Investigation of the Stability of the Ruthenium based Dye (N719) Utilizing the Polarization Properties of Dispersive Raman Modes and/or of the Fluorescent Emission

Dye-sensitized solar cells (DSCs) offer intriguing new possibilities with the integration of these into, e.g., power generating windows or facade applications. For the DSCs to constitute a viable investment, the thermal appliance with respect to the working conditions typically encountered must be considered. One of the aspects governing the long-term thermal stability of the DSC is the dye stability and hence whether or not the dye is degraded upon heating or illumination. This emphasizes the need for a sensitive and nondestructive measuring technique with which it is possible to distinguish between a dye and the degradation products possibly formed in a DSC under working conditions. We have carried out the preliminary steps in this direction by showing that a distinction between the commonly used dye N719 and the main degradation product [Ru(LH) ₂(NCS)(4-tert-butylpyridine)][N(Bu) ₄] (N719-TBP for short) is possible by exploiting a combination of the polarization properties of the dispersive Raman modes with the small spectral change in the visible absorption spectrum and/or the difference in the polarization of the fluorescence related to the difference in molecular configuration. By measuring the polarized resonance Raman spectra in the region 650-1900 cm ^-1 it is demonstrated that the polarization dispersion is sufficient for four (1021, 1060, 1313, and 1606 cm ^-1) out of ten Raman modes to discriminate between N719 and N719-TBP. It is also demonstrated that the difference in molecular configuration between N719 and N719-TBP gives rise to an even larger change in the polarization of the fluorescence in the same spectral region. It therefore follows that a discrimination between the dye and degradation product can be based on the changes observed in the polarization properties of both the Raman as well as the fluorescence spectra.