Identification of Individual Electron- And Hole-Transfer Kinetics at CoO_x/BiVO₄/SnO₂ Double Heterojunctions

The fabrication of heterojunctions with different band gap semiconductors is a promising approach to increase photoelectrochemical (PEC) activity. The PEC activity is determined by the charge separation; hence, the behaviors of charge carriers at the junctions should be elucidated. However, it has been quite challenging since the distinction of carriers located in different layers has been extremely hard. In this work, we succeeded in the identification of the individual electron- and hole-transfer kinetics at CoO_x/BiVO₄/SnO₂ double heterojunctions by measuring transient absorption (TA) from the visible to mid-IR region: we found that the absorption peaks of electrons and holes depend on the materials. From the change in spectral shape after the selective photoexcitation of BiVO₄, it was confirmed that electrons excited in the BiVO₄ rapidly transferred to the SnO₂ layer after ∼3 ps, but the holes remained in the BiVO₄ and further transferred to CoO_x in a few picoseconds. As a result, recombination of charge carriers was suppressed and 2.4 and 3.6 times a large amount of carriers are surviving at 5 μs on BiVO₄/SnO₂ and CoO_x/BiVO₄/SnO₂, respectively, compared to bare BiVO₄. For such picosecond-rapid and effective charge separation, the previously well proposed sole intralayer or interlayer charge separation mechanism is not enough. Hence the synergetic effect of these two mechanisms, the band-bending-assisted charge transfer across the heterojunction, is proposed. The enhanced PEC activity of CoO_x/BiVO₄/SnO₂ electrodes was reasonably explained by this synergistic charge separation kinetics. This fundamental knowledge of charge carrier dynamics will be beneficial for the design of superior solar energy conversion systems.