Thiophene-Functionalized Coumarin Dye for Efficient Dye

Dye-loaded film, platinum, and Ag/Ag+ (0.01 M AgNO3 + 0.1 M TBAP in AN) were ... cell composed of a crystalline silicon solar cell and an IR cutoff fi...
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J. Phys. Chem. C 2007, 111, 7224-7230

Thiophene-Functionalized Coumarin Dye for Efficient Dye-Sensitized Solar Cells: Electron Lifetime Improved by Coadsorption of Deoxycholic Acid Zhong-Sheng Wang,*,† Yan Cui,† Yasufumi Dan-oh,‡ Chiaki Kasada,‡ Akira Shinpo,‡ and Kohjiro Hara*,† National Institute of AdVanced Industrial Science and Technology (AIST), 1-1-1, Higashi, Tsukuba, Ibaraki 305-8565, Japan, and Hayashibara Biochemical Laboratories, Inc., 564-176 Fujita, Okayama 701-0221, Japan ReceiVed: NoVember 28, 2006; In Final Form: March 6, 2007

This paper reports a new coumarin dye, 2-cyano-3-(5-{2-[5-(1,1,6,6-tetramethyl-10-oxo-2,3,5,6-tetrahydro1H,4H,10H-11-oxa-3a-aza-benzo[de]anthracen -9-yl)-thiophen-2-yl]-vinyl}-thiophen-2-yl)-acrylic acid (NKX2700), and its application in dye-sensitized solar cells (DSSCs). Under illumination of simulated AM1.5G solar light (100 mW cm-2) with an aperture black mask, 5.0% of power conversion efficiency [short-circuit photocurrent density (Jsc) ) 12.0 mA cm-2, open-circuit photovoltage (Voc) ) 0.59 V, and fill factor (FF) ) 0.71] was obtained for NKX-2700 based DSSC, which was significantly improved to 8.2% (Jsc ) 15.9 mA cm-2, Voc ) 0.69 V, FF ) 0.75) upon addition of 120 mM deoxycholic acid (DCA) to the dye solution for TiO2 sensitization. Coadsorption of DCA decreased dye coverage by ∼50% but significantly improved the Jsc by 33%. The breakup of π-stacked aggregates might improve electron injection yield and thus Jsc. Electrochemical impedance data indicate that the electron lifetime was improved by coadsorption of DCA, accounting for the significant improvement of Voc. These results suggest that interfacial engineering of the organic dye-sensitized TiO2 electrodes is important for highly efficient photovoltaic performance of the solar cell.

Introduction Dye-sensitized solar cells (DSSCs), mainly comprising dye sensitizer, wide band gap semiconductor oxide (e.g., TiO2), and redox electrolyte (e.g., I-/I3-), have attracted considerable attention owing to their high efficiency and potential low cost since the pioneer study.1 A dye molecule anchored to the TiO2 surface absorbs a photon, which excites the dye molecule from the ground to the excited state, and the resultant excited dye molecule injects an electron to the conduction band of TiO2. The resultant oxidized dye molecule is quickly reduced to its original state by I- ions in the electrolyte. Concomitantly, I- is oxidized to its oxidized form, I3-, and the latter is reduced back to I- through accepting electrons at the counter electrode, and thus the electric circuit is completed by diffusion of I- and I3from and to the counter electrode, respectively.2 Extensive studies on dyes,3-10 semiconductor oxides,11-16 and redox electrolytes17-20 have been promoting device efficiency toward the theoretical maximum. The dye sensitizer, which functions as a light absorber, is mainly divided into two kinds: one is metal complexes and the other is metal-free organic dyes. The highest power conversion efficiency obtained to date for metal complexes or all dyes is ∼11%21 while the champion data for metal-free organic dye based solar cells is 9%.22 Because pure organic dyes usually have much stronger light-harvesting ability than metal complexes because of their high extinction coefficient and very rich photophysical properties, it is promising to improve photocurrent to the theoretical maximum through * To whom correspondence should be addressed. Tel: +81-29-861-4638. Fax: +81-29-861-4638. E-mail: [email protected]; [email protected]. † National Institute of Advanced Industrial Science and Technology. ‡ Hayashibara Biochemical Laboratories, Inc.

molecular design of metal-free organic dyes. Recently, more and more attention has been directed to the application of metalfree organic dyes in DSSCs because of no noble metal resource limitation, relatively facile dye synthesis, and easy molecular tailoring. Many research groups have studied organic dyes as sensitizers in DSSCs, and the power conversion efficiency has been improved smoothly to approach the efficiency level for ruthenium polypyridine complexes via molecular design and synthesis.23-31 The great advancement in efficiency of metalfree organic dye solar cells from 1% to 9% in the past ∼20 years has opened up a vast range of prospects for making use of metal-free organic dyes in DSSCs. Among the metal-free organic dyes studied in DSSCs, coumarin-based dyes32 are kind of promising sensitizer stuff for TiO2 because of their good photoresponse in the visible region, good long-term stability under one sun soaking,33 and appropriate lowest unoccupied molecular orbital (LUMO) levels matching the conduction band of TiO2. On the basis of the concept of donor-π conjugation bridge-acceptor structure, we synthesized a series of coumarin dyes by inserting various numbers of thiophene or methine moieties as π bridge between coumarin as electron donor and cyano carboxylic acid as electron acceptor.34-36 Among them, NKX-2677 containing two thiophene moieties yielded 7.4% of power conversion efficiency with an aperture mask.36 However, the absorption of NKX-2677 in the wavelength more than 600 nm is poor, limiting the photoelectric conversion in this region. To extend the photoresponse to the red region for higher photocurrent, and meanwhile ensuring that the LUMO energy level is sufficiently higher than the conduction band edge for efficient electron injection, NKX-2700 was prepared through inserting one methine (-CHdCH-) unit between the two thiophene moieties in NKX-2677. Compared with NKX-2677, NKX-2700 is

10.1021/jp067872t CCC: $37.00 © 2007 American Chemical Society Published on Web 04/26/2007

Organic Dye-Sensitized Solar Cells

Figure 1. Molecular structure of NKX-2700.

anticipated to generate higher photocurrent because of the red shift of the lowest-energy absorption peak. π-π stacking of organic dye molecules usually occurs because of the strong intermolecular interaction. Especially, the strong π-electron interaction in oligothiophene allows easy formation of stacks or lamellar structure.37 Although π-π stacking is advantageous to the light harvesting because of its broad feature in the UV-vis absorption spectrum, π-stacked aggregate usually leads to inefficient electron injection and thus results in low power conversion efficiency.38,39 Prohibition of π-π stacking with additive in the dye solution is a typical way to improve efficiency of organic dye-sensitized solar cells suffering from the π-π stacking problem. Coadsorption of dye with additives40 and structural modification of dye molecules41 is proven to be effective to dissociate π-π stacking or dye aggregation and thus to improve solar cell efficiency. Herein, we report the photovoltaic properties of NKX-2700 (structure shown in Figure 1) dye-sensitized solar cells and the effect of deoxycholic acid (DCA) coadsorption on solar cell performance. Power conversion efficiency was remarkably increased from 5.0% to 8.2% upon exposing the TiO2 film to 0.3 mM NKX2700 solution containing 120 mM DCA. To understand the role of DCA in the significant enhancement of device performance parameters, the effect of DCA coadsorption on the dye adsorption behavior, intermolecular electron transfer, and electron lifetime in DSSCs was studied, and interesting results were obtained. Experimental Section Synthesis of NKX-2700. Detailed synthesis procedures for NKX-2700 are shown in the Supporting Information. Materials and Reagents. Regent-grade LiI (Wako Pure Chemical Industries Ltd.), I2 (Wako), LiClO4 (Wako), TiCl4 (Wako), acetonitrile (AN, Wako), deoxycholic acid (DCA, Tokyo Kasei Kogyo Co. Ltd.), tetrabutylammonium perchlorate (TBAP, Tokyo Kasei), 1-decylphosphonic acid (DPA, Lancaster), and hexadecylmalonic acid (HDMA, Lancaster) were used without further purification. Dimethyl-3-n-propyl-imidazolium iodide (DMPImI), 4-tert butyl-pyridine (TBP), and tertbutanol were purchased from Tomiyama Pure Chemical Industries Ltd. The electrolyte used in this work contains 0.6 M DMPImI, 0.1 M LiI, 0.05 M I2, and 0.5 M TBP unless otherwise specified. Dye solution (0.3 mM) containing different amount of DCA, prepared in a mixture solvent of AN and tert-butanol with a volume ratio of 1/1, was used to sensitize the TiO2 electrodes. Because DCA has a low solubility in AN/tertbutanol, we first dissolved a weighed amount of DCA in a small amount of ethanol and then added 0.3 mM dye solution in it. Transparent conducting oxide (TCO, F-doped SnO2, 10 Ω/0, Nippon Sheet Glass Co.) was washed with basic solution, ethanol, and acetone successively under supersonication for 10 min each before use. Fabrication of the Dye-Sensitized TiO2 Films. A screen printing method was used to fabricate TiO2 films on TCO glass. TiO2 nanoparticles (ca. 23 nm) and large particles (ca. 100 nm) were prepared by the method reported previously.42 TiO2 nanoparticles and a mixture of nanoparticles and scattering large particles at a ratio of 6:4 were dispersed in ethanol. The TiO2

J. Phys. Chem. C, Vol. 111, No. 19, 2007 7225 pastes were prepared by mixing TiO2 suspension, ethyl cellulose, and R-terpineol in ethanol followed by the removal of the solvent with rotary evaporator at 40 °C under vacuum of 20 hPa. Pastes N and M consisted of nanoparticles and mixed particles (23 and 100 nm particles at a ratio of 6:4), respectively. Transparent TiO2 films, fabricated on TCO glass or glass slides using paste N, were used for characterizations such as electrochemical and spectroscopic measurements. Double-layer (NM) TiO2 films (25 µm), employed for photovoltaic measurements, were prepared by printing the transparent layer (18 µm) first with paste N followed by further coating with paste M (7 µm). The TiO2 films were fired at 525 °C for 2 h with a rising rate of 10 °C/min. All the films were immersed in 0.05 M TiCl4 solution for 30 min at 70 °C followed by calcinations at 450 °C for 30 min. The film thickness was measured with a Tencor Alpha-Step 500 Surface Profiler. The film size is apparently 0.5 × 0.5 cm2. Dye-sensitized TiO2 photoelectrodes were obtained by immersing the TiO2 films in dye solution overnight when they were at ∼100 °C cooled from the heating. Characterizations. The UV-vis absorption spectra of the dye-loaded transparent film and the dye solutions were recorded on a Shimadzu UV-3101PC spectrophotometer. The attenuated total reflection Fourier transform infrared (FT-IR) spectra were measured with a Perkin-Elmer Spectrum One spectrophotometer. Cyclic voltammograms were measured with a three-electrode electrochemical cell on a CH 610 Electrochemical analyzer. Dye-loaded film, platinum, and Ag/Ag+ (0.01 M AgNO3 + 0.1 M TBAP in AN) were employed as working, counter, and reference electrodes, respectively. The supporting electrolyte was 0.1 M LiClO4 in AN, which was degassed with N2 for 20 min prior to scan. The potential of the reference electrode is 0.49 V versus normal hydrogen electrode (NHE) and is calibrated with ferrocene immediately after cyclic voltammogram (CV) measurement. Electrochemical impedance spectra for DSSCs under light were measured with an impedance/Gain-Phase analyzer (Solartron SI 1260) connected with a potentiostat (Solartron SI 1286). The spectra were scanned in a frequency range of 0.1-105 Hz at room temperature with applied potential set at open circuit. The magnitude of the modulation signal was set at 10 mV. Photovoltaic Measurement. Sealed cells, the sealing procedure published elsewhere,43 were employed for photovoltaic measurements. The current-voltage characteristics of the cells were measured on a computer-controlled Voltage Current source meter (Advantest, R6243), under illumination of simulated AM1.5G solar light from an AM1.5 solar simulator (Wacom Co., Japan, WXS-80C-3 with a 300 W Xe lamp and an AM1.5 filter). The incident light intensity was calibrated by using a standard solar cell composed of a crystalline silicon solar cell and an IR cutoff filter (Schott, KG-5), giving the photoresponse range of amorphous silicon solar cell. The standard solar cell was produced and calibrated by Japan Quality Assurance Organization. With the aid of AM1.5 filter, the spectral output of our lamp matches well with the standard global AM1.5 solar spectrum. After calculation of spectral mismatch factor, the spectral output of the lamp was adjusted to 49.6 mA, corresponding to 100 mW cm-2 AM1.5G simulated solar light. Photocurrent action spectra were recorded on a CEP-99W system (Bunkoh-Keiki Co., Ltd.). To avoid the diffuse light penetrating into the active dye-loaded film, a metal black mask with an aperture area of 0.2354 cm2, measured with an optical microscope, LEICA M420, equipped with a digital camera (Nikon DXM1200), was employed to test photovoltaic performance.

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Figure 2. UV-vis absorption spectrum of 0.010 mM NKX-2700 in AN.

Figure 4. FT-IR spectra for dye powder and TiO2 films exposed to 120 mM DCA solution alone and 0.3 mM NKX-2700 containing 0 or 120 mM DCA, respectively. A bare TiO2 film was used as reference.

Figure 3. UV-vis absorption spectra for TiO2 films (1.6 µm) exposed to 0.3 mM NKX-2700 solutions containing 0, 10, 60, and 120 mM DCA.

Results and Discussion Dye Adsorption Behavior. Figure 2 shows the UV-vis absorption spectra of NKX-2700 in AN solution. The lowestenergy π-π* electron transition peak is centered at 525 nm. At this peak position, molar extinction coefficient was determined to be 7.0 × 104 M-1 cm-1. Compared with NKX-2677, one of the best metal-free organic dyes for DSSCs we reported previously,36 the maximum absorption of NKX-2700 is redshifted by ∼15 nm because of the extension of π-system by insertion of one methine unit (-CHdCH-) between the two thiophene moieties. This red shift, favorable for light harvesting in the longer wavelength region, is anticipated to increase photocurrent of DSSCs. Upon adsorption of NKX-2700 onto the TiO2 surface, the maximum absorption was blue-shifted from 525 to 480 nm, as seen in Figure 3. The blue shift is attributed to the well-known solvent effect. In addition, the deprotonation of carboxylic acid in the dye molecule4,44 upon adsorption on the TiO2 film, as discussed in the following IR analysis, may also contribute to this blue shift. When TiO2 films were exposed to dye solutions containing DCA, the dye adsorption was reduced gradually with increasing the DCA content in the dye solution, as shown in Figure 3, and meanwhile the maximum absorption red-shifted by ∼5 nm. The fact that there is no significant change in the absorption spectra when the concentration of DCA varies from 0 to 120 mM, as seen in Figure 3, indicates that the possible π-stacked aggregates are not responsible for this spectral blue shift.45 The drop of dye adsorption indicates that DCA competes for the TiO2 surface

sites with the dye molecules. The slight spectral red shift may be due to the reduction of intermolecular interaction. We cannot conclude that the π-π stack was broken up completely, but it is reasonable to assume that the intermolecular interaction is reduced by the coadsorption of DCA. Estimated from the absorbance change, the dye coverage was reduced by 49% in the case of 120 mM DCA in the dye solution. Therefore, the ratio of dye to DCA is deduced to be 1:1. If dye and DCA is arranged on the TiO2 surface alternatively, the intermolecular interaction should be weakened because of the spacing effect of DCA. Although the coadsorption of DCA reduces dye coverage by half, the maximum absorbance is 1.2 for dye-loaded 1.6 µm transparent film. When a much thicker film (e.g., 25 µm) is used as in the solar cell case, absorbance greater than 1, corresponding to >90% of light-harvesting efficiency, can be obtained in a wide spectral region. This indicates that dye with 50% coverage, if the dye coverage without DCA is 100%, is able to absorb incident light efficiently. The binding mode of dye molecules to the TiO2 surface, which is associated with the interfacial electron injection, was analyzed by the FT-IR technique. Figure 4 shows the FT-IR spectra for NKX-2700 powder and TiO2 films exposed to NKX2700 solution alone and exposed to NKX-2700 solution containing 120 mM DCA. For the dye powder, the 2214 cm-1 peak is assigned to the -CN group, and the 1711 and 1692 cm-1 peaks are attributed to the carbonyl group in the coumarin framework and in the carboxylic acid, respectively.46 The peak at 1408 cm-1 results from the in-plane bending of C-O-H, and the peak at 1211 cm-1 is assigned to the C-O stretch. The peak at 1247 cm-1 is due to the cyclic ester in the coumarin framework. After the dye was adsorbed onto the TiO2 surface, the peak, diagnostic of -COOH at 1692 and 1408 cm-1, disappeared while asymmetric and symmetric peaks for -COOgroup were observed, respectively, at 1577 and 1310 cm-1, indicating the deprotonation of carboxylic acid upon dye adsorption. The separation between the asymmetric and symmetric peak for -COO- is calculated to be 267 cm-1,

Organic Dye-Sensitized Solar Cells

Figure 5. Cyclic voltammograms of dye-loaded TiO2 films with or without coadsorbent DCA using 0.1 M LiClO4 in AN as supporting electrolyte. The scan rate was 0.1 V s-1.

comparable to that (242 cm-1) for the solid salt form,36 indicating a bidentate coordination mode of the dye to the TiO2 surface.47 The characteristic peaks for -CN, carbonyl, and cyclic ester in the coumarin framework are still observed for the dyeloaded film. The characteristic IR peaks for DCA observed in the DCAloaded film (bottom trace in Figure 4) confirm the adsorption of DCA on TiO2 surface. When a TiO2 film was exposed to dye solution containing 120 mM DCA, the IR spectrum was slightly changed in appearance because of the DCA coadsorption. The characteristic IR peaks for NKX-2700 linked to TiO2 were also observed for the DCA cografted dye-loaded film except reductions of vibration peak intensities. However, the IR peaks for DCA are not clearly discernible for dye/DCAloaded TiO2, which is probably because the IR peaks for DCA are overlapped by those for the dye. The decreased IR peak intensity again verifies that DCA coadsorbed on the TiO2 sites with the dye molecules. The relative dye amount on the TiO2 surface was estimated by the relative peak intensity of -CN group. The peak intensity of -CN reduced by ∼50% indicates that coadsorption of DCA dropped the dye coverage by about half, in excellent agreement with the UV-vis absorption result. We also measured the dye amount by dye desorption into a basic solution followed by spectroscopic determination. It was found that the dye amount decreased by 54% when 120 mM DCA was included in the dye solution. The data on dye coverage change upon DCA cografting obtained from dye desorption, UV-vis absorption, and FT-IR spectra strongly support that a mixed layer, dye/DCA, is formed on the TiO2 surface. Cyclic Voltammogram (CV). Figure 5 depicts the typical CV of NKX-2700 on the TiO2 surface using 0.1 M LiClO4 in AN as supporting electrolyte. The dye exhibited two reversible one-electron oxidation waves at the half-wave potentials of 0.82 and 1.19 V versus NHE. The first oxidative process generates a cation radical by oxidation of the π-system, while the second oxidative process produces a dication. With increasing applied potential, the dye is stepwise oxidized to the dication. The first oxidation potential of 0.82 V versus NHE is taken as the highest occupied molecular orbital (HOMO) of the dye. The LUMO is estimated to be -0.95 V versus NHE using the absorption threshold (700 nm, 1.77 eV) as the gap between HOMO and LUMO. It is evident that the LUMO of the dye is above the conduction band edge,2 ensuring electron injection from the excited dye molecules to the conduction band of TiO2 is thermodynamically favorable.2 The NKX-2700 bond to mesoporous TiO2 films can be electrochemically oxidized and reduced in a reversible fashion.

J. Phys. Chem. C, Vol. 111, No. 19, 2007 7227

Figure 6. IPCE as a function of wavelength of incident monochromatic light for TiO2 electrodes exposed to 0.3 mM NKX-2700 solution containing 0, 10, 60, or 120 mM DCA in a mixture solvent of AN/ tert-butanol with volume ratio of 1/1.

Initial oxidation of dye molecules followed by intermolecular electron hopping across the TiO2 nanoparticle surface is one accepted mechanism.48,49 Formation of π-stacks for NKX-2700 allows easy intermolecular electron hopping as evidenced by the strong CV signal in Figure 5. However, upon cografting of DCA with the dye on the TiO2 surface, almost no oxidation and reduction waves were observed.50 The reduction of dye amount by half because of coadsorption of DCA cannot account for the remarkably weakening extent of the CV signal. This means that the DCA/NKX-2700 layer is much less electrochemically active than the dye alone. At present, we cannot explain clearly this different behavior for the two electrodes. One possibility is that DCA molecules may prevent intermolecular electron hopping between the dye molecules. It seems to be surprising that DCA/NKX-2700/TiO2 had less electrochemical activity but produced better solar cell performance than NKX-2700/TiO2, as discussed below. CV was performed under dark and in the absence of redox couple but solar cell performance was measured under illumination and in the presence of redox couple. Therefore, there is no direct relation between electrochemical activity and solar cell performance. In other words, the less electrochemically active DCA/NKX2700 layer can still transact electrons with iodine redox couple in the electrolyte. Photovoltaic Performance. Action spectrum, incident photonto-electron conversion efficiency (IPCE) as a function of wavelength, is measured to evaluate the photoresponse of photoelectrode in the whole spectral region. Figure 6 compares the IPCE action spectra for TiO2 film exposed to dye alone and to both the dye and DCA with 10, 60, and 120 mM in the dye solution. The maximum IPCE is only ∼66% for NKX2700 sensitized solar cell. It is much lower than 85%, a typical value for unity electron injection taking into account the light loss (∼15%) by the reflection and absorption of TCO glass. Upon addition of DCA in the dye solution, IPCE was enhanced significantly. The maximum IPCE was improved from 66% to 75% when 10 mM DCA was included in the dye solution and was further improved to 85% with 60 mM or more DCA in the dye solution. Because monomer can inject electrons to TiO2 more efficiently than aggregate,38 the mixed dye/DCA layer obtained from exposure of TiO2 to dye solution with 10 mM DCA may contain a mixture of aggregate and monomer. On the other hand, the mixed dye/DCA layer obtained from exposure of TiO2 to dye solution with 60 mM or more DCA is mainly composed of monomer judged from the high IPCE. When 60 mM or more DCA was added to the dye solution for

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TABLE 1: Influence of DCA on the Photovoltaic Performance Parametersa DCA/mM

Jsc/mA cm-2

Voc/V

FF

η (%)

0 10 60 120 160

13.3 15.0 16.8 16.9 16.0

0.61 0.62 0.63 0.64 0.66

0.68 0.69 0.70 0.72 0.72

5.5 ( 0.1 6.4 ( 0.0 7.4 ( 0.2 7.8 ( 0.1 7.6 ( 0.2

a Illumination: 100 mW cm-2 simulated AM1.5G solar light; redox electrolyte containing 0.5 M TBP.

film sensitization, a plateau from 450 to 600 nm was observed in the IPCE action spectrum with IPCE of ∼85%. This high IPCE value means that all the three efficiencies, electron injection, light harvesting, and electron-collecting efficiency, are unity. The integrated photocurrent calculated from the overlap integration of IPCE action spectrum and the AM1.5G solar emission spectrum was increased from 14 to 17 mA cm-2 when 120 mM DCA was included in the dye solution. Fixing other conditions, the effect of DCA content in the dye solution on the solar cell performance was tested, and the data is summarized in Table 1. Short-circuit photocurrent (Jsc) increased with DCA until 60 or 120 mM and then decreased with further increasing DCA content. The Jsc changing trend agrees well with the IPCE data. When DCA was less than 120 mM, the improved IPCE was able to compensate for the dye adsorption loss and thus an increase in Jsc was observed. However, when DCA was greater than 120 mM, the loss of dye adsorption offset the improved IPCE and therefore a decrease in Jsc was observed. On the other hand, while opencircuit photovoltage (Voc) increases with DCA until 160 mM, fill factor (FF) increases with DCA until 120 mM and then remains almost unchanged. As a consequence, the highest power conversion efficiency (η) could be obtained at 120 mM DCA. It is reported that coadsorption of DPA51 or HDMA52 with ruthenium polypyridine dyes could improve both Jsc and Voc. However, they could not work on NKX-2700 in terms of efficiency improvement. Like DCA, they could also decrease the adsorption of NKX-2700 significantly. Because of this, remarkable decrease in Jsc along with comparable Voc and FF was observed upon cografting of DPA or HDMA with NKX2700. This result suggests that DPA or HDMA coadsorbs paralleling to the dye molecules and the liny alkyl chain cannot separate adjacent molecules efficiently and thus cannot prohibit the π-π stacking. Judged from the difference of structure between DPA or HDMA and DCA, a coadditive with bulky structure may be necessary for effective spacing of adjacent organic dye molecules. Effect of TBP content in the redox electrolyte on solar cell performance was also investigated. Increasing TBP concentration in the electrolyte increases Voc and FF while decreases Jsc gradually because of the suppression of charge recombination and negative shift of conduction band of TiO2. It was found that 0.7 M TBP was optimal with respect to power conversion efficiency. Figure 7 compares I-V curves for devices with and without DCA at the best condition. Without DCA, the cell produced 5.0% of power conversion efficiency (Jsc ) 12.0 mA cm-2, Voc ) 0.59 V, FF ) 0.71). With 120 mM DCA in the dye solution, the efficiency was increased to 8.2% (Jsc ) 15.9 mA cm-2, Voc ) 0.69, FF ) 0.75); the efficiency is very close to 9.0% obtained from N719 based DSSC under comparable conditions. Besides the significant Jsc enhancement by 33%, the Voc increased by 100 mV and FF increased from 0.71 to 0.75. At comparable conditions, NKX-2677 produced 7.1% of η (Jsc ) 13.5 mA cm-2, Voc ) 0.69 V, FF ) 0.76). Obviously,

Figure 7. Current density-voltage characteristics for DSSCs sensitized by 0.3 mM NKX-2700 solution containing 0 or 120 mM DCA under light (100 mW cm-2 simulated AM1.5G solar light) and dark. To avoid diffuse light, the cells were masked with a black metal mask with aperture area of 0.2354 cm-2.

structural modification from NKX-2677 to NKX-2700 results in higher efficiency, which is attributed to the significant enhancement of Jsc mainly originating from the red shift of maximum absorption. The significant efficiency improvement from NKX-2677 to NKX-2700 indicates that the molecular design in this work is successful. The above photovoltaic results indicate that coadsorption of DCA is effective to improve solar cell performance. Because adsorption of DCA leaves protons on the TiO2 surface and hence charges the surface positively, the conduction band edge should be positively shifted by the coadsorption of DCA, resulting in Voc loss.53 This is contrary to the observation by Neale et al., where adsorption of chenodeoxycholate anions charge the surface negatively and shift the conduction band edge negatively.54 Here, 100 mV of Voc increase was observed, indicating that charge recombination was suppressed by DCA. To understand the role of DCA in improving performance parameters, effect of DCA on dark current was studied as shown in Figure 7. It is obvious that the dark current onset potential shifted to a larger value, and the dark current, if any, was reduced upon coadsorption of DCA. This dark current change indicates that coadsorption of DCA leads to suppression of charge recombination between injected electrons and I3- ions in the electrolyte, favorable for Voc gain. The enhancement of Voc is usually associated with the negative shift of conduction band edge or suppression of charge recombination. Consequently, suppression of charge recombination may compensate for the Voc loss because of proton exchange from DCA to the TiO2 surface, resulting in Voc improvement. Electrochemical Impedance Spectroscopy. To further clarify the DCA effect on the Voc, electrochemical impedance spectrum (EIS), which is a powerful tool to elucidate the electronic and ionic transport processes in DSSCs, was collected under illumination of one sun at open-circuit potential. The EIS Bode plot, shown in Figure 8a, exhibits two Bode frequency peaks for electron transfer at the TiO2/dye/electrolyte interface and redox charge transfer at the counter electrode in increasing order of frequency.55,56 The low-frequency peak for diffusion of redox electrolyte is not discernible because of the overlap with the middle-frequency peak as seen in Figure 8a. The equivalent circuit for the DSSC is shown in Figure 8b. The peak frequency of the middle-frequency peak is related to the charge recombination rate, and its reciprocal is regarded as electron lifetime.57 It is obvious that the middle-frequency peak shifts to a smaller value gradually with increasing DCA content in the dye solution,

Organic Dye-Sensitized Solar Cells

J. Phys. Chem. C, Vol. 111, No. 19, 2007 7229 One can see that electron lifetime decreases with increasing Jsc or electron density, and a power law relation was observed as follows:

τ ) aJscβ

Figure 8. (a) Electrochemical impedance Bode plot for DSSCs sensitized by exposing TiO2 films to 0.3 mM NKX-2700 containing 0, 10, 60, or 120 mM DCA. (b) Equivalent circuit of the DSSC consisting of TiO2/dye/electrolyte and Pt/electrolyte interface. R1, R2, R3 are the series resistance of Pt and TCO, charge-transfer resistance at TiO2/ dye/electrolyte, and at Pt/electrolyte interface, respectively. CPE2 and CPE3 are the constant phase element for the TiO2/dye/electrolyte and Pt/electrolyte interface, respectively.

(1)

where a is a constant and β is the slope of the double-logarithmic plot in Figure 9. The electron lifetime was enhanced at any Jsc value in the studied range upon DCA cografting. Linear relationship between log(τ) and log(Jsc) was observed for solar cells with and without DCA. The slopes are -0.30, -0.31, -0.35, and -0.39 for solar cells obtained by exposing TiO2 films to dye solutions containing 0, 10, 60, and 120 mM DCA, respectively. The similar slope suggests that the charge recombination mechanism was essentially not changed upon DCA coadsorption. However, the electron lifetime was enhanced by DCA at the Jsc range from 1 to 20 mA cm-2. The varying tendency of electron lifetime is consistent with that of Voc with DCA content. Thus, the Voc improvement can be explained by the enhancement of electron lifetime. The origin of the increase in electron lifetime would come from the blocking effect of DCA. The inserted DCA layer may prevent recombination of injected electrons to the I3- ions in the electrolyte. Most recently, we found that long alkyl chains linked to the dye framework, acting as a blocking layer, could improve Voc significantly as a result of enhanced electron lifetime.58 Conclusions

Figure 9. Double-logarithmic plot of electron lifetime against Jsc, which was measured under light used for EIS measurement. TiO2 photoelectrodes were obtained by exposing TiO2 films to dye solutions containing 0, 10, 60, and 120 mM DCA.

indicating increase in electron lifetime upon DCA coadsorption. One electron loss channel is the electron capture by the electron acceptors in the electrolyte. The insertion of DCA, forming an insulating spacer, blocks the electron recombination to the I3ions and thus enhances electron lifetime. Neale et al. studied the mechanism by which coadsorbent, tetrabutylammonium chenodeoxycholate, affects N719-based solar cell performance.54 They concluded that coadsorption of chenodeoxycholate (basic form) with N719 increases charge recombination rate54 in contrast to the DCA effect that charge recombination rate is reduced by coadsortption of DCA (acid form) with NKX-2700. Figure 9 shows a double-logarithmic plot of electron lifetime against Jsc, which was measured at various light intensities used for EIS measurement. Jsc is proportional to electron density, and we employ Jsc as abscissa for convenience. The electron lifetime evidently depends exponentially on the electron density.

In summary, we synthesized NKX-2700 by inserting one methine unit (-CHdCH-) between the two thiophenes in NKX-2677. Expansion of π-system by insertion of one methine unit is able to red-shift maximum absorption and hence increases Jsc as compared to a conventional coumarin dye, NKX-2677. Coadsorption of DCA reduced the dye loading by about half but improved Jsc remarkably. The breakup of π-stacked aggregates might improve electron injection yield and thus Jsc.38 The improvement of Voc is attributed to the suppressed charge recombination, revealed by the increase in electron lifetime. Power conversion efficiency of 8.2% is very close to 9.0% for N719 based DSSC under comparable conditions. Comparison of NKX-2700 with N719 shows that NKX-2700 produces higher Jsc and FF but lower Voc than N719. If the Voc for this kind of dye can be comparable or can exceed that for N719, a much higher efficiency is expected. To achieve this, new molecular design toward higher Voc while keeping Jsc and FF is underway. Acknowledgment. This work was supported by Industrial Technology Research Grant Program in 2005 from New Energy and Industrial Technology Development Organization (NEDO) of Japan. Supporting Information Available: Synthesis procedures for NKX-2700. 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) Hagfeldt, A.; Gra¨tzel, M. Chem. ReV. 1995, 95, 49. (3) Nazeerudin, 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. (4) Nazeeruddin, M. K.; Pe´chy, P.; Renouard, T.; Zakeeruddin, S. M.; Humphry-Baker, R.; Comte, P.; Liska, P.; Cevey, L.; Costa, E.; Shklover,

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