The photo-, thermal, and water stability of dyes is indispensable for the commercialization of dye-sensitized solar cells, necessitating the development of systematic molecular design strategies to enhance the stability of the dyes. Therefore, we prepared dithieno­[3,2-b:2′,3′-d]­thiophene (DTT)-based dyes, by varying the functional group on the donor moiety (TP-1, H-; TP-2, methoxy-; TP-3, carbazolyl-; and TP-4, 2-ethylhexyloxy-). Among these dyes, TP-4 exhibits the highest power conversion efficiency of 8.86% (J sc = 15.9 mA cm–2, V oc = 0.76 V, FF = 0.73) with iodine electrolyte on a thin TiO2 active layer (3.5 μm), as well as strong resistance to photo-, thermal, and water stresses. UV–vis spectroscopy, intensity-modulated photocurrent spectroscopy, and intensity-modulated photovoltaic spectroscopy were used to analyze the dyes. On the basis of these analyses, we suggest a molecular design strategy for simultaneously enhancing the stability of photo-, thermal, and water stresses.

The dye distribution within a photo-electrode is a key parameter in determining the performances of dye-sensitized photon-to-electron conversion devices, such as dye-sensitized solar cells (DSSCs). A traditional, depth profiling investigation by destructive means including cross-sectional sampling is unsuitable for large quality control applications in manufacturing processes. Therefore, a non-destructive monitoring of the dye depth profile is required, which is the first step toward a non-destructive evaluation of the internal degradation of the device in the field. Here, we present a conceptual demonstration of the ability to monitor the dye depth profile within the light active layer of DSSCs by non-destructive means with high chemical specificity using a recently developed non-destructive/non-invasive Raman method, micro-spatially offset Raman spectroscopy (micro-SORS). Micro-SORS is able to probe through turbid materials, providing the molecular identification of compounds located under the surface, without the need of resorting to a cross-sectional analysis. The study was performed on the photo-electrode of DSSCs. This represents the first demonstration of the micro-SORS concept in the solar cell area as well as, more generally, the application of micro-SORS to the thinnest layer to date. A sample set has been prepared with varying concentrations of the dye and the thickness of the matrix consisting of a titanium dioxide layer. The results showed that micro-SORS can unequivocally discriminate between the homogeneous and inhomogeneous dye depth profiles. Moreover, micro-SORS outcomes have been compared with the results obtained with destructive time-of-flight secondary ion mass spectrometry measurements. The results of the two techniques are in good agreement, confirming the reliability of micro-SORS analysis. Therefore, this study is expected to pave the way for establishing a wider and more effective monitoring capability in this important field.

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