Dye-sensitized solar cells (DSCs) are a next-generation photovoltaic technology, whose natural transparency and good photovoltaic output under ambient light conditions afford them niche applications in solar-powered windows and interior design for energy-sustainable buildings. Their ability to be fabricated on flexible substrates, or as fibers, also makes them attractive as passive energy harvesters in wearable devices and textiles. Cosensitization has emerged as a method that affords efficiency gains in DSCs, being most celebrated via its role in nudging power conversion efficiencies of DSCs to reach world-record values; yet, cosensitization has a much wider potential for applications, as this review will show. Cosensitization is a chemical fabrication method that produces DSC working electrodes that contain two or more different dyes with complementary optical absorption characteristics. Dye combinations that collectively afford a panchromatic absorption spectrum emulating that of the solar emission spectrum are ideal, given that such combinations use all available sunlight. This review classifies existing cosensitization efforts into seven distinct ways that dyes have been combined in order to generate panchromatic DSCs. Seven cognate molecular-engineering strategies for cosensitization are thereby developed, which tailor optical absorption toward optimal DSC-device function.

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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 TiO2. 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.