Molecular Engineering of Quinoxaline-Based ... - ACS Publications

Aug 6, 2012 - Suhua Fan , Xuefeng Lu , Hong Sun , Gang Zhou , Yuan Jay Chang , Zhong-Sheng Wang .... Davies , Christopher D. Gwenin , Mateusz B. Pitak...
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Molecular Engineering of Quinoxaline-Based Organic Sensitizers for Highly Efficient and Stable Dye-Sensitized Solar Cells Xuefeng Lu, Quanyou Feng, Tian Lan, Gang Zhou,* and Zhong-Sheng Wang* Department of Chemistry & Laboratory of Advanced Materials, Fudan University, 2205 Songhu Road, Shanghai 200438, P. R. China, S Supporting Information *

ABSTRACT: A series of quinoxaline based metal-free organic sensitizers has been designed and synthesized for dye-sensitized solar cells (DSSCs). The absorption, electrochemical, and photovoltaic properties for all sensitizers have been systematically investigated. It is found that the incorporation of quinoxaline unit instead of thienopyrazine unit results in a negative shift of the lowest unoccupied molecular orbital levels for FNE44, FNE45, FNE46, and FNE47, in comparison to FNE32, which induces a remarkable enhancement of the electron injection driving force from the excited organic sensitizers to the TiO2 semiconductor. Moreover, when the alkyl substituents are removed from the spacer part in FNE44 to the donor part in FNE45 and FNE46, a more conjugated system and a bathochromically shifted maximum absorption band can be realized, which consequently results in an increased light harvesting efficiency and photogenerated current. In addition, the length of the alkyl substituents on the donor part has a certain influence on the DSSC performance. Combining the three contributions, FNE46-based DSSC with liquid electrolyte displays the highest power conversion efficiency (η) of 8.27%. Most importantly, a η of 7.14% has been achieved for FNE46 based quasi-solid-state DSSC and remained at 100% of the initial value after continuous light soaking for 1000 h, which indicates that FNE46 is appropriate for promising commercial application. Our findings will facilitate the understanding of the crucial importance of molecular engineering and pave a new path to design novel metal-free organic dyes for highly efficient and stable DSSCs. KEYWORDS: quinoxaline, organic dye, dye-sensitized solar cells, quasi-solid-state, charge recombination

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INTRODUCTION Dye-sensitized solar cells (DSSCs) have attracted everincreasing attention as potential alternatives to silicon-based solar cells due to their efficient energy conversion and relatively low cost of production.1 Among the key components of a DSSC, the sensitizer always plays as one of the most crucial elements since it exerts a significant influence on the power conversion efficiency (η) as well as the device stability. To date, ruthenium-complex-sensitized DSSCs have reached η values of more than 11%,2 whereas zinc-porphyrin cosensitized DSSCs have reaped η of 12.3%.3 Compared to the metal-complex sensitizers, alternative metal-free organic sensitizers4 have attracted much attention because of their unique advantages, such as low-cost, environmentally friendly, tunable absorption, and electrochemical properties via suitable molecular design. Recently, the performance of organic dye-based DSSCs has been remarkably improved and impressive efficiencies in the range of 8−10%5 have been achieved. However, despite the promising results obtained so far, the η value of the DSSC based on metal-free organic dye is somewhat lower because of a relatively lower photocurrent which is significantly determined by the light harvesting efficiency of the dye molecules. To overcome the relatively narrow absorption spectrum and harvest sunlight in a broad spectral range, it is essential to © 2012 American Chemical Society

extend the absorption range of the organic sensitizer. Recently, as compared to typical donor-π bridge-acceptor (D-π-A) organic dyes where cyanoacrylic acid acts as both acceptor and anchor group, an additional acceptor group has been introduced into the traditional D-π-A molecules for constructing highly efficient DSSCs.5b,6 For example, we have recently incorporated benzothiadiazole,7 benzotriazole,8 thienopyrroledione,9 and thienopyrazine10 as additional acceptor group into the D-π-A constructed organic dyes for endowing the organic sensitizers unique properties. Interestingly, the maximum absorption band located at around 600 nm for thienopyrazine based organic dyes, such as FNE3210 (Chart 1). However, although the resulted dye based DSSCs exhibited very broad IPCE spectra over the whole visible range extending into the near-infrared (NIR) region up to 900 nm, the η value was still limited because of the insufficient driving force for the electron injection from the excited organic dyes into the TiO2 film. Therefore, further structure optimization is needed to tune the energy levels of the organic sensitizers for improving the DSSC performance. Received: May 17, 2012 Revised: July 9, 2012 Published: August 6, 2012 3179

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by flash column chromatography (silica gel, DCM/petroleum ether (PE) = 1:3). Yellow oil 2a was obtained with a yield of 56% (450 mg). 1 H NMR (400 MHz, CDCl3, δ ppm): 8.89 (s, 2H), 7.83 (s, 2H), 7.43 (d, 2H, J = 5.2 Hz), 7.11 (d, 2H, J = 5.2 Hz), 2.51 (t, 4H, J = 7.8 Hz), 1.60−1.53 (m, 4H), 1.20−1.15 (m, 12H), 0.80−0.77 (m, 6H). 13C NMR (100 MHz, CDCl3, δ ppm): 144.76, 142.26, 141.87, 134.53, 132.87, 132.05, 129.02, 125.87, 31.79, 30.78, 29.50, 29.31, 22.76, 14.31. Synthesis of 2b. Compound 2b was synthesized according to previously reported literature.12 Synthesis of 3a. Under a nitrogen atmosphere, compound 2a (350 mg, 0.76 mmol) and DMF (67 mg, 0.91 mmol) were dissolved in 15 mL dichloromethane (DCM). To this solution, phosphorus oxychloride (140 mg, 0.91 mmol) was added slowly. The mixture was stirred for 20 min at room temperature and then heated to 70 °C for 8 h. After cooling to room temperature, 30 mL saturated sodium acetate solution was added to the dark red reaction solution with stirring for 20 min. The mixture was poured into ice water (50 mL) and neutralized (pH = 7) through the addition of sodium hydroxide solution. The product was extracted with DCM for three times. The combined organic solution was washed with sodium bicarbonate and sodium chloride solutions and dried over anhydrous sodium sulfate. After removal of the solvent, the residue was purified by flash column chromatography (silica gel, DCM/PE = 1:2). Yellow oil 3a was obtained with a yield of 57% (210 mg). 1H NMR (400 MHz, CDCl3, δ ppm): 9.95 (s, 1H), 8.93 (d, 1H, J = 1.5 Hz), 8.88 (d, 1H, J = 1.5 Hz), 7.88−7.83 (m, 2H), 7.79 (s, 1H), 7.45 (d, 1H, J = 5.2 Hz), 7.12 (d, 1H, J = 5.2 Hz), 2.54−2.49 (m, 4H), 1.61−1.54 (m, 4H), 1.21−1.16 (m, 12H), 0.81−0.77 (m, 6H). 13C NMR (100 MHz, CDCl3, δ ppm): 183.24, 145.14, 145.01, 143.84, 143.53, 143.16, 142.15, 142.13, 141.77, 137.85, 135.97, 132.96, 132.37, 131.98, 131.81, 129.08, 126.19, 31.77, 31.70, 30.76, 30.45, 29.51, 29.42, 29.28, 29.14, 22.73, 22.69, 14.28, 14.25. Synthesis of 3b. Compound 3b was synthesized similarly as described for synthesis of 3a, red solid, yield 71% (610 mg). 1H NMR (400 MHz, CDCl3, δ ppm): 9.99 (s, 1H), 9.01 (d, 2H, J = 9.3 Hz), 8.23−8.15 (m, 2H), 7.88−7.82 (m, 3H), 7.56 (s, 1H), 7.21 (s, 1H). 13 C NMR (100 MHz, CDCl3, δ ppm): 183.81, 144.16, 144.00, 135.96, 129.60, 128.75, 128.05, 127.72, 127.50, 127.29. Synthesis of 4a. Under a nitrogen atmosphere, compound 3a (160 mg, 0.33 mmol) was dissolved in a solution of tetrahydrofuran (THF) (25 mL). To this solution, NBS (88 mg, 0.50 mmol) was added. After the mixture was stirred at room temperature for 6 h, distilled water was added to quench the reaction. The solution was extracted with DCM for three times. The combined organic solution was washed with sodium hydroxide solution and sodium chloride solution and dried over anhydrous sodium sulfate. After removal of the solvent, the residue was purified by flash column chromatography (silica gel, DCM/PE = 1:2). Yellow oil 4a was obtained with a yield of 60% (110 mg). 1H NMR (400 MHz, CDCl3, δ ppm): 9.95 (s, 1H), 8.94−8.90 (m, 2H), 7.86−7.84 (m, 2H), 7.78 (s, 1H), 7.07 (s, 1H), 2.53−2.47 (m, 4H), 1.60−1.54 (m, 4H), 1.21−1.11 (m, 12H), 0.81− 0.78 (m, 6H). 13C NMR (100 MHz, CDCl3, δ ppm): 183.23, 145.35, 145.17, 143.59, 143.53, 143.25, 142.74, 141.87, 141.77, 137.84, 137.81, 134.57, 133.84, 133.40, 132.01, 131.83, 131.78, 31.72, 31.69, 30.61, 30.44, 29.54, 29.42, 29.19, 29.14, 22.71, 22.69, 14.27, 14.26. Synthesis of 4b. Compound 4b was synthesized similarly as described for synthesis of 4a, red solid, yield 70% (420 mg). 1H NMR (400 MHz, THF-d8, δ ppm): 9.85 (s, 1H), 8.99 (d, 1H, J = 1.8 Hz), 8.96 (d, 1H, J = 1.8 Hz), 8.35−8.27 (m, 2H), 7.96 (d, 1H, J = 4.2 Hz), 7.77 (d, 1H, J = 4.0 Hz), 7.69 (d, 1H, J = 4.1 Hz), 7.10 (d, 1H, J = 4.1 Hz). 13C NMR (100 MHz, THF-d8, δ ppm): 182.82, 144.64, 144.08, 135.32, 135.28, 129.35, 128.47, 127.39, 126.66, 125.82. Synthesis of 5a. Under a nitrogen atmosphere, a mixture of compound 4a (100 mg, 0.18 mmol), 4-(diphenylamino)phenylboronic acid (73 mg, 0.25 mmol), Pd(PPh3)4 (27 mg, 0.02 mmol) and K2CO3 (1.38 g, 0.01 mol) in a solution of water (5 mL), toluene (7.5 mL) and THF (7.5 mL) was stirred and heated at 85 °C for 15 h. When the reaction was completed, the mixture was extracted with DCM for two times. The combined organic solution was washed with sodium

Chart 1

Herein, the thiophene ring fusing on the pyrazine unit in FNE32 is replaced by a benzene ring in FNE44, FNE45, FNE46, and FNE47 (Chart 1) to up-lift the lowest unoccupied molecular orbital (LUMO) level of the organic dye. The long alkyl substituents are incorporated on the spacer or/and the donor unit to ensure sufficient solubility in common organic solvents. Moreover, the introduced alkyl chains can effectively inhibit the intermolecular aggregation and charge recombination for the purpose of reducing current and voltage losses.11 However, because of the strong steric hindrance between the quinoxaline and alkylated thiophene units, a remarkable molecular twist exists in the spacer of FNE44 and FNE47, which weakens the conjugation and hypsochromically shifts the maximum absorption wavelength. Therefore, FNE45 and FNE46 (Chart 1) containing two hexyloxy and octyloxy branches, respectively, are designed and synthesized to remove the alkyl substituents from the spacer in FNE44 to the donor part for maximizing the effective conjugation length. It is found that although FNE44, FNE45, FNE46, and FNE47 have identical molecular skeletons, the subtle structural change of the substituent position induces a significant bathochromic shift of over 160 nm at the absorption maximum when the alkyl substituents are removed from the π bridge to the donor part. As a result, in liquid electrolyte based DSSCs, the η value significantly increased from 3.27% for FNE44-based DSSC to 8.27% for FNE46-based DSSC by 2.5-fold. Additionally, a η value of 7.14% was achieved for FNE46-based quasi-solid-state device and remained at 100% of the initial value after continuous light soaking for 1000 h.

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EXPERIMENTAL SECTION

Materials and Reagents. Thiophene, benzothiadiazole, Nbromosuccinimide (NBS) were purchased from J&K Chemical Ltd., China. Organic solvents used in this work were purified using the standard process. Other chemicals and reagents were used as received from commercial sources without further purification. Transparent conductive glass (F-doped SnO2, FTO, 15 Ω/square, transmittance of 80%, Nippon Sheet Glass Co., Japan) was used as the substrate for the fabrication of TiO2 thin film electrode. Synthesis of 2a. Under a nitrogen atmosphere, a mixture of compound 112 (500 mg, 1.74 mmol), tributyl(3-hexylthiophen-2yl)stannane (2.38 g, 5.21 mmol) and Pd(PPh3)4 (90 mg, 0.08 mmol) in N,N-dimethylformamide (DMF) (8 mL) was stirred and heated at 90 °C for 15 h. After removal of the solvent, the residue was purified 3180

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Synthesis of FNE45. Dye FNE45 was synthesized similarly as described for synthesis of FNE44, black solid, yield 67% (90 mg). 1H NMR (400 MHz, DMSO-d6, δ ppm): 9.20−9.16 (m, 2H), 8.50−8.47 (m, 2H), 8.25 (s, 1H), 8.17 (d, 1H, J = 4.2 Hz), 8.09 (d, 1H, J = 4.0 Hz), 7.91 (d, 1H, J = 4.2 Hz), 7.63 (d, 2H, J = 8.6 Hz), 7.49 (d, 1H, J = 3.9 Hz), 7.11 (d, 4H, J = 8.9 Hz), 6.99 (d, 4H, J = 9.0 Hz), 6.88 (d, 2H, J = 8.7 Hz), 4.01 (t, 4H, J = 6.4 Hz), 1.81−1.74 (m, 4H), 1.51− 1.47 (m, 4H), 1.40−1.37 (m, 8H), 0.97−0.94 (m, 6H). 13C NMR (100 MHz, THF-d8, δ ppm): 156.40, 149.01, 148.72, 144.35, 144.31, 143.96, 141.04, 140.97, 139.95, 139.93, 139.79, 139.45, 136.38, 133.47, 130.14, 130.12, 128.88, 128.06, 127.19, 127.13, 127.05, 126.70, 126.64, 121.93, 120.76, 115.57, 68.32, 32.22, 29.94, 26.38, 23.16, 14.03. MALDI-TOF mass: m/z 832.48. Synthesis of FNE46. Dye FNE46 was synthesized similarly as described for synthesis of FNE44, black solid, yield 65% (140 mg). 1H NMR (400 MHz, DMSO-d6, δ ppm): 9.23−9.20 (m, 2H), 8.60−8.50 (m, 2H), 8.41 (s, 1H), 8.24 (d, 1H, J = 3.5 Hz), 8.14 (d, 1H, J = 3.7 Hz), 8.04 (s, 1H), 7.65 (d, 2H, J = 8.4 Hz), 7.52 (d, 1H, J = 4.0 Hz), 7.12 (d, 4H, J = 8.7 Hz), 6.99 (d, 4H, J = 8.7 Hz), 6.88 (d, 2H, J = 8.4 Hz), 4.01 (t, 4H, J = 6.4 Hz), 1.79−1.74 (m, 4H), 1.50−1.46 (m, 4H), 1.42−1.30 (m, 16H), 0.94−0.92 (m, 6H). 13C NMR (100 MHz, THFd8, δ ppm): 156.41, 149.07, 149.04, 144.37, 144.34, 144.02, 143.33, 140.96, 139.79, 136.39, 136.37, 136.34, 133.64, 130.08, 130.05, 128.92, 128.09, 127.90, 127.27, 127.17, 127.06, 126.72, 126.65, 121.94, 120.75, 115.57, 68.32, 32.45, 30.00, 29.98, 29.90, 26.71, 23.18, 14.07. MALDITOF mass: m/z 888.57. Synthesis of FNE47. Dye FNE47 was synthesized similarly as described for synthesis of FNE44, a red solid, yield 60% (50 mg). 1H NMR (400 MHz, THF-d8, δ ppm): 8.75−8.72 (m, 2H), 8.49 (s, 1H), 7.76 (s, 3H), 7.32 (d, 2H, J = 7.8 Hz), 7.12 (s, 1H), 6.91 (d, 4H, J = 8.0 Hz), 6.77 (d, 2H, J = 8.1 Hz), 6.72 (d, 4H, J = 8.3 Hz), 3.82 (s, 4H), 2.40−2.39 (m, 4H), 1.65−1.63 (m, 4H), 1.47−0.79 (m, 42H), 0.67−0.63 (m, 6H). 13C NMR (100 MHz, THF-d8, δ ppm): 156.02, 148.52, 145.00, 144.88, 144.64, 142.73, 142.58, 141.79, 141.30, 140.69, 137.48, 136.90, 135.49, 133.17, 131.70, 131.33, 131.16, 126.82, 126.66, 126.13, 124.49, 123.56, 120.44, 117.78, 117.75, 115.20, 113.81, 67.96, 32.08, 31.85, 31.69, 30.64, 30.36, 29.97, 29.89, 29.79, 29.62, 29.57, 29.53, 29.28, 29.18, 26.34, 22.82, 22.69, 22.66, 13.73, 13.69. MALDITOF mass: m/z 1056.37. Characterizations. UV−vis absorption spectra of dye solutions and dye-loaded films were recorded with a Shimadzu UV-2550PC spectrophotometer. The film thickness was measured by a surface profiler (Veeco Dektak 150). Cyclic voltammetry (CV) measurements were performed with an Autolab analyzer using a typical threeelectrode electrochemical cell in a solution of tetrabutylammonium hexafluorophosphate (0.1 M) in water-free acetonitrile with a scan rate of 50 mV s−1 at room temperature under argon. Dye-adsorbed TiO2 on conductive glass was used as the working electrode, a Pt wire as the counter electrode, and an Ag/Ag+ electrode as the reference electrode. The potential of the reference electrode was calibrated by ferrocene, and all potentials mentioned in this work are against normal hydrogen electrode (NHE). DSSC Fabrication and Photovoltaic Measurements. TiO2 films (12 μm) composed of 6 μm nanoparticle (20 nm) layer in direct contact with the FTO substrate and 6 μm light scattering particle (80% 20 nm TiO2 + 20% 100 nm TiO2) layer were fabricated with a screen printing method and used in this study.13 The films were sintered at 500 °C for 2 h to achieve good necking of neighboring TiO2 particles. The sintered films were then treated with 0.05 M TiCl4 aqueous solution at 70 °C for 30 min followed by calcinations at 450 °C for 30 min. When TiO2 electrodes were cooled down at around 120 °C, the electrodes were dipped in dye solutions (0.3 mM in toluene) for 24 h at room temperature for complete dye adsorption. The dye-loaded TiO2 film as working electrode and the Pt-coated FTO as counter electrode were separated by a hot-melt Surlyn film (30 μm) and sealed together by pressing them under heat. Redox electrolyte (0.6 M 1,2-dimethyl-3-n-propylimidazolium iodide (DMPImI), 0.1 M LiI, 0.05 M I2, and 0.5 M 4-tert-butylpyridine (TBP) in acetonitrile) was introduced through a hole in the counter electrode via suction through another drilled hole. Finally, the two holes were

chloride solution and dried by anhydrous sodium sulfate. After removal of the solvent, the residue was purified by flash column chromatography (silica gel, DCM/PE = 1:2). Yellow oil 5a was obtained with a yield of 70% (90 mg). 1H NMR (400 MHz, CDCl3, δ ppm): 9.94 (s, 1H), 8.95 (d, 1H, J = 1.7 Hz), 8.89 (d, 1H, J = 1.7 Hz), 7.91−7.84 (m, 2H), 7.78 (s, 1H), 7.51 (d, 2H, J = 8.7 Hz), 7.29 (s, 1H), 7.27−7.25 (m, 4H), 7.14−7.02 (m, 8H), 2.55−2.49 (m, 4H), 1.63−1.57 (m, 4H), 1.22−1.17 (m, 12H), 0.81−0.78 (m, 6H). 13C NMR (100 MHz, CDCl3, δ ppm): 183.28, 147.71, 147.50, 145.07, 144.84, 143.89, 143.54, 143.23, 143.16, 142.03, 141.83, 137.89, 135.86, 132.78, 131.90, 131.80, 131.12, 129.55, 129.48, 128.63, 126.78, 124.98, 124.76, 124.57, 124.47, 123.80, 123.32, 31.81, 31.72, 30.77, 30.48, 29.86, 29.44, 29.34, 29.16, 22.76, 22.71, 14.32, 14.28. Synthesis of 5b. Compound 5b was synthesized similarly as described for synthesis of 5a, purple solid, yield 61% (150 mg). 1H NMR (400 MHz, CDCl3, δ ppm): 9.94 (s, 1H), 8.90 (d, 2H, J = 8.8 Hz), 8.07−8.00 (m, 2H), 7.77−7.73 (m, 3H), 7.49 (d, 2H, J = 8.5 Hz), 7.21 (d, 1H, J = 3.8 Hz), 7.07 (d, 4H, J = 8.7 Hz), 6.93 (d, 2H, J = 8.5 Hz), 6.83 (d, 4H, J = 8.7 Hz), 3.93 (t, 4H, J = 6.5 Hz), 1.81−1.74 (m, 4H), 1.48−1.43 (m, 4H), 1.36−1.34 (m, 8H), 0.93−0.90 (m, 6H). 13C NMR (100 MHz, CDCl3, δ ppm): 183.80, 155.89, 148.85, 148.81, 148.10, 147.55, 145.13, 143.84, 143.70, 140.51, 139.92, 139.60, 135.95, 135.88, 134.18, 129.52, 129.11, 128.55, 127.14, 127.02, 126.66, 126.47, 126.31, 121.97, 120.36, 115.51, 68.48, 31.88, 29.58, 26.03, 22.90, 14.35. Synthesis of 5c. Compound 5c was synthesized similarly as described for synthesis of 5a, purple solid, yield 75% (280 mg). 1H NMR (400 MHz, CDCl3, δ ppm): 9.98 (s, 1H), 9.01−8.98 (m, 2H), 8.20−8.13 (m, 2H), 7.87−7.84 (m, 2H), 7.81 (d, 1H, J = 4.0 Hz), 7.51 (d, 2H, J = 8.7 Hz), 7.21 (s, 1H), 7.08 (d, 4H, J = 8.8 Hz), 6.94 (d, 2H, J = 8.7 Hz), 6.84 (d, 4H, J = 8.9 Hz), 3.94 (t, 4H, J = 6.5 Hz), 1.82− 1.75 (m, 4H), 1.50−1.43 (m, 4H), 1.35−1.25 (m, 16H), 0.91−0.88 (m, 6H). 13C NMR (100 MHz, CDCl3, δ ppm): 183.81, 155.87, 148.87, 148.83, 148.24, 145.16, 143.97, 143.84, 140.51, 140.10, 139.80, 136.00, 135.96, 134.39, 129.76, 129.18, 128.75, 127.30, 127.00, 126.72, 126.69, 126.32, 122.04, 120.38, 115.50, 68.48, 32.07, 29.62, 29.58, 29.50, 26.32, 22.91, 14.37. Synthesis of 5d. Compound 5d was synthesized similarly as described for synthesis of 5a, a red oil, yield 65% (90 mg). 1H NMR (400 MHz, CDCl3, δ ppm): 9.95 (s, 1H), 8.95−8.89 (m, 2H), 7.91− 7.84 (m, 2H), 7.79 (s, 1H), 7.45 (d, 2H, J = 8.6 Hz), 7.20 (s, 1H), 7.08 (d, 4H, J = 8.8 Hz), 6.94 (d, 2H, J = 8.6 Hz), 6.84 (d, 4H, J = 8.8 Hz), 3.94 (t, 4H, J = 6.5 Hz), 2.55−2.50 (m, 4H), 1.82−1.75 (m, 4H), 1.66−1.55 (m, 4H), 1.51−1.44 (m, 4H), 1.36−1.18 (m, 28H), 0.91− 0.88 (m, 6H), 0.80 (t, 6H, J = 6.7 Hz). 13C NMR (100 MHz, CDCl3, δ ppm): 183.22, 155.79, 148.54, 147.55, 145.31, 145.01, 144.99, 143.94, 143.51, 143.14, 142.04, 141.83, 140.67, 137.84, 136.02, 132.64, 131.89, 131.76, 130.53, 126.92, 126.61, 126.52, 124.01, 120.49, 115.49, 68.47, 32.07, 31.81, 31.71, 30.76, 30.47, 29.89, 29.63, 29.60, 29.51, 29.44, 29.34, 29.16, 26.34, 22.92, 22.76, 22.70, 14.38, 14.30, 14.26. Synthesis of FNE44. Under a nitrogen atmosphere, a mixture of compound 5a (80 mg, 0.11 mmol) and cyanoacetic acid (28 mg, 0.33 mmol) in acetonitrile (10 mL) was refluxed in the presence of piperidine (0.2 mL) for 10 h. After cooling to room temperature, poured into water and extracted with DCM, the combined organic solution was washed with water and sodium chloride solution and dried over anhydrous sodium sulfate. After removal of the solvent, the residue was purified by flash column chromatography (silica gel, DCM/MeOH = 10:1). Red solid FNE44 was obtained with a yield of 63% (55 mg). 1H NMR (400 MHz, DMSO-d6, δ ppm): 9.03 (d, 2H, J = 6.9 Hz), 8.26 (s, 1H), 7.99 (m, 2H), 7.83 (s, 1H), 7.61 (d, 2H, J = 8.4 Hz), 7.44 (s, 1H), 7.36−7.32 (m, 4H), 7.11−7.06 (m, 6H), 7.01 (d, 2H, J = 8.4 Hz), 2.48−2.44 (m, 4H), 1.53−1.44 (m, 4H), 1.09− 0.99 (m, 12H), 0.70−0.67 (m, 6H). 13C NMR (100 MHz, THF-d8, δ ppm): 147.82, 147.42, 145.08, 144.97, 144.13, 142.83, 142.69, 141.80, 141.77, 136.75, 135.40, 133.17, 131.79, 131.70, 131.37, 129.33, 129.05, 126.37, 124.80, 124.48, 124.33, 124.24, 123.81, 123.08, 31.83, 31.68, 30.61, 30.34, 29.87, 29.73, 29.24, 29.14, 22.66, 22.63, 13.67, 13.64. MALDI-TOF mass: m/z 800.57. 3181

dx.doi.org/10.1021/cm301520z | Chem. Mater. 2012, 24, 3179−3187

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Scheme 1. Synthetic Route for Compounds FNE44, FNE45, FNE46, and FNE47

sealed using additional hot melt Surlyn film covered with a thin glass slide. Quasi-solid-state gel electrolyte was prepared by mixing 5 wt % poly(vinylidenefluoride-co-hexafluoropropylene) in a redox solution containing 0.1 M LiI, 0.1 M I2, 0.6 M DMPImI and TBP in 3methoxypropionitrile (MPN) under heating until all solids were dissolved. After introducing the hot gel solution into the internal space of the cell from the two holes predrilled on the back of the counter electrode, a uniform motionless polymer gel layer was formed between the working and the counter electrodes, and then the holes were sealed with a Surlyn film covered with a thin glass slide under heat. The working performance of DSSC was tested by recording the current density−voltage (J−V) curves with a Keithley 2400 source meter (Oriel) under illumination of simulated AM1.5G solar light coming from a solar simulator (Oriel-94043A equipped with a 450 W Xe lamp and an AM1.5G filter). The light intensity was calibrated using a standard Si solar cell (Newport 91150). A black mask with aperture area of 0.2304 cm2 was used during measurement to avoid stray light. The electron lifetimes were measured with controlled intensity modulated photovoltage spectroscopy (IMVS), and charge densities at open-circuit were measured using charge extraction technique. IMVS and charge extraction analysis were carried out on an electrochemical workstation (Zahner XPOT), which includes a green light emitting diode (LED, 532 nm) and the corresponding control system. The intensity-modulated spectra were measured at room temperature with light intensity ranging from 5 to 45 W m−2, in modulation frequency ranging from 0.1 Hz to 10 kHz, and with modulation amplitude less than 5% of the light intensity. Long-term stability of the quasi-solid-state DSSCs were measured under visiblelight soaking (AM1.5G, 100 mW cm−2) at around 60 °C in ambient conditions. A 420 nm cutoff filter was put on the cell surface during illumination.

The synthetic approach to sensitizers FNE44, FNE45, FNE46, and FNE47 starting from 5,8-dibromoquinoxaline (1)14 is depicted in Scheme 1. Via a Stille coupling15 with tributyl(thiophen-2-yl)stannane or tributyl(3-hexylthiophen-2yl)stannane, spacers 2a and 2b bridging the donor and anchor groups were produced. After refluxing with a Vilsmeier reagent,16 the corresponding monoaldehyde-substituted derivatives 3a and 3b were synthesized and then converted to bromides 4a and 4b, respectively, by bromination with NBS. Electron donor, triarylamine, was attached via Suzuki coupling6a,17 and provided the precursors 5a, 5b, 5c, and 5d. In the last step, the obtained precursors were converted to the corresponding sensitizer FNE44, FNE45, FNE46. and FNE47, respectively, by Knoevenagel condensation8,18 with cyanoacetic acid through refluxing acetonitrile in the presence of piperidine. All the target dye sensitizers were characterized by 1H NMR, 13 C NMR spectroscopy, and mass spectroscopy, and were found to be consistent with the proposed structures. Dyes FNE44 and FNE47 are red in the solid state, whereas dyes FNE45 and FNE46 are black. After freely dissolving in chloroform or toluene, FNE44 and FNE47 produced orange solutions, whereas FNE45 and FNE46 provided purple solutions. UV−Vis Absorption Properties. The UV−vis absorption spectra of the resulted dyes in chloroform solutions are shown in Figure 1a, and the corresponding photophysical data are listed in Table 1. As shown in Figure 1a, FNE32 exhibits two distinct absorption bands. The absorption band at 368 nm corresponds to the π−π* electron transitions of the conjugated backbone and the other one at 592 nm can be assigned to an intramolecular charge transfer (ICT)19 between the electron donating unit and the electron-withdrawing group.20 Upon replacing thienopyrazine unit with quinoxaline unit, FNE45 and FNE46 display absorption spectra with similar shape containing two distinct absorption bands, i.e., π−π* electron transitions in the high-energy region and ICT transition in the low-energy region. However, compared with the maximum absorption band of FNE32, the absorption spectra of dyes FNE45 and FNE46 display the maximum absorption wavelength at 525 nm with a hypsochromic shift of 67 nm. This

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RESULTS AND DISCUSSION Synthesis and Characterization. In metal-free organic dyes, the rodlike configuration always induces undesirable π−π stacking and intermolecular aggregation, which may lead to selfquenching of excited states and hence inefficient electron injection.11 One effective strategy for overcoming this problem is to introduce long alkyl chains into the dye molecules.8,11 Herein, alkyl groups were introduced into the spacer and donor parts, respectively, and dyes FNE44, FNE45, FNE46, and FNE47 were constructed for the investigation of the relationship between the alkyl position and the DSSC performance. 3182

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spectral difference directly to the conjugation extension caused by the introduction of two alkoxyl groups. To understand the large bathochromic shift of the absorption maximum, we synthesized FNE47 (Chart 1) containing both alkyl chains on the spacer and alkoxyl chains on the donor part for comparison. As shown in Figure 1a and Table 1, although FNE47 consists of two octyloxy chains on the triphenylamine unit as the same as FNE46, FNE47 displays the absorption maximum at 365 nm with an overlapped shoulder at around 460 nm, which is similar to that for FNE44. Compared with FNE44 or FNE47, the distinct bathochromic shift of the maximum absorption band for FNE45 or FNE46 can be explained by the twisted spacer structure of FNE44 and FNE47 caused by the strong steric hindrance between hexylthiophene moiety and quinoxaline moiety, not by introduction of alkoxyl groups on the triarylamine moiety. Consequently, the more planar molecular structures of FNE45 and FNE46, in comparison to FNE44 and FNE47, result in better conjugated systems and stronger ICT interactions between the electron-donating group and electronwithdrawing group. To summarize, not only the chemical structure of the acceptor group but also the substituted position of the alkyl group have a significant effect on the ICT interactions and hence absorption properties of the organic sensitizers. Figure 1b shows the absorption spectra of the adsorbed organic sensitizers on TiO2. When the dye molecules were attached onto nanocrystalline TiO2 surface, a hypsochromic shift of 3, 25, 25, and 4 nm was observed for sensitizers FNE44, FNE45, FNE46, and FNE47, respectively. Such a hypsochromic shift is due to the deprotonation of the carboxylic acid and has been observed for other metal-free organic dyes.11,21 However, unlike most reported D-π-A system, where the hypsochromic shift in absorption maximum from the solution to the dye-loaded TiO2 film is much larger, the slight hypsochromic shift in this case is obviously due to the additional quinoxaline acceptor. After anchoring sensitizers onto TiO2 surface, although the anchor group was deprotonated, the ICT from the electron donor to the inserted quinoxaline unit was not weakened significantly and therefore only slight hypsochromic shift could be observed. This phenomenon is similar to our previously reported theinopyrazine dye FNE3210 and greatly benefit for the sunlight harvest. Electrochemical Properties. To investigate the possibility of the photogenerated electron injection and the sensitizer regeneration, CV was carried out in a typical three-electrode electrochemical cell with TiO2 films stained with sensitizer as the working electrode in a solution of tetrabutylammonium hexafluorophosphate (0.1 M) in water-free acetonitrile with a scan rate of 50 mV s−1 (Figure 2). The highest occupied molecular orbital (HOMO) levels of dyes FNE44, FNE45, FNE46, and FNE47, taken from the first oxidation potential, are determined to be 1.08, 0.88, 0.88, and 0.88 V (vs NHE, the same below), respectively, which are more positive than the redox potential of the I−/I3− redox couple (∼0.4 V), indicating that the reduction of the oxidized dyes with I− ions is thermodynamically feasible.22 Correspondingly, calculated from HOMO levels and the optical band gap derived from the wavelength at 10% maximum absorption intensity for the dyeloaded TiO2 film,23 the LUMO energy levels of dyes FNE44, FNE45, FNE46, and FNE47, are calculated to be −1.41, −1.05, −1.05, and −1.56 V, respectively, indicating enough driving force of the electron injection from their excited states to TiO2 films.22 The energy levels for the resulted dyes are

Figure 1. Absorption spectra of FNE32, FNE44, FNE45, FNE46, and FNE47 in (a) chloroform solutions and on (b) TiO2 films.

Table 1. Photophysical and Electrochemical Properties of the Resulting Sensitizers absorption

dye FNE32 FNE44 FNE45 FNE46 FNE47

ε λmax (nm)a (M−1 cm−1)a 592 362 525 525 365

2.0 3.6 2.0 2.4 4.0

× × × × ×

104 104 104 104 104

λmax on TiO2 (nm)

HOMO (V)b

E0−0 (eV)

LUMO (V)b

576 359 505 505 361

0.97 1.08 0.88 0.88 0.88

1.61 2.49 1.93 1.93 2.44

−0.64 −1.41 −1.05 −1.05 −1.56

a

Absorption peaks (λmax) and molar extinction coefficients were measured in chloroform solutions (∼1 × 10−5 M). bThe potentials (vs NHE) were calibrated with ferrocene.

significant hypsochromic shift is obviously due to the much weaker electron-withdrawing property of quinoxaline in comparison to thienopyrazine, which resulted in the reduced ICT interaction and higher energy absorption. The identical absorption maxima of FNE45 and FNE46 are not difficult to be understood as they have similar chemical structures except for the slightly different length of the alkyl chains attached to the triarylamine unit. However, although FNE44 have the same conjugated backbone as FNE45 and FNE46, the different alkyl substitution positions induced an undesired remarkable change in the absorption spectra. As shown in Figure 1a, the absorption spectrum of FNE44 displays the absorption maximum at 362 nm, which is assigned as the π−π* electron transition of the conjugated system. Another absorption shoulder at around 450 nm corresponding to the weaker ICT band is overlapped by the π−π* transition. As FNE44 contains two alkyl chains on the spacer part while FNE45 and FNE46 consist of two alkoxyl chains on the donor part, it is easy to attribute the significant 3183

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Solar Cell Performance. Action spectra of the incident photon-to-current conversion efficiencies (IPCE) as a function of incident wavelength for DSSCs based on the resulting organic dyes are shown in Figure 3a. The highest IPCE values

Figure 2. Cyclic voltammograms of dye-loaded TiO2 films.

summarized in Table 1. It can be found that the HOMO/ LUMO values of FNE45 and FNE46 are identical, i.e., 0.88 V for HOMO and −1.05 V for LUMO, respectively, since both of them have similar chemical structures except for the slightly different length of the alkyl chain attached on the triarylamine unit. Moreover, although the shape of the absorption spectrum for FNE47 is similar to that for FNE44, FNE47 (0.88 V) displays negatively shifted HOMO level compared with that for FNE44 (1.08 V), which is identical with that for FNE45 and FNE46. This is due to the fact that FNE45, FNE46, and FNE47 consist of the same donor unit, which indicates an identical oxidation potential for the same triarylamine moiety. Furthermore, it should be noted that as compared with FNE32 containing thienopyrazine moiety with LUMO level of −0.64 V where the driving force for electron injection from the oxdized dye molec ules into the TiO2 is insufficient,22 the incorporation of quinoxaline unit instead of thienopyrazine unit has successfully upward shifted the LUMO levels of FNE44, FNE45, FNE46, and FNE47, which resulted in an efficient photogenerated electron injection from the organic dye to the TiO2 semiconductor. Theoretical Approach. To investigate the geometrical and electronic properties of the resulted dyes, density functional calculations were conducted using the Gaussian 03 program package at the B3LYP/6-31G(d) level (see Figure S1 in the Supporting Information).24 It is found that the HOMO is distributed along the π system of the donor and the spacer parts, whereas the LUMO reflecting the excited state of the dye under light illumination is a π* orbital delocalized across the cyanoacetic unit and the spacer. The overlapping distribution of the HOMO and LUMO orbitals on the quinoxaline unit in FNE44, FNE45, FNE46, and FNE47 implies that the quinoxaline unit facilitates the electron transfer from the donor to the anchor group. Upon photoexcitation, the electrons in the organic dyes loaded on nanocrystalline TiO2 surface can be successively transferred from triarylamine to quinoxaline, then transferred to the cyanoacrylic acid unit and finally into the conduction band of TiO2. It is worth noting that the dihedral angle between the quinoxaline and neighbored thiophene unit in FNE45 and FNE46 is calculated to be around 23°, as shown in Figure S1 in the Supporting Information. However, FNE44 and FNE47 show a more twisted dihedral angle of around 59° due to the large steric effect between the hexyl chain and quinoxaline unit. This may interrupt the conjugation system and weaken the ICT interactions, resulting in a hypsochromic shift of the maximum absorption band, which is consistent with the experimental observation in the absorption spectra of FNE44 and FNE47 in comparison to those for FNE45 and FNE46.

Figure 3. (a) IPCE spectra and (b) J−V curves for DSSCs based on FNE32, FNE44, FNE45, FNE46, and FNE47 with liquid electrolyte.

of 55, 82, 83, and 54% were achieved for the DSSCs based on FNE44, FN45, FNE46, and FNE47, respectively, with liquid electrolyte (0.6 M DMPImI, 0.1 M LiI, 0.05 M I2, and 0.5 M TBP in acetonitrile). Taking the reflectance and absorption by the conductive glass into account, the maximum IPCE can be regarded as unity. The IPCE spectra for DSSCs based on FNE45 and FNE46 are much broader than those for FNE44 and FNE47, which are in good agreement with their absorption spectra. The slight difference between FNE45- and FNE46based DSSCs can be probably stemmed from their different absorption intensity on TiO2 films. Compared with FNE44, FNE47 displays a bathochromically shifted IPCE curve due to the incorporation of two octyloxy groups on the donor moiety, which induces a bathochromic shift of the ICT band in the absorption spectrum. The much lower IPCE values for FNE44and FNE47-based DSSCs are due to the lower light-harvesting efficiency in the visible region. It should be noted that although FNE32 displayed a much broader absorption spectrum on TiO2 film in comparison to those for FNE44, FNE45, FNE46, and FNE47, the IPCE spectrum of FNE32-based DSSC exhibited a maximum value below 40% in the visible region due to its unsuitable LUMO level which resulted in an insufficient electron injection driving force from the excited organic dye molecules to the conduction band of TiO2 semiconductor. The current density−voltage (J−V) characteristic of liquid DSSCs based on FNE32, FNE44, FNE45, FNE46, and FNE47 were evaluated under 100 mW cm−2 simulated AM1.5G solar light and the curves are shown in Figure 3b. Although FNE32 exhibited a broad absorption spectrum extending to the NIR 3184

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region, the DSSC based on FNE32 produced a short circuit photocurrent density (Jsc) of 6.66 mA cm−2, an open circuit voltage (Voc) of 526 mV, and a fill factor (FF) of 0.72, corresponding to a η of 2.52%. Upon the incorporation of quinoxaline unit instead of thienopyrazine unit, FNE44 and FNE47 based liquid DSSCs provided Jsc of 5.70 and 6.18 mA cm−2, Voc of 735 and 744 mV, and FF of 0.78 and 0.75, respectively, corresponding to η of 3.27 and 3.45%, respectively. Under the same condition, the liquid DSSCs based on FNE45 and FNE46 offered Jsc of 14.60 and 16.16 mA cm−2, Voc of 676 and 682 mV, and FF of 0.75 and 0.75, respectively, corresponding to η values of 7.40 and 8.27%, respectively. Compared with FNE32-, FNE44-, and FNE47-based DSSCs, the remarkably enhanced Jsc value of the FNE45- or FNE46based DSSC is consistent with the trend of the photogenerated current integrated from the IPCE spectra (Figure 3a). Although FNE44- and FNE47-based DSSCs displayed much lower η values in comparison to FNE45- and FNE46-based DSSCs, the Voc of FNE47 based DSSC was 68, and 62 mV higher than that for the DSSCs based on FNE45 and FNE46, respectively, whereas the Voc of FNE44-based DSSC was 59 and 53 mV higher than that for the DSSCs based on FNE45 and FNE46, respectively, under the same condition. Since Voc is related to the conduction band position of TiO2 and the charge recombination rate in DSSCs,25 in order to understand the significant difference of the Voc caused by different substituted positions, the relative conduction band positions and electron lifetime in the DSSCs based on the resulted sensitizers were investigated. Figure 4 shows the relation between Voc and

Figure 5. Electron lifetime as a function of charge density at open circuit for DSSCs based on the resulted sensitizers.

tionally, a power law relation but with a different slope can be observed for the curve based on the DSSC with FNE32 as sensitizer, which suggests a different charge recombination mechanism. At a fixed charge density, the electron lifetime for the DSSCs based on FNE44 and FNE47 is larger than that for FNE45 or FNE46 based DSSC by around 6- and 9-fold, respectively. This indicates that the charge recombination between electrons in TiO2 film and electron acceptors in the electrolyte is significantly retarded by the introduced alkyl substituents on the π bridge in comparison to the introduction in the donor part. The retarded charge recombination rate constant will reduce charge density at open circuit. When more charge is accumulated in TiO2, Fermi level moves upward and Voc gets larger.23,25c At 45 W m−2 LED light (532 nm), the extracted charge densities are 73.1, 36.9, 38.2, and 77.3 C cm−2 for FNE44-, FNE45-, FNE46-, and FNE47-based DSSCs, respectively. According to the Q−Voc plots, the Voc gain from FNE45- or FNE46-based DSSC to FNE47-based DSSC arising solely from the increased charge density or retarded charge recombination is calculated to be 70 and 67 mV, respectively, whereas that to FNE44-based DSSC is calculated to be 65 and 62 mV, respectively, explaining why FNE47-based DSSC produces about 68 and 62 mV higher Voc than the DSSC based on FNE45 and FNE46, respectively, while FNE44 -based DSSC provides around 59 and 53 mV higher Voc than the DSSC based on FNE45 and FNE46, respectively. In comparison to the DSSCs with liquid electrolyte, quasisolid-state DSSCs29 have shown greatly improved long-term stability as there is less or no possibility for leakage of the electrolyte. Therefore, in view of promising commercial application, quasi-solid-state DSSCs based on FNE32, FNE44, FNE45, FNE46, and FNE47 were fabricated using a quasi-solid-state gel electrolyte containing 0.1 M LiI, 0.1 M I2, 0.6 M DMPImI, and 5 wt % poly(vinylidenefluoride-cohexafluoropropylene) in MPN. To achieve the best photovoltaic performance, the content of another additive, TBP, was first optimized for FNE46-based quasi-solid-state DSSC and the performance data are summarized in Table 2. With the increase of the TBP concentration, the Jsc value slightly decreased but Voc increased significantly with the TBP concentration increasing. This is mainly attributed to the negative shift of conduction band of TiO2 caused by the adsorption of TBP on TiO2 surface,30 which lowered the driving force for the electron injection but enlarged the energy level difference between the Fermi level of TiO2 and the redox potential under light irradiation. Figure 6 compares the J−V curves for FNE32-, FNE44-, FNE45-, FNE46-, and FNE47-based quasi-solid-state DSSCs

Figure 4. Charge density at open circuit as a function of Voc for DSSCs based on the resulting sensitizers.

photoinduced charge density (Q)26 measured with charge extraction technique.27 It can be found that for DSSCs based on FNE32, FNE44, FNE45, FNE46, and FNE47, the Voc increases linearly with the logarithm of Q and all the curves have almost identical slope (95 mV). At fixed Q, the same Voc values for DSSCs based on FNE44, FNE45, FNE46, and FNE47 indicates that the adjustment of the alkyl chain position in organic sensitizers has no influence on the movement of the conduction band of TiO2. Therefore, the significant improvement of Voc from FNE45or FNE46- to FNE44- or FNE47-based DSSC should be attributed to the repression of charge recombination, which is related to electron lifetime.28 Figure 5 shows the electron lifetime as a function of charge density at open circuit. For DSSCs based on FNE44, FNE45, FNE46, and FNE47, the electron lifetime decreases with charge density following a power law relation with the same slope, suggesting the same charge recombination mechanism in the four DSSCs. Addi3185

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sensitizers demonstrated good long-term stability during light soaking. Therefore, it is concluded that sensitizer FNE46 is a promising candidate for highly efficient and stable quasi-solidstate DSSC.

Table 2. Photovoltaic Performance for FNE46-Based Quasisolid-state DSSC with Electrolytes Containing Different Concentrations of TBP CTBP (M)

Voc (mV)

Jsc (mA cm−2)

FF

η%

0.0 0.1 0.3 0.5

522 632 642 660

16.58 15.68 15.48 15.17

0.50 0.72 0.70 0.69

4.33 7.14 6.96 6.91

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CONCLUSIONS In summary, a series of quninoxaline based metal-free organic sensitizers has been designed and synthesized. Via molecular engineering, the replacement of the thiophene ring fusing on the pyrazine unit in FNE32 by a benzene ring in FNE44, FNE45, FNE46, and FNE47 lifted up the LUMO level of the organic dyes. In comparison to FNE45 and FNE46, although FNE32-loaded TiO2 film exhibited a much broader absorption spectrum extending to the NIR region, however, FNE45 and FNE46 based DSSCs displayed much higher IPCE values due to the greatly enhanced electron injection driving force from the excited organic sensitizers to the TiO2 semiconductor, which resulted in a higher photogenerated current and a higher power conversion efficiency. This indicates that not only broad spectral coverage but also suitable energy levels are both required for high performance DSSCs. Furthermore, the removal of the alkyl substituents from the spacer part to the donor part induced a more conjugated system and a bathochromically shifted maximum absorption band. Although the DSSCs based on FNE44 and FNE47 with alkyl groups substituted on the spacer bridge demonstrated increased Voc values due to the more effective repression of charge recombination between electrons in TiO2 film and electron acceptors in the electrolyte, the DSSCs based on FNE45 and FNE46 showed an improved light harvesting efficiency and therefore photogenerated current due to their much broader absorption spectra. Furthermore, after modification of the alkyl length, a η of 8.27 and 7.14%, respectively, was realized for the DSSC based on FNE46 with octyloxy substituents using liquid electrolyte and quasi-solid-state electrolyte, respectively. The η of the latter quasi-solid-state DSSC remained 100% of the initial value after continuous light soaking for 1000 h. Overall, this work helps to understand the crucial importance of molecular engineering and presents a way for designing metalfree organic dyes for highly efficient and stable DSSCs.

Figure 6. J−V curves for the quasi-solid-state DSSCs based on the resulting sensitizers.

using a quasi-solid-state gel electrolyte containing 0.1 M TBP. The resulted quasi-solid-state DSSCs based on FNE32, FNE44, FNE45, FNE46, and FNE47 offered Jsc of 11.29, 6.42, 14.33, 15.68, and 6.76 mA cm−2, Voc of 470, 669, 624, 632, and 683 mV, and FF of 0.69, 0.72, 0.72, 0.72, and 0.72, respectively, corresponding to η values of 3.66, 3.09, 6.44, 7.14, and 3.30%, respectively. The photovoltaic performance of the resulted quasi-solid-state DSSCs based on FNE44, FNE45, FNE46, and FNE47 is consistent with the trend for the DSSCs with liquid electrolyte discussed above. Compared with FNE32-based DSSC with liquid electrolyte, the improved performance of FNE32 based quasi-solid-state DSSC is probably due to the reduced TBP concentration in the electrolyte, which might positively shift the conduction band of the TiO2 and therefore enlarge the electron injection driving force.23,31 Furthermore, the stability of the quasi-solid-state DSSCs based on the resulted sensitizers was recorded over a period of 1000 h under one-sun soaking. Figure 7 displays the photovoltaic performance parameters of the quasi-solid-state DSSC employing FNE46 as sensitizer under one sun soaking. It can be found that the power conversion efficiency remained 100% of the initial value after 1000 h of one sun soaking, which indicates that the quasi-solid-state DSSCs based on the resulting

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ASSOCIATED CONTENT

S Supporting Information *

Calculated frontier molecular orbitals and experimental energy level diagram of the resulted sensitizers (PDF). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (G.Z.); [email protected]. cn (Z.-S.W.). Tel/Fax: (+86)21-5163-0345. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Basic Research Program (2011CB933302) of China, the National Natural Science Foundation of China (90922004, 50903020, and 20971025), Shanghai Pujiang Project (11PJ1401700), Shanghai Leading Academic Discipline Project (B108), and Jiangsu Major Program (BY2010147).

Figure 7. Evolutions of photovoltaic performance parameters for FNE46-based quasi-solid-state DSSC under one sun soaking. 3186

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dx.doi.org/10.1021/cm301520z | Chem. Mater. 2012, 24, 3179−3187