PhD candidate Asaf Kay, PhD. Candidate (Haifa Campus)
Faculty of Materials Science and Engineering at the Technion – Israel institue of technology
Kiriat Ha’Technion, Haifa
Given its abundance, stability in alkaline aqueous solutions, and absorption of visible light, α-Fe2O3 (hematite) is a promising candidate for photoelectrodes for solar-induced water splitting. However, hematite is notoriously known for its slow charge carrier mobility and short lifetime of minority charge carriers which gives rise to massive recombination. In this work we explore ways to improve the performance of hematite photoelectrodes and study their electrical, optical and photoelectrochemical properties. We start by introducing heterogeneous doping profiles in order to enhance charge separation. We show that using a combination of layers with different dopants, especially a p-i-n stack, can significantly improve the photoelectrochemical performance. Since the layers are very thin (< 30 nm) in order to reduce charge carrier recombination, optical interference is employed to enhance light-matter interaction and harvest broadband light in ultrathin hematite films on specular back-reflectors. A newly developed film flip and transfer process that allows for high-temperature processing without degradation of the metallic back-reflector and without the need of passivation interlayers was successfully implemented. The highest absorbed photon to current efficiency (APCE) achieved was 21.2% at the reversible potential of water oxidation (1.23 V versus RHE). Besides these efforts to enhance the performance of ultrathin hematite films, fundamental investigations of thick (~1 um) layers was carried out to understand the physics and dynamics of photogenerated charge carriers. These studies revealed that hematite, unlike other conventional semiconductors, exhibits a photocurrent which is strongly dependent on the optical excitation wavelength. The most widely used model to describe photoelectrode behavior is the Gartner model, which describes the photocurrent as a sum of all carriers photogenerated within the depletion region plus those generated in the bulk which are able to diffuse to the depletion region. We show that, in seeming contradiction to one or more of the preceding assumptions (i.e., Gartner model, diffusion length, or depletion region width), holes generated at least 700 nm away from the surface in a thick Ti-doped hematite planar photoelectrode are able to reach the surface and contribute to the photocurrent. Furthermore, by Time Resolved Microwave Conductivity and Terahertz spectroscopy we see decay processes which strongly suggest that the attributed lifetime for hematite is much longer than it is widely cited.
Prof. Avner Rothschild