Proceedings of 6th International Conference on Hybrid and Organic Photovoltaics (HOPV14)
Publication date: 1st March 2014
Dye-sensitized solar cells (DSSCs) have been considered an effective alternative to conventional p-n junction devices. Their low production cost, ability to harvest diffuse light and versatile applications, makes them a very interesting PV technology1-3. However, for the DSSC technology become a competitive alternative, higher efficiencies are required. Up to now the major drawback in increasing the DSSCs efficiency has been the excessive loss of voltage during the dye-regeneration reaction; the use of iodine/triiodide redox electrolyte limits the open-circuit potential to 0.8V5. With the current state-of-art sensitizers, such as N719, and high surface area TiO2, I-/I3- electrolytes achieve an efficiency maximum of 11.1%6. However, by replacing I-/I3- by cobalt based redox couples new opportunities for higher efficiency DSSCs have been unlocked. The best DSSCsresult achieved so far is 12.3%, accomplished by the complementary use porphyrin dyes with Co(II/III) electrolytes7. These have emerged as the future electrolytes for liquid state DSSC7-13. In comparison to I-/I3-, Co(II/III) complexes possess higher reduction potentials, originating Voc close to 1V; they also have several advantages such as non-corrosiveness and negligible visible absorption7. Even though the remarkable 12.3% world record, higher efficiency is desired to commercialize the DSSC technology14. The drawbacks hampering the efficiency in Co(II/III) devices are mass-transfer limitations due to electrolyte’s high viscosity and very fast electron recombination with the Co(II/III) species12. In fact in these devices, the photoanode’s thickness and porosity was found to be crucial in their performance13. In this work we use phenomenological modeling to optimize the photoelectrode thickness, considering the slower Co(II/III) ionic diffusion coefficient (1.4x10-6 vs 5x10-6 cm2.s-1 found in I-/I3-)15. It is shown that a photoanode with higher electronic mobility should allow increasing its optimal thickness in current Co(II/III) DSSCs. New photoanode architecture is proposed, that tackles the shortcomings of cobalt-based electrolytes. Our approach uses high surface area particles with enhanced electron mobility due to the exposed (001) facets of TiO2 crystals. By increasing the crystals size in (001) direction it is possible to create high electrical conductivity material capable of adsorbing large amounts of dye; these unique structures show higher electron diffusion with lower recombination rates even in the presence of Co(II/III). This works shows that careful optimization of the TiO2 crystal features is a promising route for increasing the thickness of photoanodes without increasing recombination and mass transfer resistance; and thus opening a new route for a higher efficient DSSC.
Figure 1. Influence of recombination and transport are crucial in optimizing the photoelectrodes thickness
References 1. J. Ciesielska, Global Market Outlook for Photovolatics until 2015. European Photovoltaic Industry Association 2011. 2. Goncalves, L. M.; Bermudez, V. d. Z.; Ribeiro, H. A.; Mendes, A. M., Dye-sensitized solar cells: A safe bet for the future. Energy & Environmental Science 2008, 1, (6), 655-667. 3. Nazeeruddin, M. K.; Baranoff, E.; Grätzel, M., Dye-sensitized solar cells: A brief overview. Solar Energy 2011, 85, (6), 1172-1178. 4. Grätzel, M., Dye-sensitized solar cells. Journal of Photochemistry and Photobiology C: Photochemistry Reviews 2003, 4, (2), 145-153. 5. McGehee, M. D., Paradigm Shifts in Dye-Sensitized Solar Cells. Science 2011, 334, (6056), 607-608. 6. Chiba, Y.; Islam, A.; Watanabe, Y.; Komiya, R.; Koide, N.; Han, L., Dye-Sensitized Solar Cells with Conversion Efficiency of 11.1%. Japanese Journal of Applied Physics 45, (25), L638. 7. Yella, A.; 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 2011, 334, (6056), 629-634. 8. Xiang, W.; Huang, W.; Bach, U.; Spiccia, L., Stable high efficiency dye-sensitized solar cells based on a cobalt polymer gel electrolyte. Chem. Commun. 2013, 49, (79), 8997-8999. 9. Miettunen, K.; Saukkonen, T.; Li, X.; Law, C.; Sheng, Y. K.; Halme, J.; Tiihonen, A.; Barnes, P. R. F.; Ghaddar, T.; Asghar, I.; Lund, P.; O'Reganb, B. C., Do counter electrodes on metal substrateswork with cobalt complex based electrolyte in dye sensitized solar cells? Journal of the Electrochemical Society 2013, 160, (2), H132-H137. 10. Liu, Y.; Jennings, J. R.; Zakeeruddin, S. M.; Grätzel, M.; Wang, Q., Heterogeneous Electron Transfer from Dye-Sensitized Nanocrystalline TiO2 to [Co(bpy)3]3+: Insights Gained from Impedance Spectroscopy. Journal of the American Chemical Society 2013, 135, (10), 3939-3952. 11. Yum, J.-H.; Baranoff, E.; Kessler, F.; Moehl, T.; Ahmad, S.; Bessho, T.; Marchioro, A.; Ghadiri, E.; Moser, J.-E.; Yi, C.; Nazeeruddin, M. K.; Grätzel, M., A cobalt complex redox shuttle for dye-sensitized solar cells with high open-circuit potentials. Nat Commun 2012, 3, 631. 12. 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. Journal of the American Chemical Society 2010, 132, (46), 16714-16724. 13. Nakade, S.; Makimoto, Y.; Kubo, W.; Kitamura, T.; Wada, Y.; Yanagida, S., Roles of Electrolytes on Charge Recombination in Dye-Sensitized TiO2 Solar Cells (2): The Case of Solar Cells Using Cobalt Complex Redox Couples. J. Phys. Chem. B 2005, 109, (8), 3488-3493. 14. Maçaira, J.; Andrade, L.; Mendes, A., Review on nanostructured photoelectrodes for next generation dye-sensitized solar cells. Renewable and Sustainable Energy Reviews 2013, 27, (0), 334-349. 15. Macaira, J.; Andrade, L.; Mendes, A., Modeling, simulation and design of dye sensitized solar cells. RSC Advances 2014, 4, (6), 2830-2844.
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