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A 2-quinolinecarboxylate-substituted ruthenium(II) complex as a new type of sensitizer for dye-sensitized solar cells
Inorganica Chimica Acta 362 (2009) 2519–2522
Contents lists available at ScienceDirect
Inorganica Chimica Acta journal homepage: www.elsevier.com/locate/ica
Note
A 2-quinolinecarboxylate-substituted ruthenium(II) complex as a new type of sensitizer for dye-sensitized solar cells Takashi Funaki a,*, Masatoshi Yanagida a, Nobuko Onozawa-Komatsuzaki a, Kazuyuki Kasuga a, Yuji Kawanishi b, Hideki Sugihara a,* a
Energy Technology Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), AIST Tsukuba Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan b Nanotechnology Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), AIST Tsukuba Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan
a r t i c l e
i n f o
Article history: Received 7 July 2008 Received in revised form 15 October 2008 Accepted 21 October 2008 Available online 29 October 2008 Keywords: Dye-sensitized solar cell Ruthenium(II) complex 2-Quinolinecarboxylate ligand
a b s t r a c t A new type of ruthenium(II) complex containing a 2-quinolinecarboxylate ligand was designed and synthesized as a sensitizer for dye-sensitized solar cells, and its photophysical and photochemical properties were characterized. The solar cells created with this complex exhibited efficient panchromatic sensitization over the entire visible wavelength range extending into the near-IR region. An overall conversion efficiency of 8.2% was attained under standard air mass 1.5 irradiation (100 mW cm2) with the short-circuit photocurrent density of 18.2 mA cm2, the open-circuit photovoltage of 0.63 V and the fill factor of 0.72. Ó 2008 Elsevier B.V. All rights reserved.
Dye-sensitized solar cells (DSSCs) created from nanocrystalline TiO2 films have been intensively investigated over the past decade [1–4]. Since the sensitizers used in DSSCs are critical to the cell’s photovoltaic performance, extensive efforts have been focused on the synthesis of new, highly efficient sensitizers. Among the numerous sensitizers developed for this purpose, ruthenium(II) polypyridine complexes have received much attention owing to their superior performance in DSSCs. Grätzel and coworkers reported the most successful of these complexes, black dye, which contains ruthenium-4,40 ,400 -tricarboxy-2,20 :60 ,200 -terpyridine (tctpy), tetra-n-butylammonium (TBA) and monodentate isothiocyanato (NCS) functionalities; DSSCs incorporating black dye achieve 10% conversion efficiency from solar light to electricity [5]. In these ruthenium(II) complexes, NCS ligands tune the spectral and redox properties of the complexes by destabilizing the metal t2g orbital. Although the NCS ligand is a suitable donor ligand, it can undergo photosubstitution or photodegradation reactions, which decrease the long-term stability of the complexes. The occurrence of these reactions can be reduced through the replacement of NCS ligands with other donor ligands, such as bidentate ligands. Several researchers have attempted to replace pairs of NCS ligands with single bidentate donor ligands. For example, both
* Corresponding authors. Tel.: +81 29 861 4892; fax: +81 29 861 6771 (T. Funaki), tel.: +81 29 861 6273; fax: +81 29 861 6771 (H. Sugihara). E-mail addresses: [email protected] (T. Funaki), [email protected] (H. Sugihara). 0020-1693/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2008.10.019
Bignozzi et al. and our research group have utilized dithiocarbamates and dithiolates as donor ligands [6,7]. The spectral responses of the complexes containing these ligands were expanded to longer wavelengths by changing the electron-donating ability of these ligands. Recently, we reported the synthesis of b-diketonato ruthenium(II) complexes and an ethylenediamine ruthenium(II) complex [8–10]. Although these complexes show efficient panchromic sensitization of DSSCs, the conversion efficiencies did not increase as expected. However, Han and coworkers have reported that conversion efficiency is improved by a modification of b-diketonato ligands [11]. In addition to the ruthenium(II) complexes mentioned above, we have also investigated 2-pyridinecarboxylate ligands, which have been examined as an ancillary ligand of iridium(III) complexes [12–15]. 2-Pyridinecarboxylate ligands are an anionic bidentate ligand. Although the electron-donating ability of a single 2-pyridinecarboxylate ligand is inferior to that of two NCS ligands, the electron-donating ability of the former can be changed by introducing a substituent on the pyridyl group [16]. Thus, modified 2-pyridinecarboxylate ligands might be suitable donor ligands for ruthenium(II) complexes. Before the examination of modified 2-pyridinecarboxylate ligands, a ruthenium(II) complex containing a 2-pyridinecarboxylate ligand was synthesized due to the complex has two isomers (Fig. 1). A relative disposition of the pyridyl group on the 2-pyridinecarboxylate ligand and the central pyridyl group on the 2,20 :60 ,200 -terpyridine ligand is different between each isomer. The isomers are labeled cis and trans conformation here. A
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Fig. 1. Schematic diagrams of cis and trans conformation.
ruthenium(II) complex containing a 2-pyridinecarboxylate ligand was synthesized under the same procedures as shown in Scheme 1. The only one isomer was obtained after 2-pyridinecarboxylate ligand was introduced. This isomer was estimated to be trans conformer [17]. The cis conformer was, however, generated during a replacement reaction of chloride with NCS ligand. It was not able to isolate each isomer by column chromatography and recycling preparative high-performance liquid chromatography. To obtain pure products, a control of the cis–trans isomerization reaction should be necessary. It was assumed that a steric hindrance between 2,20 :60 ,200 -terpyridine ligand and bidentate ligand prevents the generation of cis conformer. Therefore, 2-quinolinecarboxylate was chosen as a ligand. We report here the synthesis and photoelectrochemical properties of a new ruthenium(II) complex containing a 2-quinolinecarboxylate ligand, and we also investigate its application as a sensitizer in DSSCs. As shown in Scheme 1, complex 1 was synthesized by slight modification of literature procedures [9,17,18]. The structures of all complexes have been identified by nuclear magnetic resonance (NMR) spectroscopy and electrospray ionization mass spectrum (ESI-MS) analyses. In the case of 2-quinolinecarboxylate ligand, the only one isomer was obtained. The 1H NMR spectroscopy analysis suggested that the isomer was trans conformer [19]. This is most likely because of a bulky structure of a quinolyl group. The absorption spectrum of 1 is shown in Fig. 2. For comparison, the absorption spectrum of black dye is also shown in Fig. 2. The measurements were carried out in 1 103 M NaOH aqueous solution. Strong p–p* absorptions for the coordinating ligand are observed in the UV region. A broad metal-to-ligand charge transfer (MLCT) absorption is observed at lower energy region. The MLCT absorption maximum of 1 is 518 nm (e = 8900 M1 cm1), which is slightly blue-shifted compared with that of black dye. The blue
Scheme 1. Synthesis of 1: (a) 2-quinolinecarboxylic acid, LiCl, Et3N, EtOH; (b) (1) NH4NCS, DMF, H2O; (2) Et3N, H2O.
Fig. 2. Absorption spectra of 1 (—) and black dye (- - -) in 1 103 M NaOH aqueous solution.
shift might have been caused by the replacement of two NCS ligands with 2-quinolinecarboxylate ligand, since the electrondonating ability of a single 2-quinolinecarboxylate ligand might be inferior to that of two NCS ligands [16]. Although the absorption maximum of 1 is blue-shifted, the spectrum at wavelengths longer than 550 nm is almost identical to that of black dye. Notably, the absorption intensity from 400 to 550 nm for 1 is larger than that observed for black dye. The electrochemical behavior of 1 adsorbed on TiO2 was investigated by cyclic voltammetry in acetonitrile containing 0.1 M LiClO4. A quasi-reversible wave due to the Ru3+/2+ redox reaction is observed between 0.5 and 0.8 V versus a saturated calomel electrode (SCE). The peak potential of the differential pulse voltammograms for 1 is 0.71 V versus SCE. This observed potential is sufficiently low for rapid electron transfer from iodide/triiodide couple to the oxidized complex (Fig. 3). The emission of 1 is centered at 940 nm as measured in acetonitrile: N,N-dimethylformamide (99:1, v/v) solution at 298 K, and the excited-state lifetime is estimated to be 12 ns. The 0–0 transition energy (E0–0) is approximately 1.59 eV [20]. Therefore, the excited-state redox potential (Eox ) was calculated to be 0.88 V versus SCE, which is 0.07 eV more positive than that of black dye. Although this Eox value is relatively close to the TiO2 conduction band edge, a thermodynamic driving force for electron injection might be provided by the energy difference between Eox and TiO2 conduction band edge.
Fig. 3. Energy level diagram of TiO2, 1, black dye and I 3 /I .
T. Funaki et al. / Inorganica Chimica Acta 362 (2009) 2519–2522
A nanocrystalline TiO2 photoelectrode (area: 0.25 cm2; thickness: 36 lm) was prepared by screen printing on conductive glass as previously described [21]. Complex 1 and black dye were separately dissolved in ethanol at concentrations of 1 104 M and 2 104 M, respectively. Owing to the low solubility of 1 in ethanol, the solution of 1 contained two equivalents of tetrabutylammonium hydroxide. To suppress dye aggregation on TiO2 surfaces, deoxycholic acid has been employed as a co-adsorbate in DSSCs, and it was also used here [22]. The concentration of deoxycholic acid in the solutions of 1 and black dye were 1 102 and 2 102 M, respectively. The TiO2 films were immersed in the dye solutions for 15 h at 25 °C. Photoelectrochemical measurements were performed with a sealed sandwich-type twoelectrode solar cell consisting of the dye-coated TiO2 electrode, a Pt counter electrode, a polymer film spacer and an electrolyte solution consisting of 0.6 M 1,2-dimethyl-3-propylimidazolium iodide, 0.05 M iodine and 0.1 M lithium iodide in acetonitrile. The photocurrent action spectra of DSSCs containing 1 and black dye are shown in Fig. 4. The incident photon-to-current conversion efficiency (IPCE) for each DSSC is plotted as a function of wavelength. The broad IPCE curve for 1 covers over the whole visible range extending into the near-IR region. A maximum IPCE value of 65% was observed at 540 nm. Under similar condition, complex 1 shows higher IPCE values in the 420–610 nm and 750–850 nm regions than that of black dye. This result is consistent with the absorption spectra of 1 and black dye. Fig. 5 shows a photocurrent–voltage curve for the maximum performances of the DSSC containing 1 and black dye under standard air mass 1.5 irradiation (100 mW cm2). The measurements were carried out using a black metal mask with an aperture of 0.175 cm2. The electrolyte solution consisted of 0.6 M 1,2-dimethyl-3-propylimidazolium iodide, 0.05 M iodine, 0.1 M lithium iodide and 0.2 M tert-butylpyridine in acetonitrile. The DSSC containing 1 gave an 8.2% efficiency, with a short-circuit photocurrent density (Jsc) of 18.2 mA cm2, an open-circuit photovoltage (Voc) of 0.63 V, and a fill factor (ff) of 0.72. For comparison, the efficiency of DSSC containing black dye was 8.8%, with a Jsc of 18.1 mA cm2, a Voc of 0.65 V, an ff of 0.74. Although the DSSC containing 1 showed a slightly lower efficiency than that of black dye, the cell fabrication used this study was before optimization for 1. Moreover, the electric and steric environments in the dye can be modified by the introduction of a substituent on the 2-quinolinecarboxylate ligand. We believe that ruthenium(II) complexes containing these ligands are a promising new class of sensitizers for DSSCs.
Fig. 4. Photocurrent action spectra for DSSCs containing 1 (—) and black dye (- - -). The electrolyte solution consisted of 0.6 M 1,2-dimethyl-3-propylimidazolium iodide, 0.05 M iodine and 0.1 M lithium iodide in acetonitrile.
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Fig. 5. Photocurrent–voltage curves of the DSSCs containing 1 (—) and black dye (- - -). The electrolyte solution consisted of 0.6 M 1,2-dimethyl-3-propylimidazolium iodide, 0.05 M iodine, 0.1 M lithium iodide and 0.2 M tert-butylpyridine in acetonitrile.
In summary, a new type of ruthenium(II) complex containing a 2-quinolinecarboxylate ligand was synthesized as a sensitizer for DSSCs. This complex showed strong absorption over the whole visible range extending into the near-IR region. We are currently investigating the optimization of ligand structure and cell fabrication as well as long-term stability of the dye. Acknowledgements We are grateful to Dr. Ryuzi Katoh and to Dr. Motohiro Kasuya for the measurement of the emission spectrum. This work was supported by the Incorporated Administrative Agency New Energy and Industrial Technology Development Organization (NEDO) under Ministry of Economy, Trade and Industry (METI). References [1] B. O’Regan, M. Grätzel, Nature 353 (1991) 737. [2] M.K. Nazeeruddin, A. Key, I. Rodicio, R. Humphry-Baker, E. Müller, P. Liska, N. Vlachopoulos, M. Grätzel, J. Am. Chem. Soc. 115 (1993) 6382. [3] K. Kalyanasundaram, M. Grätzel, Coord. Chem. Rev. 77 (1998) 347. [4] M. Grätzel, Inorg. Chem. 44 (2004) 6841. [5] M.K. Nazeeruddin, P. Péchy, T. Renouard, S.M. Zakeeruddin, R. Humphry-Baker, P. Comte, P. Liska, L. Cevey, E. Costa, V. Shklover, L. Spiccia, G.B. Deacon, C.A. Bignozzi, M. Grätzel, J. Am. Chem. Soc. 123 (2001) 1613. [6] R. Argazzi, C.A. Bignozzi, G.M. Hasselmann, G.J. Meyer, Inorg. Chem. 37 (1998) 4533. [7] A. Islam, H. Sugihara, K. Hara, L.P. Singh, R. Katoh, M. Yanagida, Y. Takahashi, S. Murata, H. Arakawa, J. Photochem. Photobiol. A 145 (2001) 135. [8] Y. Takahashi, H. Arakawa, H. Sugihara, K. Hara, A. Islam, R. Katoh, Y. Tachibana, M. Yanagida, Inorg. Chim. Acta 310 (2000) 169. [9] A. Islam, H. Sugihara, M. Yanagida, K. Hara, G. Fujihashi, Y. Tachibana, R. Katoh, S. Murata, H. Arakawa, New J. Chem. 26 (2002) 966. [10] T. Yamaguchi, M. Yanagida, R. Katoh, H. Sugihara, H. Arakawa, Chem. Lett. 33 (2004) 986. [11] A. Islam, F.A. Chowdhury, Y. Chiba, R. Komiya, N. Fuke, N. Ikeda, L. Han, Chem. Lett. 34 (2005) 344. [12] S. Lamansky, P. Djurovich, D. Murphy, F. Abdel-Razzaq, R. Kwong, I. Tsyba, M. Bortz, B. Mui, R. Bau, M.E. Thompson, Inorg. Chem. 40 (2001) 1704. [13] C. Adachi, R.C. Kwong, P. Djurovich, V. Adamovich, M.A. Baldo, M.E. Thompson, S.R. Forrest, Appl. Phys. Lett. 79 (2001) 2082. [14] S.-J. Yeh, M.-F. Wu, C.-T. Chen, Y.-H. Song, Y. Chi, M.-H. Ho, S.-F. Hsu, C.H. Chen, Adv. Mater. 17 (2005) 285. [15] Y. You, S.Y. Park, J. Am. Chem. Soc. 127 (2005) 12438. [16] K. Kalyanasundaram, M.K. Nazeeruddin, Chem. Phys. Lett. 193 (1992) 292. [17] A. Llobet, P. Dopplet, T.J. Meyer, Inorg. Chem. 27 (1988) 514. [18] Ru(4,40 ,400 -trimethoxycarboxnyl-2,20 :60 ,200 -terpyridine)(2-quinolinecarboxylato)Cl was prepared by the addition of 2-quinolinecarboxylic acid (0.813 mmol), LiCl (8.13 mmol) and 0.4 mL of triethylamine to a solution of Ru(4,40 ,400 -trimethoxycarbonyl-2,20 :60 ,200 -terpyridine)Cl3 (0.813 mmol) in 75 mL ethanol. The reaction mixture was refluxed for 6 h, and the solution was concentrated to 20 mL with a rotary evaporator. The precipitated complex was collected, washed with ethanol and dried (yield 45%). The synthesis of 1
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T. Funaki et al. / Inorganica Chimica Acta 362 (2009) 2519–2522 then proceeded as follows: Ru(4,40 ,400 -trimethoxycarbonyl-2,20 :60 ,200 terpyridine)(2-quinolinecarboxylato)Cl (0.279 mmol) in 5 mL of N,Ndimethylformamide and 2 mL of an aqueous solution of ammonium thiocyanato (0.279 mmol) were refluxed for 5 h. After the ester groups of the terpyridine ligand were hydrolyzed, the reaction mixture was purified by column chromatography using Sephadex LH-20 as a column support and 5 mM TBA(OH) in ethanol/water (1:1, v/v) as an eluent (yield 22%).Data for 1: 1 H NMR (400 MHz, DMSO-d6, Me4Si): 9.34–9.32 (1H, m), 9.18 (2H, s), 9.09 (2H, s), 8.88 (1H, d, J 8.4), 8.42–8.40 (1H, m), 8.21(1H, d, J 8.4), 8.00–7.94 (4H, m), 7.84 (2H, d, J 5.7). MS (ESI-MS): m/z 325.5 [MNCS3H+H2O]2, 347.5 [M2H]2, 367.6 [M3H+Na+H2O]2, 469.6 [M3H+TBA]2.
[19] Although it was not able to isolate each isomer of ruthenium(II) complexes containing 2-pyridinecarboxylate ligand, the signals of 1H NMR of 2pyridinecarboxylato ligand were different between cis and trans conformers. The strong downfield shifts of cis conformer were obtained by a ring current effect of 2,20 :60 ,200 -terpyridine ligand. The signals of 1 showed similar characteristics with trans conformer. [20] Eox was calculated from the equality Eox = Eox E0–0; the E0–0 transition energy was estimated at the 5% level of maximum of emission intensity. [21] Z.-S. Wang, H. Kawauchi, T. Kashima, H. Arakawa, Coord. Chem. Rev. 248 (2004) 1381. [22] A. Kay, M. Grätzel, J. Phys. Chem. 97 (1993) 6272.
Contents lists available at ScienceDirect
Inorganica Chimica Acta journal homepage: www.elsevier.com/locate/ica
Note
A 2-quinolinecarboxylate-substituted ruthenium(II) complex as a new type of sensitizer for dye-sensitized solar cells Takashi Funaki a,*, Masatoshi Yanagida a, Nobuko Onozawa-Komatsuzaki a, Kazuyuki Kasuga a, Yuji Kawanishi b, Hideki Sugihara a,* a
Energy Technology Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), AIST Tsukuba Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan b Nanotechnology Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), AIST Tsukuba Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan
a r t i c l e
i n f o
Article history: Received 7 July 2008 Received in revised form 15 October 2008 Accepted 21 October 2008 Available online 29 October 2008 Keywords: Dye-sensitized solar cell Ruthenium(II) complex 2-Quinolinecarboxylate ligand
a b s t r a c t A new type of ruthenium(II) complex containing a 2-quinolinecarboxylate ligand was designed and synthesized as a sensitizer for dye-sensitized solar cells, and its photophysical and photochemical properties were characterized. The solar cells created with this complex exhibited efficient panchromatic sensitization over the entire visible wavelength range extending into the near-IR region. An overall conversion efficiency of 8.2% was attained under standard air mass 1.5 irradiation (100 mW cm2) with the short-circuit photocurrent density of 18.2 mA cm2, the open-circuit photovoltage of 0.63 V and the fill factor of 0.72. Ó 2008 Elsevier B.V. All rights reserved.
Dye-sensitized solar cells (DSSCs) created from nanocrystalline TiO2 films have been intensively investigated over the past decade [1–4]. Since the sensitizers used in DSSCs are critical to the cell’s photovoltaic performance, extensive efforts have been focused on the synthesis of new, highly efficient sensitizers. Among the numerous sensitizers developed for this purpose, ruthenium(II) polypyridine complexes have received much attention owing to their superior performance in DSSCs. Grätzel and coworkers reported the most successful of these complexes, black dye, which contains ruthenium-4,40 ,400 -tricarboxy-2,20 :60 ,200 -terpyridine (tctpy), tetra-n-butylammonium (TBA) and monodentate isothiocyanato (NCS) functionalities; DSSCs incorporating black dye achieve 10% conversion efficiency from solar light to electricity [5]. In these ruthenium(II) complexes, NCS ligands tune the spectral and redox properties of the complexes by destabilizing the metal t2g orbital. Although the NCS ligand is a suitable donor ligand, it can undergo photosubstitution or photodegradation reactions, which decrease the long-term stability of the complexes. The occurrence of these reactions can be reduced through the replacement of NCS ligands with other donor ligands, such as bidentate ligands. Several researchers have attempted to replace pairs of NCS ligands with single bidentate donor ligands. For example, both
* Corresponding authors. Tel.: +81 29 861 4892; fax: +81 29 861 6771 (T. Funaki), tel.: +81 29 861 6273; fax: +81 29 861 6771 (H. Sugihara). E-mail addresses: [email protected] (T. Funaki), [email protected] (H. Sugihara). 0020-1693/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2008.10.019
Bignozzi et al. and our research group have utilized dithiocarbamates and dithiolates as donor ligands [6,7]. The spectral responses of the complexes containing these ligands were expanded to longer wavelengths by changing the electron-donating ability of these ligands. Recently, we reported the synthesis of b-diketonato ruthenium(II) complexes and an ethylenediamine ruthenium(II) complex [8–10]. Although these complexes show efficient panchromic sensitization of DSSCs, the conversion efficiencies did not increase as expected. However, Han and coworkers have reported that conversion efficiency is improved by a modification of b-diketonato ligands [11]. In addition to the ruthenium(II) complexes mentioned above, we have also investigated 2-pyridinecarboxylate ligands, which have been examined as an ancillary ligand of iridium(III) complexes [12–15]. 2-Pyridinecarboxylate ligands are an anionic bidentate ligand. Although the electron-donating ability of a single 2-pyridinecarboxylate ligand is inferior to that of two NCS ligands, the electron-donating ability of the former can be changed by introducing a substituent on the pyridyl group [16]. Thus, modified 2-pyridinecarboxylate ligands might be suitable donor ligands for ruthenium(II) complexes. Before the examination of modified 2-pyridinecarboxylate ligands, a ruthenium(II) complex containing a 2-pyridinecarboxylate ligand was synthesized due to the complex has two isomers (Fig. 1). A relative disposition of the pyridyl group on the 2-pyridinecarboxylate ligand and the central pyridyl group on the 2,20 :60 ,200 -terpyridine ligand is different between each isomer. The isomers are labeled cis and trans conformation here. A
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Fig. 1. Schematic diagrams of cis and trans conformation.
ruthenium(II) complex containing a 2-pyridinecarboxylate ligand was synthesized under the same procedures as shown in Scheme 1. The only one isomer was obtained after 2-pyridinecarboxylate ligand was introduced. This isomer was estimated to be trans conformer [17]. The cis conformer was, however, generated during a replacement reaction of chloride with NCS ligand. It was not able to isolate each isomer by column chromatography and recycling preparative high-performance liquid chromatography. To obtain pure products, a control of the cis–trans isomerization reaction should be necessary. It was assumed that a steric hindrance between 2,20 :60 ,200 -terpyridine ligand and bidentate ligand prevents the generation of cis conformer. Therefore, 2-quinolinecarboxylate was chosen as a ligand. We report here the synthesis and photoelectrochemical properties of a new ruthenium(II) complex containing a 2-quinolinecarboxylate ligand, and we also investigate its application as a sensitizer in DSSCs. As shown in Scheme 1, complex 1 was synthesized by slight modification of literature procedures [9,17,18]. The structures of all complexes have been identified by nuclear magnetic resonance (NMR) spectroscopy and electrospray ionization mass spectrum (ESI-MS) analyses. In the case of 2-quinolinecarboxylate ligand, the only one isomer was obtained. The 1H NMR spectroscopy analysis suggested that the isomer was trans conformer [19]. This is most likely because of a bulky structure of a quinolyl group. The absorption spectrum of 1 is shown in Fig. 2. For comparison, the absorption spectrum of black dye is also shown in Fig. 2. The measurements were carried out in 1 103 M NaOH aqueous solution. Strong p–p* absorptions for the coordinating ligand are observed in the UV region. A broad metal-to-ligand charge transfer (MLCT) absorption is observed at lower energy region. The MLCT absorption maximum of 1 is 518 nm (e = 8900 M1 cm1), which is slightly blue-shifted compared with that of black dye. The blue
Scheme 1. Synthesis of 1: (a) 2-quinolinecarboxylic acid, LiCl, Et3N, EtOH; (b) (1) NH4NCS, DMF, H2O; (2) Et3N, H2O.
Fig. 2. Absorption spectra of 1 (—) and black dye (- - -) in 1 103 M NaOH aqueous solution.
shift might have been caused by the replacement of two NCS ligands with 2-quinolinecarboxylate ligand, since the electrondonating ability of a single 2-quinolinecarboxylate ligand might be inferior to that of two NCS ligands [16]. Although the absorption maximum of 1 is blue-shifted, the spectrum at wavelengths longer than 550 nm is almost identical to that of black dye. Notably, the absorption intensity from 400 to 550 nm for 1 is larger than that observed for black dye. The electrochemical behavior of 1 adsorbed on TiO2 was investigated by cyclic voltammetry in acetonitrile containing 0.1 M LiClO4. A quasi-reversible wave due to the Ru3+/2+ redox reaction is observed between 0.5 and 0.8 V versus a saturated calomel electrode (SCE). The peak potential of the differential pulse voltammograms for 1 is 0.71 V versus SCE. This observed potential is sufficiently low for rapid electron transfer from iodide/triiodide couple to the oxidized complex (Fig. 3). The emission of 1 is centered at 940 nm as measured in acetonitrile: N,N-dimethylformamide (99:1, v/v) solution at 298 K, and the excited-state lifetime is estimated to be 12 ns. The 0–0 transition energy (E0–0) is approximately 1.59 eV [20]. Therefore, the excited-state redox potential (Eox ) was calculated to be 0.88 V versus SCE, which is 0.07 eV more positive than that of black dye. Although this Eox value is relatively close to the TiO2 conduction band edge, a thermodynamic driving force for electron injection might be provided by the energy difference between Eox and TiO2 conduction band edge.
Fig. 3. Energy level diagram of TiO2, 1, black dye and I 3 /I .
T. Funaki et al. / Inorganica Chimica Acta 362 (2009) 2519–2522
A nanocrystalline TiO2 photoelectrode (area: 0.25 cm2; thickness: 36 lm) was prepared by screen printing on conductive glass as previously described [21]. Complex 1 and black dye were separately dissolved in ethanol at concentrations of 1 104 M and 2 104 M, respectively. Owing to the low solubility of 1 in ethanol, the solution of 1 contained two equivalents of tetrabutylammonium hydroxide. To suppress dye aggregation on TiO2 surfaces, deoxycholic acid has been employed as a co-adsorbate in DSSCs, and it was also used here [22]. The concentration of deoxycholic acid in the solutions of 1 and black dye were 1 102 and 2 102 M, respectively. The TiO2 films were immersed in the dye solutions for 15 h at 25 °C. Photoelectrochemical measurements were performed with a sealed sandwich-type twoelectrode solar cell consisting of the dye-coated TiO2 electrode, a Pt counter electrode, a polymer film spacer and an electrolyte solution consisting of 0.6 M 1,2-dimethyl-3-propylimidazolium iodide, 0.05 M iodine and 0.1 M lithium iodide in acetonitrile. The photocurrent action spectra of DSSCs containing 1 and black dye are shown in Fig. 4. The incident photon-to-current conversion efficiency (IPCE) for each DSSC is plotted as a function of wavelength. The broad IPCE curve for 1 covers over the whole visible range extending into the near-IR region. A maximum IPCE value of 65% was observed at 540 nm. Under similar condition, complex 1 shows higher IPCE values in the 420–610 nm and 750–850 nm regions than that of black dye. This result is consistent with the absorption spectra of 1 and black dye. Fig. 5 shows a photocurrent–voltage curve for the maximum performances of the DSSC containing 1 and black dye under standard air mass 1.5 irradiation (100 mW cm2). The measurements were carried out using a black metal mask with an aperture of 0.175 cm2. The electrolyte solution consisted of 0.6 M 1,2-dimethyl-3-propylimidazolium iodide, 0.05 M iodine, 0.1 M lithium iodide and 0.2 M tert-butylpyridine in acetonitrile. The DSSC containing 1 gave an 8.2% efficiency, with a short-circuit photocurrent density (Jsc) of 18.2 mA cm2, an open-circuit photovoltage (Voc) of 0.63 V, and a fill factor (ff) of 0.72. For comparison, the efficiency of DSSC containing black dye was 8.8%, with a Jsc of 18.1 mA cm2, a Voc of 0.65 V, an ff of 0.74. Although the DSSC containing 1 showed a slightly lower efficiency than that of black dye, the cell fabrication used this study was before optimization for 1. Moreover, the electric and steric environments in the dye can be modified by the introduction of a substituent on the 2-quinolinecarboxylate ligand. We believe that ruthenium(II) complexes containing these ligands are a promising new class of sensitizers for DSSCs.
Fig. 4. Photocurrent action spectra for DSSCs containing 1 (—) and black dye (- - -). The electrolyte solution consisted of 0.6 M 1,2-dimethyl-3-propylimidazolium iodide, 0.05 M iodine and 0.1 M lithium iodide in acetonitrile.
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Fig. 5. Photocurrent–voltage curves of the DSSCs containing 1 (—) and black dye (- - -). The electrolyte solution consisted of 0.6 M 1,2-dimethyl-3-propylimidazolium iodide, 0.05 M iodine, 0.1 M lithium iodide and 0.2 M tert-butylpyridine in acetonitrile.
In summary, a new type of ruthenium(II) complex containing a 2-quinolinecarboxylate ligand was synthesized as a sensitizer for DSSCs. This complex showed strong absorption over the whole visible range extending into the near-IR region. We are currently investigating the optimization of ligand structure and cell fabrication as well as long-term stability of the dye. Acknowledgements We are grateful to Dr. Ryuzi Katoh and to Dr. Motohiro Kasuya for the measurement of the emission spectrum. This work was supported by the Incorporated Administrative Agency New Energy and Industrial Technology Development Organization (NEDO) under Ministry of Economy, Trade and Industry (METI). References [1] B. O’Regan, M. Grätzel, Nature 353 (1991) 737. [2] M.K. Nazeeruddin, A. Key, I. Rodicio, R. Humphry-Baker, E. Müller, P. Liska, N. Vlachopoulos, M. Grätzel, J. Am. Chem. Soc. 115 (1993) 6382. [3] K. Kalyanasundaram, M. Grätzel, Coord. Chem. Rev. 77 (1998) 347. [4] M. Grätzel, Inorg. Chem. 44 (2004) 6841. [5] M.K. Nazeeruddin, P. Péchy, T. Renouard, S.M. Zakeeruddin, R. Humphry-Baker, P. Comte, P. Liska, L. Cevey, E. Costa, V. Shklover, L. Spiccia, G.B. Deacon, C.A. Bignozzi, M. Grätzel, J. Am. Chem. Soc. 123 (2001) 1613. [6] R. Argazzi, C.A. Bignozzi, G.M. Hasselmann, G.J. Meyer, Inorg. Chem. 37 (1998) 4533. [7] A. Islam, H. Sugihara, K. Hara, L.P. Singh, R. Katoh, M. Yanagida, Y. Takahashi, S. Murata, H. Arakawa, J. Photochem. Photobiol. A 145 (2001) 135. [8] Y. Takahashi, H. Arakawa, H. Sugihara, K. Hara, A. Islam, R. Katoh, Y. Tachibana, M. Yanagida, Inorg. Chim. Acta 310 (2000) 169. [9] A. Islam, H. Sugihara, M. Yanagida, K. Hara, G. Fujihashi, Y. Tachibana, R. Katoh, S. Murata, H. Arakawa, New J. Chem. 26 (2002) 966. [10] T. Yamaguchi, M. Yanagida, R. Katoh, H. Sugihara, H. Arakawa, Chem. Lett. 33 (2004) 986. [11] A. Islam, F.A. Chowdhury, Y. Chiba, R. Komiya, N. Fuke, N. Ikeda, L. Han, Chem. Lett. 34 (2005) 344. [12] S. Lamansky, P. Djurovich, D. Murphy, F. Abdel-Razzaq, R. Kwong, I. Tsyba, M. Bortz, B. Mui, R. Bau, M.E. Thompson, Inorg. Chem. 40 (2001) 1704. [13] C. Adachi, R.C. Kwong, P. Djurovich, V. Adamovich, M.A. Baldo, M.E. Thompson, S.R. Forrest, Appl. Phys. Lett. 79 (2001) 2082. [14] S.-J. Yeh, M.-F. Wu, C.-T. Chen, Y.-H. Song, Y. Chi, M.-H. Ho, S.-F. Hsu, C.H. Chen, Adv. Mater. 17 (2005) 285. [15] Y. You, S.Y. Park, J. Am. Chem. Soc. 127 (2005) 12438. [16] K. Kalyanasundaram, M.K. Nazeeruddin, Chem. Phys. Lett. 193 (1992) 292. [17] A. Llobet, P. Dopplet, T.J. Meyer, Inorg. Chem. 27 (1988) 514. [18] Ru(4,40 ,400 -trimethoxycarboxnyl-2,20 :60 ,200 -terpyridine)(2-quinolinecarboxylato)Cl was prepared by the addition of 2-quinolinecarboxylic acid (0.813 mmol), LiCl (8.13 mmol) and 0.4 mL of triethylamine to a solution of Ru(4,40 ,400 -trimethoxycarbonyl-2,20 :60 ,200 -terpyridine)Cl3 (0.813 mmol) in 75 mL ethanol. The reaction mixture was refluxed for 6 h, and the solution was concentrated to 20 mL with a rotary evaporator. The precipitated complex was collected, washed with ethanol and dried (yield 45%). The synthesis of 1
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T. Funaki et al. / Inorganica Chimica Acta 362 (2009) 2519–2522 then proceeded as follows: Ru(4,40 ,400 -trimethoxycarbonyl-2,20 :60 ,200 terpyridine)(2-quinolinecarboxylato)Cl (0.279 mmol) in 5 mL of N,Ndimethylformamide and 2 mL of an aqueous solution of ammonium thiocyanato (0.279 mmol) were refluxed for 5 h. After the ester groups of the terpyridine ligand were hydrolyzed, the reaction mixture was purified by column chromatography using Sephadex LH-20 as a column support and 5 mM TBA(OH) in ethanol/water (1:1, v/v) as an eluent (yield 22%).Data for 1: 1 H NMR (400 MHz, DMSO-d6, Me4Si): 9.34–9.32 (1H, m), 9.18 (2H, s), 9.09 (2H, s), 8.88 (1H, d, J 8.4), 8.42–8.40 (1H, m), 8.21(1H, d, J 8.4), 8.00–7.94 (4H, m), 7.84 (2H, d, J 5.7). MS (ESI-MS): m/z 325.5 [MNCS3H+H2O]2, 347.5 [M2H]2, 367.6 [M3H+Na+H2O]2, 469.6 [M3H+TBA]2.
[19] Although it was not able to isolate each isomer of ruthenium(II) complexes containing 2-pyridinecarboxylate ligand, the signals of 1H NMR of 2pyridinecarboxylato ligand were different between cis and trans conformers. The strong downfield shifts of cis conformer were obtained by a ring current effect of 2,20 :60 ,200 -terpyridine ligand. The signals of 1 showed similar characteristics with trans conformer. [20] Eox was calculated from the equality Eox = Eox E0–0; the E0–0 transition energy was estimated at the 5% level of maximum of emission intensity. [21] Z.-S. Wang, H. Kawauchi, T. Kashima, H. Arakawa, Coord. Chem. Rev. 248 (2004) 1381. [22] A. Kay, M. Grätzel, J. Phys. Chem. 97 (1993) 6272.