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4 th Hybrid and Organic Photovoltaic Conference -Uppsala 2012 31 B1 - Dye Regeneration by Single Electron Redox Couples Torben Daeneke, Udo Bach, Leone Spiccia Monash University, Calyton, 3800, AU Dye regeneration is a crucial step in the charge generation cycle of dye sensitized solar cells as the reaction has to be rapid to minimize recombination between the oxidized dye and the injected electrons. The driving force for dye regeneration arising from the potential difference between the redox mediator and the oxidized dye however needs to be kept minimal in order to ensure efficient use of the solar spectrum and high open circuit potentials. As a result the ideal redox couple should be matched to the sensitizing dye, providing just the right amount of driving force to ensure fast dye regeneration. To date however, little data is available on the driving force dependency of the dye regeneration reaction. Previous studies are limited to a small selection of redox couples or dyes, never exceeding six data points. In this study we utilized a group of 10 ferrocene derivatives covering a redox potential range of over 800 mV. The regeneration of six structurally analog ethylcarbazole dyes featuring different oxidation potentials was studied by nanosecond transient absorption spectroscopy, leading to overall 60 different conditions. This is allowing us for the first time to extract reliable relations on how the dye regeneration reaction kinetics are dependant on the provided driving force. We found that the regeneration rapidly accelerates with increasing driving force showing Marcus-normal behavior in the regime of small driving forces until it reaches a threshold potential. Above the threshold potential the reaction enters diffusion control. Structural modifications of the dye as well as the redox mediator had surprisingly little influence on the overall regeneration reaction. All in all, the rate controlling parameter was found to be the provided driving force. In conclusion our observations provide certain selection rules which are applicable in general to all single electron redox couples including cobalt and copper polypyridyl redox shuttles as well as ferrocenes. © SEFIN 2012
4 th Hybrid and Organic Photovoltaic Conference -Uppsala 2012 32 B2 - Dye Regeneration Kinetics in Solar Cells using Cobalt Polypyridine Redox Couples Sandra Feldt, Peter Lohse, Hanna Ellis, Gerrit Boschloo, Anders Hagfeldt Department of Chemistry - Ångström Laboratory, Box 523, Uppsala, 751 20, SE Recently, there has been significant progress in dye-sensitized solar cell (DSC) research on alternative redox mediators to the conventional iodide/triiodide (I - /I3 - ) couple. The major drawback of I - /I3 - is the large driving force needed for dye regeneration, which limits the voltage output and thus the power conversion efficiency of the DSC. Cobalt polypyridine redox couples with more positive redox potential can replace I - /I3 - , when used in combination with sensitizers that have suitable steric properties to prevent fast recombination processes. 1, 2 The Nernst potential of cobalt complexes can be tuned over a large range by changing the coordination sphere. A record efficiency of 12.3% has been reported for a cobalt-based DSC. 3 Figure 1 Regeneration of the triphenylamine-based organic dye, D35, by cobalt tris(2,2´- bipyridine). The recombination and regeneration kinetics in D35-sensitized TiO2 electrodes was investigated for a series of different cobalt redox couples. 4 Open circuit potentials of more than 1 V were obtained by using cobalt phenanthroline complexes. The photocurrent of the devices decreased, however, with increasing Nernst potential of the redox couples, because of slower regeneration of the oxidized dye and faster recombination of photoinjected electrons with Co(III) species. Here we will present new results of photovoltaic performance and regeneration kinetics of cobalt-based DSCs for a new series of structurally modified triphenylamine dyes, as well as for co-sensitized DSCs. The Marcus theory is applied to explain the observed electron transfer kinetics. References [1] Feldt, S.M.; Gibson, E. A.; Gabrielsson, E.; Sun, L.; Boschloo, G.; Hagfeldt, A. "Design of Organic Dyes and Cobalt Polypyridine Redox Mediators for High-Efficiency Dye-Sensitized Solar Cells". J. Am. Chem. Soc. 132, 16714-16724 (2010). [2] Tsao, H. N.; Yi, C.; Moehl, T.; Yum, J.-H.; Zakeeruddin, S. M.; Nazeeruddin, M. K.; Grätzel, M. "Cyclopentadithiophene Bridged Donor–Acceptor Dyes Achieve High Power Conversion Efficiencies in Dye- Sensitized Solar Cells Based on the tris-Cobalt Bipyridine Redox Couple". ChemSusChem. 4. 591-594 (2011). [3] Aswani , Y.; Lee, H.-W.; Tsao, H. N.; Yi, C.; Chandiran, A. K.; Nazeeruddin, M. K.; Diau, E. W.-G.; Yeh, C.-Y.; Zakeeruddin, S. M.; Grätzel, M. "Porphyrin-Sensitized Solar Cells with Cobalt (II/III)–Based Redox Electrolyte Exceed 12 Percent Efficiency" Science. 334, 629-634 (2011). © SEFIN 2012
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