New Diketopyrrolopyrrole (DPP) Dyes for Efficient Dye-Sensitized

Dec 17, 2009 - (10) On the other hand, DPP-containing materials are bright and strongly fluorescent with exceptional photochemical, mechanical, and th...
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J. Phys. Chem. C 2010, 114, 1343–1349

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New Diketopyrrolopyrrole (DPP) Dyes for Efficient Dye-Sensitized Solar Cells Sanyin Qu, Wenjun Wu, Jianli Hua,* Cong Kong, Yitao Long, and He Tian* Key Laboratory for AdVanced Materials and Institute of Fine Chemicals, East China UniVersity of Science & Technology, 130 Meilong Road, Shanghai 200237, People’s Republic of China ReceiVed: October 12, 2009; ReVised Manuscript ReceiVed: NoVember 29, 2009

Two novel metal-free dyes (DPP-I and DPP-II) with a diketopyrrolopyrrole (DPP) core were synthesized for dye-sensitized solar cells (DSSCs). The absorption spectra and electrochemical and photovoltaic properties of DPP-I and DPP-II were extensively investigated. Electrochemical measurement data indicate that the tuning of the HOMO and LUMO energy levels can be conveniently accomplished by alternating the π-conjugated systems. Besides, coadsorption of chenodeoxycholic acid (CDCA) can hinder the formation of dye aggregates and might improve electron injection yield and, thus, Jsc. This has also led to a rise in the photovoltage, which is attributed to the decrease of charge recombination. The DSSC based on dye DPP-I showed better photovoltaic performance: a maximum monochromatic incident photon-to-current conversion efficiency (IPCE) of 80%, a short-circuit photocurrent density (Jsc) of 9.78 mA cm-2, an open-circuit photovoltage (Voc) of 605 mV, and a fill factor (FF) of 0.69, corresponding to an overall conversion efficiency of 4.14% under standard global AM 1.5 solar light condition. This work suggests that the metal-free dyes based on a DPP core are promising candidates for improvement of the performance of DSSCs. 1. Introduction Dye-sensitized solar cells (DSSCs) have received considerable attention as the most promising candidates for renewable energy systems in recent years, owing to their high conversion efficiency and low cost of production.1 DSSCs based on Rucomplex photosensitizers, such as N3, N719, and the black dye, have shown a record solar-energy-to-electricity conversion efficiency of 11%.2 On the other hand, metal-free organic dyes have also been developed for DSSCs due to their high molar absorption coefficient, simple synthesis process, low cost, and structure adjustability. Metal-free dyes, such as perylene dyes,3 cyanine dyes,4 merocyanine dyes,5 coumarin dyes,6 hemicyanine dyes,7 and indoline dyes,8 have been investigated as sensitizers for DSSCs, and great progress has been made in this field. The sensitizer is a crucial element in DSSCs, exerting significant influence on the power conversion efficiency as well as the stability of the cells. Generally, organic dyes used for efficient solar cells are required to possess high charge carrier mobility and broad and intense spectral absorption in the visiblelight region.9 Here, we present two novel efficient dyes (DPP-I and DPP-II) (shown in Figure 1) with triphenylamine as the donor, 2,5-dibutylpyrrolo-[3,4-c]pyrrole-1,4(2H,5H)-dione (DPP) as a π-conjugated system, and carboxylic acid moiety acting as acceptor. The triphenylamine can improve the hole-transporting ability of the materials, and the nonplanar structure of triphenylamine can also prevent the formation of dye aggregates.10 On the other hand, DPP-containing materials are bright and strongly fluorescent with exceptional photochemical, mechanical, and thermal stability and are, therefore, used in industrial applications as high performance pigments in paints, plastics, and inks.11 DPP-based molecular materials, however, are not soluble in most common organic solvents due to the * To whom correspondence should be addressed. E-mail: tianhe@ ecust.edu.cn (H.T.), [email protected] (J.H.). Fax: 86-21-64252756 (H.T.), 86-21-64252758 (J.H.). Tel: 86-21-64252756 (H.T.), 86-21-64250940 (J.H.).

Figure 1. Molecular structures of DPP-I and DPP-II dyes.

concurrent strong H bonding and π-π intermolecular interactions in the solid state.12 Soluble derivatives, however, can be made by attaching long alkyl chains on the lactam N atom positions of the DPP moiety.13 Also, the introduction of long alkyl chains into DPP rings can also inhibit the charge recombination and increase the electron lifetime.14 Finally, the highly absorbing DPP moiety and the extended absorption at long wavelengths due to intramolecular charge transfer between the thiophene units and the DPP core increase their potential applicability as dye-sensitizer for DSSCs.15 However, to the best of our knowledge, small chromophores containing DPP structural motifs have not been exploited for DSSCs. On the basis of the above consideration, we have designed and synthesized the two novel organic sensitizers, DPP-I and DPP-II, and applied them successfully to sensitize nanocrystalline TiO2-based solar cells.

10.1021/jp909786k  2010 American Chemical Society Published on Web 12/17/2009

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2. Experimental Section 2.1. Materials and Reagents. Tetra-n-butylammonium hexafluorophosphate (TBAPF6), 4-tert-butylpyridine (4-TBP), lithium iodide, and 2-cyanoacetic acid were purchased from Fluka. THF was predried over 4 Å molecular sieves and distilled under argon atmosphere from sodium benzophenone ketyl immediately prior to use. All other solvents and chemicals were purchased from Aldrich and used as received without further purification. 3,6-bis-(4-bromophenyl)-2,5-di-n-butyl-pyrrolo[3,4c]-pyrrole-1,4-dione (1), 3,6-bis-(5-bromo-thiophen-2-yl)-2,5di-n-butyl-pyrrolo[3,4-c]-pyrrole-1,4-dione (2), and 4-(bis(4methoxyphenyl)-amino)phenylboronic acid were synthesized according to the corresponding literature methods.16 Transparent FTO conducting glass (fluorine-doped SnO2, transmission >90% in the visible, sheet resistance ) 15 Ω/square) was obtained from Geao Science and Educational Co. Ltd. of China. Commercial TiO2 (P25) was used for the preparation of the nanocrystalline films. 2.2. Spectroscopic Measurements. Proton NMR spectra were obtained with a Bru¨cker AM 500 spectrometer (relative to TMS). Mass spectra were recorded with a Waters Micromass LCT mass spectrometer. The absorption spectra of the dyes in solution and adsorbed on TiO2 films were measured with a Varian Cary 500 spectrophotometer. 2.3. Synthesis of Dyes. 2.3.1. Synthesis of 3-[4-[4-(N,NBis(4-methoxyphenyl)amino)phenyl]phenyl]-6-(4-bromophenyl)-2,5-di-n-butyl-pyrrolo[3,4-c]pyrrole-1,4-dione (3). Compound 1 (0.84 g, 1.50 mmol), Pd(PPh3)4 (10 mg, 0.01 mmol), and Na2CO3 (1.02 g, 0.01 mol) in 30 mL of THF and 5 mL of H2O were heated to 45 °C under a nitrogen atmosphere for 30 min. A solution of 4-(bis(4-methoxyphenyl)-amino)phenylboronic acid (0.63 g, 1.80 mmol) in THF (10 mL) was added slowly, and the mixture was refluxed for a further 12 h. After cooling to room temperature, the mixture was extracted with 50 mL of CH2Cl2. The organic portion was combined and removed by rotary evaporation. The residue was purified by column chromatography on silica (CH2Cl2/petroleum ether ) 1/1, v/v) to give an orange solid (0.41 g). Yield: 34.9%. 1H NMR (500 MHz, CDCl3) δ: (ppm) 7.89 (d, J ) 8.29 Hz, 2H), 7.72 (d, J ) 8.34 Hz, 4H), 7.66 (d, J ) 8.51 Hz, 2H), 7.48 (d, J ) 8.60 Hz, 2H), 7.12 (d, J ) 8.81 Hz, 4H), 6.99 (d, J ) 8.58 Hz, 2H), 6.86 (d, J ) 8.82 Hz, 4H), 3.82 (s, 6H), 3.75 (t, J ) 7.47 Hz, 7.63 Hz, 4H), 1.55-1.65 (m, 4H), 1.20-1.35 (m, 4H), 0.85 (m, 6H). 2.3.2. Synthesis of 3-[5-[4-(N,N-Bis(4-methoxyphenyl)amino)phenyl]thiophen-2-yl]-6-(5-bromo-thiophen-2-yl)-2,5-di-nbutyl-pyrrolo[3,4-c]pyrrole-1,4-dione (4). The synthesis method resembles that of compound 3, and the compound was purified by column chromatography on silica (CH2Cl2/petroleum ether ) 5/1, v/v) to give a blue purple solid. Yield: 33.3%. 1H NMR (500 MHz, CDCl3) δ: (ppm) 9.13 (d, J ) 4.20 Hz, 1H), 8.62 (d, J ) 4.15 Hz, 1H), 7.47 (d, J ) 8.53 Hz, 2H), 7.34 (d, J ) 4.18 Hz, 1H), 7.23 (d, J ) 4.15 Hz, 1H), 7.09 (d, J ) 8.88 Hz, 4H), 6.93 (d, J ) 8.58 Hz, 2H), 6.85 (d, J ) 8.85 Hz, 4H), 4.12 (m, 4H), 3.83 (s, 6H), 1.72 (m, 4H), 1.43 (m, 4H), 0.96 (t, J ) 7.34 Hz, 7.39 Hz, 6H). 2.3.3. Synthesis of 5-[4-[3-[4-(4-(N,N-Bis(4-methoxyphenyl)amino)phenyl)phenyl]-2,5-di-n-butyl-pyrrolo[3,4-c]pyrrole1,4-dione]phenyl]thiophene-2-carbaldehyde (5). Compound 3 (0.40 g, 0.51 mmol), Pd(PPh3)4 (10 mg, 0.01 mmol), and Na2CO3 (1.02 g, 0.01 mol) in 10 mL of THF and 5 mL of H2O were heated to 45 °C under a nitrogen atmosphere for 30 min. A solution of 5-formylthiophen-2-yl boronic acid (0.17 g, 1.10 mmol) in THF (5 mL) was added slowly, and the mixture was

Qu et al. refluxed for a further 12 h. After cooling to room temperature, the mixture was extracted with 30 mL of CH2Cl2. The organic portion was combined and removed by rotary evaporation. The residue was purified by column chromatography on silica (CH2Cl2/ethyl acetate ) 1/70, v/v) to give a red solid. Yield: 46.6%. 1H NMR (500 MHz, CDCl3) δ: (ppm) 9.90 (s, 1H), 7.93 (m, 4H), 7.82 (d, J ) 8.29 Hz, 2H), 7.78 (d, J ) 3.84 Hz, 1H), 7.71 (d, J ) 8.25 Hz, 2H), 7.51 (d, J ) 3.85 Hz, 1H), 7.48 (d, J ) 8.57 Hz, 2H), 7.12 (d, J ) 8.81 Hz, 4H), 6.99 (d, J ) 8.54 Hz, 2H), 6.87 (d, J ) 8.84 Hz, 4H), 3.81 (m, 10H), 1.58-1.69 (m, 4H), 1.21-1.38 (m, 4H), 0.82-0.91 (m, 6H). 13C NMR (125 MHz, CDCl ) δ: 183.4, 163.5, 163.3, 156.9, 3 153.3, 149.8, 149.6, 147.1, 144.3, 143.9, 141.1, 138.0, 135.8, 131.7, 130.2, 130.0, 129.7, 128.2, 127.6, 127.3, 127.1, 126.5, 125.7, 120.7, 115.5, 111.2, 110.2, 56.2, 42.6, 42.5, 32.3, 32.2, 20.7, 14.3. 2.3.4. Synthesis of 5-[5-[3-[5-(4-(N,N-Bis(4-methoxyphenyl)amino)phenyl)thiophene-2-yl]-2,5-di-n-butyl-pyrrolo[3,4c]pyrrole-1,4-dione]thiophene-2-yl]thiophene-2-carbaldehyde (6). The synthesis method resembles that of compound 5, and the compound was purified by column chromatography on silica (CH2Cl2) to give a green solid. Yield: 43.2%. 1H NMR (500 MHz, CDCl3) δ: (ppm) 9.90 (s, 1H), 9.08 (d, J ) 4.20 Hz, 1H), 8.85 (d, J ) 4.17 Hz, 1H), 7.72 (d, J ) 3.97 Hz, 1H), 7.49 (d, J ) 2.58 Hz, 2H), 7.48 (d, J ) 1.80, 1H), 7.39 (d, J ) 3.94 Hz, 1H), 7.35 (d, J ) 4.19 Hz, 1H), 7.10 (d, J ) 8.88 Hz, 4H), 6.92 (d, J ) 8.76 Hz, 2H), 6.86 (d, J ) 8.91, 4H), 4.12 (m, 4H), 3.83 (s, 6H), 1.77 (m, 4H), 1.48 (m, 4H), 0.98 (m, 6H). 13C NMR (500 MHz, CDCl3) δ: 184.0, 163.2, 162.6, 158.2, 153.4, 151.4, 147.1, 144.5, 142.7, 141.6, 141.5, 139.8, 138.8, 138.6, 137.1, 128.8, 128.6, 127.0, 124.9, 121.2, 116.5, 57.2, 55.1, 43.8, 33.8, 33.7, 21.9, 15.4. 2.3.5. Synthesis of 2-cyano-3-[5-[4-[3-[4-(4-(N,N-Bis(4methoxyphenyl)amino)phenyl)phenyl]-2,5-di-n-butyl-pyrrolo[3,4c]pyrrole-1,4-dione]phenyl]thiophene-2-yl]acrylic acid (DPPI). Compound 5 (0.10 g, 0.12 mmol), 2-cyanoacetic acid (0.11 g, 1.29 mmol), and piperidine (0.5 mL) in 20 mL of THF were heated to reflux under a nitrogen atmosphere for 6 h. After cooling to room temperature, the precipitate was filtered. The residue was purified by column chromatography on silica (CH2Cl2/ethanol ) 10/1, v/v) to give a dark red solid. Yield: 73.4%. 1H NMR (500 MHz, DMSO) δ: (ppm) 8.45 (s, 1H), 8.18 (d, 1H), 7.98 (d, 1H), 7.89-7.96 (m, 4H), 7.82-7.86 (m, 4H), 7.65 (d, J ) 8.47 Hz, 2H), 7.12 (d, J ) 8.76 Hz, 4H), 6.98 (d, J ) 8.82 Hz, 4H), 6.82 (d, J ) 8.61 Hz, 2H), 3.71-3.82 (m, 10H), 1.64 (m, 4H), 1.20 (m, 4H), 0.69 (m, 6H). 13C NMR (125 MHz, DMSO) δ: 163.1, 162.9, 156.7, 151.1, 149.6, 149.2, 147.1, 143.5, 141.1, 137.5, 135.2, 131.7, 130.1, 130.0, 129.7, 128.1, 127.5, 126.5, 126.4, 126.3, 125.2, 120.7, 115.4, 110.6, 109.8, 56.1, 45.4, 42.0, 32.5, 31.9, 30.3, 30.0, 23.4, 23.1, 20.6, 14.8, 14.2. HRMS (m/z): [M + H]+ calcd for (C54H48N4O6S), 881.3295; found, 881.3285. 2.3.6. Synthesis of 2-Cyano-3-[5-[5-[3-[5-(4-(N,N-bis(4methoxyphenyl)amino) phenyl)thiophene-2-yl]-2,5-di-n-butylpyrrolo[3,4-c]pyrrole-1,4-dione]thiophene-2-yl]thiophene-2yl]acrylic acid (DPP-II). The synthesis method resembles that of compound DPP-I, and the compound was purified by column chromatography on silica (CH2Cl2/ethanol ) 10/1, v/v) to give a dark green solid. Yield: 56.8%. 1H NMR (500 MHz, DMSO) δ: (ppm) 8.92 (d, J ) 2.92, 1H), 8.78 (d, J ) 3.70, 1H), 8.13 (s, 1H), 7.72 (d, 1H), 7.68 (d, 1H), 7.62 (d, 1H), 7.56 (d, 1H), 7.53 (d, J ) 8.78 Hz, 2H), 7.04 (d, J ) 8.41 Hz, 4H), 6.93 (d, J ) 8.57 Hz, 4H), 6.72 (d, J ) 8.18 Hz, 2H), 4.02 (m, 4H), 3.76 (s, 6H), 1.64 (m, 4H), 1.38 (m, 4H), 0.94 (t, J ) 7.13 Hz,

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SCHEME 1: Synthesis of the DPP Sensitizers

7.38 Hz, 6H). 13C NMR (125 MHz, DMSO) δ: 162.8, 162.2, 158.0, 151.2, 151.0, 146.8, 144.5, 142.3, 141.9, 141.5, 137.9, 137.2, 136.2, 136.0, 134.3, 128.8, 128.5, 127.0, 124.9, 121.1, 116.4, 57.1, 56.5, 54.6, 43.3, 34.0, 33.4, 32.1, 31.8, 24.6, 24.3, 21.8, 15.9, 15.3. HRMS (m/z): [M + H]+ calcd for (C50H44N4O6S3), 893.2501; found, 893.2502. 2.4. Electrochemical Measurements. The oxidation potentials of dyes adsorbed on TiO2 films were measured in a normal three-electrode electrochemical cell. TiO2 films stained with sensitizer were used as the working electrode, a platinum wire was the counter electrode, and a regular calomel electrode in saturated KCl solution was the reference electrode. The measurements were performed using a potentiostat/galvanostat model K0264 (Princeton Applied Research). The supporting electrolyte was 0.1 M TBAPF6 (tetra-n-butylammonium hexafluorophosphate) in acetonitrile as the solvent. 2.5. Preparation of Solar Cells. The dye-sensitized TiO2 electrode was prepared by following the procedure reported in the literature.17 A TiO2 colloidal dispersion was made employing commercial TiO2 (P25, Degussa AG, Germany) as material. Films of nanocrystalline TiO2 colloidal on FTO were prepared by sliding a glass rod over the conductive side of the FTO. Sintering was carried out at 450 °C for 30 min. Before immersion in the dye solution, these films were soaked in the 0.2 M aqueous TiCl4 solution overnight in a closed chamber, which can significantly increase the short-circuit photocurrent. The thickness of the TiO2 film was about 12 µm. After being washed with deionized water and fully rinsed with ethanol, the films were heated again at 450 °C, followed by cooling to 80 °C and dipping into a 3 × 10 -4 M solution of dyes in CH2Cl2 for 12 h at room temperature. For the coadsorption, chenodeoxycholic acid was added. The adsorbed TiO2 electrode and Pt counter electrode were assembled into a sealed sandwich-type cell by heating with a hot-melt ionomer film (Surlyn 1702, DuPont). The redox electrolyte was placed in a drilled hole in

the counter electrode by capillary force and was driven into the cell by means of vacuum backfilling. Finally, the hole was sealed using a UV-melt gum and a cover glass. Two electrolytes were used for device evaluation, in which one was composed of 0.1 M lithium iodide, 0.6 M 1,2-dimethyl-3-n-propylimidazolium iodide (DMPImI), 0.1 M I2, and 0.5 M 4-tert-butylpyridine (4TBP) in acetonitrile; the other was composed of 0.1 M lithium iodide, 0.6 M methyl-propylimidazolum iodide (MPII), and 0.05 M I2 in the mixed solvent of acetonitrile and 3-methoxypropionitrile (7:3, v/v). 2.6. Photoelectrochemical Measurements. Photovoltaic measurements employed an AM 1.5 solar simulator equipped with a 300 W xenon lamp (model no. 91160, Oriel). The power of the simulated light was calibrated to 100 MW/cm2 using a Newport Oriel PV reference cell system (model 91150 V). I-V curves were obtained by applying an external bias to the cell and measuring the generated photocurrent with a Keithley model 2400 digital source meter. The voltage step and delay time of photocurrent were 10 mV and 40 ms, respectively. The cell active area was tested with a mask of 0.158 cm2. The photocurrent action spectra were measured with an IPCE test system consisting of a model SR830 DSP Lock-In Amplifier and model SR540 Optical Chopper (Stanford Research Corporation, U.S.A.), a 7IL/PX150 xenon lamp and power supply, and a 7ISW301 spectrometer. Impedance spectra were done using a CHI-660c electrochemical station.) 2.7. Electrical Impedance Measurements. Electrical impedance experiments were performed in the dark with a CHI660C electrochemical workstation, with a frequency range from 0.01 Hz to 100 kHz and a potential modulation of 10 mV. The applied bias potential was held at -0.5 V. 3. Results and Discussion 3.1. Synthesis. The synthetic route to DPP dyes containing thiophene is depicted in Scheme 1. The butyl chain on the DPP

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Figure 2. Normalized absorption spectra of DPP-I and DPP-II in CH2Cl2 and on TiO2 film.

nucleus can improve the solubility and form a tightly packed insulating monolayer blocking the I3- or cations approaching the TiO2.14 A 4-methoxy-N-(4-methoxyphenyl)-N-phenylbenzenamine group as donor is attached to the core DPP by the Suzuki coupling reaction. The reaction produced the disubstituted side products; fortunately, monocapped compounds can be easily separated by column chromatography. In the next step, this bromo-exposed intermediate reacted with 5-formylthiophen2-ylboronic acid by the Suzuki coupling reaction. Finally, the target products (DPP-I and DPP-II) were synthesized via the Knoevenagel condensation reaction of the respective carbaldehydes with cyanoacetic acid in the presence of piperidine. All the key intermediates and two novel organic DPP sensitizers have been confirmed by 1H NMR, 13C NMR, and HRMS. 3.2. Absorption Properties in Solutions and on TiO2 Film. Normalized absorption spectra of the two dyes, DPP-I and DPPII, in a diluted solution of CH2Cl2 and TiO2 films were shown in Figure 2. The absorption maxima of the dyes, DPP-I and DPP-II, appeared at 524 and 627 nm in diluted solution, respectively. The latter was observed to be largely shifted to a longer wavelength than the former DPP-I because of the two

thiophenes instead of the benzene in the π-conjugated linker part. Compared with the spectrum in dichloromethane solution, the absorption spectra of the two dyes attached to TiO2 film were broadened significantly and a nearly 100 nm red shift of the maxima absorption peak was observed, which can be attributed to the formation of a J-type aggregate. We have measured the adsorbed amount of DPP dyes for the electrodes with and without addition of the CDCA, in which there is further proof to support this comment that the coadsorption of CDCA can retard the aggregation of the DPP dyes. As seen in Table 3, when TiO2 films were exposed to DPP dye solutions containing CDCA, the DPP dyes adsorption was reduced gradually with increasing the CDCA content in the dye solution. The drop of DPP dyes’ adsorption indicates that CDCA competes for the TiO2 surface sites with the DPP dye molecules. Strong interaction between the adsorbed dye molecules and the oxide molecules in the TiO2 surface leads to aggregate formation, and consequently, broadening of the absorption spectrum was observed during the dye absorption. This might be correlated with dyesTiO2 interactions, dyesdye interactions, or both, and coadsorption of CDCA diminished these interactions on the TiO2 surface.18 The broadened absorption of two dyes loaded on the TiO2 film is advantageous for light harvesting of the solar spectrum. The absorption peak showed smaller red shifts when two DPP dyes coadsorbed with CDCA compared with the spectrum on TiO2 film, revealing that the aggregation of DPP dye molecules on TiO2 is true. This result further proved the comment that the coadsorption of CDCA can retard the aggregation of DPP dyes. 3.3. Electrochemical Properties. To evaluate the possibility of electron transfer from the excited dye to the conduction band of TiO2, cyclic voltammetry was performed in acetonitrile solution, using 0.1 M tetra-n-butylammonium hexafluorophosphate as the supporting electrolyte and TiO2 films stained with sensitizer as the working electrode. The examined highest occupied molecular orbital (HOMO) levels and the lowest unoccupied molecular orbital (LUMO) levels are collected in Table 1. The SCE reference electrode was calibrated using a ferrocene/ferrocenium (Fc/Fc+) redox couple as an external standard. The excitation transition energy (E0-0) of DPP-I and DPP-II was estimated from their absorption thresholds of dyesensitized TiO2 films to be 1.65 and 1.55 eV, respectively. The HOMO values of DPP-I and DPP-II corresponding to their first redox potential are 0.78 and 0.99 V versus NHE, respectively. The HOMO levels of the two dyes are much more positive than the iodine/iodide redox potential value (0.4 V), ensuring that there is enough driving force for the dye regeneration efficiently through the recapture of the injected electrons from I- by the dye cation radical. As listed in Table 1, the HOMO value of DPP-II is significantly higher than that of DPP-I, which should be ascribed to the better electrontransfer character of the two thiophene units. The estimated excited-state potential corresponding to the LUMO levels of DPP-I and DPP-II, calculated from EHOMO s E0s0, are -0.87

TABLE 1: Photophysical Parameters of the Dyes dye

λmaxa/nm (ε × 10-4 M-1 cm-1)

λmaxb (nm)

HOMOc/V (vs NHE)

E0s0d (eV)

LUMOe/V (vs NHE)

DPP-I DPP-II

524(4.21) 627(3.32)

605 702

0.78 0.99

1.65 1.55

-0.87 -0.56

a Absorption maximum in CH2Cl2 solution (3 × 10-5 M). b Absorption maximum on TiO2 film (without CDCA). c The HOMO was measured in acetonitrile with 0.1 M tetra-n-butylammonium hexafluorophosphate (TBAPF6) as electrolyte (working electrode, FTO/TiO2/dye; reference electrode, SCE; calibrated with ferrocene/ferrocenium (Fc/Fc+) as an external reference. Counter electrode: Pt). d E0-0 was estimated from the absorption thresholds from absorption spectra of dyes adsorbed on the TiO2 film. e The LUMO is estimated by subtracting E0-0 from the HOMO.

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Figure 3. Photocurrent action spectra of the TiO2 electrodes sensitized by DPP-I and DPP-II.

TABLE 2: Influence of 4-TBP on the Photovoltaic Performance Parametersa dye

4-TBP

Jsc/mA cm-2

Voc/V

FF

η (%)

DPP-I

0 0.5 M 0.8 M 0 0.5 M 0.8 M

10.81 9.78 3.89 7.65 1.71 0.85

0.483 0.531 0.532 0.346 0.419 0.443

0.52 0.62 0.64 0.55 0.64 0.56

2.70 3.24 1.32 1.45 0.46 0.21

DPP-II

a Illumination: 100 mW cm-2 simulated AM 1.5 G solar light. Other redox electrolyte contained 0.1 M LiI + 0.6 M MPII + 0.05 M I2 in the mixed solvent of acetonitrile and 3-methoxypropionitrile (7:3, v/v).

Figure 4. Photocurrent density vs voltage curves for DSSCs based on DPP-I and DPP-II with various concentrations of 4-TBP in the redox electrolyte under irradiation of AM 1.5 G simulated solar light (100 mW cm-2).

Figure 5. Photocurrent density vs voltage curves for DSSCs based on DPP-I and DPP-II with various concentrations of CDCA under irradiation of AM 1.5 G simulated solar light (100 mW cm-2) with liquid electrolyte.

and -0.56 V versus NHE, respectively. Judging from the LUMO value, the two dyes are more negative than the bottom of the conduction band of TiO2 (-0.5 V), indicating that the electron injection process from the excited dye molecule to TiO2 conduction band is energetically permitted.19 3.4. Photovoltaic Performance of DSSCs. Figure 3 shows the action spectra of incident photon-to-current conversion efficiency (IPCE) for DSSCs based on DPP-I and DPP-II. The IPCE data of DPP-I sensitizer exhibit a high efficiency of more than 60% in the range of 400-600 nm with a maximum value of 80% at 550 nm. On the other hand, the IPCE value for DPPII was obviously decreased at 400 to 600 nm, suggesting the worst electron injection ability from the excited dye of DPP-II to the TiO2 conduction band. The IPCE spectra of them are consistent with the absorption spectra on the TiO2 film. The maximum IPCE value of DPP-II is significantly lower than that of DPP-I, while in the long-wavelength region, DPP-II shows higher IPCE. To be mentioned, owing to the two thiophene units in the molecule, the action spectra of DPP-II even extend to the near-infrared region (>800 nm), which is rare in previous studies. The photovoltaic performance of the DSSCs was measured at 100 mW cm-2 under simulated AM 1.5 G solar light conditions. The DPP-I-sensitized cell gave a short-circuit photocurrent density (Jsc) of 10.81 mA cm-2, an open-circuit voltage (Voc) of 0.483 V, and a fill factor (FF) of 0.50, corresponding to an overall conversion efficiency (η) of 2.70%. Under the same conditions, the DPP-II-sensitized cell gave a Jsc value of 7.65 mA cm-2, a Voc of 0.346 V, and an FF of 0.53, corresponding to the η value of 1.45% (see Table 2). The

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TABLE 3: Influence of CDCA on the Photovoltaic Performance Parametersa,b dye

CDCA

Jsc/mA cm-2

Voc/V

FF

η (%)

amount (10-8 mol cm-2)

DPP-I

0 1.5 mM saturated 0 1.5 mM saturated

9.36 9.78 9.74 1.39 2.39 2.09 16.10

0.572 0.605 0.588 0.459 0.508 0.483 0.689

0.64 0.70 0.67 0.67 0.68 0.68 0.67

3.44 4.14 3.81 0.43 0.83 0.69 7.47

5.80 4.98 3.08 6.36 4.79 4.17

DPP-II N719

a Illumination: 100 mW cm-2 simulated AM 1.5 G solar light. b Electrolyte containing 0.1 M LiI + 0.6 M DMPImI + 0.1 M I2 + 0.5 M 4-TBP in acetonitrile.

lower efficiency of the DSSC based on DPP-II as compared with the DPP-I-based DSSC is due to its lower extinction coefficient, photocurrent, and IPCE data in the range of 400-600 nm. To improve the open-circuit potential (Voc), 0.5 and 0.8 M 4-tert-butylpyridine (4-TBP) were added to the electrolyte, and the data were summarized in Table 2. As shown in Figure 4, the cell voltages in the DSSCs with both DPP-I and DPP-II were largely improved with an addition of 4-TBP in the electrolyte, while the photocurrent density was decreased. This is because the tert-butyl chain of 4-TBP prevents recombination between the injected electrons and I3- ions on the TiO2 surface, resulting in an improved voltage. However, 4-TBP adsorbed on the TiO2 surface negatively shifts the conduction band level of TiO2, which suggests that relatively large energy gaps between the LUMO levels of the dyes and the conduction band edge level of TiO2 should be requested in the use of 4-TBP.20 The reduction potential of DPP-I obtained in CH2Cl2 is -0.87 V (Table 1), which is largely more negative than the value of -0.5 V for the TiO2 electrode, indicating an improvement of total efficiency with the use of 4-TBP at a suitable concentration. It was found that 0.5 M 4-TBP was optimal for DPP-I with respect to power conversion. On the contrary, the TiO2 conductive band levels are relatively close in energy to the excited-state levels of the dye DPP-II (Table 1) when 4-TBP is present in the electrolyte, so the Jsc value of DPP-II is drastically decreased. As a consequence, the highest power conversion efficiency of DPP-II was obtained without 4-TBP contained in the redox electrolyte (Table 2). Chenodeoxycholic acid (CDCA) is a transparent, organic compound that binds strongly to the surface of nanostructured TiO2, being able to displace dye molecules from the semiconductor surface, therefore, hindering the formation of dye aggregates.21 It is known to improve DSSC efficiency, generally attributed to its preventing dye aggregation, resulting in improved electron injection efficiency and thus leading to an increase in the device photocurrent.22 Moreover, in some cases, the use of CDCA has also led to a rise in the photovoltage. This latter effect has been attributed to a decrease of recombination of the injected electrons.23 To solve the problem of dye aggregation, the effect of CDCA content in the dye solution on the solar cell performance was also investigated, and the data were summarized in Table 3. As shown in Figure 5, the shortcircuit photocurrent (Jsc) of DPP-I and DPP-II increased with 1.5 mM CDCA and then slightly decreased with further increasing CDCA to a saturated concentration. One explanation is that the amount of adsorbed dye on the TiO2 surface was reduced by coadsorbents of CDCA, resulting in a loss in light harvesting and thus decreased short-circuit photocurrent. The open-circuit voltage in the DSSCs with both DPP-I and DPPII was improved, indicating that the charge recombination process was effectively prohibited with a content of chenodeoxycholic acid. However, further increases in the CDCA

Figure 6. Electrochemical impedance for DSSCs sensitized by DPP-I and DPP-II containing various concentrations of CDCA.

concentration to saturated lead to a decrease in the Voc value; this is because adsorption of CDCA leaves protons on the TiO2 surface and, hence, charges the surface positively shifted by the coadsorption of CDCA, resulting in Voc loss.24 The enhancement of the Voc is usually associated with the negative shift of the conduction band edge or suppression of charge recombination. In consequence, suppression of charge recombination may be counteracted by the Voc loss because of proton exchange from CDCA to the TiO2 surface,6a Thus, the open-circuit voltage would be somewhat determined by the concentration of CDCA. As shown in Table 3, the highest power conversion efficiency could be obtained at 1.5 mM CDCA. 3.5. Electrochemical Impedance Spectroscopy. Electrochemical impedance spectroscopy (EIS) analysis was performed to clarify the CDCA effect on the Voc further. Figure 6 compares the impedance spectra for DPP-I- and DPP-II-sensitized cells measured in the dark under a forward bias of -0.50 V with a frequency range of 0.1 Hz to 100 kHz, respectively. The important differences for the two organic dye solar-sensitized

Diketopyrrolopyrrole Dyes for Efficient DSSCs cells were found for the conductivity. The electron lifetime values derived from curve fitting of DPP-I are 21.50, 31.75, and 26.62 ms with increasing concentration of CDCA until saturated, whereas the data of DPP-II are 12.29, 14.34, and 13.33 ms. The longer electron lifetime observed with DPP-I relative to DPP-II indicated more effective suppression of the back reaction of the injected electron with I3- in the electrolyte and was also reflected in the improvements seen in the photocurrent, yielding substantially enhanced device efficiency. The Bode phase plots shown in Figure 6 likewise support the differences in the electron lifetime for TiO2 film derivatives with the two dyes. On the other hand, the middle-frequency peak shifts to a smaller value with an addition of CDCA content in the dye solution, as indicated in Figure 6, indicating an increase in electron lifetime upon CDCA coadsorption, and the smallest middle-frequency value was obtained at 1.5 mM CDCA, which is also in agreement with the observed shift in the Voc value under standard global AM 1.5 illumination. 4. Conclusion We have successfully designed and synthesized two novel metal-free dyes (DPP-I and DPP-II) by linking different π-conjugated systems with the diketopyrrolopyrrole (DPP) core, in combination with the triphenylamine donor and carboxylic acid moiety acceptor. Introduction of the diketopyrrolopyrrole core into the dye molecules extended absorption at long wavelength, thus improving the range of the IPCE conversion region. Coadsorption of CDCA hindered the formation of dye aggregates and might improve the electron injection yield and, thus, Jsc. The enhancement of the photovoltage is attributed to the decrease of charge recombination, confirmed by the increase in electron lifetime. The total efficiency of these new DPP-dyesensitized solar cells should have room for improvement and optimization. The molecular modification toward higher Voc and Jsc is in progress. Acknowledgment. This work was supported by the NSFC/ China (20772031), the National Basic Research 973 Program (2006CB806200), and the Scientific Committee of Shanghai. References and Notes (1) (a) O’Regan, B.; Gra¨tzel, M. Nature 1991, 353, 737. (b) Gra¨tzel, M. Inorg. Chem. 2005, 44, 6841. (2) (a) Wang, P.; Klein, C.; Humphry-Baker, R.; Zakeeruddin, S. M.; Gra¨tzel, M. J. Am. Chem. Soc. 2005, 127, 808. (b) Schmidt-Mende, L.; Kroeze, J. E.; Durrant, J. R.; Nazeeruddin, Md. K.; Gra¨tzel, M. Nano Lett. 2005, 5, 1315. (c) Karthikeyan, C. S.; Wietasch, H.; Thelakkat, M. AdV. Mater. 2007, 19, 1091. (d) Staniszewski, A.; Ardo, S.; Sun, Y.; Castellano, F. N.; Meyer, G. J. J. Am. Chem. Soc. 2008, 130, 11586. (3) (a) Ferrere, S.; Zaben, A.; Gregg, B. A. J. Phys. Chem. B 1997, 101, 449. (b) Edvinsson, T.; Li, C.; Pschirer, N.; Scho1neboom, J.; Eickemeyer, F.; Sens, R.; Boschloo, G.; Herrmann, A.; Mu¨llen, K.; Hagfeldt, A. J. Phys. Chem. C 2007, 111, 15137. (4) (a) Tang, J.; Wu, W. J.; Hua, J. L.; Li, J.; Tian, H. Energy EnViron. Sci. 2009, 2, 982. (b) Sayama, K.; Hara, K.; Ohga, Y.; Shinpou, A.; Suga, S.; Arakawa, H. New J. Chem. 2001, 25, 200. (c) Ma, X. M.; Hua, J. L.; Wu, W. J.; Tian, H. Tetrahedron 2008, 64, 345.

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