Charge carrier trapping, recombination and transfer in hematite (α-Fe2O3) water splitting photoanodes

Abstract

Hematite is currently considered one of the most promising materials for the conversion and storage of solar energy via the photoelectrolysis of water. Whilst there has been extensive research and much progress in the development of hematite structures with enhanced photoelectrochemical (PEC) activity, relatively limited information has been available until recently concerning the dynamics of photogenerated charge carriers in hematite and their impact upon the efficiency of water photoelectrolysis. In this perspective we present an overview of our recent studies of the dynamics of photoinduced charge carrier processes in hematite, derived primarily from transient absorption spectroscopy of nanostructured photoanodes. The relationship between PEC activity and transient measurements are discussed in terms of a phenomenological model which rationalizes the observations and in particular the impact of external potential bias on the relative rates of charge carrier trapping, recombination and interfacial transfer in hematite photoanodes for water oxidation.

Effect of Internal Electric Fields on Charge Carrier Dynamics in a Ferroelectric Material for Solar Energy Conversion

“…Our studies thus demonstrate that internal fi elds can indeed result in a spatial separation of charge carriers, and thereby a reduction in charge recombination losses, potentially enhancing solar conversion device performance. For example, the lifetimes observed here match the rate constants of photoelectrochemical water oxidation reported in the literature, [ 25,43 ] indicating that ferroelectrics could have a particularly high impact in solar water splitting systems. Further, we have previously shown that even an order of magnitude increase carrier lifetime can result in 100 mV increase in ) there is no polarization and thus no band bending (for clarity, band bending due to atmospheric effects is omitted here, although it is likely that some upward band bending exists above T C ).…”

“…[ 25 ] We also note that the absence of a further sharp reduction in carrier lifetime as the temperature is increased above T C is most probably due to charge trapping effects becoming the dominant limitation on charge carrier recombination on these timescales, as has been reported for other metal oxides.…”

“…This smaller apparent activation energy is consistent with detrapping-limited charge recombination, with a trap state depth of a few hundred meV, typical for such oxide materials. [ 25 ] We attribute the much stronger temperature dependence of the charge carrier lifetime in SC-BaTiO 3 to the temperature dependent ferroelectric dipole-induced band bending in this material.…”

“…The fast decay dynamics observed for the TF-BaTiO 3 are typical of those observed for non-ferroelectric metal oxides, and assigned to rapid bulk recombination losses. [ 25,31,33 ] Such rapid recombination losses have been reported for numerous non-ferroelectric metal oxides with a range of crystal phases, crystallinities, and morphologies. In contrast, lifetimes on the seconds timescale comparable to the SC-BaTiO 3 have only previously reported in metal oxide systems in the presence of strong electrical bias or sacrifi cial reagents to suppress recombination of photogenerated electrons and holes.…”

“…We focus on the charge carrier dynamics on longer (μs-s) timescales as these correspond to carrier lifetimes necessary to drive most photocatalytic or artifi cial photosynthetic processes, such as water splitting. [ 25 ] We fi nd that the tetragonal phase of BaTiO 3 exhibits remarkably slow charge carrier recombination ( t 50% = 0.12 s). Such…”