Intrinsic Charge-Carrier Transport Limitations in ZnFe₂O₄ Revealed by Time-Resolved Microwave Conductivity

ZnFe₂O₄ has attracted significant interest as a photoanode material for solar water splitting, yet its photoelectrochemical performance remains inferior to that of well-studied materials such as hematite (α-Fe₂O₃). Here, we investigate charge carrier dynamics in epitaxial ZnFe₂O₄ thin films using time-resolved microwave conductivity (TRMC). TRMC measurements show that ZnFe₂O₄ exhibits a carrier yield-mobility product roughly an order of magnitude lower than that of hematite. Furthermore, the microwave photoconductance action spectrum deviates strongly from the optical absorption spectrum, independent of illumination direction, revealing an excitation-wavelength-dependent, subunity yield of mobile charge carriers. Near-band-edge excitation results in reduced photoconductivity, indicating that a substantial fraction of absorbed photons do not generate mobile charge carriers on nanosecond timescales, accounting for the wavelength-dependent external quantum efficiency (EQE) behavior commonly observed in ZnFe₂O₄ photoanodes. Overall, these findings reveal inherent limitations in both carrier yield and transport properties, offering a mechanistic basis for the photoelectrochemical performance of ZnFe₂O₄.