The “Rust” Challenge: On the Correlations between Electronic Structure, Excited State Dynamics, and Photoelectrochemical Performance of Hematite Photoanodes for Solar Water Splitting

Abstract

In recent years, hematite's potential as a photoanode material for solar hydrogen production has ignited a renewed interest in its physical and interfacial properties, which continues to be an active field of research. Research on hematite photoanodes provides new insights on the correlations between electronic structure, transport properties, excited state dynamics, and charge transfer phenomena, and expands our knowledge on solar cell materials into correlated electron systems. This research news article presents a snapshot of selected theoretical and experimental developments linking the electronic structure to the photoelectrochemical performance, with particular focus on optoelectronic properties and charge carrier dynamics.

Effect of Doping and Excitation Wavelength on Charge Carrier Dynamics in Hematite by Time‐Resolved Microwave and Terahertz Photoconductivity

“…Since the measured photoconductance in the TRMC technique is a product of the photogeneration yield and the charge carrier mobility, the difference between the excitation wavelengths must arise from a change in one or both of these values. Possible reasons for this include a change in the mobility of the charge carriers themselves due to the different excitation energies or it may be a result of a change in the photogeneration yield due to localized LF transitions which do not yield mobile charge carriers . Table as well as Figures S3 and S4 (Supporting Information) show that the decay times for each dopant are nearly identical between the 355 and 532 nm excitation wavelengths, suggesting that for both excitation energies, the TRMC measurement probes the same physical process.…”

“…Possible explanations for this anomalous wavelength dependence have been attributed to nonmobile ligand field (LF or d–d) excitations which do not contribute to the photocurrent, or alternatively, wavelength‐dependent mobility and lifetimes of photogenerated polarons . In the first case, the presence of nonmobile LF excitations has been suggested to result in a wavelength dependent photogeneration yield (ξ), defined as the probability that an absorbed photon will generate a mobile charge carrier.…”

Identifying protons trapped in hematite photoanodes through structure–property analysis

“…The highly variable performance of a-Fe 2 O 3 photoanodes for PEC water oxidation is mirrored by signicant dispersion in experimentally measured properties. 56,57 Structural defects are oen cited to explain the experimental variability as they are known to exert signicant inuence over band structure and photophysics. A lack of information regarding the specic chemical nature of these defects inhibits attempts to remove or avoid them.…”

Polaronic structure of excess electrons and holes for a series of bulk iron oxides

“…1,2 Hematite (a-Fe 2 O 3 ) in particular has received much attention as a photoanode material for water splitting, however problems remain including low mobility and short carrier lifetimes due to electron-hole recombination. 3 While there has been experimental 4,5 and theoretical [6][7][8][9][10][11][12] investigations of charge transport in these materials, a complete atomistic level understanding of their polaron structures is still lacking.…”