Ultrathin film Fe2O3 photoanodes for solar energy conversion and storage
Hen Dotan a, Avner Rothschild a
a Technion - Israel Institute of Technology, Faculty of Materials Engineering, Haifa, 32000, Israel
Invited Speaker, Avner Rothschild, presentation 009
Publication date: 16th April 2014
Iron oxide (α-Fe2O3, hematite) is a promising photoanode candidate for solar-powered water splitting for sustainable hydrogen production. In recent years, significant progress has been made towards resolving the main shortcomings that hindered, for several decades, the development of efficient Fe2O3 photoanodes, namely, unfavourable energy band alignment with respect to the water decomposition potentials (especially the hydrogen evolution reaction) and massive charge carrier recombination. The former shortcoming is successfully resolved by coupling the photoanode in tandem with a PV cell that provides sufficient voltage to split water. Surface recombination is effectively mitigated by adding co-catalysts or hole transport overlayers and interfacial recombination with the substrate (typically FTO-coated glass) is reduced by the use of electron transport underlayers. The conventional route to reduce bulk recombination employs mesoporous thick (> 500 nm) layers that decouple the optical and charge transport path lengths. This approach has led to significant improvement in the charge generation and collection efficiency, reaching photocurrent density of 4 mA cm-2 at 1.5 VRHE under simulated solar radiation [1]. However, it is difficult to control the microstructure of these layers in order to mitigate deleterious effects such as surface and grain boundary recombination and grain boundary resistance that are amplified by the mesoporous morphology.

We explore an alternative approach using ultrathin (20-30 nm) compact (i.e., nonporous) films of high crystalline order and small surface roughness. This approach benefits from the ability to control the microstructure and chemical composition of the films very precisely using PVD techniques such as sputtering or PLD, thereby mitigating deleterious recombination processes. It also provides the opportunity to tailor the light intensity and doping profiles in multilayer structures designed for optimal light harvesting and charge separation. We already achieved a photocurrent density of 4 mA cm-2 at 1.6 VRHE using ultrathin Fe2O3 films on silver-coated substrates [2], and we are now heading towards higher photocurrent at lower potentials by optimizing the fabrication process and tailoring the photoanode structure and chemical composition. We also explore new device layouts designed for optimal coupling of our photoanodes with PV cells to construct tandem cells for solar hydrogen production. We believe that this approach is the key to exploring the ultimate limits of Fe2O3 photoanodes because it enables efficient light harvesting in ultrathin films that can be made nearly free of defects, similarly to thin film PV cells and other optoelectronic devices. In this talk I will present our most recent progress in this front.                
[1] S. C. Warren et al., Nature Materials 12, 842 (2013).
[2] H. Dotan et al., Nature Materials 12, 158 (2013).

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