Micro-Raman analysis of reverse bias stressed dye-sensitized solar cells
The degradation mechanisms of Reverse Bias (RB) stressed Dye Solar Cells (DSCs), sensitized with cis-bis(isothiocyanato)bis(2,2′-bipyridyl-4,4′-dicarboxylato)-ruthenium(II)bis-tetrabutylammonium (N719, Red Dye) and with cis-dicyano-bis(2,2′-bipyridyl-4,4′-dicarboxylic acid) ruthenium(II) (Ru505, Orange Dye) have been studied by means of resonance micro-Raman and UV-Vis spectroscopy. For N719 sensitized devices, the visible degradation induced by the stress tests involves both electrolytic solution and the sensitizer: the electrolyte suffers gas bubble formation and loss of solvent, while the dye cannot be regenerated and undergoes irreversible chemical changes. Confocal Raman imaging and UV-Vis absorption spectra confirmed that in regions where the electrolyte was absent, the detachment of the thiocyanate ligand (SCN−) from the dye is favored. On the other hand, measurements carried out on DSCs realized with the bis-cyano dye (Ru505) do not show dye modifications during the RB stress. We also clarify that the apparent N719 dye bleaching in particular zones of the cell active area, is not related to dye desorption from the TiO2 layer, but to loss of solvent and to dye chemical changes, which are responsible for a characteristic blue shift in the absorption spectrum.