Computational Spectroscopy Characterization of the Species Involved in Dye Oxidation and Regeneration Processes in Dye-Sensitized Solar Cells

We investigate the spectroscopic signature of oxidized dyes and of their complexes with I- occurring in the oxidation/regeneration process in dye-sensitized solar cells. Following the transient absorption spectroscopy study by Clifford et al. [J. Phys. Chem. C, 2007, 111, 6561-6567], we report a DFT/TDDFT study on the geometrical, electronic, and optical properties of the neutral and cationic [RuII(dcbpy)2(X)2] complexes, where dcbpy = 4,4?-dicarboxy-2,2?-bipyridyl and X = NCS and CN, and of their adducts with I-. We focus on the interaction between the oxidized dyes and iodide and perform DFT/TDDFT calculations using a continuum model of solvation on the complexes of interest, thus providing a picture of the electronic structure of the oxidized dyes and their adducts with I-. We find a good agreement between the simulated and the experimental absorption spectra, including the absorption band in the IR region that is experimentally assigned to the [dye+:I-] complexes. In line with Clifford et al., we find such band in the IR to overlap substantially with that of the dye cation for X = NCS, whereas it is readily identified for X = CN, for which the LMCT band of the cation has negligible intensity. Our results confirm the assignment of the IR band to a charge-transfer transition occurring within the [dye+:I-] complex, corresponding to shift of electron density from the Ru-X HOMOs to the LUMO based on the I atom. We expect our study to provide the required theoretical basis for interpretation of further transient spectroscopic data of interest in the DSCs field.