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4 th Hybrid and Organic Photovoltaic Conference -Uppsala 2012 49 B14 - DFT/TDDFT computational investigation of Ru(II) and Os(II) panchromatic dye sensitizers Simona Fantacci , Maria Grazia Lobello, Filippo De Angelis Istituto CNR di Scienze e Tecnologie Molecolari (CNR-ISTM), via Elce di Sotto n. 8, Perugia, 6123, IT A new and bright interest has been addressed towards the Ru(II) panchromatic dyes as sensitizers in Dye-sensitized Solar Cells (DSCs). These dyes show a broad absorption of light ranging the entire visible region up to near-IR and the idea is to exploit the extended light harvesting for enhancing the DSC efficiency. Black Dye, [Ru(H3tcterpy)(NCS)3] - (tcterpy=4,4’,4”tricarboxy-2,2’:6’,2”-terpyridine), despite to be one of the first panchromatic dyes employed in DSC, 1,2 is still widely investigated, since only a deepened comprehension of its acid-base chemistry, its photophysics and photochemistry allow to design new panchromatic dyes with specific target functionalities. Here the results obtained in an extensive Density Functional Theory (DFT) and Time Dependent DFT study on Black Dye (BD) and analogous panchromatic dyes are presented. BD and its possible deprotonated formshave been investigated with the aim to understand and rationalize their acid-base, electrochemical and optical properties and finally to define a relationship between these properties and the dye-sensitized solar cell (DSC) working efficiency parameters. Figure 1 Comparison between the theoretical and experimental absorption spectrum. Optimized geometry of BD@TiO2. The pKa associated to the stepwise deprotonation of the dye carboxylic groups have been calculated, assigning the deprotonation order. The ground and excited state oxidation potentials have been evaluated and compared to the available experimental data. The influence of deprotonation on the absorption and emission processes has been analyzed and the spectral changes as a function of the pH have been assigned. The adsorption geometry of the BD with extended TiO2 models is presented, along with a detailed analysis of the excited states energy and localization at the dye/semiconductor interface. Calculations with inclusion of the spin-orbit coupling have been performed on BD and on the analogous Os-dye. Our results suggest a minor role of singlet-triplet excitation in the light- © SEFIN 2012
4 th Hybrid and Organic Photovoltaic Conference -Uppsala 2012 50 harvesting of the Ru-system, while a substantial absorption at the read-end of the spectrum in the Os-dye is due to spin-forbidden transitions. New design rules for panchromatic Ru dyes are finally presented. References [1] Nazeeruddin, M. K.; Pèchy, P.; Renouard, T.; Zakeeruddin, S. M.; Humphry-Baker, R.; Comte, P.; Liska, P.; Cevey, L.; Costa, E.; Shklover, V.; Spiccia, L.; Deacon, G. B.; Bignozzi, C. A.; Grätzel, M. "Engineering of Efficient Panchromatic Sensitizers for Nanocrystalline TiO2-Based Solar Cells". J. Am. Chem. Soc. 123, 1613-1624 (2001). [2] Bauer, C.; Boschloo, G.; Mukhtar, E.; Hagfeldt, A. "Interfacial Electron-Transfer Dynamics in Ru(tcterpy)(NCS)3- Sensitized TiO2 Nanocrystalline Solar Cells". J. Phys. Chem. B 106, 12693-12704 (2002). © SEFIN 2012
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