Development of optimum bandgap photoanodes for tandem water-splitting cells based on doped complex metal oxides and III-V semiconductors coupled to water oxidation electrocatalysts

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One of the most promising approaches for the development of highly efficient solar water-splitting systems is the use of photoelectrochemical devices comprising of a tandem of photocathodes and photoanodes with optimized optical (bandgap), photoelectrochemical (quasi-Fermi levels, current matching), and surface catalytic properties. For example, such tandem cells could provide solar-to-hydrogen efficiencies of around 25% when using two absorbers with bandgaps of 1.1 eV and 1.8 eV. While highly efficient low-bandgap photocathodes based on crystalline silicon (1.1 eV) are available, a major challenge in bridging the gap to working devices is the development of efficient and stable photoanodes with well-matched characteristics: bandgap of 1.8 eV, and photocurrent maximum at potentials as low as 0.4 V vs. RHE (i.e., photocurrent onset at ca. 0.2 V vs. RHE). However, there are currently no materials fulfilling such criteria, and new materials must be developed. In addressing this challenge, this collaborative project aims to investigate two novel and distinct types of photoanodes based on two different classes of light absorbers, selected as highly promising on the basis of rational considerations and theoretical (DFT) calculations: i) doped copper tungstates, and ii) antimony-doped gallium nitride. High degree of control over the composition, structure, morphology, and crystallinity is crucial for efficient light harvesting and charge separation. Therefore, thin films with various morphologies (porous, compact, epitaxial layers, nanopillars) will be prepared by sol-gel and metalorganic chemical vapor deposition (MOCVD) techniques. In order to ensure fast kinetics of water oxidation, thin layers of highly efficient amorphous electrocatalysts for oxygen evolution will be deposited by atomic layer deposition and by inherently low temperature methods (electrochemical, photoelectrochemical, and photochemical metalorganic deposition) in order to avoid formation of defects by thermally activated interlayer atomic diffusion.Detailed mechanistic investigations will be employed to identify the bottlenecks in photoelectrochemical performance of photoanodes. A time-resolved terahertz photoconductivity probe and photo-induced transient absorption spectroscopy will be used to study the charge transport properties and recombination dynamics. These investigations will drive the rational design of the photoanodes and are expected to provide unique knowledge on the charge dynamics in different materials classes and architectures with significance far beyond the scope of this project.