Photoelectrochemical Effects of Guanidinium Thiocyanate on Dye

J. Phys. Chem. C 2009, 113, 21779–21783

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Photoelectrochemical Effects of Guanidinium Thiocyanate on Dye-Sensitized Solar Cell Performance and Stability Changneng Zhang, Yang Huang, Zhipeng Huo, Shuanghong Chen, and Songyuan Dai* Key Laboratory of NoVel Thin Film Solar Cells, Institute of Plasma Physics, Chinese Academy of Sciences, P.O. Box 1126, Hefei, Anhui 230031, People’s Republic of China ReceiVed: August 5, 2009; ReVised Manuscript ReceiVed: NoVember 16, 2009

In this study, we investigated the photoelectrochemical effect of guanidinium thiocyanate (GuSCN) in the base electrolyte composed of 1-methylbenzimidazole (0.45 M) and 3-methoxypropionitrile on the efficiency of electron injection (Φinj), interfacial recombination kinetics, and photovoltaic performance of dye-sensitized solar cells (DSCs). A significant increase in the photocurrent for DSCs with GuSCN was observed, which was higher than that for DSCs with the base electrolyte. The dependence of the short-circuit photocurrent density on the illumination intensity indicated that the large increase in Φinjcould be attributed to the positive shift in the flatband potential of the TiO2 electrode and could increase the electron injection yield. The results from electrochemical impedance spectroscopy (EIS) for DSCs indicated that guanidinium cation chemisorbed on the TiO2 surface could passivate the surface recombination sites and enhance the electron lifetime in the nanostructured TiO2 film to give an improved open-circuit photovoltage. The photostability of DSCs with 0.1 M GuSCN could retain over 98% of its initial photoelectric conversion efficiency value under one sun light soaking over the time of 3000 h. It is indicated that GuSCN chemisorbed on TiO2 surface could keep the interface of DSCs stable. 1. Introduction Dye-sensitized solar cells (DSCs) based on a nanoporous TiO2 electrode sensitized to visible light with dye sensitizer have been extensively studied because of their high photon-to-electron conversion efficiency, low manufacturing cost, and high-stability semiconductor and sensitizer.1-11 A DSC is composed of a dyed TiO2 electrode, a platinized counter electrode, and an electrolyte between the two electrodes behaving as a conducting medium with I-/I3- redox couple. The adsorbed dye molecules become excited under the irradiation of visible light and inject electrons into the conduction band of the semiconductor. Then, the oxidized dye molecules are reduced by I- ions. The effective charger transport in the electrolyte plays an important role in the regeneration of the oxidized dye to achieve high efficiency for DSCs. For practical application of DSCs, a large research and development effort on electrolyte composition has been undertaken to improve the efficiency and long-term stability. Many studies have found that the use of ionic liquids and quasisolid-state electrolytes could improve DSC stability.12-14 However, research on the influence of cations in the electrolyte on the solar cell stability is very sparse. Gra¨tzel and co-workers reported that Li+ ion chemisorbed on the TiO2 surface positively shifts the energetics of the TiO2 conduction band with an increase of Li+ concentration in the electrolyte.15 Meyers and co-workers investigated the influence of Li+ ions on the photoelectron injection yields from the excited dye into the TiO2 conduction band and found that the regeneration rate of the oxidized dye molecules depends on the nature and concentration of the cations in the electrolyte.16,17 But inorganic cations such as Li+ could intercalate into the TiO2 crystal lattice under normal solar light intensities, resulting in a drop in cell performance.18 * Corresponding author. Tel: +86-551-5591377; fax: +86-551-5591377; e-mail: [email protected].

A recent study has found that GuSCN could improve the solar cell performance by slowing the surface recombination.3,4,19-23 Several mechanisms have been proposed to explain the effects of guanidinium cation in DSCs. Gra¨tzel reported that the addition of guanidinium cation to the electrolyte could control the self-assembly of the N3 dye at the TiO2 interface and suppress the dark current.4 Frank and his co-workers investigated the effect of the adsorbent guanidinium cation on the recombination rate and band-edge movement measured by the transient photovoltage.23 They found that a positive shift in the TiO2 conduction band and slower recombination produced a net improvement in the photovoltage of about 20 mV for DSCs with 0.1 M guanidinium thiocyanate in the electrolyte. GuSCN does not intercalate into TiO2 and is expected to replace inorganic iodide such as LiI. But there is no detailed study about GuSCN effect on the efficiency of electron injection in the nanostructured TiO2 electrode and the long-term stability of DSCs. In our work, we focus on the effect of GuSCN in the electrolyte on the efficiency of electron injection (Φinj) and interfacial recombination kinetics of DSCs. The photovoltaic performance and photostability of DSCs after the addition of GuSCN to the electrolyte were also discussed. 2. Experimental Section Materials. 3-Methoxypropionitrile (MePN) and iodine were purchased from Fluka. Guanidinium thiocyanate (Figure 1) and 1-methylbenzimidazole (MBI) were obtained from Aldrich. 1,2Dimethyl-3-propylimidazolium iodide (DMPII) were synthesized as reported previously.24 Four different electrolytes were employed for DSCs as shown in Table 1. DSC Assembly. DSCs were assembled in a sandwich configuration. The colloidal TiO2 nanoparticles were prepared by hydrolysis of titanium tetraisopropoxide as described

10.1021/jp909732f  2009 American Chemical Society Published on Web 12/10/2009

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Figure 1. Structure of guanidinium thiocyanate (GuSCN) used in this work.

TABLE 1: Composition and Concentration of Various Electrolytes Used in This Study electrolyte

composition and concentration

A

0.45 M 1-methylbenzimidazole (MBI), 0.6 M 1,2-dimethyl-3-propylimidazolium iodide (DMPII), 0.1 M I2 in MePN 0.1 M guanidinium thiocyanate (GuSCN), 0.45 M MBI, 0.6 M DMPII, 0.1 M I2 in MePN 0.2 M GuSCN, 0.45 M MBI, 0.6 M DMPII, 0.1 M I2 in MePN 0.4 M GuSCN, 0.45 M MBI, 0.6 M DMPII, 0.1 M I2 in MePN

B C D

elsewhere.24,25 Nanocrystalline electrodes of about 10 µm in thickness were immersed in an ethanol solution (0.5 mM) of dye N719 [cis-dithiocyanate-N,N′-bis-(4-carboxylate-4′-tetrabutylammonium carboxylate-2,2′-bipyridine)ruthenium(II)] at room temperature for 12 h. The platinized counter electrode was placed directly on top of the dyed TiO2 film sealed with a 60 µm thermal adhesive film (Surlyn, Dupont). The electrolyte was filled from a hole made on the counter electrode, which was sealed with a Surlyn adhesive film under a cover glass by heating. Methods. The photocurrent-photovoltage (J-V) characteristics of DSCs with an active area of 4.0 cm2 were measured under an illumination of AM 1.5 (100 mW cm-2) which was realized on a solar simulator (Changchun Institute of Optics Fine Mechanics and Physics, Chinese Academy of Science, calibrated with standard crystalline silicon solar cell; spectral mismatch was not considered) with a Keithley 2420 source meter. During a successive one sun light soaking experiment, the solar cells covered with a 50 µm thick polymer film as a UV cutoff filter (up to 394 nm) were irradiated at open circuit under a 450 W xenon lamp (XQ3000, 100 mW/cm2, Shanghai B.R. Science Instrument Co., Ltd, China), the air temperature was set to approximately 50 °C, and the temperature of the devices during testing and stress under illumination was continuous. Impedance measurements were carried out with an IM6ex electrochemical workstation (Zahner-Elektrick, Germany) in the frequency range of 20 mHz to 1000 kHz at room temperature. The working electrode was a dyed TiO2 electrode of the DSC, and the auxiliary electrode and the reference electrode were a platinized counter electrode of the DSC. The amplitude of the alternative signal was 5 mV. 3. Results and Discussion Photovoltaic Performance of DSCs. Figure 2 presents the J-V curves of DSCs with the base electrolyte containing different contents of GuSCN under 100 mW · cm-2 illumination. It is observed that the addition of 0.1 M GuSCN to the electrolyte can improve the Jsc and Voc and slightly decrease the FF, which achieved a higher photovoltaic performance than that of solar cells with base electrolyte. The photovoltaic performance parameters of the solar cells were not influenced by the addition of GuSCN at concentrations from 0.1 to 0.4 M (Figure 2a and 2b). In DSCs, MBI chemisorbed on the TiO2 surface could raise

Figure 2. J-V characteristics (a) and photovoltaic performance parameters (b) of the Ru(dcbpy)2(SCN)2-sensitized solar cells with different GuSCN concentrations in the base electrolyte composed of 0.45 M 1-methylbenzimidazole, 0.6 M DMPII, and 0.1 M I2 in MePN under 100 mW cm-2 simulated sunlight.

Figure 3. Dependence of short-circuit current on illumination intensity for DSCs with various concentrations of GuSCN in the base electrolyte composed of 0.45 M 1-methylbenzimidazole, 0.6 M DMPII, and 0.1 M I2 in MePN. Insert: The slope k as a function of [GuSCN].

the conduction band edge of TiO2, but guanidinium chemisorbed on the TiO2 surface led to the shift in the conduction band edge of TiO2 to positive potentials, which increased the electron injection rate. As a result, the Jsc of the cells was obviously improved after the addition of GuSCN in the electrolyte. The increased Voc resulted from guanidinium cation chemisorbed on the TiO2 surface to decrease the charge recombination compared to DSCs with the base electrolyte. Effect of GuSCN on the Photoinjected Electrons from Dye to TiO2. Figure 3 shows the photocurrent density at short circuit (Jsc) of DSCs with guanidinium cations in the electrolytes varied in direct proportion to the illumination intensity (I). In the short circuit, the electron recombination in the dye-sensitized solar cells is generally considered to be negligible. The observed Jsc is thus given by26

Jsc ) qAφI ) kI

(1)

where q is the electron charge, A is a constant and depends on the light harvesting efficiency of dye N719 at each wavelength,

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Figure 4. Transmission line model of the DSC used to fit the EIS experimental data. rct, RPt, and RFTO: charge-transfer resistance at the dyed TiO2/electrolyte, the electrolyte/Pt interface, and the TiO2/ electrolyte interface, respectively. Cµ, CPt, and CFTO: the chemical capacitance at the dyed TiO2/electrolyte, the electrolyte/Pt interface, and the TiO2/electrolyte interface, respectively. rt is the transport resistance of the electrons in the TiO2 film. Zd(sol) is the Warburg element showing the Nernst diffusion of I3- in the electrolyte. Rs is the series resistance, including the sheet resistance of the FTO glass and the contact resistance of the cell.

φ corresponds to the absorbed photon-to-current conversion efficiency at each wavelength under solar illumination, and the slope k ) qAφ. From Figure 3, it is clearly indicated that the slope k dependence on the illumination intensity I increased with the addition of GuSCN and cannot increase with the increase of guanidinium cation concentration compared to the base electrolyte. The k depended on the φ value for the DSC and was found to indicate the efficiency of electron injection (Φinj).26 The efficiency of electron injection (Φinj) increased with the addition of GuSCN and was not significantly affected with the further increase in concentration of GuSCN. Recently, Frank et al. investigated the effect of guanidinium cations on the photoinduced charge density at the open circuit of dye-sensitized solar cells and found that guanidinium cations could control the surface properties of the TiO2 electrode, resulting in the positive shift in Vfb of the TiO2 electrode.23 However, the addition of the adsorbent MBI to the electrolyte led to a significant upward shift of Vfb of the TiO2 electrode.25 The electron injection yield is strongly dependent upon the shifts of the TiO2 conduction band energetics.27 It was suggested that the large increase in Φinj could be attributed to the positive shift in Vfb after the addition of GuSCN to the electrolytes and that the increase in electron injection efficiency yields a higher Jsc. Interface Recombination under the Applied Potentials. Impedance spectroscopy of DSCs measured in the dark is a wellknown technique to study the electron recombination from the conduction band of TiO2 to I3- ions.9,28,29 The equivalent circuit is shown as the transmission line model in Figure 4. This equivalent circuit indicates that the injected electrons in the TiO2 conduction band can transport through TiO2 with a resistance Rt or transfer to the I3- ions at the dyed TiO2/electrolyte interface.28,29 In DSCs, the electron transport resistance through the semiconductor is defined as Rt ) rtd (d is the thickness of the TiO2 film, and rt is the transport resistance of the electrons in the TiO2 film) and the interfacial charge recombination reaction of conduction band electrons with the I3- ions can be described by a charge transfer resistance, Rct () rct/d, and rct is the charge-transfer resistance of the charge recombination between the TiO2 conduction band electrons and I3- in the electrolyte) and a chemical capacitance, Cµ () cµd), respectively.9,28 Regeneration of I3- at the counter electrode is characterized by the charge transfer resistance, RPt, and the double-layer capaci-

Figure 5. Bode plots (a) and Nyquist plots (b) for the corresponding DSC with various concentrations of GuSCN in the dark at an applied forward bias of -0.71 V: the symbols show the experimental data; the lines show the experimental fitting.

tance, CPt, at the counter electrode/electrolyte interface, and the interfacial charge recombination resistance, RFTO, and the chemical capacitance, CFTO, at the FTO/TiO2/electrolyte interface. The ZView software was used to fit the Nyquist experimental data of DSCs with the addition of GuSCN measured at a forward bias of -0.71 V as shown in Figure 5b. From the fitted data, the electron lifetime τ (τ ) RctCµ), and the effective diffusion coefficient of electrons in the TiO2 semiconductor Dn (Dn ) d2/RtCµ) can be obtained.9,28 Figure 5 presents some useful parameters to understand the underlying mechanisms for DSCs with the addition of GuSCN to the electrolyte. As shown in Figure 5a, the high-frequency peak position for the DSC with GuSCN in the electrolyte was slightly shifted to lower frequencies. This indicated that the higher charge-transfer resistance RPt at the counter electrode is obtained for DSCs with GuSCN (in Figure 5b). RPt increased from 1.52 Ω to 2.09 Ω by the addition of 0.1 M GuSCN and slightly decreased to 1.84Ω with increase of the GuSCN concentration up to 0.4 M. The double layer capacitance at the counter electrode CPt increased with the addition of 0.1 M GuSCN and started to saturate and changed slightly with increase of GuSCN concentration in the electrolyte (in Figure 6a). Apparently, the increase of GuSCN concentration had little influence on the electron exchange reaction at the Pt/electrolyte interface of the DSC. From the electrochemical impedance spectroscopy (EIS) resonance frequency of the middle frequency peak position shown in Figure 5a, the lower frequencies for DSCs with GuSCN in the electrolyte, compared to those without GuSCN, revealed that the transfer time of the electrons in the nanostructured TiO2 film is indeed prolonged after the addition of GuSCN to the electrolyte. The increased electron lifetime is mainly ascribed to a major increase in Rct (in Figure 6b) for the sample with GuSCN compared to DSCs with the base electrolyte. The

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Figure 7. Photovoltaic parameter evolution with aging time for DSCs in the presence of 0.1 M GuSCN under one sun light soaking.

Rt decreased slightly from 1.74 Ω to 1.29 Ω after the addition of GuSCN to the electrolytes, and increasing the GuSCN concentration from 0.1 to 0.4 M was found to have no influence on the Rt as shown in Figure 6b. From eq 2, it was found that the slightly decreased Rt for the DSC with GuSCN was probably associated with the surface adsorption of guanidinium cation, resulting from the downward shift of the conduction band in TiO2. From the values of Rt and Cµ, the Dn of electrons within the network of TiO2 nanoparticles was calculated and showed (Figure 6c) little change with increasing GuSCN concentration, as reported in Frank’s work on photovoltage transient measurements.23 The transport time τc can be estimated from the diffusion coefficient D with the equation18,30

τc )

Figure 6. Characteristic parameters of the ac impedance spectra of DSCs with various concentrations of GuSCN in the dark at an applied forward bias of -0.71 V.

chemical capacitance Cµ increased from 9.21 to 9.81 mF and the electron transfer resistance Rct increased from 3.67 Ω to 6.37 Ω, yielding a lower electron recombination lifetime (τ) of 34 ms for DSCs without GuSCN electrolyte compared to that of GuSCN electrolyte, i.e., 62 ms (in Figure 6c). τ values (recombination lifetime) slightly increased with the increasing of the amount of GuSCN. As a very weakly acidic, guanidinium cation chemisorbed with oxygen on the surface of the nanostructured TiO2 electrode through the central carbon atom. As a result, GuSCN could decrease the electron recombination at the dyed TiO2/electrolyte interface by passivating surface recombination sites in the DSC. In DSCs, the electron diffusion resistance, Rt, in the TiO2 film was smaller than the electron transfer resistance, Rct, and Rt is determined by the following equation:28

(

Rt ) R0 exp -

Efb - Ecb kT

)

(2)

where R0 is an approximate constant for the DSC, Efb the position of the Fermi level in the TiO2 semiconductor, Ecb the position of the lower edge of the conduction band of the TiO2 electrode, T the temperature, and k the Boltzmann constant. The

d2 2.35D

(3)

where d is the TiO2 film thickness. From eq 3, it was found that the addition of GuSCN to the electrolyte had little influence on the transport time, τc, in the nanostructured TiO2 film. Photostability Tests. Figure 7 shows the photovoltaic parameter evolution during long-term accelerated aging tests under one sun light soaking with DSCs containing 0.1 M GuSCN electrolyte. After the first week of aging, a slight increase in Jsc and FF led to the moderately enhanced efficiency η. The efficiency η retaining 98% of its initial value after 3000 h is mainly due to the fact that the decrease of 40-50 mV in Voc was well compensated by the increase in Jsc and FF. At the end of the light soaking test, the Jsc and FF were even larger than the initial value. In order to study the photoelectrochemical effect of GuSCN on the photovoltaic parameter evolution with aging time for DSCs, EIS was applied to investigate the interface performance. Figure 8 shows the Bode polts and Nyquist plots of DSCs with 0.1 M GuSCN measured at a forward bias of -0.71 V in the dark. It is indicated that the middle frequency peak position slightly shifts to higher frequency (Figure 8), revealing a decrease in the electron recombination time. The chemical capacitance Cµ at the dyed TiO2/electrolyte interface retained a rather constant value of about 5.1 mF, and the electron transfer resistance Rct decreased from 8.47 Ω to 5.42 Ω, yielding a decreased electron recombination lifetime from 43.2 to 28.7 ms. The decrease of electron lifetime explains the drop of Voc observed upon aging the cells over the time of 3000 h under one sun light soaking. The position of the high frequency peak corresponding to the charge transfer reaction at Pt electrode was slightly shifted to higher frequencies after aging tests, indicating an increase in fill factor for the DSCs and a better stability of the Pt/electrolyte interface. A stability test of DSCs for a longer

Dye-Sensitized Solar Cell Performance and Stability

J. Phys. Chem. C, Vol. 113, No. 52, 2009 21783 Research and Development Program of China (Grant No. 2009AA050603), and Foundation of the Chinese Academy of Sciences (Grant No. KGCX2-YW-326) are greatly appreciated for financial support. References and Notes

Figure 8. Bode plots and Nyquist plots for DSCs with aging time for DSCs in the presence of 0.1 M GuSCN under one sun light soaking, measured at a forward bias of -0.71 V in the dark.

period under thermal stress and detailed degradation studies are now in progress. 4. Conclusions In summary, the influence of GuSCN on the photoelectron injection efficiency and interface dynamics of electron transfer/ transport in DSCs was studied. Analysis of Jsc vs the light intensity for DSCs indicated that the high efficiency of electron injection (Φinj) for DSCs with GuSCN in the electrolyte could be obtained in this work. The increase of GuSCN concentration from 0.1 to 0.4 M had no significant effect on the efficiency of electron injection (Φinj). Jsc is obviously improved after the addition of GuSCN to the electrolyte, due to the positive shift in Vfb of the TiO2 electrode resulting in the increased Φinj. From impedance spectra of the solar cells, it can be concluded that the addition of GuSCN in electrolytes could decrease the interfacial recombination reaction on the TiO2 electrode. It is indicated that guanidinium cation could suppress the surface recombination and shift the conduction band to positive potentials. The accelerated aging tests showed that DSCs with 0.1 M GuSCN could retain over 98% of its initial photoelectric conversion efficiency value under one sun light soaking over the time of 3000 h. Acknowledgment. National Basic Research Program of China (Grant No. 2006CB202600), National High Technology

(1) O’Regan, B.; Gra¨tzel, M. Nature 1991, 353, 737–740. (2) Nazeeruddin, M. K.; Kay, A.; Rodicio, I.; Baker, R. H.; Miiller, E.; Liska, P.; Vlachopoulos, N.; Gra¨tzel, M. J. Am. Soc. Chem. 1993, 115, 6382–6390. (3) Huang, S. Y.; Schlichthorl, G.; Nozik, A. J.; Gratzel, M.; Frank, A. J. J. Phys. Chem. B 1997, 101, 2576–2582. (4) Gratzel, M. J. Photochem. Photobiol., A 2004, 164, 3–14. (5) Gratzel, M. J. Photochem. Photobiol., A 2003, 4, 145–1153. (6) Nazeeruddin, M. K.; Angelis, F. D.; Fantacci, S.; Selloni, A.; Viscardi, G.; Liska, P.; Ito, S.; Takeru, B.; Gra¨tzel, M. J. Am. Chem. Soc. 2005, 127, 16835–16847. (7) Chiba, Y.; Islam, A.; Watanabe, Y.; Komiya, R.; Koide, N.; Han, L. Y. Jpn. J. Appl. Phys. 2006, 45, L638–L640. (8) Wang, Z. S.; Yanagida, M.; Sayama, K.; Sugihara, H. Chem. Mater. 2006, 18, 2912–2916. (9) Wang, Q.; Ito, S.; Gra¨tzel, M.; Fabregat-Santiago, F.; Mora-Sero, I.; Bisquert, J.; Bessho, T.; Imail, H. J. Phys.Chem. B 2006, 110, 25210– 25221. (10) Ito, S.; Murakami, T. N.; Comte, P.; Liska, P.; Gratzel, Mohammad, C.; Nazeeruddin, K.; Gra¨tzel, M. Thin Solid Films 2008, 516, 4613–4619. (11) 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, 10720–10728. (12) Lu, S.; Koeppe, R.; Gunes, S.; Sariciftci, N. S. Sol. Energy Mater. Sol. Cells 2007, 91, 1081–1086. (13) Kuang, D.; Wang, P.; Ito, S.; Zakeeruddin, S.-M.; Gra¨tzel, M. J. Am. Chem. Soc. 2006, 128, 7732–7733. (14) Biancardo, M.; West, K.; Krebs, F. C. Sol. Energy Mater. Sol. Cells 2006, 90, 2575–2588. (15) Pelet, S.; Moser, J.-E.; Gra¨tzel, M. J. Phys. Chem. B 2000, 104, 1791–1795. (16) Kelly, C. A.; Farzad, F.; Thompson, D. W.; Stipkala, J. M.; Meyer, G. J. Langmuir 1999, 15, 7047–7054. (17) Watson, D. F.; Meyer, G. J. Coord. Chem. ReV. 2004, 248, 1391– 1406. (18) Kopidakis, N.; Benkstein, K. D.; Lagemaat, J.; Frank, A. J. J. Phys. Chem. B 2003, 107, 11307–11315. (19) Zhang, Z.; Zakeeruddin, S. M.; O’Regan, B. C.; Humphry-Baker, R.; Gra¨tzel, M. J. Phys. Chem. B 2005, 109, 21818–21824. (20) Wang, P.; Zakeeruddin, S. M.; Humphry-Baker, R.; Gra¨tzel, M. Chem. Mater. 2004, 6, 2694–2696. (21) Wang, P.; Zakeeruddin, S. M.; Moser, J. E.; Humphry-Baker, R.; Gra¨tzel, M. J. Am. Chem. Soc. 2004, 126, 7164–7165. (22) Wang, P.; Humphry-Baker, R.; Moser, J.-E.; Zakeeruddin, S. M.; Gra¨tzel, M. Chem. Mater. 2004, 16, 3246–3251. (23) Kopidakis, N.; Neale, N. R.; Frank, A. J. J. Phys. Chem. B 2006, 110, 12485–2489. (24) Zhang, C.; Huo, Z.; Huang, Y.; Guo, L.; Sui, Y.; Hu, L.; Kong, F.; Pan, X.; Dai, S.; Wang, K. J. Electroanal. Chem. 2009, 632, 133–138. (25) Zhang, C.; Dai, J.; Huo, Z.; Pan, X.; Hu, L.; Kong, F.; Huang, Y.; Sui, Y.; Fang, X.; Wang, K.; Dai, S. Electrochim. Acta 2008, 53, 5503– 5508. (26) Yanagida, M.; Yamaguchi, T.; Kurashige, M.; Hara, K.; Katoh, R.; Sugihara, H.; Arakawa, H. Inorg. Chem. 2003, 42, 7921–7931. (27) Koops, S. E.; O’Regan, B. C.; Barnes, P. R. F.; Durrant, J. R. J. Am. Chem. Soc. 2009, 131, 4808–4818. (28) Fabregat-Santiago, F.; Bisquert, J.; Garcia-Belmonte, G.; Boschloo, G.; Hagfeldt, A. Sol. Energy Mater. Sol. Cells 2005, 87, 117–131. (29) Fabregat-Santiago, F.; Garcia-Belmonte, G.; Bisquert, J.; Zaban, A.; Salvador, P. J. Phys. Chem. B 2002, 106, 334–339. (30) Nakade, S.; Saito, Y.; Kubo, W.; Kitamura, T.; Wada, Y.; Yanagida, S. J. Phys. Chem. B 2003, 107, 8607–8611.

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