An Extremely High Molar Extinction Coefficient Ruthenium Sensitizer

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J. Phys. Chem. C 2009, 113, 14559–14566

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An Extremely High Molar Extinction Coefficient Ruthenium Sensitizer in Dye-Sensitized Solar Cells: The Effects of π-Conjugation Extension Qingjiang Yu,† Shi Liu,† Min Zhang,† Ning Cai,†,‡ Yuan Wang,† and Peng Wang*,† State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China and Graduate School, Chinese Academy of Sciences, Beijing 100039, China ReceiVed: May 3, 2009; ReVised Manuscript ReceiVed: June 22, 2009

We report a heteroleptic ruthenium complex (C107) featuring the electron-rich 5-octyl-2,2′-bis(3,4ethylenedioxythiophene) moiety conjugated with 2,2′-bipyridine and exhibiting 10.7% power conversion efficiency measured at the AM1.5G conditions, thanks to the enhanced light-harvesting that is closely related to photocurrent. This C107 sensitizer has an extremely high molar extinction coefficient of 27.4 × 103 M-1 cm-1 at 559 nm in comparison to its analogue C103 (20.5 × 103 M-1 cm-1 at 550 nm) or Z907 (12.2 × 103 M-1 cm-1 at 521 nm) with the corresponding 5-hexyl-3,4-ethylenedioxythiophene- or nonyl-substituted bipyridyl unit. The augmentation of molar extinction coefficients and the bathochromic shift of low-energy absorption peaks along with the π-conjugation extension are detailed by TD-DFT calculations. The absorptivity of mesoporous titania films grafted with Z907, C103, or C107 sublinearly increases with the molar extinction coefficient of sensitizers, which is consistent with the finding derived from the surface coverage measurements that the packing density of those sensitizers decreases with the geometric enlargement of ancillary ligands. When the dye-coated titania film is immersed in a high-efficiency redox electrolyte, a lower density molecule grafting on titania nanocrystals leads to more deep electronic states and a faster charge recombination at the titania/dye/electrolyte interface at a given electron Fermi level, explaining the observation of a larger dark current and a lower open-circuit photovoltage. Electrical impedance analysis further reveals that the electron diffusion length in nanocrystalline titania films is shortened along with the π-conjugation extension of ancillary ligands. 1. Introduction The sensitization of a wide bandgap semiconductor with a small organic molecule plays a vital role in photoelectrochemical cells for stable conversion of light energy to electricity.1 This fascinating process enables the system to absorb visible and even near-infrared photons, and generate charges from the closely bound Frenkel excitons at the energy-offset semiconductor and molecule interface. The introduction of a high-quality mesoporous titania film2 into dye-sensitized solar cells (DSCs) by Gra¨tzel’s group led to the breakthrough of employing carbonbased materials for the high-efficiency solar energy conversion, smartly addressing the long-term pending dilemma of lightharvesting and exciton dissociation in organic materials in contrast to inorganic semiconductors.3 In the past years, the mesoporous DSC has attracted much attention as a promising low-cost candidate for the future photovoltaic market, because of its validated efficiency of 11.1%4 at the AM1.5G conditions and remarkable stabilities under prolonged light and thermal dual stress.5 During the continuous development of DSCs, we have witnessed the critical contribution from molecular engineering of ruthenium sensitizers6 to the boosting of device performance. In this respect, a small-area cell showing over 11% efficiency can be made by only four ruthenium sensitizers7 so far while impressive device efficiencies have been reached with some other metal complexes8 and metal-free organic dyes.9 * To whom correspondence should be addressed. E-mail: peng.wang@ ciac.jl.cn. † Changchun Institute of Applied Chemistry. ‡ Graduate School.

The elegant design of amphiphilic ruthenium sensitizers10 similar to the N719 dye and well exemplified by the Z907 dye has made a pivotal contribution to realizing thermally stable DSCs, apart from an engineered electrolyte without any lithium additive.11 A further strategy of extending the π-conjugation of the ancillary ligand (L), in Ru(dcbpy)(L)(NCS)2 (where dcbpy is 4,4′-dicarboxylic acid-2,2′-bipyridine) photosensitizers, was explored to enhance the optical absorptivity of a mesoporous titania film higher than that stained with Z907 or N719.12 The subsequent study on a slight modification of Z910 to K19 by fairly comparing device efficiencies as well as stabilities has confirmed the feasibility of using ruthenium sensitizers with a large organic moiety, removing some previous concerns on the instability of fragile conjugated units during the long-term device operation.13 This has considerably encouraged the flexible molecular tailoring of ruthenium sensitizers14 on this line and has most prominently fostered the new DSC efficiency record of 11.9% achieved with the C101 dye.15 However, we remark that very few of these high molar extinction coefficient ruthenium sensitizers can really generate higher efficiencies than that (11.18%)16 of the high-quality N719 dye. Some comparisons must be very careful because the sources of N719 also heavily affect the cell efficiencies. In a previous work, we have demonstrated that 2,2′-bis(3,4ethylenedioxythiophene) can be used as a stable building block for an efficient metal-free organic D-π-A sensitizer.17 Here we report a novel heteroleptic ruthenium complex coded C107 and shown in Figure 1. This new sensitizer features the electronrich 5-octyl-2,2′-bis(3,4-ethylenedioxythiophene) unit conjugated

10.1021/jp904096g CCC: $40.75  2009 American Chemical Society Published on Web 07/08/2009

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Figure 1. Molecular structures of Z907, C103, and C107.

with 2,2′-bipyridine, in contrast to C103 and the standard hydrophobic dye Z907 with 5-hexyl-3,4-ethylenedioxythiophene and nonyl attached, respectively. With the aid of TD-DFT calculations, we have detailed the origins of molar extinction coefficient enhancement and bathochromic shift along with the extension of π-conjugation in the spectrator ligands. Transient photoelectrical decay measurements together with electrical impedance analysis have been performed to scrutinize the effects of a gradual enlargement of sensitizer geometry on the cell photovoltage. 2. Experimental Section 2.1. Materials. All solvents and reagents, unless otherwise stated, were of analytical quality and used as received. Tetran-butylammonium hexafluorophosphate, dichloro(p-cymene)ruthenium(II) dimer, n-butyllithium, and 4,4′-dicarboxylic acid2,2′-bipyridine were purchased from Aldrich. Sephadex LH-20 was obtained from Pharmacia. Guanidinium thiocyanate (GNCS) and tert-butylpyridine (TBP) were purchased from Fluka. The 400-nm-sized TiO2 light-scattering paste was received as a gift from Dyesol. The solvent-free synthesis of 1,3-dimethylimidazolium iodide (DMII) was described in our previous paper.18 4,4′-Dibromo-2,2′-bipyridine,19 2,2′-bis(3,4-ethylenedioxythiophene),20 Z907,21 and C1035 were prepared according to the literature methods. 2.2. Synthesis of 5-Octyl-2,2′-bis(3,4-ethylenedioxythiophene). A solution of 2,2′-bis(3,4-ethylenedioxythiophene) (4.500 g, 15.94 mmol) in anhydrous THF (100 mL) was cooled to -78 °C and n-butyllithium (1.6 M in n-hexane, 11.0 mL, 1.1 equiv) was added dropwise under argon. The mixture was

Yu et al. stirred for 1 h and n-bromooctane (3.3 mL, 1.2 equiv) was added. The resulting solution was warmed to room temperature and stirred overnight. The reaction was terminated by the addition of water. After extraction with ether, the combined organic layers were dried over anhydrous Na2SO4. Solvents were removed under a reduced pressure with a rotoevaporator and the crude product was purified with column chromatography (ethyl acetate/petroleum ether, 1:5) on silica gel to afford a white solid (2.630 g, 41.8% yield). 1H NMR (400 MHz, DMSO-d6, δH) 6.47 (s, 1H), 4.27 (m, 4H), 4.21 (m, 4H), 2.57 (t, 2H), 1.53 (m, 2H), 1.25-1.28 (m, 10H), 0.86 (t, 3H). HR-MS m/z calcd for (C20H26O4S2) 394.12725, found 394.12728. Anal. Calcd for C20H26O4S2: C, 60.88; H, 6.64. Found: C, 60.87; H, 6.62. 2.3. Synthesis of 4,4′-Bis(5-octyl-2,2′-bis(3,4-ethylenedioxythiophen)-5′-yl)-2,2′-bipyridine (L7). 5-Octyl-2,2′-bis(3,4ethylenedioxythiophene) (1.010 g, 2.56 mmol) was dissolved in 30 mL of anhydrous THF and cooled to -78 °C. After addition of n-butyllithium (2.4 mL, 1.6 M in hexane, 1.5 equiv), the solution was stirred under Ar at -78 °C for 1 h. Tributylstannyl chloride (1.1 mL, 1.6 equiv) was added dropwise via a syringe. The mixture was warmed to room temperatue and stirred for 6 h. The reaction mixture was quenched with water and extracted with ether. The combined organic layers were dried over anhydrous Na2SO4. After the removal of solvents, the unpurified 5-tributylstannyl-5′-octyl-2,2′-bis(3,4-ethylenedioxythiophene) and 4,4′-dibromo-2,2′-bipyridine (0.265 g, 0.84 mmol) were dissolved in toluene (100 mL). A catalytic amount of Pd(PPh3)2Cl2 (0.033 g, 0.05 equiv) was added and the reaction mixture was refluxed under Ar overnight. After rotoevaporation of toluene under a reduced pressure, the crude compound was purified by column chromatography on silica gel with methanol/ chloroform (1/10) as eluent to afford L7 (0.511 g, 64.2% yield). 1 H NMR (400 MHz, CDCl3, δH) 8.66 (br, 2H), 8.61 (br, 2H), 7.67 (br, 2H), 4.45-4.26 (m, 16H), 2.66 (t, 4H), 1.62 (m, 4H), 1.43-1.28 (m, 0.88 (t, 6H). HR-MS m/z calcd for (C50H56N2O8S2) 940.29195, found 940.29189. Anal. Calcd for C50H56N2O8S2: C, 63.80; H, 6.00, N, 2.98. Found: C, 63.71; H, 6.05, N, 3.04. 2.4. Synthesis of NaRu(4,4′-bis(5-octyl-2,2′-bis(3,4-ethylenedioxythiophen)-5′-yl)-2,2′-bipyridine)(4-carboxylic acid4′-carboxylate-2,2′-bipyridine)(NCS)2 (C107). Dichloro(pcymene)ruthenium(II) dimer (0.042 g, 0.0689 mmol) and L7 (0.130 g, 0.1378 mmol) were dissolved in chloroform (30 mL). The reaction mixture was refluxed for 4 h under Ar in the dark. After the removal of chloroform under a reduced pressure, 4,4′dicarboxylic acid-2,2′-bipyridine (0.034 g, 0.1378 mmol) and DMF (30 mL) were added into the flask and the reaction mixture was stirred at 140 °C for 4 h. Finally, an excess of NH4NCS (0.458 g, 5.512 mmol) was added to the resulting dark solution and the reaction continued for another 4 h at the same temperature. Then, the reaction mixture was cooled to room temperature and the solvent was removed on a rotoevaporator. Water was added to obtain a suspended solution. The solid was collected on a sintered glass crucible by suction filtration, washed with water and ether, and dried under vacuum. The crude complex was dissolved in basic methanol (NaOH) and purified on a Sephadex LH-20 column with methanol as eluent. The collected main band was concentrated and slowly titrated with an acidic methanol solution (HNO3) to pH 4.6. The precipitate was collected on a sintered glass crucible by suction filtration and dried in air. Yield with three times column purification: 57.3%. 1H NMR (400 MHz, DMSO-d6 + NaOH δH) 9.41 (d, 1H), 9.09 (m, 2H), 8.92 (s, 1H), 8.69 (s, 1H), 8.52 (s, 1H), 8.28 (d, 1H), 8.03 (d, 1H), 7.88 (d, 1H), 7.62 (d, 1H),

A Ru Sensitizer in Dye-Sensitized Solar Cells 7.34 (d, 1H), 7.28 (d, 1H), 4.41 (m, 16H), 2.74 (t, 2H), 2.56 (t, 2H), 1.65 (m, 2H), 1.55 (m, 2H), 1.39-1.25 (m, 20H), 0.89 (t, 3H), 0.84 (t, 3H). HR-MS m/z calcd for (NaRuC64H63N6O12S6) 1424.17694, found 1401.18703 [M - Na]-. Anal. Calcd for NaRuC64H63N6O12S6 · 2H2O: C, 52.62; H, 4.62; N, 5.75. Found: C, 52.70; H, 4.65; N, 5.68. 2.5. UV-Vis, ATR/FTIR, and Voltammetric Measurements. Electronic absorption spectra were recorded on a UNICO WFZ UV-2802PC/PCS spectrometer. The ATR-FTIR spectra were measured with a BRUKER Vertex 70 FTIR spectrometer. A CHI660C electrochemical workstation was used for squarewave voltammetric measurements in combination with a mini three-electrode electrochemical cell equipped with a 5-µm-radius Pt ultramicroelectrode as the working electrode. A Pt wire and a silver wire were used as the counter and quasireference electrodes, respectively. The redox potentials were calibrated with ferrocene as the internal reference. 2.6. Computation. The ground-state geometries of dnbpy (4,4′-dinonyl-2,2′-bipyridine), Nadcbpy (4-sodium carboxylate4′-carboxylic acid-2,2′-bipyridine), L3 (4,4′-bis(2-hexyl-3,4ethylenedioxylthiophene-5-yl)-2,2′-bipyridine), L7, Z907, C103, and C107 were fully optimized by the density functional theory (DFT) method22 with Beck’s three-parameter functional and the Lee-Yang-Parr functional (B3LYP).23 In all calculations, the LANL2DZ basis set was used for ruthenium and the 6-31G basis set for other atoms.24 The absorption wavelengths in acetonitrile media were calculated by time-dependent density functional theory (TD-DFT) method25 based on the optimized ground-state structures associated with the polarized continuum model (PCM).26 All calculations were performed with the Gaussian03 program package. 2.7. Device Fabrication. A screen-printed single or double layer film of interconnected TiO2 particles was used as the negative electrode. A transparent layer of 20-nm-sized TiO2 particles was first printed on the fluorine-doped SnO2 (FTO) conducting glass electrode and further coated by a 5-µm-thick second layer of 400-nm-sized light scattering anatase particles if needed. The film thickness was measured by a benchtop Ambios XP-1 stylus profilometer. The detailed preparation procedures of TiO2 nanocrystals, pastes for screen-printing, and nanostructured TiO2 film have been reported in a previous paper.27 A cycloidal TiO2 electrode (∼0.28 cm2) was stained by immersing it into a dye solution containing Z907, C103, or C107 sensitizer (300 µM) in a mixture of acetonitrile and tertbutanol (volume ratio 1/1) overnight. After washing with acetonitrile and drying by air flow, the sensitized titania electrode was assembled with a thermally platinized FTO electrode. The electrodes were separated by a 30-µm-thick Bynel (DuPont) hotmelt gasket and sealed by heating. The internal space was filled with a liquid electrolyte, using a vacuum backfilling system. The electrolyte-injecting hole on the counter electrode glass substrate made with a sand-blasting drill was sealed with a Bynel sheet and a thin glass cover by heating. 2.8. Photovoltaic Characterization. A Keithley 2400 source meter and a Zolix Omni-λ300 monochromator equipped with a 500 W xenon lamp were used for photocurrent action spectrum measurements, with a wavelength sampling interval of 10 nm and a current sampling time of 2 s under the full computer control. Monochromatic incident photon-to-collected electron conversion efficiency (IPCE) is defined by IPCE(λ) ) hcJsc/ eφλ, where h is the Planck constant, c is the light speed in vacuum, e is the electronic charge, λ is the wavelength, Jsc is the short-circuit photocurrent density, and φ is the incident radiative flux. A Hamamatsu S1337-1010BQ silicon diode used

J. Phys. Chem. C, Vol. 113, No. 32, 2009 14561 for IPCE measurements was calibrated at the National Institute of Metrology, China. A model LS1000-4S-AM1.5G-1000W solar simulator (Solar Light Company, USA) in combination with a metal mesh was employed to give an irradiance of 100 mW cm-2. The light intensity was tested with a PMA2144 pyranometer and a calibrated PMA 2100 dose control system. J-V characteristics were obtained by applying a bias potential to a testing cell and measuring dark current and photocurrent with a Keithley 2400 source meter under full computer control. The measurements were fully automated by using Labview 8.0. A metal mask with an aperture area of 0.158 cm2 was covered on a testing cell during all measurements. Importantly, the shortcircuit photocurrent densities measured under this solar simulator are very consistent with the integral of IPCEs with the AM1.5G spectrum (ASTM G173-03), within 5% error for Z907, C103, and C107 based cells. 2.9. Transient Photoelectrical Measurements. In transient photoelectrical decay experiments, a steady light was supplied with a white light-emitting diode (LED) array while a perturbing light pulse was provided with a green LED array controlled by a fast solid-state switch. Both white and green lights were irradiated on the photoanode side of a testing cell. The green pulse was carefully controlled by the driving potential of diodes to keep the modulated photovoltage below 5 mV. We used green light as a probe to generate a photovoltage perturbation near the open-circuit photovoltage (Voc) of the cell under the white light and measured the voltage decay process thereafter. Normally, the transient signals follow a monoexponential decay, thus the recombination rate constant, kr, can be extracted from the slope of the semilogarithmic plot. The capacitance (Cµ) of the titania interface at the Voc is calculated by Cµ ) ∆Q/∆V, where ∆V is the peak of the photovoltage transient and ∆Q is the number of electrons injected during the green light flash. The latter is obtained by integrating a short-circuit photocurrent transient generated from an identical green pulse. This method may underestimate the actual injected electrons by the fraction that is lost due to recombination during the electron collection. The error is thought to be less than 30% in the worst case. More critically, it does not affect the shape of the calculated capacitance versus potential but only the magnitude. 2.10. Electrical Impedance Measurements. Electrical impedance experiments were carried out in the dark with an IM6ex electrochemical workstation, with a frequency range from 50 mHz to 100 kHz and a potential modulation of 10 mV. The obtained impedance spectra were fitted with the Z-view software (v2.80, Scribner Associates Inc.) in terms of an appropriate equivalent circuit. 3. Results and Discussion For the long-term operation of dye-sensitized solar cells, it is necessary to avoid the direct band excitation of the wide bandgap TiO2 nanocrystals. This consideration stems from our observation on a serious deterioration of the device efficiency even when solvent-free ionic liquid electrolytes are employed in the hermetically sealed cells if UV photons are not filtered. We deduce that this is probably due to the structural modification of dye molecules attacked by the high-energy holes in titania. In this respect, an antireflection coating film can be used to filter UV photons and can simultaneously reduce the reflection of visible photons. Therefore, in the practical DSC, the sensitizer normally functions as the only light-harvesting component making a contribution to charge generation although recently Grimes et al.28 have proposed the concept of Fo¨rster resonance energy transfer by adding a dyestuff in the electrolyte.

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Figure 2. (A) Electronic absorption spectra of Z907, C103, and C107 dissolved in DMF. (B) Absorption spectra of Z907, C103, and C107 anchored on a 4.9-µm-thick mesoporous titania film. (C) Plots of film absorbance versus molar extinction coefficient. Here the low-energy maximum absorbance of C103 or C107 is normalized relative to that of Z907. The calculated red line is based on the linear relationship between film absorbance and molar extinction coefficient of low-energy absorption peaks.

Here we first measured the electronic absorption spectrum of the C107 dye dissolved in DMF so as to have a preliminary evaluation of its light-harvesting capacity. As presented in Figure 2A, there are two intense metal-to-ligand charge-transfer transition (MLCT) bands in the visible region. In DMF, the lowenergy MLCT transition absorption of C107 peaks at 559 nm, which is 38 and 9 nm red-shifted compared to Z907 and C103, respectively. The measured molar extinction coefficient (ε) at 559 nm for C107 is 27.4 × 103 M-1 cm-1, which is obviously higher than the corresponding values for Z907 (12.2 × 103 M-1 cm-1 at the peak of 521 nm), N3 (14.2 × 103 M-1 cm-1 at the peak of 538 nm), and C103 (20.5 × 103 M-1 cm-1 at the peak of 550 nm).5,10 Moreover, C107 exhibits a strong absorption peak at 453 nm with a molar extinction coefficient of 54.3 × 103 M-1 cm-1. The absorption profile of C107 is very similar to that of the ruthenium trimer dye designed by Scandola et al.,6c which resulted in the first efficient DSC.2 However, the molar extinction coefficients of C107 have been remarkably enhanced due to the use of a large organic ligand. We are also curious to know that besides the molar extinction coefficient, does the molecular diameter also have any influence on the optical absorptivity of a stained mesoporous film, which is more closely related to the light-harvesting of dye-sensitized solar cells. ATR-FTIR spectra (Figure S1 in the Supporting Information) of Z907, C103, and C107 anchored on TiO2 film clearly show the bands at 1608 cm-1 for the asymmetric stretching mode of the carboxylate group, indicating that the carboxylic acid is deprotonated and involved in the adsorption of these dyes on the surface of TiO2. From these ATR-FTIR data we can infer that these dyes are anchored on the surface through the carboxylate group via a bidentate chelation or a bridging of surface titanium ions rather than an ester-type linkage. The NCS signal remains at 2102 cm-1, indicating that

Figure 3. Molecular geometries of Z907, C103, and C107 optimized at the B3LYP level.

NCS coordinated to the ruthenium center through the N atom is unaffected by the adsorption process. The impressive improvements through extending the π-conjugation of ancillary ligands in heteroleptic ruthenium complexes are evident by observing the red-shifted absorption peaks and enhanced absorbance from Figure 2B, which depicts the absorption spectra in the visible region of Z907, C103, and C107 anchored on a 4.9 µm thick transparent nanocrystalline TiO2 film. Notably, C107 confers a very strong absorption in the blue region, which can compete with the dissipative absorption of triiodide in the DSC electrolytes. However, the film absorbance ratio of lowenergy peaks for Z907, C103, and C107 sensitizers is 1:1.45: 1.73, which sublinearly follows the augment of molar extinction coefficients (1:1.68:2.25) as presented in Figure 2C, indicating a lowest packing density of C107 on titania among these three sensitizers. This can be caused by a gradually enlarged lateral diameter (Z907, C103, and C107) as shown in Figure 3. To solidify this ratiocination, we also calculated the surface coverage of sensitizer on titania from the surface area (68.3 m2 g-1) measurement of our titania film and ICP-OES analysis on ruthenium contents (Z907: 6.573 mg g-1; C103: 4.562 mg g-1; C107: 2.613 mg g-1) of dye-coated films. The surface coverages of Z907, C103, and C107 are 9.58 × 10-11, 6.64 × 10-11, and 3.80 × 10-11 mol cm-2, respectively. Our analysis has explicitly

A Ru Sensitizer in Dye-Sensitized Solar Cells advised that in the further molecular engineering of Ru(dcbpy)(L)(NCS)2 sensitizers for shortening the light absorption length of a stained mesoporous film, there may be a trade-off between enhancing molar extinction coefficient and geometric enlargement. While a good light-harvesting yield is a basic requirement for any high-efficiency solar cells, the immediate charge generation yield from the excited state of sensitizers also has a direct influence on the device operation. In dye-sensitized solar cells, this process is triggered at the interface between a donor component and an acceptor component, where a favorite energyoffset between dye and titania is needed. We measured the redox potentials of these sensitizers with a platinum ultramicroelectrode using the square-wave voltammetry. The HOMO and LUMO values were transformed via the equation ELUMO/HOMO ) -e(4.88 + Vredox),29 where Vredox is the onset potential vs. ferrocene of reduction or oxidization of sensitizers dissolved in DMF. The measured LUMO (-3.35 eV for Z907; -3.37 eV for C103; -3.38 eV for C107 vs. vacuum) relative to the conduction band edge (-4.00 eV vs. vacuum) of TiO21a,c provides a thermodynamic driving force for charge generation. The HOMO (-4.99 eV for Z907; -4.98 eV for C103; -4.95 eV for C107 vs. vacuum) in comparison with that (-4.60 eV vs. vacuum) of iodide30 could supply a negative Gibbs energy change for dye regeneration. To understand the molar extinction coefficient augment and bathochromic shift of low-energy MLCT absorptions along with the π-conjugation extension, we detailed the electron transitions by calculating the electronic states of these sensitizers as well as their bipyridyl ligands. The only difference in the molecular structures of Z907, C103, and C107 lines in their ancillary ligands. In contrast to dnbpy, Nadcbpy, L3, and L7 all exhibit a narrower HOMO/LUMO energy gap (Figure S2 in the Supporting Information) owing to the π-conjugation extension. Moreover, among these ligands Nadcbpy with the carboxylate anchoring group has the lowest LUMO, endowing an efficient electronic coupling of excited dye molecules and titania nanocrystals. Insertion of 3,4-ethylenedioxythiophene and bis(3,4ethylenedioxythiophene) between pyridine and alkyl gradually depresses the LUMO and evidently lifts the HOMO. Additionally, it is easy to see from Figure S3 in the Supporting Information that for Nadcbpy deprotonated carboxylate is a weaker electron-withdrawing group than carboxylic acid. The calculated electron absorptions (Figure S4 in the Supporting Information) are in close agreement with experimental data presented in Figure 2. Transition involved molecular orbitals are shown in Figures S4-S6 in the Supporting Information and detailed transition assignments are listed in Tables S1-S3 of the Supporting Information. Energy diagrams of occupied (HOMO to HOMO-3) and unoccupied (LUMO to LUMO+3) frontier orbitals of Z907, C103, and C107 are shown in Figure 4. The occupied (HOMO to HOMO-3) orbitals of these sensitizers all own ruthenium t2g character with sizable contribution from the thiocyanate ligand. It is further noted that there is no obvious variation of their LUMO energies because of the sole distribution on Nadcbpy. However, the energy of ancillary ligand-localized LUMO+1 orbitals has been significantly depressed along with the π-conjugation extension, making a predominant contribution to the experimentally measured lowenergy transition. It is valuable to note that the HOMO to LUMO transition (674 nm for Z907; 679 nm for C103; 686 nm for C107) is red-shifted with the use of a more electrondonating ancillary ligand (L3 or L7) to uplift the HOMO. Unfortunately, this transition has very low oscillator strength

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Figure 4. Energy diagram of occupied (HOMO to HOMO-3) and unoccupied (LUMO to LUMO+3) frontier orbitals of Z907, C103, and C107.

and does not contribute too much to the light-harvesting of DSCs until a double-layer film is introduced. This scenario can be understood in terms of the Franck-Condon principle, stating that during an electronic transition, a change from one vibrational energy level to another will be less likely to happen if the two wave functions overlap less significantly. Overall, the calculated oscillator strengths listed in Tables S1-S3 in the Supporting Information echo the measured molar extinction coefficients of Z907, C103, and C107. To comparatively evaluate the performance of these sensitizers, we employed a high-quality double-layer titania film (7.8 + 5 µm) and a high-efficiency electrolyte7c to construct dyesensitized solar cells. To see the effect of light-harvesting enhancement from the π-conjugation extension, photocurrent action spectra were first measured as depicted in Figure 5A. The monochromatic incident photon-to-collected electron conversion efficiencies of C107 exceed 80% from 440 to 660 nm and exhibit a broad plateau of ∼90% in the region between 500 and 610 nm. Considering the light absorption and scattering loss by the conducting glass, the absorbed photon-to-collected electron conversion efficiencies (APCE) are close to unity over a broad spectral range, suggesting a very high charge collection yield. A close look at the IPCE ratios (Figure 5B) of C107 to Z907 and C107 to C103 has explicitly demonstrated the remarkable merit of extending the π-conjugation of ancillary ligands in polypyridyl ruthenium sensitizers, especially in the weak absorption red region as well as the blue region where competitive triiodide absorption is dissipative. J-V characteristics were further measured under an irradiance of AM1.5G full sunlight. As presented in Figure 5C, the shortcircuit photocurrent density (Jsc), open-circuit photovoltage (Voc), and fill factor (FF) of a cell made from C107 are 19.18 mA cm-2, 739 mV, and 0.751, respectively, yielding an overall conversion efficiency (η) of 10.7%. In contrast, the photovoltaic parameters (Jsc, Voc, FF, and η) of the cells with Z907 and C103 are 16.46 mA cm-2, 769 mV, 0.718, 9.1% and 18.35 mA cm-2, 760 mV, 0.748, 10.4%, respectively. While the short-circuit photocurrents are closely in agreement with the IPCE data, it is noted that the open-circuit photovoltage becomes low along with the geometric enlargement of sensitizers. This adverse effect stemming from π-conjugation extension is generally consistent with our previous observation on cells with metalfree D-π-A organic dyes.31 Overall, the enhanced Jsc due to better light harvesting is partially compromised with the loss in Voc. We also measured J-V characteristics of cells made from

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Figure 5. (A) Photocurrent action spectra of cells made from Z907, C103, and C107. (B) Relative monochromatic incident photon-to-collected electron conversion efficiencies. (C) J-V characteristics of cells made with Z907, C103, and C107 measured in the dark and under an irradiance of 100 mW cm-2 AM1.5G sunlight. Double layer film: 7.8 + 5 µm. The electrolyte composition is as follows: 1.0 M DMII, 50 mM LiI, 30 mM I2, 0.5 M tert-butylpyridine, and 0.1 M GNCS in the mixed solvents of acetonitrile and valeronitrile (v/v 85/15). An antireflection film was adhered to the cell during measurements.

Z907, C103, and C107 employing titania films of different thicknesses. Detailed cell parameters are listed in Table S4 of the Supporting Information. Note that with a thin titania film, the photovoltage can be improved for all these sensitizers mainly due to an enhanced electron density in the mesoporous titania film under a given light intensity. To clarify the origins of the dye-Voc relationship, we resorted to the transient photoelectrical decay technique32 to measure the chemical capacitance (Cµ) and the charge recombination at the titania/electrolyte interface. As shown in Figure 6A, the capacitances of cells all increase exponentially along with the increase in Voc, which was generated by using a gradually enhanced light intensity. It is also known that the chemical capacitance of dye-sensitized solar cells under relatively strong light is mainly related to the density of surface states (DOS) below the conduction band edge. As the same titania film and electrolyte were used for the evaluation of these three sensitizers, we can derive from the chemical capacitance measurements that the density of surfaces states increases in the order of Z907 < C103 < C107. This can be explained by the relative packing density. Furthermore, a low-density packing of dye molecules will also result in a fast charge recombination at the titania/ electrolyte interface, as shown in Figure 6B, mainly due to more electron-occupied deep states at a given potential. At a given Voc, the charge recombination rate constant increases in the order of Z907 < C103 < C107, which is in good agreement with the tendency of the reverse saturation currents (Js) derived from the dark J-V curves presented in Figure 5C by the following diode equation:33

( (

J ) Js exp

) )

V - JRs q(V - JRs) -1 + nkT Rsh

(1)

Figure 6. Plots of (A) chemical capacitance and (B) charge recombination rate constant versus open-circuit photovoltage for cells made from Z907, C103, and C107.

where J is the current density, V is the voltage, q is the electronic charge, n is the ideality factor, k is the Boltzmann constant, T is the temperature, and Rs and Rsh are the series resistance arising from the internal resistance and resistive contacts of the cell, and parallel resistance from the leakage of the current, respec-

A Ru Sensitizer in Dye-Sensitized Solar Cells

Figure 7. Plots of (A) electron diffusion coefficient, (B) electron lifetime, and (C) electron diffusion length versus DOS for cells made from Z907, C103, and C107.

tively. The reverse saturation current values of cells made from Z907, C103, and C107 in the dark are 5.42 × 10-7, 6.98 × 10-7, and 9.32 × 10-7 mA cm-2, respectively. This may explain the Voc trend observed for these three sensitizers, because the open-circuit voltage decreases logarithmically with an increase in the reverse saturation current. We further measured the dark electrical impedance spectra34 of cells at various bias potentials to scrutinize the effects of π-conjugation extension on the electron collection in the mesoporous titania film. The electron transport in our titania films may be described by the multiple trapping-detrapping (MTD) model. Along with the increase of bias potentials, deep traps in the mesoporous titania film will be filled with electrically injected electrons. The detrapping of electrons from shallow traps is fairly fast, resulting in a high electron diffusion coefficient (Dn) as shown in Figure 7A. However, it does not seem that a different molecule grafting has a strong influence on the electron mobility. As presented in Figure 7B, at a given DOS, the electron lifetime of cells made from these three sensitizers decreases in the order of Z907 > C103 > C107, which is very consistent with the above transient photovoltage decay measurements. Overall, the electron diffusion lengths are shortened with an increase of the lateral diameters of sensitizers, showing that there is a trade-off between the light absorptivity and the electron diffusion length for the π-conjugation extension of ancillary ligands in ruthenium sensitizers. We remark that in our cells the electron diffusion length even with C107 is still much larger than the film thickness; however, for a low-fluidity electrolyte and/or a poor-quality titania film, a high-density molecule packing is desirable to realize a quantitative charge collection yield, because in that case the electron diffusion length will be considerably shortened. 4. Conclusions To summarize, an extremely high molar extinction coefficient ruthenium sensitizer exhibiting a 10.7% power conversion efficiency at the AM1.5G conditions has been synthesized through the conjugation of the electron-rich 2,2′-bis(3,4ethylenedioxythiophene) unit with bipyridine. Our comparative experiments have shown clearly that the absorbance of dyecoated nanocrystalline films may only sublinearly increase with the molar extinction coefficient of sensitizers achieved by extending the π-conjugation of ancillary ligands in chargetransfer ruthenium complexes. We have found that the packing densities of sensitizers on titania are closely related to their lateral geometric diameters. While the monochromatic incident photon-to-collected electron conversion efficiencies in the blue, red, and near-infrared regions have been improved thanks to the enhancement of film absorptions, an undesirable reduction

J. Phys. Chem. C, Vol. 113, No. 32, 2009 14565 in the photovoltage has been observed. Our transient photoelectrical measurements have revealed that along with the use of larger dye molecules, there will be more deep electrontrapping states and faster charge recombination at the titania/ electrolyte interface, explaining the observed photovoltage tendency. Further electrical impedance analysis has indicated that the electron diffusion lengths in the dye-coated titania films could be shortened with the increase of the lateral diameters of sensitizers. In the further dye design, the light absorptivity and the charge diffusion length must be synchronously enhanced to boost the practical application of dye-sensitized solar cells. Acknowledgment. The National Key Scientific Program (No. 2007CB936700), the CAS Knowledge Innovation Program (No. KGCX2-YW-326), and the CAS Hundred Talents Program have supported this work. Q.Y. thanks the Postdoctoral Science Foundation of China (No. 20080440148). We are grateful to Dyesol for supplying the scattering paste and to DuPont Packaging and Industrial Polymers for supplying the Bynel film. Supporting Information Available: Calculation details and additional ATR-FTIR and photovoltaic data. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) Hagfeldt, A.; Gra¨tzel, M. Chem. ReV. 1995, 95, 49. (b) Hagfeldt, A.; Gra¨tzel, M. Acc. Chem. Res. 2000, 33, 269. (c) Gra¨tzel, M. Nature 2001, 414, 338. (2) O’Regan, B.; Gra¨tzel, M. Nature 1991, 353, 737. (3) Pope, M.; Swenberg, C. E. Electronic Processes in Organic Crystals and Polymers, 2nd ed.; Oxford University Press: Oxford, UK, 1999. (4) Chiba, Y.; Islam, A.; Watanabe, Y.; Komiya, R.; Koide, N.; Han, L. Jpn. J. Appl. Phys., Part 2 2006, 45, L638. (5) Shi, D.; Pootrakuchote, N.; Li, R.; Guo, J.; Wang, Y.; Zakeeruddin, S. M.; Gra¨tzel, M.; Wang, P. J. Phys. Chem. C 2008, 112, 17046. and some references cited therein. (6) (a) Desilvestro, J.; Gra¨tzel, M.; Kavan, L.; Moser, J.; Augustynski, J. J. Am. Chem. Soc. 1985, 107, 2988. (b) Liska, P.; Vlachopoulos, N.; Nazeeruddin, M. K.; Comte, P.; Gra¨tzel, M. J. Am. Chem. Soc. 1988, 110, 3686. (c) Amadelli, R.; Argazzi, R.; Bignozzi, C. A.; Scandola, F. J. Am. Chem. Soc. 1990, 112, 7099. (7) (a) Nazeeruddin, M. K.; Kay, A.; Rodicio, I.; Humphry-Baker, R.; Mu¨ller, E.; Liska, P.; Vlachopoulos, N.; Gra¨tzel, M. J. Am. Chem. Soc. 1993, 115, 6382. (b) Nazeeruddin, M. K.; Pe´chy, P.; Renouard, T.; Zakeeruddin, S. M.; Humphry-Baker, R.; Comte, P.; Liska, P.; Cevey, L.; Costa, E.; Shklover, V.; Spiccia, L.; Beacon, G. B.; Bignozzi, C. A.; Gra¨tzel, M. J. Am. Chem. Soc. 2001, 123, 1613. (c) 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. (d) Cao, Y.; Bai, Y.; Yu, Q.; Cheng, Y.; Liu, S.; Shi, D.; Gao, F.; Wang, P. J. Phys. Chem. C 2009, 113, 6290. (8) (a) Ferrere, S.; Gregg, B. A. J. Am. Chem. Soc. 1998, 120, 843. (b) Sauve´, G.; Cass, M. E.; Doig, S. J.; Lauermann, I.; Pomykal, K.; Lewis, N. S. J. Phys. Chem. B 2000, 104, 3488. (c) Islam, A.; Sugihara, H.; Hara, K.; Singh, L. P.; Katoh, R.; Yanagida, M.; Takahashi, Y.; Murata, S.; Arakawa, H. Inorg. Chem. 2001, 40, 5371. (d) Geary, E. A. M.; Yellowlees, L. J.; Jack, L. A.; Oswald, L. D. H.; Parsons, S.; Hirata, N.; Durrant, J. R.; Robertson, N. Inorg. Chem. 2005, 44, 242. (e) Altobello, S.; Argazzi, R.; Caramori, S.; Contado, C.; Da Fre´, S.; Rubino, P.; Chone´, C.; Larramona, G.; Bignozzi, C. A. J. Am. Chem. Soc. 2005, 127, 15342. (f) Bessho, T.; Constable, E. C.; Gra¨tzel, M.; Redondo, A. H.; Housecroft, C. E.; Kylberg, W.; Nazeeruddin, M. K.; Neuburger, M.; Schaffner, S. Chem. Commun. 2008, 3717. (9) (a) Mishra, A.; Fischer, M. K. R.; Ba¨uerle, P. Angew. Chem. 2009, 48, 2474, and references cited therein. (b) Zhang, G.; Bala, H.; Cheng, Y.; Shi, D.; Lv, X.; Yu, Q.; Wang, P. Chem. Commun. 2009, 2198. (10) Zakeeruddin, S. M.; Nazeeruddin, M. K.; Humphry-Baker, R.; Pe´chy, P.; Quagliotto, P.; Barolo, C.; Viscard, G.; Gra¨tzel, M. Langmuir 2002, 18, 952. (11) Wang, P.; Zakeeruddin, S. M.; Moser, J.-E.; Nazeeruddin, M. K.; Sekiguchi, T.; Gra¨tzel, M. Nat. Mater. 2003, 2, 402. (12) Wang, P.; Zakeeruddin, S. M.; Moser, J.-E.; Humphry-Baker, R.; Comte, P.; Aranyos, V.; Hagfeldt, A.; Nazeeruddin, M. K.; Gra¨tzel, M. AdV. Mater. 2004, 16, 1806.

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