# Disentangling Structural and Electronic Contributions to Photogenerated Mobile Charge Carrier Yield and Transport in Fe2O3 Polymorphs

## TL;DR

This study investigates how crystal structure and electronic configuration affect the efficiency of charge carriers in iron oxide photoelectrodes.

## Contribution

The paper uses α- and β-Fe2O3 polymorphs to separate structural and electronic effects on charge carrier behavior.

## Key findings

- Structural factors influence charge transport, with α-Fe2O3 showing longer hole transport lengths than β-Fe2O3.
- Both polymorphs have similar spectral profiles for mobile-carrier generation, indicating electronic structure dominates over crystal symmetry.
- Ligand-field states are identified as intrinsic nonproductive relaxation pathways in open d-shell metal oxides.

## Abstract

Iron oxides exhibit poor photoconversion efficiencies
as photoelectrodes
for solar water splitting, generally attributed to short carrier diffusion
lengths and subunity yields of photogenerated mobile charge carriers
caused by ultrafast relaxation through ligand field (LF) states. However,
the extent to which crystal structure or electronic configuration
governs these loss mechanisms remains unclear. Here, epitaxial thin-film
photoanodes of the α- and β-Fe2O3 polymorphs, which share the same Fe3+ (3d5) electronic configuration yet possess distinct crystal symmetries,
are employed as model systems to disentangle the relative influence
of electronic configuration and crystal structure on charge carrier
yields and transport. Using a computational method that combines optical
and photoelectrochemical measurements, we determine both the wavelength-dependent
efficiency of mobile charge carrier generation and the depth-dependent
charge collection probability. We find that structural factors influence
charge carrier transport, with the α-Fe2O3 films exhibiting a larger hole transport length than the β-Fe2O3 thin films. In contrast,
both polymorphs show an essentially identical spectral profile for
mobile-carrier generation, indicating that the energies of the ligand-to-metal
charge-transfer (LMCT) transitions that produce mobile carriers are
largely unaffected by the difference in crystal structure. These results
suggest that carrier yields are governed predominantly by the local
Fe–O electronic structure associated with the octahedrally
coordinated Fe3+ centers and are consistent with the view
that ligand-field states act as intrinsic nonproductive relaxation
pathways in open d-shell metal oxides.