Bimolecular Self-Trapped Exciton Formation in Bismuth Vanadate

Bismuth vanadate (BiVO₄) is a promising photoanode material for solar-driven water splitting, and knowledge of the photocarrier dynamics in BiVO₄ could offer guidance to propel the development of the photoanode performance. Herein, we uncovered the nature of various photogenerated transient species in BiVO₄ and extracted their respective dynamics. We found spectral and dynamic evidence that the electrons in the conduction band collapsed into severely localized small electron polarons on a subpicosecond time scale, while the holes in the valence band remained delocalized and accounted for the photoconductivity. In the following tens to hundreds of picoseconds, the electron polaron captured the hole to form a self-trapped exciton via a bimolecular reaction mechanism, and in consequence, the hole was immobilized. Our finding suggests that exciton dissociation strategies should be taken into account in the design of the BiVO₄-based water-splitting applications in order to enhance charge transport and suppress charge recombination.