Influence of the Electrolytes on Electron Transport in Mesoporous TiO2

Feb 20, 2002 - Diffusion coefficients of electrons in mesoporous TiO2−electrolyte systems are determined by laser pulse induced photocurrent measure...
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J. Phys. Chem. B 2002, 106, 2967-2972

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Influence of the Electrolytes on Electron Transport in Mesoporous TiO2-Electrolyte Systems Shingo Kambe,† Shogo Nakade,‡ Takayuki Kitamura,† Yuji Wada,† and Shozo Yanagida*,† Material and Life Science, Graduate School of Engineering, Osaka UniVersity, Suita, Osaka 565-0871, Japan, and Nokia Research Center, Nokia-Japan Co., Ltd., 2-13-5, Nagata-cho, Chiyoda-ku, Tokyo, Japan ReceiVed: September 5, 2001

Diffusion coefficients of electrons in mesoporous TiO2-electrolyte systems are determined by laser pulse induced photocurrent measurements in the presence of a wide range of concentration of Li+, Na+, Mg2+, tetrabutylammonium cation (TBA+), or dimethylhexylimidazolium cation (DMHI+) in electrolytes. The total amount of photocharge generated by a laser pulse increases in the order of DMHI+< TBA+ < Na+ < Li+. The diffusion coefficients (DAmb) increase with increasing the cation density. In the case of TBA+, the increase of the diffusion coefficients is interpreted with ambipolar diffusion mechanism. In the case of Li+, Na+, Mg2+, and DMHI+, each diffusion coefficient increases to some extent with the cation density as in the case of TBA+, but does not fit well with ambipolar diffusion mechanism with the assumption of constant diffusion coefficient and electrons at the high density of the cations. Electron diffusion in the systems is discussed in terms of the surface cation density arising from the adsorption of cations on the films.

Introduction The mechanism of dye-sensitized mesoporous TiO2 solar cells has been extensively studied due to the high-energy conversion efficiency up to 10%. This high conversion efficiency has been explained by the combination of following factors. The adsorbed dye on the TiO2 surface absorbs the solar light and then injects photogenerated electrons into the TiO2 conduction band in the femtosecond time frame.1-4 This is much faster than the deactivation rate of the excited state to the ground state of the dye, resulting in a high quantum yield of dye-sensitization. Injected electrons in the conduction band of TiO2 are transported by diffusion without large loss owing to back electron transfer to either the oxidized dye or the redox electrolyte when the dyed-TiO2 electrode is immersed in the I-/I3- electrolyte solution.5-8 Recently, we demonstrated the contribution of effective charge transport of the redox I-/I3- electrolyte to the efficiency in the dye-sensitized TiO2 solar cells.9 It has been known that several cations existing in the electrolyte solution as the countercation of I- and I3- also play important roles for the high-energy conversion efficiency. For example, interaction of Li+ with TiO2 surface enhances the electron transfer from the sensitized dye to the TiO2 and also the electron transfer from I- to the oxidized dye, leading to high photocurrent.10-12 The effects of cations on TiO2 surface were also discussed for electron screening in dye-sensitized solar cells.13,14 Although interaction between cations and TiO2 has been well studied,15-22 there is no detailed study on the effects of the cationic species on electron transport properties in the mesoporous TiO2-electrolyte systems. The dynamics of electron transport in mesoporous TiO2 films has been studied as electron diffusion with trapping events.23-26 Reported values of electron diffusion coefficients in mesoporous * Corresponding author. E-mail: [email protected]. Phone: +81-6-6879-7924. Facsimile: +81-6-6879-7875. † Osaka University. ‡ Nokia-Japan Co., Ltd.

TiO2 range from 10-8 to 10-4 cm2 s-1, depending on light intensity.7,27 These values are much lower than the diffusion coefficient calculated from the electron mobility measured for single-crystal TiO2.26,28 The low values of the electron diffusion coefficient have been attributed to electrons that spend a large fraction of their transit time in trap sites. It is assumed that the trap sites would be located at the surface of TiO2 particles because of the large surface-to-volume ratio of the mesoporous TiO2 films.29 Nelson proposed a random walk mechanism for electron transport in TiO2, in which each electron can move after a waiting time which can be determined by the activation energy of the trap site.30 Therefore, if the adsorption or the intercalation of cations changes the number of electron trap states or the energy level of the trap states, the diffusion coefficient will depend on the cations. Literature research revealed that the adsorption sequence for K+, Li+, Mg2+, and tetramethylammonium cation (TMA+) on TiO2 in water was Mg2+ . Li+ > K+ ≈ TMA+19,22 If there is specific interaction between cationic species and TiO2 surface, the diffusion coefficient would be strongly influenced by the cationic species. In this report, the diffusion coefficients of electrons in the mesoporous TiO2-electrolyte systems were determined by laserinduced time transient photocurrent measurements in the presence of different cations in electrolyte. Alkaline metal, alkaline earth metal, and ammonium perchlorates were used as a cationic species in electrolytes. An imidazolium bromide was also used as an electrolyte because imidazolium cations are essential components to achieve high performance of dye-sensitized solar cells.31-33 Changes in electron transport properties were discussed in view of the difference in the adsorptive behavior of the cations on the TiO2 electrodes. Experimental Section Spectroscopic-grade ethanol and acetonitrile and perchlorate and bromide salts of Li and tetra-n-butylammonium (denoted

10.1021/jp013397h CCC: $22.00 © 2002 American Chemical Society Published on Web 02/20/2002

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as TBA) were obtained from Nacalai Tesque. The sodium perchlorate was obtained from Kanto Chemical, and magnesium perchlorate and tetra-n-butylammoniumbromide were obtained from Wako Pure Chemical. The bromide salt of 1,2-dimethyl3-hexyl-imidazolium (denoted as DMHI) was obtained from Shikoku Kasei. TBA+Br- was not used for the determination of diffusion coefficients but for absorption spectroscopy as a reference for DMHI+Br-. Acetonitrile was distilled over calcium hydride under argon atmosphere before use. The water content in acetonitrile was estimated to be less than 0.6 wt %. Anatase TiO2 nanocrystallites, consisting of particles with diameter of ca. 18 nm, were prepared by a hydrothermal method, which was direct hydrolysis of titanium n-butoxide in organic solvent at high temperature under high pressure.35 Anatase crystallites (6 g) were mixed with larger TiO2 particles (ca. 100 nm in diameter, 1.2 g, Fluka), acetylacetone (0.2 mL; Wako Pure Chemical), and distilled water (10 mL), and then the mixture was mechanically dispersed to give a paste. The TiO2 paste was then spread on the top of conducting glass plate (F-doped SnO2 overlayer, 10 Ω/square, 1.4 × 2.3 cm2; Asahi Glass). The TiO2 coated glass was heated in air at 450 °C for 30 min and then allowed to cool. The thickness of the resulting films was fixed at 7.2 ( 0.1 µm, which was determined by a surface profiler (Sloan, Dektak3). A setup of the time transient photocurrent measurements and detailed analysis to obtain the diffusion coefficient of electron were described in previous reports.36-38 The TiO2 mesoporous electrode as a working electrode was fixed at a hole on a quartz cell with a rubber O-ring, and a platinum wire was used as a counter electrode. The concentration of the salt in electrolyte ranged from 0.3 mM to 1.0 M, where the number of cationic species as cation density ranged from 2.0 × 1017 to 6.0 × 1020 cm-3 for monovalent cations, but that is twice of concentration for di-valent cation. Cation density was calculated from cation concentration considering porosity of the TiO2 film as 50%. The two electrodes electrochemical setup was connected to a potentiostat (Toho Technical Research, Potentiostat/Galvanostat 2000). The transient current was monitored using a digital oscilloscope (Tektronix, TDS 3052). TiO2 electrode was irradiated from an electrolyte side by a laser pulse (Quanta-Ray, INDI Series Pulsed Nd:YAG Lasers, pulse width ) 7 ns, wavelength ) 355 nm). An aperture was placed in front of the cell, and the irradiated area of the electrode was fixed as 0.093 cm2. All measurements were performed with argon bubbling and at least 3-min interval between measurements. From the peak time (tpeak) of the photocurrent transient and film thickness (W), the electron diffusion coefficient (denoted as DAmb) was calculated from

DAmb ) W2/2 tpeak

(1)

Electrons are assumed to diffuse in a one-dimensional space. To compare the DAmb for different cations, comparable electron density estimated from the amount of photogenerated-electron is required in the case of each cation for all cation density range. This is because the electron diffusion coefficient depends on an electron density in TiO2.7,26,27 Therefore, the laser intensity was controlled to obtain the comparable electron density. We have already observed D of mesoporous TiO2-ethanolic electrolyte system.37,38 In this paper, since aprotic solvents such as acetonitrile are generally used as a solvent in electrochemical measurements and in dye-sensitized solar cells, diffusion coefficients are determined as a function of the cation density exclusively in acetonitrile.39

Figure 1. Photocurrent transients induced by pulsed UV irradiation for 7.2 µm thick TiO2 mesoporous electrodes in ethanol with 0.5 M of salts. Salts used are LiClO4 (bold line), NaClO4 (solid line), TBA+ClO4(dotted line), and DMHI+Br- (broken line). Inset shows amounts of photogenerated electron as a function of the pulse intensity in the presence of Li+ (circles), Na+ (squares), TBA+ (triangles), and DMHI+ (diamonds).

Results and Discussion Electron Transport in Mesoporous TiO2-Electrolyte Systems. Laser pulse induced photocurrent transients in mesoporous TiO2 electrodes were performed first in ethanolic electrolyte solution with 0.5 M of various salts under different laser pulse energies. Figure 1 shows the photocurrent transients in the presence of Li+, Na+, TBA+, and DMHI+ under the same pulse intensity (0.98 mJ cm-2).34 The inset in the Figure 1 shows the amount of photogenerated electrons as a function of the pulse intensity in the presence of each cation, where the total amount of the photogenerated charges was obtained by integrating the photocurrent transients. It clearly shows that the maximum of photocurrent and the total photogenerated electron increased in the order of DMHI+ < TBA+ < Na+ < Li+. Figure 2 shows the photocurrent transients in acetonitrileelectrolytes consisting of various density of TBA+ at photoelectron density of n ) 2.1 × 1017 cm-3.39 The shape of photocurrent transients changed in series, and the peak of the photocurrent increased with an increase of TBA+ density at the comparable photogenerated electron density. Figure 3 shows diffusion coefficients (DAmb) as a function of TBA+ density at different electron density (n). Different n gives different DAmb, and it was larger at the higher n. This difference indicates trapfilling effect of electrons.7,23,25,26,40 Ambipolar diffusion mechanism was recently introduced for the electron transport in mesoporous TiO2-electrolyte system.27 Photoinjected electrons in TiO2 were surrounded by an electrolyte consisting of various kinds of ionic species. The ambipolar diffusion coefficient is expressed by

DAmb ) (n + p)/(n/Dp + p/Dn)

(2)

where n and Dn are the negative charge density and diffusion coefficient, respectively, and p and Dp are the positive ones.27 A solid line obtained by fitting DAmb according to eq 2 is shown

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Figure 2. Photocurrent transients for 7.2 µm thick TiO2 mesoporous electrodes in acetonitrile with various concentrations of TBA+ClO4at average electron density n ) 2.1 × 1017 cm-3. From below 0.3, 0.7, 1.0, 2.0, 5.0, 10, 20, 40, 80, 100, 200, 500, and 1000 mM of the cation concentration. Inset shows longer time scale at low concentrations of TBA+ (0.3, 0.7, 1.0, and 2.0 mM from below).

Figure 3. Diffusion coefficients as a function of TBA+ClO4- density in acetonitrile at different electron density n, 2.1 × 1017 cm-3 (circle) and 5.0 × 1016 cm-3 (square). A line is calculated from eq 2 where n ) 2.1 × 1017 cm-3, Dn ) 2.1 × 10-4 cm2 s-1, and Dp ) 2.8 × 10-6 cm2 s-1.

in Figure 3. The fitted curve of DAmb agreed closely with measured DAmb at Dn ) 2.1 × 10-4 cm2 s-1 and Dp ) 2.8 × 10-6 cm2 s-1. The Dp obtained by the curve fitting is close to the limiting cation diffusion coefficient Dp∞ ) 2.32 × 10-6 cm2 s-1 derived from the Nernst’s equation using the limiting molar conductance Λ0 ) 61.63 S cm2 mol-1.41 DAmb as a function of the cation density was also determined for the case of Li+, Na+, Mg2+, and DMHI+, as shown in Figure 4. The DAmb depends on kinds of the cation and increased in the order of DMHI+ > TBA+ > Na+ > Li+ > Mg2+. The values of cation diffusion coefficient Dp, limiting molar conductance (Λ0) and limiting cation diffusion coefficient (Dp∞) are summarized in Table 1. For each cation, Dp calculated at low cation density range agreed with Dp∞ in order. Each DAmb increases with the cation density, but at high cation density above 1020 cm-3, the DAmb did not fit with the calculated curvature using the Dn and the positive charge density p estimated from the cation concentration. These behaviors were in contrast with that in the case of TBA+. From the ambipolar

Figure 4. Diffusion coefficients as a function of cation density of LiClO4 (a), NaClO4 (b), Mg(ClO4)2 (c), and DMHI+ClO4- (d) when n ) 2.1 × 1017 cm-3 (Li+), 1.6 × 1017 cm-3 (Na+), 2.1 × 1017 cm-3 (Mg2+), and 1.8 × 1017 cm-3 (DMHI+) in acetonitrile. Lines are calculated from eq 2 at lower cation density.

diffusion mechanism, DAmb is close to Dp in low cation density conditions, and the values of DAmb in low cation density agreed well with the Dp∞. Thus, we should consider other effects of cation under higher density conditions on electron transport in the TiO2 in addition to the ambipolar diffusion. The limiting molar conductance of each ions in acetonitrile are reported, 103.6 S cm2 mol-1 for ClO4-, 100.7 for Br-, 61.63 for TBA+, 69.77 for Li+, 77.00 for Na+, and 194.2 for Mg2+, suggesting that mobility of ions is not so different from each

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TABLE 1: Average Density of Photogenerated Electron n, Calculated Cation Diffusion Coefficient Dp, Limiting Molar Conductance Λ0, and Limiting Cation Diffusion Coefficient Dp∞ na (×1017 cm3)

DAmbb (×10-4 cm2 s-1)

Dpc (×10-6 cm2 s-1)

Λ0d (S cm2 mol-1)

Dp∞ e (×10-6 cm2 s-1)

TBA+ 2.1 2.0 2.8 61.63 2.32 Li+ 2.0 1.3 3.8 69.77 2.63 1.6 1.1 2.6 77.00 2.89 Na+ 2+ Mg 1.5 0.32 4.6 194.2 1.83 + 1.8 2.2 5.1 DMHI a Obtained by integrating the time transient curve in time. b Measured at the highest cation density. c Calculated at lower cation density range. d In ref 41. e Estimated from limiting molar conductance.

other except Mg2+.41 Accordingly, the relative size of the anion to the cation should hardly affect the electron transport in this study. In fact, the molecular size of DMHI+ is comparable to that of TBA+. The photogenerated charge was adjusted as the constant value for wide range of cation density as shown in Table 1 in accuracy of (20% in all cases. Thus, the anomalous increase of DAmb should be attributed to the nature of the cation. At low cation density range, DAmb is well expressed with the ambipolar diffusion model in all cases. At high-density conditions, however, the effect of adsorption of the cations should be considered. One can expect that adsorption of ions creates the extra trap states on the surface of TiO2; in this case, the electron diffusion should be slowed. Contrary to expectation, the diffusion coefficient increased especially in the case of adsorptive cations, i.e., Li+ and DMHI+ (as discussed at latter section, DMHI+ adsorbed enormously on the TiO2 surface). One possible explanation for the anomalous increase of DAmb might be due to much more increase of the cation concentration on the TiO2 surface than that in the bulk electrolyte. The cations having an adsorptive interaction with TiO2 might affect strongly ambipolar diffusion in the mesoporous TiO2-electrolyte system. This effect needs more investigation. The presence of Mg2+ gave decreased diffusion coefficients, and no reproducible transient response was obtained, especially at the high-density conditions (>2.0 × 1020 cm-3). Recently, it was reported that use of MgI2 as additive in the dye-sensitized solar cells improved the stability against UV light but resulted in the poor performance of the cells.42,43 The decreased electron transport in the presence of Mg2+ may explain the poor performance in the presence of MgI2. Effects of Lithium Ion on Electron Transport in Mesoporous TiO2-Electrolyte Systems. DAmb in the presence of Li+ increased dramatically at the cation density more than 1020 cm-3 and exceeded DAmb in the presence of Na+. This observation supported that in the extremely high Li+ density, a large number of Li+ should be adsorbed strongly on TiO2.15,18,21 In order to study the adsorption (or shallow intercalation) effect on the electron transport, diffusion coefficients were measured by changing Li+ density from the low to the high density and then from the high to the low density using the same electrode. Each measurement was repeated after rinsing the electrode, dried, and left standing for 3 min. DAmb’s measured from the high Li+ density were larger than those measured from the low Li+ density, as shown in Figure 5, giving the DAmb value even in neat acetonitrile, i.e., in the absence of Li+ in the electrolyte. This hysteresis behavior was not observed for the case of TBA+ (Figure 5b). These results support that the adsorption of Li+ in the TiO2 electrodes increase the local cation density at the TiO2 surface. Or the results suggest that the adsorption might form the effective trap sites in the TiO2 and influence the electron transport. We have reported previously that D measured was well interpreted with ambipolar diffusion mechanism for a wide range

Figure 5. Diffusion coefficients as a function of cation density of LiClO4 (a) and TBA+ClO4- (b) measured from the low to the high cation density (circles) and from the high to the low cation density (squares) in acetonitrile.

of Li+ density in ethanolic electrolyte.38 The difference observed in acetonitrile and in ethanol could be explained by the difference in Li+ adsorption behavior. Because Li+ is hard to solvate in aprotic solvents, its adsorption on the TiO2 should occur easily in aprotic solvent like acetonitrile. In fact, it was reported that the flat band potential of TiO2 was independent of the Li+ density in protic solvents because of the selective solvation of Li+ by the protic molecules such as water and ethanol but depended on it in aprotic solvents.15,16 Specific Interaction of Imidazolium Cation on Mesoporous TiO2 Surfaces. Because no specific interaction exists between TBA+ and the TiO2,10,22,44 Dn should be constant in the wide density range of TBA+. Therefore, the change of DAmb in Figure 3 was well expressed with ambipolar diffusion mechanism in the wide range of the examined density. On the other hand, in the case of DMHI+, the largest increase in DAmb was observed at the high cation density conditions (>1020 cm-3). The behavior of the DAmb as a function of the cation density in the case of DMHI+ was similar to that in the case of Li+ rather than TBA+, although DMHI+ is a quaternary ammonium cation. Taking into account the similarity of DAmb in the case of DMHI+ and Li+, the DMHI+ adsorption on the TiO2 was expected. To investigate the possibility of the adsorption of DMHI+, absorption spectra of TBA+Br- and DMHI+Br- in acetonitrile were measured before and after an addition of small amount of dehydrated TiO2

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Figure 6. Absorption spectra of 1.0 M TBA+Br- (bold line) and 0.1 M DMHI+Br- (thin line) in acetonitrile. Broken lines show the centrifuged solutions after the addition of TiO2 (100 mg for 5 mL of TBA+Br- and 10 mg for 5 mL of DMHI+Br-). Inset shows magnified spectra for TBA+Br-.

(P25) to the solutions and centrifugation. Absorption shoulders at the UV region of solid lines in Figure 6 were assigned to the absorption of TBA+ (bold line) and DMHI+ (thin line). After the addition of TiO2 and the centrifugation, the absorbance of DMHI+ in the supernatant solution was decreased (thin broken line), whereas that of TBA+ was not changed (bold broken line). The absorbance change of DMHI+ in the solution was attributed to the effective adsorption of DMHI+ on the TiO2. The amount of the adsorbed DMHI+ on the TiO2 surface was estimated to be ca. 100 molecules/nm2 from the concentration of the solution, the difference in absorbance, and the BET surface area of the TiO2. This value is too large for the monolayer of DMHI+ on the TiO2, suggesting multilayered adsorption of DMHI+. The low yield in photogeneration of electron in the presence of DMHI+ (Figure 1) might be explained as due to the multilayered adsorption of DMHI+ on oxidation sites at the TiO2 surface. Competitive Effects of Lithium and Imidazolium Cations. The adsorption of Li+ in the mesoporous TiO2 seems to promote the photogeneration of charges and to increase the diffusion coefficient to an appreciable extent at the high Li+ density. On the other hand, the multilayered adsorption of DMHI+ leads to much higher increase of the diffusion coefficient but with much less photogeneration of electron. When both cations were added into the electrolyte solution, the mesoporous TiO2-electrolyte systems may be expected to give a large diffusion coefficient followed by the most favorable photogeneration of electrons. Figure 7 shows the amount of photogenerated electron and the diffusion coefficients as a function of the ratio of Li+ to DMHI+ in the total electrolyte concentration of 1.0 M, which is generally employed for the fabrication of the dye-sensitized solar cells. DAmb’s were unchanged in all mixed electrolytes at the same electron density (1.5 × 1017 cm-3), and the electrons were generated almost constantly by the same power laser irradiation when the concentration of Li+ is above 0.7 M (the ratio 7:3), as is seen in Figure 7. The sudden decrease in the amount of photogenerated electron at the Li+ concentration below 0.7 M is apparently due to the adsorption of DMHI+. These observations suggest that the adsorption of Li+ that occurs in preference to the adsorption of DMHI+ should play a decisive role in the charge generation and electron diffusion in the mesoporous TiO2-electrolyte systems. As for the role of the imidazolium cation in dye-sensitized solar cells, we recently revealed that the presence of imidazolium cation contributes to enhancement of conductivity of the I-/I3- electrolyte.9 Conclusions Photocurrent transients observed by a UV pulsed laser irradiation of mesoporous TiO2-electrolyte systems were

Figure 7. Amounts of photogenerated electron and diffusion coefficients as a function of the ratio of LiClO4 to DMHI+Br- (Li+: DMHI+) in the total electrolyte concentration of 1.0 M. Amounts of photogenerated electron are measured under constant light irradiation conditions and diffusion coefficients are measured at n ) 1.5 × 1017 cm-3.

analyzed in the presence of LiClO4, NaClO4, Mg(ClO4)2, TBA+ClO4- and DMHI+Br- in ethanol or in acetonitrile. Diffusion coefficient and the amount of photogenerated electron were evaluated by changing the light intensity (electron density) or the concentration of salts, revealing the following facts. First, the photocurrent can be explained with ambipolar diffusion model in the presence of TBA+ClO4- and at low concentration for the presence of LiClO4, NaClO4, and DMHI+Br- in acetonitrile. Second, at high concentration of LiClO4 or DMHI+Br-, which is comparable to the electrolyte concentration of dye-sensitized solar cell systems, the electron diffusion coefficient increased remarkably in the TiO2-electrolyte systems with the concentration of salts. Third, the increased diffusion coefficient could be explained as due to the adsorption of Li+ and DMHI+ to the TiO2 at high concentration. At last, the adsorption of Li+ enhances the photogeneration of electrons, while the multiadsorption of DMHI+ occur on photooxidation sites at the TiO2 surface, suppressing the photogeneration of electron. When combined with the contribution of DMHI+ to the conductivity of the redox electrolyte, these facts and observations explain well why the composition and concentration of the electrolyte for the dye-sensitized TiO2 solar cells; i.e., high concentration of LiI and imidazolium salts is beneficial for the cell performance. References and Notes (1) Asbury, J. B.; Ellingson, R. J.; Ghosh, H. N.; Ferrere, S.; Nozik, A. J.; Lian, T. J. Phys. Chem. B 1999, 103, 3110. (2) Tachibana, Y.; Moser, J. E.; Gra¨tzel, M.; Klug, D. R.; Durrant, J. R. J. Phys. Chem. 1996, 100, 20056. (3) Tachibana, Y.; Haque, S. A.; Mercer, I. P.; Durrant, J. R.; Klug, D. R. J. Phys. Chem. B 2000, 104, 1198. (4) Hannappel, T.; Burfeindet, B.; Storck, W.; Willig, F. J. Phys. Chem. B 1997, 101, 6799. (5) Nazeeruddin, M. K.; Kay, A.; Rodicio, I.; Humphry, B. R.; Mueller, E.; Liska, P.; Vlachopoulos, N.; Gra¨tzel, M. J. Am. Chem. Soc. 1993, 115, 6382. (6) Huang, S. Y.; Schlichthorl, G.; Nozik, A. J.; Gra¨tzel, M.; Frank, A. J. J. Phys. Chem. B 1997, 101, 2576.

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Kambe et al. (33) Nazeeruddin, M. K.; Pe′chy, P.; Renouard, T.; Zakeeruddin, S. M.; Humphry-Baker, R.; Comte, P.; Liska, P.; Cevey, L.; Costa, E.; Shklover, V.; Spiccia, L.; Deacon, G. B.; Bignozzi, C. A.; Gra¨tzel, M. J. Am. Chem. Soc. 2001, 123, 1613. (34) The photocharges should be effectively formed on TiO2 by the perfect current doubling of perchlorate anion as far as perchlorate salts (Li+, Na+, TBA+) are used as electrolytes in ethanol [Morrison, S. R. Electrochemistry at Semiconductor and Oxidized Metal Electrodes; Plenum Press: New York, 1980; p 213]. However, even when taking into account the current doubling effect, more than 10 times smaller amount of photocharge in the presence of DMHI+ as bromide than in the presence of LiClO4 is worth noting. (35) Kambe, S.; Murakoshi, K.; Kitamura, T.; wada, Y.; Yanagida, S.; Kominami, H.; Kera, Y. Solar Energy Mater., Solar Cells 2000, 61, 427. (36) Solbrand, A.; Lindstrom, H.; Rensmo, H.; Hagfeldt, A.; Lindquist, S.; Sodergren, S. J. Phys. Chem. B 1997, 101, 2514. (37) Kambe, S.; Nakade, S.; Wada, Y.; Kitamura, T.; Yanagida, S. J. Mater. Chem., in press. (38) Nakade, S.; Kambe, S.; Kitamura, T.; Wada, Y.; Yanagida, S. J. Phys. Chem. B 2001, 105, 9150. (39) When acetonitrile is employed as a solvent, water in acetonitrile should be a main hole scavenger. Oxidative hole trapping of water gives H-OH•+, which fragments rapidly to a surface-bound hydroxy radical and an adsorbed proton [Fox, M. A.; Dulay, M. T. Chem. ReV. 1993, 93, 3, 341]. Even if photooxidation of acetonitrile itself occur, it will not strongly affect the result. Because the amount of oxidized acetonitrile is on the order of only about 1 pmol per one shot, when all generated electrons are from oxidation of acetonitrile. We also confirmed the reproducibility of the amount of photogenerated electrons during the continuous laser irradiation for several times, which indicates that the effect of the photooxidated products on electron generation is negligible. (40) There is little electron acceptor in the system since the system is filled with an inert gas (Ar gas). In fact, no significant photovoltage decay after the pulse UV irradiation was observed in ms time scale under the open circuit conditions [ref 38]. Therefore, the effect of charge recombination on DAmb is negligible. (41) Berthel, J.; Iberl, L.; Rossmaier, J.; Gores, H. J.; Kaukal, B. J. Solut. Chem. 1990, 19, 321. (42) Meyer, T. B.; Meyer, A. F.; Ginestoux, D. Proc. SPIE 2001, 4108, 8. (43) Hinsch, A.; Kroon, J. M.; Spa¨th, M.; van Roosmalen, J. A. M.; Bakker, N. J.; Sommeling, P.; van der Burg, N.; Kinderman, R.; Kern, R.; Ferber, J.; Schill, C.; Schubert, M.; Meyer, A.; Meyer, T.; Uhlendorf, I.; Holzbock, J.; Niepmann, R. Proc. 16th PV Sol. En. Conference, Glasgow 2000. (44) Cahen, D.; Hodes, G.; Gra¨tzel, M.; Guillemoles, J. F.; Riess, I. J. Phys. Chem. B 2000, 104, 2053.