Energy-Level and Molecular Engineering of Organic D-π-A Sensitizers

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J. Phys. Chem. C 2008, 112, 19770–19776

Energy-Level and Molecular Engineering of Organic D-π-A Sensitizers in Dye-Sensitized Solar Cells Mingfei Xu,† Renzhi Li,† Nuttapol Pootrakulchote,‡ Dong Shi,† Jin Guo,† Zhihui Yi,† 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: September 17, 2008; ReVised Manuscript ReceiVed: October 21, 2008

A series of organic D-π-A sensitizers composed of different triarylamine donors in conjugation with the thienothiophene unit and cyanoacrylic acid as an acceptor has been synthesized at a moderate yield. Through tuning the number of methoxy substituents on the triphenylamine donor, we have gradually red-shifted the absorption of sensitizers to enhance device efficiencies. Further molecular engineering by the substitution of two hexyloxy chains in place of the methoxy groups allows fabricating a solvent-free dye-sensitized solar cell with a power conversion efficiency of 7.05% measured under the air mass 1.5 global sunlight. Time- and frequency-domain photoelectrical techniques have been employed to scrutinize the aliphatic chain effects with a close inspection on effective electron lifetime, diffusion coefficient, and diffusion length. 1. Introduction It is of urgency to develop renewable energy resources for the growing global energy demand considering the limited storage of fossil fuels and disastrous environmental problems from their combustion. Solar energy has a widely recognized potential to appease a substantial part of sustainable energy supplies. Without a doubt, the performance/price ratio will play a pivotal role in the future choice of various photovoltaic devices.1 During the last two decades the dye-sensitized solar cell2 (DSC) has attracted much attention as a promising lowcost candidate for the photovoltaic market, because of its validated efficiency of 11.1%3 and a remarkable stability4 under the prolonged light and thermal dual stress. The DSC efficiency could be further improved through the exploration of new sensitizers with a good spectral match with the solar emission. At the moment a major drawback of the DSC technology is the used volatile electrolytes. This hurdle has precluded large-scale outdoor application and integration into flexible structures. Among various efforts to develop solvent-free DSCs to address this issue, the use of roomtemperature ionic liquid electrolytes has proved to be the most successful strategy by far.5 However, with a low fluidity ionic liquid electrolyte the charge collection yield in DSCs becomes low due to the shortened electron diffusion length.4 Enhancing the optical absorption coefficient of a stained mesoporous film to allow the use of a thin photoactive layer can counter this effect. This has been exemplified by our recent work,6 showing that with a high molar extinction coefficient organic sensitizer there is a negligible photocurrent difference between two DSCs with a volatile acetonitrile-based electrolyte and a solvent-free electrolyte. Apart from the higher absorption coefficient of organic sensitizers compared to polypyridyl ruthenium dyes, the * Corresponding author. E-mail: [email protected]. † Changchun Institute of Applied Chemistry, CAS. ‡ Swiss Federal Institute of Technology.

flexibility in tailoring organic dyes at the molecular level could inject a new momentum for the boosting of dye-sensitized solar cells.7 The combination of bithiophene,7e thienothiophene,8 or dithienothiophene6 with the bisfluorenylaniline fragment has been demonstrated as a triumphant strategy to produce organic sensitizers for making stable DSCs with solvent-free ionic liquid electrolytes. However, our subsequent studies in synthesizing more red-absorption and higher molar extinction coefficient sensitizers with a larger conjugated unit met the problem of purification of target molecules, owing to their low solubility in traditional organic solvents. Tuning the 9,9-substituents with flexible chains should be a sensible strategy for the feasibility of purification but may cause the disordered packing of sensitizers on titania nanocrystals, which is not desirable for the high DSC performance. Here we report four D-π-A sensitizers shown in Scheme 1 with several simple triarylamine electron donors to tune their energy levels. Further tailoring the molecular structure with the addition of two more aliphatic chains on the triphenylamine unit has enhanced the device efficiency. Through a close inspection on the electrical physics, this enhancement will be detailed by measuring the intensitymodulated photocurrent and photovoltage spectroscopies as well as transient photoelectrical decay. 2. Results and Discussion 2.1. Synthesis. The synthetic route of C207, C202, C208, and C206 dyes is shown in Scheme 1, and the detailed syntheses are described in the Supporting Information. The triarylamines 3a, 3b, 2c, and 3d were prepared by the copper-catalyzed Ullman coupling of electron deficient aryliodides with nucleophilic arylamines. In our experiments, we have found that the direct reaction of 2,5-dimethoxy-4-bromoaniline and 4-methoxybenzyliodide does not produce 3c in a good yield perhaps due to the self-coupling of 2,5-dimethoxy-4-bromoaniline in the harsh condition of Ullman reaction. Thus we first synthesized the triarylamine 2c and subsequently converted it to 3c by means of the bromination reaction. The Suzuki coupling of N,N-bisaryl-

10.1021/jp808275z CCC: $40.75  2008 American Chemical Society Published on Web 11/12/2008

Organic D-π-A Sensitizers in Dye-Sensitized Solar Cells

J. Phys. Chem. C, Vol. 112, No. 49, 2008 19771

SCHEME 1: Synthetic Route of the C207 (6a), C202 (6b), C208 (6c), and C206 (6d) Sensitizersa

a (i) Sn, HCl; (ii) aryl iodide, 1,10-phenothroline, CuCl; (iii) NBS; (iv) (thieno[3,2-b]thiophen-2-yl)boronic acid, Pd[P(Ph)3]4; (v) DMF, POCl3; (vi) cyanoacetic acid, piperidine.

4-bromoanilines 3 and thieno[3,2-b]thiophen-2-yl)boronic acid gave 5-[N,N-bisarylphenyl]thieno[3,2-b]thiophenes 4, which were subsequently converted to their corresponding carbaldehydes 5 by performing the Vilsmeier-Haack reaction. Finally, the aldehydes were condensed with cyanoacetic acid to yield the target compounds 6 via the Knoevenagel reaction in the presence of piperidine. 2.2. Spectral Response and Energy Level. In the dyesensitized solar cells, sensitizer is a pivotal and unique component with a function of light-harvesting. Its spectral response overlapped with the solar emission will affect the device photocurrent to a large extent; thus, we measured the UV-vis absorption of these new dyes both dissolved in solution and anchored on the mesoporous titania film. Due to the low solubility of these molecules in ethanol, we prefer to employ chloroform as a solvent to test their molar extinction coefficients. Our conductivity measurements also indicate that these cyanoacrylic acid dyes may dissociate partially to form free ions in a high-polarity solvent such as ethanol. This will result in a considerably blue-shifted absorption, precluding a correct spectral characterization. The normalized electronic absorption and emission spectra of the C207, C202, C208, and C206 dyes in chloroform are shown in Figure 1A, and some physical parameters are listed in Table 1. We have noted that all these four new dyes lack a strong absorption in the UV region compared to the previously reported C201 dye,8 but this should not affect device conversion efficiency of DSCs. Actually, the total UV photon flux in the standard AM 1.5 solar emission is pretty small. Moreover, it may be impractical to use UV light for DSCs in view of the direct band-edge excitation of titania nanocrystals, which can photocatalytically decompose some organic materials in the device. We note that the UV light is always filtered off by an antireflection coating even for inorganic

Figure 1. (A) Normalized electronic absorption and emission spectra of the C207, C202, C208, and C206 sensitizers in chloroform. (B) Normalized absorption spectra of dyes anchored the nanocrystalline titania film. The absorption from the titania and glass substrate has been subtracted.

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TABLE 1: Experimental Spectral and Electrochemical Data of Organic Dyes dye

maxa λabs (nm)

maxa εabs (103 M-1 m-1)

maxa λpl (nm)

E0-0b (eV)

φS/S+c (V)

φS/S-c (V)

φS*/S+d (V)

HOMOe (eV)

LUMOe (eV)

C207 C202 C208 C206

493 512 517 516

36.6 40.5 33.7 41.9

644 712 717 712

2.164 2.105 2.056 2.102

0.490 0.300 0.250 0.300

-1.530 -1.540 -1.520 -1.540

-1.674 -1.805 -1.806 -1.802

-5.26 -5.09 -5.03 -5.09

-3.44 -3.42 -3.45 -3.43

a max max The absorption maximum wavelength (λabs ), maximum molar extinction coefficient (εabs ), and photoluminescence maximum wavelength max ) were derived from the absorption and photoluminescence spectra of dyes dissolved in chloroform. b The 0-0 transition energy (E0-0) (λpl was estimated from the crossing point of normalized absorption and photoluminescence spectra. c The ground-state oxidation (φS/S+) and reduction potentials (φS/S-) are reported with Fc/Fc+ as reference. d The excited-state reduction potential (φS*/S+) was calculated from the ground-state oxidation potential and 0-0 transition energy by equation φS*/S+ ) φS/S+ - E0-0/F without considering any entropy change during the light excitation. e The LUMO and HOMO values were estimated vs vacuum by equation ELUMOorHOMO) -4.88 - φredoxF.

Figure 2. Square-wave voltammograms of the C207, C202, C208, and C206 sensitizers dissolved in DMF.

silicon solar cells. The molar extinction coefficients of their visible absorption peaks are in the range of 36.6-41.9 × 103 M-1 cm-1. The origins of these electronic absorptions are detailed by calculating the singlet electronic transitions with the TDDFT method in Gaussian03W program suite. Calculations show that these visible bands are mainly attributed to the electronic transition from the highest occupied molecular orbitals (HOMOs) to the lowest unoccupied molecular orbitals (LUMOs). The visible absorption behaviors of the C202 and C206 sensitizers match well that of the previously reported C201.8 It is interesting to see a continuous red-shifting of absorption peaks along with introducing more electron-donating methoxy group to the donor of these D-π-A dyes (C207, C202, and C208). The more red spectral response from C202 to C208 is offset by a low molar extinction coefficient, which could be understood by low oscillator strength of the charge-transfer transition according to the Franck-Condon principle. The absorption spectra (Figure 1B) of these dyes anchored on mesoporous titania film show blue-shifted responses compared to those in solution, suggesting the carboxylate-titanium unit is a weaker acceptor compared to the carboxylic acid. Knowing the spectral response of an organic compound does not guarantee its function as an efficient sensitizer in DSCs. A sensitizer must form energy offsets with the conduction band edge of titania and the iodide electron donor, i.e., two “typeII” heterointerfaces (titania/dye and dye/electrolyte) to overcome the exciton binding energy in an organic material for charge generation. We employed square-wave voltammetry in combination with the ultramicroelectrode technique to measure the redox potentials of these new sensitizers accurately. As shown in Figure 2 and Table 1, the LUMOs of our sensitizers are in the range from -3.42 to -3.45 eV and are all higher than the conduction band edge (-4.0 eV vs vacuum) of titania, providing sufficient thermodynamic driving force for electron injection from the excited dyes to titania. In addition, the ground-state oxidation potentials of these sensitizers range from 0.25 to 0.49

V vs Fc/Fc+, which are higher than the redox potential (-0.1 V vs Fc/Fc+) of the iodide/triiodide couple. This could lead to a fast dye-regeneration, avoiding the geminate charge recombination between oxidized sensitizers and photoinjected electrons in the nanocrystalline titania film. It is noted that introducing the methoxy substituent has a remarkable influence on the ground-state oxidation potentials and HOMOs but a small effect on the ground-state reduction potentials and LUMOs. This is well consistent with the calculation results shown in Figure 3. The different influence on the HOMOs and LUMOs by introducing a stronger donor narrows the energy-gap of organic sensitizers. We remark that an excited dye can be electron donor as well as electron acceptor like a semiconducting polymer once an energy-offset interface is formed.9 Our transient spectral measurements have shown that if the charge generation occurs preferentially at the dye/electrolyte interface, the negatively charged sensitizer with a long lifetime does not inject electron to titania but recombines with triiodide, causing a photocurrent loss channel.10 This can be avoided by rational molecular engineering of sensitizers, differentiating the kinetics of charge generation at these two heterointerfaces. For example, functionalization of sensitizers with the carboxylate group ensures a chemical binding to the titania for strong electron coupling while the dye/electrolyte interface is just a physical contact. We performed DFT calculation to gain an insight to the electronic states of these sensitizers. As presented in Figure 3, HOMOs of C207, C202 (C206 not shown), and C208 sensitizers are mainly populated over the triarylamine moiety while their LUMOs are mainly populated on the anchoring moiety. This orientationally spatial separation of HOMOs and LUMOs is an ideal condition for dye-sensitized solar cells, which not only facilitates the ultrafast interfacial electron injection from the excited dyes to the titania conduction band but also slows down the recombination of injected electrons in the mesoporous titania film 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 efficient dye-regeneration at a moderate but right rate compared to the ultrafast charge generation at the titania/dye interface. Moreover, both HOMOs and LUMOs have the overlapping extension on the thienothiophen fragment, enhancing the electronic coupling parallel to the electronic transition dipole moment between the two states, which in turn results in certain oscillator strength between these two electronic states. 2.3. Photovoltaic Performance. The plot of incident photonto-collected electron conversion efficiencies (IPCE) versus wavelength of DSCs with the C207, C202, C208, and C206 sensitizers are shown in Figure 4A. The photocurrent action spectra of all these sensitizers exhibit a high plateau of over 85%, which are very impressive in view that a solvent-free ionic

Organic D-π-A Sensitizers in Dye-Sensitized Solar Cells

J. Phys. Chem. C, Vol. 112, No. 49, 2008 19773

Figure 3. Schematic energy levels and isodensity plots of HOMOs and LUMOs of the C207, C202, and C208 sensitizers.

TABLE 2: Detailed Photovoltaic Parametersa Measured under the Illumination of AM 1.5G Full Sunlight (100 mW cm-2) of Solvent-Free DSCs with Organic Dyes dye

Jsc, mA cm-2

Voc, mV

FF

η, %

C207 C202 C208 C206

11.74 13.38 13.56 14.49

661 664 673 693

0.764 0.736 0.745 0.702

5.93 6.54 6.80 7.05

a The spectral distribution of our measurement system simulates AM 1.5G solar emission. Incident power intensity: Pin. Short-circuit photocurrent density: Jsc. Open-circuit photovoltage: Voc. Maximum electricity output power density: Pmax. Fill factor: FF)Pmax/Pin. Total power conversion efficiency: η. Cell area tested with a metal mask: 0.158 cm2. The solvent-free electrolyte composition is DMII/ EMII/EMITCB/I2/NBB/GNCS (molar ratio: 12/12/16/1.67/3.33/ 0.67).

Figure 4. (A) Photocurrent action spectra of solvent-free DSCs with the C207, C202, C208, and C206 sensitizers. (B) J-V characteristics measured under the illumination of AM 1.5G full sunlight (100 mW cm-2). Cells were tested using a metal mask with an aperture area of 0.158 cm2. The solvent-free electrolyte composition is DMII/EMII/ EMITCB/I2/NBB/GNCS (molar ratio: 12/12/16/1.67/3.33/0.67).

liquid electrolyte was used for device fabrication. Considering the light absorption and scattering loss by the conducting glass, the maximum efficiency for absorbed photon-to-collected electron conversion efficiency (APCE) is unity for the cell with the C206 sensitizer over a broad spectral range, suggesting a quantitative charge collection yield. The enhanced IPCE of C206 relative to C202 by tailoring the sensitizer with two aliphatic alkoxy chains is probably due to the reduced charge recombination at the titania/dye/electrolyte interface. The red-shifted photocurrent action spectra along with an increase of the number of methoxy groups are consistent with the absorption measurements. As listed in Table 2, the short-circuit photocurrent densities (Jsc) as well as power conversion efficiencies (η) for dye-sensitized solar cells of C207, C202, and C208 increase gradually along with the red-shifted photocurrent response. As presented in Figure 4B, the short-circuit photocurrent density (Jsc), open-circuit photovoltage (Voc), and fill factor (FF) of dyesensitized solar cells with the C202 sensitizer under an irradiance of AM 1.5G full sunlight are 13.38 mA cm-2, 664 mV, and 0.738, respectively, yielding an overall conversion efficiency (η) of 6.54%. In contrast, the improved photovoltaic parameters (Jsc, Voc, and η) of cells with the C206 sensitizer are 14.49 mA

Figure 5. Detailed photovoltaic parameters of a cell with the C206 sensitizer and a solvent-free ionic liquid electrolyte measured under the irradiance of AM 1.5G sunlight during successive one sun visiblelight soaking at 60 °C.

cm-2, 693 mV, and 7.05%, respectively. This efficiency is impressive for a dye-sensitized solar cell with a solvent-free ionic liquid electrolyte. As presented in Figures 5 and S1 (Supporting Information), cells with the C206 and C202 sensitizers in combination with a stable solvent-free ionic liquid electrolyte5 both showed good stability when submitted to the accelerated testing in a solar simulator with a light irradiance of 100 mW cm-2. After 1000 h of light soaking at 60 °C, only a small decrease of device efficiency from 6.79 to 6.13% was observed for the C206 cell covered with a UV absorbing polymer film while the shortcircuit photocurrent and fill factor do not drop, proving the robustness of these organic sensitizers. The variation of device efficiency is mainly attributed to a 65 mV drop in open-circuit potential, reflecting the surface state augment of the mesoporous

19774 J. Phys. Chem. C, Vol. 112, No. 49, 2008 titania film. This is a frequently met problem during the thermal aging of dye-sensitized solar cells with both ruthenium and organic sensitizers. Previously we have demonstrated that the drop of photovoltage can be attenuated by the cografting of 1-decylphosphonic acid with an amphiphilic ruthenium sensitizer.11 This strategy will be employed for our further device engineering with innovative dyes showing a feature of a wider spectral response. 2.4. Charge Recombination, Transport, and Collection. In the previous discussion, we have shown that molecular engineering of organic sensitizers by increasing the length of aliphatic chains of alkoxy groups can enhance the photocurrent, photovoltage, and efficiency of DSCs with a solvent-free ionic liquid electrolyte. This is well consistent with our earlier observation on solvent-free DSCs with two well-known ruthenium sensitizers (N719 and Z907).12 The previously presumed reason for this improvement is that the long hydrocarbon chains of the Z907 dye interacts laterally to form an aliphatic network, thereby increasing the distance between triiodide and electrons trapped on the TiO2 surface and thus hindering the adverse charge recombination between photoinjected electrons and the triiodide present in the electrolytes. Here, in this study we are curious to understand the device physics of DSCs with the C202 and C206 sensitizers. In order to scrutinize the origins of Voc and Jsc differences, we first measured photoelectrical transients13 to have a close look on the surface states of the dye-coated titania nanocrystals as well as the charge recombination at the titania/electrolyte interface. As presented in Figure 6A, the chemical capacitance (Cµ) of devices with the C202 and C206 sensitizers both arise exponentially with the increase of Voc. Knowing that density of states (DOS) including surface and bulk traps is proportional to Cµ, we have further calculated the DOS with an exponential distribution profile as depicted in Figure 6B. The density of states are calculated by DOS ) 6.24 × 1018∆Q/[∆Vd(1 - P)], where ∆Q is the number of electrons injected during the red light flash, ∆V is the peak of the photovoltage transient, and d and P are the thickness and porosity of titania films. Apparently, more surface states below the conduction band edge, due to the presence of unsaturated titanium species of the mesoporous titania film, have been passivated in the C206 case. In addition, we observed the same phenomenon for the C202 and C206 devices without 3R,7R-dihyroxy-5β-cholic acid as a coadsorbent. The uplifted conduction band-edge of sensitized titania film and a higher photocurrent density will result in a higher electron quasi-Fermi level in the titania film under the same illumination, explaining the observed higher Voc for a cell with the C206 dye in contrast to C202 when an identical electrolyte is used. It is known that the charge recombination at the titania/ electrolyte interface depends on the thermodynamic driving force as well as charge densities. By adjusting output light intensities of white light-emitting diodes to generate different Voc, we measured the pseudo-first-order recombination rate (kr) from the transient photovoltage decay by a fast and small light perturbation. Figure 6C presents the semilogarithmic plot of kr versus Voc for cells with the C202 and C206 dyes, respectively. The linear fitting of all the data gave a similar slope value of 11.0 ( 0.6 for both dyes, indicative of a similar recombination mechanism. Obviously, along with the increase of Voc, the recombination rates become higher due to the higher electron densities in the titania film as well as a larger driving force for recombiantion. At a given Voc, the device with the C206 sensitizer has a slower charge recombination rate compared to that with C202.

Xu et al.

Figure 6. (A) Plots of chemical capacitances versus Voc for cells with the C202 and C206 sensitizers. (B) DOS profiles. (C) The dependence of recombination rate on Voc.

We further resorted to the intensity-modulated photovoltage spectroscopy (IMVS)14 and intensity-modulated photocurrent spectroscopy (IMPS)15 to scrutinize the charge collection in devices with the C202 and C206 sensitizers by measuring the effective electron lifetime, diffusion coefficient, and diffusion length. IMPS measures the periodic photocurrent response of a testing cell to a small sinusoidal perturbation of the light intensity superimposed on a larger steady background level, providing information about the dynamics of charge transport and back reaction under short circuit conditions. IMVS experiment uses the same intensity perturbation but measures the periodic modulation of the photovoltage, giving the information on electron lifetime under open-circuit conditions. As shown in Figure 7A, under the same light intensities, the cell made with the C206 dye always shows a feature of longer electron lifetime than the C202 cell, which is well consistent with the

Organic D-π-A Sensitizers in Dye-Sensitized Solar Cells

Figure 7. Plots of (A) effective electron lifetime, (B) diffusion coefficient, and (C) diffusion length versus light intensity for cells with the C202 and C206 sensitizers.

above transient photovoltage decay measurements. It can be inferred from the fitted curves that τn is inversely proportional to I00.79. The electron transport in mesoporous titania film can be described by the proposed multiple trapping-detrapping (MTD) model.16 Along with the increase of light intensity, 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 electron diffusion coefficient (Dn) as shown in Figure 7B. From the fitted curves we can derive that Dn is proportional to I00.59. The apparently higher electron diffusion coefficient in the case of C206 in contrast to C202 can be understood as the fewer traps in the titania film at a given energy level, which can be perceived from the DOS profile plot presented in Figure 6B. The values of electron diffusion coefficient and lifetime can be used for the calculation 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 in coherence with the high measured Jsc for the C206 cell. 3. Experimental Section A screen-printed double layer film of interconnected TiO2 particles was used as mesoporous negative electrode. A 7 µm thick film of 20-nm-sized 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-sized light scattering anatase particles. The detailed preparation procedures of TiO2 nanocrystals, pastes for screen-printing, and doublelayer nanostructured TiO2 film have been reported in our previous paper.17 A TiO2 electrode was stained by immersing it into a dye solution containing 300 µM of an organic dye (C207, C202, C208, or C206) and 2 mM 3R,7R-dihyroxy-5βcholic acid in the mixture of acetonitrile and tert-butanol (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 hotmelt gasket and sealed up by heating. Unless specified, the internal space was filled with a liquid electrolyte using a vacuum back filling system. The electrolyte-injecting 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. Photovoltaic characterization was performed as reported previously.4 Solar cells covered with a 50 µm thick polyester film (Preservation Equipment Ltd., U.K.) as a 400 nm UV cutoff filter was irradiated at open circuit under a Suntest CPS plus

J. Phys. Chem. C, Vol. 112, No. 49, 2008 19775 lamp (ATLAS GmbH, 100 mW cm-2) in ambient air at 60 °C. Photoelectrochemical measurements were carried out at room temperature after allowing the cells to cool down and equilibrate during 2 h. All photovoltaic data were measured by using a metal mask with an aperture area of 0.158 cm2. For the transient photovoltage and photocurrent experiments, we used a homemade white light-emitting diode (LED) array tuned by varying the driving voltage to shine steady-state lights on a testing cell. In addition, a red LED array controlled with a fast solid-state switch was used to generate a perturbation pulse with a width of 200 ms. Both pulsed red lights and steady-state white light were irradiated on a testing cell from the photoanode side of testing cells. Note that the pulsed light intensity was controlled by the driving potential of red diodes to keep the modulated photovoltage (∆V) below 5 mV. We used red diodes as a probe to generate a perturbation near the open-circuit photovoltages (Voc) of the cell under the steady-state white light and measured the voltage decay process thereafter. Normally, the transient photovoltage decay follows closely a monoexponential form, and the recombination rate, kr, can be extracted from the slope of the semilogarithmic plot. An identical red pulsed light was employed to generate the modulated photocurrent near the short-circuit photocurrents. The TiO2/electrolyte interface capacitance at a given Voc is calculated as Cµ)∆Q/ ∆V. Here ∆Q is the number of electrons injected during the red light flash and obtained by integrating a short-circuit photocurrent transient generated from an identical red-light pulse. Note that this method may underestimate the actual injected electrons by the fraction that is 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 but not the shape of the calculated capacitance versus potential curves. The homemade system was controlled with a Keithley model 2602 digital source meter and fully automated using the Labview 8.2 software. IMVS and IMPS were carried out on a ZAHNER CIMPS system. Stationary as well as sinusoidal modulated light was supplied with a green LED with a maximum wavelength at 546 nm. The LED was controlled by a potentiostatic feedback loop. The selected alternating current (AC) amplitude is in the range of 5% to 15% of the stationary direct current (DC) value. The transfer function for the 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 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 the Levenberg-Marquardt algorithm. 4. Conclusions We have prepared four new organic sensitizers composed of differenttriarylaminedonorsinconjugationwiththethienothiophene unit and cyanoacrylic acid as an acceptor and gained some insight into their structure-property relationships. We have found that for D-π-A sensitizers using a strong electron donor can lift up the HOMOs considerably but the LUMOs slightly, resulting in a lower band gap light-harvesting dye. We have also demonstrated that increasing the aliphatic chain length of the alkoxy group can enhance the electron collection yield in dye-sensitized solar cells to be unity. Along with a solventfree ionic liquid electrolyte, we have demonstrated a ∼7% cell with the amphiphilic C206 sensitizer showing an excellent stability measured under the thermal and light-soaking dual

19776 J. Phys. Chem. C, Vol. 112, No. 49, 2008 stress. We further predict that amphiphilic organic sensitizers with a high molar extinction coefficient will considerably enhance the device efficiency of all-solid-state dye-sensitized solar cells where the charge collection length is normally shorter than the liquid cells. Acknowledgment. The National Key Scientific Program (No. 2007CB936700), the National Science Foundation of China (No. 50773078), and the “100-Talent Program” and the “Knowledge Innovation Program” of Chinese Academy of Sciences have supported this work. N.P, S.M.Z., and M.G. thank the U.S. Air Force project (FA 8655-08-C-4003) for financial support. Supporting Information Available: Experimental details and additional data. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Lewis, N. S. Science 2007, 315, 798. (2) Gra¨tzel, M. Nature 2001, 414, 338. (3) Chiba, Y.; Islam, A.; Watanabe, Y.; Komiya, R.; Koide, N.; Han, L. Jpn. J. Appl. Phys., Part 2 2006, 45, L638. (4) 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. (5) Bai, Y.; Cao, Y.; Zhang, J.; Wang, M.; Li, R.; Wang, P.; Zakeeruddin, S. M.; Gra¨tzel, M. Nat. Mater. 2008, 7, 626. (6) 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. (7) (a) Hara, K.; Kurashige, M.; Dan-oh, Y.; Kasada, C.; Shinpo, A.; Suga, S.; Sayama, K.; Arakawa, H. New J. Chem. 2003, 27, 783. (b) Horiuchi, T.; Miura, H.; Sumioka, K.; Uchida, S. J. Am. Chem. Soc. 2004, 126, 12218. (c) Hagberg, D. P.; Edvinsson, T.; Marinado, T.; Boschloo, G.; Hagfeldt, A.; Sun, L. Chem. Commun. 2006, 2245. (d) Koumura, N.; Wang, Z.-S.; Mori, S.; Miyashita, M.; Suzuki, E.; Hara, K. J. Am. Chem. Soc. 2006, 128, 14256. (e) Kim, S.; Lee, J. W.; Kang, S. O.; Ko, J.; Yum, J. H.; Fantacci, S.; De Angellis, F.; Di Censo, D.; Nazeeruddin, M. K.;

Xu et al. Gra¨tzel, M. J. Am. Chem. Soc. 2006, 128, 16071. (f) Wang, Z.-S.; Cui, Y.; Hara, K.; Dan-oh, Y.; Kasada, C.; Shinpo, A. AdV. Mater. 2007, 19, 1138. (g) Wang, Z.-S.; Koumura, N.; Cui, Y.; Takahashi, M.; Sekiguchi, H.; Mori, A.; Kubo, T.; Furube, A.; Hara, K. Chem. Mater. 2008, 20, 3993. (h) Li, G.; Jiang, K.-J.; Li, Y.-F.; Li, S.-L.; Yang, L.-M. J. Phys. Chem. C 2008, 112, 11591. (i) Wang, Z.-S.; Cui, Y.; Dan-oh, Y.; Kasada, C.; ShinpoA.; Hara, K. J. Phys. Chem. C 2008, 112, in press. (8) Wang, M.; Xu, M.; Shi, D.; Li, R.; Gao, F.; Zhang, G.; Yi, Z.; Humphry-Baker, R.; Wang, P.; Zakeeruddin, S. M.; Gra¨tzel, M. AdV. Mater. 2008, 20, in press. (9) Wong, H. M. P.; Wang, P.; Abrusci, A.; Svensson, M.; Andersson, M. R.; Greenham, N. C. J. Phys. Chem. C 2007, 111, 5244. (10) Wang, P.; Wenger, B.; Humphry-Baker, R.; Moser, J.-E.; Teuscher, J.; Kantlehner, W.; Mezger, J.; Stoyanov, E. V.; Zakeeruddin, S. M.; Gra¨tzel, M. J. Am. Chem. Soc. 2005, 127, 6850. (11) Wang, P.; Zakeerruddin, S. M.; Humphry-Baker, R.; Moser, J. E.; Gra¨tzel, M. AdV. Mater. 2003, 15, 2101. (12) Wang, P.; Zakeeruddin, S. M.; Moser, J.-E.; Gra¨tzel, M. J. Phys. Chem. B 2003, 107, 13280. (13) (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. (14) (a) Schlichtho¨rl, G.; Huang, S. Y.; Sprague, J.; Frank, A. J. J. Phys. Chem. B 1997, 101, 8141. (b) Schlichtho¨rl, G.; Park, N. G.; Frank, A. J. J. Phys. Chem. B 1999, 103, 782. (c) Park, N.-G.; Schlichtho¨rl, G.; van de Lagemaat, J.; Cheong, H. M.; Mascarenhas, A.; Frank, A. J. J. Phys. Chem. B 1999, 103, 3308. (d) Kru¨ger, J.; Plass, R.; Gra¨tzel, M.; Cameron, P. J.; Peter, L. M. J. Phys. Chem. B 2003, 107, 7536. (15) (a) Dloczik, L.; Ileperuma, O.; Lauermann, I.; Peter, L. M.; Ponomarev, E. A.; Redmond, G.; Shaw, N. J.; Uhlendorf, I. J. Phys. Chem. B 1997, 101, 10281. (b) Peter, L. M.; Wijayantha, K. G. U. Electrochim. Acta 2000, 45, 4543. (16) Nelson, J.; Haque, S. A.; Klug, D. R.; Durrant, J. R. Phys. ReV. B 2001, 63, 205321. (17) Wang, P.; Zakeeruddin, S. M.; Comte, P.; Charvet, R.; HumphryBaker, R.; Gra¨tzel, M. J. Phys. Chem. B 2003, 107, 14336.

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