Proceedings of 6th International Conference on Hybrid and Organic Photovoltaics (HOPV14)
Publication date: 1st March 2014
Hole hopping between dyes anchored on a surface occurs for molecules commonly used in the field of dye sensitized solar cells (DSSCs).1,2This process could open the possibility to design new concepts of electrochemical systems,3and it could also play a central role in existing solar cell devices.4 For example, in high performance DSSCs, efficient collection of holes left on the dyed surface upon photo induced electron injection in the TiO2 is a primary requirement. The process of hole collection involves, first of all, electron transfer from an hole transport material (HTM) to the dye. For this reason, high contact area between the HTM phase and the dyed titania is desirable. One common interpretation of the low collection efficiency of devices relies on the loss of those holes generated on "uncovered" dye molecules due to incomplete pore filling by the HTM in the dyed TiO2 nanostructure. However, this view neglects any contribution to charge transport that may be coming from the dye monolayer.
We address this question, by analyzing solid state DSSC structures made of dye sensitized nanocrystals of TiO2 with no additional HTM (see figure 1). We investigated the behavior of the dye monolayer in this test architecture and interpret the observed photo-conversion efficiency in terms of hole hopping between dye molecules. We support this analysis with measurements of solar cells quantum efficiency and electron hole recombination dynamics. By including these results in a drift diffusion model we estimate the hole diffusion coefficient across the dye monolayer that we compare to our previous investigations of hole percolation in such systems.5
To conclude, we present preliminary results from transient absorption spectroscopy describing holes dynamics in complete solid state DSSCs, which will also give insight into dye monolayer conformation.
Figure 1: (a) Structure of a DSSC structure where no additional hole transport material is deposited on the dyed mesoporous TiO2 film. (b) In this device, electrons injected in the TiO2 upon dye absorption are transported in the conduction band of the nanocrystal. The remaining hole on the dye hops across the monolayer of dyes to reach the metal contact.
1. Bonhôte, P.; Gogniat, E.; Tingry, S.; Barbe, C.; Vlachopoulos, N.; Lenzmann, F.; Comte, P.; Graetzel, M. Efficient Lateral Electron Transport inside a Monolayer of Aromatic Amines Anchored on Nanocrystalline Metal Oxide Films J. Phys. Chem. B, 1998, 5647, 1498–1507. 2. Ardo, S.; Meyer, G. J. Direct observation of photodriven intermolecular hole transfer across TiO2 nanocrystallites: lateral self-exchange reactions and catalyst oxidation. J. Am. Chem. Soc., 2010, 132, 9283–5. 3. Wang, Q.; Evans, N.; Zakeeruddin, S. M.; Exnar, I.; Grätzel, M. Molecular wiring of insulators: charging and discharging electrode materials for high-energy lithium-ion batteries by molecular charge transport layers. J. Am. Chem. Soc., 2007, 129, 3163–7. 4. Weisspfennig, C. T.; Hollman, D. J.; Menelaou, C., Stranks, S. D.; Joyce, H. J.; Johnston, M. B.; Snaith, H. J.; Herz, L. M. Dependence of Dye Regeneration and Charge Collection on the Pore-Filling Fraction in Solid-State Dye-Sensitized Solar Cells. Adv. Funct. Mater. 2013. doi: 10.1002/adfm.201301328 5. Moia, D.; Vaissier, V.; López-Duarte, I.; Torres, T.; Nazeeruddin, M. K.; O’Regan, B. C.; Nelson, J.; Barnes, P. R. F. The reorganization energy of intermolecular hole hopping between dyes anchored to surfaces. Chem. Sci., 2014, 5, 281.
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