Dye sensitization of four low index TiO2 single crystal photoelectrodes with a series of dicarboxylated cyanine dyes.

Four dicarboxylated cyanine dyes were used to sensitize single-crystal anatase (001), anatase (101), rutile (001), and rutile (100) surfaces. Incident photon to current efficiencies (IPCE) spectra and isotherms were gathered for the different combination of dyes and surfaces. The maximum coverage of the surface-bound dyes on the TiO2 crystal surfaces was determined by photochronocoulometric measurements. The IPCE spectra of the surface-bound dyes revealed that both the dye monomers and H-aggregates were both present and generated photocurrent. The relative abundance of dye monomers and H-aggregates was found to be strongly dependent on the crystallographic face used as the substrate for sensitization. The ratio of dye monomer to H-aggregate was quantified by fitting the IPCE spectra with a sum of the dye monomer and H-aggregate solution spectra. The trends in surface coverage were explained using a simple "lattice matching" model where the distance between the coordinatively unsaturated Ti binding sites on the various TiO2 crystallographic surfaces was compared with the distance between the carboxylate groups on the dyes. The rutile (100) surface had the highest coverage for all the dyes in agreement with the predictions of the lattice-matching model. Absorbed photon-to-current-efficiencies (APCEs) were calculated from the incident photon current efficiencies, the extinction coefficients and the measured surface coverages. The factors that affect the APCE values such as the relative injection yield for monomers and aggregate, the relative surface coverage values for monomers and aggregates, and semiconductor doping levels are discussed.

Dye sensitization of single crystal semiconductor electrodes.

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 to fully understand the charge transfer from the localized electronic states of the sensitizer to the extended states of the semiconductor.

Regenerator dependent photoinduced desorption of a dicarboxylated cyanine dye from the surface of single-crystal rutile.

A photon-initiated desorption of a dicarboxylated thiacarbocyanine dye from a dye-sensitized semiconducting oxide crystal has been observed when hydroquinone is used as a regenerator. No desorption was found under the same conditions when KI was used as the regenerator. Intermittent illumination experiments suggest that the oxidation products of the hydroquinone regenerator compete for dye adsorption sites. By comparing the photocurrent decay at both the dye monomer sensitization maximum and the dimer sensitization maximum, a rearrangement of monomer into dimer was observed. A kinetic model for the photocurrent decay as a function of desorption time was derived, and the desorption rate constants were obtained by fitting the experimental data to the model.

Photochronocoulometric measurement of the coverage of surface-bound dyes on titanium dioxide crystal surfaces.

Atomically flat terraced single-crystal anatase and rutile surfaces can be prepared allowing for the reproducible adsorption of covalently attached sensitizing dyes. Once reproducible surfaces and dye coverages are achieved, a photochronocoulometric technique is developed to measure the surface coverage of the dyes, an important parameter in determining the efficiency of sensitization. The surface-bound dyes are irreversibly oxidized by exposure to a light pulse with the n-type oxide semiconductor electrode held in depletion. A double-exponential decay of the subsequent photocurrent is then measured, where the integration of the faster decay is associated with the adsorbed dye coverage and the second much slower decay is attributed to trace regenerators, including water, in the nonaqueous electrolyte. The ruthenium-based N3 dye shows the expected linear dependence of the rate constant on light intensity whereas a dicarboxylated thiacyanine dye shows a square root dependence of its photooxidation rate on light intensity. The sublinear response of the thiacyanine dye is discussed in terms of the more complex surface chemistry that is known for this family of sensitizing dyes.