Periodic Reporting for period 1 - DREAM (Design Rules for Efficient Photogeneration in Metal Oxides)
Periodo di rendicontazione: 2023-01-01 al 2025-06-30
Photoelectrochemical water splitting offers a green approach for hydrogen production, but its efficiency is limited by the lack of suitable photoelectrode materials for water oxidation and reduction reactions. Metal oxides demonstrate exceptional stability in aqueous electrolytes, but those with band gaps optimised for visible light typically have open d shell configurations and suffer from low photoconversion efficiencies. Unlike in conventional semiconductors where absorbed photons generate mobile electrons and holes, in metal-oxides with open d shell configuration, many of the absorbed photons give rise to localized electronic transitions that do not generate charge carriers that contribute to the photocurrent.
The goal of DREAM is to address this challenge and provide a leap forward in understanding the photogeneration processes in metal-oxide photoelectrodes and their effect on photoconversion efficiency. To achieve these goals, we couple systematic control of crystallographic structure, d orbital occupancy, and local cation environment using heteroepitaxial thin film growth together with wavelength and temperature-resolved characterization of the photogeneration yield spectrum. The knowledge gained by these fundamental investigations is expected to lead to new design rules, which will be employed to engineer new metal-oxides with near unity photogeneration yield, and integrate them into novel device architectures, enabling highly efficient PEC-PV tandem cells for unassisted solar water splitting.
The goal of DREAM is to address this challenge and provide a leap forward in understanding the photogeneration processes in metal-oxide photoelectrodes and their effect on photoconversion efficiency. To achieve these goals, we couple systematic control of crystallographic structure, d orbital occupancy, and local cation environment using heteroepitaxial thin film growth together with wavelength and temperature-resolved characterization of the photogeneration yield spectrum. The knowledge gained by these fundamental investigations is expected to lead to new design rules, which will be employed to engineer new metal-oxides with near unity photogeneration yield, and integrate them into novel device architectures, enabling highly efficient PEC-PV tandem cells for unassisted solar water splitting.
In the first two years of the project, we have systematically investigated the effect of crystallographic structure, d orbital occupancy, and local cation environment on photogeneration processes in metal-oxide photoabsorbers. We grew epitaxial thin films of polymorphic materials with identical chemical composition to systematically examine the effect of crystallographic structure on the photogeneration yield spectrum. Additionally, we grew isomorphic thin films with the same crystalline structure but varied chemical composition to systematically study the effect of d-orbital occupancy on photogeneration processes.
Among the materials we studied is ZnFe2O4, which has been suggested to be a promising photoanode material. In our recent publication (“Quantification of mobile charge carrier yield and transport lengths in ultrathin film light-trapping ZnFe2O4 photoanodes”, Miriyala et al, DOI: 10.1039/D4TA05448B) we extracted the charge carrier photogeneration yield spectrum and spatial collection efficiency of this material. The results confirmed our hypothesis that reduction in mobile charge carrier yield due to the existence of ligand field states is a dominant loss mechanism for metal-oxides containing metal centers with open d-shell configuration.
We have developed a new methodology for temperature-resolved extraction of the mobile charge carrier photogeneration yield spectrum and spatial charge collection profiles in photoelectrodes under operating conditions. We are currently applying this methodology to several photoelectrode materials developed in the project.
Among the materials we studied is ZnFe2O4, which has been suggested to be a promising photoanode material. In our recent publication (“Quantification of mobile charge carrier yield and transport lengths in ultrathin film light-trapping ZnFe2O4 photoanodes”, Miriyala et al, DOI: 10.1039/D4TA05448B) we extracted the charge carrier photogeneration yield spectrum and spatial collection efficiency of this material. The results confirmed our hypothesis that reduction in mobile charge carrier yield due to the existence of ligand field states is a dominant loss mechanism for metal-oxides containing metal centers with open d-shell configuration.
We have developed a new methodology for temperature-resolved extraction of the mobile charge carrier photogeneration yield spectrum and spatial charge collection profiles in photoelectrodes under operating conditions. We are currently applying this methodology to several photoelectrode materials developed in the project.
In Miriyala et al, 2025, we reported, to our knowledge, the highest photocurrent density for planar compact (non-nanostructured) ZnFe2O4 thin film photoanodes under solar illumination to date. Furthermore, we extracted the charge carrier photogeneration yield spectrum and spatial collection efficiency of this material, demonstrating that reduction in mobile charge carrier yield due to the existence of ligand field states is a dominant loss mechanism for metal-oxides containing Fe metal centers with open d-shell configuration.
Our results generated in the first two years of the project have deepened our fundamental understanding of how material and electronic structure affect photogeneration processes in metal-oxide photoabsorbers. We anticipate that these insights will lead us to design of metal-oxides with reduced photogeneration losses and improved photoelectrochemical performance for green hydrogen production beyond the state of the art.
Our results generated in the first two years of the project have deepened our fundamental understanding of how material and electronic structure affect photogeneration processes in metal-oxide photoabsorbers. We anticipate that these insights will lead us to design of metal-oxides with reduced photogeneration losses and improved photoelectrochemical performance for green hydrogen production beyond the state of the art.