Ga-based III-V semiconductor photoanodes for solar fuels and novel techniques to investigate their photocorrosion.
Authors:   ,   University of Louisville, Pishgar Sahar 
Published: February 2023

Solar energy is one of the most abundant renewable energy sources. However, the diurnal variation of the sun as well as seasonal and weather effects, limits the widespread global implementation of solar energy. Thus, Cost-effective energy storage is critical to overcome the intermittent nature of solar energy available on the earth. Photoelectrolysis of water to oxygen and hydrogen fuel is a promising large-scale solution to store intermittent solar energy in a dense and portable form. Photoelectrochemical (PEC) water-splitting, or artificial photosynthesis, research strives to develop a semiconductor photoelectrode with both high efficiency and long-term stability. Semiconductors of the III–V class are among the most promising materials for high efficiency solar fuels applications. However, they suffer from severe instability in acidic and alkaline electrolyte and fundamental understanding of the corrosion mechanism of III-V semiconductors is of significant importance for the solar fuels community. This dissertation is focused on study of photocorrosion of Gallium based III-V semiconductors. A thorough review of important in-situ analytical techniques for the investigation of materials stability is given. The review explains some of the main in-situ electrochemical characterization techniques, briefly explaining the principle of operation and the necessary modifications for in-situ operation, and highlighting key relevant work in applying the method for the investigation of material stability and interfacial properties for electrocatalysts and photoelectrode materials. Next, in this dissertation, the corrosion of n-GaP, a promising III–V material for tandem top subcells, was investigated in strongly acidic electrolyte using an in-situ UV-Vis spectroscopy technique to monitor dissolved Ga and P species as a function of applied bias and time. The changing faradaic efficiency of the electrochemical GaP oxidation reaction was calculated from this data and used to interpret the corrosion process in conjunction with SEM and XPS characterization. In addition, corrosion measurements were made with thin conformal coatings of TiO2 as a protective barrier layer on the GaP surface. Although the protective coating slowed the rate of GaP dissolution, the TiO2 layers produced herein contributed significant charge-transfer resistance and still showed similar trends in the corrosion faradaic efficiency vs. time as the bare n-GaP. Further, photocorrosion of n-GaAs, one of the most well-developed and efficient III-V semiconductors was studied in strongly acidic electrolyte. Three type of Ir, OER co-catalyst, were tested to investigate their affect on photocorrosion of n-GaAs. In-situ UV-Vis spectroscopy was utilized to monitor the corrosion faradaic efficiency and the results showed decreased dissolution faradaic efficiency to a small degree over the first 15 minutes for samples with thin layers of Ir. SEM and XPS characterization have also been used to understand the photocorrosion mechanism. To develop high efficiency and stable water splitting systems new semiconductor materials with appropriate band gap, band edge positions, charge carrier mobility and chemical stability are demanded. Synthesis of ternary III-V alloys enable us to tune the band gap of III-V semiconductor with changing the compositions according to the requirements of PEC systems. Herein, optical and electrical properties of a novel III-V ternary alloy GaSbxP(1-x), synthesized in Conn Center for Renewable Energy Research by Halide Vapor Phase Epitaxy (HVPE) is reported. The effect of Sb addition on the band gap of the semiconductor was studied utilizing diffuse reflectance spectroscopy and photoluminescence spectroscopy. Band gap of HVPE-grown GaSbxP(1-x) film, with x=0.03-0.06 is decreased due to Sb incorporation to the lattice of GaP indicating that it can be a promising photoabsorber for PEC systems. In addition, incorporation of Sb to the lattice of GaP was estimated using Vegard’s law and X-ray diffraction spectrum of samples. Finally, resistivity and Hall effect measurements were performed to study the electrical properties of GaSbxP(1-x) films.