Disentangling Structural and Electronic Contributions to Photogenerated Mobile Charge Carrier Yield and Transport in...

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.