Langmuir 2001, 17, 2743-2747
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Phthalocyanine-Sensitized Nanostructured TiO2 Electrodes Prepared by a Novel Anchoring Method Jianjun He, Anders Hagfeldt, and Sten-Eric Lindquist* Department of Physical Chemistry, Uppsala University, S-751 21 Uppsala, Sweden
Helena Grennberg Department of Organic Chemistry, Uppsala University, S-751 21 Uppsala, Sweden
Ferenc Korodi, Licheng Sun, and Bjo¨rn Åkermark Department of Organic Chemistry, Stockholm University, S-106 91 Stockholm, Sweden Received November 28, 2000. In Final Form: February 1, 2001
A novel method for anchoring phthalocyanines substituted with ester groups onto nanostructured TiO2 films is described. Such phthalocyanines did not adsorb on nanostructured TiO2 film by the ordinary methods. In our new method, the TiO2 film is pretreated with (CH3)3COLi to change the surface hydroxyl groups (-OH) into oxygen anions (-O-), thus making the surface more reactive toward the ester functionalities of the dye. The dye can then be anchored onto the semiconductor surface through the produced carboxylate group(s). The amount of anchored dye on the semiconductor shows a dependence on both the time of base treatment and the time of dye treatment. Electrodes treated with the free base phthalocyanine and zinc phthalocyanine were characterized by absorption spectroscopy, photocurrent action spectroscopy, and photocurrent-photovoltage measurements. The homogeneous blue-green color and the absorption bands in the far-red region are indicative of an attachment of the dye on TiO2 film. A monochromatic incident photo-to-current conversion efficiency of 4.3% was achieved at 690 nm for a cell where the base-treated electrode was treated with ZnPcBu.
Introduction Phthalocyanines possess intensive absorption in the farred/near-IR region, are known for their excellent chemical, light, and thermal stability, and have appropriate redox properties for the sensitization of large band-gap semiconductors, for example, TiO2,1 rendering them attractive for dye-sensitized nanostructured solar cells. The parent phthalocyanine macrocycle itself is insoluble in almost every solvent. By proper choice of substituents at the periphery of the macrocycle, this problem may be diminished or even eliminated. Phthalocyanines substituted by carboxylic acid as well as sulfonic acid groups have been tested previously as sensitizers of wide band-gap oxide semiconductors.2-7 A solar cell based on a zinc tetracarboxyl phthalocyanine (ZnTcPc)-coated nanostructured TiO2 electrode had the highest incident photon-tocurrent conversion efficiency (IPCE) of 4% at the wave* Corresponding author. (1) Phthalocyanines: Properties and applications; Leznoff, C. C., Lever, A. B. P., Eds.; VCH: New York; 1993, Vols. 1-3; 1996, Vol. 4. (2) Nazeeruddin, M. K.; Humphry-Baker, R.; Gra¨tzel, M.; Wo¨hrle, D.; Schnurpfeil, G.; Schneider, G.; Hirth, A.; Trombach, N. J. Porphyrins Phthalocyanines 1999, 3, 230. (3) Fang, J.; Su, L.; Wu, J.; Shen, Y.; Lu, Z. New J. Chem. 1997, 21, 1303. (4) Shen, Y.; Wang, L.; Lu, Z.; Wei, Y.; Zhou, Q.; Mao, H.; Xu, H. Thin Solid Films 1995, 257, 144. (5) Fang, J.; Wu, J.; Lu, X.; Shen, Y.; Lu, Z. Chem. Phys. Lett. 1997, 270, 145. (6) Yanagi, H.; Chen, S.; Lee, P. A.; Nebesny, K. W.; Armstrong, N. R.; Fujishima, A. J. Phys. Chem. 1996, 100, 5447. (7) Deng, H.; Lu, Z.; Mao, H.; Xu, H. Chem. Phys. 1997, 221, 323.
length of 690 nm.4 When surface aggregation of the sensitizer was avoided, a strikingly high IPCE of 45% in the near-infrared region was obtained.2 The carboxylic acid substituents in ruthenium(II) polypyridyl complexes are essential for the anchoring of the dye on the surface of TiO2.8,9 The carboxylate groups establish good electronic coupling with the Ti(3d) conduction band orbital manifold. However, phthalocyanines with carboxylic acid groups have poor solubility in organic solvents, for example, ethanol and chloroform. On the other hand, liquid ammonia has been used to dissolve zinc tetracarboxyl phthalocyanine for the dye-coating of a nanostructured TiO2 film.4 Because of the poor solubility in organic solvents, it is difficult to synthesize, separate, and purify carboxylated phthalocyanines. In contrast, phthalocyanines with ester groups are readily synthesized and purified in good yields as they are reasonably soluble in less polar solvents, for example, chloroform. However, phthalocyanines substituted by ester groups, for example, free-base and zinc 2,9,16,23-tetra(n-butoxycarbonyl)phthalocyanines (see molecular structures below), do not adsorb on nanostructured TiO2 film by means of ordinary methods.10 Moreover, they were surprisingly resistant toward hydrolysis by for example aqueous NaOH. (8) Hagfeldt, A.; Gra¨tzel, M. Chem. Rev. 1995, 95, 49. (9) Hagfeldt, A.; Gra¨tzel, M. Acc. Chem. Res. 2000, 33, 269. (10) Nazeeruddin, 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.
10.1021/la001651b CCC: $20.00 © 2001 American Chemical Society Published on Web 03/28/2001
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The surface of TiO2 is covered with hydroxyl groups (Ti-OH),11-13 which can be deprotonated to give oxygen anions (Ti-O-) on the surface by treatment with a sufficiently strong base with a nonpolar corresponding acid which can be washed away by a nonpolar solvent. A thus-activated surface would be reactive toward the ester functionalities of the phthalocyanines, leading to an in situ formation of the desired carboxylate group(s) and attachment of the dye onto the surface. Experimental Section 1. Synthesis. (i) 2,9,16,23-Tetra(n-butoxycarbonyl)phthalocyanine (PcBu). 4-Butoxycarbonylphthalonitrile (1.32 g, 6 mmol) was refluxed in butanol (30 mL) in the presence of 1,8diazabicyclo[5.4.0]undec-7-ene (DBU, 1.02 g, 6 mmol) for 3 h. The mixture was left standing at room temperature overnight. The precipitate was collected, washed four times with methanol, and dried in an exsiccator over silica gel to obtain 0.58 g (42%) of the compound PcBu as a dark purple solid. For further purification, 0.5 g of the product was put on a silica gel column (25 g) and was eluted with chloroform to achieve 0.45 g of analytically pure PcBu. UV/vis (CHCl3) λmax (nm): 704, 666, 637, 343, 292. 1H NMR (CDCl3) δ (ppm): 0.00 (s, 2H, 2 × NH), 1.03 (s /br/, 12H, 4 × CH3), 1.45 (s /br/, 8H, 4 × CH2), 1.70 (s /br/, 8H, 4 × CH2), 4.11 (s /br/, 8H, 4 × CH2), 6.2-7.2 (br, 12H, Ar-H). (ii) 2,9,16,23-Tetra(n-butoxycarbonyl)phthalocyaninato Zinc(II) (ZnPcBu). A mixture of PcBu (0.182 g, 2 mmol) and zinc(II) acetate dihydrate (0.88 g, 4 mmol) was refluxed in butanol (10 mL) for 3 h. The solvent was then removed in a vacuum, and the residue was treated with 10 mL of hot chloroform. The white precipitate was removed by filtration and washed with chloroform, and the blue chloroform solution was evaporated to give 0.198 g of ZnPcBu as a bluish-purple solid which was further purified by column chromatography using silica gel packing (20 g) and chloroform eluent to give 0.160 g (82%) of pure product. UV/vis (CHCl3) λmax (nm): 688, 616, 349. 1H NMR (CDCl3) δ (ppm): 1.21 (s /br/, 12H, 4 × CH3), 1.76 (s /br/, 8H, 4 × CH2), 2.08 (s /br/, 8H, 4 × CH2), 4.67 (s /br/, 8H, 4 × CH2), 8.58 (s /br/, 4H, Ar-H), 8.91 (br, 4H, Ar-H), 9.47 (br, 4H, Ar-H). 2. Film Preparation. The nanostructured TiO2 films were fabricated by a similar procedure to that described in the literature.10 Briefly, commercial powder of TiO2 (Degussa P25) was dispersed by grinding with water and adding acetylacetone as particle stabilizer and Triton X-100 as wetting agent. The viscous suspension was spread onto transparent conducting glass sheets (Libbey Owens Ford, fluorine-doped SnO2 glass, sheet resistance 8Ω/0) using scotch tape as a spacer. A thin film was obtained by raking off the excess of suspension with a glass rod. After the tape was removed and the samples were air-dried, they were sintered in air at 450 °C for 30 min to form a nanostructured TiO2 film electrode. The thickness of the film, recorded by Dektak3 Surface Profile Measuring System, was about 10 µm. 3. Film Treatment and Dye-Anchoring. All experiments were carried out in a water- and oxygen-free glovebox at 24 °C. A nanostructured TiO2 film was immersed in a solution of lithium tert-butoxide ((CH3)3COLi) in hexane (1.0 M) for a certain time (see Results and Discussion). The base-treated film was washed (11) Fox, M. A.; Dulay, M. T. Chem. Rev. 1993, 93, 341. (12) Sato, S. Langmuir 1988, 4, 1156. (13) Zang, L.; Liu, C.-Y.; Ren, X.-M. J. Chem. Soc., Chem. Commun. 1994, 1865.
He et al. with hexane thoroughly and then immersed into a chloroform solution of PcBu or ZnPcBu (1.0 × 10-4 M) for a certain time (see Results and Discussion), followed by a wash with chloroform and ethanol. 4. Optical Measurements. Absorption spectra of the dyes in chloroform were recorded on a HP8453 UV/vis spectrophotometer. Absorbances of ZnPcBu adsorbed on TiO2 films at the wavelength of 690 nm for studying the processes of film treatment and dyeanchoring were recorded with a bare TiO2 film as a reference. Absorption spectra of the dyed nanostructured TiO2 film electrodes for photoelectric measurements were determined using a Beckman UV 5240 spectrophotometer equipped with an integrating sphere by recording the reflectance and transmittance. These measurements were carried out for dry films, and no corrections were made for optical effects due to the presence of the electrolyte. 5. Photoelectric Characterization. Photocurrent action spectra and photocurrent-photovoltage characteristics of dyed nanostructured TiO2 electrodes were measured with sandwichtype cells. The working electrode with the dye-coated film on conducting glass was gently squeezed together with a platinized conducting glass using a spring and illuminated from the substrate side of the dyed film. The electrolyte, 0.5 M LiI/0.05 M I2 in propylene carbonate, was attracted into the cavities of the dye-coated TiO2 electrode by capillary forces. A 450 W xenon lamp with a monochromator was used as a light source for the action spectra measurements. The IPCE values were determined at 10 nm intervals between 400 and 800 nm. A 1000 W xenon lamp sun-simulator with a 10 cm water filter was used as a light source for the photocurrent-photovoltage characteristics. The light intensity was measured by a pyranometer (Kipp & Zonen CM 11). The active electrode area was typically 0.25 cm2.
Results and Discussion 1. Treatment of the TiO2 Film and Dye-Anchoring. PcBu and ZnPcBu cannot be anchored onto TiO2 by the conventional method for dye-coating of nanostructured TiO2,10 as reheating of the film at 450 °C for 20 min followed by immersion in a solution of of PcBu or ZnPcBu (1.0 × 10-4 M) for 3 days did not result in any adsorption of phthalocyanine to our electrodes. In contrast, the glovebox pretreatment of the nanostructured TiO2 film with (CH3)3COLi in hexane followed by immersion in the dye solution gave electrodes with a homogeneous blue-green color. The color did not rinse off with chloroform. This we interpret as a successful anchoring of our phthalocyanines onto the TiO2 particle surface. Further proof of anchoring is absorption in the far-infrared region and the monochromatic incident photo-to-current conversion efficiency of 4.3% achieved at 690 nm for a cell where the base-treated electrode was treated with ZnPcBu. Also, the solar cells based on these dyed TiO2 electrodes have apparent photocurrent action spectra, which resemble the absorption spectra of the dyes on the surface. As a control, a nanostructured TiO2 film was immersed in pure hexane for 3 days. The film was then soaked in the chloroform solution of PcBu or ZnPcBu for 3 days and washed with chloroform and ethanol, respectively. No adsorption of the dye was observed. Neither did deposition of dye onto TiO2 electrodes by spin-coating using chloroform solution of PcBu or ZnPcBu result in any adsorption, as the precipitated dye could be washed away easily and completely with chloroform. Furthermore, solar cells based on these electrodes did not show any IPCE response. The experimental results described above provided evidence for a reaction between the TiO2 film and the base and between the base-treated film and the phthalocyanine dyes. In particular, the IPCE results are proof of an electronic coupling between the dye molecule and TiO2 surface. Previous studies12,13 indicated that the surface of metal oxide semiconductors, for example, TiO2, is covered, more or less, with surface hydroxyls. Upon
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Figure 1. Time dependence of the surface treatment of nanostructured TiO2 film in 1.0 M (CH3)3COLi hexane solution.
Figure 2. Time dependence of dye-anchoring of the treated nanostructured TiO2 film in 1.0 × 10-4 M ZnPcBu chloroform solution.
treatment with a solution of lithium tert-butoxide, some of the surface hydroxyls of the TiO2 film are deprotonated (eq 1). We propose that surface anions of the treated film react with the ester unit(s) of the phthalocyanines, leading to the formation of surface-bound carboxylate unit(s) (eq 2) and thus to an attachment of the phthalocyanine to the surface.
-Ti-OH + (CH3)3COLi f -Ti-O- Li+ + (CH3)3COH (1) -Ti-O- Li+ + Pc-COOBu f -Ti-OOC-Pc + BuOLi (2) The amount of dye on the electrode depends on the basetreatment time and on the dye-treatment time. Figure 1 shows the amount of ZnPcBu on the electrodes, measured as absorbance at 690 nm, as a function of base-treatment time (see spectrum of ZnPcBu in Figure 3b). The absorbances of the treated films were examined after soaking in the 1.0 × 10-4 M ZnPcBu chloroform solution for 72 h after the base treatment. It is seen that the absorbance increases with increasing time in the (CH3)3COLi solution. A plateau is reached after ca. 40 h. This indicates that no further useful reactions occur after 40 h. Figure 2 shows the amount of ZnPcBu on the electrodes as a function of dye-treatment time for electrodes treated with base for 72 h. We can see that the absorbance at 690 nm increases with the dye-treatment time and reaches saturation at about 20 h. The saturation points can probably be reached more quickly at higher temperatures. We have tried to characterize the proposed covalent bond(s) formed between the dye molecule and the TiO2
Figure 3. Absorption spectra of PcBu (a) and ZnPcBu (b) in chloroform solution (curve 1, [dye] ) 1.0 × 10-6 M) and on nanostructured TiO2 film with maximum adsorption (curve 2). The spectra of the dyes on TiO2 films are recorded using an integrating sphere setup. The light is incident on the conducting glass side, similar to the substrate-side illumination in the photoelectric measurements. No electrolyte or counter electrode is present.
surface by comparing Fourier transform infrared (FTIR) and Raman spectra of dye powders and dyes anchored on TiO2. However, the FTIR and Raman peaks of the dyes anchored on TiO2 had low intensities and were not well resolved. Ongoing work is focused on characterizing the surface state of the dye on the TiO2 film surface and/or the electronic coupling between the dye molecule and the surface using techniques such as XPS, extended X-ray absorption fine structure (EXAFS), and X-ray absorption near-edge structure (XANES) spectroscopies. 2. Absorption Spectra. The absorption spectra (in terms of absorbance and absorption = 1 - T - R, where T is the transmission and R the reflectance) of PcBu in chloroform solution (1.0 × 10-6 M) and on nanostructured TiO2 film with maximum adsorption are shown in Figure 3a. In dilute chloroform solution (curve 1), PcBu is present mainly as monomers, characterized by the sharp absorption bands in the Soret (350 nm) and in the Q-band region (around 680 nm).14,15 For the sample anchored on nanostructured TiO2 film (curve 2), the Q-band is broadening and the maximum shifts considerably to around 620 nm, which is ascribed to the Q-band(s) of the face-to-face PcBu dimer or polymer aggregates.14 Figure 3b shows the absorption spectra of ZnPcBu in chloroform solution (1.0 × 10-6 M) and on nanostructured TiO2 film at maximum (14) Phthalocyanine Materials; McKeown, N. B., Ed.; Cambridge University Press: New York, 1998. (15) Schlettwein, D.; Oekermann, T.; Yoshida, T.; Tochimoto, M.; Minoura, H. J. Electroanal. Chem. 2000, 481, 42.
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adsorption. The absorption spectrum of ZnPcBu in dilute solution (curve 1) indicates typical absorption of monomeric metal phthalocyanines with a sharp Soret-band maximum at the wavelength of 350 nm and a Q-band maximum at the wavelength of 690 nm.15 The absorption spectrum of ZnPcBu on nanostructured TiO2 film (curve 2) shows two bands. One is at the same position as the Q-band of the monomeric ZnPcBu in solution, which can be attributed to the Q-band of monomer species on the film. The blue-shifted band with a maximum at the wavelength of 620 nm can generally be assigned as the characteristic Q-band of the ZnPcBu dimer or higher-order aggregate.15-17 This system is therefore characterized by adsorption of a mixture between monomers and aggregates. It should be noted that the absorbances (1 - R - T) of PcBu and ZnPcBu on nanostructured TiO2 films should go to zero near 800 nm, because neither TiO2 nor the dye absorbs light at this wavelength. The value found, however, is caused by optical losses, such as light scattering through the glass edges.18 3. Photocurrent Action Spectra. The incident monochromatic photo-to-current conversion efficiency was registered from experiments using a sandwich solar cell. The working electrode of the maximum PcBu- or ZnPcBuanchored nanostructured TiO2 film on conducting glass was squeezed together with a platinized conducting glass using a spring and illuminated from the substrate side. The electrolyte, 0.5 M LiI/0.05 M I2 in propylene carbonate, was attracted into the cavities of the dye-coated TiO2 electrode by capillary forces. The cell was operated in the short-circuit mode. The IPCE was then calculated from
IPCE )
1240 iph[µA] P[µW] λ[nm]
where iph and P are the photocurrent and power of the incident radiation per unit area, respectively, and λ is the wavelength of the monochromatic light. No corrections were made for absorption and reflection in the substrate. The results for PcBu- and ZnPcBu-anchored nanostructured TiO2 electrodes are shown in parts a and b of Figure 4, respectively. For comparison and to obtain the absorbance of the dye on the film electrode, the absorption spectrum of a bare TiO2 electrode and the respective absorption spectrum of the dyed nanostructured TiO2 electrode are also shown in the figure. The photocurrent action spectra resemble the absorption spectra except for a slight red-shift. The reason for the red-shift of the photocurrent action spectra compared to the absorption spectra may be that the absorption spectra were recorded using dry films, whereas the photocurrent action spectra were registered in the presence of electrolyte. The highest IPCE for the PcBu-coated TiO2 electrode is 0.30% at 620 nm (Figure 4a), and for the ZnPcBu-coated TiO2 electrode the highest value is 4.3% at 690 nm (Figure 4b). The absorbance of the dye at a definite wavelength is approximately determined as the difference between the absorbance of the dyed TiO2 electrode and that of a bare TiO2 electrode. The quantum efficiencies (φ ) IPCE/(1 R - T)), defined as electrons measured in the external circuit per absorbed photon, for PcBu- and ZnPcBu-coated TiO2 electrodes are 0.6% at 620 nm and 22% at 690 nm, respectively. The quantum efficiency of 22% for the (16) Nevin, W. A.; Liu, W.; Lever, A. B. P. Can. J. Chem. 1987, 65, 855. (17) Snow, A. W.; Jarvis, N. L. J. Am. Chem. Soc. 1984, 106, 4706. (18) Keis, K.; Roos, A.; Lindquist, S.-E.; Hagfeldt, A. To be published.
He et al.
Figure 4. Photocurrent action spectra (curve 1) of maximum PcBu-anchored (a) and maximum ZnPcBu-anchored (b) nanostructured TiO2 electrodes in sandwich-type measurements. The absorption spectra of a bare TiO2 film electrode (curve 2) and the corresponding dyed TiO2 electrodes (curve 3) are also depicted.
ZnPcBu-coated TiO2 electrode is in agreement with, or even slightly higher than, the previous report of the quantum efficiency (ca. 20%) of a ZnTcPc-coated TiO2 electrode.4 Considering that quantum efficiencies of approximately 100% have been found in other dyed nanostructured TiO2 systems,10,19,20 the reason for the relatively low quantum efficiency should be explained. In our cases, it could be that the electron injection is less efficient because of aggregate formation and/or that the back electron transfer reactions occur at higher rates. The significant difference in quantum efficiency between PcBuand ZnPcBu-coated TiO2 electrodes may be due to not only the formation of aggregates but also the different energy levels in ground and excited states, which result in different efficiencies in electron injection and back electron transfer reactions. From Figure 4b, one can see that the absorbance of the Q-band from the ZnPcBu aggregates at the wavelength of 640 nm is higher than that from the ZnPcBu monomers at the wavelength of 690 nm, but the IPCE of the former is lower than that of the latter. From the IPCE value (4.2%) and the absorbance (0.23) at 640 nm, a quantum efficiency of 18% can be estimated. The lower quantum efficiency of the aggregates, compared to 22% of the monomers at 690 nm, can be explained by losses due to energy transfer21,22 or charge-transfer reactions23,24 be(19) Kay, A.; Gra¨tzel, M. J. Phys. Chem. 1993, 97, 6272. (20) Hagfeldt, A.; Bjo¨rkste´n, U.; Lindquist, S.-E. Sol. Energy Mater. Sol. Cells 1992, 27, 293. (21) Schneider, G.; Wo¨hrle, D.; Spiller, W.; Stark, J.; Schulz-Ekloff, G. Photochem. Photobiol. 1994, 60, 333.
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of a sandwich solar cell based on PcBu- and ZnPcBuanchored TiO2 electrodes, respectively, illuminated by light from a sun-simulator (light intensity, 100 mW/cm2). For a PcBu-coated TiO2 cell (Figure 5a), the short-circuit photocurrent (ISC) is 0.0316 mA/cm2, the open-circuit photovoltage (VOC) is 277 mV, and the fill factor (FF) is 47%, and for a ZnPcBu-coated TiO2 cell (Figure 5b) the corresponding values are ISC ) 0.353 mA/cm2, VOC ) 407 mV, and FF ) 61%, giving overall efficiencies of 0.0042% and 0.088%, respectively.
Figure 5. Photocurrent-photovoltage characteristics of the sandwich solar cells with maximum PcBu-anchored (a) and maximum ZnPcBu-anchored (b) nanostructured TiO2 films as working electrodes. The light intensity is 100 mW/cm2.
tween molecules in the aggregates. This effect was also observed by Schlettwein et al. for tetrasulfophthalocyaninatozinc-ZnO systems.15 4. Photocurrent-Photovoltage Characteristics of PcBu- and ZnPcBu-Coated TiO2 Electrodes. Figure 5 shows the photocurrent-photovoltage characteristics (22) Spiller, W.; Kliesch, H.; Wo¨hrle, D.; Hackbarth, S.; Roeder, B. J. Porphyrins Phthalocyanines 1998, 2, 145. (23) Klessinger, M.; Michl, J. Excited States and Photochemistry of Organic Molecules; VCH: New York, 1995. (24) Rodgers, M. A. J. Photosensitization; Moreno, G., Pottier, R. H., Truscott, T. G., Eds.; Springer: Berlin, 1988.
Conclusion A novel method for preparing phthalocyanine-sensitized nanostructured TiO2 electrodes is described. Through this method, the easily synthesized, purified, and chloroformsoluble phthalocyanines with four ester groups can be anchored onto nanostructured TiO2 film. The saturation point of the surface treatment of a 10 µm thick nanostructured TiO2 film at room temperature (24 °C) can be reached after 40 h when immersing the film in 1.0 M (CH3)3COLi hexane solution, and saturated dye-anchoring of the treated film in 1.0 × 10-4 M dye chloroform solution needs about 20 h. PcBu is present on the nanostructured TiO2 film mainly as aggregates, whereas ZnPcBu exists as monomers and aggregates. The quantum efficiency from monomers is higher than that from aggregates. The highest IPCE for the PcBu-coated TiO2 electrode is 0.30% at 620 nm, and for the ZnPcBu-coated TiO2 electrode the highest IPCE is 4.3% at 690 nm, corresponding to a quantum efficiency of 22%, which is one of the highest IPCE values reported for phthalocyanine photovoltaic devices. To improve the efficiency of these systems, further work should focus on avoiding aggregation on the oxide surface. Acknowledgment. The Swedish National Energy Administration, MISTRA, the Swedish National Science Research Council, and the Swedish Research Council for Engineering Sciences are acknowledged. We also thank Eva Magnusson for fabricating nanostructured TiO2 films and Dr. Arne Roos at Ångstro¨m Solar Center, Uppsala University, for his kind assistance with the reflectance and transmittance measurements. Dr. Gerrit Boschloo and Dr. Heli Wang at the Department of Physical Chemistry, Uppsala University, are acknowledged for their helpful discussions. LA001651B
