Proceedings of 6th International Conference on Hybrid and Organic Photovoltaics (HOPV14)
Publication date: 1st March 2014
We reported a joint theoretical and experimental investigation on the effects of the interactions which occur when D5 dye molecules are adsorbed on TiO2 surface. For this investigation we have used spectroscopic techniques, photovoltaic characterization of DSSCs and high-level theoretical calculations.
Going from the dye in solution to sensitized TiO2, various factors may shift the position of the UV-vis absorption maximum, both towards longer and shorter wavelengths. The overall UV-vis spectral shift from solution to dye-sensitized TiO2 film is in fact due to the interplay of several factors: i) the protonation/deprotonation state of the dye in solution;1, 2 ii) the effect of the dye/semiconductor interactions; and iii) the dye-dye intermolecular interactions occurring upon adsorption.3 Here, in order to separate this factors, we have focused on the effect of dye aggregation on TiO2, solvent effects and surface protonation. We used different sensitization conditions to vary the dye loading and thus the extent of dye aggregation and, for each sensitization condition, we explored protonated and native TiO2 films and also the effects of solvent. The protonation of TiO2 films showed the most interesting effects: spectral red-shift of UV-vis absorption spectra of dye@TiO2, increase of dye load and, in working devices, enhancement of generated photocurrent density. On the basis of our calculations we were able to attribute the enhancement of generated photocurrent density to a combined effect of spectroscopic changes (red-shifted absorption, increase of absorbance due to dye load) coupled to higher electronic coupling between dye molecules and semiconductor surface, and higher injection rates. Moreover, computational modeling of different dimeric aggregates, with increasing intermolecular interactions, and simulation of the associated optical responses also confirmed the observed spectral shifts. Also our results showed that the presence of solvent stabilize the excited state of adsorbed dye molecules, causing a marked red-shift of the absorption maximum.
Left panel. UV-vis absorption spectra of D5@TiO2 at high (black line) and low (red line) dye load values. A dimer configuration used in calculation is also reported. Right panel. UV-vis absorption spectra of D5 dye anchored on native (black line) and protonated (red line) TiO2.A magnification of protonated tiO2 surface is also reported.
1. Pastore, M.; Mosconi, E.; De Angelis, F.; Grätzel, M. A Computational Investigation of Organic Dyes for Dye-Sensitized Solar Cells: Benchmark, Strategies, and Open Issues. J. Phys. Chem. C, 2010, 114, 7205-7212. 2. Agrawal, S.; Pastore, M.; Marotta, G.; Reddy, M. A.; Chandrasekharam, M.; De Angelis, F. Optical Properties and Aggregation of Phenothiazine-based Dye Sensitisers for Solar Cells Applications: A Combined Experimental and Computational Investigation. J. Phys. Chem. C, 2013, 117, 9613-9622. 3. Pastore, M.; Fantacci, S.; De Angelis, F. Modeling Excited States and Alignment of Energy Levels in Dye-Sensitized Solar Cells: Successes, Failures, and Challenges. J. Phys. Chem. C, 2013, 117, 3685-3700
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