טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
M.Sc Thesis
M.Sc StudentHadar Mor
SubjectUsing Underlayers to Improve Iron Oxide
Photoelectrodes for Solar Water Splitting
DepartmentDepartment of Materials Science and Engineering
Supervisor Professor Avner Rothschild


Abstract

The global energy demand is increasing each year, with oil, natural gas and coal meeting most of this demand. Because these energy sources are not eco-friendly and since they are also limited, the need for renewable energy sources, like the sun, is increasing. If we will be able to harvest the energy from the sun efficiently, it can supply the global demand for energy.

Photoelectrochemical (PEC) water splitting produces hydrogen and oxygen using solar energy, providing an elegant route to produce renewable fuel. The PEC solar cell consists of two electrodes immersed in electrolyte and connected through an external circuit. In order to harvest solar energy, at least one of the electrodes should be made of photo-active material, that absorbs visible light. The photoelectrode should also be stable in aqueous electrolytes and have appropriate energy band positions for water oxidation and/or reduction.

Iron oxide (α-Fe2O3), also known as hematite, is considered as a promising candidate for solar water splitting. It has a favorable band gap of 2.1eV, it is stable in alkaline solutions, and it is an abundant and low-cost material. Despite these merits, hematite photoelectrodes display lower efficiency than expected. One strategy to improve the performances of these photoanodes is by adding thin underlayers at the back interface of the hematite layer.

The addition of an underlayer to the photoanode can lead to structural changes, difference in the optical absorbance or it can influence the charge carrier dynamics inside the photo-active layer (the hematite) and at its interface with the substrate. In some of the cases reported in the literature, multiple effects were assigned simultaneously, and this led to the understanding that in order to analyze the electronic effect of an underlayer on hematite photoanode, systematic research is needed, which will enable to separate this effect from other possible effects. 

This work examines the influence of metal oxide underlayers on the photoelectrochemical properties of hematite photoanodes. Different metal oxides were deposited as underlayers, with nanometric thicknesses, on top of transparent conductive substrates, such as fluorine doped tin oxide (FTO) or (001) oriented sapphire (Al2O3).

The structure, surface morphology and crystalline quality were examined as a function of  underlayer addition. Their photoelectrochemical performances were also investigated, using photocurrent vs. potentials measurements, intensity modulated photocurrent spectroscopy (IMPS) and incident photon to current efficiency (IPCE) analyses. The absorption at the hematite layer was calculated for selected photoanodes in order to examine how the underlayer may affect this absorption.

Among the different underlayers that were scrutinized, Nb-doped TiO2 underlayers were found most effective to improve thin film hematite photoanodes, without changing significantly the photoanode layered structure. The combination of the different analysis methods used in this work, led to the understanding that the absorption in the hematite layer changes as a function of this underlayer. In addition, the analysis of the experimental results showed that this underlayer decreased the recombination at the interface between the hematite layer and the substrate, which led to cathodic shift in the potential at which the photocurrent rises.