טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentKay Asaf
SubjectIron Oxide (ALPHA-Fe2O3)Thin Film Photoelectrodes for Water
Splitting
DepartmentDepartment of Materials Science and Engineering
Supervisor Professor Avner Rothschild
Full Thesis textFull thesis text - English Version


Abstract

Fossil fuel is the prime suspect for climate change and many other harmful effects on our environment. Solar energy can easily provide all our energy needs if it can be efficiently harvested and converted to other forms of useful energy such as electricity and fuel. Using batteries for energy storage is an expensive solution and essential material resources such as cobalt are limited. An alternative route is to store the solar energy in chemical bonds, with hydrogen (H2) as a leading candidate.

One of the main routes of converting solar energy directly to hydrogen is by using photoelectrochemical (PEC) solar cells. The main challenge lies in the material selection of suitable photoelectrode candidate. Iron oxide in its hematite phase ( ) is a promising candidate that meets the material selection criteria, but despite many years of research it still falls short from its theoretical potential and there are many open questions regarding the link between its physical properties and photoelectrochemical performance. Although much research has been conducted on hematite photoelectrodes, conclusive answers are still missing and researchers suggest different approaches to explain the empirical observations.

This research aims to improve the fundamental understanding of the correlation between physical and photoelectrochemical properties and provide new routes to improve hematite performance as a photoanode for solar water splitting. We demonstrated improved photoelectrochemical performance in both the photocurrent and photovoltage of thin film stacks with p-i-n like doping profile. We developed an efficient way to fabricate these stacks on a mirror-like substrate in order to significantly enhance the light harvesting by employing the resonant light trapping method that was developed previously in our group. This approach demonstrated a remarkable enhancement in the light harvesting efficiency as compared to the conventional fabrication process.

Our thin film stacks absorb significant amount of light and with the heterogeneous doping profile they were expected to yield effective charge separation so as to convert most of the absorbed photons to charge carriers that contribute to the photocurrent. However, in practice less than 3 0% of the absorbed photons contributed to the photocurrent in our top performing structures. This observation triggered complete reassessment of our basic hypotheses and assumptions. Our results show that the charge carrier separation and collection yield in hematite is not only position dependent, as previously thought, but is also wavelength dependent. We show that photogenerated charge carriers can be collected from a large distance from the surface, much larger than the commonly cited diffusion length for hematite photoelectrodes. We show that the charge carriers in hematite have several contributions in different time scales from picoseconds to nanoseconds, indicating complex charge carrier dynamics that go beyond the simple model that was assumed so far. Further research in this direction should be carried out to understand the mechanism of charge carrier generation in hematite and the fate of the photo-generated charge carriers. This would lead to better understanding of the losses in hematite photoelectrodes, which is essential to improving their photoelectrochemical performance.