Efficient Structural Modification of Triphenylamine-Based Organic

Jul 9, 2008 - A series of new organic D−π−A dyes, coded as DS-1, DS-2, DS-3, and DS-4, was designed, synthesized, and characterized by 1H NMR, 13...
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J. Phys. Chem. C 2008, 112, 11591–11599

11591

Efficient Structural Modification of Triphenylamine-Based Organic Dyes for Dye-Sensitized Solar Cells Gang Li,†,‡ Ke-Jian Jiang,*,† Ying-Feng Li,† Shao-Lu Li,† and Lian-Ming Yang*,† Beijing National Laboratory for Molecular Sciences (BNLMS), Laboratory of New Materials, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China, and Graduate School of the Chinese Academy of Sciences, Beijing 100049, China ReceiVed: March 19, 2008; ReVised Manuscript ReceiVed: May 13, 2008

A series of new organic D-π-A dyes, coded as DS-1, DS-2, DS-3, and DS-4, was designed, synthesized, and characterized by 1H NMR,13C NMR, infrared spectroscopy, mass spectrometry, and elemental analysis. These dyes consist of a di(p-tolyl)phenylamine moiety as an electron donor, a cyanoacetic acid moiety as an electron acceptor/anchoring group, and different types of conducting thiophene units as electron spacers to bridge the donor and acceptor. It was found that both the use of di(p-tolyl)phenylamine donor and the variation of electron spacers in the D-π-A dyes played an essential role in modifying and/or tuning physical properties of organic dyes. These dyes were developed as sensitizers for the application in dye-sensitized TiO2 nanocrystalline solar cells (DSSCs), and their photophysical and electrochemical properties were investigated. The DSSCs based on the dyes gave good performance in terms of incident photon-to-current conversion efficiency (IPCE) in the range of 400-700 nm. A solar-energy-to-electricity conversion efficiency (η) of 7.00% was obtained with the DSSC based on 5-[[2-[p-(di-p-tolylamino)]styryl]thiophene-yl]thiophene-2cyanoacrylic acid (DS-2) under simulated AM 1.5 G irradiation (100 mW/cm2): short-circuit current density (Jsc) of 15.3 mA cm-2; open-circuit voltage (Voc) of 0.633 V; fill factor (FF) of 0.725. The density functional theory (DFT) calculation suggests that the electron-transfer distribution moves from the donor unit to the acceptor under light irradiation, which means efficient intramolecular charge transfer. 1. Introduction Energy demands and environment pollution resulting in global warming have led to a greater focus all over the world on renewable energy sources over the past decades. Dye-sensitized solar cells (DSSCs), the most efficient1 and stable2 excitonic photocells currently, have received wider attention since the first report in 1991 by O’Regan et al.3 due to their high performances and low cost of production and have become one of the most promising alternatives to the conventional solid silicon-based photovoltaic devices for the photovoltaic conversion of solar energy. Central to this device is a thick TiO2 nanoparticle film that provides a large surface area for the adsorption of a photosensitizer, which absorbs visible light and injects an electron into the conduction band of the TiO2 after light excitation and generates the photocurrent. Several Ru(II) polypyridyl complexes have achieved power conversion efficiencies above 11% in standard global air mass 1.5 and show favorable stabilities.4 Although Ru complexes are suitable as photosensitizers, the limited availability and environmental issues would limit their extensive application. In the meantime, alternative metal-free organic chromophores are increasingly studied in DSSCs. Organic dyes as photosensitizers have many advantages; for example, they have high molar absorption coefficients due to intramolecular π-π* transitions, a wide variety of structures can be designed more easily and economically, and they are cost-efficient because no noble metal is involved. Varieties of organic dyes, such as perylene,5 cyanine,6b,7 xanthene,8 mero* Corresponding authors. E-mail: [email protected] (L.-M.Y.), kjjiang@ iccas.ac.cn (K.-J.J.). Fax: 86-010-62559373. Tel: 86-010-62565609. † Institute of Chemistry, Chinese Academy of Sciences. ‡ Graduate School of the Chinese Academy of Sciences.

cyanine,9 coumarin,10 hemicyaine,11 indoline,12 and triphenylamine13 dyes, have been reported, making organic dyes fruitful in the application of DSSCs. And impressive photovoltaic performances have been obtained using organic dyes that showed efficiencies in the range of 5-9%.6,14 Although remarkable advance has been made in the organic dyes as sensitizers for DSSCs, it is still needed to optimize their chemical structures for further improvement on performances. The organic dyes commonly consist of donor, linker, and acceptor groups (i.e., a D-π-A molecular structure). Their various properties could be finely tuned by alternating independently or matching the different groups of D-π-A dyes. It has been found that triarylamine moieties and a cyanoacetic acid moiety are desirable units in the design of dyes as electron donor and electron acceptor/anchoring group, respectively. Bridge groups, as electron spacers to connect the donor and acceptor, are not only to decide light absorption regions of the DSSCs but also influence the electron injection from excited dyes to the TiO2 surface. Accordingly, here we reported on the design, synthesis, and application of four new organic dyes, in which 4,4′-dimethylphenylamine was used as the electrondonating moiety, cyanoacrylic acid as the electron-withdrawing/ anchoring moiety, and different types of thiophene groups (i.e., thiophene, bithiophene, (E)-1,2-bis(2-thienyl)ethene, and thienothiophene) as π-conjugation bridges. 4,4′-Dimethylphenylamine was selected as the donor on the basis of three factors: (1) the triphenylamine moiety was expected to greatly locate the cationic charge from the TiO2 surface and efficiently restrict recombination between the photoelectron and the oxidized sensitizer;12c (2) the tolyl segment, compared to a phenyl group, has much more steric hindrance and hence might more powerfully prevent unfavorable dye aggregation;15 (3) the p-tolyl

10.1021/jp802436v CCC: $40.75  2008 American Chemical Society Published on Web 07/09/2008

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Figure 1. Molecular structures of four new organic dyes.

moiety is more strongly electron-donating than phenyl. The assynthesized dyes, referred to as DS-1-DS-4 (Figure 1), demonstrated excellent photovoltaic performances. Among all the dyes, DS-2 exhibited the highest overall solar-energy-toelectricity conversion efficiency of 7.00% with a short-circuit photocurrent density of 15.3 mA/cm2, an open-circuit voltage of 633 mV, and a fill factor of 0.725 under AM 1.5 irradiation with 100 mW/cm2 simulated sunlight. The present results indicate that the new organic dyes are promising in the development of DSSCs. 2. Experimental Section 2.1. Materials and Reagents. All reactions were carried out under nitrogen atmosphere. THF was distilled from sodium/ benzophenone. DMF, dichloromethane, and POCl3 were distilled from CaH2 under N2 atmosphere. 1H NMR spectra were obtained on a Bruker Avance 400 (400 MHz) or Bruker DMX 300 (300 MHz) spectrometer using tetramethylsilane as internal standard. 13C NMR spectra were recorded on a Bruker DMX 300 (75 MHz) spectrometer using DMSO-d6 as solvent. IR spectra were measured in a Bruker EQUINOX 55 spectrometer. MALDI-TOF mass spectrometric measurements were performed on Bruker Biflex III MALDI-TOF. Elemental analyses were performed on a FLASH EA1112 elemental analyzer. UV-vis spectra of the dyes in DMF solutions were recorded in a quartz cell with 1 cm path length on an HP-8453 spectrometer. Potassium tert-butoxide, cyanoacetic acid, 2-thiophenecarbaldehyde, 2,2′-bithiophene-5-carbaldehyde, tetrabutylammonium perchlorate, chenodeoxycholic acid (DCA), Triton X-100, and 4-tert-butylpyridine (TBP) were purchased commercially. The N3 dye was purchased from Solaronix Company. (E)-1,2-Bis(5-formyl-2-thienyl)ethene16,17 and thieno[3,2-b]thiophene-2,5dicarbaldehyde18 were synthesized according to the reported procedures. All other solvents and chemicals used in this work were analytical grade and used without further purification. All chromatographic separations were carried out on silica gel (200-300 mesh). 2.1.1. Dye Synthesis. 2.1.1.1. 5-[2-[p-(Di-p-tolylamino)phenyl]Vinyl]thiophene (1). A mixture of [p-(di-p-tolylamino)phenyl]triphenylphosphonium bromide (3.77 g, 6 mmol) and 2-thiophene-carbaxaldehyde (0.467 mL, 5 mmol) was dispersed in anhydrous THF (30 mL) and stirred at ambient temperature under nitrogen atmosphere for 30 min. A solution of t-BuOK (0.785 g, 7 mmol) in anhydrous THF was added dropwise, and then the mixture was stirred overnight. The reaction was

quenched by water, and the mixture was extracted with CH2Cl2 (50 mL × 3). The combined extracts were dried over anhydrous Na2SO4 and then filtered. Solvents were removed by rotary evaporation, followed by a simple separation over a silica gel column with CH2Cl2-petroleum ether (1:8) as eluent, to give a yellow solid (1.42 g) of the product which was used directly in the next step. MS-EI m/z: 381 (M+). 2.1.1.2. 5-[p-(Di-p-tolylamino)styryl]thiophene-2-carbaldehyde (2). POCl3 (0.867 mL, 9.55 mmol) was added dropwise with stirring to a solution of dried CH2Cl2 (20 mL) and dried DMF (8 mL) at 0-5 °C under nitrogen. After stirring for 30 min, a solution of 1 (1.42 g, 3.47 mmol) in CH2Cl2 (10 mL) was added slowly. The mixture was allowed to warm to room temperature and then refluxed for 12 h. After cooling to room temperature, the mixture was added to 50 mL of iced water and stirred for 10 min, followed by neutralization with aqueous NaOH (10%). The mixture was extracted with CH2Cl2, and the organic phase was washed with water and dried over Na2SO4. Solvents were removed by rotary evaporation, and the residue was purified by silica gel column chromatography with ethyl acetate-petroleum ether (1:10) as eluent to yield 2 as a red solid (1.0 g, 65%). 1H NMR (400 MHz, CDCl3): δ 2.32 (s, 6H), 6.96 (d, 2H, J ) 8.7 Hz), 7.01 (d, 4H, J ) 8.4 Hz), 7.05-7.08 (m, 7H), 7.31 (d, 2H, J ) 8.7 Hz), 7.63 (d, 1H, J ) 3.9 Hz), 9.82 (s, 1H). MS-EI m/z: 409 (M+). 2.1.1.3. 5-[p-(Di-p-tolylamino)styryl]thiophene-2-cyanoacrylic Acid (DS-1). A 20 mL acetonitrile solution of 2 (0.24 g, 0.58 mmol), cyanoacetic acid (0.1 g, 1.17 mmol), and piperidine (0.2 mL) was charged sequentially in a three-necked flask and heated to reflux under N2 atmosphere for 8 h. After cooling to room temperature solvents were removed by rotary evaporation, and the residue was absorbed on silica gel and purified by column chromatography using CH2Cl2-acetic acid (100:1) as eluent to yield DS-1 as a dark-purple solid (0.22 g, 79%). Mp: 231 °C. 1H NMR (300 MHz, DMSO-d ): δ 2.06 (s, 6H), 6.82 (d, 2H, J 6 ) 8.5 Hz), 6.96 (d, 4H, J ) 8.2 Hz), 7.15 (d, 4H, J ) 8.0 Hz), 7.18 (d, 1H, J ) 16 Hz), 7.37 (d, 1H, J ) 4 Hz), 7.39 (d, 1H, J ) 16 Hz), 7.52 (d, 2H, J ) 8.5 Hz), 7.92 (d, 1H, J ) 3.9 Hz), 8.44 (s, 1H). 13C NMR (75 MHz, DMSO-d6): δ 164.3, 153.4, 148.8, 146.9, 144.6, 141.9, 133.9, 133.7, 133.1, 130.7, 128.9, 128.8, 127.0, 125.6, 120.9, 118.9, 117.2, 97.6, 20.9. IR (KBr, cm-1): 1416, 1504, 1567, 1683, 2216, 2916, 3024. MSESI: m/z 477.3 [(M + H)+], 499.3 [(M + Na)+], 515.3 [(M + K)+]. Anal. Calcd for C30H24N2O2S: C, 75.60; H, 5.08; N, 5.88. Found: C, 75.37; H, 5.07; N, 5.70.

Triphenylamine-Based Organic Dyes for DSSCs 2.1.1.4. 5-[[2-[p-(Di-p-tolylamino)]styryl]thiophene-yl]thiophene (3). A mixture of [p-(di-p-tolylamino)benzyl]triphenylphosphonium bromide (3.77 g, 6 mmol) and 2,2′-bithiophene5-carbaldehyde (0.9 g, 4.63 mmol) was dispersed in anhydrous THF (30 mL) and stirred at ambient temperature under nitrogen atmosphere for 30 min. A solution of t-BuOK (0.785 g, 7 mmol) in anhydrous THF was added dropwise. The reaction mixture was stirred overnight at ambient temperature. Then water was added, followed by extraction with CH2Cl2 (50 mL × 3). The combined extracts were dried over anhydrous Na2SO4. Solvents were removed by rotary evaporation, followed by silica gel column chromatography with CH2Cl2-petroleum ether (1:10) as eluent, to yield a yellow solid (1.61 g) of the product which was used directly in the next step without further purification. MS-EI m/z: 463 (M+). 2.1.1.5. 5-[[2-[p-(Di-p-tolylamino)]styryl]thiophene-yl]thiophene-2-carbaldehyde (4). POCl3 (0.867 mL, 9.55 mmol) was added dropwise with stirring to a solution of dried CH2Cl2 (20 mL) and DMF (8 mL) at 0-5 °C under nitrogen. After stirring for 30 min, a solution of 3 (1.61 g, 3.27 mmol) in CH2Cl2 (10 mL) was added slowly whereupon the mixture was heated to reflux for 12 h. After cooling to room temperature, the reaction mixture was poured into an aqueous solution of sodium acetate (1 M, 200 mL), stirred for 2 h, and then extracted with DCM (50 mL × 3). The combined organic phases were dried over anhydrous Na2SO4. Solvents were removed by rotary evaporation, and the residue was purified by silica gel column chromatography with toluene-petroleum ether (2:1) as eluent to yield 4 as a red solid (0.86 g, 58%). 1H NMR (400 MHz, CDCl3): δ 2.34 (s, 6H), 6.91 (d, 1H, J ) 16 Hz), 6.96 (d, 1H, J ) 4 Hz), 6.99 (d, 2H, J ) 8 Hz), 7.01 (d, 4H, J ) 8 Hz), 7.03 (d, 1H, J ) 16 Hz), 7.1 (d, 4H, J ) 8 Hz), 7.23 (d, 1H, J ) 4 Hz), 7.31 (d, 2H, J ) 8 Hz), 7.6 (d, 2H, J ) 4 Hz), 9.8 (s, 1H). MS-EI: m/z 491 (M+). 2.1.1.6. 5-[[2-[p-(Di-p-tolylamino)]styryl]thiophene-yl]thiophene-2-cyanoacrylic acid (DS-2). A 20 mL acetonitrile solution of 4 (0.285 g, 0.58 mmol), cyanoacetic acid (0.1 g, 1.17 mmol), and piperidine (0.2 mL) was charged sequentially in a three-necked flask and heated to reflux overnight under N2 atmosphere. After cooling to room temperature, solvents were removed by rotary evaporation, and the residue was absorbed on silica gel and purified by column chromatography using CH2Cl2-acetic acid (200:1) as eluent to yield DS-2 as a darkpurple solid (0.30 g, 93%). Mp: 251 °C. 1H NMR (300 MHz, DMSO-d6): δ 2.27 (s, 6H), 6.85 (d, 2H, J ) 8.6 Hz), 6.95 (d, 4H, J ) 8.3 Hz), 6.99 (d, 1H, J ) 16.2 Hz), 7.14 (d, 4H, J ) 8.3 Hz), 7.20 (d, 1H, J ) 3.8 Hz), 7.28 (d, 1H, J ) 16.1 Hz), 7.45 (d, 2H, J ) 8.7 Hz), 7.55-7.57 (m, 2H), 7.97 (d, 1H, J ) 4.2 Hz), 8.47 (s, 1H). 13C NMR (75 MHz, DMSO-d6): δ 163.6, 147.6, 146.2, 145.8, 145.3, 144.2, 141.5, 133.7, 132.78, 132.71, 130.1, 129.5, 129.1, 127.8, 127.6, 124.7, 121.0, 118.9, 116.6, 97.7, 20.4. IR (KBr, cm-1): 1412, 1503, 1571, 1683, 2217, 3024. MS (MALDI-TOF): m/z 558.0 (M+). Anal. Calcd for C34H26N2O2S2: C, 73.09; H, 4.69; N, 5.01. Found: C, 72.60; H, 4.65; N, 5.02. 2.1.1.7. 5-[2-[p-(Di-p-tolylamino)styryl]thiophene-2-ylVinyl]thiophene-2-carbaldehyde (5). A mixture of [p-(di-p-tolylamino)benzyl]triphenylphosphonium bromide (1.89 g, 3 mmol) and (E)-1,2-bis(5-formyl-2-thienyl)ethene (0.6 g, 2.42 mmol) was dispersed in anhydrous THF (30 mL) and stirred at ambient temperature under nitrogen atmosphere for 30 min. t-BuOK (0.5 g, 4 mmol) was dissolved in anhydrous THF and added dropwise, and the mixture was stirred for 1 h at ambient temperature and then heated at 50 °C overnight. The reaction

J. Phys. Chem. C, Vol. 112, No. 30, 2008 11593 mixture was allowed to cool to room temperature, and then water was added, followed by extraction with CH2Cl2 (50 mL × 3). The combined extracts were dried over anhydrous Na2SO4 and filtered. Solvents were removed by rotary evaporation, followed by silica gel column chromatography with toluene-petroleum ether (1:1) as eluent, to yield a red solid of product (0.45 g, 36%). 1H NMR (300 MHz, CDCl3): δ 2.32 (s, 6H), 6.85 (d, 1H, J ) 16 Hz), 6.90-7.01 (m, 9H), 7.05-7.09 (m, 6H), 7.20 (d, 1H, J ) 15.3 Hz), 7.28 (d, 2H, J ) 8.6 Hz), 7.63 (d, 1H, J ) 3.8 Hz), 9.83 (s, 1H). MS-EI: m/z 517 (M+). 2.1.1.8. 5-[2-[p-(Di-p-tolylamino)styryl]thiophene-2-ylVinyl]thiophene-2-cyanoacrylic Acid (DS-3). A 20 mL acetonitrile solution of 5 (0.45 g, 0.869 mmol), cyanoacetic acid (0.29 g, 3.4 mmol), and piperidine (0.2 mL) was charged sequentially in a three-necked flask and heated to reflux overnight under N2 atmosphere. After cooling to room temperature, solvents were removed by rotary evaporation, and the residue was absorbed on silica gel and purified by column chromatography using CH2Cl2-acetic acid (100:1) as eluent to yield the product DS-3 as a dark solid (0.3 g, 60%). Mp: 254 °C. 1H NMR (400 MHz, DMSO-d6): δ 2.28 (s, 6H), 6.89 (d, 2H, J ) 8.6 Hz), 6.96 (d, 4H, J ) 8.2 Hz), 7.14 (d, 4H, J ) 8.2 Hz), 7.21 (d, 1H, J ) 15.8 Hz), 7.37 (s, 2H), 7.42 (d, 1H, J ) 3.9 Hz), 7.46 (d, 1H, J ) 16.2 Hz), 7.52 (d, 2H, J ) 8.6 Hz), 7.92 (d, 1H, J ) 3.9 Hz), 8.44 (s, 1H). 13C NMR (75 MHz, DMSO-d6): δ 163.7, 151.7, 147.5, 144.3, 144.1, 141.3, 139.5, 133.9, 132.7, 130.1, 129.3, 127.5, 124.7, 121.1, 119.7, 116.6, 97.5, 20.4. IR (KBr, cm-1): 1412, 1504, 1572, 1676, 2217, 3018. MS (MALDITOF): m/z 584.0 (M+). Anal. Calcd for C36H28N2O2S2: C, 73.94; H, 4.83; N, 4.79. Found: C, 73.46; H, 4.88; N, 4.75. 2.1.1.9. 5-[p-(Di-p-tolylamino)styryl]thieno[3,2-b]thiophene2-carbaldehyde (6). A mixture of thieno[3,2-b]thiophene-2,5dicarbaldehyde (0.196 g, 1 mmol), anhydrous K2CO3 (0.276 g, 2 mmol), and 18-crown-6 ether (15 mg) was charged sequentially in a three-necked flask and then dried in vacuo for 30 min. Dried DMF (40 mL) was added. [p-(Di-p-tolylamino)benzyl]triphenylphosphonium bromide (0.628 g, 1 mmol) was dissolved in dried DMF (10 mL) and added dropwise to the above solution with stirring under N2 atmosphere. The mixture was stirred for another 3 h at ambient temperature. The reaction mixture was poured into ice-water (200 g), leading to a precipitate of yellow solids. The precipitate was filtered off and then purified by silica gel column chromatography with CH2Cl2-petroleum ether (1:2) as eluent to yield a yellow solid of the product 6 (0.32 g, 69%). 1H NMR (400 MHz, CDCl3): δ 2.32 (s, 6H), 6.95 (d, 1H, J ) 15 Hz), 7.01 (d, 4H, J ) 8.3 Hz), 7.07-7.11 (m, 6H), 7.2 (d, 1H, J ) 15 Hz), 7.25 (s, 1H), 7.31 (d, 2H, J ) 8.6 Hz), 7.84 (s, 1H), 9.91 (s, 1H). MS-EI m/z: 465 (M+). 2.1.1.10. 5-[p-(Di-p-tolylamino)styryl]thieno[3,2-b]thiophene2-cyanoacrylic Acid (DS-4). A 20 mL ethanol solution of 6 (0.32 g, 0.688 mmol), cyanoacetic acid (0.14 g, 1.738 mmol), and piperidine (0.25 mL) was charged sequentially in a three-necked flask and heated to reflux overnight under N2 atmosphere. After cooling to room temperature, solvents were removed by rotary evaporation and the residue was absorbed on silica gel and purified by column chromatography using CH2Cl2-acetic acid (50:1) as eluent to yield the product DS-4 as a dark solid (0.33 g, 90%). Mp: 247 °C. 1H NMR (400 MHz, DMSO-d6): δ 2.28 (s, 6H), 6.84 (d, 2H, J ) 8.7 Hz), 6.96 (d, 4H, J ) 8.3 Hz), 7.06 (d, 1H, J ) 16.1 Hz), 7.14 (d, 4H, J ) 8.3 Hz), 7.37 (d, 1H, J ) 16.1 Hz), 7.49 (d, 2H, J ) 8.7 Hz), 7.61 (s, 1H), 8.25 (s, 1H), 8.52 (s, 1H). 13C NMR (75 MHz, DMSO-d6): δ 163.7, 152.1, 148.0, 147.2, 147.1, 144.1, 137.1, 136.9, 133.0, 132.2,

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SCHEME 1: Synthetic Routes to Four New Dyesa

a

(a) THF/KOtBu, rt. (b) DMF/POCl3, CH2Cl2, reflux. (c) CNCH2COOH, Piperidine, MeCN, reflux. (d) DMF/18-Crown-6, K2CO3, rt.

130.9, 130.1, 128.6, 128.0, 124.9, 120.6, 119.6, 118.8, 116.6, 97.1, 20.4. IR (KBr, cm-1): 1405, 1481, 1505, 1566, 1688, 2217, 3023. MS (MALDI-TOF): m/z 531.9 (M+). HRMS-SIMS (m/ z): [M-] calcd for C32H24N2O2S2, 532.1279; found, 532.1274; [M - COOH]- calcd for C32H24N2O2S2, 487.1303; found, 487.1299. Anal. Calcd for C32H24N2O2S2: C, 72.15; H, 4.54; N, 5.26. Found: C, 71.77; H, 4.50; N, 5.44. 2.2. Electrochemical Measurements. To investigate the electrochemical properties of the dyes, cyclic voltammetry measurements (CV) were carried out in a three-electrode

Figure 2. UV-vis absorption spectra of the four dyes measured in DMF solution.

measuring device. Both the working and counter electrodes were Pt wires, and the reference electrode was a Hg/Hg2Cl2 electrode (BAS 100B/W electrochemical system). The electrolyte is 0.1 M (n-C4H9)4NPF6 in DMF. Ferrocene was added to each sample solution at the end of the experiments, and the ferrocenium/ ferrocene (Fc/Fc+) redox couple was used as an internal potential reference. 2.3. Fabrication of the Dye-Sensitized Nanocrystalline TiO2 Electrodes. For fabrication of the devices, two layers of TiO2, the main layer and the scattering layer, were prepared by screen-printing TiO2 pastes on FTO glass substrate (10 Ω/sq, Nippon Sheet Glass). The main layer (thickness, 14 µm; TiO2 particle size, 18 nm) and scattering layer (thickness, 8 µm; TiO2 particle size, 400 nm) were prepared from two different TiO2 colloids (PST-18NR and PST-400C from CCI in Japan). After screen-printing, the TiO2 film was heated at 500 °C for 30 min and treated with 0.04 M TiCl4 aqueous solution as reported by Ito et al.19 The four new dyes and N3 dye solutions were prepared in DMF at a concentration of 0.5 mM. A solution of 1 mM chenodeoxycholic acid in tert-butyl alcohol/acetonitrile (1:1, v/v) was prepared as reported,20 the use of which is able to effectively prevent unfavorable dye aggregation on TiO2 surface. The TiO2 films were left in the solution at room temperature for 15 h. A Pt-sputtered FTO glass was used as a counter electrode. The electrolyte was composed of 0.6 M 1-propyl-3-methylimidazolium iodide, 0.1 M LiI, 0.05 M I2, and 0.75 M tert-butylpyridine in a mixture of acetonitrile and

Triphenylamine-Based Organic Dyes for DSSCs

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TABLE 1: Absorption, Emission, and Electrochemical Properties of the Four As-Synthesized Dyes dye

λab-max/nma

εmax/M-1 cm-1 b

λex-max/nmc

λab-max/nmd

Eox/V vs NHEe

E0-0/V vs NHEf

Ered/V vs NHEg

Egap/V vs NHEh

DS-1 DS-2 DS-3 DS-4

437 457 473 443

65 000 47 000 34 000 48 000

597 618 641 591

444 485 507 493

1.10 1.05 1.04 1.09

2.41 2.34 2.27 2.39

-1.31 -1.29 -1.23 -1.30

0.81 0.79 0.73 0.80

a Absorption spectra were measured in DMF solution with the concentration of 5.0 × 10-4 M at room temperature. b The molar extinction coefficient at λmax of the absorption spectra. c Absorption spectra were measured in DMF solution with the concentration of 5.0 × 10-4 M at room temperature. d Absorption spectra of dyes adsorbed on TiO2. e The oxidation potentials of the dyes were measured in DMF with 0.1 M (n-C4H9)4NPF6 as electrolyte (scanning rate, 100 mV/s; working electrode and counter electrode, Pt wires; reference electrode, Hg/Hg2Cl2). f E0-0 was estimated from the intersection between the absorption and emission spectra. g The reduction potential of the dye was calculated from Eox - E0-0. h Egap is the gap between the Ered of the dye and the conduction band level of the TiO2 (-0.5 V vs NHE).

Figure 3. Normalized absorption spectra (solid line) and emission spectra (dashed line) of the DS-2 dye in DMF solution.

Figure 5. Cyclic voltammetry of the four as-synthesized dyes: scanning rate is 100 mV/s; working electrode and counter electrode, Pt wires; reference electrode, Hg/Hg2Cl2.

illumination (Yamashita Denso, Yss-80) of AM 1.5 (100 mW/ cm2) conditions at 25 °C. The IPCE values were measured at 5 nm intervals according to the following equation:

IPCE(λ) )

Figure 4. Normalized absorption spectra of the as-synthesized dyes adsorbed on TiO2 films.

where Jsc is the short-circuit photocurrent generated by monochromatic light, λ is the wavelength of incident monochromatic light, and φ is the incident light intensity. The losses of light reflection and absorption by the conducting glass were not corrected. The solar-energy-to-electricity conversion efficiency (η) of the DSSCs is calculated from Jsc, the open-circuit photovoltage (Voc), the fill factor (FF), and the intensity of the incident light (Pin) according to the following equation:21

η) valeronitrile (1:1, v/v). The photovoltaic performance of the devices was recorded under 100 mW/cm2 simulated air mass (AM) 1.5 solar light illumination. 2.4. Photovoltaic Measurements. The action spectra of monochromatic incident photon-to-current conversion efficiency (IPCE) for the solar cells were performed by using a commercial setup for IPCE measurement (PV-25 DYE, JASCO) under 5 mW/cm2 monochromic light illumination. The irradiation source for the photocurrent-voltage (I-V) measurement is an AM 1.5 solar simulator (YSS-50A, Yamashita Denso Co. Ltd.). A 500 W Xe lamp serves as the light source in combination with a band-pass filter (400-800 nm) to remove ultraviolet and infrared light and to give 100 mW/cm2. The current-voltage curves were obtained by the linear sweep voltammetry method using an electrochemical workstation using a PC-controlled voltage-current source meter (R6246, Advantest) under a solar simulator

2 1240 (eV · nm) Jsc (mA/cm ) λ (nm) φ (mW/cm2)

[Jsc (mA/cm2)][Voc (V)][FF] Pin (mW/cm2)

3. Results and Discussion 3.1. Synthesis. The structures of the four D-π-A dyes are shown in Figure 1. All of these dyes have been prepared according to several classical reactions. The synthetic protocols, illustrated in Scheme 1, involve the Wittig reaction, Vilsmeier reaction, and Knoevenagel condensation reaction. [p-(Di-p-tolylamino)benzyl]triphenylphosphonium bromide reacted with 2-thiophene-carbaldehyde and 2,2′-bithiophene-5carbaldehyde under Wittig reaction conditions, obtaining corresponding intermediates 1 and 3, respectively. Then both were subjected to a Vilsmeier reaction in the presence of DMF and POCl3 in refluxing dichloromethane, affording selectively aldehydes 2 and 4 in moderate yields. Subsequent Knoevenagel condensation reactions with cyanoacetic acid in the presence

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Figure 6. IPCE curves for DSSCs based on the four dyes. The electrolyte used was 0.6 M 1-propyl-3-methylimidazolium iodide, 0.1 M LiI, 0.05 M I2, and 0.75 M tert-butylpyridine in the mixture of acetonitrile and valeronitrile (1:1, v/v).

Figure 7. Current density-voltage characteristics for DSSCs from the four dyes under illumination of simulated solar light (AM 1.5, 100 mW/cm2); the electrode used was the same as that shown in Figure 6.

TABLE 2: Photovoltaic Performances of DSSCs Based on the DS-1-DS-4 and N3 Dyea dye

Jsc/mA cm-2

Voc/mV

FF

η (%)

DS-1 DS-2 DS-3 DS-4 N3

14.3 15.3 11.1 15.4 16.87

654 633 630 610 680

72.5 72.5 73.2 70 68

6.71 7.00 5.12 6.57 7.82

a Performances of DSSCs were measured with 0.2 cm2 working area.

of piperidine gave the target dyes DS-1 and DS-2 in good yields, respectively. (E)-1,2-Bis(2-thienyl)ethene was prepared by McMurry coupling reaction of 2-thiophene-carboxaldehyde,16 whereas thienothiophene was obtained in three steps from 3-bromothiophene. Both the spacers were reacted with 2 equiv of n-BuLi and then treated with anhydrous DMF to give the corresponding diformylated derivatives, (E)-1,2-bis(5-formyl2-thienyl)ethene and 2,5-diformylthienothiophene,17,18 respectively, in good yield. Then the two diformylated intermediates reacted with [p-(di-p-tolylamino)benzyl]triphenylphosphonium bromide leading to 5 and 617 through mono-Wittig reaction. After Knoevenagel condensation reaction with cyanoacetic acid, DS-3 and DS-4 were gained in moderate to good yields, respectively. The four dyes are soluble in common organic solvents, such as dichloromethane, THF, DMF, and DMSO. 3.2. Photophysical Properties. The absorption spectra of the dyes in DMF solutions are displayed in Figure 2, and the data

are collected in Table 1. In DMF solution, all the dyes give two distinct absorption bands: one relatively weak band in the near-UV region (300-310 nm) corresponds to the π-π* electron transition; and the other is a strong absorption in the visible region (430-480 nm) that can be assigned to an intramolecular charge transfer (ICT) between the triarylamine donating unit and the cyanoacrylic acid anchoring moiety,22 producing an efficient charge-separated state. The spectra of the four as-synthesized dyes are quite similar, but those of the latter three dyes (DS-2-DS-4) are red-shifted in comparison with that of DS-1, which could be attributed to introduction of more thiophene units resulting in the expansion of the π-conjugation system. The red-shifted value for DS-4 is relatively smaller than that of DS-2 or DS-3 because the DS-4’s π-conjugation system is a little shorter. Noticeably, the molar extinction coefficients (>3 × 104 M-1 cm-1) of the four as-synthesized dyes are higher than that of N3 dye (1.6 × 104 M-1 cm-1),23 indicating a good ability of light harvesting. Figure 3 shows the absorption and emission spectra of DS-2 in DMF solution. The excitation wavelength for emission was 450 nm. The maximum absorption and emission in DMF are at 457 and 618 nm, respectively. The photoelectric data are also summarized in Table 1. The emission wavelengths were not affected strongly by modifying the electron spacer part. However, the maximum emission was shifted toward the longerwavelength region by expansion of the spacers (597 nm for DS-1 and 641 nm for DS-4). Figure 4 shows the absorption spectra of the four dyes adsorbed on transparent TiO2 films, and the data are collected in Table 1. In comparison to the spectra in DMF solution, the absorption spectra were broadened and red-shifted. The similar phenomenon was also observed for other organic dyes.24 The trend may be ascribed to J-type aggregation of the dyes on the TiO2 surface.7b The red-shifting and broadening favor the light harvesting of the solar cells and thus increase the photocurrent response region, resulting in the increase of Jsc. The red-shifted values of different dyes varied and depended on the different electron spacers in the dyes. The absorption spectrum of DS-1 was red-shifted only by 7 nm (from 437 to 444 nm), indicating that DS-1 molecules were assembled on the TiO2 electrode mainly in a monomeric state with very little partial aggregation. For the other dyes, DS-2, DS-3, and DS-4, the red-shift values are 28, 34, and 50 nm, respectively. Apparently, DS-4 dye has a stronger tendency to aggregate on the TiO2 film, and the thienothiophene moieties might be responsible for the tendency. Therefore, the incorporation of coadsorbents such as chenodeoxycholic acid is necessary to prevent dyes from aggregating on TiO2 films. 3.3. Electrochemical Properties. To judge the possibilities of electron transfer from the excited dye molecule to the conductive band of TiO2 and the dye regeneration, their redox potentials were measured via CV (as shown in Figure 5). The measurement was carried out in DMF solution with 0.1 M tetrabutylammonium hexafluorophosphate as the electrolyte. The oxidation potential versus NHE (Eox) was obtained by averaging the anodic and cathodic peak potentials from the cyclic voltamgram, corresponding to the highest occupied molecular orbital (HOMO). The reduction potential versus NHE (Ered), which corresponds to the lowest unoccupied molecular orbital (LUMO), can be obtained from the first oxidation potential and the E0-0 value determined from the intersection of absorption and emission spectra, namely, Eox - E0-0. Table 1 summarizes the electrochemical properties of the as-synthesized four dyes. The four dyes are redox-stable, one-electron reversible at a

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Figure 8. Frontier orbitals of the four dyes optimized at the B3LYP/6-31+G (D) level.

moderately high first oxidation potential. The first oxidation and reduction peaks are clearly observed, suggesting that the first oxidized states of the dyes are stable.25 The negative shifts of the Eox (0.05, 0.06, and 0.01 V) and the positive shifts of the Ered (0.05, 0.11, and 0.01 V) were observed for DS-2, DS-3, and DS-4 versus DS-1, respectively, leading to the decrease of their energy gaps between the HOMO and LUMO compared with DS-1. That could be attributed to a larger expansion of their π-conjugated systems than that of DS-1. As showed in Table 1, both the HOMO and LUMO levels of these dyes agree well with the requirement as an efficient photosensitizer: the HOMO levels of the four dyes were more positive than the (I-/I3-) redox potential (0.42 V),27 and thus the oxidized dyes could be regenerated by the reduced part (I-) in the electrolyte to guarantee an efficient charge separation; and the reduction potentials are more negative than the conduction band level of TiO2 (-0.5 V vs NHE).27 Provided that energy gap of 0.2 eV is necessary for efficient electron injection,28 these driving forces are sufficient for efficient charge injection. Thus, the electron injection process from the excited dye molecule to the TiO2 conduction band and the subsequent dye regeneration are energetically permitted. Therefore, these four dyes can be used as sensitizers because electron transfer in DSSCs is feasible. 3.4. Photovoltaic Performances of the DSSCs. Figure 6 shows the IPCE as a function of the wavelength for the sandwiched DSSCs based on the four new dyes as sensitizers.

The shapes of the action spectra of these dyes are very similar to the absorption spectrum of the dye-absorbed thin TiO2 layer (Figure 4). The spectra cover the whole visible region in the range of 300-750 nm, facilitating the DSSCs to efficiently convert solar light to electricity. The IPCE of DS-1, DS-2, and DS-4 are more than 70% in the spectral range from 330 to 580 nm, and DS-2 reaches its maximum of 86% at 472 nm. When reflection and absorption losses of the FTO glass substrate are considered, the net photon-to-electron conversion efficiencies of the three dyes (DS-1, DS-2, and DS-4) would almost exceed 90% in their spectral ranges. In contrast, the IPCE of DS-3 gives a relatively low value with the maximum of 67% at 470 nm, which might be due to the poor injection efficiency arising from unfavorable binding or orientation of DS-3 onto the TiO2 surface.26 Figure 7 shows the I-V curves of the four dye-based DSSCs with electrolyte composed of 0.6 M 1-propyl-3-methylimidazolium iodide, 0.1 M LiI, 0.05 M I2, and 0.75 M tertbutylpyridine in the mixture of acetonitrile and valeronitrile (1: 1, v/v) under simulated AM 1.5 illumination (100 mW/cm2). Following previous reports, 1 mM chenodeoxycholic acid was added in each dye solution for preventing unfavorable aggregation on the TiO2 surface. It needs to be pointed out that the effect of chenodeoxycholic acid on the DSSCs performance was not carefully conducted here. Their detailed photovoltaic parameters are listed in Table 2, where N3 is included for comparison. Consistent with the results of IPCE, the photovoltaic data of DS-1, DS-2, and DS-4 performed superior to that

11598 J. Phys. Chem. C, Vol. 112, No. 30, 2008 of DS-3. As demonstrated in Table 2, DS-3 gives a lower overall conversion efficiency of 5.12%, and 7.00% of solar-energy-toelectricity conversion efficiency (η) based on DS-2 was achieved (short-circuit photocurrent density, Jsc ) 15.3 mA/cm2; the opencircuit photovoltage, Voc ) 633 mV; and fill factor, FF ) 72.5); this conversion efficiency is higher compared to the published results of triphenylamine-based DSSCs, which reported the efficiencies in the range of 2.47-6.15%.12c,13c,24c Furthermore, the efficiency is higher than these of 6.71% and 6.57% obtained from the DSSCs based on the dye DS-1 and DS-4, respectively. According to the data, it is clear that the efficiencies of the DSSCs can be strongly affected by the electron spacers in the dye molecules. In comparison to DS-1, the efficiency of the DS-2-based device is significantly improved by introducing another thiophene unit through a single bond. In comparison to the DS-1-based device, a little increased Jsc was found in the DS-2-based device. The increase can also be reflected from their photocurrent action spectra (IPCE), where some stronger photocurrent response was observed for the DS-2-based device in the near-IR region (630-750 nm). For DS-4, its device efficiency is lower than those for DS-1 and DS-2, although it gives the highest current density (Jsc, 15.4 mA/cm2). The lower efficiency may result from the stronger aggregation of DS-4 on the TiO2 surface due to the introduction of the thienothiophene electron spacer in the dye. Generally, a monolayer of dye molecules adsorbed on the TiO2 surface is necessary for efficient electron injection from the excited dye to TiO2. The aggregation leads to unfavorable back-electron transfer, and decreases the open-circuit voltage of the device, which is consistent with the lowest Voc value of 610 mV for the DS-4-based device. Among the four dye-based DSSCs, the DS-3-based device gives the lowest efficiency although the dye gave the broadest light harvesting range. In the DS-3 dye molecule, a vinyl unit in the space (from (E)-1,2-bis(5-formyl-2-thienyl)ethane) may lead to a twisted intramolecular charge-transfer geometry of the dye excited state, which would decrease the excited-state energy and cause a lower efficiency of electron injection as reported previously.26 In addition, the introduction of the vinyl unit may also easily cause the dye aggregation on the TiO2 film and hence decrease Voc as shown in Table 2. For comparison, the device with standard N3 dye presents 16.87 mA/cm2 of Jsc, 680 mV of Voc, 68 of FF, and 7.82% of η under the similar conditions. The results suggest that the four as-synthesized organic dyes are a promising alternative to the ruthenium polypyridyl dyes used in DSSCs, especially for DS-2. 3.5. Molecular Orbital Calculations. To get an insight into the molecular structure and electron distribution of the organic dyes, their geometries have been optimized using density functional theory (DFT) calculations at a B3LYP/6-31+G (D) level. The electron distribution of the HOMO and LUMO of the selected dyes is shown in Figure 8; the HOMO is delocalized over the π system with the highest electron density centered at the central nitrogen atom, and the LUMO is located in anchoring groups through the π-bridge. It reveals that the cyanoacrylic acid group is essentially coplanar with respect to the thiophene or thienothiophene acceptor groups. We notice that the HOMO-LUMO excitation induced by light irradiation could move the electron distribution from the whole molecules to the anchoring moieties. Assuming similar molecular orbital geometry when anchored to TiO2, the position of the LUMO close to the anchoring group would enhance the orbital overlap with the titanium 3d orbital and thus favor electron injection from dye to TiO2.

Li et al. 4. Conclusion We have designed and synthesized four new metal-free organic dyes (DS-1, DS-2, DS-3, and DS-4) featuring di(ptolyl)phenylamine as the electron-donating moiety and cyanoacrylic acid as the electron-withdrawing moiety. Different types of thiophene-containing conjugation moieties are introduced into the molecules and serve as electron spacers. The maximum absorption peaks of the four dyes range from 437 to 473 nm with high molar extinction coefficients. Their photovoltaic performances in DSSCs were investigated. These DSSCs exhibited the efficiencies ranging from 5.12% to 7.00%, which reached 65-89% with respect to that of an N3-based device fabricated under similar conditions. Thus, all of four dyes show good performance for solar cells. Especially, a maximum solarenergy-to-electricity conversion efficiency based on the DS-2 sensitizer reaches a high level of 7.00% under AM 1.5 irradiation (100 mW/cm2). The power conversion efficiency was shown to be sensitive to the structural modifications of bridging thiophene groups. Considerably high conversion efficiency for the dyes reveals that the as-synthesized metal-free organic dyes are promising in the development of DSSCs. Further optimization of chemical structure of an organic dye needs not only to broaden its absorption spectrum for enhancing the light harvesting ability through increasing its π-conjugating system but also to avoid the tendency of the dye aggregation on the TiO2 surface.29 Acknowledgment. The authors thank the National Natural Science Foundation of China (Project Nos. 20672116 and 20774103) for financial support of this work. References and Notes (1) Nazeeruddin, M. K.; Pe´chy, P.; Renouard, T.; Zakeeruddin, S. M.; HumphryBaker, R.; Comte, P.; Liska, P.; Cevey, L.; Costa, E.; Shklover, V.; Spiccia, L.; Deacon, G. B.; Bignozzi, C. A.; Gra¨tzel, M. J. Am. Chem. Soc. 2001, 123, 1613. (2) Wang, P.; Zakeeruddin, S. M.; Moser, J. E.; Nazeeruddin, M. K.; Sekiguchi, T.; Gra¨tzel, M. Nat. Mater. 2003, 2, 402. (3) (a) O’Regan, B.; Gra¨tzel, M. Nature 1991, 353, 737. (b) Gra¨tzel, M. Inorg. Chem. 2005, 44, 6841. (4) (a) Nazeeruddin, M. K.; De Angelis, F.; Fantacci, S.; Selloni, A.; Viscardi, G.; Liska, P.; Ito, S.; Takeru, B.; Gra¨tzel, M. J. Am. Chem. Soc. 2005, 127, 16835. (b) Gra¨tzel, M. J. Photochem. Photobiol., C 2003, 4, 145. (c) Gra¨tzel, M. Prog. PhotoVoltaics. 2006, 14, 429. (5) (a) Ferrere, S.; Zaban, A.; Gregg, B. A. J. Phys. Chem. B 1997, 101, 4490. (b) Ferrere, S.; Gregg, B. A. New J. Chem. 2002, 26, 1155. (6) (a) Wang, Z. S.; Cui, Y.; Hara, K.; Dan-oh, Y.; Kasada, C.; Shinpo, A. AdV. Mater. 2007, 19, 1138. (b) Hara, K.; Sato, T.; Katoh, R.; Furube, A.; Ohga, Y.; Shinpo, A.; Suga, S.; Sayama, K.; Sugihara, H.; Arakawa, H. J. Phys. Chem. B 2003, 107, 597. (c) Sayama, K.; Tsukagoshi, S.; Hara, K.; Ohga, Y.; Shinpou, A.; Abe, Y.; Suga, S.; Arakawa, H. J. Phys. Chem. B 2002, 106, 1363. (d) Wang, Z. S.; Li, F. Y.; Huang, C. H.; Wang, L.; Wei, M.; Jin, L. P.; Li, N. Q. J. Phys. Chem. B 2000, 104, 9676. (e) Ehret, A.; Stuhl, L.; Spitler, M. T. J. Phys. Chem. B 2001, 105, 9960. (7) (a) Hara, K.; Kurashige, M.; Danoh, Y.; Kasada, C.; Shinpo, A.; Suga, S.; Sayama, K.; Arakawa, H. New J. Chem. 2003, 27, 783. (b) Hara, K.; Danoh, Y.; Kasada, C.; Shinpo, A.; Suga, S.; Arakawa, H.; Sugihara, H. J. Phys. Chem. B 2005, 109, 3907. (8) Hara, K.; Horiguchi, T.; Kinoshita, T.; Sayama, K.; Sugihara, H.; Arakawa, H. Chem. Lett. 2000, 29, 316. (9) (a) Khazraji, A. C.; Hotchandani, S.; Das, S.; Kamat, P. V. J. Phys. Chem. B 1999, 103, 4693. (b) Sayama, K.; Hara, K.; Mori, N.; Satsuki, M.; Suga, S.; Sukagoshi, S.; Abe, Y.; Sugihara, H.; Arakawa, H. Chem. Commun. 2000, 13, 1173. (10) (a) Hara, K.; Sato, T.; Katoh, R.; Furube, A.; Ohga, Y.; Shinpo, A.; Suga, S.; Sayama, K.; Sugihara, H.; Arakawa, H. J. Phys. Chem. B 2003, 107, 597. (b) Hara, K.; Tachibana, Y.; Ohga, Y.; Shinpo, A.; Suga, S.; Sayama, K.; Sugihara, H.; Arakawa, H. Sol. Energy Mater. Sol. Cells 2003, 77, 89. (c) Hara, K.; Dan-oh, Y.; Kasada, C.; Ohga, Y.; Shinpo, A.; Suga, S.; Sayama, K.; Arakawa, H. Langmuir 2004, 20, 4205. (d) Hara, K.; Wang, Z.-S.; Sato, T.; Furube, A.; Katoh, R.; Sugihara, H.; Dan-oh, Y.; Kasada, C.; Shinpo, A.; Suga, S. J. Phys. Chem. B 2005, 109, 15476.

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