Dye-Sensitized Nanocrystalline TiO2 Solar Cells Based on Ruthenium

cell sensitized by Ru(dcphen)2(NCS)2(TBA)2 exceeded that of ZnO, SnO2, and In2O3 solar cells. A solar energy to ... photovoltage (Voc) of 0.74 V, and ...
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5992

Langmuir 2001, 17, 5992-5999

Dye-Sensitized Nanocrystalline TiO2 Solar Cells Based on Ruthenium(II) Phenanthroline Complex Photosensitizers Kohjiro Hara, Hideki Sugihara, Yasuhiro Tachibana, Ashraful Islam, Masatoshi Yanagida, Kazuhiro Sayama, and Hironori Arakawa* National Institute of Advanced Industrial Science and Technology (AIST), Photoreaction Control Research Center (PCRC), 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan

Gaku Fujihashi, Takaro Horiguchi, and Tohru Kinoshita Sumitomo Osaka Cement Co. Ltd., 585 Toyotomi, Funabashi, Chiba 274-8601, Japan Received March 6, 2001. In Final Form: June 29, 2001 We have synthesized four carboxylated Ru(II) phenanthroline complexes with different numbers of carboxyl groups, cis-bis(4,7-dicarboxy-1,10-phenanthroline)dithiocyanato ruthenium(II) (Ru(dcphen)2(NCS)2), cis-bis(4-monocarboxy-1,10-phenanthroline)dithiocyanato ruthenium(II) (Ru(mcphen)2(NCS)2), cis-(4,7-dicarboxy-1,10-phenanthroline)(1,10-phenanthroline)dithiocyanato ruthenium(II) (Ru(dcphen)(phen)(NCS)2), and cis-(4-monocarboxy-1,10-phenanthroline)(1,10-phenanthroline)dithiocyanato ruthenium(II) (Ru(mcphen)(phen)(NCS)2), as photosensitizers for oxide semiconductor solar cells. We have studied photovoltaic properties of dye-sensitized nanocrystalline semiconductor solar cells based on Ru phenanthroline complexes and an iodine redox electrolyte. The photovoltaic performance of the TiO2 solar cell sensitized by Ru(dcphen)2(NCS)2(TBA)2 exceeded that of ZnO, SnO2, and In2O3 solar cells. A solar energy to electricity conversion efficiency (η) of 6.6% was obtained under the standard AM 1.5 irradiation (100 mW cm-2, JIS A class) with a short-circuit photocurrent density (Jsc) of 12.5 mA cm-2, an open-circuit photovoltage (Voc) of 0.74 V, and a fill factor (ff) of 0.71. Monochromatic incident photon to current conversion efficiency was 78% at 526 nm. Deoxycholic acid as a coadsorbate and decreasing film thickness improved Voc especially due to suppression of the dark current reaction corresponding to the reduction of triiodide ions with injected electrons. The improved photovoltaic property due to the added coadsorbate suggests that some aggregates of the Ru complex suppress efficient electron injection to the semiconductor. The position and number of carboxyl groups attached to the phenanthroline ligand as an anchor affect photosensitizer performance significantly, suggesting that the anchoring configuration of Ru phenanthroline complexes on the semiconductor surface is important to efficient photovoltaic cell performance. Two carboxyl groups attached to phenanthroline ligands are necessary for effective electron injection.

Introduction A dye-sensitized solar cell based on cis-bis(4,4′-dicarboxy-2,2′-bipyridine)dithiocyanato ruthenium(II) (Ru(dcbpy)2(NCS)2), a nanocrystalline TiO2 thin film photoelectrode, and an I-/I3- redox electrolyte (Gra¨tzel cell) is being studied intensively due to its high cell performance. A highly efficient solar energy to electricity conversion efficiency (η) exceeding 10% was attained under AM 1.5 irradiation.1 Maximum monochromatic photon to current conversion efficiency (IPCE) was 80% at 450-600 nm. Up to now, Ru complexes having ligands such as polypyridine have been extensively studied as a photosensitizer for dye-sensitized nanocrystalline oxide semiconductor solar cells.1-12 The trithiocyanato 4,4′4′′-tricarboxy-2,2′:6′,2′′terpyridine ruthenium(II) complex (black dye), which is * Corresponding author. E-mail: [email protected]. (1) Nazeeruddin, M. K.; Kay, A.; Rodicio, I.; Humphry-Baker, R.; Muller, E.; Liska, P.; Vlachopoulos, N.; Gra¨tzel, M. J. Am. Chem. Soc. 1993, 115, 6382. (2) Argazzi, R.; Bignozzi, C. A.; Heimer, T. A.; Castellano, F. N.; Meyer, G. J. J. Inorg. Chem. 1994, 33, 5741. (3) Nazeeruddin, M. K.; Mu¨ller, E.; Humphry-Baker, R.; Vlachopoulos, N.; Gra¨tzel, M. J. Chem. Soc., Dalton Trans. 1997, 4571. (4) Nazeeruddin, Md. K.; Pechy, P.; Gra¨tzel, M. J. Chem. Soc., Chem. Commun. 1997, 1705. (5) Argazzi, R.; Bignozzi, C. A.; Hasselmann, G. M.; Meyer, G. J. Inorg. Chem. 1998, 37, 4533. (6) Ruile, S.; Kohle, O.; Pettersson, H.; Gra¨tzel, M. New J. Chem. 1998, 25. (7) Zakeeruddin, S. M.; Nazeeruddin, Md. K.; Humphry-Baker, R.; Gra¨tzel, M. Inorg. Chem. 1998, 37, 5251.

able to absorb light extending into the near-IR region up to 920 nm, also showed good performance as a photosensitizer for the nanocrystalline TiO2 solar cell.4,12 Ru phenanthroline complexes absorbing the visible light region are expected to be a good photosensitizer for dyesensitized solar cells as well as bipyridine complexes. We reported that the nanocrystalline TiO2 solar cell based on the cis-bis(4,7-dicarboxy-1,10-phenanthroline)dithiocyanato ruthenium(II) complex (Ru(dcphen)2(NCS)2) showed good solar cell performance with an efficient η of 6.1% under AM 1.5 irradiation (100 mW cm-2) and a maximum IPCE of 70% at 540 nm.13,14 Schwarz et al. also studied the dye-sensitized nanocrystalline TiO2 solar cell using a Ru phenanthroline complex photosensitizer.15 They reported a 3.8% η for the Ru(dcphen)2(NCS)2-sensitized TiO2 (8) Jing, B.; Zhang, H.; Zhang, M.; Lu, Z.; Shen, T. J. Mater. Chem. 1998, 8, 2055. (9) Islam, A.; Hara, K.; Singh, L. P.; Katoh, R.; Yanagida, M.; Murata, S.; Takahashi, Y.; Sugihara, H.; Arakawa, H. Chem. Lett. 2000, 490. (10) Sauve´, G.; Cass, M. E.; Coia, G.; Doig, S. J.; Lauermann, I.; Pomykal, K. E.; Lewis, N. S. J. Phys. Chem. B 2000, 104, 6821. (11) Kimberly, A. M.; Sykora, M.; DeSimone, J. M.; Meyer, T. J. Inorg. Chem. 2000, 39, 71. (12) Nazeeruddin, M. K.; Pe´chy, P.; Renouard, T.; Zakeerudin, S. M.; Humphy-Baker, R.; Comte, P.; Cevey, L.; Costa, E.; Shklover, V.; Spiccia, L.; Deacon, G. B.; Bignozzi, C. A.; Gra¨tzel, M. J. Am. Chem. Soc. 2001, 123, 1613. (13) Sugihara, H.; Singh, L. P.; Sayama, K.; Arakawa, H.; Nazeeruddin, Md. K.; Gra¨tzel, M. Chem. Lett. 1998, 1005. (14) Yanagida, M.; Singh, L. P.; Sayama, K.; Hara, K.; Katoh, R.; Islam, A.; Sugihara, H.; Arakawa, H.; Nazeerddin, Md. K.; Gra¨tzel, M. J. Chem. Soc., Dalton Trans. 2000, 2817.

10.1021/la010343q CCC: $20.00 © 2001 American Chemical Society Published on Web 08/18/2001

Dye-Sensitized Nanocrystalline TiO2 Solar Cells

Figure 1. Structure of Ru phenanthroline complexes having different numbers of carboxyl groups as anchors.

solar cell under AM 0 irradiation while that for the Ru(dcbpy)2(NCS)2 solar cell was 6.1% and concluded that solar cell performance depends strongly on absorption properties of Ru polypyridine complexes on the TiO2 electrode surface.15 We consider that η for the Ru(dcphen)2(NCS)2-sensitized TiO2 solar cell can be improved by optimizing semiconductor electrode performance, complex adsorption conditions, and complex and electrolyte purity, using coadsorbate, and so forth. Thus, we studied this solar cell in detail, attaining a η of 6.6%, the highest efficiency for the Ru(dcphen)2(NCS)2-sensitized TiO2 solar cell under standard AM 1.5 irradiation. We report the effects of the materials of the semiconductor electrode, film thickness, and coadsorbate on the photovoltaic performance of the Ru phenanthroline complex sensitized solar cell. To understand the effect of the adsorbing state of the Ru phenanthroline complex on solar cell performance, we have synthesized carboxylated Ru(II) phenanthroline complexes with different numbers of carboxyl groups and studied the photovoltaic performance of nanocrystalline TiO2 solar cells sensitized by these complexes. We also report the effect of the number and position of carboxyl groups on solar cell photovoltaic performance. Experimental Section Preparation of Ligand and Ru Phenanthroline Complexes. We used four carboxylated Ru(II) phenanthroline complexes (Figure 1), cis-bis(4,7-dicarboxy-1,10-phenanthroline)dithiocyanato ruthenium(II) (Ru(dcphen)2(NCS)2, DCP2), cisbis(4-monocarboxy-1,10-phenanthroline)dithiocyanato ruthenium(II) (Ru(mcphen)2(NCS)2, MCP2), cis-(4,7-dicarboxy-1,10-phenanthroline)(1,10-phenanthroline)dithiocyanato ruthenium(II) (Ru(dcphen)(phen)(NCS)2, DCPP), and cis-(4-monocarboxy-1,10phenanthroline)(1,10-phenanthroline)dithiocyanato ruthenium(II) (Ru(mcphen)(phen)(NCS)2, MCPP). All materials were reagent grade and used as received. Solvents for reactions (15) Schwarz, O.; van Loyen, D.; Joskusch, S.; Turro, N. J.; Du¨rr, H. J. Photochem. Photobiol., A 2000, 132, 91.

Langmuir, Vol. 17, No. 19, 2001 5993 were predried carefully, and all reactions were carried out under Ar atmosphere. 1. Synthesis of Ru(dcphen)2(NCS)2 (DCP2). The synthesis of 4,7-dicarboxy-1,10-phenanthroline (dcphen) and Ru(dcphen)2(NCS)2 complex is as described elsewhere.13,14 The Ru(dcphen)2(NCS)2 complex having two protons of its four carboxyl groups exchanged to two tetrabutylammonium cations (TBA), Ru(dcphen)2(NCS)2(TBA)2 (DCP2-2TBA), was also prepared and used to measure solar cell performance.13,14 2. Synthesis of Ru(dcphen)(phen)(NCS)2 (DCPP). DCPP was prepared by slightly modifying the method described by Meyer et al.11 A mixture of RuCl2 (DMSO)4 (484 mg, 1 mmol), dcphen (280 mg, 1.05 mmol), and phen (208 mg, 1.05 mmol) in DMF was heated for 30 min to form a pale-orange complex. To this reaction mixture, we added 760 mg (10 mmol) of NH4NCS dissolved in 10 mL of H2O and monitored the reaction by UV-vis spectroscopy. After 5 h, the reaction was stopped, and the mixture was cooled and filtered. DMF was evaporated, and the solid complex obtained was washed well with water, acetone, and ether. The complex was purified on a silica column using methanol-acetonitrile as the eluent and then over a Sephadex LH20 column. 1H NMR, δ: 9.75 (d, 1H), 9.70 (d, 1H), 8.73 (d, 1H), 8.64 (d, 1H), 8.60 (d, 1H), 8.24 (d, 1H), 8.16 (d, 1H), 8.10 (d, 1H), 8.07 (d, 1H), 7.97 (d, 1H), 7.68 (d, 1H), 7.65 (d, 1H), 7.36 (d, 1H), 7.29 (d, 1H). MS (ESIMS) m/z: 331.7 (M - 2H)2-, 663.7 (M - H)-. Anal. Calcd for C28H16N6O4S2Ru: C, 50.52; H, 2.42; N, 12.63. Found: C, 50.62; H, 2.42; N, 12.73. 3. Synthesis of 4-Carboxy-1,10-phenanthroline (mcphen). 4-Methyl-1,10-phenanthroline (1 g) was refluxed for 2 h with selenium oxide (2.5 g) in dioxane containing 4% water and filtered through Celite 521 while hot. The product was recrystallized from THF as pale-white crystals (yield 70%). The aldehyde obtained was oxidized with HNO3 (70%) to give 4-carboxy1,10-phenanthroline (mcphen) (70%). Mp: 227 °C. 1H NMR, δ: 8.98 (d, J ) 4.6 Hz, 3-1H), 8.79 (dd, J ) 4.6 Hz, 9-1H), 8.02 (dd, J ) 8.2 Hz, 2-1H), 7.88 (d, J ) 9.3 Hz, 5-1H), 7.72 (d, J ) 4.3 Hz, 7-1H), 7.54 (d, J ) 9.3 Hz, 6-1H), 7.47 (dd, J ) 4.3 Hz, 8-1H). MS (EIMS) m/z: 224.8 (M + H)+. Anal. Calcd for C13H8N2O2Na (H2O): C, 59.10; H, 3.43; N, 10.60. Found: C, 58.95; H, 3.57; N, 10.73. 4. Synthesis of Ru(mcphen)2(NCS)2 (MCP2). RuCl3‚3H2O (523 mg, 2 mmol) was dissolved in 50 mL of DMF under N2. We added mcphen (862 mg, 3.85 mmol), and the mixture was refluxed for 3 h in the dark. It was cooled to room temperature and filtered, DMF was evaporated in vacuo, and the resulting solid was washed with a mixture of 1:4 acetone and diethyl ether. The purple complex obtained was stirred with 100 mL of 2 mol HCl for 4 h and filtered through a membrane filter to give Ru(mcphen)2Cl2. 1H NMR, δ: 8.60 (d, 2H), 8.48 (d, 2H), 8.24 (d, 2H), 8.12 (d, 4H), 7.62 (d, 4H). MS (EIMS) m/z: 308.5 (M - 2H)2-, 618.7 (M - H)-. Anal. Calcd for C26H16N4O4Cl2Ru: C, 50.32; H, 2.58; N, 9.03. Found: C, 50.42; H, 2.57; N, 9.22. Ru(mcphen)2Cl2 (310 mg, 0.5 mmol) was dissolved in 50 mL of DMF under reduced light. To this solution was added 10 mL of 0.1 M aqueous NaOH to deprotonate carboxyl groups. NH4NCS (380 mg, 5.0 mmol) was separately dissolved in 5 mL of H2O and added to the above solution. The reaction mixture was heated to reflux in an Ar atmosphere during magnetic stirring. After 6 h, it was cooled, and the solvent was removed using a rotary evaporator. The resulting solid was dissolved in water and filtered through a membrane filter, and the pH of the filtrate was lowered to 2.5 by adding HNO3 to yield a dense precipitate. This was placed in a refrigerator for 12 h, and then the solid was collected on a membrane filter, washed well with H2O/acetone-ether, and air-dried (yield: 320 mg, 85%). The complex was purified on a Sephadex LH20 column to give Ru(mcphen)2(NCS)2 (MCP2). 1H NMR, δ: 9.85 (d, 2H), 8.92 (d, 1H), 8.75 (d, 2H), 8.40 (d, 1H), 8.35 (d, 1H), 8.22 (d, 2H), 8.10 (d, 1H), 7.80 (d, 2H), 7.55 (d, 1H), 7.35 (d, 1H). MS (EIMS) m/z: 331.7 (M - 2H)2-, 664.9 (M - H)-. Anal. Calcd for C28H16N6O4S2Ru: C, 50.52; H, 2.41; N, 12.63. Found: C, 50.62; H, 2.37: N, 12.42. Any isomeric separation of MCP2 was not carried out, so MCP2 was an isomeric mixture of cis and trans complexes. 5. Synthesis of Ru(mcphen)(phen)(NCS)2 (MCPP). To 10 mL of 1 N HCl, we added 100 mg (0.38 mmol) of RuCl3‚H2O and 90 mg (0.45 mmol) of 1,10-phenanthroline. After stirring in the dark

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for 30 min and maintaining an Ar atmosphere, we allowed the reaction to stand overnight. The product (Ru(phen)Cl4) was isolated by filtration, washed with water, and dried in vacuo. MS (ESIMS) m/z: 424.1 (M + H)+. A mixture of Ru(phen)Cl4 (423 mg, 1 mmol) and mcphen (224 mg, 1 mmol) was then dissolved in 50 mL of DMF and refluxed in the dark for 6 h while maintaining a nitrogen atmosphere. After cooling, the reaction mixture was filtered, the solvent was evaporated, and the solid mixture was washed with a 1:4 acetone-ether mixture to yield Ru(mcphen)(phen)Cl2. MS (ESIMS) m/z: 541.4 (M - Cl)+. Ru(mcphen)(phen)(NCS)2 was prepared by refluxing a mixture of 576 mg (1 mmol) of Ru(mcphen)(phen)Cl2 and 760 mg (10 mmol) of NH4NCS in DMF. After 6 h, the reaction was stopped, the mixture was cooled to room temperature and filtered, and the solvent was removed using a rotary evaporator. The solid mixture was washed with water, acetone, and ether and dried. The complex was purified on a silica column (packed with hexane) using acetonitrile and methanol as eluent. The complex was purified on a Sephadex LH20 column. 1H NMR, δ: 10.15 (d, 1H), 9.75 (d, 1H), 9.54 (d, 1H), 8.65 (d, 1H), 8.50 (d, 1H), 8.32 (d, 1H), 8.05 (d, 1H), 7.82 (d, 1H), 7.75 (d, 1H), 7.30 (m, 4H), 6.90 (d, 2H). MS (ESIMS) m/z: 621.0 (M - H)-. Anal. Calcd for C27H16N6O2S2Ru: C, 52.17; H, 2.59; N, 13.52. Found: C, 52.01; H, 2.57; N, 13.75. Any isomeric separation of MCPP was not carried out. Preparation of Dye-Coated Semiconductor Films. TiO2 nanoparticles prepared by the method reported by Gra¨tzel et al.16,17 and ZnO nanoparticles (Sumitomo Osaka Cement, no. 100) were used for semiconductor thin film electrodes. Preparation of SnO2 and In2O3 nanoparticles was as reported elsewhere.18 Semiconductor films were prepared by screen printing organic paste containing semiconductor nanoparticles, binder, and solvent on a transparent conducting oxide (TCO, F-doped SnO2) coated glass (Nippon Sheet Glass Co., 10 Ω/sq, transparency 80%) and then sintered at 420 °C for ZnO and 500 °C for other semiconductors for 1 h.18 TiO2 film was dipped in a 0.1 mol dm-3 (M) TiCl4 (Wako) aqueous solution for over 18 h at 25 °C and sintered at 450 °C for 30 min in air (TiCl4 treatment). Semiconductor films measured by an Alpha-Step 300 profiler (Tencor Instruments) varied from 4 to 13 µm thick. Ru phenanthroline complexes were dissolved in dehydrated ethanol (Wako Chemicals) or tert-butylalcohol (Kanto Chemicals)acetonitrile (Kanto, dehydrated) mixture solvent (50:50) with a concentration of 3 × 10-4 M. Solvents were used without further purification. Semiconductor films were immersed into the dye solution and maintained at 25 °C for over 18 h to adsorb dye on the semiconductor surface. Characterization. Absorption spectra of complexes in ethanol and adsorbed on oxide semiconductor films were measured using Shimadzu UV-3101PC with transmission and in diffuse reflectance mode. The amount of complex adsorbed on the semiconductor surface was evaluated from the absorbance intensity in solution after dye was desorbed by a 1.0 mM NaOH ethanolH2O (50:50) solution. Emission spectra of complexes were measured with a Hitachi F-4500. FT-IR absorption spectra were measured using model Spectrum One (Perkin-Elmer) with attenuated total reflectance (ATR) with a ZnSe prism and a diffuse reflectance accessory. The oxidation potentials of complexes in the ethanol solution were estimated using a conventional threecompartment cell consisting of a carbon or a Au working electrode, a Pt counter electrode, and an Ag/AgCl reference electrode in saturated KCl solution. Measurement was made using an electrochemical measurement system, BAS100B and a potentio/ galvanostat (Hokuto Denko Ltd., HA-501), an arbitrary function generator (Hokuto, HB-105), and a XY recorder (Riken Dennshi Co. Ltd., F-5C). Photoelectrochemical Measurements. The sandwich twoelectrode electrochemical cell for photovoltaic measurement consisted of a dye-coated semiconductor film electrode, counter electrode, spacer, and organic electrolyte. The counter electrode, a Pt film sputtered on a TCO-coated glass, was prepared using

UV-Vis and FT-IR Absorption Spectra. UV-vis absorption spectra of the Ru phenanthroline complex DCP2 in ethanol13,14 and adsorbed on a transparent TiO2 film (4 µm thick) are shown in Figure 2. The absorption spectrum of the complex adsorbed on a TiO2 film is expanded to the long wavelength region compared to that in ethanol. This result could indicate an interaction between complexes and/or strong electronic coupling between adsorbed DCP2 and TiO2. Figure 3 shows an FT-IR (ATR) absorption spectrum of DCP2-2TBA adsorbed on a TiO2 film. The absorption peak at 2100 cm-1 is attributed to the C-N stretching band of the NCS group.2,20,21 Peaks at 1380 and 1600 cm-1 are assigned to O-C-O symmetric and asymmetric stretching bands of carboxylate COO- bidentate, respectively.2,20,21 No CdO stretching band near 1700 cm-1

(16) Barbe´, C. J.; Arendse, F.; Comte, P.; Jirousek, M.; Lenzmann, F.; Shklover, V.; Gra¨tzel, M. J. Am. Ceram. Soc. 1997, 80, 3157. (17) Kalyanasundaram, K.; Gra¨tzel, M. Coord. Chem. Rev. 1998, 77, 347. (18) Hara, K.; Horiguchi, T.; Kinoshita, T.; Sayama, K.; Sugihara, H.; Arakawa, H. Sol. Energy Mater. Sol. Cells 2000, 64, 115.

(19) Bonhoˆte, P.; Dias, A. P.; Papageorgiou, N.; Kalyanasundram, K.; Gra¨tzel, M. Inorg. Chem. 1996, 35, 1168. (20) Finnie, K. S.; Bartlett, J. R.; Woolfrey, J. L. Langmuir 1998, 14, 2744. (21) Nazeeruddin, Md. K.; Amirnasr, M.; Comte, P.; Mackay, J. R.; McQuillan, A. J.; Houriet, R.; Gra¨tzel, M. Langmuir 2000, 16, 8525.

Figure 2. Absorption spectra of Ru(dcphen)2(NCS)2 in ethanol (dashed line) and adsorbed on TiO2 film (solid line) . an ion coater (Eiko engineering, IB-5). The spacer was a polyethylene thin film (30 µm thick). The apparent dye-sensitized semiconductor electrode was about 0.25 cm2 (0.5 cm × 0.5 cm). The electrolyte solution was a mixture of 0.6 M 1,2-dimethyl3-propylimidazolium iodide (DMPImI), 0.1 M LiI, 0.05 M I2, and 0.5 M tert-butylpyridine (TBP) in methoxyacetonitrile. Reagent grade chemicals such as LiI (Wako Chemicals) and I2 (Wako) were used for electrolyte components. TBP (Aldrich) and methoxyacetonitrile (Aldrich and Tokyo Kasei) were purified by distillation before use. DMPImI was synthesized from 1,2dimethylimidazolium (Tokyo Kasei) and n-propyl iodide (Tokyo Kasei) as reported elsewhere.19 Photoelectrochemical performance of the solar cell was measured with a source meter (Keithley, model 2400, and Advantest, R6246). The light source was a standard AM1.5 solar simulator with 300 and 1000 W Xe lamp (Wacom, WXS-80C-3, and Yamashita Denso Co., YSS-150A). Incident light intensity was calibrated with a thermopile (Eppley Lab., Inc., Newport, RI) and a standard solar cell for amorphous silicon solar cell produced by Japan Quality Assurance Organization (JQA). A 500 W halogen lamp, monochromator (JASCO, CT-10), scanning controller (JASCO, SMD-25C), and multimeter (Keithley, model 2000) were used for IPCE measurement of the solar cell. The intensity of monochromatic light was estimated by an optical power meter (Advantest, TQ8210).

Results and Discussion

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Langmuir, Vol. 17, No. 19, 2001 5995

Figure 3. An FT-IR absorption spectrum of Ru(dcphen)2(NCS)2(TBA)2 adsorbed on TiO2 film measured by ATR mode.

corresponding to ester bonding was observed, indicating that DCP2 is adsorbed on the TiO2 surface with carboxylate COO- bidentate coordination. No change in the spectrum was observed for DCP2-2TBA adsorbed on a ZnO film, showing an absence of ester bonding and suggesting that the binding configuration of the Ru phenanthroline complex is independent of semiconductor materials. For Ru(dcbpy)2(NCS)2 adsorbed on semiconductors, both CdO stretching bands for ester bonding and C-O carboxylate as the linkage condition were assigned from FT-IR spectra.2,20-24 When the complex was adsorbed on the TiO2 film under reflux, the ester linkage dominated.22 Carboxylate bidentate coordination was mainly observed for Ru(dcbpy)2(NCS)2 adsorbed on ZnO films.23,24 For Ru(dcbpy)2(NCS)2(TBA)2 adsorbed on TiO2 films, both CdO stretching bands and absorption due to carboxylate condition were observed and absorption due to CdO stretching disappeared for 4 TBA complex.20,21 In our experiment, strong peaks at 1380 and 1600 cm-1 were also observed for Ru(dcbpy)2(NCS)2(TBA)2 adsorbed on the TiO2 film, whereas no CdO stretching band at 1700 cm-1 corresponding to ester bonding was observed, being similar to DCP2-2TBA. We consider that binding coordination depends markedly on preparation of semiconductor films and dye adsorption conditions. Energy Diagram for the Solar Cell. Figure 4 shows the energy diagram for nanocrystalline TiO2 solar cells sensitized by DCP2 with an iodine redox electrolyte. The potentials of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of DCP2 estimated electrochemically are 1.1 and -0.8 V versus NHE, respectively,14 while those for Ru(dcbpy)2(NCS)2 are 0.8 and -0.9 V versus SCE (1.0 and -0.7 V vs NHE), respectively.17,25 The conduction band of the TiO2 electrode is -0.5 V, and the redox potential of I-/I3- is 0.4 V versus NHE.17,25 Energy loss processes in photon to current conversion in this solar cell are attributed to (1) low light harvesting efficiency due to low adsorbed complex; (2) low electron injection yield due to the presence of comparable fast relaxation of excited electrons, the adsorbed complex which cannot participate in electron injection such as aggregates, and weak electronic coupling (22) Murakoshi, K.; Kano, G.; Wada, Y.; Yanagida, S.; Miyazaki, H.; Matsumoto, M.; Murasawa, S. J. Electroanal. Chem. 1995, 396, 27. (23) Sayama, K.; Sugihara, H.; Arakawa, H. Chem. Mater. 1998, 10, 3825. (24) Keis, K.; Lindgren, J.; Lindquist, S.-E.; Hagfeldt, A. Langmuir 2000, 16, 4688. (25) Hagfeldt, A.; Gra¨tzel, M. Chem. Rev. 1995, 95, 49.

Figure 4. Energy diagram for the Ru(dcphen)2(NCS)2sensitized TiO2 solar cell with the iodine redox electrolyte. Table 1. Photovoltaic Performance of Nanocrystalline Oxide Semiconductor Solar Cells Sensitized by Ru(dcphen)2(NCS)2(TBA)2a film adsorbed thickness/ amount of dye/ Jsc/ Voc/ -8 electrode µm 10 mol cm-2 µm-1 mA cm-2 V TiO2 ZnO SnO2 In2O3

4.2 10.8 6.3 9.4

1.2 1.7 0.8 1.3

7.6 6.6 4.2 3.8

0.75 0.57 0.49 0.29

ff

η/ %

0.67 0.51 0.65 0.16

3.8 1.9 1.3 0.2

a Conditions: light intensity, the standard AM 1.5 condition (100 mW cm-2, JIS A class); cell size, 0.25 cm2; electrolyte solution, 0.6 M DMPII, 0.1 M LiI, 0.05 M I2, and 0.5 M TBP in methoxyacetonitrile.

between the complex and semiconductor; (3) fast recombination between injected electrons and oxidized complexes; and (4) notable dark current corresponding to rereduction of triiodide ions with injected electrons. Photovoltaic Performance. Photovoltaic performance of nanocrystalline oxide semiconductor (TiO2, ZnO, SnO2, and In2O3) solar cells sensitized by DCP2-2TBA photosensitizer and an iodine redox electrolyte under the standard AM 1.5 irradiation (100 mW cm-2) is shown in Table 1, where Jsc is the short-circuit photocurrent density, Voc is open-circuit photovoltage, ff is the fill factor, and η is solar energy to current conversion efficiency. These semiconductor films were almost transparent because large semiconductor particles were not included as a scattering center. The electrolyte solution was a mixture of 0.6 M DMPImI, 0.1 M LiI, 0.05 M I2, and 0.5 M TBP in methoxyacetonitrile. The amounts of the complex adsorbed on semiconductor films for 1 µm thickness are shown in Table 1. Similarly to Ru(dcbpy)2(NCS)2 adsorbed on TiO2 film,1 adsorbed DCP2-2TBA was 1-1.6 × 10-7 mol cm2 for 10 µm thickness of each semiconductor film. The solar cell performance depended markedly on the species of semiconductor materials, and the highest efficiency, 3.8%, was obtained for the TiO2 electrode. Dependence of open-circuit photovoltage on semiconductor materials, TiO2 > ZnO > SnO2 > In2O3 (Table 1), is mainly

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Figure 5. Photocurrent action spectrum obtained with the nanocrystalline TiO2 solar cell sensitized by Ru(dcphen)2(NCS)2(TBA)2 with the electrolyte of 0.6 M DMPImI, 0.1 M LiI, 0.05 M I2, and 0.5 M TBP in methoxyacetonitrile. IPCE is represented as a function of wavelength.

due to differences in the conduction band levels of semiconductors.18,23 Photocurrent performance of a DCP2-2TBA/ZnO solar cell is lower than that of a DCP2-2TBA/TiO2 solar cell, whereas the amount of adsorbed complex exceeds that on the TiO2 film. The binding coordination of the complex on the ZnO and TiO2 surfaces is almost the same, as shown in FT-IR spectra. Photovoltaic performance of the Ru(dcbpy)2(NCS)2/ZnO solar cell is also inferior to that of the Ru(dcbpy)2(NCS)2/TiO2 solar cell.24,26,27 Keis et al. concluded that dye agglomerates formed with Zn2+, which cannot contribute to electron injection, lead to lower electron injection yield.24 Asbury et al. directly observed much slower electron injection from Ru(dcbpy)2(NCS)2 in ZnO compared to that in TiO2 by femtosecond mid-IR absorption spectroscopy.28 They suggested that different electron injection kinetics in Ru(dcbpy)2(NCS)2/ZnO and Ru(dcbpy)2(NCS)2/TiO2 solar cells may be due to the difference in electronic coupling between the π* orbital of the dye and accepting orbitals in ZnO and TiO2 and/or their density of states. The states near the conduction band edge of ZnO consist of 4s orbitals of Zn2+, while those of TiO2 consist of 3d orbitals of Ti4+, resulting in a difference in electronic coupling with the π* orbital of the dye.28 We consider that the difference in performance of DCP22TBA/TiO2 and DCP2-2TBA/ZnO solar cells may be due to the aggregation of DCP2-2TBA on the ZnO electrode as that of Ru(dcbpy)2(NCS)2 and/or the difference in electronic coupling between the π* orbital of DCP2 and the accepting orbitals in ZnO and TiO2. By optimizing TiO2 film in film thickness and introducing TiO2 large particles as a scattering center, we improved solar cell performance mainly due to increased photocurrent. Figure 5 shows monochromatic incident photon to current conversion efficiency for the DCP2-2TBAsensitized TiO2 solar cell as a function of wavelength. This solar cell converts visible light at 400-750 nm to current with a maximum efficiency of 78% at 526 nm. Maximum short-circuit photocurrent density (Jsc) obtained for this (26) Redmond, G.; Fitzmaurice, D.; Gra¨tzel, M. Chem. Mater. 1994, 6, 686. (27) Rensmo, H.; Keis, K.; Lindstro¨m, H.; So¨dergren, S.; Solbrand, A.; Hagfeldt, A.; Lindquist, S.-E. J. Phys. Chem. B 1997, 101, 2598. (28) Asbury, J. B.; Wang, Y. Q.; Lian, T. J. Phys. Chem. B 1999, 103, 6644.

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Figure 6. Typical photocurrent-voltage curve for the Ru(dcphen)2(NCS)2(TBA)2-sensitized TiO2 solar cell under the standard AM 1.5 irradiation (100 mW cm-2). η is 6.6% with Jsc of 12.5 mA cm-2, Voc of 0.74 V, and ff of 0.71.

solar cell was 14.1 mA cm-2, depending strongly on conditions such as the TiO2 film and electrolyte. A typical photocurrent-voltage curve for the DCP2-2TBA/TiO2 solar cell under standard AM 1.5 irradiation (100 mW cm-2) is shown in Figure 6. Solar energy to electricity conversion efficiency, η, of 6.6% was attained with Jsc of 12.5 mA cm-2, Voc of 0.74 V, and ff of 0.71. Under the same experimental conditions, we gained η of 7.7% for the Ru(dcbpy)2(NCS)2(TBA)2/TiO2 solar cell with Jsc of 15.7 mA cm-2, Voc of 0.70 V, and ff of 0.70, indicating that η for the DCP2-2TBA/TiO2 solar cell is about 1% inferior to that of the Ru(dcbpy)2(NCS)2/TiO2 solar cell. We consider that IPCE in the two regions near 400 and 650-800 nm for the DCP2-2TBA/TiO2 solar cell is lower than that for the Ru(dcbpy)2(NCS)2/TiO2 solar cell due to the lower absorption property of DCP2-2TBA, resulting in smaller Jsc and lower η values compared to those for the Ru(dcbpy)2(NCS)2 solar cell. The aggregation of DCP2-2TBA, described above, suppressing electron injection, may be a factor contributing to lower photocurrent performance of the DCP2-2TBA/TiO2 solar cell. Dark Current in the Solar Cell. Figure 7 shows the dependence of photovoltaic performance (Jsc and Voc) for the DCP2-2TBA/TiO2 solar cell on the thickness of the TiO2 film electrode. Jsc increased from 3.1 to 8.7 mA cm-2 with increasing thickness of the TiO2 film from 2 to 9.8 µm. This dependence is due to increased adsorbed complex with increasing TiO2 thickness, improving light harvesting efficiency. Voc decreased from 0.77 to 0.69 V with increasing thickness from 2 to 9.8 µm. Increasing thickness increases the series resistance of the TiO2 film, decreasing photovoltage and ff. Voc for the dye-sensitized solar cell decreases with enhanced back electron transfer reaction (dark current reaction) corresponding to reduction of I3- by injected electrons to I-.1,25,29 The decrease in Voc with increasing film thickness is also explained by the enhanced dark current reaction with increasing reaction sites where dark current reaction preferably takes place. Currentvoltage curves obtained in the dark condition corresponding to the dark current for DCP2-2TBA-adsorbed TiO2 electrodes having different film thickness are shown in Figure 8. The X axis represents applied bias versus the (29) Huang, S. Y.; Schlichtho¨rl, G.; Nozik, A. J.; Gra¨tzel, M.; Frank, A. J. J. Phys. Chem. B 1997, 101, 2576.

Dye-Sensitized Nanocrystalline TiO2 Solar Cells

Figure 7. Dependence of Jsc and Voc for the Ru(dcphen)2(NCS)2(TBA)2-sensitized TiO2 solar cell on thickness of the TiO2 film electrode. The electrolyte solution is 0.6 M DMPImI, 0.1 M LiI, 0.05 M I2, and 0.5 M TBP in methoxyacetonitrile: (O) Jsc and (b) Voc.

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Figure 9. Dark current characteristics obtained for the Ru(dcphen)2(NCS)2(TBA)2-sensitized TiO2 solar cell with and without deoxycholic acid (DCA) as coadsorbate. The electrolyte solution is 0.6 M DMPImI, 0.1 M LiI, 0.05 M I2, and 0.5 M TBP in methoxyacetonitrile: (dashed line) no DCA and (solid line) 25 mM DCA. The X axis represents applied bias versus the Pt counter electrode.

Figure 8. Current-voltage curves obtained in the dark condition corresponding to the reduction of triiodide ions for the Ru(dcphen)2(NCS)2(TBA)2-sensitized TiO2 electrodes having different thicknesses: (solid line) 2 µm, (dashed line) 6 µm, and (dotted line) 10 µm. The X axis represents applied bias versus the Pt counter electrode.

Figure 10. Dependence of Jsc and Voc for the Ru(dcphen)2(NCS)2(TBA)2-sensitized TiO2 solar cell on the concentration of DCA in dye solution: (O) Jsc and (b) Voc.

Pt counter electrode. The onset of dark current shifted negatively with decreasing film thickness, indicating the dark current reaction easily takes place on the thicker TiO2 electrode. Suppression of the dark current reaction is important for improving the solar cell performance, especially Voc and ff. Kay et al. used cholic acid derivatives as the coadsorbate for nanocrystalline TiO2 solar cells sensitized by porphyrinderivative photosensitizers to improve solar cell performance.30 Photovoltaic performance, especially Voc, was markedly improved by using the coadsorbate, suggesting the coadsorbate prevented the dark current reaction by adsorption on TiO2 sites. Huang et al. reported that treating Ru(dcbpy)2(NCS)2-coated TiO2 electrodes with pyridine derivatives significantly improves both Voc and solar cell performance, indicating pyridine compounds reduce the rate constant for back electron transfer (i.e., dark current).29 Figure 9 shows dark current character-

istics for the TiO2 solar cell sensitized by DCP2-2TBA with and without deoxycholic acid (DCA) as the coadsorbate. DCA was simultaneously adsorbed on the TiO2 film under dye-coating of the film in DCA (25 mM) and the Ru phenanthroline complex (0.3 mM) mixed solution. Adsorbed DCP2-2TBA on the TiO2 electrode decreased with increasing DCA concentration in the solution. The onset of dark current shifted negatively due to coadsorption of DCA, indicating prevention of dark current (Figure 9). The dependence of Jsc and Voc for the solar cell on the concentration of DCA in the solution is shown in Figure 10. Voc increased from 0.67 to 0.74 V with increasing DCA concentration to 25 mM. In concentrations over 2 mM of DCA, Jsc decreased with increasing DCA concentration due to decreasing adsorbed DCP2-2TBA. Maximum η was obtained in a 5 mM DCA solution. These results indicate that coadsorbates such as DCA play a significant role in improving solar cell performance, especially Voc, for the DCP2-2TBA/TiO2 solar cell. In addition to suppressing the dark current, the coadsorbate may also prevent dye aggregation. Coadsor-

(30) Kay, A.; Gra¨tzel, M. J. Phys. Chem. 1993, 97, 6272.

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and HOMO does not determine low APCE performance of the MCPP/TiO2 solar cell. Aranyos et al. prepared new Ru bipyridine complexes having new anchoring functionality malonate and measured photoelectrochemical properties of dye-TiO2 solar cells.32 They reported that Jsc and IPCE for a complex having 1 carboxyl group as an anchor were inferior to those for a complex containing a malonate anchoring group (i.e., 2 carboxyl groups). They considered that the differences in the Jsc and IPCE performance are most likely due to differences in the efficiency of electron injection from excited dye to TiO2 and should be related to the number of carboxylates involved in binding.32 Electron injection from excited molecules adsorbed on the semiconductor surface to the conduction band is an important primary process in the solar cell. The rate constant of electron injection, kinj, is expressed as33 Figure 11. Action spectra of APCE for the TiO2 solar cell sensitized by Ru phenanthroline complexes with the electrolyte of 0.6 M DMPImI, 0.1 M LiI, 0.05 M I2, and 0.5 M TBP in methoxyacetonitrile: (O) DCP2, (b) DCPP, (]) MCP2, and ([) MCPP.

bates such as cholic acid derivatives were also used for a Ru terpyridine complex (black dye)4,12 and Ru phthalocyanine photosensitizers31 to prevent formation of aggregates. We consider that aggregation of DCP2-2TBA reduces the electron injection yield in this solar cell due to intermolecular quenching, as suggested in the Ru(dcbpy)2(NCS)2/ZnO solar cell. Suppression of aggregation due to the coadsorbate could also play an important role in improving photovoltaic performance. Effect of Carboxyl Group as the Anchor. We studied the effect of the number and position of carboxyl groups attached to the phenanthroline ligand on solar cell performance. The carboxyl group plays a role as an anchor to the TiO2 surface. We synthesized three Ru phenanthroline complexes having 1 or 2 carboxyl groups, Ru(mcphen)2(NCS)2 (MCP2), Ru(dcphen)(phen)(NCS)2 (DCPP), and Ru(mcphen)(phen)(NCS)2 (MCPP), in addition to Ru(dcphen)2(NCS)2 (DCP2) (Figure 1). No remarkable difference in absorption spectra of the four complexes was seen. Figure 11 shows absorbed photon to current conversion efficiency (APCE) as a function of wavelength for TiO2 solar cells sensitized by the four Ru phenanthroline complexes. The TiO2 electrode is transparent at about 4 µm thick. APCEs are evaluated from IPCEs and light harvesting efficiency, 1 - T (T is transmittance), based on the following equation:

APCE ) IPCE/(1 - T)

(1)

Figure 11 shows that the APCE performance of MCPP with 1 carboxyl group is lower than that for other complexes with 2 or more carboxyl groups, indicating that APCE performance depends strongly on the anchoring geometry of Ru phenanthroline complexes on the TiO2 surface and 2 carboxyl groups as the anchor are necessary for effective electron injection into the TiO2 electrode. The LUMO and HOMO of the ground state of MCPP are about -1.2 and 0.9 V versus NHE, respectively, estimated from cyclic voltammogram and the 0-0 energy gap.13,14 These energy levels of LUMO and HOMO are able to inject energetically electrons into the conduction band of TiO2 and accept electrons from iodide ions, respectively. This indicates that mismatching of the energy levels of LUMO (31) Nazeerddin, Md. K.; Humphry-Baker, R.; Gra¨tzel, M.; Murrer, A. Chem. Commun. 1998, 719.

kinj )

( )

4π2 |V|2 F(E) h

(2)

where V is electronic coupling between the excited molecule and semiconductor and F(E) is the density of states of the conduction band. It was reported that kinj from the excited Ru(dcbpy)2(NCS)2 complex, which has 4 carboxyl groups, to the TiO2 film is very large.34 A Ru complex having more than 2 carboxyl groups was expected to be attached to the surface with 2 carboxyl groups. Shklover et al. studied the crystal structure of the Ru(dcbpy)2(NCS)2 complex anchored to the TiO2 surface by X-ray diffraction analysis.35 They reported that the anchoring geometry of the complex to the TiO2 anatase surface with 2 carboxyl groups of both bipyridine ligands is most favorable thermodynamically,35 indicating that the anchoring geometry with 2 carboxyl groups is favorable for electron injection, that is, electronic coupling V is large. A complex having just 1 carboxyl group, such as MCPP, however, does not have the same geometry as that of one having 2 or more carboxyl groups. Electronic coupling V is sensitive to the relative configuration between the excited molecule and the semiconductor surface. Therefore, we consider that the low efficiency of the MCPP/TiO2 solar cell is due to unfavorable anchoring geometry for electron injection. Note that organic dyes having only 1 carboxyl group as the anchoring group such as mercurochorme,18,36 eosin Y,37 merocyanine,38 and coumarin39 also performed as efficient photosensitizers for ZnO and TiO2 electrodes. This clearly indicates that the number of carboxyl groups, which is necessary for favorable anchoring geometry to inject electrons effectively, depends on the molecular structure of photosensitizers. One carboxyl group as an anchor is sufficient to rigidly fix small molecules such as organic dyes and consequently inject electrons effectively into the conduction band with a large electronic coupling between (32) Aranyos, V.; Grennberg, H.; Tingry, S.; Lindquist, S.-E.; Hagfeldt, A. Sol. Energy Mater. Sol. Cells 2000, 64, 97. (33) Moser, J.-E.; Gra¨tzel, M. Chimia 1998, 52, 160. (34) Tachibana, Y.; Moser, J. E.; Gra¨tzel, M.; Klug, D. R.; Durrant, J. R. J. Phys. Chem. 1996, 100, 20056. (35) Shklover, V.; Ovchinnikov, Yu. E.; Braginsky, L. S.; Zakeeruddin, S. M.; Gra¨tzel, M. Chem. Mater. 1998, 10, 2533. (36) Hara, K.; Horiguchi, T.; Kinoshita, T.; Sayama, K; Sugihara, H.; Arakawa, H. Chem. Lett. 2000, 316. (37) Sayama, K.; Sugino, M.; Sugihara, H.; Abe, Y.; Arakawa, H. Chem. Lett. 1998, 753. (38) Sayama, K.; Hara, K.; Mitsuzuka, Y.; Mori, N.; Satsuki, M.; Suga, S.; Tsukagoshi, S.; Abe, Y.; Sugihara, H.; Arakawa, H. Chem. Commun. 2000, 1173. (39) Hara, K.; Sayama, K.; Ohga, Y.; Shinpo, A.; Suga, S.; Arakawa, H. Chem. Commun. 2001, 569.

Dye-Sensitized Nanocrystalline TiO2 Solar Cells

the dyes and the semiconductor. Two anchoring groups are considered necessary for Ru phenanthroline complexes, large and bulky molecules compared to organic dyes, to fix molecules on the TiO2 surface with favorable anchoring geometry injecting electrons effectively into the TiO2 electrode. Photophysical properties of photosensitizers, such as the character of singlet and triplet excited states, HOMO-LUMO levels, and so forth, determine electron injection performance. However, the performance of MCPP is much lower than that for other complexes (Figure 11), while photophysical and photoelectrochemical properties of MCPP and other complexes do not differ greatly. We therefore conclude that the molecular structure and anchoring geometry of the photosensitizer markedly influences electron injection performance in addition to photophysical properties of photosensitizers. Interestingly, the APCE performance for MCP2, which has 1 carboxyl group on each phenanthroline ligand (i.e., 2 carboxyl groups for the complex), is higher than that for DCP2, as shown in Figure 11. At the present stage, the factor determining the different APCE performance of MCP2 and DCP2 remains to be clarified. The LUMO levels of MCP2 and DCP2 are -1.0 and -0.8 V versus NHE, respectively.40 F(E) depends on the energy gap, ∆G, between the conduction band edge level and the LUMO level and increases with increasing ∆G.41 Electron injection from MCP2 into the conduction band of TiO2 would take place with a larger kinj due to a larger ∆G, resulting in a larger F(E) compared to that of DCP2, based on eq 2. Thus, it is possible that different LUMO levels (i.e., ∆G) of MCP2 and DCP2 determine different APCE performance of solar cells sensitized by the two complexes. As mentioned in the Experimental Section, MCP2 is an isomeric mixture. Each isomer may have different photophysical properties, resulting in different performance as photosensitizers. Photophysical properties and performance as photosensitizers of isomers are not yet clear. We are now currently investigating electron injection kinetics in nanocrystalline solar cells based on Ru (40) Hara, K.; Sugihara, H.; Singh, L. P.; Islam, A.; Katoh, R.; Yanagida, M.; Sayama, K.; Murata, S.; Arakawa, H. J. Photochem. Photobiol., A, submitted. (41) Sakata, T.; Hashimoto, K.; Hiramoto, M. J. Phys. Chem. 1990, 94, 3040.

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phenanthroline complexes using a nanosecond transient absorption spectroscopy to understand mechanisms of solar cells in greater detail. Conclusions A Ru phenanthroline complex, Ru(dcphen)2(NCS)2(TBA)2 (DCP2-2TBA), showed high performance as a photosensitizer in the dye-sensitized nanocrystalline TiO2 solar cell. The solar cell performance of the TiO2 solar cell exceeded that of ZnO, SnO2, and In2O3 solar cells, indicating that the combination of the photosensitizer and semiconductor materials is important for attaining a highly efficient dye-sensitized solar cell. Solar energy to electricity conversion efficiency under standard AM 1.5 irradiation (100 mW cm-2) for the DCP2-2TBA/TiO2 solar cell was 6.6% (Jsc ) 12.5 mA cm-2, Voc ) 0.74 V, and ff ) 0.71). Maximum monochromatic incident photon to current conversion efficiency was 78% at 526 nm. Deoxycholic acid as coadsorbate and decreasing film thickness improved Voc especially due to suppression of dark current corresponding to the reduction of triiodide ions with injected electrons. Improvement of the photovoltaic property due to the coadsorbate also suggests that aggregates of the complex formed on the semiconductor surface suppress efficient electron injection. The photocurrent performance of Ru(mcphen)(phen)(NCS)2 (MCPP) with only 1 carboxyl group as an anchoring group was inferior to that for other complexes with 2 or more carboxyl groups. This suggests that the anchoring geometry of the complex on the semiconductor surface determining electronic coupling between the complex and semiconductor are important for efficient electron injection. We found that 2 anchoring groups attached to the phenanthroline ligand are necessary for effective electron injection. Acknowledgment. We thank Professor Michael Gra¨tzel and co-workers in EPFL, Switzerland, for their invaluable advice on synthesis of the Ru phenanthroline complex and preparation of the nanocrystalline TiO2 thin film electrode. This work was supported by the Science and Technology Agency, Center of Excellence Development Project (COE project), Japan. LA010343Q