Interfacial charge carrier dynamics of Bi2Fe4O9 photoanodes: interplay between charge transfer and recombination

Abstract

The charge carrier dynamics of Bi2Fe4O9 thin-film photoanodes are investigated using electrochemical impedance spectroscopy (EIS) and intensity-modulated photocurrent spectroscopy (IMPS) as a function of applied potential, light intensity, and charge transfer rate, modulated by adding sulfite as a hole-accepting redox species. The photocurrent strongly depends on sulfite concentration, reflecting a transition from surface recombination limited to charge transfer dominated behavior. EIS indicates that the effective interfacial capacitance includes semiconductor contributions and is comparable in magnitude to the Helmholtz capacitance. IMPS reveals a second characteristic time constant under suppressed recombination at more positive applied potentials and higher sulfite concentration. Numerical IMPS simulations reproduce the evolution from a pronounced 1st quadrant contribution to two distinct loops in the 4th quadrant, showing that the second loop can emerge when charge transfer prevails over recombination and the semiconductor and Helmholtz capacitance values are similar. These results establish a framework for interpreting multiple time constant behavior in photoelectrochemical systems.