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J. Phys. Chem. C 2008, 112, 4767-4775

4767

Photoinduced Formation of Polythiophene/TiO2 Nanohybrid Heterojunction Films for Solar Cell Applications Yasuhide Otsuka, Yuko Okamoto, Hitomi Y. Akiyama, Kazuya Umekita, Yasuhiro Tachibana,* and Susumu Kuwabata Department of Applied Chemistry, Graduate School of Engineering, Osaka UniVersity, Yamada-oka, Suita, Osaka 565-0871, Japan ReceiVed: October 10, 2007; In Final Form: December 1, 2007

Formation of nanostructured polythiophene/TiO2 heterojunction films, using photoinduced polymerization of thiophene inside TiO2 nanopores, was investigated. The resultant film possesses nanohybridization and electronic connection within the TiO2 nanoporous domain. Photopolymerization proceeded in three stages: (i) photoexcitation of bithiophene covalently attached to the TiO2 surface, (ii) an electron injection reaction from the surface attached thiophene to the TiO2, and (iii) an electron transfer from a thiophene reactant in an electrolyte to the surface-attached bithiophene. Initial rapid photopolymerization and subsequent slow polymer growth were explained by analysis of a series of experiments, e.g., with respect to light irradiation time, applied bias, electrolyte types, thiophene reactant type, and their morphology. Electrochemical measurements for the bithiophene adsorbed on TiO2 revealed a wide distribution of redox potentials. This was explained by influence of the local electric field on the TiO2 surface in addition to strong interaction between the surfacebound bithiophene and the TiO2. The nanohybrid film was applied to a sensitized-type photoelectrochemical solar cell, substantiating direct application of the nanohybrid film to electronic devices. The solar cell performance was closely associated with the interfacial structure in the nanohybrid film and the photopolymerization degree.

Introduction Organic/inorganic polymer hybrid materials have a wide variety of attractive potentialities to introduce novel structural design in material sciences1-3 and to derive novel functions for device applications.4-6 In particular, a hybrid structure based on a conducting polymer/metal oxide semiconductor is one of the most advantageous combinations for photoelectronic devices. As organic materials, conducting polymers possess distinctive properties such as economical viability, light and easy processability, and high suitability for various types of electronic devices, e.g., display devices,7 lasers,8 FETs,9 and photovoltaics.10 In contrast, metal oxides such as titanium dioxide, zinc oxide, and tin oxide are environmentally viable with excellent chemical and physical stability, having been studied for transparent electrodes,11 electrochromism,12 photocatalysis,13 and solar cells.14 By combination of these materials with nanometer size control, an efficient electron-transfer reaction or energy transfer reaction can be derived,6,15-17 i.e. introducing further novel functions in addition to their individual attractive properties. Application of conducting polymer/metal oxide nanometer size controlled hybrids, nanohybrids, to photovoltaic devices has recently been studied. Bulk heterojunction type is one of the most attractive structures since enhanced light absorption, charge separation, and charge transportation can be achieved.16,18-24 Sensitization type photoelectrochemical cells using a liquid electrolyte have also been developed owing to their simple fabrication process.25-29 * To whom correspondence should be addressed. Tel and fax: +81-(0) 6-6879-7374, e-mail: [email protected].

Fabrication of polymer/metal oxide nanohybrid films has generally been accomplished using the blend method and the penetration method. In the blend method, conducting polymer and metal oxide nanoparticles are mixed in solution, and subsequently nanostructured films were prepared by spin coating.16,21,22 This method is attractive owing to the simple process and can be introduced to diversified applications. In addition, dense hybridization throughout the film can be expected. For the penetration method, metal oxide nanostructured films were first prepared, and then the conducting polymer solution was placed on top of the metal oxide film. The conducting polymer penetrates into the pores of the film until the polymer solution evaporates.17,19,23,24 In this method, the electronic connection within the metal oxide domain is maintained with the hybridization occurring with ease, facilitating the device assembly. For the application of the polymer/metal oxide nanohybrid films to electronic devices, a dense heterojunction (hybridization) and an electronic connection within the metal oxide throughout the film are essential, since the device function is often influenced by charge-transfer reaction at the polymer/metal oxide interface and by charge carrier transport through the metal oxide domain. In this regard, using the blend method, one may find difficulty to obtain the electric contact between the metal oxide particles throughout the film, since the polymer chain may interfere with the connection formed within the metal oxide. Complete hybridization may not be obtained in the penetration method, as the polymer may not be filled owing to limited penetration depth of the conducting polymer layer. For example, Huisman et al. reported the penetration depth of 1 µm when dodecylthiophene is employed to fill inside the pores in the TiO2 nanocrystalline film.19

10.1021/jp7099064 CCC: $40.75 © 2008 American Chemical Society Published on Web 03/05/2008

4768 J. Phys. Chem. C, Vol. 112, No. 12, 2008 In this paper, an alternative method to construct conducting polymer/metal oxide nanohybrid films, by photoinduced polymerization of thiophene inside metal oxide nanopores, is investigated. Initially, thiophene monomers possessing a carboxyl group are adsorbed on the TiO2 surface, and a selective light excitation of the adsorbed thiophene induces an electron injection from the thiophene to the TiO2. Thiophene polymers grow from the surface attached thiophene by sequential electrontransfer reactions inside the TiO2 pore. The TiO2 nanoparticle film, preserving the electronic contact between the nanoparticles, has been employed. The polythiophene, covalently bound to the TiO2 surface through a carboxyl group, is formed inside the TiO2 pore. Thus, this method thoroughly provides nanohybridization and electronic connection within metal oxide. The photopolymerization mechanism will be discussed by determining potential energy of the states (such as the TiO2, the surface attached thiophene, and the thiophene reactants) involving the reaction and by observing electron-transfer reactions at the interfaces. Experimental Methods Samples. TiO2 nanocrystalline films, thickness 6-7 µm, were prepared on a slide glass or a fluorine doped tin oxide glass, FTO, (Asahi glass, type-U, 10 Ω/square) by a screen printer. The TiO2 paste, Ti-Nanoxide, were purchased from Solaronix SA. The film, after printing, was leveled for 15 min, heated up to 500 °C at 17.5 °C/min, and calcined at 500 °C for 1 h in an air flow oven. Al2O3 nanoporous films, thickness: 7 µm, were prepared using a similar method. Degussa aluminum oxide (Alu C, particle diameter of approximately 13 nm), ethyl cellulose 10 (Wako), ethyl cellulose 45 (Wako), and R-terpineol (Kanto Chemical) were mixed to obtain the appropriate viscosity for the printing paste. We discovered that transparent crack-free films can be obtained by adjusting composition of these materials. The printed Al2O3 films were calcined at 500 °C for 1 h in air. A series of thiophene monomers, reactants, were compared regarding photopolymerization behaviors on the TiO2 surface. Monomeric thiophene, T, (98%), 3-methylthiophene, MT (98%), 2,2′-bithiophene, BT (98%), and 3-n-dodecylthiophene, DT (98%), were purchased from Tokyo Chemical Industry, and 2,2′bithiophene-5-carboxylic acid, BTC (97%), and 3,3′-dimethyl2,2′-bithiophene, DMBT (98%), were purchased from Maybridge and Acros Organics, respectively. These thiophene reactants were used without further purification. Photopolymerization and electrochemical measurements were performed by using three different types of electrolyte: (i) 1-buthyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide, BMITFSI, an ionic liquid, (ii) 0.1 M lithium perchlorate, LiClO4, (Wako) in propylene carbonate, PC, (Wako), and (iii) 0.1 M LiClO4 in acetonitrile (Wako). BMITFSI was synthesized following the previously reported method.30,31 The BMITFSI was dried at 105 °C in vacuum for at least 1 h. Photopolymerization of Thiophene. Polythiophene was photoelectrochemically synthesized from BTC adsorbed on TiO2 (BTC/TiO2). Attachment of BTC on the TiO2 surface was performed by dipping a TiO2 film in 10 mM BTC dissolved in ethanol for 10 min at room temperature. Prior to this attachment, the film was treated with titanium tetrachloride aqueous solution to coat the surface with a thin TiO2 layer.32 Photopolymerization was performed by exciting the BTC, after the BTC/TiO2 electrode was placed in a three-electrode cell, containing a thiophene reactant in the electrolyte, under white light from a Xe lamp through a > 400 nm high wavelength pass filter (320

Otsuka et al. mW/cm2) with application of +0.5 V vs Ag/AgCl. This filter was intentionally inserted in the light path to avoid the TiO2 excitation by UV light. A Pt plate (0.4 cm2) and an Ag/AgCl electrode were used as a counter electrode and a reference electrode, respectively. After the photopolymerization, the hybrid film was washed with ethanol and dried in air. Characterization of Nanohybrid Films. Chronoamperometry using a potentiostat (Hokuto Denko, HSV-100) were employed to detect photocurrents from the TiO2 electrode. The amount of electrons collected from the TiO2 electrode was calculated by integrating the data to estimate the photopolymerization yield. Absorption and emission spectra were measured by a UV/vis absorption spectrometer (JASCO, V-670) and an emission spectrometer (Horiba, FluoroLog-3), respectively. Morphology of the nanohybrid film was observed by FE-SEM (Hitachi, S-5200). The oxidation potential of the thiophene reactant was determined as the onset potential of the thiophene oxidation in linear sweep voltammograms (ALS 660C) using a threeelectrode cell consisting of a gold working electrode (diameter: 100 µm), a Pt plate counter electrode, and an Ag/AgCl reference electrode in BMITFSI or PC with 0.1M LiClO4. The voltage was applied from 0 V to +2.0 V at 50 mV/s. A redox potential of the BTC was determined by the similar configuration as the above; the only difference is the electrolyte containing 10 mM BTC instead of the thiophene reactant. The value was obtained as a half-wave potential.33 Alternatively, the redox potential of the BTC/TiO2 electrode in the threeelectrode cell containing 0.1 M LiClO4 in PC was determined by linear sweep voltammetry and differential pulse voltammetry (ALS 660C) according to the method reported previously.34 The linear sweep voltammogram was measured with a bias application between 0 and +1.8 V with a sweep rate of 50 mV/s. The differential pulse voltammograms were obtained with a pulse width of 50 mV at 50 mV/s. All electrochemical data were presented against the Ag/AgCl potential. Application to Photoelectrochemical Solar Cells. Sandwich type solar cells were fabricated by binding a redox electrolyte with the nanohybrid electrode and the Pt counter electrode.32 The electrolyte was prepared by dissolving 0.6 M dimethylpropylimidazolium iodide (Tomiyama Pure Chemical), 0.05 M iodine (99.8%, Wako), 0.1 M lithium iodide (99.995%, Wako), and 0.5 M tert-butylpyridine (99%, Aldrich) in dried acetonitrile (99%, Wako). A Xe lamp with a monochromator (Bunko Keiki, SM-25) was used to characterize IPCE spectra. An I-V measurement was performed by an AM1.5 solar simulator (100 mW/cm2, Yamashita Denso, YSS-50A). Results Photoelectrochemical Formation of Polythiophene/TiO2 Nanohybrid Films. Prior to photoelectrochemical polythiophene formation, attachment of a visible light-sensitive thiophene molecule on the TiO2 was investigated to achieve a heterojunction between a polymerized thiophene and the TiO2. A molecule possessing a carboxyl group is known to form a chemical bond with the TiO2 surface,14 and thus we considered thiophene molecules with a carboxyl group. We found by introducing a commercially available BTC on the TiO2 surface, the film turned yellow. Figure 1 compares an absorption spectrum and a photograph of the BTC/TiO2 with those of a TiO2 film alone. An absorption spectrum of 100 µM BTC in ethanol exhibits no coloration as shown in Figure 1. Photopolymerization was performed by exposing light (>400 nm) to the cell in order to excite only the surface-attached BTC

Polythiophene/TiO2 Nanohybrid Heterojunction Films

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Figure 2. Anodic photocurrent profile for the PBT photopolymerization inside the BTC/TiO2 nanopores. The inset shows linear sweep voltammograms of BTC/TiO2 electrode with (a, solid line) and without (b, dotted line) light irradiation. The voltage was applied from -0.5 V to +1.5 V at 50 mV/s. Figure 1. Absorption spectra and photographs (inset) of the TiO2 film (‚‚‚‚‚‚‚), BTC/TiO2 (- - -), and PBT-BTC/TiO2 (ss). The paste containing large TiO2 particles (diameter: ∼50 nm) was used for the photographs. An absorption spectrum of 100 µM BTC in ethanol (-‚‚-‚‚-) is also shown as comparison.

with application of +0.5 V. As the light continuously irradiated, the color of the film, in contact with the electrolyte, turns from yellow to dark blue, typically observed for doped polythiophene.35 Note that the bias application in the range of 0 to +0.5 V is necessary to collect the electrons for the oxidation polymerization, see below. This dark blue color readily turned red by reducing the doped thiophene with application of a negative bias for a long time. An absorption spectrum of a dedoped polythiophene/TiO2 nanohybrid film, PBT-BT/TiO2, prepared with 0.1M BT as a thiophene reactant in BMITFSI is shown in Figure 1. In this case, the polymerization was performed for 80 min. The absorption is featureless and broad at 530 nm, the IPCE initially increases with time and decreases after

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Figure 7. (a) Influence of a thiophene reactant type on an absorption spectral change of the TiO2 film after the photopolymerization for 10 min. (b) Absorption difference, (polythiophene-BTC/TiO2) - (BTC/TiO2), was calculated from the spectra shown in a.

SCHEME 1: Potential Energy Diagram of the TiO2 Electronic States, the Adsorbed BTC and the Thiophene Reactantsa

Figure 8. Voltammograms measured for BTC in PC containing 0.1 M LiClO4. (a) Linear sweep voltammograms obtained for 0.01 M BTC using Au electrode. (b) Linear sweep voltammograms obtained for BTC/ TiO2 (solid line) and TiO2 alone (dotted line). (c) A differential pulse voltammogram observed for a BTC/TiO2 (solid line). A voltammogram for ferrocene carboxylic acid/TiO2 is also shown as a reference (dotted line). The scan rate for the measurements was 50 mV/s.

the excess polymerization time. Similar results were obtained for PBT-BTC/TiO2 (please refer to a Supporting Information). I-V characteristics obtained for PDMBT-BTC/TiO2, photopolymerized for 10 min, are presented in the inset of Figure 10. The short circuit photocurrent density, Jsc, the open circuit photovoltage, Voc, the fill factor, FF, and the solar-to-electric conversion efficiency, η, are 0.92 mA/cm2, 0.38 V, 0.46, and 0.16%, respectively. On comparison with the previously reported data,25-29 the efficiency is similar. Discussion In this study, photoinduced polymerization of thiophene, inside TiO2 nanopores, has been introduced as a novel method to construct nanostructured polythiophene/TiO2 bulk heterojunction films. Following light irradiation, with application of a relatively positive bias (0∼+0.5 V), dark blue doped polythiophene readily appeared on the TiO2 film. This color immediately turned red when the negative bias (-0.3 to -0.4 V) was applied to the TiO2 electrode. This color change behavior implies that the TiO2 nanoporous domain and the polythiophene domain are electronically connected, i.e., interparticle connection and electronic conjugation are established, respectively. Nanohybridization is also achieved throughout the film. This method

a The oxidation potentials of the thiophene reactants are listed in Table 1. The distribution of the BTC redox potentials is shown as a result of the differential pulse voltammogram. The density of the TiO2 electron acceptor states is presented based on the spectroelectrochemical measurement of the TiO2 film in PC with 0.1 M LiClO4 (see Figure S2 in the Supporting Information).

therefore offers not only a dense hybridization but also an electronic connection within the metal oxide or the conducting polymer. Relationship between the Reaction Condition and the Photopolymerization. In this study, the photopolymerization is initiated by the light excitation of the BTC attached to the TiO2 surface. We confirmed that no photopolymerization reaction occurred for the TiO2 film without the BTC attachment; hence, the BTC excitation must be necessary for the polymerization. The BTC in a solvent phase exhibits no coloration (Figure 1); however, only when the BTC is adsorbed on the TiO2 surface, the coloration changed to yellow. Two factors can be considered for this color change: (i) aggregation of BTC molecules on the TiO2 surface, (ii) formation of the charge transfer state from the BTC to the TiO2. An aggregation of the adsorbed BTC molecules may occur since thiophene oligomers, particularly the one without alkyl side chains, are known to form intermolecular electronic interaction.41,42 A self-assembly of the BTC molecules on the metal oxide surface is expected; this method is commonly applied for preparing dye-sensitized semiconductor films.14 However, we did not see any color change when Al2O3 films were employed. If the color change

Polythiophene/TiO2 Nanohybrid Heterojunction Films

Figure 9. Absorption difference, (polythiophene-BTC/TiO2) - (BTC/ TiO2), at a peak wavelength (from Figure 7b) plotted as a function of ∆Gapp.

results solely from the aggregation of BTC molecules, then it is irrelevant whatever metal oxide nanoporous films are employed. In contrast, a BTC-TiO2 charge transfer state may form following the tight adsorption of the BTC molecules to the TiO2 surface. Appearance of similar absorption bands using molecules with enediol moiety were previously reported43,44 and were assigned to CT absorption bands. In this study, the photopolymerization proceeds by the reaction between the oxidized BTC and the oxidized thiophene reactant generated from the sequential electron-transfer reactions. Factors such as electrolyte viscosity, electron-transfer rate from the thiophene reactant to the BTC/TiO2, and photopolymerization time are expected to influence the photopolymerization efficiency. Several electrolytes were employed to compare the influence of the viscosity. If the viscosity is high, the oxidized thiophene reactant present in the electrolyte slowly diffuses away from the TiO2 surface, providing higher probability to react at the surface. The polymerization using BT in BMITFSI (viscosity: 52 cP45) was compared with PC (viscosity: 2.48-2.51 cP46) and acetonitrile (viscosity: 0.341 cP47). In the acetonitrile electrolyte, the photopolymerization was hardly noticeable for 10 min light irradiation while a similar polymerization behavior was clearly observed for both BMITFSI and PC. We found that with acetonitrile to achieve a similar red color requires approximately 3 h. These results clearly show that the viscosity plays an important role for the nanohybrid fabrication. In contrast, light irradiation time significantly influences the polymerization behavior. The chronoamperometric data represented in Figure 2 indicates a sharp decrease in the photoanodic current within 1 min irradiation. This strongly implies an initial rapid polymerization and subsequent gradual polymerization. The absorption difference does not increase linearly with the electrons collected during the photopolymerization (Figure 3c). This nonlinearity shows that the oxidized thiophene reactants do not react with the oxidized BTC or surface-attached oxidized thiophene oligomer, and we suspected that they in fact react with each other to form oligomers or decompose in the electrolyte, or the reactants may no longer have access inside the pore after the prolonged light irradiation. Pore filling by the polythiophene was clearly observed by the FE-SEM measurements in Figure 5. Influence of a Potential Energy Level on a Photopolymerization. The photopolymerization in this study is largely influenced by the electron-transfer efficiency at each reaction step. Following the BTC excitation, the electron injection from the BTC to the TiO2 conduction band is expected to occur on femtosecond or picosecond time scales. This speculation can

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Figure 10. IPCE spectra observed for the sensitized type solar cell based on PDMBT-BTC/TiO2. The PDMBT was photopolymerized for 0 (s), 1 (‚‚‚‚), 10 (- - -) and 30 min (-‚‚-‚‚-). The IPCE obtained for the TiO2 alone is also shown (s s ). The inset shows I-V curves in dark (‚‚‚‚) and under AM1.5 solar simulated light with the light power of 100 mW/cm2 (s).

be justified by the report published by Janssen et al.38 that the electron transfer from the terthiophene attached TiO2 nanoparticle was 99% according to the emission quenching data, Figure 6. This implies fast charge recombination processes, being significant loss processes compared to the ruthenium dye sensitization solar cells.53 Palomares reported54 that the distance between the dye and the TiO2 largely influences charge recombination rates for the dye-sensitized TiO2 films; the shorter the distance, the faster the charge recombination rate. A bithiophene unit is directly linked to the TiO2 through the carboxylic moiety, suggesting that the relatively fast charge recombination rate is expected. The preliminary data regarding the charge recombination rate observed by the transient absorption spectrometer exhibited a half-life time of about 100 µs, being slightly faster than the ruthenium dye/TiO2 observed for the dye-sensitized solar cells.55,56 This result implies that the BTC regeneration rate is similar to this charge recombination rate, or the charge recombination rate between the electron in the TiO2 and the electrolyte is relatively fast. The detailed kinetic studies at the interface are currently in progress, and the results will be published shortly. The photocurrent generated in the solar cell is largely dependent on the photopolymerization degree, Figure 10. After the light irradiation for 1 min, the IPCE spectrum is remarkably different from the BTC/TiO2 film. This drastic change is expected since the photopolymerization rapidly occurs within 1 min and the polymer grows vertically from the TiO2 surface. The IPCE decreases with the light irradiation time although the light harvesting efficiency, light absorption by the film, improves after the photopolymerization. This tendency is explained by the following factors: (i) less accessibility of the redox electrolyte (I3-/I-) inside pores, (ii) slow rereduction of the surface attached polythiophene due to the negative potential shift of the polythiophene, (iii) accelerated charge recombination rate between the oxidized polymer and the electron in the TiO2. (i) As already discussed, the pore inside a TiO2 film is largely occupied by the photopolymerized thiophene after the reaction for the prolonged period; see the FE-SEM images in Figure 5. This decreased pore size hinders the redox active couple, i.e., I3-/I-, to be transported inside the TiO2 pore. Following the light excitation, probability of the oxidized polythiophene to be reduced by the I- significantly decreases. Thus, decreased IPCEs are expected as the photopolymerization proceeds. (ii) When the polymerization occurs, the oxidation potential of the surface attached thiophene shifts negatively.51,52 This negative shift reduces the free energy difference between the oxidized polythiophene and the I-, resulting in slow polythiophene rereduction reaction by the I-. (iii) Following the electron injection from the polymer to the TiO2, the oxidized polymer may be doped by the electrolyte anion. This leads to the formation of the polythiophene conducting state. Since the polythiophene is directly attached to the TiO2 surface, a fast charge recombination between the oxidized thiophene and the electron in the TiO2 is anticipated. Consequently, the structural control at the polymer/TiO2 interface may be vital for improvement of the solar cell performance, thereby clarifying the difference in cell performance between our current results and other relatively superior results based on polythiophene-sensitized solar cells.29,57 Once

Otsuka et al. the electron-transfer processes are kinetically controlled, the performance will be expected to increase. Acknowledgment. We acknowledge Dr. Takao Sakata from the Research Center for Ultra-High Voltage Electron Microscopy in Osaka University for FE-SEM measurements, and Professor Hikaru Kobayashi from the Institute of Scientific and Industrial Research in Osaka University for using the AM1.5 solar simulator. We also thank Dr. Norio Nagayama from Osaka University for the emission measurements. This work was financially supported by Grant-in-Aid for Scientific Research, 18201022 and 18685002, from the Ministry of Education, Culture, Sports, Science and Technology, Japan, and the Nippon Sheet Glass Foundation for Materials Science and Engineering. Supporting Information Available: Spectroelectrochemical data for the TiO2 films and IPCE spectra for the sensitized type solar cell based on PBT-BTC/TiO2. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Vendamme, R.; Onoue, S.-Y.; Nakao, A.; Kunitake, T. Nat. Mater. 2006, 5, 494-501. (2) Valle, K.; Belleville, P.; Pereira, F.; Sanchez, C. Nat. Mater. 2006, 5, 107-111. (3) Lin, Y.; Boeker, A.; He, J.; Sill, K.; Xiang, H.; Abetz, C.; Li, X.; Wang, J.; Emrick, T.; Long, S.; Wang, Q.; Balazs, A.; Russell, T. P. Nature 2005, 434, 55-59. (4) Li, L.; Beniash, E.; Zubarev, E. R.; Xiang, W.; Rabatic, B. M.; Zhang, G.; Stupp, S. I. Nat. Mater. 2003, 2, 689-694. (5) Moeller, S.; Perlov, C.; Jackson, W.; Taussig, C.; Forrest, S. R. Nature 2003, 426, 166-169. (6) Tajima, K.; Li, L.-S.; Stupp, S. I. J. Am. Chem. Soc. 2006, 128, 5488-5495. (7) Cao, Y.; Parker, I. D.; Yu, G.; Zhang, C.; Heeger, A. J. Nature 1999, 397, 414-417. (8) Tessler, N.; Denton, G. J.; Friend, R. H. Nature 1996, 382, 695697. (9) Stutzmann, N.; Friend, R. H.; Sirringhaus, H. Science 2003, 299, 1881-1885. (10) Schmidt-Mende, L.; Fechtenkotter, A.; Mullen, K.; Moons, E.; Friend, R. H.; MacKenzie, J. D. Science 2001, 293, 1119-1122. (11) Kawazoe, H.; Yasukawa, M.; Kyodo, H.; Kurita, M.; Yanagi, H.; Hosono, H. Nature 1997, 389, 939-942. (12) Lee, S.-H.; Deshpande, R.; Parilla, P. A.; Jones, K. M.; To, B.; Mahan, H.; Dillon, A. C. AdV. Mater. 2006, 18, 763-766. (13) Fujishima, A.; Honda, K. Nature 1972, 238, 37-38. (14) O’Regan, B.; Gra¨tzel, M. Nature 1991, 353, 737-739. (15) Liu, Y.; Summers, M. A.; Edder, C.; Frechet, J. M. J.; McGehee, M. D. AdV. Mater. 2005, 17, 2960-2964. (16) Beek, W. J. E.; Wienk, M. M.; Janssen, R. A. J. AdV. Funct. Mater. 2006, 16, 1112-1116. (17) Ravirajan, P.; Haque, S. A.; Durrant, J. R.; Poplavskyy, D.; Bradley, D. D. C.; Nelson, J. J. Appl. Phys. 2004, 95, 1473-1480. (18) Ravirajan, P.; Peiro, A. M.; Nazeeruddin, M. K.; Gra¨tzel, M.; Bradley, D. D. C.; Durrant, J. R.; Nelson, J. J. Phys. Chem. B 2006, 110, 7635-7639. (19) Huisman, C. L.; Goossens, A.; Schoonman, J. Chem. Mater. 2003, 15, 4617-4624. (20) Arango, A. C.; Johnson, L. R.; Bliznyuk, V. N.; Schlesinger, Z.; Carter, S. A.; Horhold, H.-H. AdV. Mater. 2000, 12, 1689-1692. (21) Slooff, L. H.; Wienk, M. M.; Kroon, J. M. Thin Solid Films 2004, 451-452, 634-638. (22) Kwong, C. Y.; Djurisic, A. B.; Chui, P. C.; Cheng, K. W.; Chan, W. K. Chem. Phys. Lett. 2004, 384, 372-375. (23) Grant, C. D.; Schwartzberg, A. M.; Smestad, G. P.; Kowalik, J.; Tolbert, L. M.; Zhang, J. Z. J. Electroanal. Chem. 2002, 522, 40-48. (24) Coakley, K. M.; McGehee, M. D. Appl. Phys. Lett. 2003, 83, 33803382. (25) Nogueira, A. F.; Alonso-Vante, N.; De Paoli, M.-A. Synth. Met. 1999, 105, 23-27. (26) Liu, J.; Kadnikova, E. N.; Liu, Y.; McGehee, M. D.; Frechet, J. M. J. J. Am. Chem. Soc. 2004, 126, 9486-9487. (27) Senadeera, R.; Fukuri, N.; Saito, Y.; Kitamura, T.; Wada, Y.; Yanagida, S. Chem. Commun. 2005, 2259-2261. (28) Huisman, C. L.; Huijser, A.; Donker, H.; Schoonman, J.; Goossens, A. Macromolecules 2004, 37, 5557-5564.

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