Optimization of Multiple Electron Donor and Acceptor in Carbazole

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Optimization of Multiple Electron Donor and Acceptor in Carbazole-Triphenylamine-Based Molecules for Application of Dye-Sensitized Solar Cells Chien-Hsin Yang,*,† Shao-Hong Liao,† Yu-Kuang Sun,† Yao-Yuan Chuang,‡ Tzong-Liu Wang,† Yeong-Tarng Shieh,† and Wen-Churng Lin§ Department of Chemical and Materials Engineering and Department of Applied Chemistry, National UniVersity of Kaohsiung, Kaohsiung 811, Taiwan, and Department of EnVironmental Engineering, Kun Shan UniVersity, Tainan 710, Taiwan ReceiVed: July 7, 2010; ReVised Manuscript ReceiVed: NoVember 7, 2010

Organic dyes have been synthesized which containing multiple electron donors (carbazole) and electron acceptors (rhodaniline-3-acetic acid) on triphenylamine (TPA). Photophysical, electrochemical, and theoretical computational methods have categorized these compounds. Nanocrystalline TiO2-based dye-sensitized solar cells (DSSCs) are fabricated using these dye molecules as light-harvesting sensitizers. The overall efficiency of sensitized cells is high (4.64%) as compared to a cis-di(thiocyanato)-bis(2,2′-bipyridyl)-4,4′-dicarboxylate ruthenium(II) (N3 dye)-sensitized device (7.83%) fabricated and measured under the same conditions. Both electron donor (carbazole) and acceptor (rhodaniline-3-acetic acid) play a key role in the increased efficiency. One carbazole and two rhodaniline-3-acetic acid-based dye appears to help convey the charge transfer from the excited dye molecules to the conduction band of TiO2, leading to a higher efficiency of the assembled devices using such a dye. Electrochemical impedance measurements support this dye’s effect on enhancing charge transfer of TiO2 (e-). Computation on this CTPAR2 compound also indicates a larger charge transfer efficiency in the electronically excited state. Introduction Over the past two decades, the first reported TiO2 nanostructural photovoltaic device with an efficiency of 7.1-7.9% under similated solar light,1,2 a dye-sensitized solar cell (DSSC), has shown the ability to convert solar energy into electricity. The inorganic-organic hybrid DSSCs have been extensively investigated in an effort to design more efficient dyes and electron mediators,2 to fabricate better nanostructured films,3 and to increase an understanding of the interfacial charge-transfer process.3-5 In a DSSC device, light is absorbed by the dye anchored on the TiO2 surface and then electrons from the excited dye inject into the conduction band of the TiO2, generating an electric current. At the same time, the ground state of the dye is regenerated by the electrolyte to give efficient charge separation.6 Thus, the dye in DSSCs is essential for efficient light harvesting and electron generation/transfer. The electrolyte, containing a redox couple I3-/I- mediator, is injected into the slit between the anode and the catalytic Pt counter electrode. A dye sensitizer needs to have efficient charge injection with photoexcitation.2 To obtain high conversion efficiencies, the photogenerated electrons must flow into the oxide film with minimal losses to interfacial recombination.3-5 The typical Ru complex sensitizers such as N3, N719, and black dye have a demonstrated photoconversion efficiency of up to 11% under AM 1.5 irradiation,2 these sensitizers are grafted onto the semiconductor through anchoring groups, such as carboxylate, which bind strongly to the oxide by coordinating the titanium ions on the surface. However, the costs associated * Corresponding author. Tel: 886-7-5919420. E-mail:[email protected]. † Department of Chemical and Materials Engineering, National University of Kaohsiung. ‡ Department of Applied Chemistry, National University of Kaohsiung. § Kun Shan University.

with large-scale production are prohibitive, so organic dyes have been synthesized as an alternative.7-12 An organic sensitizer is designed to link the electron donor and the electron acceptor. A TiO2 surface anchoring group, such as carboxylate, is integrated at the acceptor end. When irradiated, these dipolar molecules induce intramolecular charge transfer from the donor to the acceptor. Then, the electron is injected into the TiO2 via the anchoring group. Several dipolar organic dyes have been reported as sensitizers in DSSCs, including coumarin-,8 perylene-,7 xanthene-,13 thiophene-,14 oligoene-,15 indoline,11,12 and triarylamine-based16,17 dyes. An ideal sensitizer needs to meet a sufficiently high LUMO energy level for efficient electron injection into the TiO2 and a sufficiently low HOMO energy level for efficient regeneration of the oxidized state. This process also should have an absorption band ranging from the visible region to near-IR as well as sufficient spatial separation between the positive charge density on the dye and the electron injection. We recently reported a series of efficient DSSCs using new dipolar triarylamine/rhodanine-3-acetic acid sensitizers.16,17 Though there are several reported examples of incorporating the carbazole ring in organic18 and ruthenium-complex19 sensitizers, use of carbazole incorporating in triarylamine/rhodanine-3-acetic acid (TPAR)-based dye is still rare. In this work, we used carbazole as the extra-electron donor of dipolar TPAR sensitizer because carbazole can provide more effective electron density to the triarylamine donor center. Furthermore, one or two of the rhodanine-3-acetic acids were strategically incorporated with the triarylamine donor at the C2 and C3 positions (see Scheme 1), in such a way that delocalization of the positive charge between the C2 and C3 substituents, after electron injection, prevented electron back transfer from TiO2 to the oxidized dye. Moreover, the hydrophobic nature of the carbazole substituent may suppress electron transfer from TiO2 to the electrolyte (i.e., dark current). Herein, we report the photophysical and electro-

10.1021/jp106275v  2010 American Chemical Society Published on Web 11/23/2010

Carbazole-Triphenylamine-Based Molecules

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SCHEME 1: Synthesis of CTPAR Dyes

chemical characteristics of a new series of carbazole/triarylamine/rhodanine-3-acetic acid dyes (CTPAR). The effect of carbazole and rhodanine-3-acetic acid numbers in the dyes is discussed in detal in this study. Using these dyes as the sensitizers, DSSCs were also assembled. Study of electrochemical impedance is in support of the preventive charge transfer of TiO2 (e-) with both oxidized sensitizer and I3-. Experimental Section General Information. All reactions and manipulations were carried out under N2 using standard inert atmosphere and Schlenk techniques. Solvents were dried by standard procedures. All column chromatography was performed under N2 using silica gel (230-400 mesh, Macherey-Nagel GmbH& Co.) as stationary phase in a column 30 cm long and 2.0 cm in diameter. The starting materials 4-formaltriphenylamine (1) and 4,4′diformaltriphenylamine (2) were prepared by adopting previous work.16 The synthetic pathway of the three dyes (CTPAR1, CTPAR2, and CTPAR3) is shown in Scheme 1. 4-Formyltriphenylamine (1). A Vilsmeier reagent was first prepared as follows: phosphorus oxychloride (3.13 g, 20.38 mmol) was added dropwise to N, N-dimethylformamide (DMF, 80 mL) in ice bath, and then the temperature was risen to 25 °C and kept mixing for an additional 1 h. At 25 °C, triphenylamine (5 g, 20.38 mmol) was slowly added over 15 min to the above Vilsmeier reagent and stirred overnight. The reaction was quenched by the addition of cooled aqueous sodium acetate solution (3 M) in ice bath. The precipitate product was collected and recrystallized with ethanol. Yield: 4.57 g (82%). Green solid. 1 H NMR (DMSO-d6): δ 6.88 (d, J ) 8.8 Hz, 2H, ArH), δ 7.18-7.24 (q, 6H, ArH), δ 7.42 (t, J ) 8.0 Hz, 4H, ArH), δ 7.71 (d, J ) 8.4 Hz, 2H, ArH), δ 9.76 (s, 1H, sCHdO). 4,4′-Diformyltriphenylamine (2). A Vilsmeier reagent was first prepared as follows: phosphorus oxychloride (7.81 g, 50.94

mmol) was added dropwise to DMF (80 mL) in ice bath, and then the temperature was increased to 25 °C and left to mix overnight. Triphenylamine (5 g, 20.38 mmol) was slowly added over 15 min to the above Vilsmeier reagent at 25 °C and refluxed at 95 °C for 4 h. The reaction solution was quenched by the addition of cooled aqueous sodium acetate solution (3 M). The product was extracted repeatedly from the above mixture with dichlorometane and collected the organic phase. After concentrating the product solution under vacuum, it was then purified by column chromatography on silica gel using n-hexane/ethyl acetate mixture (4:1) as eluent. Yield: 3.62 g (59%). Yellow solid. 1H NMR (DMSO-d6): δ 7.154 (d, J ) 8.8 Hz, 4H, ArH), δ 7.19-7.21 (q, 2H, ArH), δ 7.31 (d, J ) 7.2 Hz, 1H, ArH), δ 7.43-7.48 (m, 2H, ArH), δ 7.81-7.85 (m, 4H, ArH), δ 9.87 (s, 2H, sCHdO). 4-((4-Bromophenyl)phenylamino)benzaldehyde (3). A DMF (90 mL) solution of 1 (5.1 g, 18.67 mmol) was cooled to -10 °C under N2 atmosphere. A DMF (60 mL) solution of Nbromosuccinimind (NBS, 3.33 g, 18.70 mmol) was added dropwise over 30 min to the above mixture while the temperature was risen and kept at 50 °C. After reacting for overnight, 2 N HCl aqueous solution was added at once to quench the reaction. The organic product was extracted with diethyl ether. The collected diethyl ether extracts were dried over MgSO4 and filtered, and evaporation of the filtrate produced the product. Yield: 5.85 g (89%). Yellow solid. 1H NMR (DMSO-d6): δ 6.92 (d, 2H, ArH), δ 7.08-7.11 (d, 2H, ArH), δ 7.11-7.19 (d, 2H, ArH), δ 7.21-7.25 (t, 1H, ArH), δ 7.21-7.25 (t, 1H, ArH), δ 7.39-7.43 (t, 2H, ArH), δ 7.54-7.57 (d, 2H, ArH), δ 7.71-7.74 (d, 2H, ArH), δ 9.78 (s, 1H, sCHdO). 4-Bromo-4′,4′′-diformyltriphenylamine (4). A DMF (60 mL) solution of 2 (2.0 g, 6.6 mmol) was cooled to -10 °C under N2 atmosphere. A DMF (22 mL) solution of NBS (1.19 g, 6.68 mmol) was added dropwise over 30 min to the above mixture

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while the temperature was increased and kept at 50 °C. After reacting overnight, 2 N HCl aqueous solution was added at once to quench the reaction. The organic product was extracted with diethyl ether. The collected diethyl ether extracts were dried over MgSO4 and filtered, and evaporation of the filtrate produced the product. Yield: 2.28 g (90%). Yellow solid. 1H NMR (DMSO-d6): δ 7.12-7.19 (d, 2H, ArH), δ 7.25-7.27 (d, 2H, ArH), δ 7.59-7.63 (d, 2H, ArH), δ 7.82-7.91 (d, 2H, ArH), δ 9.90 (s, 1H, sCHdO). 4,4′-Dibromo-4′′-formyltriphenylamine (5). A DMF (70 mL) solution of 1 (3.0 g, 10.9 mmol) was cooled to -10 °C under N2 atmosphere. A DMF (60 mL) solution of N-bromosuccinimind (NBS, 3.9 g, 21.8 mmol) was added dropwise over 30 min to the above mixture while the temperature was risen and kept at 50 °C. After reacting for overnight, 2 N HCl aqueous solution was added at once to quench the reaction. The organic product was extracted with diethyl ether. The collected diethyl ether extracts were dried over MgSO4 and filtered, and evaporation of the filtrate produced the product. Yield: 4.04 g (85%). Yellow viscous liquid. 1H NMR (DMSO-d6): δ 6.97-6.99 (d, 2H, ArH), δ 7.08-7.12 (d, 4H, ArH), δ 7.54-7.58 (d, 4H, ArH), δ 7.73-7.76 (d, 2H, ArH), δ 9.80 (s, 1H, sCHdO). 4-((4-(9H-Carbazol-2-yl)phenyl)phenylamino)benzaldehyde (6). A tetrahydrofuran (THF, 80 mL) solution of 2-(4,4,5,5tetramethyl-1,3,2-dioxaborolan-2-yl)-9H-carbazole (2.2 g, 7.51 mmol) was stirred at 25 °C under N2 atmosphere. 3 (2.61 g, 7.41 mmol) and a 2 M K2CO3 aqueous solution (10 mL) were added to the above solution. At 30 °C, a catalyst Pd(PPh3)4 (0.03 g) was added to the above mixture while the temperature was increased to 55 °C. After reacting for 72 h, 2 N HCl aqueous was added at once to quench the reaction. The organic product was extracted with diethyl ether. The collected diethyl ether extracts were dried over MgSO4 and filtered, and evaporation of the filtrate produced the crude product, which was further purified by column chromatographty on silica gel using a n-hexane/dichloromethane (1:3) as eluent: Yield: 1.81 g (56%). Yellow solid. 1H NMR (DMSO-d6): δ 9.78 (s, 1H, sCHdO), δ 11.31 (s, 1H, C2-NH). 4,4′-(4-(9H-Carbazol-2-yl)phenylazanediyl)dibenzaldehyde (7). A THF (80 mL) solution of 2-(4,4,5,5-tetramethyl-1,3,2dioxaborolan-2-yl)-9H-carbazole (1.36 g, 4.65 mmol) was stirred at 25 °C under N2 atmosphere. 4 (1.76 g, 4.61 mmol) and a 2 M K2CO3 aqueous solution (10 mL) were added to the above solution. At 30 °C, a catalyst Pd(PPh3)4 (0.03 g) was added to the above mixture while temperature was increased to 55 °C. After reacting for 72 h, 2 N HCl aqueous was added at once to quench the reaction. The organic product was extracted with diethyl ether. The collected diethyl ether extracts were dried over MgSO4 and filtered, and evaporation of the filtrate produced the crude product, which was further purified by column chromatographty on silica gel using a n-hexane/dichloromethane (1:4) as eluent: Yield: 1.20 g (56%). Yellow solid. 1H NMR (DMSO-d6): δ 9.90 (s, 2H, sCHdO), δ 11.32 (s, 1H, C2N-H). 4-(Bis-(4-(9H-carbazol-2-yl)-phenyl)-amino)benzaldehyde (8). A tetrahydrofuran (THF, 120 mL) solution of 2-(4,4,5,5tetramethyl-1,3,2-dioxaborolan-2-yl)-9H-carbazole (2.82 g, 9.65 mmol) was stirred at 25 °C under N2 atmosphere. 5 (2.09 g, 4.82 mmol) and a 2 M K2CO3 aqueous solution (10 mL) were added to the above solution. At 30 °C, a catalyst Pd(PPh3)4 (0.06 g) was added to the above mixture while temperature was increased to 55 °C. After reacting for 72 h, 2 N HCl aqueous was added at once to quench the reaction. The organic product was extracted with diethyl ether. The collected diethyl ether

Yang et al. extracts were dried over MgSO4 and filtered, and evaporation of the filtrate produced the crude product, which was further purified by column chromatographty on silica gel using a n-hexane/dichloromethane (3:4) as eluent: Yield: 1.36 g (47%). Yellow solid. 1H NMR (DMSO-d6): δ 9.81 (s, 1H, sCHdO), δ 11.32 (s, 2H, C2-N-H). (5-(4-((4-(9H-Carbazol-2-yl)phenyl)phenylamino)-benzylidene)4-oxo-2-thioxo-thiazolidin-3-yl)-acetic acid (CTPAR1). To 100 mL of glacial acetic acid were added 1.0 g (2.28 mmol) of 6 and 0.44 g (2.30 mmol) of rhodanine-3-acetic acid, and the solution was refluxed for 3 days in the presence of 1.5 g of ammonium acetate. After cooling to 2 °C, the precipitate was filtered and washed with cooled distilled water (2 °C) and then dried under vacuum to obtain red crystals of CTPAR1 (Yield: 1.22 g, 88%). 1H NMR (DMSO-d6): δ (ppm) 4.71-4.81 (s, 2H, N-CH2sC), δ11.33(s, 1H, C2-NH), δ13.41 (s, 1H, -COOH). 13 C NMR (DMSO-d6): δ (ppm) 45.0 (C1), 108.5 (C2), 110.9 (C3), 117.3 (C4), 117.6 (C5), 118.7 (C6), 119.7 (C7), 120.2 (C8), 120.6 (C9), 121.7 (C10), 122.1 (C11,11′), 124.7 (C12,12′), 125.4 (C13,13′), 126.2 (C14), 128.3 (C15), 130.0 (C16), 132.9 (C17,17′), 133.9 (C18), 136.9 (C19), 137.7 (C20), 140.2 (C21), 140.3 (C22), 144.5 (C23), 145.4 (C24), 150.0 (C25), 166.4 (C26), 167.3 (C27), 192.8 (C28). (5-(4-((4-(9H-Carbazol-2-yl)phenyl)-(4-(3-carboxymethyl-4oxo-2-thioxo-thiazolidin-5-ylidenemethyl)phenyl)amino)benzylidene)4-oxo-2-thioxo-thiazolidin-3-yl)-acetic acid (CTPAR2). To 100 mL of glacial acetic acid were added 1.0 g (2.14 mmol) of 7 and 0.83 g (4.34 mmol) of rhodanine-3-acetic acid, and the solution was refluxed for 3 days in the presence of 1.5 g of ammonium acetate. After cooling to 2 °C, the precipitate was filtered and washed with cooled distilled water (2 °C) and then dried under vacuum to obtain red crystals of CTPAR2 (Yield: 1.44 g, 83%). 1H NMR (DMSO-d6): δ 4.79 (s, 4H, N-CH2-C), δ 11.32 (s, 1H, C2-NH). 13C NMR (DMSO-d6): δ (ppm) 45.0 (C1,1′), 108.5 (C2,2′), 110.9 (C3), 117.3 (C4,4), 117.6 (C5), 119.2 (C6), 120.2 (C7), 120.8 (C8), 121.9 (C9), 122.1 (C10), 123.2 (C11,11′), 125.7 (C12,12′, 12“,12”′), 127.4 (C13,13′), 128.3 (C14,14′), 132.8 (C15,15′,15“,15”′), 133.4 (C16), 136.5 (C17,17′), 138.6 (C18), 140.4 (C19), 144.0 (C20,20′), 148.2 (C21,21′,21”), 166.4 (C22,22′), 167.3 (C23,23′), 192.8 (C24,24′). (5-(4-(Bis-(4-(9H-carbazol-2-yl)phenylamino)benzylidene)4-oxo-2-thioxo-thiazolidin-3-yl)-acetic acid (CTPAR3). To 100 mL of glacial acetic acid were added 0.8 g (1.32 mmol) of 8 and 0.26 g (1.36 mmol) of rhodanine-3-acetic acid, and the solution was refluxed for 3 days in the presence of 1.3 g of ammonium acetate. After cooling to 2 °C, the precipitate was filtered and washed with cooled distilled water (2 °C) and then dried under vacuum to obtain red crystals of CTPAR3 (Yield: 0.80 g, 78%). 1H NMR (DMSO-d6): δ 4.71 (s, 2H, N-CH2-C), δ 11.32 (s, 2H, C2-NH). 13C NMR (DMSO-d6): δ (ppm) 45.0 (C1,1′), 108.5 (C2,2′,2“,2”′), 110.9 (C3,3′), 117.6 (C4), 118.7 (C5,5′), 120.2 (C6,6′), 120.6 (C7,7′), 121.8 (C8,8′,8“,8”′), 122.1 (C9,9′), 125.6 (C10,10′,10“,10”′), 126.3 (C11,11′), 126.5 (C12,12′, 12“,12”′), 128.4 (C13), 131.4 (C14,14′), 133.0 (C15,15′), 133.8 (C16,16′), 136.9 (C17,17′), 138.0 (C18,18′), 140.3 (C19), 144.5 (C20,20′,20”), 166.5 (C21), 167.5 (C22), 192.4 (C23). TiO2 Mesoporous Electrode. The hydrothermal processed TiO2 colloid was synthesized. In brief, 3 g of TiO2 nanoparticles (P25, Degussa AG, Germany, a mixture of ca. 30% rutile and 70% anatase) were dispersed in 100 mL of 10 N NaOH and heated to 130 °C in autoclave for 1 day. The precipitate was then redispersed in 100 mL of 1 N HNO3. This suspension was subsequently subjected to autoclaving at 240 °C for 12 h to give the TiO2 colloid. The TiO2 specimens were baked at 450

Carbazole-Triphenylamine-Based Molecules °C in air for 30 min. The Raman spectra of TiO2 nanoparticles, revealing that there existed a pure anatase phase corresponding to the wavenumbers of 150, 400, 517, and 638 cm-1. X-ray diffraction (XRD) patterns of the titanate-derived TiO2 also proved a pure anatase phase. The average crystal size was calculated, according to Scherrer’s equation, to be approximately 20 nm. The solution of TiO2 colloid was mixed with polyethylene glycol (PEG-2000) (Fluka) to form a viscous TiO2 dispersion at a 0.15 of PEG/TiO2 ratio, which was spin-coated onto a fluorine-doped tin oxide (FTO)-coated glass (Merck, sheet resistance of 10 Ω/sq) to form a TiO2 film of 0.25 cm2. The thickness of TiO2 film was controlled at ca. 7 µm. The film was dried in air at 120 °C for 30 min and calcined at 450 °C for 30 min. Assembly and Characterization of DSSCs. The counter electrode of platinum was coated onto an FTO glass (10Ω/ square, Hartford) by spin-coated processes. The dye-loaded anode and a Pt counter electrode were sealed together with a sealing material, SX1170 (Solaronix), around the TiO2 active area of 0.25 cm2. The electrolyte contained 0.6 M 1-propyl2,3-dimethylimidazolium iodide (DMPII), 0.1 M lithium iodide, 0.05 M iodine, and 0.5 M 4-tert-butylpyridine (TBP) in 3-methoxypropionitrile (MPN). Measurements. 1H and 13C NMR spectra were measured on a Bruker AV400 FT-NMR spectrometer (400 MHz). Infrared spectra were recorded on a Perkin-Elmer RXI FT-IR spectrometer. Absorption spectra were obtained with a Perkin-Elmer Lambda 25 UV-visible spectrophotometer. The thickness of TiO2 films was determined by an R-step instrument (Surfocorder TE 2400M, Tosaka Lab. Ltd.). Electrochemical experiments were performed using the electrochemical analyzer (PGSTAT 30, AUTOLAB Electrochemical Instrument, The Netherlands). All measurements were carried out at room temperature with a conventional three-electrode configuration consisting of a platinum working electrode, a platinum wire auxiliary electrode, and a nonaqueous Ag/AgNO3 reference electrode. The E1/2 values were determined as (Epa + Epc)/2, where Epa and Epc are the anodic and cathodic peak potentials, respectively. The potentials are quoted against the ferrocene internal standard. The solvent in electrochemical experiments was acetonitrile, and the supporting electrolyte was 0.1 M tetrabutyl ammonium perchlorate (Bu4N ClO4). The current density-voltage (J-V) characteristics of the DSSCs were measured using Keithley2400 digital source meter controlled by a computer at a scan rate of 10 mVs-1. An Oriel 500 W xenon lamp served as the light source in connection with an AM 1.5 Globe filter (Oriel 81094) to remove ultraviolet and infrared radiation to give 100 mW cm-2 at the surface of the test cell. Calibration was performed by a USB 400 plug-and-play miniature fiber optic spectrometer (Ocean, USA) to give an AM 1.5 simulated sunlight. The active area of testing cell was 0.25 cm2. Theoretical Approach Methodology. The Gaussian 03 package20 was used for carrying out theoretical calculations. To model the electronic state of TPAR dyes on the TiO2 surface, the dye’s potassium salt was employed to simulate the dye bonded to TiO2 surface in its carboxylate form.16 The B3LYP method with 6-31G(d) basis set was applied to optimize ground state geometries of CTPAR1, CTPAR2, and CTPAR3. The minimum energy structures were confirmed by no imaginary frequency. Results and Discussion Photophysical and Electrochemical Properties. Figure 1A shows the absorption spectra of as-synthesized dyes in ethanol.


Figure 1. (A) Absorption spectra of dyes dissolved in ethanol. (B) Absorption spectra of the as-synthesized dyes adsorbed on TiO2 films, which were measured in the diffuse reflectance mode.

Table 1 displays this data. These dyes exhibit three distinct absorption bands at around 260, 335, and 470 nm, respectively. The absorption peaks at around 335 nm which corresponds to the π f π* electron transition. Strong absorption peaks at around 470 nm can be assigned to an intramolecular charge transfer between the TPA-based donor and the rhodanine-3acetic acid,21 providing efficient charge-separation at an excited state. Compared TPA-rhodaniline-3-acetic acid dyes (TPAR),16 the last two peaks are shifted toward a longer wavelength. This is because the increase of electron donors (carbazole groups) in the dye molecules is beneficial to intramolecular charge transfer. It is noteworthy that there exist weak absorption bands at same wavelength of 260 nm for the three dyes. These bands arise from the split of more congested π f π* transition bands with the introduction of additional electron donors in these dyes. The former two absorption peaks have the same positions, indicating that there are similar conditions of π f π* transition in one-carbazole dyes (CTPAR1 and CTPAR2). In the two-carbazole dye (CTPAR3), the second absorption band is shifted from 335 to 350 nm, implying that the density of π f π* transition is higher than that in one-carbazole dyes. Note that the third absorption band is red-shifted from 470 to 495 nm with an increase in the rhodaniline-3-acetic acid groups (two rhodaniline-3-acetic acid-bearing on CTPAR2). This is due to the increase of electron acceptors (rhodaniline-3-acetic acid) in dye molecules, which enhances intramolecular charge transfer. Moreover, an intermediate shoulder occurred at ca. 435 nm in CTPAR2. This arose from the intervalence charge transfer (IVCT) interactions with the adjacent amino groups in the

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TABLE 1: Maximum Absorption Data, Energy Levels, and Electrochemical Properties of the As-Synthesized Dyes dye

ε, M-1 cm-1

CTPAR1 CTPAR2 CTPAR3

35564 28702 20170

a

UV λmax, nma

UV λmax, nmb

reduction (V vs Ag/Ag+)

oxidation (V vs Ag/Ag+)

HOMO, eV

Eg, eV

LUMO, eV

470 491 477

504 539 510

0.254 0.234 0.184

0.637 0.667 0.758

-4.991 -5.001 -5.092

2.12 1.96 2.01

-2.87 -3.04 -3.08

Absorption spectra of dyes measured in EtOH with the concentration of 5 × 10-5 M; ε is the extinction coefficient at λmax of absorption. Absorption spectra of dyes adsorbed on TiO2. a

b

Figure 2. Plot of (Rhν)2 vs hν.

triarylamine derivatives, the aminyl radical cations usually show absorption in the UV-vis region when the triarylamine derivatives are dissolved in acid-containing solvents.22 The IVCT interactions increase with increasing rhodanine-3-acetic acid because the doping between carboxylic acid and triarylamine is more significant with an increase of the rhodanine-3-acetic acid. When the dyes were adsorbed on the TiO2 surface, the absorption spectra were generally broadened and red-shifted as compared to the dyes dissolved in ethanol, implying that most of the dyes were adsorbed on the TiO2 surface with only partial J-type aggregates.21 Note, in Figure 1B, that the absorption spectra of the adsorbed CTPAR2 dye on the TiO2 surface exhibit an obvious red shifts. In contrast, when these dyes were dissolved in ethanol, these three absorption bands combined to form one broad band. This indicates that most of the CTPAR2 dye was adsorbed on the TiO2 surface with only partial J-type aggregates and that two anchoring groups of carboxylic acid in CTPAR2 adsorbed on TiO2 formed a stable excited state. CTPAR1 and CTPAR3 dyes were adsorbed on TiO2 surface which show a shoulder around 380 nm, implying that the two dyes demonstrate an unstable excited state. These phenomena are related to the efficiency of the assembled DSSC devices (see later). Table 1 lists the maximum absorption data. This shows that the extinction coefficient at maximum absorption increases with the increase of the rhodanine ring. Based on the Tauc relation,23 the energy gap (Eg) can be obtained by plotting (hν)2 vs ohν and extrapolating the linear portion of (Rhν)2 to zero as shown in Figure 2. The lowest energy band gap among these dyes indicates the red shift of the CTPAR2 dye. The two anchoring groups (rhodaniline-3acetic acid) in the CTPAR2 dye adsorbed on the TiO2 surface resulted in a more efficient excitation of electrons and to a lower Eg. On the other hand, the Eg values of the CTPAR3 is close to that of CTPAR1, implying that the number of electron acceptors dominates the energy band gap in the dye molecules, and that the number of carbazole (electron donor) is insignificant effect on the energy band gap in these dyes.

Figure 3. Frontier orbitals of the potassium salt of CTPAR dyes optimized at the B3LYP/6-31+G(d) level.

We further calculate the molecular structures with bidentate carboxylate coordination to potassium as shown in Figure 3. At the ground state (HOMO) for these dyes, electrons are homogeneously distributed in the electron donor (TPA/carbazole) groups. Illumination produces an excited state (LUMO) in these dyes, and intramolecular charge transfer occurs, resulting in the electron movement from the donor groups to the acceptor groups (rhodanine rings). Since the CTPAR2 has more acceptor groups than CTPAR1/CTPAR3, it exhibits more



Figure 4. CVs of CTPAR1, CTPAR2, and CTPAR3 films in CH3CN/ AcOH containing 0.1 M tetrabutylammonium perchlorate at a scan rate of 10 mVs-1.

fluent movement of electrons coupled with a lower Eg value which, in turn, leads to a more efficient injection of electrons into the TiO2 photoelectrode. Cyclic voltammetry is a preliminary characterization technique to determine the redox properties of organic and polymeric materials. Cyclic voltammetry and UV-visible spectroscopy (Figure 2) obtained experimental values for the highest occupied molecular orbital (HOMO), the lowest unoccupied molecular orbital (LUMO) energy levels, and the band gap (Eg) of each of the compounds were obtained by.24 The dyes can be reversibly oxidized at moderately high oxidation potentials. Figure 4 shows the representative cyclic voltammograms (CVs) of CTPAR dyes adsorbed on TiO2 films in acetonitrile/AcOH (7/1, V/V) solution containing 0.1 M tetrabutyl ammonium perchlorate (Bu4N ClO4). The detection of one redox couple was detected in these dyes, indicated that a one-electron oxidation wave of the amine existed in the range of 600-800 mV (vs Ag/Ag+). The electrochemical data of the dye compounds are listed in Table 1. The HOMO energy level can be calculated from the onset oxidation potential [Eox(onset)] based on the reference energy level of ferrocene (4.8 eV below the vacuum level, which is defined as zero) according to eq 1,

HOMO ) -[Eox(onset) - EFc + 4.8] eV

(1)

LUMO ) HOMO + Eg

(2)

wherein EFc is the potential of the internal standard, the ferrocene/ferricenium ion (Fc/Fc+) couple. The value of EFc, determined under the same experimental conditions, was about 0.25 V vs Ag/Ag+. The excited state (LUMO) of the sensitizer was estimated from the ground state (HOMO) and the band gap (Eg) derived from the absorption onset (Figure 2). The deduced LUMO values (-2.87 to ∼-3.35 eV, see Table 1) were higher than the conduction-band-edge energy level of the TiO2 electrode, -4.0 eV,25 indicating that the electron injection process is energetically favorable. Figure 5 shows the related energy levels. Overall, the oxidation potentials of the dyes (0.85-0.97 V vs NHE) were more positive than the I-/I3- redox couple (∼ 0.4 V vs NHE).26 The sufficiently low HOMO energy level of the dye ensures more effective dye regeneration and suppresses the recapture of the injected electrons by the dye cation radical.

Figure 5. Energy levels of related materials.

Figure 6. IPCE spectra for the DSSCs based on different dyes under AM 1.5 irradiation (100 mW cm-2).

Photovoltaic Devices. DSSCs were fabricated with an effective area of 0.25 cm2 using these dyes as the sensitizers, nanocrystalline anatase TiO2 particles, and an electrolyte composed of 0.6 M DMPII/0.05 M I2/0.1 M LiI/0.5 M TBP in 3-methoxypropionitrile solution. Light irradiation from a 500 W xenon lamp was focused through a monochromator onto the testing photovoltaic cell. The incident photon-to-current conversion efficiency (IPCE) of a DSSC was estimated between 400 and 800 nm according to the following equation:5

IPCE(λ) ) [1240Jsc(mAcm-2)]/[λ(nm)Ψ(mW cm-2)] (3) where λ is the wavelength and Ψ is the power of the incident radiation per unit area. Light intensity was measured using a USB 400 fiber optic spectrometer. Figure 6 shows the IPCE of the DSSCs as a function of the wavelength. The photocurrent response of CTPAR2-sensitized DSSC had a higher value exceeding 75% between 470 and 500 nm as compared to the other dyes. The maximum IPCE value of 80% occurred at 475 nm. The decrease of the IPCE above 600 nm in the longwavelength region can be attributed to the decrease in light harvesting for these dyes. The lower photocurrent response of CTPAR1 and CTPAR3-sensitized devices is due to the blue shift dye with a higher energy band gap as compared to the CTPAR2 dye. A similar data acquisition system was used to

21792


Yang et al. concentrate of electron density on TPA molecular frame. The concentrated electrons cannot be fluently conveyed via only one channel of rhodanine-3-acetic acid on the CTPAR3 dye. In contrast, the comparatively higher-lying HOMO of CTPAR1 and CTPAR2 should lead to larger Voc values, whereas the comparatively lower-lying HOMO of CTPAR3 leads to a lower Voc value. Cells of different TiO2 thickness also optimized cell performance. The film thickness decreased with a slight increase in Voc and a decrease in Jsc. A cell with a 7 µm film thickness exhibited the best conversion efficiency and filled factor. The thicker film apparently results in better light harvesting at the expense of dark current suppression. An absorbance near 600 nm for the CTPAR2 dye is relatively high (Figure 1B), and the IPCE of this solar cell was about 20%. This result suggests a strong interaction between the dye and TiO2. The absorbance of a TiO2 film was calculated from the difference of a dye adsorbed TiO2 film and a blank semitransparent TiO2 film. Therefore, the reflection part by TiO2 could be minimized, and the lightharvesting efficiency was estimated directly from the absorbance.27 As we assumed that the TiO2 film was uniform and the reflection part of incident light was small, the lightharvesting efficiency for the dye on a 7 µm TiO2 film was then estimated to be 32% at 600 nm, 93% at 500 nm, and 97% at 550 nm. This derivation is in agreement with the IPCE values measured for the device of CTPAR2 (23% at 600 nm, 69% at 500 nm, and 75% at 550 nm). AC Impedance Analysis. CTPAR2-sensitized DSSCs are about 24-64% more efficient than the other dye-sensitized DSSCs, for reasons similar to those that caused the increase in IPCE. To clarify the above results, the kinetic parameters of photoinjected electrons within the oxidized dye in the DSSCs were measured by using the electrochemical impedance spectroscopy (EIS). In these cells, the electrons are transported through the mesoporous TiO2 network and are reacted with I3-. At the same time, I- is oxidized to I3- at the counter electrode. Figure 8A shows the Nyquist plots of DSSCs with different dyes and which all had an anode thickness of 7.0 ( 0.2 µm. Three semicircles were observed in the Nyquist plots. The smaller semicircle at high frequencies (>1 × 102 Hz) indicated the redox charge transfer response at the Pt/electrolyte interface. The larger semicircle at intermediated frequencies (1-102 Hz) represented the electron transfer impedance at the TiO2/dye/electrolyte interface. The remaining semicircle, which occurred at low frequencies ( Voc (CTPAR1) > Voc (CTPAR3) and Jsc (CTPAR2) > Jsc (CTPAR1) > Jsc (CTPAR3). We thus speculate that the extra second rhodaniline-3-acetic acid may enhance the injection of electrons from the excited dye molecules into the conduction band of TiO2, resulting in the suppression of the dark current and decreasing the approach of not only proton but also I3(and maybe Li+) species, on the TiO2 surface. The adsorbed dye density is the same order in different cells, therefore dye aggregation alone cannot account for the relatively low efficiency of the CTPAR3 device. We believe that the comparatively higher-lying HOMO of CTPAR3 (estimated from the CV) also plays an important role due to its smaller driving force in the reduction of the oxidized dye. This, in turn, will lead to a faster back electron transfer from TiO2 to the dye and result in a smaller Voc, arising from two extra electron donors (carbazole groups) around triphenylamine (main electron donor frame) that

TABLE 2: Photovoltaic Performance and Electron Transport Properties of DSSCs Sensitized with CTPAR Dyes dye

keff, s-1

τ, s

R k, Ω

Rw, Ω

L, µM

CTPAR1 CTPAR2 CTPAR3 N3

114 50 87 34

0.0087 0.0200 0.0114 0.0295

39 24 34 15

0.009 0.015 0.230 0.002

6.85 6.83 6.92 6.87

ns, cm-3

Deff, cm2 s-1

Jsc, mAc m-2

FF

Voc, V

efficiency, %

× × × ×

0.0228 0.0369 0.0059 0.0455

10.60 12.70 5.06 19.87

0.58 0.61 0.59 0.59

0.579 0.598 0.561 0.669

3.54 4.63 1.67 7.83

2.12 7.85 3.18 3.19

1017 1017 1017 1018


J. Phys. Chem. C, Vol. 114, No. 49, 2010 21793 the same, the high Jsc of CTPAR2-based DSSCs shows that the CTPAR2 dye has a lower resistance of electron injection into the TiO2 anode than the other two dyes. This is responsible for the higher FF of CTPAR2 dye in J-V curves. In contrast, this diffusion resistance is reflected in low FFs for CTPAR1 and 3 dyes in J-V curves that suppress the Jsc. Conclusions We have synthesized a new class of dyes featuring carbazole and triphenylamines as electron donors and rhodanine-3-acetic acids as electron acceptors. These dyes were designed with differing contents of carbazole and rhodanine-3-acetic acid in their molecular structures to exhibit differing photovoltaic performance. The performance achieved for the DSSCs constructed from this new series dyes is of significance. Incorporation of one carbazole and two rhodanine-3-acetic acids on a TPA molecular frame (CTPAR2 dye) was demonstrated to be beneficial in retarding the electron transfer from TiO2 to the oxidized dye or electrolyte and to enhance the charge transfer efficiency in the excited state. It is likely that incorporation of appropriate numbers of extra electron donors and electron acceptors in the TPA structure will further improve the performance of the cells. Acknowledgment. Financial support from the National Science Council in Taiwan (NSC 98-2221-E-390-018 and NSC 99-2221-E-390-029-MY2) is gratefully acknowledged.

Figure 8. (A) Nyquist plots of DSSCs. (B) Suggested equivalent circuit of the DSSCs. Rw ()rwL) is the electron transport resistance in the anode (L is the thickness of the anode), Rk ()rk/L) is the charge transfer resistance related to recombination of an electron at the interface, Cµ ()cµL) is the chemical capacitance, and Rs is a lumped series resistance for the transport resistance of FTO and all resistances out of the cell, W1 is the impedance of diffusion of the redox species in the electrolyte, and R1 and CPt are the charge transfer resistance and the interfacial capacitance at the counter electrode/electrolyte interface, respectively.

electron in the photoanode of a DSSC can be estimated by using the equation

Deff ) (Rk/Rw)(L2 /Τ)

(5)

Supporting Information Available: Proton (S 1) and carbon NMR (S 2) spectra of (A) CTPAR1, (B) CTPAR2, and (C) CTPAR3 dyes in DMSO-d6. S 3(A) FTIR spectra of (a) TPA, (b) 1, (c) 2, (d) 3, (e) 4, and (f) 5 compounds; (B) FTIR spectra of (a) 6, (b) 7, (c) 8, (d) CTPAR1, (e) CTPAR2, and (f) CTPAR3 compounds. S4. Elemental analysis of CTPAR1, CTPAR2, and CTPAR3 dyes. The AC impedance data as Figure 8A were subjected to fit by using the SigmaPlot software, obtaining three semicircle of fitting curves as shown. The calculation of the related parameters is demonstrated. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes

where L represents the film thickness of the photoanode. In general, the Deff is a lumped key criterion that determines DSSC efficiency;21 the greater the Deff of an electron in the photoanodes the higher DSSC efficiency.21 Figure 8B shows a suggested equivalent circuit representing the DSSCs. This equivalent circuit, based on the diffusionrecombination model,22,30 was used to fit the impedance spectra. The red lines in Figure 8A depict the results. Table 2 shows that in CTPAR2-based DSSC, the smaller values of keff and Rw collaborate with the greater values of ns, Deff, and τ, to reflect the consistency of lower Eg for the CTPAR2 dye. Moreover, the CTPAR2-based device has a longer electron lifetime (τ) due to the slower recapture of the conduction band electrons by I3-, thereby increasing the device efficiency. This outcome is consistent with our previous argument that incorporation of two rhodanine-3-acetic acid groups in CTPAR2 dye decreases the possibility of electron capture by I3-. Compared to CTPAR1 and CTPAR3, the lower Eg of CTPAR2 dye produces a red shift in the absorption spectra (Figure 1B), which, in turn, produces a higher IPCE. Since Jsc reflects the integrated IPCE amplitude, it should be higher for the CTPAR2-based DSSCs. As TiO2 film thickness and the loading of these dyes are almost

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