Photochemistry and Photophysics of Coordination Compounds

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190 S. Campagna et al. Fig. 19 Schematic operation principle of a dye-sensitized solar cell semiconductor oxide obtained via a sol–gel procedure. Dye coverage of semiconductor nanoparticles is generally obtained from alcoholic solutions of the sensitizer, in which the sintered film is left immersed for a few hours. Sensitizers are usually designed to have functional groups such as – COOH, – PO3H2, or – B(OH)2 for stable adsorption onto the semiconductor substrate. The dyecovered film is in intimate contact with an electrolytic solution containing a redox couple dissolved in a suitable solvent. The electron donor member of the redox couple must reduce quickly and quantitatively the oxidized sensitizer, so closing the circuit. A variety of solvents with different viscosity and of redox mediators have been the object of intense studies, the most commonly used being the couple I – 3 /I– in acetonitrile or methoxypropionitrile solution. The counter electrode is a conductive glass covered with a few clusters of metallic platinum, which has a catalytic effect in the reduction process of the electron mediator. Further details on the cells and on their preparation can be found in the literature [403–405]. The complete photoelectrochemical cycle of the device can be outlined as follows. The adsorbed sensitizer molecules (S) are brought into their excited state (S∗ ) by photon absorption and inject one electron into the empty conduction band of the semiconductor in a timescale of femtoseconds. Injected electrons percolate through the nanoparticle network and are collected by the conductive layer of the photoanode electrode, while the oxidized sensitizer (S + ) in its ground state is rapidly reduced by I – ions in solution. Photoinjected electrons flow in the external circuit where useful electric work is produced and are available at the counter electrode for the reduction of the electron mediator acceptor I – 3 . The entire cycle consists in the quantum conversion of photons to electrons. S+hν → S∗ photoexcitation (25) S∗ +TiO2→S + +(e – ,TiO2) electron injection (26) 2S + +3I – → 2S+I3 – sensitizer regeneration (27) I3 – +2e – → 3I – electron donor regeneration . (28)
Photochemistry and Photophysics of Coordination Compounds: Ruthenium 191 Photoinjected electrons should escape from any recombination process in order to have a unit charge collection efficiency at the photoelectrode back contact. The two major waste processes in a dye-sensitized solar cell are due to (1) back electron transfer, at the semiconductor/electrolyte interface, between electrons in the conduction band and the oxidized dye molecules (Eq. 29), and (2) reduction of the electron relay (I – 3 , in this case) at the semiconductor nanoparticle surface (Eq. 30). S + +(e – ,TiO2) → S back electron transfer (29) I3 – +2(e – ,TiO2) → 3I – electron capture from mediator . (30) A detailed knowledge of all the kinetic mechanisms occurring in a photoelectrochemical cell under irradiation is an essential feature toward optimization of the process. 7.2 Ruthenium-Sensitized Photoelectrochemical Solar Cells A major breakthrough in the field relied on the performance of dye-sensitized solar cells employing Ru(II) complexes as sensitizers [401–405, 409–411]. Several reasons are at the basis of the success of Ru(II) polypyridine complexes in playing this leading role: 1. Strong absorption throughout all the visible region, which can also extend to the near-IR. This result is obtained by means of intense MLCT bands due to a judicious choice and combination of ligands [1]. 2. Strong electronic coupling between the MLCT excited state of the chromophore and the semiconductor conduction band. To fulfill this requirement, it has to be noted that the polypyridine ligand connected to the semiconductor via suitable functionalization of the ligand (usually carboxylated ligands) must be that involved in the lowest-lying MLCT state. 3. Tunability of the excited-state redox properties. This allows the preparation of compounds whose excited-state oxidation potential can ensure an efficient electron injection in the semiconductor conduction band. In this regard, it should be considered that to estimate a “reduction potential” (Ecb) for the semiconductor conduction band is not an easy task [404, 412– 414], and in nonaqueous solvents adsorption of cations, which are present as electrolytes, also has a significant effect on Ecb values. For example, Ecb for nanostructured TiO2 has been reported to be – 1.0 VvsSCEin 0.1M LiClO4/acetonitrile and about – 2.0 VwhenLi + cations are replaced by tetrabutylammonium [413, 414]. 4. Stability of the Ru(II) polypyridine complexes, in the ground state as well as in the excited and redox states. However, it is useful to note that photostability is not a strict requisite here, since the excited state is rapidly deactivated by electron injection. The same applies to chromophores hav-
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280 Topics in Current Chemistry Edi
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Photochemistry and Photophysics of
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Volume Editors Prof. Vincenzo Balza
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Topics in Current Chemistry Also Av
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X Preface nation Compounds. The ven
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Contents Photochemistry and Photoph
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258 Author Index Volumes 251-280 Be
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Subject Index Alkyne bridges 148 An
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Subject Index 273 Rh(III) cyclometa
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