New Ruthenium Sensitizer with Carbazole Antennas for Efficient and

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J. Phys. Chem. C 2009, 113, 20752–20757

New Ruthenium Sensitizer with Carbazole Antennas for Efficient and Stable Thin-Film Dye-Sensitized Solar Cells Chia-Yuan Chen,† Nuttapol Pootrakulchote,‡ Shi-Jhang Wu,† Mingkui Wang,‡ Jheng-Ying Li,† Jia-Hung Tsai,† Chun-Guey Wu,*,† Shaik M. Zakeeruddin,*,‡ and Michael Gra¨tzel*,‡ Department of Chemistry, National Central UniVersity, Jhong-Li, 32001, Taiwan, Republic of China and Laboratory for Photonics and Interfaces, Swiss Federal Institute of Technology, CH 1015, Lausanne, Switzerland ReceiVed: September 15, 2009

A new heteroleptic ruthenium complex, coded CYC-B13, incorporating an antenna ligand composed of the sequential connection of a conjugated segment and carbazole hole-transport moiety was prepared. This new sensitizer exhibits the lower energy MLCT band centered at 547 nm with a high molar absorption coefficient of 1.93 × 104 M-1 cm-1. Thin-film cells based on this new sensitizer show good conversion efficiency (>8%) and excellent durability under light soaking at 60 °C in simulated sunlight for 1000 h. An all-solid state device based on CYC-B13 is also demonstrated to have a conversion efficiency of 3.8%. The photovoltaic data of DSCs sensitized with CYC-B13 suggested that carbazole is a photostable hole-transporting moiety to be used in dye-sensitized solar cells. 1. Introduction Dye-sensitized solar cells (DSCs) have been regarded as promising candidates for practical photovoltaic applications by virtue of their low manufacturing cost and impressive conversion efficiency.1 At present, DSCs based on the ruthenium sensitizers have achieved remarkable conversion efficiencies over 11%.2,3 In addition, heat-stable DSCs were demonstrated for the first time in 2003 by employing the amphiphilic ruthenium dye, Z907.4 Since the self-assembled monolayer of sensitizer on the surface of the n-type oxide semiconductor is a vital component for DSCs, molecular engineering of the sensitizers to achieve high photovoltaic performance as well as good durability is critical. Progress in optimizing the ruthenium-based sensitizers for DSCs has mainly been focused on enhancing on the lightharvesting ability to match the solar radiation5 and on multiplying the specific functionalities for improving the thermal resistance6 or retaining the photoinduced interfacial charge separation between the dye-sensitized n-type semiconductor and electrolyte.7 Recently, we demonstrated that the efficient ruthenium supersensitizer (CYC-B6S) endowed with the ancillary ligand consisting of a conjugated segment and a carbazole holetransporting unit can enhance the spectral response and therefore the conversion efficiency of the DSCs based on it.8 However, the long-term stability of cells based on the ruthenium dyes incorporating hole-transporting moieties such as carbazole has not been reported so far. To further improve the light harvesting capacity of this class of sensitizers and at the same time test the merit of the carbazole-containing ruthenium dyes, we have designed a new ruthenium sensitizer, coded CYC-B13 (Figure 1), endowed with a similar antenna function. The photovoltaic performance of CYC-B13 in dye sensitized solar cells at different aspects was investigated. * To whom correspondence should be addressed. E-mail: t610002@ cc.ncu.edu.tw; [email protected];[email protected]. † Department of Chemistry, National Central University. ‡ Laboratory for Photonics and Interfaces, Swiss Federal Institute of Technology.

Figure 1. The molecular structure of CYC-B13.

2. Experimental Section 2.1. Materials. All reagents were obtained from the commercial sources and used as received unless specified otherwise. Solvents were dried over sodium or CaH2 before use. DINHOP was synthesized as reported in the earlier publication.9 The structures of CYC-B13 dye and its intermediates were identified with 1H- NMR spectra. The structure of CYC-B13 dye was further confirmed by FAB-MS and elemental analysis. 2.2. Synthesis of 3,6-Bis-tert-Butyl-9-(3,4-(ethylenedioxy)thienyl) Carbazole. The starting materials, 3,6-bis-tert-butyl carbazole and 2-iodo-3,4-(ethylene-dioxy) thiophene, were prepared according to the literature.8,10 Then, 2.69 g (9.63 mmol) 3,6-bis-tert-butyl carbazole, 1.61 g (8.74 mmol) Cu-bronze, and 1.57 g (11.4 mmol) K2CO3 were added into the solution consisting of 2.34 g (8.74 mmol) 2-iodo-3,4-(ethylenedioxy) thiophene dissolved in 90 mL nitrobenzene. After the mixture was refluxed under argon for 36 h, the nitrobenzene was removed by distillation under vacuum. The reaction was terminated by suspending the residue in the mixture of 300 mL CHCl3, 100 mL H2O, and 150 mL saturated NH4OH aqueous solution. The organic layer was extracted with CHCl3 after the mixture was stirred continuously for 2 h. Then the collected organic layer was washed further with excess water and

10.1021/jp9089084 CCC: $40.75  2009 American Chemical Society Published on Web 11/02/2009

New Ruthenium Sensitizer with Carbazole Antennas saturated NaCl aqueous solution, respectively, and then dried over Na2SO4. After rotary evaporation of the solvent, the crude product was purified by chromatography on silica gel eluted with hexane/ethyl acetate (10:1) and then purified further by recrystallization from hexane for several times to afford 1.06 g (28.8% yield) of the pure product. 1H NMR (300 MHz, δH/ ppm in CDCl3): 8.07 (d, 2H), 7.47 (dd, 2H), 7.27 (d, 2H), 6.38 (s, 1H), 4.27 (m, 2H), 4.19 (m, 2H), 1.45 (s, 18H). 2.3. Synthesis of 4,4′-Bis(3,6-bis-tert-butyl-9-(3,4-(ethylenedioxy)thien-2-yl) Carbazole)-2,2′-bipyridine (Ligand-13). Ligand-13 was prepared by Stille coupling of 3,6-bis-tertbutyl-9-(5-trimethyl-stannyl-2-(3,4-(ethylenedioxy)thienyl))carbazole and 4,4′-dibromo-2,2′-bipyridine. The former was prepared according to the following procedures: A 0.9-mL portion of n-BuLi (2.5 M in hexane) was added into the solution of 3,6-bis-tert-butyl-9-(3,4-(ethylenedioxy)thienyl) carbazole (0.785 g, 1.87 mmol) dissolved in 30 mL anhydrous THF at -78 °C. The mixture was stirred for 2 h, and then 0.485 g (2.43 mmol) trimethyltin chloride in anhydrous THF was added. The mixture was stirred overnight at room temperature. The reaction was terminated by adding saturated NaCl aqueous solution and the product was extracted with CH2Cl2. The organic layer was collected and the solvent removed to afford 1.04 g (95.5% yield) crude product. This crude product was mixed with 0.255 g (0.81 mmol) 4,4′-dibromo-2,2′- bipyridine dissolved in 55 mL anhydrous DMF and then added 0.11 g (97 µmol) Pd(PPh3)4 as a catalyst. The mixture was refluxed under argon for 48 h. After cooling to room temperature, 5 wt % NH4Cl(aq) was added to terminate the reaction, and the product was extracted with CHCl3. The organic layer was washed with saturated NaHCO3(aq), distilled water and saturated NaCl(aq), respectively, and then dried under vacuum. The crude product was purified with a Soxhlet using hexane as solvent to remove the nonreacted 3,6-bis-tert-butyl9-(5-trimethylstannyl-2-(3,4- (ethylene-dioxy)thienyl)) carbazole then using ethyl acetate to extract the product Ligand-13 (65.8% yield). 1H NMR (300 MHz, δH/ppm in CDCl3): 8.73 (s, 2H), 8.68 (d, 2H), 8.10 (d, 4H), 7.71 (d, 2H), 7.50 (dd, 4H), 7.37 (d, 4H), 4.51 (m, 4H), 4.31 (m, 4H), 1.47 (s, 36H). MS: m/z 990.4 ([M]+); LRFAB-MS found: m/z 991.7 (m) ([M+H]+). 2.4. Synthesis of CYC-B13 (TBA(Ru[(4-Carboxylic Acid4′-carboxylate-2,2′-bipyridine) (Ligand-13) (NCS)2]). CYCB13, was synthesized using the one-pot synthetic procedure similar to what we previously reported:5c The reaction consisted of 0.162 g (0.264 mmol) [RuCl2(p-cymene)]2, 0.529 g (0.535 mmol) Ligand-13, 0.130 g (0.535 mmol) dcbpy (4,4′-dicarboxylic acid-2,2′- bipyridine), and excess NH4NCS. The crude product was dissolved in the mixture of methanol and tetrabutyl ammonium hydroxide (TBAOH) aqueous solution (40 wt % in H2O) and then purified on a Sephadex LH-20 column using methanol as an eluent. The main band was collected and the pH value of the collected solution was lowered to ∼5.8 by adding dilute HNO3. The collected precipitate was washed with water and dried under vacuum. After purification, 0.309 g (0.182 mmol, 34.1% yield) CYC-B13 was obtained. MS: m/z 1693.6 ([M]+) LRFAB-MS found: m/z 1452.4 (m) ([M+H-N(C4H9)4]+); 1394.3 (s) ([M+H-N(C4H9)4-NCS]+). HRFAB-MS found: 1452.3245, ∆ + 0.2 mmu ([M+H-N(C4H9)4]+). Elemental analysis: calcd. for C92H105N9O8RuS4 · 3H2O: C, 63.21; H, 6.40; N, 7.21; S, 7.34%. Found: C, 62.75; H, 5.97; N, 7.36; S, 7.49%. 1H NMR (500 MHz, δH/ppm in d6-DMSO, J Hz): 9.38 (d, J ) 5.5 Hz, 1H); 9.22 (d, J ) 6.1 Hz, 1H); 9.11 (s, 1H); 8.96 (s, 1H); 8.86 (s, 1H); 8.67 (s, 1H); 8.29 (s, 2H); 8.24 (m, 4H); 7.85 (d, J ) 5.5 Hz, 1H); 7.64 (d, J ) 5.5 Hz, 1H); 7.54 (d, J )

J. Phys. Chem. C, Vol. 113, No. 48, 2009 20753 9.1 Hz, 2H); 7.48 (d, J ) 9.1 Hz, 2H); 7.46 (d, J ) 9.3 Hz, 2H); 7.42 (d, J ) 8.5 Hz, 2H); 7.30 (d, J ) 8.5 Hz, 2H); 4.67 (m, 2H); 4.53 (m, 2H); 4.43 (m, 2H); 4.33 (m, 2H); 3.15 (m, 8H); 1.55 (m, 8H); 1.42 (s, 18H); 1.38 (s, 18H); 1.31 (m, 8H); 0.93 (t, J ) 7.3 Hz,12H). 2.5. Device Fabrication. A mesoscopic TiO2 film composed of an 8-µm thick transparent layer of 20 nm sized TiO2 anatase nanoparticles onto which a second 5 µm thick scattering layer of 400 nm sized TiO2 was superimposed. The detailed methods for TiO2 film preparation, device fabrication, and the photocurrent-voltage measurements can be found in the earlier report.11 The double layer films were heated to 520 °C and sintered for 30 min, then cooled to 80 °C and immersed into the dye solution (300 µM) containing 10% DMSO in acetonitrile and tert-butyl alcohol (volume ratio: 1:1) mixture, respectively with 75 µM DINHOP as a coadsorbent for 16 h. For high performance device, chlorobenzene was used as a solvent for the dye with equimolar ratio of chenodeoxycholic acid being added as a coadsorbent and an acetonitrile-based electrolyte (1.0 M DMII, 50 mM LiI, 20 mM I2, 0.5 M tBP, and 0.1 M GNCS in the mixed solvent of acetonitrile and valeronitrile (v/v, 85/ 15)) was used. 2.6. Physicochemical and Photovoltaic Performance Measurements. 1H NMR spectra were recorded with Bruker 200 or 500 MHz NMR spectrometer in CDCl3 or d6-DMSO. FAB-MS spectra were obtained using JMS-700 HRMS. UV-vis spectra were measured using a Cary 300 Bio spectrometer. Voltammetric measurement was performed in a singlecompartment, three-electrode cell with a platinum disk working electrode and a Pt wire counter electrode. The reference electrode was Ag/Ag+ and the supporting electrolyte was 0.1 M TBAPF6 (tetrabutylammonium hexafluorophosphate) in DMF. The square-wave voltammogram was recorded using an Autolab system (PGSTAT 30, Autolab, Eco-Chemie, The Netherlands), and the ferrocene/ferrocinium redox couple was used as a calibration standard. Elemental analysis was carried out with a Heraeus CHN-O-S Rapid-F002 analysis system. The photovoltaic performance of cells as well as the device stability was measured using the setup as reported in the literature.12 2.7. Transient Photoelectrical Measurements. In the transient photovoltage decay experiments, different steady-state light levels were provided by a homemade white light-emitting diode array tuning the driving voltage. A red light-emitting diode array controlled with a fast solid-state switch was used to generate a perturbation pulse of 50 ms duration. The pulsed red- and steady-state white-light were both incident on the working electrode side of the test cell. The intensity of the red light pulse was carefully controlled by the driving potential of the red diode array to keep the modulated photovoltage below 10 mV. The cell was maintained at open circuit voltage under the white light and the transient photovoltage decay following the red light pulse was monitored. Normally, the decay closely follows a monoexponential form, thus the recombination rate constant can be extracted from the slope of the semilogarithmic plot. 2.8. Electrochemical Impedance Spectroscopy (EIS) Measurements. Electrochemical impedance spectra of the DSC were measured using the PGSTAT 30 frequency analyzer from Autolab (Eco Chemie B.V, Utrecht, The Netherlands) together with the Frequency Response Analyzer module providing voltage modulation in the desired frequency range. The Z-view software (v2.8b, Scribner Associates Inc.) was used to analyze the impedance data. The EIS experiments were performed at a constant temperature of 20 °C in the dark. The impedance spectra of the DSC devices were

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Chen et al. TABLE 1: Detailed Photovoltaic Parameters of CYC-B13 and CYC-B6S-Sensitized Devices under Full Sunlight Intensitya device

electrolyte

CYC-B13 CYC-B6S CYC-B13 CYC-B6S

Z946 Z946 Spiro-MeOTAD Spiro-MeOTAD

Jsc (mA/cm2) Voc (V) 10.26 9.22 7.74 5.84

0.728 0.731 0.844 0.878

FF

η (%)

0.68 0.70 0.58 0.67

5.1 4.7 3.8 3.5

a The spectral distribution of incident sunlight simulates AM 1.5G solar irradiation.

Figure 2. Electronic absorption spectra of CYC-B13, its anchoring and ancillary ligands measured in DMF.

recorded at potentials varying from -0.85 to -0.35 V at frequencies ranging from 0.02 Hz to 200 kHz, and the oscillation potential amplitude was adjusted to 10 mV. The photoanode (TiO2) was used as the working electrode and the Pt counter electrode (CE) was used as both the auxiliary electrode and the reference electrode. 3. Results and Discussion 3.1. Synthesis and Physical Properties of CYC-B13. The antennas in CYC-B13 are formed by connecting the electronrich 3,4-(ethylenedioxy) thiophene (EDOT) to alkyl-substituted carbazole units which have been successfully prepared for the first time using the Ullmann N-arylation.13 The electronic absorption spectra of CYC-B13, its anchoring and ancillary ligands measured in DMF are displayed in Figure 2. CYC-B13 shows three absorption bands centered at 295 nm, 397 and 547 nm, respectively. The band at 295 nm is assigned to the overlap of intraligand π-π* transitions of 4,4′-dicarboxylic acid-2,2′-bipyridyl anchoring ligand and that of the ancillary ligand.14 Another band centered at 397 nm also contains two components: the π-π* transition of ancillary ligand and one of the metal-to-ligand charge transfer (MLCT) transitions for CYC-B13. The molar absorption coefficient () of lower energy MLCT band centered at 547 nm is 1.93 × 104 M-1 cm-1. Compared to its predecessor, CYC-B6S,8 the  value of CYC-B13 increased by ∼20%. This significant augmentation of the MLCT absorption cross section is due to the dioxyethylene pendent group of EDOT increasing both the electron donating ability and extension of π-conjugation to the thiophene moiety.5d The oxidation potential of CYC-B13 measured by the squarewave voltammetry with the condition reported previously8 is 0.94 V versus NHE, which is 0.54 V more positive than the redox potential of the iodide/triiodide couple used in the liquid electrolyte. The optical transition energy, E0-0, of CYC-B13 determined from its absorption onset is 1.58 eV. Neglecting the entropy changes during excitation, the excited-state redox potential, φ 0(S+/S*) is -0.64 V versus NHE, which is more negative than the potential of the TiO2 (anatase) conduction band edge (-0.50 V versus NHE). Thus upon photoexcitation, CYCB13 self-assembled onto the TiO2 film can inject electrons into the TiO2 conduction band and then the reduction of oxidized CYC-B13 by the electrolyte will occur spontaneously in this regenerative photovoltaic device.

3.2. The Photovoltaic Performance of the Devices Based on CYC-B13 as well as CYC-B6S. The open-circuit photovoltage of a DSC decreases with increasing film thickness, due to the augmentation of the surface area enhancing the undesired dark current. Hence, initially the ultrathin TiO2 films of only 3 µm thickness were used to fabricate photovoltaic devices to take advantage of the enhanced optical cross section of CYC-B13 with respect to the standard Z907Na sensitizer. These cells employed an electrolyte coded Z946, containing 3-methoxypropionitrile as a solvent and 1.0 M 1,3-dimethylimidazolium iodide (DMII), 0.15 M I2, 0.5 M N-butyl benzimidazole (NBB) as well as 0.1 M guanidinium thiocyanate (GNCS) as solutes. Encouragingly, even with such a thin titania film and a low-volatility electrolyte, the thin-film device based on CYC-B13 in the presence of DINHOP as a coadsorbent (4:1 molar ratio in the dye solution) provides a Jsc of 11.9 mA/cm2, a Voc of 0.734 V and a FF of 0.69, yielding an overall conversion efficiency of 6.1% under illumination with AM 1.5G simulated sunlight (100 mW/cm2). Under the same conditions, the efficiency obtained with Z907Na sensitized ultrathin film cell is only 5.3%, the Jsc, Voc, and FF being 9.5 mA/cm2, 0.763 V and 0.74, respectively. The major difference in the photovoltaic performance between these two cells is the Jsc, and this arises from the 58% higher peak molar absorption coefficient of CYC-B13 compared to that of Z907Na dye (1.22 × 104 M-1 cm-1)6 and a red shift in the absorption. We also compared the photovoltaic performance of the CYCB13 with CYC-B6S dye using the same electrolyte and a 2.7 µm thin mesoporous TiO2 film without any coadsorbent. Photovoltaic parameters are summarized in Table 1. The 20% increase in the molar extinction coefficient of CYC-B13 dye over CYC-B6S enhances significantly the Jsc value, i.e., from 9.22 to 10.26 mA/cm-2. Consequently, the photovoltaic performance of CYC-B13 is increased ∼10%. These results suggest that CYC-B13 dye may be attractive in the all-solid-state and plastic dye sensitized solar cells where thinner TiO2 films are essential.15 Due to the difficulty of filling the pores of TiO2 film with the hole conductor and faster charge carrier recombination, only thin films (here 1.8 µm) can be used, which limit the Jsc accordingly. Hence, it is advantageous to use high molar extinction coefficient dyes in solid-state dye sensitized solar cells (SSDSC). A SSDSC device with Spiro-OMeTAD as a hole transporting material gave efficiencies of 3.8% (this value is slightly higher than the SSDSC based on TPA (triphenyl amine)antenna dye16a and close to that based on Z90716b) and 3.5% with the CYC-B13 and CYC-B6S dye, respectively (Figure 3). The detailed photovoltaic parameters of SSDSCs based on these two different dyes are also tabulated in Table 1. The Jsc for SSDSC with CYC-B13 is 1.9 mA/cm2 higher than that for SSDSC with CYC-B6S dye, the results consist to those obtained with the liquid devices. A more efficient device based on CYC-B13 employed a double-layered TiO2 film (8 + 5) µm, chenodeoxycholic acid

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Figure 3. Photocurrent density-voltage characteristic curve under full sunlight (AM 1.5G, 100 mW cm-2) and in the dark of devices sensitized with CYC-B13 and CYC-B6S using a solid hole conductor. (Thickness of TiO2 film thickness is 1.8 µm.)

Figure 5. Evolution of the photovoltaic parameters (Jsc, Voc, FF, and η) for the device based on (8 + 5 µm) TiO2 film sensitized with CYCB13 and low-volatile liquid electrolyte (Z946) during the visible lightsoaking (1 sun; 100 mW/cm2) at 60 °C.

Figure 4. Photocurrent density-Voltage characteristic curve of a cell with CYC-B13 measured under AM 1.5G simulated sunlight illumination (100 mW /cm2) and in dark. The inset is its photon-to-current action spectrum. (Thickness of TiO2 film: (8 + 5 µm); cell active area tested with a mask: 0.158 cm2).

coadsorbent, (1:1 molar ratio in chlorobenzene) and an acetonitrile-based volatile electrolyte were also fabricated. The incident phototo-current conversion efficiency (IPCE) spectrum shown in the inset of Figure 4 exhibits a plateau of over 80% from 455 to 620 nm, with the maximum of 90% at 550 nm. The corresponding I-V characteristic curve of the solar cell under illumination with standard AM 1.5G simulated sunlight (100 mW/cm2) is also displayed in Figure 4. The photovoltaic parameters derived are Jsc )17.1 mA/cm2, Voc ) 0.760 V and FF ) 0.73, yielding an overall power conversion efficiencies (η) of 9.6%. The Jsc agrees with the value of 17.3 mA/cm2 calculated from the overlap integral of the IPCE spectrum with standard AM 1.5G solar emission spectrum, showing that the spectral mismatch is less than 2%. Further cell optimization is in progress and the results will be reported elsewhere. 3.3. The Long-term Stability of the Devices Sensitized with CYC-B13. In order to explored the cell stability based on CYC-B13, a mixture of acetonitrile, tert-butyl alcohol (volume ratio: 1:1) and 10% DMSO was used as a solvent for the dye solution and a low-volatility MPN-based Z946 were used as an electrolyte. The corresponding cell covered with a 50-µm thick polyester film acting as a UV cutoff filter for the accelerated test was performed in the illumination with the visible light (1 sun; 100 mW/cm2) at 60 °C. As shown in Figure 5, the photovoltaic parameters Jsc, Voc, FF, and η of the CYC-B13sensitized cell were 16.1 mA/cm2, 0.713 V, 0.72, and 8.3%, respectively in the initial state. After 1000 h of light soaking and thermal stress, the photovoltaic parameters Jsc, Voc, and FF of the cell are slightly changed to 16.0 mA/cm2, 0.673 V, and

0.72, respectively, the η value retained 95% of its initial value. These results reveal that the antennas on CYC-B13 can efficiently prevent the self-assembled dye molecules from desorption which may be induced by water during the aging process. For comparison, under identical conditions, devices prepared using CYC-B6S and Z946 electrolyte. The photovoltaic parameters of the CYC-B6S- sensitized cell were 16.5 mA/ cm2, 0.704 V, 0.69 and 8.0% for Jsc, Voc, FF, and η, respectively. With double layer films, it was not obvious to see the influence of the high molar extinction coefficient of sensitizer on the photovoltaic performance as it was attenuated by the presence of reflecting particles in the double layer structured TiO2 film. It was not surprising that the long-term stability of devices based on CYC-B6S under light soaking at 60 °C behaves similar to that with CYC-B13. To the best of our knowledge, this is the first time that a cell based on the ruthenium sensitizer incorporating a conjugated segment and a carbazole hole-transporting motif as the antennas has achieved such high conversion efficiency and noteworthy durability under long-term lightsoaking stress at elevated temperature. The long-term stability of cell based on CYC-B13 is comparable with that of the stateof-the-art robust DSCs.17 3.4. Exploring the Changes of the Individual Circuit Elements after Photo and Thermal Aging with Photovoltage Transient Spectroscopy. To diagnose any changes in the individual circuit elements during the long-term light soaking test, photovoltage transient measurements were performed for both fresh and aged DSC devices. Figure 6(a) displays the opencircuit voltage as a function of the photoinduced charge density accumulated in the TiO2 nanoparticles at open circuit condition. The value of the photoinduced charge was extracted from the experimental measurements by collecting the electrons when switching the device from open circuit to short circuit conditions. At identical photoinduced charge densities, the fresh cell shows higher Voc compared to the aged cell. This result indicates that after light soaking, the conduction band edge of the TiO2 in the aged device undergoes a downward shift (to more positive potentials), resulting in the slightly decrease of the open circuit voltage. This effect is quite commonly observed in DSCs and has been attributed to the photoinduced proton intercalation into the titania.18 At identical photoinduced charge densities, the recombination lifetime for the fresh and aged devices has similar

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Figure 8. Electron transport resistance (Rt) in the TiO2 film obtained from the impedance measurements in the dark at 20 °C.

Figure 6. Transient photovoltage decay measurements of the fresh and aged device: Effect of light soaking on the relationship (a) between the photoinduced charge density and open circuit voltage and (b) between the photoinduced charge density and the recombination lifetime.

ment of the Fermi level for the electrons (EFn) with respect to the conduction band edge (Ecb).21 Figure 8 illustrates the electron transport resistance (Rt) vs applied voltage for the TiO2 film obtained from the impedance measurements in the dark at 20 °C. There is a small downward shift of the TiO2 conduction band edge by about 20 mV for the aged device compared to that of the fresh device. As we did not observe a change in the diffusion and redox overpotentials of the electrolytes on the platinized counter electrode by impedance measurements, the decrease of open circuit voltage of the aged device should be mainly related to surface protonation during the light soaking period. This energy shift principally influenced the open circuit voltage of the device. Both photovoltage transient and electrochemical impedance data suggest that CYC-B13 is stable during the light soaking and thermal stressing treatments. 4. Conclusions

Figure 7. (a) Impedance spectra (Nyquist plot) of the fresh and aged devices at forward bias of -0.50 V under dark conditions at 20 °C. (b) The enlarge part of plot (a) in the high frequency range. The TiO2 film was cosensitized with CYC-B 13 and DINHOP (molar ratio 4:1). The solid line corresponds to the derived values using the fitting model.

values (Figure 6b) revealing that no change in the distribution of the surface trap states or rate for interfacial charge recombination during the 1000 h light soaking test. The slight decrease in the efficiency after light-soaking and thermal stressing of CYC-B13 sensitized device is mainly due to the change of Voc. 3.5. Electrochemical Impedance Spectra of Both Fresh and Photothermal Aged Devices. Moreover, electrochemical impedance spectroscopy (EIS) was also employed to scrutinize the effect of light soaking on the photovoltaic performance of the devices. Figure 7(a) depicts the Nyquist plots of the fresh and aged devices at a forward bias of -0.5 V. These spectra follow well the transmission line model suggested by Bisquert et al.19,20 The EIS spectrum of the aged device shown in Figure 7(b) displays a smaller electron transport resistance (Rt) in the nanocrystalline TiO2 at high frequencies range (from 10 kHz to 200 Hz) compared to that for the fresh device. Under forward bias, electrons are transferred from the FTO substrate into the TiO2 film, allowing electron propagation through the individual TiO2 particles. The logarithm of the electron transport resistance, ]) in the which depends on the number of free electrons ([ecb conduction band, shows parallel behavior for various devices. This implies that the shift of the resistances for the steady state electron transport of those devices are caused by the displace-

In summary, we have reported a new ruthenium sensitizer, CYC-B13, with an antenna consisting of the electron-rich EDOT and alkyl-substituted carbazole. The cell based on this new sensitizer with a volatile liquid electrolyte exhibits a 9.6% conversion efficiency under AM 1.5G sunlight. The remarkable cell stability under prolonged light soaking and thermal stress is also demonstrated with a low-volatility liquid electrolyte. Furthermore, studies on CYC-B13 and CYC-B6S-sensitized cells using organic hole-transporting mediator and low volatile solvent electrolyte demonstrated the advantage of having high molar extinction coefficient sensitizer by increasing the photocurrent density with thinner TiO2 films. Photovoltage transient and electrochemical impedance data of fresh and aged devices suggest that carbazole- containing CYC-B13 dye is stable under the light soaking and thermal stressing treatments. Acknowledgment. Financial support from the National Science Council, Taiwan, ROC, and the Swiss National Science Foundation is gratefully acknowledged. References and Notes (1) O’Regan, B.; Gra¨tzel, M. Nature 1991, 353, 737. (2) 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) Gao, F.; Wang, Y.; Shi, D.; Zhang, J.; Wang, M.; Jing, X.; Humphry-Baker, R.; Wang, P.; Zakeeruddin, S. M.; Gra¨tzel, M. J. Am. Chem. Soc. 2008, 130, 10720. (c) Cao, Y.; Bai, Y.; Yu, Q.; Cheng, Y.; Liu, S.; Shi, D.; Gao, F.; Wang, P. J. Phys. Chem. C 2009, 113, 6290. (3) Chiba, Y.; Islam, A.; Watanabe, Y.; Komiya, R.; Koide, N.; Han, L. Jpn. J. Appl. Phys. 2006, 45, L638. (4) Wang, P.; Zakeeruddin, S. M.; Moser, J.-E.; Nazeeruddin, M. K.; Sekiguchi, T.; Gra¨tzel, M. Nat. Mater. 2003, 2, 402. (5) (a) Wang, P.; Zakeeruddin, S. M.; Moser, J.-E.; Humphry-Baker, R.; Comte, P.; Aranyos, V.; Hagfeldt, A.; Nazeeruddin, M. K.; Gra¨tzel, M.

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