ARTICLE pubs.acs.org/JPCC
Preventing Dye Aggregation on ZnO by Adding Water in the Dye-Sensitization Process Rebecka Sch€olin,† María Quintana,‡ Erik M. J. Johansson,§ Maria Hahlin,† Tannia Marinado,‡ Anders Hagfeldt,§ and Hakan Rensmo*,† †
Department of Physics and Astronomy, Molecular and Condensed Matter Physics, Uppsala University, Box 516, SE-751 20 Uppsala, Sweden Inorganic Chemistry, Center of Molecular Devices, Chemical Science and Engineering, Royal Institute of Technology, SE-100 44 Stockholm, Sweden § Department of Physical and Analytical Chemistry, Uppsala University, Box 259, SE-751 05 Uppsala, Sweden ‡
ABSTRACT: ZnO based dye-sensitized solar cells have been studied using N719 and Z-907 as sensitizing dyes, with and without including water to the dye solution. The solar cells have been characterized with photoelectric measurements and the interface between the dye and the ZnO surface has been studied using photoelectron spectroscopy. It was shown that water in the dye solution greatly reduces surface dye aggregation and thereby enhances the solar cell performance for N719. For Z-907 where no sign of dye aggregation could be found, the presence of water had minor effect on the surface structure and solar cell performance.
’ INTRODUCTION Dye-sensitized solar cells are widely studied as an alternative to conventional solar cells.1 A mesoporous film of a semiconductor sensitized with a dye together with an electrolyte and a back contact forms the complete solar cell. The most common semiconductor material is TiO2, but others are also of interest, where ZnO is one of the best candidates. A similar energy position of the conduction band and a similar band gap as TiO22 has been the principal motivation. Even though ZnO possess higher electron mobility and larger flexibility concerning different structural morphologies3 (i.e., nano particles, nano rods, nano wires) it is still not performing as good as TiO2 in dye senstitized solar cells. Several dyes have been used for sensitized ZnO electrodes, e.g., eosin Y,4,5 mercurochrome,6 squaraine,7 organic,8 and Ruthenium dyes, N719 being a common dye for ZnO dye-sensitized solar cells.3,9 Earlier studies using Ru-dyes and ZnO show a decrease in solar cell performance with increasing immersing time of the ZnO electrodes in the dye bath for ruthenium dyes.1014 According to these studies, the dye sensitization process on ZnO electrodes entails a small amount of Zn2+ ions dissolving into the dye bath, with a subsequent surface aggregation of the dye and Zn2+ ions occurring. Formation of aggregates on ZnO has also been observed for organic dyes15 and indoline dyes.16 In general, when such aggregates are present on the working electrode, the cell performance is low. Hence, to understand how dye aggregation takes place in the sensitization process, and learn how to avoid it, are crucial aspects to achieve efficient ZnO solar cells. Approaches to avoid dye aggregation have been the addition of coadsorbents in the dye bath17 or modification of the dye molecule.18,19 r 2011 American Chemical Society
In this paper we study the dependence of water for dye aggregation on ZnO electrodes using the ruthenium dyes N719 and Z-907, the first being soluble in water while the latter is not. Earlier studies of TiO2 shows that water can affect the bonding behavior to the mesoporous electrode20 and also the solar cell performance.2124 For ZnO, it has been showed that water affect the growth rate and the crystallinity of the nanocrystals.25 Some basic measurement techniques has been used in this study to characterize the function of the solar cells, e.g., incident photon to current conversion efficiency (IPCE), current voltage (IV), absorption measurements, and intensity modulated photocurrent and photovoltage spectroscopy. However, the main focus of the characterization is effects on surface electronic and molecular structures. The work therefore contains a more detailed study of the electronic energy levels and the molecular orientation relative to the oxide surface, using photoelectron spectroscopy (PES) techniques.
’ EXPERIMENTAL SECTION Preparation of ZnO Films. ZnO films were prepared from a ZnO colloid with a ZnO particle size of 20 nm, presented in earlier publications.26,27 Transparent nanostructured ZnO electrodes were obtained by depositing the colloid onto conducting glass substrates (TEC8, Pilkington) by doctor blading, followed by heating in oven at 400 °C for 1 h. Film thickness was determined by profilometry. To match the requirements for Received: June 27, 2011 Revised: August 23, 2011 Published: August 29, 2011 19274
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The Journal of Physical Chemistry C the photoelectron spectroscopy measurements, the ZnO films had to be limited in thickness and were made 3.5 μm thick. Dye Sensitization and Solar Cell Assembly. The ZnO electrodes were left overnight (15 h) in dye baths consisting of 0.5 mM cis-diisothiocyanato-bis(2,20 -bipyridyl-4,40 -dicarboxylato) ruthenium(II) bis(tetrabutylammonium) (N719) solved in ethanol or ethanol/water at a volume ratio of 1:1 and 0.15 mM cis-disothiocyanato-(2,20 -bipyridyl-4,40 -dicarboxylic acid)-(2,20 bipyridyl-4,40 -dinonyl) ruthenium(II) (Z-907) solved in acetonitrile/tertbutanol at a volume ration of 1:1 or acetonitrile/ tertbutanol/water at a volume ratio of 1:1:2. The dyes were purchased from Solaronix. The dyed electrodes were rinsed with ethanol and dried in air. Absorption measurements were performed before the cells were assembled with a platinized conducting glass counter electrode using a 50 μm thick thermoplastic frame (Surlyn 1702). The electrolytes were inserted into the cell through holes in the counter electrode and the electrolyte composition was as follows: 0.1 M LiI, 0.6 M tetrabutyl ammonium iodide (TBAI), 0.5 M 4-tert butylpyridine (4-TBP), and 0.1 M I2 in 3-methoxypropionitrile (MPN). The complete cell was then sealed with thermoplastic (Surlyn 1702). For photoelectron spectroscopy analysis, a shorter sensitization time of 10 min was also used for comparison. The dye concentrations and solvents were the same as described above except that a dye concentration of 0.3 mM were used for Z-907 for short sensitization time and for longer time when no water was included in the solution. The dye coverage of Z-907 will be shown not to be affected by this difference in concentration. Characterization Methods. The setups for recording incident photon to current conversion efficiency (IPCE) spectra and IV curves under simulated sunlight consist of a Keithley 2400 source/meter as a current meter and a xenon arc lamp (300 W Cermax, ILC Technology), followed by a 1/8 m monochromator (CVI Digikrom CM 110) as a light source, and has been described in more detail elsewhere.2 The absorbance spectra of the dye-sensitized semiconductor films were measured with an absorption spectrometer (Perkin-Elmer, Lambda 750). Correction for scattering was made by subtracting the absorbance of the unsensitized electrodes. The electronic transport in the ZnO solar cells was studied, using intensity modulated white light LED and measuring the photovoltage or the photocurrent response. The method has been described in more detail earlier.28 Photoelectron spectroscopy measurements were carried out on the sensitized ZnO electrodes at beamline I411 at the Swedish national synchrotron facility MAX-lab, in Lund.29 The beamline was equipped with a Scienta R4000 WAL electron analyzer. The electron take off angle was 70°, and the electron take off direction was collinear with the e-vector of the incident photon beam. Quantitative measurements of the amounts of elements at the surface was performed with the in house ESCA300 spectrometer which is using Al Kα X-rays (1487 eV) and is calibrated for cross section and analyzer transmission.30 All photoelectron spectroscopy spectra are energy calibrated relative the N 1s peak arising from the bipyridyl nitrogen, and this peak is set to binding energy 400 eV. The spectra are intensity calibrated using the Ru 3d peak if not stated otherwise.
’ RESULTS AND DISCUSSION Solar Cell Characterization. The plotted IV curves are presented in Figure 1 and the corresponding IV data are shown
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Figure 1. IV measurements of ZnO/N719 and ZnO/Z-907 solar cells with and without water in the dye solution.
Table 1. IV Measurements of ZnO Dye-Sensitized Solar Cells Showing the Overall Efficiency η, the Open Circuit Voltage Voc, the Short Circuit Current Isc and the Fill Factor FFa cell
η
Voc
Isc
[%]
[V]
[mA/cm2]
FF
λ
Abs IPCE
[nm] [%]
[%]
ZnO/N719
0.03
0.60
0.22
0.23
520
53
3
ZnO/N719 H2O
1.5
0.65
4.15
0.57
520
38
35
ZnO/Z-907
1.3
0.64
3.64
0.54
520
44
32
ZnO/Z-907 H2O 1.3
0.61
4.20
0.50
520
49
38
a
Light intensity was 1000 mW/cm2. Included are also absorption and IPCE at the given wavelength, i.e., the position of IPCE maxima. Film thickness was 3.5 μm and sensitization time 15 h for all solar cells.
in Table 1, η being the overall efficiency, Voc the open circuit voltage, Isc the short circuit current and FF the fill factor. As can be seen in Table 1, for ZnO/N719, the solar cell efficiency increases greatly when using water in the dye solution. The open circuit voltage and the short circuit current is also increased. For ZnO/Z-907 there is also increased short circuit current with water in the dye solution, but since the open circuit voltage at the same time decreases, the efficiency remains the same. Incident photon to current conversion efficiency (IPCE) can be seen in Figure 2. Despite slightly higher absorption than the rest of the ZnO solar cells, ZnO/N719 without water has an IPCE of less than 10%, which for example could indicate an inefficient electron injection from the dye to the ZnO. This is also reflected in the low short circuit current described above. For ZnO/N719 with water and for ZnO/Z-907 with and without water, IPCE are similar and the maximum values are around 35%. Absorption properties are given in Table 1, and the values of IPCE maxima reflect the low absorption of these rather thin ZnO films. The wavelength at maximum IPCE are shown in Table 1. For TiO2/N719 water has been showed to give a redshift of the IPCE maximum,20 but here, no clear redshift can be observed. Prior to the intensity modulated studies, the photocurrent was checked to be linear with the light intensity. The electron transport through the ZnO solar cells was then studied using intensity modulated photocurrent measurements under shortcircuit conditions, and the results can be found in Figure 3a. The 19275
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Figure 2. Incident photon to current conversion efficiency measurements of ZnO/N719 and ZnO/Z-907 solar cells with and without water in the dye solution.
Figure 3. Transport time (a) and lifetime (b) measurements of ZnO/ N719 and ZnO/Z-907 with and without water in the dye solution.
photocurrent time constant, τ, can be interpreted as the electron transport time in the solar cell.28 The transport time is similar for both systems, with and without water. Hence, in this case, the transport time in the solar cell shows no clear dependence on dye and solvent used for sensitization. The transport time is seen to decrease with increasing Isc. Such change in the mobility has previously been explained by trap states below the conduction band edge where conduction band electrons are trapped/ detrapped upon illumination.28,31 Intensity modulated photovoltage measurements were performed under open-circuit conditions, see Figure 3b, where the decay time constant, τe, can be regarded as the electron lifetime in the ZnO .28 Only a smaller change is observed for ZnO/Z-907 when including water or not. For N719 on the other hand, the time constant is larger at a particular voltage for ZnO/N719 with water, indicating a longer lifetime than without water. A longer lifetime for ZnO/N719 with water supports the better performance of the cell. The shorter lifetime for ZnO/N719 without water may be explained by intensified recombination pathways. One such pathway could be between the ZnO conduction band and the electrolyte due to an incomplete dye layer formation within the mesoporous film. Other sources for decreased lifetime could be inefficient electron injection or regeneration of the oxidized dye from the electrolyte due to multilayer formation. The high absorption, low IPCE and short lifetime can altogether
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be signs of dye multilayer formation, or dye aggregation, at the electrode surface. This will be discussed further in the following section. PES Measurements. Photoelectron spectroscopy measurements were performed on dye-sensitized ZnO electrodes to study the dye coverage and the anchoring to the ZnO surface. Similarities and differences to dye-sensitized TiO2 electrodes are discussed. Amount of Dye on the Surface. Figure 4 shows overview spectra of ZnO/N719 taken after a sensitization time of 10 min (Figure 4a) and 15 h (Figure 4b). The spectra are intensity normalized by setting the Zn 3p peaks to equal intensity. After 10 min, when using ethanol as solvent, the carbon C 1s and nitrogen N 1s peaks are more intense compared to when ethanol/water was used as solvent, which indicates more dye on the surface in the sensitization without water. After 15 h of sensitization, this effect is enhanced and there is substantially more dye on the surface when ethanol is used as solvent. An evaluation of the surface coverage is found in Table 2 where also values for ZnO/Z-907 are included. From the table it can be seen that for N719 the longer time doubles the dye/substrate ratio if water is present, and without water this ratio is more than 10 times higher after 15 h, indicating a multilayer formation. For Z-907 the increase of the dye/substrate ratio for longer immersion time is around 2030% both with and without water. Another effect for Z-907 is that the ratios are slightly higher when water is present (for both sensitization times). This means that water increases the amount of dye for Z-907, which can be a consequence from Z-907 not being soluble in water. However, for all samples that have been in the dye bath for 10 min, the amounts of dye are rather similar. In this study, surface induced aggregation is assumed to be the reason for the large amount of dye for ZnO/N719 when the solvent contains no water .10 A sign that points in this direction is the fact that Zn still can be seen for long sensitizaton time (Figure 4b) despite the high dye coverage, which would not be the case for a strictly multilayer adsorption. Instead, Zn2+ ions may have dissolved from the surface to form aggregates with the dye on top of the ZnO surface. Such mechanism explain why a small Zn signal can be seen also for the multilayers. When water is added to the dye bath, this aggregation mechanism cannot be observed and this could be the reason for the enhanced solar cell performance in this case. For ZnO/Z-907, there is no sign of aggregation. The formation of Zn2+/dye aggregates has earlier been found to increase as the dye solution becomes more acidic;12 that is, the dye contains more carboxyl groups. N719 contains four carboxyl groups, whereas Z-907 contains two, and this can be a reason why no aggregates are formed in the latter case. In addition to this, the long alkyl chains of Z-907 can prevent multilayer formation. It is also of interest to compare the amount of adsorbed dye molecules at ZnO and TiO2 surfaces. For TiO2 sensitized with N719, it has been showed that N719 anchors to the TiO2 surface through the carboxyl groups,32 and that this interaction is strong enough to favor monolayer adsorption .33 The surface amount of a molecule at two different substrates can be compared by measuring the photoemission intensity from a spectroscopically well-defined core level, such as Ru 3d5/2, relative to a substrate element (Ti 3p or Zn 3p). However in such a procedure one has to take into account the differences in photoionization cross section, analyzer transmission and substrate element density. Measurements of ZnO and TiO2 electrodes (with similar particle 19276
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Figure 4. Overview spectra of ZnO/N719 electrodes with/without water in the dye solution sensitized during 10 min (a) and 15 h (b) respectively. The photon energy used was 758 eV.
Table 2. Ratios between Ru 3d5/2 and Zn 3p intensities (Ru/Zn) with and without water in the dye solution and sensitized for 15 h and 10 min, respectivelya
a
dye and solvent
15 h
10 min
N719, H2O N719, no H2O
0.19 1.72
0.10 0.13
Z-907, H2O
0.15
0.11
Z-907, no H2O
0.13
0.08
A photon energy of 758 eV was used.
sizes) sensitized with N719 were performed with the ESCA300 setup with a photon energy of 1487 eV. The dye/substrate ratios for ZnO/N719 (sensitized during 10 min) and TiO2/N719 reveals the ratios Ru 3d5/2/Zn 3p to be 0.021 and Ru 3d5/2/Ti 3p to be 0.025. These numbers are corrected for different photoionization cross section and analyzer transmission for Zn and Ti, whereas in Table 2 no such corrections were needed. The ratios can be used as a comparison of the surface coverage. However, the density of the metal atoms are different for the different metal oxides, giving different numbers of Zn compared to Ti atoms per area unit. A simple calculation gives the Zn and Ti density to be related as 7:5. (The density of ZnO is 5.6 g/cm3 of which the contribution from Zn atoms is 80 wt %. The Zndensity is then 4.5 g/cm3, or 0.07 mol/cm3. A corresponding calculation for Ti in TiO2 anatase, with a density of 3.9 g/cm3,
gives a Ti density of 0.05 mol/cm3. Assuming the same relationship on the surface as in the bulk gives the 7:5 relation.) The slightly lower dye/substrate ratio for ZnO is therefore balanced by the higher Zn density, giving altogether that the packing of the dye molecules are similar on ZnO and TiO2. Bonding to the ZnO Surface. Even though both dyes are assumed to bind to the semiconductor surface through the carboxyl groups, there can still be differences in the interaction with the surface and neighboring molecules. The following section deals with water-induced changes of the adsorbed dye molecules. The following surface characterization was done on samples sensitized for 10 min. Based on the small differences in coverage observed at this time; these measurements represent the surface structure for the dye layer closest to the ZnO surface the best. N 1s. The nitrogen N 1s spectra for N719 shows mainly three features which corresponds to the nitrogen atoms in the thiocyanate ligands (NNCS) at a binding energy of 397398 eV, the bipyridyl ligand (Nbpy) at a binding energy of 400 eV, and the TBA+ counterions (NTBA) at a binding energy of 402.5 eV,32 see Figure 5. The Z-907 dye contains no TBA+ counterion and consequently, no such peak can be seen in Figure 6. Comparing the N 1s spectra with and without water, some differences are observed. For N719, the NTBA decreases when water is included in the solvent. This can either be a sign of that the TBA counterions are located closer to the ZnO surface, i.e., deeper into the molecular surface layer, or that less TBA ions are 19277
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Figure 5. N 1s spectra of N719 on ZnO measured with photon energies 540 eV (the upper part of the spectra) and 758 eV (the lower part of the spectra). The dashed line indicating the position of the NNCS peak in a multilayer of the dye.34 Figure 7. S 2p spectra measured with a photon energy of 454 eV.
Figure 6. N 1s spectra of Z-907 on ZnO measured with photon energies 540 eV (the upper part of the spectra) and 758 eV (the lower part of the spectra).
present. The decrease is independent of photon energy, which favors the latter explanation. The same effect has previously been observed in similar measurements on TiO2.20 Z-907 shows a decrease of the total N 1s peak intensity when water is a part of the solvent. A difference in the bonding geometry could cause the long carbon chains in the molecule to be either pointing away from the surface or in the surface direction causing a larger screening of the nitrogen atoms in the first case.
The NNCS peak can be discussed in more detail for both dyes. The peak is broad and can be divided into two peaks, a lower binding energy peak and a higher binding energy peak. For a multilayer of N719, there is only one contribution from NNCS and the position of this peak, relative the bipyridyl peak, is indicated with a line in Figure 5 at about 398 eV.34 The multilayer NNCS peak coincides well with the higher binding energy peak seen in this study, while the lower binding energy peak seen here, appears after adsorbing to the ZnO surface. This lower binding energy peak is also observed when N719 and Z-907 are adsorbed to TiO2.20 However this peak is much more intense for ZnO and the shift between the lower and higher binding energy peaks are larger for ZnO than for TiO2. Hence, this spectroscopic result show that the molecules bonds differently to ZnO compared to TiO2. Moreover, this difference is due to a strong interaction with the S atom as observed by the large shift (1 eV) in the NNCS signal. For 540 eV the shape of the NNCS peaks are similar for the different solvents and the lower binding energy peak is more prominent for both dyes. When increasing the photon energy to 758 eV, the higher binding energy peak becomes more noticeable indicating that this signal arises from an atom with a position deeper into the molecular layer. The differences in peak intensity can be interpreted as a sign of a mixing of binding configurations onto the ZnO surface. S 2p. Sulfur S 2p spectra are found in Figure 7. The sulfur S 2p level has a spin orbit split of 1.18 eV with intensity ratio 1:2 between the S 2p1/2 and S 2p3/2 peak areas. Using this as fitting parameters, two chemically inequivalent sulfur features can be seen for all samples. A multilayer of N719 shows only one chemical state of sulfur, whereas a higher binding energy peak has been shown to appear after adsorbing onto TiO2.32 The higher binding energy peaks seen here can therefore be attributed to interactions 19278
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The Journal of Physical Chemistry C at the ZnO surface. The relative intensities of the S 2p peaks are different for N719, whereas for N719 without water, the lower binding energy peak has 66% of the total intensity and with water, this number is 44%. This indicates different ways to interact with the surface if using water in the dye solution or not. For Z-907 this number is 63% in both cases. For N719, when the sulfur is intensity calibrated versus the Ru 3d peak, the total intensity of the peak, corresponding to no water in the solvent, is larger than when water is used in the solvent. The opposite behavior is seen for Z-907. A lowering of the total sulfur signal can be a sign of that some of the NCS groups has disappeared. Since the PES measurements are very surface sensitive, the decrease can also correspond to that NCS groups are closer to the surface. The same measurements performed for N719, using the in house ESCA setup with a photon energy of 1487 eV, showed similar amounts of S 2p compared to Ru 3d (the sulfur to ruthenium ratio was 1.9 without water and 1.8 with water, which both are close to the expected value of 2) which favors the latter explanation; that is, there is no ligand exchange but rather more NCS groups closer to the ZnO surface, due to a change of the dye binding geometry.
’ CONCLUSIONS Water in the dye solution prevents the formation of dye aggregations at the electrode surface and greatly enhances the solar cell performance for ZnO/N719 where surface dye multilayer formation, or aggregation, is a problem. Using water in the dye solution results in an increased short circuit current and incident photon to current conversion efficiency (IPCE). A longer electron lifetime also explains the enhancement of the solar cell characteristics. For ZnO/Z-907, there is no major changes in overall efficiency when including water in the dye solution, and the effects on IPCE are small. The N 1s core level originating from the NCS ligand in N719 and Z-907 is different on ZnO compared to on TiO2, indicating a different interaction at ZnO and TiO2 surfaces. The dye coverage of N719 are however similar on ZnO and TiO2 electrodes if there is no aggregation. Including water in the dye solution changes the bonding behavior onto ZnO. This is most noticeable in the S 2p spectra for N719, where a lower binding energy state is enhanced with water. This result exemplifies how the solvent used for dye adsorption changes the surface molecular and electronic structures. Such effects will affect the function of the dye molecule at the surface. However, a main reason for the increase in solar cell performance with water for the ZnO/N719 is most probably that water prevents the formation of Zn2+/dye aggregates at the electrode surface. ’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT The experimental work was supported by the Swedish Research Council (VR), the G€oran Gustafsson Foundation, the Carl Trygger Foundation, and the Swedish Energy Agency. We thank the staff at MAX-lab for their assistance.
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’ REFERENCES (1) Gr€atzel, M. Nature 2001, 414, 338–344. (2) Bauer, C.; Boschloo, G.; Mukhtar, E.; Hagfeldt, A. J. Phys. Chem. B 2001, 105, 5585–5588. (3) Gonzalez-Valls, I.; Lira-Cantu, M. Energy Environ. Sci. 2009, 2, 19–34. (4) Hosono, E.; Fujihara, S.; Kimura, T. Electrochim. Acta 2004, 49, 2287–2293. (5) Lee, W. J.; Suzuki, A.; Imaeda, K.; Okada, H.; Wakahara, A.; Yoshida, A. Jpn. J. Appl. Phys. 2004, 43, 152–155. (6) Ku, C. H.; Wu, J. J. Appl. Phys. Lett. 2007, 91, 093117. (7) Otsuka, A.; Funabiki, K.; Sugiyama, N.; Yoshida, T. Chem. Lett. 2006, 35, 666–667. (8) Quintana, M.; Marinado, T.; Nonomura, K.; Boschloo, G.; Hagfeldt, A. J. Photochem. Photobiol. A 2009, 202, 159–163. (9) Zhang, Q.; Dandeneau, C.; Zhou, X.; Cao, G. Adv. Mater. 2009, 21, 4087–4108. (10) Westermark, K.; Rensmo, H.; Siegbahn, H.; Keis, K.; Hagfeldt, A.; Ojam€ae, L.; Persson, P. J. Phys. Chem. B 2002, 106, 10102–10107. (11) Keis, K.; Bauer, C.; Boschloo, G.; Hagfeldt, A.; Westermark, K.; Rensmo, H.; Siegbahn, H. J. Photochem. Photobiol. A 2002, 148, 57–64. (12) Keis, K.; Lindgren, J.; Lindquist, S. E.; Hagfeldt, A. Langmuir 2000, 16, 4688–4694. (13) Horiuchi, H.; Katoh, R.; Hara, K.; Yanganida, M.; Murata, S.; Arakawa, H.; Tachiya, M. J. Phys. Chem. B 2003, 107, 2570–2574. (14) Ke, L.; Dolmanan, S. B.; Shen, L.; Pallathadk, P. K.; Zhang, Z.; Lai, D. M. Y.; Liu, H. Sol. Energy Mater. Sol. C 2010, 94, 323–326. (15) Baumeler, R.; Rys, P.; Zollinger, H. Helv. Chim. Acta 1973, 56, 2450–2460. (16) Sakuragi, Y.; Wang, X.-F.; Miura, H.; Matsui, M.; Yoshida, T. J. Photochem. Photobiol. A 2010, 216, 1–7. (17) Kay, A.; Gr€atzel, M. J. Phys. Chem. 1993, 97, 6272–6277. (18) Khazraji, A. C.; Hotchandani, S.; Das, S.; Kamat, P. V. J. Phys. Chem. B 1999, 103, 4693–4700. (19) Sayama, K.; Tsukagoshi, S.; Hara, K.; Ohga, Y.; Shinpou, A.; Abe, Y.; Suga, S.; Arakawa, H. J. Phys. Chem. B 2002, 106, 1363–1371. (20) Hahlin, M.; Johansson, E. M. J.; Sch€olin, R.; Siegbahn, H.; Rensmo, H. J. Phys. Chem. C 2011, 115, 11996–12004. (21) Liu, Y.; Hagfeldt, A.; Xiao, X.-R.; Lindquist, S.-E. Sol. Energy Mater. Sol. C 1998, 55, 267–281. (22) Mikoshiba, S.; Murai, S.; Sumino, H.; Kado, T.; Kosugi, D.; Hayase, S. Curr. Appl. Phys. 2005, 5, 152–158. (23) Jung, Y.-S.; Yoo, B.; Lim, M. K.; Lee, S. Y.; Kim, K.-J. Electrochim. Acta 2009, 54, 6286–6291. (24) Law, C.; Pathirana, S. C.; Li, X.; Anderson, A. Y.; Barnes, P. R. F.; Listorti, A.; Ghaddar, T. H.; O’Regan, B. C. Adv. Mater. 2010, 22, 4505–4509. (25) Ali, M.; Winterer, M. Chem. Mater. 2010, 22, 85–91. (26) Bauer, C.; Boschloo, G.; Mukhtar, E.; Hagfeldt, A. Chem. Phys. Lett. 2004, 387, 176–181. (27) Quintana, M.; Edvinsson, T.; Hagfeldt, A.; Boschloo, G. J. Phys. Chem. C 2007, 111, 1035–1041. (28) Boschloo, G.; Hagfeldt, A. J. Phys. Chem. B 2005, 109, 12093–12098. (29) B€assler, M.; Forsell, J. O.; Bj€ orneholm, O.; Feifel, R.; Jurvansuu, M.; Aksela, S.; Sundin, S.; Sorensen, S. L.; Nyholm, R.; Ausmees, A.; Svensson, S. J. Electron Spectrosc. 1999, 101103, 953. (30) Gelius, U.; Wannberg, B.; Baltzer, P.; Fellner-Feldegg, H.; Carlsson, G.; Johansson, C.-G.; Larsson, J.; M€unger, P.; Vegerfors, G. J. Electron Spectrosc. 1990, 52, 747–785. (31) Paulsson, H.; Kloo, L.; Hagfeldt, A.; Boschloo, G. J. Electroanal. Chem. 2006, 586, 56–61. (32) Johansson, E. M. J.; Hedlund, M.; Siegbahn, H.; Rensmo, H. J. Phys. Chem. B 2005, 109, 22256–22263. (33) Nazeeruddin, M. K.; Kay, A.; Rodicio, I.; Humphry-Baker, R.; M€uller, E.; P-Liska; Vlachipolous, N.; Gr€atzel, M. J. Am. Chem. Soc. 1993, 115, 6382–6390. (34) Johansson, E. M. J.; Hedlund, M.; Odelius, M.; Siegbahn, H.; Rensmo, H. J. Chem. Phys. 2007, 126, 244303. 19279
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