Origin of the Different Photoelectrochemical Performance of Mesoporous BiVO₄ Photoanodes between the BiVO₄ and the FTO Side Illumination

Understanding charge separation and charge transport in mesoporous semiconductor films is crucial to designing high efficiency photoelectrochemical water splitting cells. In the present work, we systematically study the origin of the higher photoelectrochemical performance of mesoporous BiVO₄ film under FTO-side illumination (F-illumination) than that under BiVO₄-side illumination (B-illumination). Via intensity-modulated photocurrent spectroscopy in conjunction with modeling simulation of electron diffusion inside mesoporous BiVO₄ films with different thicknesses, we find that the F-illumination is more tolerant to recombination than the B-illumination, leading to a higher charge separation efficiency of the former. Specifically, we have identified a trap-free electron transport region of BiVO₄ vicinal to the FTO substrate and a trap-limited transport region farther away under F-illumination, whereas only a trap-limited transport exists under B-illumination. Simulated results accord well with the experimental data and further provide a deep insight of the detailed electron transport behavior: it is the higher electron density in the region proximal to the FTO under F-illumination that has led to the greater recombination tolerance than under B-illumination. Such a photogenerated electron transport characteristic in mesoporous films is expected to be common for other semiconductors and will inspire practicle strategies for designing high efficiency semiconductor nanostructure-based photoelectrochemical devices.