Surface - Gated Charge Carrier - Selective Nanocontacts in Photoelectrochemical Catalysis

• Applicant: Sebastian Zeki Oener, Ph.D.
• Subject Area: Solid State and Surface Chemistry, Material Synthesis ; Electrical Energy Systems, Power Management, Power Electronics, Electrical Machines and Drives ; Experimental Condensed Matter Physics ; Synthesis and Properties of Functional Materials ; Physical Chemistry of Solids and Surfaces, Material Characterisation
• Term: from 2018 to 2020
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 408246589

The transition from fossil fuels to a sustainable energy infrastructure is impeded by the lack of chemical fuels with high energy-to-weight ratios made from renewable energies. Energy-dense fuels are needed for long-term energy storage (from seasons to years) and for long-distance or heavy-duty transportation.Photoelectrochemical water splitting is a direct route to store the sunlight’s energy in the chemical fuels hydrogen and oxygen gas. Recently, an interdisciplinary team from the USA (Caltech) and Germany (TU Ilmenau, Fraunhofer ISE, Helmholtz Zentrum Berlin) realized a system with an impressive solar-to-hydrogen efficiency of 19%, i.e. with 85% of the maximum theoretical efficiency achievable for the semiconductors used.The presented results clearly prove the feasibility of converting sunlight directly and efficiently into hydrogen and oxygen, however they lack one important ingredient required for an application: long-term stability. For stable operation the semiconductor must be protected from corrosion in harsh aqueous electrolyte solutions while the catalyst integrity is maintained. Stability issues represent the most significant challenge facing practical devices.To prevent corrosion, semiconductor surfaces are often coated conformably with a thermodynamically stable oxide film. This film must not only provide chemical protection but also electrical transport between semiconductor and electrocatalyst. This creates a fundamental trade-off: thick or non-conductive films can prevent corrosion but those do not sufficiently conduct current.During my PhD, I developed new charge carrier-selective contacts for nanowire solar cells based on the surface-gating/pinch-off effect. Those contacts use a surface layer to control the carrier-selectivity of an adjacent metal nanoscale point contact (by donating or accepting charge carriers), while the metal point contact itself facilitates the carrier extraction. Here I propose to use those nanoscale point contacts in water-splitting devices. Such contacts are predicted to generate large photovoltages, and hence high performances due to the “pinch-off” effect. Importantly, they can be surrounded by a thick (100 nm – 1 µm) and even non-conductive oxide layer for robust chemical protection while simultaneously guaranteeing sufficient electrical transport, a combination of properties that seems out of reach for conventional geometries. The work consists of two specific aims. In the first, I will study fundamental aspects of the “pinch-off” effect in model systems. In the second, I will apply the new fundamental knowledge to develop a high-performance, but chemically robust, carrier-collecting nanostructured interface using area-selective oxide deposition methods.To accomplish the work I will combine my expertise in nanofabrication and nanoscale interface properties from my PhD with the world-leading expertise on semiconductor-catalyst interfaces of the American host, Prof. Shannon Boettcher.

• DFG Programme: Research Fellowships
• International Connection: USA
• Host: Professor Dr. Shannon W. Boettcher