Sensitization of n-Type TiO2 Electrode by a Novel Isoquinoline Ruthenium(II) Polypyridyl Complex

Article

Se nsitization of n-Type TiO₂ Electrode by a Novel Isoquinoline Ruthenium(II) Polypyridyl Complex

Christian Graziani Garcia^a, Neyde Yukie Murakami Ihaa^*, Roberto Argazzi^b, and Carlo Alberto Bignozzib^**

^aInstituto de Química - Universidade de São Paulo, C.P. 26.077,

05599-970 São Paulo - SP, Brazil;

^bDipartimento di Chimica - Università di Ferrara - Via L. Borsari, 46,

44100 Ferrara, Italy

Received: October 30, 1997

A sensibilização de TiO₂ à luz visível pela espécie cis-[(dcbH₂)2Ru(isq)₂](ClO4)₂, onde dcbH₂ = 4,4'-(CO2H)₂-2,2'-bipiridina e isq = isoquinolina, é relatada. O espectro de ação de fotocorrente, obtido para células solares regenerativas com filmes de TiO₂ nanocristalinos recobertos com o corante, resultou em valores de IPCE de até 40%. Este novo composto atua eficientemente como sensibilizador de fotoanodo de TiO₂ em células solares do tipo sanduíche constituídas de camada delgada.

The effective sensitization of TiO₂ to visible light by cis-[(dcbH₂)₂Ru(isq)₂](ClO4)₂, where dcbH₂ = 4,4'-(CO₂H)₂-2,2'-bipyridine and isq = isoquinoline, is reported. The photocurrent action spectrum obtained for the regenerative solar cell with dye coated nanocrystalline TiO₂ films resulted in IPCE values up to 40%. This novel compound acts as an efficient TiO₂ photoanode sensitizer in thin-layer sandwich-type solar cells.

Keywords: TiO₂ sensitization, solar cell, energy conversion, Ru-dye

Introduction

The development of efficient systems for the conversion of solar energy into electricity is a very active research field. Spectral sensitization of wide band-gap semiconductors in photoelectrochemical cells has been an attractive approach, with efficient results.^1-4 Many cis-[(dcbH₂)₂RuLL'] complexes have been studied for this purpose^5-8, and the best solar to electric power conversion has been achieved by thiocyanate derivatives^5,6. In the present work we extended our investigations onsemiconductor sensitization to the Ru(II) complex with isoquinoline as the ancillary ligand. Previous studies on the fac-[ClRe(CO)3(isq)2] complex have revealed interesting spectral, photophysical and photochemical properties⁹ which could be conveniently extended to another compound. In order to verify if the use ofthis azine ligand leads to a good ruthenium carboxylate dye, we have prepared the novel sensitizer, cis-[(dcbH₂)₂Ru(isq)₂](ClO4)₂, and determined its effectiveness. This species has the requirements for the sensitization of n-type TiO₂ semiconductors and can present promising efficiency in TiO₂-based solar cells.

Experimental

The cis-[(dcbH₂)₂RuCl₂] complex was synthesized following a slight modification of the procedure previously described^5. To a solution of 37 mg (15.2 mmol) of dcbH₂ (Aldrich) in 10 mL of DMF (Aldrich), 53 mg (2.6 mmol) of RuCl₃.xH₂0 (Aldrich) was added. The reaction mixture was refluxed for 8 h, then concentrated to 2 mL and cooled. The complex was precipitated by addition of acetone (Merck). The solid product was filtered off and dried under vacuum.

The cis-[(dcbH₂₎₂Ru(isq)₂](ClO4)₂ complex was obtained starting from the anionic form of [(dcbH₂₎₂RuCl₂] ^6. To a solution of 200 mg (0.27 mmol) ofNa₄[(dcb)₂RuCl₂] in 50 mL of methanol (Merck), 1.38 g (10.7 mmol) of isoquinoline (Aldrich) was added. The reaction mixture was refluxed for 14 h, rotary-evaporated to 3 mL, and then added to acetone (Aldrich). The solid was filtered off, redissolved in water and precipitated by addition of HClO₄ (Mallinckrodt). After washing with a pH 1.9 HClO₄ solution, the solid was dried under vacuum. Anal. Calcd. for RuC₄₂H₃₀N_6O16Cl₂.3H₂O: C, 45.83; N, 7.63; H, 3.30. Found: C, 45.90; N, 8.02; H, 3.55.

Spectra and photocurrent measurements were performed as described elsewhere^6,10.

Results and Discussion

The absorption spectrum of the cis-[(dcbH₂)₂ Ru(isq)₂]²+ complex in methanol is shown in Fig. 1. Analogously to similar complexes, it exhibits p ® p* intra ligand transition in the UV region and metal to ligand (MLCT), dp ® p* bands in the visible range^5,6,11,12.

Transparent, nanocrystalline TiO₂ electrodes for photoelectrochemical measurements were prepared using titanium isopropoxide as previously described^{5. The} complex was attached to theTiO₂ surface by immersing the processed electrode in a ~10^-4 M solution of the dye in ethanol. Photoelectrochemical experiments were carriedout using the dye sensitized TiO₂ film in a thin-layer sandwich-type solar cell. This cell consists of a TCO, fluorine doped SnO₂ glass, with the TiO₂ film sensitized by the complex, as a photoanode, I_2/LiI solution in acetonitrile, as an electrolyte relay and a transparent Pt film on a conducting TCO glass as a counter electrode. The spectrum for the dye adsorbed on the electrode is shown in Fig. 2.

Figure 3 shows the photocurrent action spectrum of the solar cell where the incident photon to current conversion efficiency (IPCE) is plotted as a function of the irradiation wavelength. The IPCE values were calculated by using the equation

A comparison between Figs. 2 and 3 shows that the photocurrent action spectrum closely resembles the optical density spectrum for the dye adsorbed on the electrode. The photocurrents obtained in the wavelength region up to 540 nm are quite good, with efficiencies ranging up to 40%. These values confirm that the dye adsorbs efficiently to the nanoporous TiO₂ with large internal surface area, and are comparable to those obtained by other ruthenium(II) polypyridyl-based sensitizers previously studied^6,11,13.

Conclusions

Enhanced spectral response of TiO₂ to visible light has been accomplished with the use of cis-[(dcbH₂)₂Ru (isq)₂](ClO₄)₂ as a dye. This novel compound presents good light harvesting properties and performs efficiently in the charge-transfer sensitization of nanocrystalline TiO₂. When employed in regenerative solar cells, this complex converts light into electricity effectively, showing a good spectral sensitization of the wide band-gap semiconductor to the visible light. This has been an attractive strategy for the conversion of sun light to electricity with efficiencies close to those obtained with traditional solid-state photovoltaic devices.

Acknowledgments

This work was supported by Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) and Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq).

References

1. Gerfin, T.; Grätzel, M.; Walder, L. Progr. Inorg. Chem. 1997, 44, 345.

2. Bignozzi, C.A.; Schoonover, J.R.; Scandola, F. Progr. Inorg. Chem. 1997, 44, 1; Murakami Iha, N.Y.; Nakano, A.K.; Garcia, C.G. An. Assoc. Bras. Quím. 1998, 47, 46. Y

3. Kalyanasundaram, K.; Grätzel, M.; Eds.; Photosensitization Using Inorganic and Organometallic Compounds, Kluwer Academic Pub., Dordrecht, 1993.

4. Kamat, P.V. Chem. Rev. 1993, 93, 267.

5. Nazeeruddin, M.K.; Kay, A.; Rodicio, I.; Humphry-Baker, R.; Muller, E.; Liska, P.; Vlachopoulos, N.; Grätzel, M. J. Am. Chem. Soc. 1993, 115, 6382.

6. Argazzi, R.; Bignozzi, C.A.; Heimer, T.A.; Castellano, F.N.; Meyer, G.J. Inorg. Chem. 1994, 33, 5741.

7. Péchy, P.; Rotzinger, F.P.; Nazeeruddin, M.K.; Kohle, O.; Zakeeruddin, S.M.; Baker, R.H.; Grätzel, M. J. Chem. Soc. Chem. Commun. 1995, 65.

8. Nazeeruddin, M.K.; Péchy, P.; Grätzel, M. Chem. Commun. 1997, in press.

9. Murakami Iha, N.; Ferraudi, G. J. Chem. Soc. Dalton. Trans. 1994, 2565.

10. Lima, J.F.; Murakami Iha, N.Y. Can. J. Chem. 1996, 74, 476.

11. Liska, P.; Vlachopoulos, N.; Nazeeruddin, M.; Comte, P.; Grätzel, M. J. Am. Chem. Soc. 1988, 110, 3686.

12. Heimer, T.A.; Bignozzi, C.A.; Meyer, G.J. J. Phys. Chem. 1993, 97, 11987.

13. Vlachopoulos, N.; Liska, P.; Augustynski, J.; Grätzel, M. J. Am. Chem. Soc. 1988, 110, 1216.

FAPESP helped in meeting the publication costs of this article

** [e-mail]

Publication Dates

• Publication in this collection
  17 Mar 2008
• Date of issue
  Feb 1998

History

• Received
  30 Oct 1997
• Accepted
  30 Oct 1997