New chromophores were explored for use in dye-sensitized solar cells. The attachment of various
di-carboxylated cyanine dyes to nanocrystalline TiO2 was examined spectroscopically and through their
performance in a sensitized solar cell. It was found that aggregated forms of these cyanine dyes sensitized
with an efficiency equal to that of the monomer form and that combinations of cyanine dyes could be used
to sensitize solar cells over the entire visible spectrum. With the high extinction coefficients of the organic
dyes, short circuit photocurrents for these cyanine-sensitized systems was maximized at a 4 μm thickness of
the nanocrystalline TiO2. Short circuit photocurrents for these 4 μm solar cells were found to exceed that of
ruthenium coordination complexes.
Even though investigations of dye-sensitized nanocrystalline semiconductors in solar cells has dominated research on dye-sensitized semiconductors over the past two decades, single crystal electrodes represent far simpler model systems for studying the sensitization process with a continuing train of studies dating back more than 40 years. Even today single crystal surfaces prove to be more controlled experimental models for the study of dye-sensitized semiconductors than the nanocrystalline substrates. This Account analyzes the scientific advances in the model sensitized single crystal systems that preceded the introduction of nanocrystalline semiconductor electrodes. It then follows the single crystal research to the present, illustrating both their striking simplicity of use and clarity of interpretation relative to nanocrystalline electrodes. Researchers have employed many electrochemical, photochemical, and scanning probe techniques for studying monolayer quantities of sensitizing dyes at specific crystallographic faces of different semiconductors. These methods include photochronocoulometry, electronic spectroscopy, and flash photolysis of dyes at potential-controlled semiconductor electrodes and the use of total internal reflection methods. In addition, we describe the preparation of surfaces of single crystal SnS(2) and TiO(2) electrodes to serve as reproducible model systems for charge separation at dye-sensitized solar cells. This process involves cleaving the SnS(2) electrodes and a photoelectrochemical surface treatment for TiO(2) that produces clean surfaces for sensitization (as verified by AFM) resulting in near unity yields for electron transfer from the molecular excited dyes into the conduction band. In recent experiments with ruthenium complexes at TiO(2) and with carboxylated cyanine dyes, we demonstrate the promise of this simple model for understanding dye-sensitized solar cells. In each of these systems, we can observe and analyze the complex photochemistry in a quantitative manner. Molecules of the well-known N3 ruthenium complex attach to four different crystallographic faces of anatase and rutile TiO(2) at different rates and to a different extent. With carboxylated cyanine dye sensitizers on these surfaces, molecular aggregation on the surface is a function of molecular structure and crystallographic face. In contrast with the N3 sensitizer these organic dyes undergo a photoinduced dimerization and desorption reaction when hydroquinone regenerators are present. With both classes of sensitizers, we demonstrate a new photochronocoulometric technique that quantifies the amount of attached dye on the electrode surface. We have completed initial experiments examining quantum dot sensitization of TiO(2) crystals, which could eventually lead to sensitizers with higher stability and absorption coefficients. Although these single crystal electrode models show promise for providing insights and predictive value in understanding the sensitization process, more sophisticated models will be needed t...
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