducing alkoxy group of TPA unit, because the bulky alkoxy group is a strong donating group for the more red shift and for reducing aggregation of dyes in TiO2 film. The structure-property relationship was investigated. Under standard global AM 1.5 G illumination, a maximum photo-to-electron conversion efficiency of 7.30% was achieved with the DSSC based on dye HKK-BTZ4 (JSC ¼ 17.9 mA/cm 2, VOC ¼ 0.62 V, FF ¼ 0.66), while the Ru dye N719-sensitized DSSC showed an efficiency of 7.82% with a JSC of 17.5 mA/cm 2, a VOC of 0.62 V, and a FF of 0.72. Ó 2011 Elsevier Ltd. All rights reserved. Keywords: Metal-free organic dyes Low band-gap chromophore Benzothiadiazole unit Triphenylamine (TPA) unit p-Bridge units Dye-sensitized solar cells 1. Introduction Over the last two decade, dye-sensitized solar cells (DSSCs), based on ruthenium complexes endowed with appropriate ligands and anchoring groups as the most widely used choice of charge transfer sensitizers for mesoscopic solar cells, have attracted significant attention as an alternative to the conventional solar cells due to their low-cost of production and high performance [1], since Grätzel and co-workers reported very high solar cell performances [2]. Several ruthenium-based sensitizers have achieved remarkable power conversion efficiency of 10-11% under standard global air mass 1.5 (AM 1.5G) illumination [3e5]. However, the rarity and high cost of the ruthenium metal may limit their development for large-scale applications. Consequently, many researchers have focused on developing metal-free organic sensitizers, and some of these endeavors have enhanced the solar-to-electric power conversion efficiencies reaching ca. 10% [6e10]. The advantages of * Corresponding author. Tel.: þ82 41 860 1493; fax: þ82 41 867 5396. E-mail address: hkk777@korea.ac.kr (H.K. Kim). 0143-7208/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.dyepig.2011.03.015 organic dyes are: 1) easy preparation and purification, and lower cost, 2) they have higher molar absorption coefficients than the Ru(II) complexes, and 3) the wide variety of the structures and their facile modification provides potential for molecular design, with the introduction of substituents onto the chromophore skeletons allowing for easy control not only of their photophysical and electrochemical properties but also of their stereochemical structures [5e9]. Many organic dyes, based on the donor-(p-spacer)-acceptor (D-p-A) system, exhibiting relatively high DSSC performances, have so far been designed and developed. They include coumarin dyes [11e15]. triarylamine dyes [16e20], hemicyanine dyes [21,22], thiophene-based dyes [23], indoline dyes [24e30], heteropolycyclic dyes [31], porphyrin dyes [32e36], and phthalocyanine [37]. But, one of the main drawbacks of organic sensitizers is still the sharp and narrow absorption bands in the blue region of the visible region, impairing their light-harvesting capabilities. In this paper, we synthesized a series of new p-conjugated metal-free D-p-A organic dyes comprising triphenylamine (TPA) moieties as the electron donor and benzothiadiazole moieties as the electron acceptor/anchoring groups: First, the use of a p-extended benzothiadiazole derivative bridging unit between the TPA donor and the cyanoacrylic acid D.H. Lee et al. / Dyes and Pigments 91 (2011) 192e198 anchoring group leads to higher efficiency, more red shift of absorption and emission bands than a D-p-A sensitizer based on benzothiadiazole (3.77%) reported by Ho and coworkers [38]. Second, the present p-extended BTZ dyes, associate with different links, such as single, double, and triple bonds have been designed and synthesized to investigate the structure-property relationship between the conversion efficiency and p-bridging system, which has not been wellestablished, yet [39e42]. Third, HKK-BTZ1 was modified by introducing alkoxy group of TPA donor unit onto triphenylamine unit, because the bulky alkoxy group is a strong donating group for reducing the gap between the HOMO and the LUMO resulting more red shift of the pep* charge transfer transitions. Also, the branched alkoxy groups reduce aggregation of dyes in TiO2 films. 2. Experimental Details of the synthesis of the TPA donor and BTZ acceptor moieties are provided in the supplementary data. 2.1. Synthesis 2.1.1. 5-(7-(5-(4-(Diphenylamino)phenyl)thiophen-2-yl) benzo[c] [1,2,5] thiadiazol-4-yl)thiophene-2-carb aldehyde (15) A mixture of 2 (1.5 g, 3.68 mmol), 5-(7-(5-bromothiophen-2-yl) benzo[c][1,2,5]thiadiazol-4-yl)thiophene-2-carbaldehyde 12 (2.13 g, 7.37 mmol), Pd(PPh3)4 (0.15 g), Na2CO3 (0.5 g) was dissolved in toluene (50 mL)eEtOH (20 mL)eH2O (20 mL) and the mixture was refluxed for 12 h. After evaporating the solvent under reduced pressure, H2O (50 mL) and methylene chloride (50 mL) were added. The organic layer was separated and dried in MgSO4. The solvent was removed under reduced pressure. The pure product 15 was obtained by column chromatography on silica gel using CH2Cl2. Yield: 35% (0.74 g, 1.3 mmol). 1H-NMR (300 MHz; CDCl3): d ¼ 9.97 (s, 1H), 8.21e8.20 (d, 1H), 8.18e8.17 (d, 1H), 8.01e7.99 (d, 1H), 7.91e7.88 (d, 1H), 7.85e7.84 (d, 1H), 7.58e7.55 (d, 2H), 7.35e7.33 (d, 2H), 7.31e7.26 (m, 4H), 7.15e7.06 (m, 8H). MS-EI: m/z 571 (Mþ). lmax,abs in THF: 365 nm, 516 nm. lmax,em in THF: 725 nm. 2.1.2. (2-Cyano-3-(5-(7-(5-(4-(diphenylamino)phenyl) thiophen-2yl)benzo[c][1,2,5]thiadiazol-4-yl) thiophen-2-yl)acrylic acid) (HKK-BTZ1) The compound 15 (0.2 g, 0.35 mmol), dissolved in CHCl3 (20 mL), and acetonitrile (20 mL) was condensed with 2-cyanoacetic acid (0.044 g, 0.52 mmol) in the presence of piperidine (0.17 mL, 2.38 mmol). The mixture was refluxed for 18 h under a nitrogen atmosphere. After cooling to room temperature, the mixture was washed with 2 M aqueous HCl and extracted with CHCl3. The solid was then washed with H2O, dichloromethane, ethyl acetate and dried under vacuum for ca. 20 h to afford the product of HKK-BTZ1 in 53% yield (0.12 g, 0.19 mmol). 1H-NMR (300 MHz; DMSOd): d ¼ 8.53 (s, 1H), 8.35e8.33 (d, 2H), 8.31e8.30 (d, 1H), 8.25e8.24 (d, 1H), 8.21e8.18 (d, 1H), 8.11e8.09 (d, 2H), 7.71e7.68 (d, 2H), 7.60e7.59 (d, 2H), 7.38e7.33 (m, 4H), 7.13e7.07 (m, 6H), 7.03e7.00 (m, 2H). MS (MALDI-TOF): 638.1 (Mþ). lmax,abs in THF:361 nm, 533 nm. lmax,em in THF:745 nm. 2.1.3. 5-(7-(5-(4-(Diphenylamino)styryl)thiophen-2-yl) benzo[c] [1,2,5]thiadiazol-4-yl)thiophene-2-carbaldehyde (16) A solution of tris(o-tolyl)phosphine (0.6 g, 0.03 mmol) and palladium acetate (0.03 g, 0.02 mmol) in dry DMF (150 mL) was added dropwise at 80  C under a nitrogen atmosphere to a solution of 12 (1.98 g, 0.7 mmol), 4 (1.88 g, 1 mmol) in triethylamine (30 ml) and dry DMF (200 mL). After the mixture was heated at 110  C for 12 h, the reaction mixture was poured into water (50 mL), and extracted with chloroform (40 mL  3). The combined organic 193 layers were neutralized with 1.2 N aqueous HCl 20 mL, washed with brine (30 mL  3), dried over anhydrous magnesium sulfate, and evaporated in vacuum to dryness. The product 16 was purified by silica gelcolumn chromatograph. Yield: 12% (0.5 g, 0.84 mmol). 1 H-NMR (300 MHz; CDCl3): d ¼ 9.97 (s, 1H), 8.22e8.21 (d, 1H), 8.13e8.11 (d, 1H), 8.02e7.99 (d, 1H), 7.91e7.88 (d, 1H), 7.85e7.84 (d, 1H), 7.39e7.36 (d, 2H), 7.30e7.28 (m, 3H), 7.18e7.13 (m, 6H), 7.11e7.03 (m, 6H). MS-EI: m/z 597 (Mþ). lmax,abs in THF:382 nm, 528 nm. lmax,em in THF:753 nm. 2.1.4. (2-Cyano-3-(5-(7-(5-((E)-4-diphenylamino)styryl) thiophen2-yl)benzo[c][1,2,5]thiadiazol-4-yl) thiophen-2-yl)acrylic acid) (HKK-BTZ2) The compound 16 (45 mg, 0.08 mmol), dissolved in CHCl3 (20 mL) and acetonitrile (20 mL), was condensed with 2-cyanoacetic acid (9 mg, 0.52 mmol) in the presence of piperidine (0.04 mL, 0.51 mmol). The mixture was refluxed for 18 h. After cooling to room temperature, the mixture was washed with 2 M aqueous HCl and extracted with CHCl3. The solid was then washed with H2O, dichloromethane, ethyl acetate and dried under vacuum for ca. 20 h to afford the product of HKK-BTZ2 in 53% yield (0.12 g, 0.18 mmol). 1H-NMR (300 MHz; DMSOd): d ¼ 8.46 (s, 1H), 8.34e8.32 (d, 1H), 8.31e8.29 (d, 1H), 8.21e8.20 (d, 1H), 8.20e8.16 (d, 1H), 8.05 (s, 1H), 7.56e7.53 (d, 2H), 7.37e7.31 (m, 5H), 7.15e7.05 (m, 8H), 7.96e7.93 (d, 2H). MS (MALDI-TOF): 664.2 (Mþ). lmax,abs in THF:391 nm, 546 nm. lmax,em in THF:760 nm. 2.1.5. 5-(7-(5-((4-(Diphenylamino)phenyl)ethynyl)thiophen-2-yl) benzo[c][1,2,5]thiadiazol-4-yl)thiophene-2-carbaldehyde (17) The compound 7 (0.1 g, 0.37 mmol), 14 (0.18 g, 0.39 mmol), PPh3 (1 mg, 0.004 mmol), Pd(PPh3)2Cl2 (3 mg, 0.004 mmol), and CuI (1 mg, 0.01 mmol) were added into 20 ml solution (TEA:THF ¼ 2:1). The mixture was stirred in a argon atmosphere under reflux for 24 h. After the solvent was removed by rotary evaporator, the residue was extracted with dichloromethane and water. The organic layer was dried with anhydrous magnesium sulfate overnight. The product was purified by silica gel column chromatograph (dichloromethane) Yield: 54% (0.12 g, 0.21 mmol). 1H-NMR (300 MHz; CDCl3): d ¼ 9.98 (s, 1H), 8.22e8.21 (d, 2H), 8.07e8.06 (d, 1H), 8.02e7.99 (d, 1H), 7.91e7.88 (d, 1H), 7.85e7.84 (d, 1H), 7.39e7.36 (d, 2H), 7.33e7.31 (m, 4H), 7.29e7.27 (m, 4H), 7.14e7.05 (m, 7H), 7.03e7.00 (d, 2H). MS-EI: m/z 595 (Mþ). lmax,abs in THF:362 nm, 496 nm. lmax,em in THF:721 nm. 2.1.6. (2-Cyano-3-(5-(7-(5-((4-(diphenylamino)phenyl)ethynyl) thiophen-2-yl) benzo[c] [1,2,5]thiadiazol-4-yl)thiophen-2-yl)acrylic acid) (HKK-BTZ3) The compound 17 (180 mg, 0.3 mmol), dissolved in CHCl3 (20 mL), and acetonitrile (20 mL), was condensed with 2-cyanoacetic acid (38 mg, 0.45 mmol) in the presence of piperidine (0.15 mL, 2.05 mmol). The mixture was refluxed for 18 h under a nitrogen atmosphere. After cooling to room temperature, the mixture was washed with 2 M aqueous HCl and extracted with CHCl3. The solid was then washed with H2O, dichloromethane, ethyl acetate and dried under vacuum for ca. 20 h to afford the product of HKK-BTZ3 in 45% yield (0.09 g, 0.13 mmol). 1H-NMR (300 MHz; DMSOd): d ¼ 8.46 (s, 1H), 8.36e8.34 (d, 1H), 8.32e8.31 (d, 1H), 8.29e8.26 (d, 1H), 8.20e8.18 (d, 1H), 8.06 (s, 1H), 7.52e7.51 (d, 2H), 7.47e7.44 (d, 2H), 7.41e7.35 (m, 4H), 7.18e7.10 (m, 6H), 6.92e6.89 (d, 2H). MS (MALDI-TOF): 662.1 (Mþ). lmax,abs in THF:359 nm, 515 nm. lmax,em in THF:666 nm. 2.1.7. 5-(7-(5-(4(bis(4-(2-ethylhexyloxy)phenyl)amino)phenyl) thiophen-2 yl)benzo(c)[1,2,5]thiadiazole-4-yl)thiophene-2carbaldehyde (19) A mixture of 18 (1.11 g, 1.77 mmol), 12 (0.48 g, 1.18 mmol), Pd(PPh3)4 (0.05 g, 0.08 mmol), Na2CO3 (0.25 g, 2.36 mmol) was 194 D.H. Lee et al. / Dyes and Pigments 91 (2011) 192e198 tetramethylsilane as an internal standard. Infrared spectra were measured on KBr pellets using a Perkin-Elmer Spectrometer. The mass spectra were taken by a JEOL JMS-AX505WA mass spectrometer. The absorption and photoluminescence spectra were recorded on a Perkin-Elmer Lambda 2S UV-visble spectrometer and a Perkin LS fluorescence spectrometer, respectively. Cyclic voltammetry was carried out with a Versa STAT3 (AMETEK). The cyclic voltammogram curves were obtained from a three electrode cell in 0.1 M TBAPF6 in CH3CN at the scan rate of 50 mV s 1, using dye coated TiO2 electrode as a working electrode and Pt wire counter electrode and Ag/AgCl (saturated KCl) reference electrode (þ0.197 V vs NHE) and calibrated with ferrocene. All of the measured potentials were converted to the NHE scale. Photovoltaic data were measured using a 1000 W xenon light source (Oriel, 91193) that was focused to give 1000 W/m2, the equivalent of one sun at Air Mass (AM) 1.5 G, at the surface of the test cell. The light intensity was adjusted with a Si solar cell that was double-checked with an NREL-calibrated Si solar cell (PV Measurement Inc.). The applied potential and cell current were measured using a Keithley model 2400 digital source meter. The currentevoltage characteristics of the cell under these conditions were determined by biasing the cell externally and measuring the generated photocurrent. This process was fully automated using Wavemetrics software. dissolved in THF 50 mL, EtOH 20 mL, H2O 20 mL and the mixture was refluxed for 12 h. After evaporating the solvent under reduced pressure, H2O (50 mL) and methylene chloride (50 mL) were added. The organic layer was separated and dried in MgSO4. The solvent was removed under reduced pressure. The pure product 19 was obtained by column chromatography on silica gel using CH2Cl2. Yield: 35% (0.74 g, 1.3 mmol). 1H-NMR (300 MHz; CDCl3): d ¼ 9.95 (s, 1H), 8.18e8.16 (d, 1H), 8.15e8.14 (d, 1H), 7.96e7.94 (d, 1H), 7.85e7.81 (d, 2H), 7.49e7.46 (d, 2H), 7.28e7.26 (d, 1H), 7.10e7.07 (m, 2H), 6.95e6.92 (d, 2H), 6.87e6.83(d, 4H), 3.83(d, 4H), 1.40 (m, 2H), 1.57e1.18 (m, 16H), 0.93 (t, 12H,). 2.1.8. (Z)-3-(5-(7-(5(-4(bis(4-(2-ethylhexyloxy)phenyl)amino) phenyl)thiophen-2-yl)benzo(c)[1,2,5] thiadiazol-4-yl)thiophene-2cyanoacrylic cid (HKK-BTZ4) The compound 19 (0.45 g, 0.54 mmol) dissolved in CHCl3 20 mL and acetonitrile 20 mL was condensed with 2-cyanoacetic acid (0.068 g, 0.8 mmol) in the presence of piperidine (0.31 mL, 3.69 mmol). The mixture was refluxed for 18 h under a nitrogen atmosphere. After cooling to room temperature, the mixture was washed with 2 M aqueous HCl and extracted with CHCl3. The solid was then washed with H2O, dichloromethane, ethyl acetate and dried under vacuum for ca. 20 h to afford a product HKK-BTZ4 in 53% yield (0.12 g, 0.19 mmol). 1H-NMR (300 MHz; DMSOd): d ¼ 8.21e8.08 (m, 4H), 7.82e7.78 (d, 1H), 7.61e7.52 (d, 2H), 7.50e7.44 (d, 1H), 7.11e7.01 (d, 4H), 6.69e6.88 (d, 4H), 6.82e6.76 (d, 2H), 3.83(d, 4H), 1.40 (m, 2H), 1.57e1.18 (m, 16H), 0.93 (t, 12H), MS (MALDI-TOF): 894.15 (Mþ). lmax,abs in THF:378 nm, 542 nm. lmax,em in THF:740 nm. 2.3. Fabrication of dye-sensitized solar cells The preparation of TiO2 electrodes and the fabrication of the sealed cells for photovoltaic measurement were performed by following the procedures previously reported by Grätzel and coworkers. Fluorine-doped tin oxide (FTO) glass plates (Pilkington TEC Glass-TEC 8, Solar 2.3 mm thickness) were cleaned in a detergent solution using an ultrasonic bath for 30 min and then rinsed with water and ethanol. Then, the plates were immersed in 40 mm 2.2. Measurement 1 H NMR was recorded with the use of Varian Oxford 300 MHz spectrometer; chemical shifts were reported in ppm units with OH N B + Br R CHO OH (i) (v) N N N S HOOC N CN S S R CHO 2 12 15 HKK-BTZ1 HOOC (ii) N + Br R CHO S N N (v) N N CN S S R CHO 12 4 HKK-BTZ2 16 HOOC N H 6 + I R CHO S N N (v) (iii) N 17 14 HKK-BTZ3 O O O O B O N (iv) (v) N R CHO + Br R CHO CN S S N R CHO N N S HOOC S S O CN N O O 18 12 19 HKK-BTZ4 N R= S S N S Scheme 1. Chemical structures and synthesis of HKK-BTZ dyes. (i) Pd(PPh3)4, Na2CO3, THF, Toluene; (ii) Tris(o-tolyl)phosphine, Pd(OAc), Triethylamine, DMF; (iii) Pd(PPh3)2Cl2 , CuI, PPh3, Triethylamine; (iv)Pd(PPh3)4, Na2CO3, Toluene, EtOH; (v) Cyanoacetic acid, piperidine, CHCl3, Acetonitrile). 195 D.H. Lee et al. / Dyes and Pigments 91 (2011) 192e198 3. Results and discussion Scheme 1 shows the synthetic protocol used for the p-extended BTZ dyes. The absorption, emission, and electrochemical properties of the HKK-BTZ1-4 are listed in Table 1. Fig. 1 shows the UVeVis spectra of the dyes in THF solutions, where the compounds exhibit two absorption maxima in regions of 361e378 nm and 533e542 nm, corresponding to triphenylamine unit to p-extended benzothiadiazole charge transfer transitions, respectively. It should be pointed out that their UVevisible absorption behavior was quite different from the similar dye structures based on fused thiophene derivatives with triphenlyamine unit [42]. It may indicate that the HKK-BTZ 1-4 (Dp-A) organic dyes have a tilted structure between triphenylamine and p-extended benzothiadiazole units, yielding two strong absorption bands, which is in a good agreement with the result from Table 1 Absorption, emission and electrochemical properties of the HKK-BTZ dyes. Absorption Dye Emission Electrochemical data lmaxa/nm lmaxb/nm lmaxc/nm EOxb/V E0-0c/V ELUMOd /V (3b/M 1cm 1) (TiO2) (vs NHE) (vs NHE) HKK-BTZ1 361(22047), 533(30791) HKK-BTZ2 391(20840), 546(23198) HKK-BTZ3 359(24651), 515(27169) HKK-BTZ4 378(22581), 542(31035) a 518 745 1.05 1.97 0.92 528 760 0.93 1.93 1.01 501 666 1.19 2.11 0.92 525 740 0.98 1.93 0.95 Absorption was measured in THF solutions (1.0  10 5 M) at room temperature. The oxidation potential of the dye on TiO2 was measured in acetonitrile with 0.1 M TBAPF6 with a scan rate between 50 mV s 1 (working electrode and counter electrode: Pt wires, and reference electrode: Ag/AgCl). c E0-0 was determined from intersection of absorption and emission spectra in THF. d LUMO was calculated by EoxeE0-0. b 1.4 HKK-BTZ1 HKK-BTZ2 HKK-BTZ3 HKK-BTZ4 1.2 1.0 Abs. TiCl4 (aqueous) at 70  C for 30 min and washed with water and ethanol. A transparent nanocrystalline layer was prepared on the FTO glass plates by a doctor blade method using TiO2 paste (Solaronix, Ti-Nanoxide T/SP), which was then dried for 2 h at 25  C. Then 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 200 surface profilometer (Tencor Instruments, San Jose, CA). A paste containing 400 nm sized anatase particles (CCIC, PST-400C) was deposited by means of doctor blade method on top of the transparent TiO2 electrodes, to obtain a scattering layer. The deposited film then dried for 2 h at 25  C. The TiO2 electrodes were treated again with TiCl4 at 70  C for 30 min and sintered at 500  C for 30 min. The resulting film was composed of 8 mm thick transparent layer and 8 mm thick scattering layer. The, films were immersed in dye solution (0.3 mM dye and 80 mM DCA in THF) of HKK-BTZ1, HKK-BTZ-2, HKK-BTZ3 and HKK-BTZ4 and kept at room temperature for 24 h. FTO plates for the counter electrodes were cleaned in an ultrasonic bath in H2O, acetone, and 0.1 M aqueous HCl, respectively. The counter electrodes were prepared by placing a drop of an H2PtCl6 solution (2 mg Pt in 1 mL ethanol) on an FTO plate and heating it (at 400  C) for 15 min. The dye-adsorbed TiO2 electrodes and the Pt counter electrodes were assembled into a sealed sandwich-type cell by heating at 80  C, using a hot-melt ionomer film (Surlyn) as a spacer between the electrodes. A drop of the electrolyte solution was placed in the drilled hole of the counter electrode and was driven into the cell via vacuum backfilling. Finally, the hole was sealed using additional Surlyn and a cover glass (0.1 mm thickness). 0.8 0.6 0.4 0.2 0.0 300 400 500 600 700 800 Wavelength(nm) Fig. 1. Absorption spectra of HKK-BTZ dyes in THF at room temperature (conc. ¼ 1.0  10 5 M). DFT calculation. The UV/visible spectrum of HKK-BTZ1 displays two absorption maxima at 533 nm (e ¼ 30,791 dm3 mol 1 cm 1) and 361 nm, which are assigned as the pep* transitions of the conjugated system. When a triphenylamine unit is bridged to benzothiadiazole with a double bond, the absorption maximum is red-shifted and the e value is decreased compared to the HKK-BTZ1 dye that has a single bond. Also, the absorption maximum of HKK-BTZ3 that contains a triple bond is blue-shifted relative to HKK-BTZ1 and HKKBTZ2 sensitizers. This may be attributed to the fact that, in the alkene bridged chromophores, all the carbon atoms on the branches are sp2 hybridized to give a relatively longer conjugation. However, in the case of the alkyne chromophores, the carbon atoms are in both sp and sp2 hybridized. This results in poorer p-orbital overlap and mismatch in energy of the p-orbitals, leading to a blue shift [43,44]. HKK-BTZ4 dye with bulky alkoxy group was red-shifted, compared to HKK-BTZ1, due to the strong donating ability of the bulky alkoxy group. The donating effect has the more influence on triphenylamine unit rather than the benzothiadiazole unit, due to the direct attachment of the bulky alkoxy group onto triphenylamine unit, resulting in more red-shift. Electrochemical properties of the HKK-BTZ1, HKK-BTZ2, HKKBTZ3, and HKK-BTZ4 were scrutinized by cyclic voltammetry in acetonitrile with 0.1 M tetrabutylammonium hexafluorophosphate by adsorbing onto TiO2 films and the results are summarized in Table 1. The estimated LUMO of the HKK-BTZ1, HKK-BTZ2, HKKBTZ3, and HKK-BTZ4 from the oxidation potential and the energy at the intersection point of absorption and emission spectra are 0.92 V, 1.01 V, 0.92 V and 0.95 V vs NHE, respectively. Surprisingly, the LUMO level of HKK-BTZ3 is lower than that of HKK-BTZ2, while the LUMO level of HKK-BTZ3 is expected to be higher than that of HKK-BTZ2 as follows: The absorption maximum of HKK-BTZ3 is blue-shifted relative to HKK-BTZ2 sensitizer. It leads to a larger band-gap between HOMO and LUMO level, so that the LUMO level generally rises up and the HOMO level goes down further. By contrast, it may be ascribed to the more electron negativity of sp character in HKK-BTZ3 than that of sp2 character in HKK-BTZ2, leading to the lower LUMO level [45]. In addition, introducing the bulky alkoxy substituent has a strong influence on the ground-state oxidation potentials and HOMO potentials and HOMO but a small effect on the ground-state reduction potentials and LUMO. The strong influence on the HOMO by introducing a stronger donor narrows the energy-gap of organic 196 D.H. Lee et al. / Dyes and Pigments 91 (2011) 192e198 HKK-BTZ1 HKK-BTZ2 HKK-BTZ3 HKK-BTZ4 Current Density (mA/cm2) 20 Dye DCA JSC (mA/cm2) VOC (V) FF h (%) HKK-BTZ1 0 mM 40 mM 80 mM 80 mM 80 mM 80 mM 0 mM 80 mM 11.9 13.9 15.0 10.6 13.0 17.9 17.2 17.5 0.54 0.56 0.58 0.54 0.56 0.62 0.71 0.62 0.59 0.61 0.65 0.59 0.63 0.66 0.72 0.72 3.81 4.81 5.72 3.37 4.55 7.30 8.97 7.82 15 HKK-BTZ2 HKK-BTZ3 HKK-BTZ4 N719 10 TiO2 thickness: 16 mm (8 mm þ 8 mm: active layer þ scattering layer); working area: 0.16 cm2, electrolyte condition: 0.6 M DMPII, 0.1 M LiI, 0.05 M I2, 0.5 M TBP in acetonitrile solution, 0.3 mM of dye was dissolved in THF. 5 0 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 Voltage (V) Fig. 2. Photocurrentevoltage characteristics of representative TiO2 electrodes sensitized with HKK-BTZ dyes under AM 1.5 simulated sunlight (100 mW/cm2). sensitizers. HKK-BTZ4 dye with bulky alkoxy group has the HOMO level of 0.98 V (vs NHE) and the LUMO level of 0.95 V (vs NHE). This is quite same from the previous result of the similar dye structures based on fused thiophene derivatives with triphenlyamine unit reported by Peng group [42]. All LUMO are all higher than the conduction band edge of TiO2 film, providing sufficient thermodynamic driving force for electron injection from the excited dyes to TiO2 film. The photovoltaic performance characteristics of HKK-BTZ1- 4 were shown in Figs. 2 and 3. The incident monochromatic photonto-current efficiency (IPCE) and currentevoltage (J/V) characteristics were obtained with a sandwich cell comprising of 0.6 M 1,2-dimetyl-3-propyl imidazolium iodide, 0.05 M iodine, 0.1 M LiI, and 0.5 M tert-butylpyridine in acetonitrile. It was introduced into the inter-electrode space from the counter electrode side through predrilled holes. The drilled holes were sealed with a microscope cover slide and Surlyn to avoid leakage of the electrolyte solution. Typically, both the incident photon-to-electron conversion efficiency (IPCE) and the photocurrents in organic-based devices have been improved by the addition of deoxycholic acid (DCA) to break up dye aggregates [46e49]. Such an unwanted redox process is HKK-BTZ1 HKK-BTZ2 HKK-BTZ3 HKK-BTZ4 70 60 50 IPCE(%) Table 2 Dye sensitized solar cell performance data of HKK-BTZ dyes. 40 30 retarded by the hydrophobic spacer and, as a result, the dark current is reduced and high open circuit voltage (VOC) is obtained [50e53]. Table 2 shows the effect of the co-adsorbent DCA concentration on the HKK-BTZ1 sensitized cell performance on under standard global AM 1.5 solar condition. As the co-adsorbent DCA concentration increases, the HKK-BTZ1 sensitized cell performance enhances by slowing charge recombination, due to the prevention of dye aggregation [46,47]. At the co-adsorbent DCA concentration of 80 mM, the HKK-BTZ1 sensitized cell gave a better result with the short circuit photocurrent density (JSC) of 15.0 mA cm 2, open circuit voltage (VOC) of 0.58 V, and a fill factor (FF) of 0.65, corresponding to an overall conversion efficiency h of 5.72%, derived from the equation: h ¼ JSCVOCFF/light intensity. Under the same conditions, the HKK-BTZ2 sensitized cell gave a JSC of 10.6 mA cm 2, VOC of 0.54 V, and a FF of 0.59, and the HKK-BTZ3 sensitized cell gave a JSC of 13.0 mA cm 2, VOC of 0.56 V, and FF of 0.63, corresponding to an overall conversion efficiency h of 3.37% and 4.55%, respectively. However, The IPCE data of HKK-BTZ4 plotted as a function of excitation wavelength exhibits a wide wavelength over 800 nm. The important red shift in the photocurrent response is attributed to the introduction of alkoxy group of TPA donor unit, because the bulky alkoxy group is a strong donating group for the more red shift and for reducing aggregation of dyes. The HKK-BTZ4 sensitized cell under standard global AM 1.5 solar condition gave a JSC of 17.9 mA cm 2, VOC of 0.62 V, and FF of 0.66, corresponding to an overall conversion efficiency h of 7.30%, while the Ru dye N719sensitized TiO2 showed an efficiency of 7.82% with a JSC of 17.5 mA/ cm 2, a VOC of 0.62 V, and a FF of 0.72, under the same DCA concentration of 80 mM (see Table 2). To get a further insight into the difference in performance of DSSCs sensitized by all the dyes, density functional theory (DFT) calculations were performed at B3LYP/6-31G* level and at TDDFT calculations performed by MPW1 K/631G* in THF by means of CPCM model for the geometry optimization. The DFT/TDDFT calculations performed provide useful insights into the molecular and electronic structures of the dyes [54]. For HKK-BTZ1, we calculated (Table 3) absorption wavelenghts of 574 (protonated) and 544 nm (deprotonated) compared to the experimental value of 533 nm, possibly suggesting the dissociation of the cyanoacrylic 20 Table 3 Computed lmax (nm), and HOMO/LUMO energies (eV) for the (a) protonated and (b) deprotonated HKK-BTZ dyes in THF. 10 0 300 400 500 600 700 800 900 Wavelength(nm) Fig. 3. Typical action spectra of incident photon-to-current conversion efficiencies (IPCE) obtained for nanocrystalline TiO2 solar cells sensitized by HKK-BTZ dyes. Dye lmax (nm) HKK-BTZ1 HKK-BTZ2 HKK-BTZ3 HKK-BTZ4 574a, 544b 571 551 554 3HOMO/3LUMO (eV) 5.66b/ 5.54/ 5.69/ 5.50/ 2.26b 2.28 2.34 2.25 D.H. Lee et al. / Dyes and Pigments 91 (2011) 192e198 197 However, in the case of the alkyne chromophores, the carbon atoms are in both sp and sp2 hybridized. It results in poorer porbital overlap and mismatch in energy of the p-orbitals, leading to a blue shift. Also, the D-p-A system shows easily polarizable and electron-deficient bridge between the push-pull chromophores. The photovoltaic performance based on the single-bond bridged unit in HKK-BTZ1 dye is better than double and triple bond bridging, due to the higher charge separation. The introduction of bulky alkoxy group to TPA donor has a strong donating effect for the more red shift and for reducing aggregation of dyes. And the introducing of the bulky alkoxy substituent could lead to a fast dyeregeneration in order to avoid the geminate charge recombination between oxidized sensitizers and photoinjected electrons in the nanocrystalline TiO2 film, thus enhancing the HKK-BTZ1 sensitized cell performance. The HKK-BTZ4 sensitized cell under standard global AM 1.5 solar condition exhibited the better photovoltaic performance with JSC of 17.9 mA cm 2, VOC of 0.62 V, and a FF of 0.66, corresponding to an overall conversion efficiency h of 7.30%, while the Ru dye N719sensitized TiO2 showed an efficiency of 7.82% with a JSC of 17.5 mA/ cm 2, a VOC of 0.62 V, and a FF of 0.72. Acknowledgements Fig. 4. Plots of the isodensity surfaces (MPW1K/6-31G* in THF) of HOMO and LUMO of HKK-BTZ1, HKK-BTZ2 and HKK-BTZ3. (The HOMO and LUMO of HKK-BTZ4 have the same spatial distribution of HKK-BTZ1.) acid in THF. For the deprotonated HKK-BTZ2 and HKK-BTZ3 dyes, we obtained absorption maxima at 571 and 551 nm, respectively, which confirm the red-shifted absorption in HKK-BTZ2 and the blue-shift in HKK-BTZ3 compared to HKK-BTZ1, respectively. Introduction of OeR substituents to the TPA moiety of HKK-BTZ1 leads in HKK-BTZ4 to a slight red-shift of the absorption spectrum, perfectly in line with the experimental trend. This red-shift is essentially due to the TPA-based HOMO destabilization in HKKBTZ4, due to the electron donating effect of the OeR substituents, while the LUMO energy is essentially unaltered, see Table 3. The optimized structures of the present dyes revealed that the introduction of both double and triple bonds forces the N-phenyl of TPA to be coplanar with the BTZ and cyanoacrylic units, while in HKKBTZ1 they are distorted by ca. 22 . Thus, the HOMO in HKK-BTZ2 and HKK-BTZ3 result to be delocalized from the donor to the acceptor (Fig. 4), reducing the effectiveness of charge separation and yielding to lower IPCE values and poorer performances. 4. Conclusions A series of new p-conjugated metal-free organic dyes, comprising triphenylamine (TPA) moieties as the electron donor and benzothiadiazole moieties as the electron acceptor/anchoring groups, showed red-shift of absorption band in UVevisible spectrum because of long p conjugation and narrow band gap. The absorption maximum of the HKK-BTZ2 dye with the double-bond bridged unit is red-shifted, compared to the HKK-BTZ1 dye that has a single bond. Also, the absorption maximum of HKK-BTZ3 that contains a triple bond is blue-shifted relative to HKK-BTZ1 and HKK-BTZ2 sensitizers. This may be due to the fact that, in the alkene chromophore, all the carbon atoms on the branches are sp2 hybridized to give a relatively longer conjugation. 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