Journal of Photochemistry and Photobiology A: Chemistry 147 (2002) 143–148 Short communication Time-resolved experiments in dye-sensitized solar cells using [(dcbH 2 ) 2 Ru(ppy) 2 ](ClO 4 ) 2 as a nanocrystalline TiO 2 sensitizer Christian Graziani Garcia a , Cornelis J. Kleverlaan b , Carlo Alberto Bignozzi b , Neyde Yukie Murakami Iha a,∗ a Instituto de Qu´ ımica, Universidade de São Paulo, Av. Prof. Lineu Prestes, 748-05508-900 São Paulo, SP, Brazil b Dipartimento di Chimica, Università di Ferrara, Via Luigi Borsari, 46-44100 Ferrara, Italy Received 8 November 2001; accepted 19 November 2001 Abstract The complex cis-[(dcbH 2 ) 2 Ru(ppy) 2 ] 2+ , in which dcbH 2 is 4,4 ′ -(CO 2 H) 2 -2,2 ′ -bipyridine and ppy is 4-phenylpyridine, was anchored to nanocrystalline TiO 2 films deposited onto FTO substrate. Transient UV–visible absorption spectra with the resulting electrodes were carried out in order to investigate the electron-transfer process across the excited dye/semiconductor interface, as well as the charge recombination and quenching processes. The formation of the oxidized complex [(dcbH 2 ) 2 Ru III (ppy) 2 ] 3+ upon light excitation was monitored by transient absorption difference spectra. The compound performs successful conversion of visible light into electricity as a molecular sensitizer of nanocrystalline n-type TiO 2 in photoelectrochemical solar cells. A fast quenching of the oxidized complex in the presence of iodide emphasizes the importance of a proper concentration of donor species in the redox mediator for the effective regeneration of the oxidized sensitizer. Kinetics data of electron injection obtained by time-resolved experiments are discussed in parallel with photoelectrochemical properties of the dye-sensitized anatase TiO 2 . © 2002 Published by Elsevier Science B.V. Keywords: TiO 2 sensitization; 4-Phenylpyridine; Transient absorption spectra 1. Introduction Efficient conversion of visible light into electricity is achieved by photoelectrochemical solar cells based on dye sensitization of wide bandgap semiconductors. Dye sensiti- zation presents advantages over direct band to band excita- tion as in conventional solar cells. It allows electron injection from the photoexcited dye molecule into the semiconductor conduction band (CB) with energy lower than the bandgap and the semiconductor valence band is not involved in the light absorption process. Therefore, the electron–hole pair is separated by the semiconductor–sensitizer interface with the harvesting of a large fraction of sunlight by coordination complexes having a broad absorption band [1–7]. The concept of the dye sensitization was already con- sidered more than 30 years ago [8,9], but the experimental efficiency achieved by the cells was restricted to the low ab- sorptivities presented by monolayers of dyes on electrodes ∗ Corresponding author. Fax: +55-11-3815-5579. E-mail address: neydeiha@quim.iq.usp.br (N.Y. Murakami Iha). of low surface roughness. Significant improvements have been achieved with the evolution of the synthetic route to effectively bind the molecular sensitizer to the semiconduc- tor surface [10,11] followed by the development of fractal polycrystalline TiO 2 films achieved a decade later [12]. In a porous film, consisting of nanometric semiconductor parti- cles, the active surface area is greatly increased, resulting in an effective light absorption. As a result of these advances, the development of low-cost efficient solar cells became possible [13,14]. n-TiO 2 is widely used as semiconductor material in dye-sensitized photoelectrochemical solar cells, owing to its favorable energetics, stability, low price and simple processing [14–18]. Ruthenium(II) polypyridyl complexes with carboxylated ligands are commonly employed as TiO 2 sensitizers in such cells [19–21] and outstanding results have been achieved using thiocyanate derivatives as ancil- lary ligands [22–24]. As a result of visible light excitation, dye species are electronically excited resulting in electron transfer through the dye complex to the semiconductor in the femto/sub-picosecond time-domain [2,6,22,25]. 1010-6030/02/$ – see front matter © 2002 Published by Elsevier Science B.V. PII:S1010-6030(01)00643-8