Non-destructive Monitoring of Dye Depth Profile in Mesoporous TiO₂ Electrodes of Solar Cells with Micro-SORS

Abstract

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.

Non‐destructive analysis of concentration profiles in turbid media using micro‐spatially offset Raman spectroscopy: A physical model

“…The model was developed to be applicable to real situations encountered in Cultural Heritage, but its use can be extended to materials of interest in several other contexts, such as biomedical field, solar cells and semiconductors technologies. [ 38 ]…”

“…The model was developed to be applicable to real situations encountered in Cultural Heritage, but its use can be extended to materials of interest in several other contexts, such as biomedical field, solar cells and semiconductors technologies. [38] For each of these applications, one would need to ensure that assumptions used for deriving the above equations still hold-for example, the high turbidity medium as discussed in section 4 and negligible absorption at laser and Raman wavelengths and sample semi-infinity in dimensions. The uniformity of photon transport length in sample volume also needs to be present.…”

Advanced portable micro-SORS prototype coupled with SERDS for heritage science

“…Micro-SORS applications have been extended also to other analytical fields such as food science, 5 biomedicine 6 and solar energy. 7 In micro-SORS laser light and collected Raman signal pass through microscope objective(s), to render laser illumination spot and Raman collection areas on a micrometre scale. To date, several micro-SORS modalities have been developed: (a) defocusing micro-SORS, performed by moving the sample from the imaged position away from the microscope objective 3 and (b) full-micro-SORS where complete separation between the illumination and collection areas on samples surface is achieved.…”

“…Micro-SORS applications have been extended also to other analytical fields such as food science, 5 biomedicine 6 and solar energy. 7…”

“…To date, several micro-SORS modalities have been developed: (a) defocusing micro-SORS, performed by moving the sample from the imaged position away from the microscope objective 3 and (b) full-micro-SORS where complete separation between the illumination and collection areas on samples surface is achieved. The full micro-SORS can be implemented either using (i) "beam steering" optics, which allows achieving the lateral offset between the collection and illumination areas on sample surface by moving the laser beam by means of the motorized beam-steer alignment mirrors; 6,7 (ii) by reading different zones on the sample surface on different parts of CCD sensor; 8 or (iii) by using an external laser delivery optical accessory for driving the laser beam onto the sample and setting desirable spatial offset by moving this delivery accessory. 9 One of the most important needs in heritage science is performing analytical measurements in situ, in museum collections, conservation yards and archaeological sites since artworks often cannot be moved from their location or across national boundaries.…”