Understanding Charge Transport Control in Water Splitting Dye-Sensitized Photoelectrochemical Cells
Date of Award
Doctor of Philosophy (PhD)
Understanding the dynamics of the charge transfer processes that underlie the operation of solar energy conversion devices is crucial for the development of next generation materials. This dissertation focuses on materials used for photoelectrochemical water splitting, namely the water splitting dye-sensitized photoelectrochemical cell (WS-DSPEC). In a prototypical WS-DSPEC, water oxidation is driven at a photoanode and proton reduction occurs concurrently at a dark cathode to form hydrogen, a reduced chemical fuel. WS-DSPECs accomplish visible light-driven water-splitting by functionalizing wide bandgap semiconductors (i.e., metal oxides) with visible light harvesting dyes and catalysts. The modular nature of components comprising a WS-DSPEC makes it possible to tune specific degrees of freedom in the material, providing insight into how interfacial electron transfer (IET) processes can be controlled to improve material efficiency. Specifically, this dissertation focuses on the dye-metal oxide donor-acceptor dyad, providing insight into the fundamentals and tunability of IET in a model system applicable to WS-DSPECs. A number of spectroscopic methods were utilized to probe changes in IET processes, each well suited to a specific aspect of the materials of interest. These techniques include terahertz (THz), UV-visible, and X-ray spectroscopy, in both time-resolved and steady-state variants. In the context of dye-sensitized metal oxides, THz spectroscopy is useful as a non-contact conductivity probe, allowing IET processes to be studied from the perspective of the semiconducting metal oxide. In contrast, UV-visible spectroscopy probes the dye sensitizer adsorbed on the metal oxide surface, allowing IET processes to be studied from the perspective of the chromophore. In addition, X-ray spectroscopy performed at large-scale facilities (i.e., synchrotrons) was utilized to probe changes in the electronic structure of the metal oxide with element specificity. To develop a better understanding of the mechanism of IET in dye-sensitized metal oxides, fundamental studies were performed on dye-sensitized SnO2 and anatase TiO2 using a combination of the spectroscopic methods mentioned above. The combined use of these techniques provided unique insights into the mechanism of IET in these materials (e.g., directly differentiating recombination and trapping processes). Furthermore, the process of trapping was investigated in greater detail for anatase TiO2 and likely occurs by electron transfer into low-mobility states below the conduction band minimum. One of the degrees of freedom easily accessible in WS-DSPECs is the molecular structure of the dye sensitizer. The anchoring group is particularly important as it affects stability in the aqueous oxidizing conditions required for water oxidation and has the potential to play a role in determining IET behavior. The IET behavior in SnO2 sensitized with a series of dye sensitizers with different linkers and anchoring groups was studied using time-resolved spectroscopy and theoretical methods. Ultimately, the anchoring group was determined to have little effect on IET for the porphyrin sensitizers used, suggesting that the choice in anchoring group should be largely based on stability and synthetic viability. Of the main components that comprise WS-DSPECs, the metal oxide substrate that acts as the electron “acceptor” in these systems has been largely ignored. In addition to supporting both the dye and catalyst, the metal oxide plays a major role in tuning IET behavior and by extension the overall performance of the WS-DSPEC. Using the mixed metal oxide solid solution SnxTi1-xO2 (where 0≤x≤1) as a photoanode material alters both the composition of the conduction band (i.e., density of states) and affords nearly monotonic tunability of the conduction band minimum with composition. Using a combination of time-resolved spectroscopic techniques, the influence of the conduction band on IET was studied as a function of composition in dye-sensitized SnxTi1-xO2. The mechanisms behind the differences in IET were explored further by theory and steady-state spectroscopic techniques, which suggested that IET was largely dependent on the character of the density of states in the conduction band of the metal oxide.
Spies, Jacob Alan, "Understanding Charge Transport Control in Water Splitting Dye-Sensitized Photoelectrochemical Cells" (2022). Yale Graduate School of Arts and Sciences Dissertations. 526.