Dye-Anchoring Modes at the Dye···TiO₂ Interface of N3- and N749-Sensitized Solar Cells Revealed by Glancing-Angle Pair Distribution Function Analysis

The efficient transport of electrons from the sunlight-harvesting dye molecules into the electrical circuit of a dye-sensitized solar cell (DSSC) is imperative to its effective operation. A dye···semiconductor interface comprises the working electrode of a DSSC. Dye molecules adsorb onto the semiconductor surface, whereupon they transfer electronic charge into the conduction band of the semiconductor; this process initiates the electrical circuit. It is therefore important to characterize this interfacial structure in order to understand how efficiently the dye binds, or anchors, onto the semiconductor surface and imparts charge transfer to it. Armed with such knowledge, the performance of DSSCs may then be improved systematically. The structural determination of a thin-film interface is nonetheless a challenging task. We herein report the results of a glancing-angle pair distribution function (gaPDF) experiment that generated synchrotron X-ray diffraction patterns of DSSC working electrodes sensitized by the archetypal ruthenium-based DSSC dye complexes N3 and N749 . This gaPDF experimental approach represents the first diffraction-based strategy for the characterization of intact DSSC working electrodes. The gaPDF structural signatures were compared with PDFs simulated from possible interfacial structures that were computed using density functional theory. The differences between the experimental observation and these simulated structures revealed a preference for each dye, N3 and N749, to adopt a bidentate-bridging dye anchoring mode when sensitized onto TiO 2 . Our results also suggest that this anchoring mode is sometimes supported by an auxiliary anchor, in the form of a monodentate carboxylic acid. This work not only demonstrates the successful application of a gaPDF method to DSSC research, but it also advocates the applicability of a gaPDF to many types of thin-film samples.