Dye-Sensitized Solar Cells with a High Absorptivity Ruthenium

Mar 25, 2009 - Naphtho[2,3-c][1,2,5]thiadiazole and 2H-Naphtho[2,3-d][1,2 ... Hsien-Hsin Chou , Kamani Sudhir K. Reddy , Hui-Ping Wu ...... Langmuir 2...
0 downloads 0 Views 1MB Size
Download PDF
6290

J. Phys. Chem. C 2009, 113, 6290–6297

Dye-Sensitized Solar Cells with a High Absorptivity Ruthenium Sensitizer Featuring a 2-(Hexylthio)thiophene Conjugated Bipyridine Yiming Cao,†,‡ Yu Bai,†,‡ Qingjiang Yu,† Yueming Cheng,† Shi Liu,† Dong Shi,† Feifei Gao,† 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: January 23, 2009; ReVised Manuscript ReceiVed: February 12, 2009

We conjugated 2-(hexylthio)thiophene with bipyridine to construct a new heteroleptic polypyridyl ruthenium sensitizer exhibiting a charge-transfer band at 550 nm with a molar extinction coefficient of 18.7 × 103 M-1 cm-1. In contrast to its analogues Z907 and C101, a mesoporous titania film stained with this new sensitizer featured a short light absorption length, allowing for the use of a thin photoactive layer for efficient lightharvesting and conversion of solar energy to electricity. With a preliminary testing, we have reached 11.4% overall power conversion efficiency measured at the air mass 1.5 global conditions. Transient photoelectrical decays and electrical impedance spectra were analyzed to picture the intrinsic physics of temperature-dependent photovoltage and photocurrent. 1. Introduction In the past 20 years, considerable research efforts1 have been devoted to the mesoscopic dye-sensitized solar cell (DSC) since the seminal demonstration2 of its feasibility as a low-cost photovoltaic technology. On the basis of several material combinations, DSC has achieved a respectable high efficiency and a remarkable stability under the prolonged accelerating testing conditions. Note that so far 11% efficiency cells can be fabricated with only three polypyridyl ruthenium sensitizers3-5 in combination with some toxic and volatile acetonitrile-based electrolytes. The smart design of amphiphilic ruthenium sensitizers6 embodied by the Z907 dye, which was prepared via a convenient one-pot procedure, has made a pivotal contribution to attaining thermally stable cells, apart from an engineered electrolyte without any lithium additive.7 A further strategy of extending the conjugation length of the ancillary ligand (L) in Ru(dcbpy)(L)(NCS)2 (where dcbpy is 4,4′-dicarboxylic acid2,2′-bipyridine) photosensitizers was explored to enhance the light-harvesting capacity.8 Among several following studies5a,9 of high molar extinction coefficient ruthenium sensitizers on this track, the C101 dye with a 4,4′-bis(5-hexylthiophen-2-yl)2,2′-bipyridine ligand (L1) has set a new DSC efficiency record of 11.9%,10 becoming the first sensitizer triumphing over the well-known N3 dye reported 15 years ago. We remark that this high-efficiency cell was realized by using an optimized titania film. In the near past, 2-(hexylthio)phenylvinyl was conjugated with bipyridine to assemble a higher molar extinction coefficient ruthenium sensitizer9h compared to its analogue, the K19 dye possessing the 2-(hexyloxy)phenylvinyl fragment.11 Here we report a C101 analogue with a sulfur atom inserted between hexyl and thiophene as presented in Figure 1 and code this new dye C106. With the aid of DFT-TDDFT calculations, we have * To whom correspondence should be addressed. E-mail: peng.wang@ ciac.jl.cn. † Changchun Institute of Applied Chemistry. ‡ Graduate School.

Figure 1. Molecular structures of Z907, C101, and C106.

detailed the effect of this sulfur insertion on energy levels and electronic transitions of ruthenium complexes. On the basis of a series of parallel experiments, we have proved that C106 is more applicable to make efficient solar cells with very thin photoactive layers in comparison with its counterparts, Z907 and C101. With a preliminary optimization, a cell with C106 as sensitizer has reached 11.4% high efficiency at 30 °C measured at the air mass 1.5 global (AM1.5G) conditions.

10.1021/jp9006872 CCC: $40.75  2009 American Chemical Society Published on Web 03/25/2009

Solar Cells with a High Absorptivity Ru Sensitizer Electrical impedance and transient photoelectrical measurements have been performed to take a close look on the origins of temperature-dependent J-V characteristics of a high-efficiency cell based on the C106 dye. 2. Experimental Section 2.1. Materials. All solvents and reagents, unless otherwise stated, were of puriss quality and were used as received. Dichloro(p-cymene)ruthenium(II) dimer was purchased from Aldrich. Sephadex LH-20 was obtained from Pharmacia. Guanidinium thiocyanate (GNCS), 3R,7R-dihyroxy-5β-cholic acid (cheno), and tert-butylpyridine (TBP) were purchased from Fluka. 1-Ethyl-3-methylimidazolium tetracyanoborate (EMITCB) and 400-nm-sized TiO2 light-scattering paste were received as gifts from Dyesol. The solvent-free synthesis of 1,3-dimethylimidazolium iodide (DMII) and 1-ethyl-3-methylimidazolium iodide (EMII) was described in our previous paper.12 4,4′Dibromo-2,2′-bipyridine,13 tributyl(5-(hexylthio)thiophen-2-yl)stannane,14 and N-butylbenzimidazole (NBB)15 were prepared according to the literature methods. 2.2. Synthesis of 4,4′-Bis(5-(hexylthio)thiophen-2-yl)-2,2′bipyridine (L6). To 150 mL of toluene were added tributyl(5(hexylthio)thiophen-2-yl)stannane (3.32 g, 6.79 mmol), 4,4′dibromo-2,2′-bipyridine (0.88 g, 2.83 mmol), and Pd(PPh3)2Cl2 (0.11 g, 0.14 mmol) and the mixture was refluxed under Ar overnight. After rotoevaporation of toluene under a reduced pressure, the resulting solid was purified by column chromatography on silica gel with methanol/chloroform (1/10) as eluent to afford L6 (1.12 g, 71.3% yield). 1H NMR (400 MHz, CDCl3, δH) 8.67 (m, 4H), 7.55 (br, 2H), 7.47 (d, 2H), 7.11 (d, 2H), 2.90 (t, 4H), 1.66-1.69 (m, 4H), 1.41-1.45 (m, 4H), 1.27-1.31 (m, 8H), 0.89 (t, 6H). MS (ESI) m/z calcd for (C30H36N2S4) 552, found 553 [M + H]+. 2.3. Synthesis of NaRu(4,4′-bis(5-(hexylthio)thiophen-2yl)-2,2′-bipyridine)(4-carboxylic acid-4′-carboxylate-2,2′-bipyridine)(NCS)2 (C106). Dichloro(p-cymene)ruthenium(II) dimer (0.11 g, 0.18 mmol) and L6 (0.20 g, 0.36 mmol) were dissolved in DMF (50 mL). The reaction mixture was stirred at 60 °C for 4 h under Ar in the dark. Subsequently, 4,4′-dicarboxylic acid2,2′-bipyridine (0.09 g, 0.36 mmol) was added into the flask and the reaction mixture was stirred at 140 °C for 4 h. Next, an excess of NH4NCS (0.82 g, 10.80 mmol) was added to the resulting dark solution and the reaction was 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 5.2. Yield with four times column purification: 49%. The precipitate was collected on a sintered glass crucible by suction filtration and dried in air. 1H NMR (400 MHz, CD3OD + NaOH δH) 9.69 (d, 1H), 9.23 (d, 1H), 9.08 (s, 1H), 8.90 (s, 1H), 8.49 (s, 1H), 8.40 (d, 1H), 8.33 (s, 1H), 8.15 (d, 1H), 7.97 (d, 1H), 7.71 (d, 1H), 7.60 (d, 1H), 7.57 (d, 1H), 7.43 (d, 1H), 7.33 (d, 1H), 7.29 (d, 1H), 6.96 (d, 1H), 3.17 (t, 2H), 3.08 (t, 2H), 1.83-1.90 (m, 2H), 1.73-1.81 (m, 2H), 1.50-1.70 (m, 4H), 1.37-1.50 (m, 8H), 1.01 (t, 3H), 0.96 (t, 3H). Anal. Calcd for NaRuC44H43N6O4S6 · 2H2O: C, 49.28; H, 4.42; N, 7.84. Found: C, 49.36; H, 4.40; N, 7.89. MS (ESI) m/z calcd for (NaRuC44H43N6O4S6) 1036, found 1013 [M - Na]-.

J. Phys. Chem. C, Vol. 113, No. 15, 2009 6291 2.4. UV-Vis, Photoluminescence, and Voltammetric Measurements. Electronic absorption spectra were recorded on a UNICO WFZ UV-2802PC/PCS spectrometer. Emission spectra were performed with a Perkin-Elmer LS55 luminescence spectrometer. The emitted light was probed with a Hamamatsu R928 red-sensitive photomultiplier. A computer-controlled CHI660C electrochemical workstation was employed for squarewave voltammetric measurements with a three-electrode electrochemical cell. 2.5. Computation. Density functional theory (DFT) and time dependent density functional theory (TDDFT) calculations, implementing the Gaussian03 program package,16 employing the B3LYP/3-21G* functional and basis set,17 were performed to gain insights on geometric structures and electronic transition properties of Z907, C101, and C106 from the molecular orbital level. In particular, the central ruthenium(II) atom adopts a lowspin 4d65s0 electronic configuration in the quasioctahedral symmetrical ligand field.18 Without any symmetry constraint, their geometrical structures were optimized under vacuum and the lowest 50 singlet-singlet electronic transitions were further calculated in consideration of the solvent effect of acetonitrile by means of the Polarizable Continuum Model (PCM).19 The TDDFT results were incorporated in the SWizard program (http://www.sg-chem.net/swizard/) to calculate the absorption profile as a sum of the Gaussian band.20 2.6. Device Fabrication. A screen-printed single or double layer film of interconnected TiO2 particles was used as a mesoporous negative electrode. The 20-nm-sized TiO2 particles were 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.21 A cycloidal TiO2 electrode (∼0.28 cm2) was stained by immersing it into a dye solution containing Z907, C101, or C106 sensitizer (300 µM) and cheno (2 mM) in a mixture of acetonitrile and tertbutyl alcohol (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 with use of 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. 2.7. Photovoltaic Characterization. A LS100 solar simulator (Solar Light Com. Inc., USA) was used to give an irradiance of 100 mW cm-2 (the equivalent of one sun at AM1.5G) at the surface of a testing cell. The current-voltage characteristics were obtained by applying external potential bias to the cell and measuring the dark current and photocurrent with a Keithley model 2602 digital source meter. This process was fully automated by using Labview 8.0. A similar data acquisition system was used to control the incident photon-to-collected electron conversion efficiency (IPCE) measurement. Under full computer control, light from a 1000 W xenon lamp was focused through a monochromator onto the photovoltaic cell under test. A computer-controlled monochromator (Omni λ300) was incremented through the spectral range (300-900 nm) to generate a photocurrent action spectra with a sampling interval of 10 nm and a current sampling time of 2 s. IPCE is defined by IPCE(λ) ) hcJsc/eφλ, where h is the Planck constant, c is the

6292 J. Phys. Chem. C, Vol. 113, No. 15, 2009

Cao et al.

Figure 2. (A) Electronic absorptions of Z907, C101, and C106 dissolved in DMF and emission spectrum of C106. (B) Absorption spectra of Z907, C101, and C106 anchored on a 7.6-µm-thick mesoporous titania film.

light speed in vacuum, e is the electronic charge, λ is the wavelength (m), Jsc is the short-circuit photocurrent density (A m-2), and φ is the incident radiative flux (W m-2). Photovoltaic performance was measured by using a metal mask with an aperture area of 0.158 cm2. A homemade heating-cooling system was used for temperature-dependent J-V measurements. 2.8. Transient Photoelectrical Measurements. In the transient photoelectrical decay experiment, steady-state light was supplied with a homemade white light-emitting diode (LED) array by tuning the driving voltage, and a green LED array controlled with a fast solid-state switch was used to generate a perturbing pulse with a width of 200 ms. The pulsed green light and steady-state white light were both incident 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 opencircuit 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 TiO2/ electrolyte 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. The electron density in the titania film under a given white light intensity was determined by charge extraction technique. A homemade heating-cooling system was used for temperature-dependent measurements. 2.9. 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 appropriate equivalent circuits. A homemade heating-cooling system was used for temperature-dependent measurements. 3. Results and Discussion 3.1. Electronic Absorption and Energy Level. Direct band excitation of the wide bandgap TiO2 nanocrystals is undesirable, because of photocatalytic decompositions of some organic materials such as dyes and electrolytes in DSC during longterm device operation. UV photons are often filtered off by using an antireflection coating film. Therefore, in the practical DSC, sensitizer functions as the only light-harvesting component

Figure 3. Square-wave voltammogram of a Pt ultramicroelectrode in a DMF solution containing the C106 dye and 0.1 M ntetrabutylammonium hexafluorophosphate as supporting electrolyte. Square-wave voltammogram of 1-ethyl-3-methylimidazolium iodide in acetonitrile with 0.1 M 1-ethyl-3-methylimidazolium bis(trifluorosulfonyl)imide supporting electrolyte was recorded with a thermally platinized FTO electrode. The LUMO and HOMO were estimated vs. vacuum: ELUMO/HOMO) -4.88 - Fφredox.

making contribution to charge generation. Here we first measured the electronic absorption spectrum of the C106 dye dissolved in DMF so as to have a preliminary evaluation on its light-harvesting capacity. As shown in Figure 2A, there are two intense absorption bands at 310 and 348 nm in the UV region, and characteristic metal-to-ligand charge-transfer transition (MLCT) bands in the visible region like other heteroleptic polypyridyl ruthenium(II) complexes.6,8,9 In DMF, the lowenergy MLCT transition absorption of C106 peaks at 550 nm, which is 29 and 3 nm red-shifted compared to Z907 and C101, respectively. The measured molar extinction coefficient () at 550 nm for C106 is 18.7 × 103 M-1 cm-1, which is higher than the corresponding values for Z907 (12.2 × 103 M-1 cm-1 at the peak of 521 nm) and C101 (17.5 × 103 M-1 cm-1 at the peak of 547 nm). Excitation of the low-energy MLCT transitions of C106 in DMF produces a weak triplet emission centered at 797 nm, whose intensity is sensitive to the concentration of dissolved oxygen. The excitation transition energy (E0-0) was roughly estimated to be 1.8 eV by taking the crossing-point of the absorption and emission spectra. Besides the molar extinction coefficient, it is known that molecular diameter and packing mode may also influence the optical absorptivity of a stained nanocrystalline film. The film is sensitized with 300 µM dye, a mixture of acetonitrile and tert-butyl alcohol (volume ratio: 1/1), overnight. The merit of introducing sulfur between hexyl and thiophene together with extending the π-conjugated system of ancillary ligands in heteroleptic ruthenium complexes can be unambiguously per-

Solar Cells with a High Absorptivity Ru Sensitizer

J. Phys. Chem. C, Vol. 113, No. 15, 2009 6293 TABLE 1: Detailed Photovoltaic Parameters of Cells Made with Z907, C101, and C106 Employing Different Films thickness/µm 2.1

dye

Z907 C101 C106 4.3 Z907 C101 C106 7.6 Z907 C101 C106 double layer 10 + 5 Z907 C101 C106

Figure 4. Energy diagram and isodensity surface plots of the first three HOMOs and the first three LUMOs of Z907, C101, and C106. All the isodensity surface values are fixed at 0.04.

ceived from Figure 2B. The film absorptivity ratio for Z907, C101, and C106 sensitizers is 1:1.38:1.56. The C106 sensitizer bestows an improved light absorption coefficient to the stained TiO2 film, ensuring a good light-harvesting efficiency even thin nanocrystalline layer must be used for efficient conversion of

Jsc/mA cm-2 Voc/mV 6.77 8.15 9.16 10.29 11.80 12.05 13.31 14.48 14.68 17.13 17.75 18.28

777 775 777 768 766 769 738 747 750 730 749 749

FF

η/%

0.737 3.88 0.764 4.82 0.769 5.47 0.740 5.85 0.775 7.01 0.776 7.19 0.731 7.18 0.778 8.42 0.773 8.51 0.724 9.05 0.777 10.33 0.772 10.57

light energy to electricity. For instance, a short charge diffusion length may confine cell performance when viscous solvent-free ionic liquid electrolytes are employed in devices based on a plastic matrix. In addition, a reduction of film thickness can improve the open-circuit photovoltage, profiting from an upgraded electron quasi-Fermi level of the titania negativeelectrode owing to an augmented electron density under the irradiance of a given light intensity. While a good light absorption is a preliminary requirement for photovoltaic cells, the following step of charge generation from a closely bound electron-hole pair (Frenkel exciton) confined to a single organic molecule22 also determines the efficient device operation. This process is triggered at the interface between a donor component and an acceptor component (dye and tiatnia in DSC), featuring a favorite energy-offset, although a hot electron injection is also possible.23 Here we measured the HOMO and LUMO of the C106 dye precisely in a nitrogen-filled glovebox by means of the ultramicroelectrode square-wave voltammetry technique. As depicted in Figure 3,

Figure 5. (A) Photocurrent action spectra of cells with the Z907 dye. (B) Photocurrent action spectra of cells with the C101 dye. (C) Photocurrent action spectra of cells with the C106 dye. (D) The ratio of monochromatic incident photon-to-collected electron conversion efficiency. The aperture area of a testing mask: 0.158 cm2. An antireflection film was adhered to the cell during measurements. 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 solvent of acetonitrile and valeronitrile (V/V, 85/15).

6294 J. Phys. Chem. C, Vol. 113, No. 15, 2009

Figure 6. J-V characteristic of a “champion” cell measured under an irradiance of 100 mW cm-2 AM1.5G sunlight. The inset is its photocurrent action spectrum. The aperture area of the testing mask: 0.158 cm2. Film thickness: 9 + 5. An antireflection film was adhered to the cell during measurements. 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 solvent of acetonitrile and valeronitrile (V/V, 85/15).

Figure 7. J-V characteristics of a “champion” cell measured in the dark (dash curves a, c, e, g, i, k, m, and o) and under an irradiance of 100 mW cm-2 AM1.5G sunlight (solid curves b, d, f, h, j, l, n, and p) at various temperatures: (a, b) 263 K; (c, d) 273 K; (e, f) 283 K; (g, h) 293 K; (i, j) 303 K; (k, l) 313 K; (m, n) 323 K; (o, p) 333 K. 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 solvent of acetonitrile and valeronitrile (V/V, 85/15).

the downhill energy offset by the measured LUMO (-3.38 eV vs. vacuum) of the C106 dye with respect to the conduction band edge (-4.00 eV vs. vacuum)24 of titania supplies a thermodynamic driving force for charge generation. Moreover, in order to reach a current collector photoinjected electrons in the titania must traverse a several micrometer-thick mesoporous film within the millisecond time domain; however, in the absence of electrolytes, the oxidized sensitizes can recapture electrons within the submillisecond time domain.25 The measured redox potential of iodide is about -0.06 V vs. Fc+/Fc. The uphill energy offset by the HOMO of C106 (-5.05 eV vs. vacuum) relative to that (-4.65 eV) of iodide could lead to fast dye regeneration, avoiding the geminate charge recombination between oxidized dye molecules and photoinjected electrons in the nanocrystalline titania film. The electrochemical HOMO/ LUMO gap (1.67 eV) is very consistent with the energy gap of 1.61 eV derived from the photocurrent action spectrum measurements in Figure 5C. To gain insights on the molar extinction coefficient tendency and red-shifted low-energy MLCT transitions observed in the preceding discussion, we detailed the electron transitions by calculating the electronic states of Z907, C101, and C106 as well as their bipyridyl ligands. The only variation in the

Cao et al. molecular structures of these three sensitizers is in their ancillary ligands. In contrast to 4,4′-dinoyl-2,2′-bipyridien (dnbpy) with two electron-donating alkyl groups, dcbpy, L1, and L6 all exhibit a narrower HOMO/LUMO energy gap as shown in Figure S1 in the Supporting Information due to the extension of π-conjugation. Moreover, among these ligands dcbpy with the carboxylate anchoring group has the lowest LUMO, facilitating an efficient electronic coupling of excited dye molecules and titania nanocrystals. In place of alkyl on dnbpy, introducing alkylthiophene not only lifts the HOMO, it also lowers the LUMO. Insertion of sulfur between thiophene and alkyl further depresses the LUMO due to the well-known electronic substituent effects of alkylthio compared to alkyl.26 The calculated electron absorption spectra in acetonitrile (Figure S2, Supporting Information) are in close agreement with experimental data shown in Figure 2. Transition involved molecular orbitals are presented in Figures S3-S5 in the Supporting Information and detailed transition assignments are summarized in Tables S1-S3 of the Supporting Information. An energy diagram and isodensity surface plots of the first three HOMOs and LUMOs are shown in Figure 4. The red-shifted MLCT bands of C101 and C106 compared to Z907 can be easily comprehended from their low-lying LUMO+1 orbitals, because the energy levels of their first three HOMOs do not differ remarkably. In comparison to C101, a slightly uplifted HOMO as well as HOMO-1 of C106 and a depressed LUMO+1 stemming from the sulfur insertion can be perceived from Figure 4, explaining the small bathochromic absorption both in solution and on film. For Z907 and C101, the first three HOMOs own ruthenium t2g character with sizable contribution from the thiocyanate ligand. However, the HOMO-1 of C106 exhibits a distinctive feature, with a certain distribution on L6, apart from the Ru-NCS combination. In addition, its HOMO-3, which is also involved in the visible excitation, is completely localized on L6 like LUMO+1. These may be responsible of its enhanced molar extinction coefficient compared to C101. 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 more likely to happen if the two wave functions overlap more significantly. Overall, the calculated oscillator strengths listed in in Tables S1-S3 in the Supporting Information echo the measured molar extinction coefficients of Z907, C101, and C106. Note that only in Z907 is the lowest energy transition from Ru-NCS combination to dcbpy, while that for C101 or C106 is from Ru-NCS combination to L1 or L6 instead. 3.2. Titania Film-Dependent Photovoltaic Performance. To fairly evaluate the potential of this new C106 sensitizer, we compared it with Z907 and C101 employing single- and doublelayer titania films with different thicknesses. Note that these films are routinely used in our laboratory for the testing of new materials. Panels A-C of Figure 5 present the typical photocurrent action spectra of cells made from the corresponding Z907, C101, and C106 dyes. We have observed that if the cells employ a thin transparent titania film, photocurrent action spectra (triangle-marked curves in the panels A-C of Figure 5) closely follow the absorption spectra (Figure 2B) of dye-coated mesoporous titania films, except for a small distortion in the blue region due to the dissipative absorption of triiodide in the electrolyte. Along with the increase of film thickness and especially the usage of light-scattering layer, external quantum efficiencies in the yellow and red regions have been significantly enhanced, and the shape of photocurrent action spectra differs from the film absorption spectra remarkably, owing to the

Solar Cells with a High Absorptivity Ru Sensitizer

J. Phys. Chem. C, Vol. 113, No. 15, 2009 6295

Figure 8. Temperature-dependent (A) open-circuit photovoltage, (B) short-circuit photocurrent density, and (C) reverse saturation current density of a “champion” cell.

Figure 9. Plots of (A) chemical capacitances versus open-circuit voltage and (B) recombination rate constants versus extracted charge densities of a “champion” cell under various temperatures: (a) 273 K; (b) 303 K; and (c) 333 K.

Figure 10. Plots of (A) electron lifetime, (B) diffusion coefficient, and (C) diffusion length of a “champion” cell versus DOS at different temperature: (a) 273 K; (b) 303 K; and (c) 333 K.

saturated absorption in the green region. A close look on the IPCE ratio (Figure 5D) of C106 to Z907 has evidently revealed the merit of enhancing optical absorptivity of stained titania films

with high molar extinction coefficient sensitizers, especially in the weak absorption red region as well as the blue region where competitive triiodide absorption is wasteful. Detailed cell param-

6296 J. Phys. Chem. C, Vol. 113, No. 15, 2009

Cao et al.

Figure 11. Detailed photovoltaic parameters of a solvent-free cell based on the C106 dye measured under an irradiance of 100 mW cm-2 AM1.5G sunlight during successive full sunlight soaking at 60 °C. Film thickness: 7 + 5. The ionic liquid electrolyte composition is DMII/ EMII/EMITCB/I2/NBB/GNCS (molar ratio: 12/12/16/1.67/3.33/0.67).

eters obtained with different titania films are summarized in Table 1, showing that the C106 dye is more efficient than Z907 and C101. We further employed a high-quality double-layer film to make a “champion” cell. As shown in Figure 6, this cell has a shortcircuit photocurrent density (Jsc) of 19.2 mA cm-2, an open-circuit photovoltage (Voc) of 776 mV, and a fill factor (FF) of 0.759, yielding an overall conversion efficiency (η) of 11.29%. The photocurrent action spectrum of this high-efficiency cell with the C106 dye is shown in the inset of Figure 6. The IPCEs exceed 80% from 450 to 670 nm, with a broad plateau of ∼90% in the region between 520 and 640 nm. Considering the light absorption and scattering loss by the conducting glass, the maximum efficiency for absorbed photon-to-collected electron conversion efficiency (APCE) is close to unity over a broad spectral range. 3.3. Temperature-Dependent Device Physics. For the outdoor application of dye-sensitized solar cells, the cell may operate at various temperatures. Thereby, we also measured the J-V characteristics (Figure 7) of this “champion” cell at different temperatures. The overall power conversion efficiencies at temperatures from 0 to 40 °C are over 11%, with a maximum of 11.4% at 30 °C. Relatively lower efficiencies at -10, 50, and 60 °C are 10.9%, 10.7%, and 10.3%, respectively. These experiments confirm that the efficiency of DSC does not vary too much with a fluctuation of temperature. As presented in Figure 8A, the open-circuit photovoltage decreases linearly along with the increase of temperature,27 reaching 836 mV at -10 °C. From the temperature-dependent J-V curves (Figure 7) measured in the dark, we derived the reverse saturation current (Js) by the following diode equation:28

( (

J ) Js exp

) )

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

(1)

cell, and parallel resistance from the leakage of the current, respectively. The reverse saturation current, which is closely related to the rectification at the titania/electrolyte interface,29 grows exponentially along with the increase of temperature, as depicted in Figure 8C. This has been previously explained by Dittrich et al.30 in terms of the notion of temperature-dependent recombination barrier height. We further resorted to the transient photoelectrical decay technique31 to take a close look at the interfacial chemical capacitance and charge recombination kinetics. As presented in Figure 9A, the capacitances (Cµ) at different temperatures all increase exponentially along with the increase of Voc, which was generated by applying a gradually enhanced light intensity. At a given Voc, the apparently higher Cµ at a higher temperature is related to more deep surface states because the calculated equilibrium potential of the electrolyte at 60 °C has a less than 5 mV negative shift relative to that at 0 °C. This tendency was also noted in an earlier paper by O’Regan et al.32 It is likely that along with the increase of temperature, the augmented deep electron-traps contribute to the enlarged reverse saturation current, which has a significant effect on open-circuit photovoltage. As shown in Figure 9B, the charge recombination becomes fast at a given extracted charge density. We imagine that this could be mainly ascribed to the enhanced diffusion coefficient of triiodide and electron trapped in the titania film. We have found that the almost constant power conversion efficiency in a moderate temperature range is a trade-off of Voc and Jsc. The raise of Jsc along with the increase of temperature is presented in Figure 8B. Note that here the triiodide diffusion flux is not a limiting factor at all for this high-efficiency cell with a low-viscosity acetonitrile-based electrolyte. We employed the electrical impedance measurements33 to see if this current enhancement is related to a better charge collection. As expected and also reported by others,34 the chemical diffusion coefficient of the electron (Dn) is augmented along with the increase of temperature as shown in Figure 10B. However, electron lifetime (τn) is shortened rapidly at a high temperature. Overall, we have found that the electron diffusion length (Ln) becomes short along with the increase of temperature, although it still may be long enough for quantitative electron collection in the temperature range discussed here. Moreover, there is no observable variation in the light-absorption of C106 stained titania film at various temperatures. Thereby we suspect that the current enhancement is caused by an improved charge generation yield at a high temperature, which needs to be clarified in our further pump-probe measurements. 3.4. Device Stability. Due to the difficulty in encapsulation of a highly volatile solvent such as acetonitrile for long-term thermal testing, a solvent-free ionic liquid electrolyte1m was used to evaluate the preliminary stability of the C106 sensitizer under moderate thermal stress and light soaking. Here, we submitted our cells covered with a UV absorbing polymer film to the 1000 h accelerated testing at 60 °C, in a solar simulator with a light intensity of 100 mW cm-2. The initial photovoltaic parameters (Jsc, Voc, FF, and η) of the testing cell are 14.2 mA cm-2, 711 mV, 0.746, and 7.5%, respectively. As presented in Figure 11, this cell exhibits a good stability, keeping 92% of its initial efficiency after 1000 h of aging. 4. Conclusions

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 Kelvin temperature, and Rs and Rsh are the series resistance arising from the internal resistance and resistive contacts of the

In summary, we have developed a high molar extinction coefficient ruthenium sensitizer featuring a hexylthiothiopheneconjugated bipyridine ligand to enhance the optical absorptivity of mesoporous titania film remarkably. For a newly developed

Solar Cells with a High Absorptivity Ru Sensitizer dye, the achievement of 11.4% power conversion efficiencies is very encouraging. We are now systematically optimizing the cell parameters to explore the full potential of this promising sensitizer. In conjunction with a solvent-free ionic liquid electrolyte, we have proved the long-term photochemical stability of this dye possessing an alkylthio fragment. We have found that the temperature-dependent Voc of dye-sensitized solar cells is mainly related to the density of surface states and correlated reverse saturation current. However, it seems that there is no direct relationship between electron diffusion length and photocurrent in a moderate temperature range for our highefficiency cell. The intrinsic physics of enhanced photocurrent at a high temperature will be addressed in our further studies. Acknowledgment. The National Key Scientific Program (No. 2007CB936700), the “CAS Knowledge Innovation Program”, and the “CAS 100-Talent Program” have supported this work. We are grateful to Dyesol for supplying EMITCB and 400nm-sized scattering paste and to DuPont Packaging and Industrial Polymers for supplying the Bynel film. Supporting Information Available: Figures and tables showing calculated molecular orbital, energy-level, electronic absorption spectroscopy, and transition assignment. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (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) Asbury, J. B.; Hao, E.; Wang, Y.; Lian, T. J. Phys. Chem. B 2000, 104, 11957. (d) Kubo, W.; Kitamura, T.; Hanabusa, K.; Wada, Y.; Yanagida, S. Chem. Commun. 2002, 374. (e) Paulsson, H.; Hagfeldt, A.; Kloo, L. J. Phys. Chem. B 2003, 107, 13665. (f) Hara, K.; Kurashige, M.; Dan-oh, Y.; Kasada, C.; Shinpo, A.; Suga, S.; Sayama, K.; Arakawa, H. New J. Chem. 2003, 27, 783. (g) Horiuchi, T.; Miura, H.; Sumioka, K.; Uchida, S. J. Am. Chem. Soc. 2004, 126, 12218. (h) 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. (i) Kato, T.; Okazaki, A.; Hayase, S. Chem. Commun. 2005, 363. (j) Martinson, A. B. F.; Elam, J. W.; Hupp, J. T.; Pellin, M. J. Nano Lett. 2007, 7, 2183. (k) 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. (l) Shi, D.; Pootrakulchote, N.; Li, R.; Guo, J.; Wang, Y.; Zakeeruddin, S. M.; Gra¨tzel, M.; Wang, P. J. Phys. Chem. C 2008, 112, 17046. (m) Bai, Y.; Cao, Y.; Zhang, J.; Wang, M.; Li, R.; Wang, P.; Zakeeruddin, S. M.; Gra¨tzel, M. Nat. Mater. 2008, 7, 626. (n) Yen, Y.-S.; Hsu, Y.-C.; Lin, J. T.; Chang, C.-W.; Hsu, C.-P.; Yin, D.-J. J. Phys. Chem. C 2008, 112, 12557. (o) O’Regan, B. C.; Lo´pez-Duarte, I.; MartinezDiaz, M. V.; Forneli, A.; Albero, J.; Morandeira, A.; Palomares, E.; Torres, T.; Durrant, J. R. J. Am. Chem. Soc. 2008, 130, 2906. (p) Larisa, A. O.; Zaban, A. J. Phys. Chem. C 2008, 112, 2779. (q) Mihi, A.; Calvo, M. E.; Anta, J. A.; Mı´guez, H. J. Phys. Chem. C 2008, 112, 13. (r) Staniszewski, A.; Heuer, W. B.; Meyer, G. J. Inorg. Chem. 2008, 47, 7062. (2) O’Regan, B.; Gra¨tzel, M. Nature (London) 1991, 353, 737. (3) (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.; 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. (4) (a) 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. (b) Chiba, Y.; Islam, A.; Watanabe, Y.; Komiya, R.; Koide, N.; Han, L. Jpn. J. Appl. Phys., Part 2 2006, 45, L638. (5) (a) 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. (b) Thampi, K. R.; Bessho, T.; Gao, F.; Zakeeruddin, S. M.; Wang, P.; Gra¨tzel, M. 23th European Photovoltaic Solar Energy Conference and Exhibition, Valencia, Spain, Sept 1-4, 2008. (6) 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. (7) Wang, P.; Zakeeruddin, S. M.; Moser, J.-E.; Nazeeruddin, M. K.; Sekiguchi, T.; Gra¨tzel, M. Nat. Mater. 2003, 2, 402.

J. Phys. Chem. C, Vol. 113, No. 15, 2009 6297 (8) 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. (9) (a) Jiang, K.-J.; Masaki, N.; Xia, J.-B.; Noda, S.; Yanagida, S. Chem. Commun. 2006, 2460. (b) Chen, C.-Y.; Wu, S.-J.; Wu, C.-G.; Chen, J.-G.; Ho, K.-C. Angew. Chem., Int. Ed. 2006, 45, 5822. (c) Karthikeyan, C. S.; Wietasch, H.; Thelakkat, M. AdV. Mater. 2007, 19, 1091. (d) Chen, C.-Y.; Wu, S.-J.; Li, J.-Y.; Wu, C.-G.; Chen, J.-G.; Ho, K.-C. AdV. Mater. 2007, 19, 3888. (e) Gao, F.; Wang, Y.; Zhang, J.; Shi, D.; Wang, M.; Humphry-Baker, R.; Wang, P.; Zakeeruddin, S. M.; Gra¨tzel, M. Chem. Commun. 2008, 2577. (f) Lee, C.; Yum, J.-H.; Choi, H.; Kang, S. O.; Ko, J.; Humphry-Baker, R.; Gra¨tzel, M.; Nazeeruddin, M. K. Inorg. Chem. 2008, 47, 2267. (g) Chen, C.-Y.; Chen, J.-G.; Wu, S.-J.; Li, J.-Y.; Wu, C.-G.; Ho, K.-C. Angew. Chem., Int. Ed. 2008, 47, 7342. (h) Mater, F.; Ghaddar, T. H.; Walley, K.; DosSanto, T.; Durrant, J. R.; O’Regan, B. J. Mater. Chem. 2008, 18, 4246. (i) Abbotto, A.; Barolo, C.; Bellotto, L.; De Angelis, F.; Gra¨tzel, M.; Manfredi, N.; Marinzi, C.; Fantacci, S.; Yum, J.-H.; Nazeeruddin, M. K. Chem. Commun. 2008, 5318. (10) Gra¨tzel, M. International Symposium on Solar cells and Solar Fuels, Dalian, China, Dec. 10-12, 2008. (11) Wang, P.; Klein, C.; Humphry-Baker, R.; Zakeeruddin, S. M.; Gra¨tzel, M. J. Am. Chem. Soc. 2005, 127, 808. (12) Cao, Y.; Zhang, J.; Bai, Y.; Li, R.; Zakeeruddin, S. M.; Gra¨tzel, M.; Wang, P. J. Phys. Chem. C 2008, 112, 13775. (13) (a) Maerker, G.; Case, F. H. J. Am. Chem. Soc. 1958, 80, 2745. (b) Wenkert, D.; Woodward, R. B. J. Org. Chem. 1983, 48, 283. (14) Roquet, S.; de Bettgnies, R.; Leriche, P.; Cravino, A.; Roncali, J. J. Mater. Chem. 2006, 16, 3040. (15) Pilarski, B. Liebigs Ann. Chem. 1983, 1078. (16) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. M.; Iyenger, S. S.; Tomasi, J.; Barone, V.; Mennueei, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonazalez, C.; Pole, J. A. Gaussian 03, revision C.02; Gaussian, Inc.: Wallingford, CT, 2004. (17) (a) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (b) Lee, C.; Yang, W.; Parr, R. G. Phys. ReV. B 1988, 37, 785. (18) Pourtois, G.; Beljonne, D.; Moucheron, C.; Schumm, S.; Mesmaeker, A. K.; Lazzaroni, R.; Bre´das, J. L. J. Am. Chem. Soc. 2004, 126, 683. (19) Foresman, J. B.; Keith, T. A.; Wiberg, K. B.; Snoonian, J.; Frisch, M. J. J. Phys. Chem. 1996, 100, 16098. (20) Hay, P. J.; Wadt, W. R. J. J. Chem. Phys. 1985, 82, 270. (21) Wang, P.; Zakeeruddin, S. M.; Comte, P.; Charvet, R.; HumphryBaker, R.; Gra¨tzel, M. J. Phys. Chem. B 2003, 107, 14336. (22) Pope, M.; Swenberg, C. E. Electronic Processes in Organic Crystals and Polymers, 2nd ed.; Oxford University Press: Oxford, UK, 1999. (23) (a) Ross, R. T.; Nozik, A. J. J. Appl. Phys. 1982, 53, 3813. (b) Ardo, S.; Meyer, G. J. Chem. Soc. ReV. 2009, 38, 115, and references cited therein. (24) (a) Hagfeldf, A.; Gra¨tzel, M. Chem. ReV. 1995, 95, 49. (b) Gra¨tzel, M. Nature (London) 2001, 414, 338. (25) Gra¨tzel, M. Inorg. Chem. 2005, 44, 6841. (26) Isaacs, N. S. Physical Organic Chemistry, 2nd ed.; Longman: London, UK, 1995. (27) Kron, G.; Egerter, T.; Werner, J. H.; Rau, U. J. Phys. Chem. B 2003, 107, 3556. (28) Sze, S. M.; Ng, K. K. Physics of Semiconductor DeVices, ed.; Wiley: NJ, 2007. (29) Gra¨tzel, M. Prog. PhotoVolt. Res. Appl. 2000, 8, 171. (30) Dittrich, T.; Beer, P.; Koch, F.; Weidmann, J.; Lauermann, I. Appl. Phys. Lett. 1998, 73, 1901. (31) (a) O’Regan, B. C.; Lenzmann, F. J. Phys. Chem. B 2004, 108, 4342. (b) Bailes, M.; Cameron, P. J.; Lobato, K.; Peter, L. M. J. Phys. Chem. B 2005, 109, 15429. (c) Kopidakis, N.; Neale, N. R.; Frank, A. J. J. Phys. Chem. B 2006, 110, 12485. (d) Walker, A. B.; Peter, L. M.; Lobato, K.; Cameron, P. J. J. Phys. Chem. B 2006, 110, 25504. (e) Quintana, M.; Edvinsson, T.; Hagfeldt, A.; Boschloo, G. J. Phys. Chem. C 2007, 111, 1035. (32) O’Regan, B. C.; Durrant, J. R. J. Phys. Chem. B 2006, 110, 8544. (33) (a) Bisquert, J. J. Phys. Chem. B 2002, 106, 325. (b) Bisquert, J. Phys. Chem. Chem. Phys. 2003, 5, 5360. (c) Bisquert, J. Phys. Chem. Chem. Phys. 2008, 10, 49. (34) (a) Kopidakis, N.; Benkstein, K. D.; van de Lagemaat, J.; Frank, A. J.; Yuan, Q.; Schiff, E. A. Phys. ReV. B 2006, 73, 045326. (b) Wang, Q.; Ito, S.; Gra¨tzel, M.; Fabregat-Santiago, F.; Mora-Sero´, I.; Bisquert, J.; Bessho, T.; Imai, H. J. Phys. Chem. B 2006, 110, 25210. (c) Bisquert, J. J. Phys. Chem. C 2007, 111, 17163.

JP9006872