Comparison of Electrode Structures and Photovoltaic Properties of

Kyoto 615-8510, Japan, Fukui Institute for Fundamental Chemistry, Kyoto ... Sakyo-ku, Kyoto 606-8103, Japan, and Institute of AdVanced Energy, Kyoto...
0 downloads 0 Views 190KB Size
Download PDF
Langmuir 2006, 22, 11405-11411

11405

Comparison of Electrode Structures and Photovoltaic Properties of Porphyrin-Sensitized Solar Cells with TiO2 and Nb, Ge, Zr-Added TiO2 Composite Electrodes Hiroshi Imahori,*,†,‡ Shinya Hayashi,† Tomokazu Umeyama,† Seunghun Eu,† Akane Oguro,† Soonchul Kang,† Yoshihiro Matano,† Tetsuya Shishido,† Supachai Ngamsinlapasathian,§ and Susumu Yoshikawa*,§ Department of Molecular Engineering, Graduate School of Engineering, Kyoto UniVersity, Nishikyo-ku, Kyoto 615-8510, Japan, Fukui Institute for Fundamental Chemistry, Kyoto UniVersity, 34-4, Takano-Nishihiraki-cho, Sakyo-ku, Kyoto 606-8103, Japan, and Institute of AdVanced Energy, Kyoto UniVersity, Uji, Kyoto 611-0011, Japan ReceiVed May 30, 2006. In Final Form: September 22, 2006 Electrode structures and photovoltaic properties of porphyrin-sensitized solar cells with TiO2 and Nb-, Ge-, and Zr-added TiO2 composite electrodes were examined to disclose the effects of partial substitution of Ti atom by the other metals in the composite electrodes. The TiO2 and Nb-, Ge-, and Zr-added TiO2 composite electrodes were prepared by sol-gel process using laurylamine hydrochloride as a template for the formation of micellar precursors yielding well-defined mesoporous nanocrystalline structures, as in the cases of the formation of silica and titania tubules and nanoparticles by the templating mechanism. The TiO2 and Nb-, Ge-, and Zr-added TiO2 composite electrodes were characterized by transmission electron microscopy, BET surface area analysis, X-ray diffraction analysis, Raman spectroscopy, and impedance measurements. The TiO2 anatase nanocrystalline structure is retained after doping a small amount (5 mol %) of Nb, Ge, or Zr into the TiO2 structure, suggesting the homogeneous distribution of the doped metals with replacing Ti atom by the doped metal. The power conversion efficiency of the porphyrin-sensitized solar cells increases in the order Zr-added TiO2 (0.8%) < Nb-added TiO2 (1.2%) < TiO2 (2.0%) < Ge-added TiO2 cells (2.4%) under the same conditions. The improvement of cell performance of the Ge-added TiO2 cell results from the negative shift of the conduction band of the Ge-added TiO2 electrode. The Ge-added TiO2 cell exhibited a maximum power conversion efficiency of 3.5% when the porphyrin was adsorbed onto the surface of the Ge-added TiO2 electrode with a thickness of 4 µm in MeOH for 1 h.

Introduction Molecular photovoltaics have been challenging research topics in recent years.1-13 In particular, dye-sensitized nanocrystalline TiO2 solar cells1-7 are a promising class of molecular photovoltaics because of their potential low cost and relatively high * Authors to whom correspondence should be addressed. E-mail: [email protected]; [email protected]. † Department of Molecular Engineering, Graduate School of Engineering. ‡ Fukui Institute for Fundamental Chemistry. § Institute of Advanced Energy. (1) (a) Hagfeldt, A.; Gra¨tzel, M. Acc. Chem. Res. 2000, 33, 269. (b) Gra¨tzel, M. Nature (London) 2001, 414, 338. (c) Lewis, N. S. Inorg. Chem. 2005, 44, 6900. (d) Bignozzi, C. A.; Argazzi, R.; Kleverlaan, C. J. Chem. Soc. ReV. 2000, 29, 87. (e) Campbell, W. M.; Burrell, A. K.; Officer, D. L.; Jolley, K. W. Coord. Chem. ReV. 2004, 248, 1363. (2) (a) Kamat, P. V.; Haria, M.; Hotchandani, S. J. Phys. Chem. B 2004, 108, 5166. (b) Benko¨, G.; Myllyperkio¨, P.; Pan, J.; Yartsev, A. P.; Sundstro¨m, V. J. Am. Chem. Soc. 2003, 125, 1118. (c) Asbury, J. B.; Ellingson, R. J.; Ghosh, H. N.; Ferrere, S.; Nozik, A. J.; Lian, T. J. Phys. Chem. B 1999, 103, 3110. (d) Clifford, J. N.; Palomares, E.; Nazeeruddin, M. K.; Gra¨tzel, M.; Nelson, J.; Li, X.; Long, N. J.; Durrant, J. R. J. Am. Chem. Soc. 2004, 126, 5225. (3) (a) Miller, R. J. D.; McLendon, G.; Nozik, A.; Schmickler, W.; Willig, F. Surface Electron-Transfer Processes; Wiley & Sons: New York, 1995. (b) Ushiroda, S.; Ruzycki, N.; Lu, Y.; Spitler, M. T.; Parkinson, B. A. J. Am. Chem. Soc. 2005, 127, 5158. (4) (a) Hirata, N.; Lagref, J.-J.; Palomares, E. J.; Durrant, J. R.; Nazeeruddin, M. K.; Gra¨tzel, M.; Di Censo, D. Chem.sEur. J. 2004, 10, 595. (b) Piotrowiak, P.; Galoppini, E.; Wei, Q.; Meyer, G. J.; Wiewior, P. J. Am. Chem. Soc. 2003, 125, 5278. (c) Nakade, S.; Matsuda, M.; Kambe, S.; Saito, Y.; Kitamura, T.; Sakata, T.; Wada, Y.; Mori, H.; Yanagida, S. J. Phys. Chem. B 2002, 106, 10004. (5) (a) Hara, K.; Kurashige, M.; Dan-oh, Y.; Kasada, C.; Shinpo, A.; Suga, S.; Sayama, K.; Arakawa, H. New. J. Chem. 2003, 27, 783. (b) Wei, M.; Konishi, Y.; Zhou, H.; Yanagida, M.; Sugihara, H.; Arakawa, H. J. Mater. Chem. 2006, 16, 1287. (c) Ito, S.; Liska, P.; Comte, P.; Charvet, R.; Pechy, P.; Bach, U.; Schmidt-Mende, L.; Zakeeruddin, S. M.; Kay, A.; Nazeeruddin, M. K.; Gra¨tzel, M. Chem. Commun. 2005, 4351.

power conversion efficiency (η). The dye sensitization is initiated by the excitation of a sensitizer adsorbed onto a surface of a semiconductor electrode, followed by electron injection into a (6) (a) Wang, Q.; Campbell, W. M.; Bomfantani, E. E.; Jolley, K. W.; Officer, D. L.; Walsh, P. J.; Gordon, K.; Humphry-Baker, R.; Nazeeruddin, M. K.; Gra¨tzel, M. J. Phys. Chem. B 2005, 109, 15397. (b) Watson, D. F.; Marton, A.; Stux, A.; Meyer, G. J. J. Phys. Chem. B 2003, 107, 10971. (c) Watson, D. F.; Marton, A.; Stux, A. M.; Meyer, G. J. J. Phys. Chem. B 2004, 108, 11680. (d) Odobel, F.; Blart, E.; Lagree´, M.; Villieras, M.; Boujtita, H.; Murr, N. E.; Caramori, S.; Bignozzi, C. A. J. Mater. Chem. 2003, 13, 502. (e) Clifford, J. N.; Yahioglu, G.; Milgrom, L. R.; Durrant, J. R. Chem. Commun. 2002, 1260. (f) Koehorst, R. B. M.; Boschloo, G. K.; Savenije, T. J.; Goossens, A.; Schaafsma, T. J. J. Phys. Chem. B 2000, 104, 2371. (7) (a) Horiuchi, T.; Miura, H.; Sumioka, K.; Uchida, S. J. Am. Chem. Soc. 2004, 126, 12218. (b) Kitamura, T.; Ikeda, M.; Shigaki, K.; Inoue, T.; Anderson, N. A.; Ai, X.; Lian, T. Q.; Yanagida, S. Chem. Mater. 2004, 16, 1806. (c) SchmidtMende, L.; Bach, U.; Humphry-Baker, R.; Horiuchi, T.; Miura, H.; Ito, S.; Uchida, S.; Gra¨tzel, M. AdV. Mater. 2005, 17, 813. (d) Wang, Z. S.; Hara, K.; Dan-Oh, Y.; Kasada, C.; Shinpo, A.; Suga, S.; Arakawa, H.; Sugihara, H. J. Phys. Chem. B 2005, 109, 3907. (e) Hara, K.; Sato, T.; Katoh, R.; Furube, A.; Yoshihara, T.; Murai, M.; Kurashige, M.; Ito, S.; Shinpo, A.; Suga, S.; Arakawa, H. AdV. Funct. Mater. 2005, 15, 246. (8) (a) Organic PhotoVoltaics; Sun, S. S., Sariciftci, N. S., Eds.; CRC Press: Boca Raton, FL, 2005. (b) Organic PhotoVoltaics; Brabec, C., Dyakonov, V., Parisi, J., Sariciftci, N. S., Eds.; Springer: Berlin, 2003. (9) (a) Tang, C. W. Appl. Phys. Lett. 1986, 48, 183. (b) Peumans, P.; Yakimov, A.; Forrest, S. R. J. Appl. Phys. 2003, 93, 3693. (c) Gregg, B. A. J. Phys. Chem. B 2003, 107, 4688. (10) (a) Yu, G.; Gao, J.; Hummelen, J. C.; Wudl, F.; Heeger, A. J. Science 1995, 270, 1789. (b) Padinger, F.; Rittberger, R. S.; Sariciftci, N. S. AdV. Funct. Mater. 2003, 13, 85. (c) Wienk, M. M.; Kroon, J. M.; Verhees, W. J. H.; Knol, J.; Hummelen, J. C.; van Hal, P. A.; Janssen, R. A. J. Angew. Chem., Int. Ed. 2003, 42, 3371. (11) (a) Halls, J. J. M.; Walsh, C. A.; Greenham, N. C.; Marseglia, E. A.; Friend, R. H.; Moratti, S. C.; Holmes, A. B. Nature (London) 1995, 376, 498. (b) Schmidt-Mende, L.; Fechtenko¨tter, A.; Mu¨llen, K.; Moons, E.; Friend, R. H.; MacKenzie, J. D. Science 2001, 293, 1119. (c) Nierengarten, J.-F. New J. Chem. 2004, 28, 1177.

10.1021/la061527d CCC: $33.50 © 2006 American Chemical Society Published on Web 11/09/2006

11406 Langmuir, Vol. 22, No. 26, 2006

conduction band of the electrode. The oxidized sensitizer is regenerated by I-/I3- couple in the electrolyte, yielding photocurrent and photovoltage in the cells. Specifically, Gra¨tzel and others developed solar cells based on the sensitization of highly porous TiO2 by ruthenium dyes with the highest power conversion efficiencies (η ) 7-11%), making practical applications feasible.1-5 Nevertheless, further improvement of the power conversion efficiency without employing an expensive metal (i.e., Ru) in the dye is required for possible future application.6,7 Although dye sensitization can be achieved with porous films of other semiconductors with high band gap such as SnO2,14 Fe2O3,15 and ZnO,16 the power conversion efficiencies are not as large as those of the TiO2 solar cells. One of the alternative promising strategies for improving the cell performance is the use of multicomponent metal oxide electrodes. They include (i) coating nanocrystalline metal oxide films with a thin overcoat of a different metal oxide with a higher conduction band edge17 and (ii) replacing or doping TiO2 by other metal oxides.18 One of the authors prepared composite TiO2 electrodes modified with other metal oxides with a surfactant-assisted mechanism using laurylamine hydrochloride (LAHC) and mixed metal alkoxides modified with an acetylacetone (ACA) system for ruthenium dye-sensitized solar cells.19 Although the photovoltaic properties were improved significantly in some cases, systematic comparison of the electrode structures and photovoltaic properties of dyesensitized solar cells with TiO2 composite electrodes modified with other metal oxides has not been performed. We report herein the first systematic comparison of the electrode structures and photovoltaic properties of porphyrin-sensitized solar cells with TiO2 composite electrodes in which the Ti atom is substituted partially by other metals (i.e., Nb, Ge, and Zr). 5-(4-Carboxyphenyl)-10,15,20-tris(2,4,6-trimethylphenyl)porphyrinatozinc(II)6c (ZnP) is chosen as a sensitizer instead of ruthenium complexes (Figure 1). Porphyrins absorb strongly in the blue region at 400-500 nm and moderately in the green region (500-700 nm), showing the potential light-harvesting ability as a dye of dye-sensitized solar cells. One carboxylic group is attached on the meso-phenyl group of tetraphenylporphyrin to guarantee the single anchorage of the porphyrin molecule on the TiO2 surface, whereas methyl groups are introduced at (12) (a) Hiramoto, M.; Fujiwara, H.; Yokoyama, M. Appl. Phys. Lett. 1991, 58, 1062. (b) Tsuzuki, T.; Shirota, Y.; Rostalski, J.; Meissner, D. Sol. Energy Mater. Sol. Cells 2000, 61, 1. (c) Xue, J.; Uchida, S.; Rand, B. P.; Forrest, S. R. Appl. Phys. Lett. 2004, 84, 3013. (d) Huynh, W. U.; Dittmer, J. J.; Alivisatos, A. P. Science 2002, 295, 2425. (e) Liu, J.; Tanaka, T.; Sivula, K.; Alivisatos, A. P.; Fre´chet, J. M. J. J. Am. Chem. Soc. 2004, 126, 6550. (13) (a) Imahori, H. J. Phys. Chem. B 2004, 108, 6130. (b) Imahori, H.; Fukuzumi, S. AdV. Funct. Mater. 2004, 14, 525. (c) Hasobe, T.; Imahori, H.; Kamat, P. V.; Ahn, T. K.; Kim, S. K.; Kim, D.; Fujimoto, A.; Hirakawa, T.; Fukuzumi, S. J. Am. Chem. Soc. 2005, 127, 1216. (d) Imahori, H.; Fujimoto, A.; Kang, S.; Hotta, H.; Yoshida, K.; Umeyama, T.; Matano, Y.; Isoda, S.; Isosomppi, M.; Tkachenko, N. V.; Lemmetyinen, H. Chem.sEur. J. 2005, 11, 7265. (e) Hasobe, T.; Imahori, H.; Fukuzumi, S.; Kamat, P. V. J. Phys. Chem. B 2003, 107, 12105. (14) Dabestani, R.; Bard, A. J.; Campion, M. A.; Fox, M. A.; Mallouk, T. E.; Webber, S. E.; White, J. M. J. Phys. Chem. 1988, 92, 1872. (15) Fitzmaurice, D. J.; Frei, H. Langmuir 1991, 7, 1129. (16) Rensmo, H.; Keis, K.; Lindstrom, H.; Sodergren, S.; Solbrand, A.; Hagfeldt, A.; Lindquist, S.-E.; Wang, L. N.; Muhammed, M. J. Phys. Chem. B 1997, 101, 2598. (17) (a) Chen, S. G.; Diamant, C. Y.; Zaban, A. Chem. Mater. 2001, 13, 4629. (b) Kay, A.; Gra¨tzel, M. Chem. Mater. 2002, 14, 2930. (c) Diamant, Y.; Chen, S. G.; Melamed, O.; Zaban, A. J. Phys. Chem. B 2003, 107, 1977. (d) Palomares, E.; Clifford, J. N.; Haque, S. A.; Lutz, T.; Durrant, J. R. J. Am. Chem. Soc. 2003, 125, 475. (e) Niinobe, D.; Makari, Y.; Kitamura, T.; Wada, Y.; Yanagida, S. J. Phys. Chem. B 2005, 109, 17892. (18) (a) Kumara, G. R. A.; Konno, A.; Tennakone, K. Chem. Lett. 2001, 180. (b) Emeline, A. V.; Furubayashi, Y.; Zhang, X.; Jin, M.; Murakami, T.; Fujishima, A. J. Phys. Chem. B 2005, 109, 24441. (19) (a) Kitiyana, A.; Ngamsinlapasathian, S.; Pavasupree, S.; Yoshikawa, S. J. Solid State Chem. 2005, 178, 1044. (b) Kitiyanan, A.; Kato, T.; Suzuki, Y.; Yoshikawa, S. J. Photochem. Photobiol. A 2006, 179, 130.

Imahori et al.

Figure 1. Porphyrin carboxylic acid (ZnP) used in this study.

2,4,6-positions of the other three meso-phenyl groups to suppress aggregation on the TiO2 surface. This is in sharp contrast with the rather complex molecular structures of ruthenium complexes with multiple binding moieties to TiO2 surfaces. Therefore, when ZnP molecules are densely packed with a perpendicular orientation to the composite TiO2 surface, we can expect to disclose the close relationship between the molecular structure, molecular arrangement on the electrode, and the photovoltaic function. Taking into account the fact that the open circuit potential (Voc) of dye-sensitized solar cells is dependent on the difference in the conduction band edge energies (ECB) of these metal oxides and the redox potential of I-/I3- couple,20 partial replacement of Ti atom by Nb, Ge, or Zr in the composite electrode would have a large impact on the photovoltaic properties including Voc as well as the composite electrode structure. Experimental Section Materials and Methods. Melting points were recorded on a Yanagimoto micro-melting point apparatus and are not corrected. 1H NMR spectra were measured on a JEOL EX-400 spectrometer. Matrix-assisted laser desorption/ionization (MALDI) mass spectra (MS) were measured on a Shimadzu KOMPACT MALDI II. All solvents and chemicals were of reagent-grade quality, purchased commercially, and used without further purification unless otherwise noted. Tetrabutylammonium hexafluorophosphate used as a supporting electrolyte for the electrochemical measurements was obtained from Fluka and recrystallized from methanol. Thin-layer chromatography and column chromatography were performed with Alt. 5554 DC-Alufolien Kieselgel 60 F254 (Merck) and Silica gel 60N (Kanto Chemicals), respectively. Tetraisopropylorthotitanate (TIPT), germanium(IV) isopropoxide (Geis), niobium(V) ethoxide, and zirconium(IV) butoxide as starting metal oxide materials were purchased from Aldrich. Transmission electron micrographic (TEM) images were recorded using a JEM-200CX transmission electron microscope (JEOL, Japan). The crystal structure of mixed metal oxides was analyzed by X-ray diffraction (XRD) with Cu KR radiation (RIGAKU-A2). BET surface area of samples was recorded using nitrogen adsorption desorption analysis (BELSORP 18 PLUS). Raman spectra were recorded on a JASCO NRS-2000 instrument with an Ar+ laser source of 514.5 nm wavelength in a macroscopic configuration. The thickness of the films was determined using surface roughness/profile measuring instrument (SURFCOM 130A, ACCRETECH). UV-visible spectra of solutions and films on electrodes were recorded using a Lambda 900 spectrophotometer (Perkin-Elmer, USA). All electrochemical measurements were carried out in a standard three-electrode system using an ALS 630a electrochemical analyzer. Redox potential of ZnP was measured in CH2Cl2 containing 0.1 M Bu4NPF6. Synthesis of Porphyrins. Porphyrins were synthesized fol(20) Sayama, K.; Sugihara, H.; Arakawa, H. Chem. Mater. 1998, 10, 3825.

Porphyrin-Sensitized Solar Cells and TiO2 Electrodes lowing the same procedures as described previously.21 5-(4Carboxyphenyl)-10,15,20-tris(2,4,6-trimethylphenyl)porphyrin (2) and ZnP have been reported, but the synthetic details have not been described.6b,c 5-(4-Methoxycarbonylphenyl)-10,15,20-tris(2,4,6-trimethylphenyl)porphyrin (1). To a solution of 2,4,6-trimethylbenzaldehyde (3.6 g, 24 mmol) in CHCl3 (800 mL) was added methyl 4-formylbenzoate (1.3 g, 8 mmol), pyrrole (2.2 mL, 32 mmol), and BF3OEt2 (1.3 mL). The solution was stirred at room temperature for 2 h. Then, p-chloranil (5.0 g) was added and stirred at room temperature for 1 h. After Et3N (1.0 mL) was added, the solvent was removed in vacuo. Column chromatography on silica gel (CH2Cl2/hexane ) 1:1) afforded 1 as a purple red solid (1.0 g, 1.3 mmol, 16% yield). mp > 300°C. 1H NMR (400 Hz, CD2Cl2): δ 8.73-8.43 (br s, 8H), 8.42 (d, J ) 8.4 Hz, 2H), 8.32 (d, J ) 8.0 Hz, 2H), 7.30 (s, 6H), 4.08 (s, 3H), 2.61 (s, 9H), 1.86 (s, 6H), 1.84 (s, 12H). MS (MALDITOF) m/z 799 (M + H+). 5-(4-Carboxyphenyl)-10,15,20-tris(2,4,6-trimethylphenyl)porphyrin (2). To a solution of 1 (1.0 g, 1.3 mmol) in THF (100 mL) was added a solution of KOH (2.5 g) in H2O (50 mL). The solution was refluxed for 2 h. After the reaction mixture was cooled to room temperature, aqueous 1 M HCl (10 mL) was added. The solution was washed with saturated aqueous NaHCO3 (50 mL) and H2O (50 mL × 3) and dried over anhydrous sodium sulfate, and then the solvent was removed in vacuo. Recrystallization from CH2Cl2/MeOH gave 2 as a purple red solid (0.92 g, 1.2 mmol, 92% yield). mp > 300 °C. 1H NMR (400 Hz, CD2Cl2 + CD3OD): δ 8.78 (br s, 2H), 8.63 (br s, 6H), 8.42 (d, J ) 8.4 Hz, 2H), 8.28 (d, J ) 8.4 Hz, 2H), 7.29 (s, 2H), 7.28 (s, 4H), 2.61 (s, 3H), 2.60 (s, 6H), 1.86 (s, 6H), 1.83 (s, 12H). MS (MALDI-TOF) m/z 786 (M + H+). 5-(4-Carboxyphenyl)-10,15,20-tris(2,4,6-trimethylphenyl)porphyrinatozinc(II) (ZnP). To a solution of 2 (0.5 g, 0.64 mmol) in CH3Cl (60 mL) was added a solution of Zn(OAc)2 (1.0 g) in MeOH (20 mL). The solution was refluxed for 2 h. The reaction mixture was washed with H2O (30 mL × 3) and dried over anhydrous sodium sulfate, and then the solvent was removed in vacuo. Recrystallization from CH2Cl2/MeOH gave ZnP as a purple red solid (0.52 g, 0.61 mmol, 95% yield). mp > 300 °C. 1H NMR (400 Hz, CD2Cl2 + CD3OD): δ 8.77 (d, J ) 4.4 Hz, 2H), 8.63 (d, J ) 4.4 Hz, 2H), 8.62 (s, 4H), 8.41 (d, J ) 8.0 Hz, 2H), 8.32 (d, J ) 7.6 Hz, 2H), 7.29 (s, 6H), 2.62 (s, 9H), 1.86 (s, 6H), 1.84 (s, 12H). MS (MALDI-TOF) m/z 849 (M + H+). Preparation of Composite Metal Oxide Gel. The synthesis of the composite metal oxide gels is based on a sol-gel process in which the main goal is to obtain homogeneous composite metal oxide nanocrystalline with a high degree of atomic dispersion of additive metal oxide in a TiO2 mesoporous network.19,22 Acetylacetone (ACA) is an organic modifier which allows control of the reactivity of the two metal alkoxides by the formation of larger, slower reacting molecules by ligand exchange. Laurylamine hydrochloride (LAHC) is an organic surfactant, which acts as a template, yielding micellar assemblies with metal alkoxide precursors to fabricate well-defined metal oxide composite nanocrystalline.19,22 Namely, LAHC in the aqueous solution forms globular aggregates which are presumed to be finely divided bilayer-like assemblies in a spongelike phase. In the presence of the alkoxide, partially hydrolyzed alkoxide penetrates the globular surfactant assemblies. Then, the polymerization of hydrolyzed alkoxide proceeds on the surface of the assemblies to yield well-defined nanostructures. Such a LAHC-mediated templating method has been well-established for the formation of well-defined silica and titania tubules and nanoparticles.22 TiO2 and 5% Nb-, Ge-, and Zr-Added TiO2 Composite Gel. To a mixture of TIPT, Geis, and ACA with a molar ratio of [TIPT (21) Luo, C.; Guldi, D. M.; Imahori, H.; Tamaki, K.; Sakata, Y. J. Am. Chem. Soc. 2000, 122, 6535. (22) (a) Adachi, M.; Harada, T.; Harada, M. Langmuir 1999, 15, 7097. (b) Adachi, M.; Harada, T.; Harada, M. Langmuir 2000, 16, 2376. (c) Adachi, M.; Murata, Y.; Harada, T.; Yoshikawa, S. Chem. Lett. 2000, 942. (d) Peng, T.; Hasegawa, A.; Qiu, J.; Hirao, K. Chem. Mater. 2003, 15, 2011. (e) Adachi, M.; Murata, Y.; Takao, J.; Jiu, J.; Sakamoto, M.; Wang, F. J. Am. Chem. Soc. 2004, 126, 14943.

Langmuir, Vol. 22, No. 26, 2006 11407 + Geis]:[ACA] ) 1:1, 0.1 M laurylamine hydrochloride (LAHC) aqueous solution was added, yielding a final molar ratio of [TIPT + Geis]:[LAHC] ) 4:1.19 The reaction mixture was stirred at 40 °C for 24 h and then aged at 80 °C for further 7 days. The reaction mixture was added to 2-propanol and centrifuged at 7500 rpm for 10 min to remove LAHC. The resultant gel was diluted with poly(ethylene glycol) (PEG) aqueous solution (PEG:H2O ) 1:1 (wt %)), yielding a wt % ratio of gel:PEG ) 4:1. The resultant gel solution was further stirred for 2 h for the deposition of the gel on electrodes. Other gels (TiO2, 5% Nb-added TiO2, or 5% Zr-added TiO2) were also prepared following the same procedure as described for the 5% Ge-added TiO2 composite gel. Preparation of Porphyrin-Modified TiO2 Electrode. Nanoporous films were prepared from the TiO2 or metal oxide composite gel.19 The gel was deposited on a transparent conducting glass (FTO, Asahi Glass, SnO2:F, 9.4 Ω/sq) by using the doctor blade technique. The films were annealed at 673 K for 10 min, followed by the same deposition and annealing to obtain TiO2 or metal oxide composite films with a suitable thickness. Then, the films were annealed at 723 K for 1.5 h. The TiO2 or composite TiO2 electrodes were immersed in 0.3 mM solution of ZnP at room temperature. After dye adsorption, the dye-coated electrodes were rinsed with the same solvent. The amounts of porphyrins adsorbed on the electrodes were determined by measuring absorbance at the Soret band of ZnP (λ ) 429.7 nm,  ) 5.03 × 105) that was dissolved from the dye-adsorbed electrodes into DMF containing 0.1 M NaOH. Impedance of TiO2 and metal oxide composite electrodes was measured using an ALS 630a electrochemical analyzer. All measurements were performed in a three-electrode configuration using a TiO2 or TiO2 composite working electrode, Pt counter electrode, and Ag/AgCl (saturated KCl) reference electrode. The electrolyte was aqueous 0.1 M Na2B4O7, the pH of which was adjusted with HCl and NaOH. Photovoltaic Measurements. The photovoltaic measurements were performed in a sandwich cell consisting of the dye-sensitized electrode as the working electrode and a platinum-coated conducting glass electrode as the counter electrode.19 The two electrodes were placed on top of each other using a thin transparent film of Surlyn polymer as a spacer to form the electrolyte space. A thin layer of electrolyte (0.1 M LiI, 0.05 M I2, 0.6 M 2,3-dimethyl-1-propylimidazolium iodide, and 0.5 M 4-tert-butylpyridine in acetonitrile) was introduced onto the interelectrode space. The IPCE values and current-voltage characteristics were determined by using a potentiostat (Bunko-Keiki Co., Ltd., Model HCSSP-25) irradiated with simulated AM 1.5 solar light (100 mW cm-2, Bunko-Keiki Co., Ltd., Model CEP-2000). All the experimental values were given as an average from five independent measurements.

Results and Discussion Characterization of TiO2 and Nb-, Ge-, and Zr-Added TiO2 Composite Metal Oxides. The gel for fabrication on the FTO electrode was prepared from tetraisopropylorthotitanate with and without germanium(IV) isopropoxide, niobium(V) ethoxide, or zirconium(IV) butoxide as starting metal oxide materials (Experimental Section).19 To compare the structures of the TiO2 composite metal oxides, relatively small and the same molar amount (5%) of additive (i.e., germanium(IV) isopropoxide, niobium(V) ethoxide, and zirconium(IV) butoxide) is employed for the preparation of TiO2-Nb2O5, GeO2, and ZrO2 metal oxide composites. Such conditions would allow us to maintain the intact TiO2 nanocrystalline structure in the TiO2 composite metal oxides largely, making it possible to compare the electrode structure and photovoltaic properties accurately (vide infra). To observe the morphology of the samples, some powders were suspended in 2-propanol and deposited on a carbon membrane for TEM analyses. Figure 2 shows the TEM micrographs for the bare TiO2 and Nb-, Ge-, and Zr-added TiO2 powders annealed at 723 K for 1.5 h. Roughly spherical nanocrystalline particles are formed. The average size of the

11408 Langmuir, Vol. 22, No. 26, 2006

Imahori et al.

Figure 2. Transmission electron microscopy images of (a) TiO2, (b) Nb-added TiO2, (c) Ge-added TiO2, and (d) Zr-added TiO2.

Figure 3. XRD patterns of (a) TiO2, (b) Nb-added TiO2, (c) Geadded TiO2, and (d) Zr-added TiO2.

particles increases in the order Zr-added TiO2 (8.2 ( 2.1 nm) < Ge-added TiO2 (9.6 ( 1.8 nm) < Nb-added TiO2 (9.9 ( 1.8 nm) < TiO2 (12.1 ( 2.8 nm). The smaller particle size for the composite metal oxide relative to the TiO2 may be attributed to mismatch of the sizes of the doped atoms and Ti atom (vide infra). In accordance with the trend on the particle size, the BET surface area decreases in the order Zr-added TiO2 (150 m2 g-1) > Ge-added TiO2 (140 m2 g-1) > Nb-added TiO2 (130 m2 g-1) > TiO2 (110 m2 g-1). Figure 3 displays the XRD patterns of the bare TiO2 and the Nb-, Ge-, and Zr-added TiO2. All the peaks in each sample can be assigned to anatase. No XRD patterns arising from rutile are seen. It should be noted here that all the samples exhibit similar XRD patterns. This implies that the TiO2 anatase nanocrystalline structure is retained after doping a small amount (5 mol %) of Nb, Ge, or Zr in the TiO2 structure. Figure 4 depicts Raman spectra of the bare TiO2 and Nb-, Ge-, and Zr-added TiO2. All the samples reveal similar Raman peaks arising from anatase structure. This is also in good agreement with the results obtained from the XRD analyses. As both nonhomogeneous distribution of the doped metals in the nanocrystal and other nanocrystalline structures except the anatase one were not detected by the TEM, XRD, and Raman spectroscopic measurements, the dopant atoms with a relatively low level of doping (5 mol %) are suggested to sit substitutionally on Ti sites in the anatase crystal structure, yielding the homogeneous distribution of the doped atoms in the anatase nanocrystalline structure.19 Mott-Schottky analyses of impedance of the bare TiO2 and Nb-, Ge-, and Zr-added TiO2 were performed to estimate the conduction band potentials (ECB).23 Mott-Schottky plots (C-2 vs V) of the electrodes are obtained at 100 Hz, which falls within

Figure 4. Raman spectra of (a) TiO2, (b) Nb-added TiO2, (c) Geadded TiO2, and (d) Zr-added TiO2.

Figure 5. Mott-Schottky plots for the Ge-added TiO2 electrode at electrolyte pH ) 1.1 (downward triangle), 5.0 (circle), 6.4 (square), 8.1 (upward triangle), and 11.2 (cross).

the capacitive regime of the Bode plot at each electrolyte pH (Figure 5 and Supporting Information S1-S3). The flat-band potentials (EFB) of the bare TiO2 and the Nb-, Ge-, and Zr-added TiO2 electrodes are estimated as -0.08, 0.03, -0.16, and -0.19 V vs NHE, respectively, at pH ) 7 from the x-intercepts of the Mott-Schottky plots.24 It is generally known that the ECB of many n-type semiconductors is 0.1-0.3 eV more negative than (23) (a) Mott, N. F. Proc. R. Soc. London, Ser. A 1939, 171, 27. (b) Schottky, W. Z. Phys. 1942, 118, 539. (c) Bolts, J. M.; Wrighton, M. S. J. Phys. Chem. 1976, 80, 2641. (24) (a) Fabregat-Santiago, F.; Garcia-Belmonte, G.; Bisquert, J.; Bogdanoff, P.; Zaban, A. J. Electrochem. Soc. 2003, 150, E293. (b) Chun, W.-J.; Ishikawa, A.; Fujisawa, H.; Takata, T.; Kondo, J. N.; Hara, M.; Kawai, M.; Matsumoto, Y.; Domen, K. J. Phys. Chem. B 2003, 107, 1798.

Porphyrin-Sensitized Solar Cells and TiO2 Electrodes

Langmuir, Vol. 22, No. 26, 2006 11409

Table 1. Photovoltaic Performance of ZnP-Sensitized Solar Cells electrode 5% Geadded TiO2 TiO2 5% Nbadded TiO2 5% Zradded TiO2

JSC/mA cm-2 (Γ total/mol cm-2)

VOC/V (ECB/V)

4.9 ( 0.3 (1.2 × 10-7) 4.6 ( 0.1 (1.4 × 10-7) 3.4 ( 0.1 (8.5 × 10-8) 1.7 ( 0.1 (1.4 × 10-7)

0.69 ( 0.01 (-0.46) 0.64 ( 0.01 (-0.38) 0.54 ( 0.01 (-0.27) 0.65 ( 0.02 (-0.49)

ff

η/%

0.71 ( 0.01

2.4 ( 0.2

0.70 ( 0.01

2.0 ( 0.1

0.63 ( 0.01

1.2 ( 0.1

0.69 ( 0.01

0.8 ( 0.1

EFB.25 Assuming the difference of 0.3 eV, the ECB values of the bare TiO2 and the Nb-, Ge-, and Zr-added TiO2 electrodes are calculated to be -0.38, -0.27, -0.46, and -0.49 V, respectively, at pH ) 7 (Table 1). The ECB value of the bare TiO2 electrode is largely similar to the previously reported values (ca. -0.5 V vs NHE).1-7,20 The valence band edge potentials (EVB) of the bare TiO2 and Nb-, Ge-, and Zr-added TiO2 electrodes are also calculated to be +2.82, +2.76, +2.88, and +2.82 V, respectively, at pH 7, from the onset of the UV-visible absorption spectra of the electrodes. Oxidation potential of the porphyrin excited singlet state (ZnP+/1ZnP*) is determined to be -1.10 V from the E00 value (2.10 eV) and the first oxidation potential of ZnP (ZnP+/ ZnP ) 1.00 V vs NHE) in CH2Cl2. From these values, photoinduced electron transfer from the porphyrin excited singlet state to the conduction band of each electrode is possible owing to the exothermic reaction. The driving force for photoinduced electron transfer increases in the order Zr-added TiO2 (0.61 eV) < Ge-added TiO2 (0.64 eV) < TiO2 (0.72 eV) < Nb-added TiO2 (0.83 eV). Adsorption of Porphyrin onto TiO2 Composite Electrodes. ZnP was adsorbed onto the TiO2 and Nb-, Ge-, and Zr-added TiO2 electrodes with a film thickness of 4 µm. First, these electrodes were immersed in a mixture of tert-butyl alcohol and acetonitrile (1:1 (v/v)) containing 0.3 mM ZnP for 12 h at room temperature. Then the dye-coated electrodes were rinsed with the same solvent. The total amounts (Γtotal) of the porphyrins adsorbed on these electrodes were determined by measuring absorbance at the Soret band of the dye molecules, which were dissolved from the dye-adsorbed electrode surface into DMF containing 0.1 M NaOH. The Γtotal values for the TiO2 and the Nb-, Ge-, and Zr-added TiO2 electrodes are determined to be 1.4 × 10-7, 8.5 × 10-8, 1.2 × 10-7, and 1.4 × 10-7 mol cm-2, respectively (Table 1). Taking into account the surface area of these electrodes, the surface coverage (Γ) for the TiO2 and Nb-, Ge-, and Zr-added TiO2 electrodes is calculated to be 1.4 × 10-10, 1.4 × 10-10, 1.3 × 10-10, and 1.4 × 10-10 mol cm-2, respectively, showing similar packing of the porphyrin molecules on each electrode surface. Assuming that the porphyrin molecules bearing one carboxylic group are densely packed onto the electrode surface where the porphyrin plane is perpendicular to the electrode surface, the Γ values are estimated to be 1.2 × 10-10 mol cm-2, which agrees well with the experimental values. Thus, one can conclude that the porphyrin molecules are wellpacked on each electrode surface to form the porphyrin monolayer on the surfaces. Figure 6 shows absorption spectra of ZnP adsorbed onto the TiO2 and the Nb-, Ge-, and Zr-added TiO2 electrodes and of ZnP in CH2Cl2. Thickness of the TiO2 electrode was adjusted to be 700-1000 nm to obtain the shape and peak position of the spectra accurately. The Soret band is split into two bands in the absorption spectra of ZnP on the TiO2 and Nb-, Ge-, and Zr-added TiO2 electrodes. Such splittings result from the exciton (25) (a) Matsumoto, Y.; Omae, K.; Watanabe, I.; Sato, E. J. Electrochem. Soc. 1986, 133, 711. (b) Matsumoto, Y. J. Solid State Chem. 1996, 126, 227.

Figure 6. UV-visible absorption spectra of (a) TiO2, (b) Nb-added TiO2, (c) Ge-added TiO2, and (d) Zr-added TiO2 electrodes modified with ZnP and of (e) ZnP in CH2Cl2. Thickness of the TiO2 electrode was adjusted to be 700-1000 nm to obtain the shape and peak position of the spectra accurately. The spectra are normalized at the Soret band for comparison.

Figure 7. Current-voltage characteristics of (a) Ge-added TiO2, (b) TiO2, (c) Nb-added TiO2, and (d) Zr-added TiO2 cells sensitized with ZnP under AM 1.5 conditions: electrolyte 0.1 M LiI, 0.05 M I2, 0.6 M 2,3-dimethyl-1-propyl imidazolium iodide, and 0.5 M 4-tert-butylpyridine in CH3CN; input power: AM 1.5 under simulated solar light (100 mW cm-2).

coupling between the closely packed porphyrins on the electrode surfaces (vide infra). Photovoltaic Properties. To compare the cell performance, current-voltage characteristics were measured using the 4-µmthick TiO2 and the Nb-, Ge-, and Zr-added TiO2 electrodes modified with ZnP and a Pt counter electrode under AM 1.5 (100 mW cm-2) conditions (Figure 7). Short-circuit photocurrent density (JSC), VOC, fill factor (ff), and η values of the TiO2 and the Nb-, Ge-, and Zr-added TiO2 cells are summarized in Table 1. The JSC value largely parallels with the amounts of porphyrin adsorbed onto the electrode except in the case of the Zr-added TiO2 cell. From the qualitative comparison of radii of r(Ti) ) 1.47 Å, r(Nb) ) 1.47 Å, r(Ge) ) 1.22 Å, and r(Zr) ) 1.60 Å, the smaller JSC value of the Zr-added TiO2 cell may originate from the steric effect of substituent by larger Zr atom, which causes the formation of defect site in the TiO2 nanocrystalline structure.26 As expected, the VOC value increases with shifting the ECB value in the negative direction, with the exception of the Zr-added TiO2 cell, which is consistent with the trend on the JSC value. A similar relationship between VOC of device and the gap between the oxidation potential of I- and the conduction band edge of oxide and between the oxidation potential of donor and (26) One possible reason for the lower short circuit current density in the Zr-doped TiO2 cell may be the loss of electron injection yield to an insufficient driving force compared to the other cells. Incomplete electron injection may result even when the driving force is nominally positive, on account of a certain degree of disorder in the site energies.

11410 Langmuir, Vol. 22, No. 26, 2006

Figure 8. Action spectra of the Nb-added TiO2 cell (circle), the TiO2 cell (cross), the Ge-added TiO2 cell (square), and Zr-added TiO2 cell (triangle). Conditions: electrolyte 0.1 M LiI, 0.05 M I2, 0.6 M 2,3-dimethyl-1-propyl imidazolium iodide, and 0.5 M 4-tertbutylpyridine in CH3CN; input power: AM 1.5 under simulated solar light (100 mW cm-2).

reduction potential of acceptor has been reported for other dyesensitized20 and bulk heterojunction27 solar cells, respectively. The fill factors of the TiO2 and the Nb-, Ge-, and Zr-added TiO2 cells are roughly similar. Overall, the η value increases in the order Zr-added TiO2 (0.8%) < the Nb-added TiO2 (1.2%) < the TiO2 (2.0%) < the Ge-added TiO2 cells (2.4%). A series of photocurrent action spectra are recorded and compared against the absorption spectra (Figure 8). The overall photoelectrochemical responses parallel the absorption features of the porphyrin on the electrodes (Figure 6), implying the involvement of the porphyrin moiety in the photocurrent generation.28 These results are in good agreement with the photoelectrochemical and photovoltaic behavior of porphyrin-sensitized TiO2 devices.6 The effect of the film thickness on the photovoltaic properties was examined to improve the cell performance of the Ge-added TiO2 cell, which exhibits the highest cell performance among the four cells under the same conditions. The η value increases initially with increasing the film thickness, reaching a maximum of η ) 2.4% with the film thickness of 4 µm and decreases gradually (Figure 9). Then the effects of solvent and its immersing time for the adsorption of ZnP were evaluated using the Geadded TiO2 cell with a thickness of 4 µm. The η value of the Ge-added TiO2 cell is strongly dependent on solvents and its immersing time for the porphyrin adsorption (Figure 10). Alcoholic solvent results in an increase of the η value (3.1% for MeOH and the immersing time of 12 h), whereas nonalcoholic solvent results in a decrease of the η value (1.1% for THF and the immersing time of 12 h). The Γ values for ZnP (1.1 × 10-10 mol cm-2 for MeOH and 1.2 × 10-10 mol cm-2 for tert-butyl alcohol and acetonitrile (1:1)) are virtually the same when the alcoholic solvents are used, whereas the value (3.3 × 10-11 mol cm-2 for THF) is low when a nonalcoholic solvent is used. Thus, the low porphyrin density on the electrode surface obtained from the nonalcoholic solvent is responsible for the small η value relative to that from the adsorption in the alcoholic solvents. The maximum η value (3.5%) of the present cell is obtained when the porphyrin is adsorbed onto the Ge-added TiO2 electrode surface in MeOH for 1 h. Namely, for the ZnP-sensitized cell, the Jsc of 7.1 mA cm-2, the Voc of 0.72 V, and the ff of 0.69 yields (27) (a) Brabec, C. J.; Cravino, A.; Meissner, D.; Sariciftci, N. S.; Fromherz, T.; Rispens, M. T.; Sanchez, L.; Hummelen, J. C. AdV. Funct. Mater. 2001, 11, 374. (b) Scharber, M. C.; Muhlbacher, D.; Koppe, M.; Denk, P.; Waldauf, C.; Heeger, A. J.; Brabec, C. AdV. Mater. 2006, 18, 789. (28) No splitting of the Soret band on the action spectra may result from the saturation behavior of IPCE value with a high value of ∼80% under which light scattering and reflection at the interface limit the IPCE value.

Imahori et al.

Figure 9. Current-voltage characteristics of the ZnP-sensitized Ge-added TiO2 cells with a film thickness of (a) 2 µm, (b) 4 µm, (c) 6 µm, and (d) 8 µm under AM 1.5 conditions: electrolyte 0.1 M LiI, 0.05 M I2, 0.6 M 2,3-dimethyl-1-propyl imidazolium iodide, and 0.5 M 4-tert-butylpyridine in CH3CN; input power: AM 1.5 under simulated solar light (100 mW cm-2).

Figure 10. Current-voltage characteristics of the ZnP-sensitized Ge-added TiO2 cells under AM 1.5 conditions. The Ge-added TiO2 electrode was immersed in (a) MeOH for 1 h, (b) MeOH for 12 h, (c) a mixed solvent of t-BuOH:CH3CN ) 1:1 for 12 h, and (d) THF for 12 h. Conditions: electrolyte 0.1 M LiI, 0.05 M I2, 0.6 M 2,3dimethyl-1-propyl imidazolium iodide, and 0.5 M 4-tert-butylpyridine in CH3CN; input power: AM 1.5 under simulated solar light (100 mW cm-2).

the η value, derived from the following equation: η ) Jsc × Voc × ff, of 3.5%. The highest η value obtained from the alcoholic solvent (i.e., MeOH) with the short immersing time (1 h) may result from a change in the orientation or the binding mode of the porphyrin molecule on the electrode surface.29

Conclusions We successfully compared the electrode structures and photovoltaic properties of porphyrin-sensitized solar cells with the TiO2 and the Nb-, Ge-, and Zr-added TiO2 composite electrodes to disclose the effects of partial substitution of Ti atom by the other metals for the first time. The microscopic and spectroscopic measurements revealed that the TiO2 anatase nanocrystalline structure is retained after doping a small amount (5 mol %) of Nb, Ge, or Zr into the TiO2 structure, suggesting the homogeneous distribution of the doped metals with replacing Ti atom by the doped metal. The power conversion efficiency of the Ge-added TiO2 cells was improved by 20% relative to that of the TiO2 cell under the same conditions due to the rise of the conduction band of the Ge-added TiO2 electrode. The Ge-added TiO2 cell exhibited a maximum power conversion efficiency of 3.5% when the porphyrin was adsorbed onto the Ge-added TiO2 (29) Pilkenton, S.; Xu, W. Z.; Raftery, D. Anal. Sci. 2001, 17, 125.

Porphyrin-Sensitized Solar Cells and TiO2 Electrodes

electrode surface with a thickness of 4 µm in MeOH for 1 h. Such systematic investigation will provide valuable and basic information on the design of dye-sensitized solar cells. Acknowledgment. This work was supported by a Grantin-Aid (No. 11740352 to H.I.) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. H.I. is also thankful for a Grant-in-Aid from MEXT, Japan (21st Century COE on Kyoto University Alliance for Chemistry),

Langmuir, Vol. 22, No. 26, 2006 11411

NEDO, Sekisui foundation, and Kurata foundation for financial support. A part of this work was supported by “Nanotechnology Support Project” of the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. Supporting Information Available: Mott-Schottky plots (S1S3) (PDF). This information is available free of charge via the Internet at http://pubs.acs.org. LA061527D