New Organic Sensitizer for Stable Dye-Sensitized Solar Cells with

Oct 15, 2008 - Julio Villanueva , Juan A. Anta , Elena Guillén and Gerko Oskam ... Abhishek Baheti , Payal Tyagi , K. R. Justin Thomas , Ying-Chan Hs...
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17478

J. Phys. Chem. C 2008, 112, 17478–17485

New Organic Sensitizer for Stable Dye-Sensitized Solar Cells with Solvent-Free Ionic Liquid Electrolytes Dong Shi,† Yiming Cao,† Nuttapol Pootrakulchote,‡ Zhihui Yi,† Mingfei Xu,† Shaik M. Zakeeruddin,‡ Michael Gra¨tzel,‡ and Peng Wang*,† State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences (CAS), Changchun 130022, China, and Laboratory for Photonics and Interfaces, Swiss Federal Institute of Technology, CH 1015, Lausanne, Switzerland ReceiVed: August 12, 2008; ReVised Manuscript ReceiVed: August 29, 2008

We report a high molar extinction coefficient metal-free sensitizer composed of a triarylamine donor in combination with the 2-(2,2′-bithiophen-5-yl)acrylonitrile conjugation unit and cyanoacrylic acid as an acceptor. In conjugation with a volatile acetonitrile-based electrolyte or a solvent-free ionic liquid electrolyte, we have fabricated efficient dye-sensitized solar cells showing a corresponding 7.5% or 6.1% efficiency measured under the air mass 1.5 global sunlight. The ionic liquid cell exhibits excellent stability during a 1000 h accelerated test under the light-soaking and thermal dual stress. Intensity-modulated photocurrent and photovolatge spectroscopies were employed along with the transient photoelectrical decay measurements to detail the electron transport in the mesoporous titania films filled with these two electrolytes. 1. Introduction In the past two decades the dye-sensitized solar cell (DSC) has attracted remarkable attention as one of the most promising technologies toward cost-effective solar energy exploitation.1,2 The archetypal DSC is composed of three key active components including mesoporous semiconducting film, sensitizer, and redox electrolyte. Promoted by continuous material innovation as well as device engineering, in conjugation with highly volatile electrolytes, the cells with ruthenium polypyridyl sensitizers have reached respectable high efficiencies up to 11.0-11.3% measured under the standard air mass 1.5 global (AM 1.5G) sunlight.3-5 Meanwhile, metal-free organic dyes have been explored for DSCs showing efficiencies up to 8-9%.6-11 Apart from the higher absorption coefficient of organic sensitizers compared to ruthenium dyes, the flexibility in tailoring organic dyes at the molecular level is a noticeable merit for the boosting of this new photovoltaic technology. The sensitizer is an important and unique component in DSCs, with a function of light-harvesting. The overlap of its spectral response with the AM 1.5G solar emission will affect the device photocurrent to a large extent. Also, it must form energy offsets with the mesoporous titania and the iodide electron donor, i.e. two “type-II” heterointerfaces (titania/dye and dye/electrolyte) to overcome the exciton binding energy in an organic material for charge generation. An excited dye can be an electron donor as well as an electron acceptor like a semiconducting polymer once an energy-offset interface is formed.12 Our transient spectral measurements have shown that if the charge generation happens preferentially at the dye/electrolyte interface, the negatively charged sensitizer with a long lifetime does not inject electrons to titania but recombines with triiodide, causing a photocurrent loss channel.13 This must be avoided by molecular engineering of sensitizers, differentiating the kinetics of charge generation at these two heterointerfaces. Keeping these in mind, * Corresponding author. E-mail: [email protected]. † Changchun Institute of Applied Chemistry, CAS. ‡ Swiss Federal Institute of Technology.

we have designed a new organic sensitizer coded “C212” and explored its potential for dye-sensitized solar cells. The synthetic route for C212 is shown in Scheme 1. 2. Experimental Section 2.1. Materials. All solvents and reagents, unless otherwise stated, were of puriss quality and used as received. Thiophene2-acetonitrile and tetra-n-butylammonium hexafluorophosphate were purchased from Aldrich. Guanidinium thiocyanate (GNCS), 2-cyanoacetic acid, and 3R,7R-dihyroxy-5β-cholic acid (cheno) were purchased from Fluka. 1-Ethyl-3-methylimidazolium tetracyanoborate (EMITCB) and 400 nm TiO2 anatase particles were received as gifts from Merck and Catalysts & Chemical Ind. Co., respectively. N-Butylbenzimidazole (NBB) was synthesized according to the literature method14 and distilled before use. The synthesis of 1,3-dimethylimidazolium iodide (DMII) and 1-ethyl-3-methylimidazolium iodide (EMII) were described in our previous paper.15 Thiophene-2-boronic acid was synthesized according to the literature method.16 Synthesis of 1-Iodo-4-methoxybenzene (1). Methoxybenzene (25 mL, 0.23 mol) and KI (40 g, 0.24 mol) were added slowly to a stirred mixture of CH3OH (400 mL) and H2SO4 (19 mL) at 0 °C. After the mixture was warmed to room temperature, a H2O2 aqueous solution (30%, 50 mL) was added dropwise. Then, the reaction was performed at 60 °C under stirring overnight. After cooling to room temperature, the mixture was poured into H2O (1000 mL) and extracted with CH2Cl2. The combined organic layers were dried over Na2SO4. After removal of the solvent, the crude product was recrystallized from ethyl acetate and petroleum ether (volume ratio: 1/10) to afford the target compound (35 g, 65% yield). 1H NMR (400 MHz, CDCl3, δ): 7.55 (d, 2 H), 6.68 (d, 2 H), 3.78 (s, 3 H). Synthesis of 4-Methoxy-N-(4-methoxyphenyl)-N-phenylbenzenamine (2). 1 (14.4 g, 62.5 mmol), aniline (2.28 mL, 25 mmol), and 1,10-phenanthroline (0.90 g, 5 mmol) were dissolved in toluene (50 mL). After the solution was heated to 100 °C, CuCl (0.495 g, 5 mmol) and KOH (11.22 g, 0.2 mol) were added under Ar. The mixture was refluxed for 12 h. After

10.1021/jp807191w CCC: $40.75  2008 American Chemical Society Published on Web 10/15/2008

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SCHEME 1: Synthetic Route of the C212 Dyea

a (i) H2SO4, CH3OH, KI, H2O2, 60 °C. (ii) Aniline, KOH, CuCl, 1,10-phenanthroline, toluene, reflux. (iii) POCl3, DMF, 90 °C. (iv) Thiophene2-acetonitrile, t-BuOK, C2H5OH, reflux. (v) NBS, DMF, room temperature. (vi) Thiophene-2-boronic acid, K2CO3, Pd(PPh3)4, THF, reflux. (vii) POCl3, DMF, 90 °C. (viii) 2-cyanoacetic acid, piperidine, CHCl3, reflux.

cooling to room temperature, acetic acid (6.8 mL, 0.12 mol) and toluene (30 mL) were added. The mixture was washed with H2O (90 mL) three times, and the organic phase was dried over Na2SO4. After removal of the solvent, the residual was purified on a silica gel column (ethyl acetate/petroleum: 1/10) to afford the target compound (4.36 g, 57% yield). 1H NMR (400 MHz, CDCl3, δ): 7.16-6.81 (m, 13 H). Synthesis of 4-(Bis(4-methoxyphenyl)amino)benzaldehyde (3). To the stirred solution of 2 (2.6 g, 7.8 mmol) in DMF (50 mL), POCl3 (1.17 mL, 12.6 mmol) was added at 0 °C under Ar. The reaction was performed at 90 °C under stirring for 3 h. After rotary evaporation of the solvent, the residual was dissolved in CHCl3 and washed with a saturated CH3COONa aqueous solution. The organic layer was dried over Na2SO4. After removal of the solvent, the crude product was purified on the silica gel column (ethyl acetate/petroleum ether: 1/1) to afford the target compound (2.68 g, 95% yield). 1H NMR (400 MHz, DMSO, δ): 9.70 (s, 1 H), 7.65 (d, 2 H), 7.21 (d, 4 H), 7.00 (d, 4 H), 6.68 (d, 2 H), 3.76 (s, 6 H). Synthesis of (E)-3-(4-(Bis(4-methoxyphenyl)amino)phenyl)2-(thiophen-2-yl)acrylonitrile (4). In the presence of t-BuOK (1.12 g, 10 mmol), 3 (2.61 g, 7.88 mmol) was condensed with thiophene-2-acetonitrile (1 mL, 9.42 mmol) in refluxed anhydrous ethanol (50 mL) overnight. After removal of the solvent, the residual was dispersed in water and extracted with chloroform. The organic layers were combined and dried over Na2SO4. After rotary evaporation of the solvent, the crude product was

purified on the silica gel column (CH2Cl2) to afford the target compound (1.76 g, 51% yield). 1H NMR (400 MHz, DMSO, δ): 7.74 (d, 2 H), 7.58 (d, 2 H), 7.33 (d, 1 H), 7.15 (m, 5 H), 6.97 (d, 4 H), 6.71 (d, 2 H), 3.76 (s, 6 H). Synthesis of (E)-3-(4-(Bis(4-methoxyphenyl)amino)phenyl)2-(5-bromothiophen-2-yl)acrylonitrile (5). To a stirred solution of 4 (0.90 g, 2.05 mmol) in DMF (40 mL), NBS (0.37 g, 2.05 mmol) was added at 0 °C under Ar. After stirring for 3 h at room temperature, the mixture was portioned into CH2Cl2 (100 mL) and washed with H2O. The organic phase was collected and dried over Na2SO4. After removal of the solvent, the crude product was purified on the silica gel column (ethyl acetate/ petroleum ethyl: 1/1) to afford the target compound (1.04 g, 98% yield). 1H NMR (400 MHz, DMSO, δ): 7.73 (d, 2 H), 7.54 (s, 1 H), 7.26 (d, 1 H), 7.15 (m, 5 H), 6.97 (d, 4 H), 6.71 (d, 2 H), 3.77 (s, 6 H). Synthesis of (E)-3-(4-(Bis(4-methoxyphenyl)amino)phenyl)2-(5-(thiophen-2-yl)thiophen-2-yl)acrylonitrile (6). To a stirred solution of 5 (1.0 g, 1.93 mmol) and thiophene-2-boronic acid (0.38 g, 2.97 mmol) in anhydrous THF (45 mL), K2CO3 (1.80 g, 13.02 mmol) and Pd(PPh3)4 (0.21 g, 0.18 mmol) were added at room temperature under Ar. The mixture was refluxed overnight. After removal of the solvent, the residual was dispersed in H2O and extracted with CH2Cl2. The organic layers were combined and dried over Na2SO4. After removal of the solvent, the crude product was purified through the silica gel column (dichloromethane/petroleum ether: 1/1) to afford the

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Shi et al.

target compound (0.90 g, 90% yield). 1H NMR (400 MHz, DMSO, δ): 7.75 (d, 2 H), 7.56 (d, 2 H), 7.36 (d, 1 H), 7.32 (d, 1 H), 7.26 (d, 1 H), 7.16 (d, 4 H), 7.12 (t, 1 H), 6.98 (d, 4 H), 6.72 (d, 2 H), 3.77 (s, 6 H). Synthesis of (E)-3-(4-(Bis(4-methoxyphenyl)amino)phenyl)2-(5-(5-formylthiophen-2-yl)thiophen-2-yl)acrylonitrile (7). To a mixture of 6 (0.90 g, 1.7 mmol) in DMF (15 mL), POCl3 (0.47 mL, 5.0 mmol) was added at 0 °C under Ar. The mixture was warmed to 90 °C and stirred for 3 h. After removal of the solvent, the residual was dissolved in CHCl3 and washed with a saturated CH3COONa aqueous solution. The organic phase was collected and dried over Na2SO4. After removal of the solvent, the crude product was purified on the silica gel column (toluene) to afford the target compound (0.58 g, 61% yield). 1H NMR (400 MHz, DMSO, δ): 9.90 (s, 1 H), 8.00 (d, 1 H), 7.78 (d, 2 H), 7.67 (s, 1 H), 7.62 (d, 1 H), 7.58 (d, 1 H), 7.36 (d, 1 H), 7.17 (d, 4 H), 6.99 (d, 4 H), 6.72 (d, 2 H), 3.77 (s, 6 H). Synthesis of (E)-3-(5-(5-((E)-2-(4-(Bis(4-methoxyphenyl)amino)phenyl)-1-cyanoWinyl)thiophen-2-yl)thiophen-2-yl)-2cyanoacrylic acid (C212). 7 (0.52 g, 0.95 mmol) dissolved in CHCl3 (12 mL) was condensed with 2-cyanoacetic acid (0.121 g, 1.42 mmol) in the presence of piperidine (0.65 mL, 6.6 mmol). The mixture was refluxed for 2 h under Ar. After cooling to room temperature, the mixture was washed with 2 M aqueous HCl and extracted with CHCl3. The organic phase was collected and dried over Na2SO4. After removal of the solvent, the crude product was purified on the silica gel column to afford the C212 dye (0.45 g, 78% yield). 1H NMR (400 MHz, DMSO, δ): 13.78 (s, 1 H), 8.48 (s, 1 H), 7.98 (d, 1 H), 7.78 (d, 2 H), 7.70 (s, 1 H), 7.63 (t, 2 H), 7.36 (d, 1 H), 7.18 (d, 4 H), 6.99 (d, 4 H), 6.72 (d, 2 H), 3.77 (s, 6 H). 2.2. Computation. All the calculations were performed by the Gaussian 03W program package.17 Without any symmetrical constraints, the geometric structure of the C212 sensitizer was optimized employing the density functional theory (DFT) method combined with Becke’s three parameter hybrid functional18 and Lee-Yang-Parr’s gradient corrected correlation functional.19 In the particular, 6-31G(d) basis set was applied for all atoms.20,21 Incorporating the optimized model in the ZINDO/S method, we calculated the lowest 40 singlet-singlet electronic transitions. Subsequently, the ZINDO/S results were submitted in the SWizard program (http://www.sg-chem.net/ swizard/) to calculate the absorption profile as a sum of the Gaussian band using the following equation:22

ε(ω) ) 2.174 × 109

f

(

I exp -2.773 ∑ ∆1/2,I I

(ω - ωI)2 ∆1/2,I2

)

(1)

where ε is the molar extinction coefficient given in units of M-1 cm-1, the energy ω of all allowed transitions included in eq 1 is expressed in cm-1, fI denotes the oscillator strength, and the half-bandwidth ∆1/2 is assumed to be 3000 cm-1. Meanwhile, with the same computational details as mentioned above, we geometry-optimized a neutral titanium complex of C212, formulated as C212Ti(OH)3, where the [Ti(OH)3]+ group was covalently bonded to each carboxyl oxygen atom on the carboxyl group of the C212 anion. Finally, the vibrational frequencies were calculated employing the optimized model for both C212 and C212Ti(OH)3. The results given by SWizard were scaled by a recommended factor of 0.9416 for the B3LYP/ 6-31G(d) method.23 2.3. UV-Vis, Emission, Fourier Transform Infrared (FTIR), and Voltammetric Measurements. Electronic absorption spectra were performed on a UNICO WFZ UV-2802PC/

PCS spectrometer. Emission spectra were recorded with a PerkinElmer LS55 luminescence spectrometer. The emitted light was detected with a Hamamatsu R928 red-sensitive photomultiplier. The FTIR spectra were measured using a BRUKER Vertex 70 FTIR spectrometer. A dye-coated film was rinsed in acetonitrile and dried prior to measuring the spectra. A computer-controlled CHI660C electrochemical workstation was used for square-wave voltammetric measurements in combination with a mini three-electrode electrochemical cell equipped with a 5 µm radium Pt ultramicroelectrode as working electrode. A Pt wire and a silver wire were used as counter and quasireference electrodes, respectively. The redox potential values vs the ferrocene internal reference were converted to those vs NHE (normal hydrogen electrode). 2.4. Device Fabrication. A screen-printed double layer film of interconnected TiO2 particles was used as a mesoporous negative electrode. A 7 µm thick film of 20 nm TiO2 particles was first printed on the fluorine-doped SnO2 conducting glass electrode and further coated by a 5 µm thick second layer of 400 nm light scattering anatase particles. The detailed preparation procedures of TiO2 nanocrystals, pastes for screen printing, and double-layer nanostructured TiO2 film have been reported in our previous paper.24 A cycloidal TiO2 electrode (∼0.2826 cm2) was stained by immersing it into a dye solution containing C212 (300 µm) and 3R,7R-dihyroxy-5β-cholic acid (2 mM) in a mixture of acetonitrile and tert-butyl alcohol (volume ratio: 1/1) for 5 h. After washing with acetonitrile and drying by air flow, the sensitized titania electrodes were assembled with thermally platinized conducting glass electrodes. The electrodes were separated by a 25 µm thick Surlyn hot-melt gasket and sealed by heating. The internal space was filled with a liquid electrolyte using a vacuum backfilling system. The electrolyteinjecting hole made with a sand-blasting drill on the counter electrode glass substrate was sealed with a Bynel sheet and a thin glass cover by heating. The electrolyte in cell I was 1.0 M DMII, 0.05 M LiI, 0.1 M GNCS, 0.03 M I2, and 0.5 M tertbutylpyridine in a mixture of acetonitrile and valeronitrile (85/ 15, v/v);5 the electrolyte in cell II was DMII/EMII/EMITCB/ I2/NBB/GNCS (molar ratio: 12/12/16/1.67/3.33/0.67).25 2.5. Photovoltaic Characterization. A 450 W xenon light source (Oriel, USA) was used to give an irradiance of 100 mW cm-2 (the equivalent of one sun at AM 1.5G) at the surface of solar cells. The spectral output of the lamp was matched in the region of 350-750 nm with the aid of a Schott K113 Tempax sunlight filter (Pra¨zisions Glas & Optik GmbH, Germany) to reduce the mismatch between the simulated and true solar spectra to less than 2%. The current-voltage characteristics of the cell under these conditions were obtained by applying external potential bias to the cell and measuring the generated photocurrent with a Keithley Model 2400 digital source meter (Keithley, USA). This process was fully automated using Wavemetrics software (http://www.wavemetrics.com/). 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 300 W xenon lamp (ILC Technology, USA) was focused through a Gemini-180 double monochromator (Jobin Yvon Ltd., U.K.) onto the photovoltaic cell under test. The monochromator was incremented through the visible spectrum to generate the IPCE (λ) as defined by IPCE (λ) ) 12400(Jsc/λφ), where λ is the wavelength, Jsc is the short-circuit photocurrent density (mA cm-2), and φ is the incident radiative flux (mW cm-2). Photovoltaic performances were measured by using a metal mask with an aperture area of 0.158 cm2. Solar cells covered

Efficient Dye-Sensitized Solar Cells with a 50 µm thick polyester film (Preservation Equipment Ltd., U.K.) as a 400 nm UV cutoff filter were irradiated at open circuit under a Suntest CPS plus lamp (ATLAS GmbH, 100 mW cm-2) in ambient air at 60 °C. Photoelectrochemical measurements were carried out at room temperature after the cells were allowed to cool down and equilibrate during 2 h. 2.6. Transient Photoelectrical Measurements. In the transient photovoltage and photocurrent experiments, different steady-state light intensities were supplied with a homemade white light-emitting diode array by varying the driving voltages. A red light-emitting diode array controlled with a fast solidstate switch was used to generate a perturbation pulse with a width of 200 ms. The pulse and steady-state white lights were both incident on the photoanode side of testing cells. The pulse light intensity was controlled by the driving potential of red diodes to keep the modulated photovoltage below 10 mV. We used red diodes as a probe to generate a perturbation near the open-circuit photovoltage (Voc) of the cell under the bias light and measured the photoelectrical decay process thereafter. The capacitance (Cµ) of the TiO2/electrolyte interface and density of states (DOS) at the Voc are calculated as Cµ ) ∆Q/∆V, where ∆V is the peak of the photovoltage transient and ∆Q is the number of electrons injected during the light flash. The latter is determined by integrating a photocurrent transient at the shortcircuit condition generated from an identical pulse. This method may underestimate the actual injected charge by the fraction of electrons that are lost due to recombination during transport. The error is thought to be less than 30% in the worst case, and more critically, it will affect only the magnitude and not the shape of the calculated chemical capacitance versus potential curves. 2.7. IMVS and IMPS Measurements. Intensity-modulated photovoltage spectroscopy (IMVS) and intensity-modulated photocurrent spectroscopy (IMPS) were measured on a ZAHNER CIMPS system. Stationary as well as sinusoidal modulated light was supplied with a green light-emitting diode with a maximum wavelength at 546 nm. The light-emitting diode was controlled by a potentiostatic feedback loop. The selected ac amplitude is in the range 5-15% of the stationary dc value. The transfer functions of IMPS and IMVS were determined by correlating the system response with the actual stimulation signal. The potential applied to the testing cell was controlled by a potentiostat. IMPS was carried out at short circuit while IMVS was carried out at open circuit. The measured short-circuit photocurrent efficiency (Φext(ω)) of IMPS and the real and imaginary parts of modulated photovoltage ∆Voc of IMVS were fitted using a nonlinear-least-squares method programmed by the Mathlab 7.0 software. 3. Results and Discussion As shown in Figure 1A, the electronic absorption spectrum of the C212 dye has two intense visible absorption bands at 417 and 527 nm with molar extinction coefficients (ε) of 21.2 × 103 and 39.8 × 103 M-1 cm-1, respectively. The absorption spectrum (Figure 1B) of the C212 dye anchored on mesoporous titania film shows a blue-shifted response compared to that in solution, indicating that the carboxylic acid is a stronger electron acceptor compared to the carboxylate-titanium unit. The emission of C212 in chloroform is centered at 724 nm, and the excitation transition energy (E0-0) was roughly estimated to be 2.0 eV by taking the crossing point of its absorption and emission spectra. The origin of these transitions was detailed by calculating the electronic states of C212 with the TDDFT method in the Gaussian 03W program suite. The absorption

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Figure 1. (A) Electronic absorption (curve a) and emission (curve c) spectra of the C212 dye dissolved in chloroform together with the calculated vertical electronic transition spectrum (curve b). The calculated molar extinction coefficient was normalized with the experimental value. (B) Absorption spectrum of the C212 dye absorbed on a transparent titania film.

peaking at 527 nm is mainly attributed to the π f π* transitions from HOMO (highest occupied molecular orbital) to LUMO (lowest unoccupied molecular orbital) and HOMO - 1 to LUMO, while the absorption at 417 nm is due to the electron transitions from HOMO to LUMO + 1, HOMO - 1 to LUMO, and HOMO - 2 to LUMO. The isodensity surface plots of frontier orbitals involved in these transitions are presented in Figure 2. Obviously, the HOMO of the C212 dye is mainly populated over the substituted N,N-dimethyoxyphenylaniline and thiophene moieties while the LUMO is delocalized through the thiophene and the cyanoacrylic acid groups with a sizable contribution from the latter. This orientationally spatial separation of HOMO and LUMO is an ideal condition for dyesensitized solar cells, which not only facilitates the ultrafast interfacial electron injection from the excited dyes to the TiO2 conduction band, but also slows down the recombination of injected electrons in TiO2 with oxidized sensitizers due to their remoteness. In addition, the hole localized on the triarylamine unit will be spatially convenient for the electron donor to approach, facilitating the fast dye regeneration. Moreover, both HOMO and LUMO have the overlapping extension on the thiophene fragment, enhancing the electronic coupling parallel to the electronic transition dipole moment between the two states, which in turn results in a certain oscillator strength between these two electronic states in view of the Franck-Condon principle. We measured the redox potentials of the C212 sensitizer accurately by employing the ultramicroelectrode technique in

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Figure 2. Isodensity surface plots of frontier molecular orbitals of the C212 dye.

Figure 3. Square-wave voltammogram of a Pt ultramicroelectrode in DMF solution containing the C212 sensitizer.

combination with square-wave voltammetry. As shown in Figure 3, the LUMO (-0.68 V vs NHE) of this new dye is more negative than the conduction band edge (-0.5 V vs NHE) of titania, providing sufficient thermodynamic driving force for electron injection from the excited dyes to titania. In addition, its ground-state oxidation potential (1.00 V vs NHE) is higher than the redox potential (0.4 V vs NHE) of the iodide/triiodide couple. This could lead to a fast dye regeneration, avoiding the geminate charge recombination between oxidized dye molecules and photoinjected electrons in the nanocrystalline titania film. The ATR-FTIR spectrum (Figure 4A) of the C212 dye anchored on TiO2 film clearly shows the bands at 1597 and 1391 cm-1 for the asymmetric and symmetric stretching modes of the carboxylate groups, indicating that the carboxylic acid group (at 1720 and 1688 cm-1 in Figure 4B) is deprotonated and involved in the adsorption of the dye on the surface of TiO2.

Shi et al. From the ATR-FTIR data we infer that the dye is anchored on the surface through the carboxylate groups via a bidentate chelation or a bridging of surface titanium ions rather than an ester type linkage.26 We calculated the infrared absorption spectra (Figure 4C,D) of the C212 dye and C212Ti(OH)3 as a model compound to mimic the titania/dye assembly. The sharp phenyl and thiophene ring modes are at 1576, 1504, 1422, and 1244 cm-1, while the C-N stretching modes of triarylamine are at 1329 and 1328 cm-1. The NtC signal remains at 2214 cm-1, indicating that cyano groups may not take part in the dye-adsorption process. The corresponding CH3- peaks are observed at 3002 cm-1, while the C-H stretching mode of aromatic units is at 3037 cm-1. The photocurrent action spectrum of cell I with a lowvolatility electrolyte is shown in Figure 5A. The plot of incident photon-to-collected electron conversion efficiencies (IPCEs) exhibits a high plateau of over 85% from 460 to 590 nm. From the overlap intergral of this curve with the standard global AM 1.5G solar emission spectrum, a short-circuit photocurrent density (Jsc) of 14.5 mA cm-2 is calculated. As shown in Figure 5B, under AM 1.5G sunlight (99.6 mW cm-2), the short-circuit photocurrent density (Jsc), open-circuit photovoltage (Voc), and fill factor (FF) of cell I with an acetonitrile-based electrolyte are 15.59 mA cm-2, 638 mV, and 0.749, respectively, yielding an overall conversion efficiency (η) of 7.5%. We have noted that the integrated photocurrent density is higher than that measured under the AM 1.5G full sunlight. By measuring the light intensity dependent short-circuit densities, we found that at low light intensities the photocurrents are nonlinear, explaining this small inconsistency in view of the weak lights used for IPCE measurements. This nonlinear photocurrent observation is sometimes observed for DSCs with organic sensitizers, probably indicating the dye aggregation. Its physical origins need to be further addressed in future work. More importantly, the photovoltaic parameters (Jsc, Voc, FF, and η) of cell II with a solvent-free ionic liquid electrolyte under AM 1.5G sunlight (99.8 mW cm-2) are 14.51 mA cm-2, 596 mV, 0.706, and 6.1%, respectively. Considering the importance of employing solvent-free ionic liquid electrolytes in DSCs, we are very interested in scrutinizing the origins of the relatively lower Voc and Jsc of cell II in contrast to that of cell I. By measuring the transient decay of Voc and Jsc caused by a swift perturbation with red light-emitting diodes,27-31 we estimated the chemical capacitance Cµ at the titania/ electrolyte interface to understand the interaction of electrolytes with uncovered titania. Knowing that the DOS including surface and bulk traps is proportional to Cµ, we derived the DOS according to the equation DOS ) (6.24 × 1018)Cµ/[d(1 - P)] (where d and P are the thickness and porosity of titania films), with an exponential distribution profile for both cells I and II as depicted in Figure 6. For the volatile electrolyte (cell I) it appears that there are fewer electron-trapping states below the conduction band edge, resulting mostly from the presence of coordinatively unsaturated titanium species at the surface of the titania nanocrystals. These have been passivated most likely by the GNCS and NBB additives.29 However, for the solvent-free ionic liquid electrolyte (cell II), the passivating action of GNCS and NBB seems to be less effective due to the extraordinary high ion concentrations. The uplifted conduction band edge of the sensitized titania film in cell I and a higher electron density will raise the electron quasi-Fermi level in the titania film for a given illumination intensity. Also, the calculated electrolyte equilibrium potential of cell I is 17 mV positively shifted

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Figure 4. (A) ATR-FTIR spectrum of the C212 dye coated on a mesoporous TiO2 film. The spectrum of a TiO2 reference film heated at 500 °C to remove surface-adsorbed water was subtracted for clarity of presentation. (B) FTIR spectrum of the C212 dye. (C) Calculated IR spectrum of C212Ti(OH)3. (D) Calculated IR spectrum of C212.

Figure 6. DOS profiles of cells I and II derived from interfacial chemical capacitance measurements.

Figure 5. (A) Photocurrent action spectrum of cell I with the C212 dye. (B) J-V characteristics of cells I (curves a and b) and II (curves c and d) measured in the dark (curves a and c) and under the irradiance of AM 1.5G full sunlight (curves b and d). The aperture area of the metal mask is 0.158 cm2.

compared to that of cell II. These together unambiguously explain the higher Voc of cell I than that of cell II. It is known that the capture of conduction band electrons by triiodide ions at the titania/electrolyte interface depends on the

thermodynamic driving force as well as electron concentration. By adjusting the output of the white light-emitting diodes used to produce a steady bias light, we generated different Voc, and we measured the pseudo-first-order recombination rate constant (kr) from the transient decay of Voc generated by the red light flash. Figure S1 in the Supporting Information presents the semilogarithmic plot of kr versus Voc for cells I and II. Obviously, along with the increase of Voc, the recombination becomes faster due to the higher electron concentration in the titania film as well as a larger driving force for charge recombination. At a given Voc, cell II with a solvent-free ionic liquid electrolyte has a much higher charge recombination rate compared with cell I. This difference is probably caused by its higher triiodide concentration of ∼0.24 M in contrast to that of 0.03 M in cell I. We note that this analysis is not completely reasonable. Actually, under a given Voc, the electron quasi-Fermi level in cell II is higher due to the 17 mV negatively shifted equilibrium potential of its electrolyte. Also considering its DOS

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Figure 7. Plots of (A) effective electron lifetime, (B) diffusion coefficient, and (C) diffusion length for cells I and II.

profile, we can derive that at the same Voc the electron density in cell II is larger compared to that in cell I. Thus we further resorted to intensity-modulated photovoltage spectroscopy (IMVS)32-35 to clarify this uncertainty. IMVS measures the periodic photovoltage response of a testing cell to a small sinusoidal perturbation of light superimposed on a large steady background level, providing information on electron lifetime under open-circuit conditions. Figure 7A presents the plot of electron lifetime versus incident light intensity. Because under a given light intensity cell II has a lower equilibrium electron density and an even smaller driving force for charge recombination, we conclude that the observed higher charge recombination rate in our ionic liquid cells is mainly due to its high concentration of triiodide ions. Intensity-modulated photocurrent spectroscopy (IMPS)36,37 experiments employ the same light perturbation but measure the periodic photocurrent response, detailing the dynamics of charge transport and back reaction under short-circuit conditions. The electron transport in mesoporous titania film can be described by the proposed multiple trapping detrapping (MTD) model.38 Along with the increase of light intensity, more deep traps will be filled with photoinjected electrons and do not retard the electron transport any more. The detrapping of electrons from shallow traps is much faster, resulting in a higher effective electron diffusion coefficient (Dn) as shown in Figure 7B. At a given light intensity, the measured effective electron diffusion coefficient in cell II filled with a solvent-free ionic liquid electrolyte is higher than that in cell I. We note that, under an identical light illumination, the electron quasi-Fermi level in cell II should be lower than that in cell I. This counterintuitive observation may imply that there is a very effective charge screening for electron transport in the mesoporous titania film due to a very high cation concentration,39,40 even though the fluidity of the ionic liquid electrolyte is considerably lower than that of the volatile acetonitrile-based electrolyte. The values of electron diffusion coefficient and lifetime can be used for the estimation of electron diffusion length, Ln ) (Dnτn)1/2, which is determined by the competition between electron transport and back reaction. Overall, the large electron diffusion length (Ln) directly related to a high charge collection yield is well consistent with the high Jsc measured for cell I. For any new materials system in the photovoltaic technology, passing a long-term stability test is a preliminary requirement to assess its potential for large-scale application. Thus, 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. Note that the contribution of total UV photon flux to the standard AM 1.5G solar emission is relatively small. UV light leads to direct band-edge excitation of titania nanocrystals, generating valence-band holes, which in turn may attack some organic materials in the device.

Shi et al. However, this reaction can be suppressed to some extent by the capture of holes by iodide in the electrolytes. Also in our light-soaking experiments UV light is filtered by the antireflection coating employed. As shown in Figure S2 in the Supporting Information, cell II with the C212 dye in conjugation with a solvent-free ionic liquid electrolyte exhibits a good stability, retaining 91% of its initial efficiency of 6.1%. While we did observe a 25 mV drop in open-circuit photovoltage and a 4% decrease of fill factor, it is worth noting that there is almost no photocurrent decrease during the whole aging process, proving the stability of the dye itself. The small photovoltage change may reflect the augmentation of the surface state of the mesoporous titania film. However, we could not measure the stability of cell I under the thermal stress due to the high volatility of its electrolyte. 4. Conclusions In summary, we have synthesized a high molar extinction coefficient metal-free sensitizer and fabricated a stable dyesensitized solar cell in conjugation with a solvent-free ionic liquid electrolyte. We have measured the intensity-modulated photocurrent and photovolatge spectroscopies as well as the transient photoelectrical decays to detail the electron transport in the mesoporous titania films filled with a solvent-free ionic liquid and a volatile acetonitrile-based electrolyte. It was found that the fast recombination in the solvent-free device is mainly caused by the high concentration of triiodide ions acting as the electron-capturing species. Also, we observed a fast electron transport in the nanocrystalline titania film immersed in an ionic liquid electrolyte, which may reflect the effective electron screening by cations. Further work is under way to realize highefficiency DSCs with solvent-free ionic liquid electrolytes by suppressing the interfacial charge recombination and improve the photovoltage. Acknowledgment. The National Key Scientific Programs Nanoscience and Nanotechnology (No. 2007CB936700) and the “100-Talent Program” of Chinese Academy of Sciences have supported this work. N.P., S.M.Z., and M.G. thank the Swiss National Science Foundation for financial support. Supporting Information Available: Figures S1 and S2 showing recombination and device stability data. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) O’Regan, B.; Gra¨tzel, M. Nature 1991, 353, 737. (2) Gra¨tzel, M. Nature 2001, 414, 338. (3) 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) Chiba, Y.; Islam, A.; Watanabe, Y.; Komiya, R.; Koide, N.; Han, L. Jpn. J. Appl. Phys. Part 2 2006, 45, L638. (5) 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. (6) Horiuchi, T.; Miura, H.; Sumioka, K.; Uchida, S. J. Am. Chem. Soc. 2004, 126, 12218. (7) Ito, S.; Zakeeruddin, S. M.; Humphry-Baker, R.; Liska, P.; Charvet, R.; Comte, P.; Nazeeruddin, M. K.; Pe´chy, P.; Takata, M.; Miura, H.; Uchida, S.; Gra¨tzel, M. AdV. Mater. 2006, 18, 1202. (8) Kim, S.; Lee, J. W.; Kang, S. O.; Ko, J.; Yum, J.-H.; Fantacci, S.; De Angellis, F.; Di Censo, D.; Nazeeruddin, M. K.; Gra¨tzel, M. J. Am. Chem. Soc. 2006, 128, 16701. (9) Wang, Z.; Cui, Y.; Dan-Oh, Y.; Kasada, C.; Shinpo, A.; Hara, K. J. Phys. Chem. C 2007, 111, 7224. (10) Qin, H.; Wenger, S.; Xu, M.; Gao, F.; Jing, X.; Wang, P.; Zakeeruddin, S. M.; Gra¨tzel, M. J. Am. Chem. Soc. 2008, 130, 9202.

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