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.

中文翻译：

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CoO _x / BiVO ₄ / SnO ₂双异质结处单个电子和空穴传输动力学的鉴定

用不同的带隙半导体制造异质结是增加光电化学（PEC）活性的有前途的方法。PEC的活性取决于电荷的分离；因此，应阐明在结处的载流子的行为。然而，由于位于不同层中的载波的区分非常困难，因此这一直是具有挑战性的。在这项工作中，我们成功地确定了CoO _x / BiVO ₄ / SnO ₂处的单个电子和空穴传输动力学通过测量从可见光到中红外区域的瞬态吸收（TA）来形成双异质结：我们发现电子和空穴的吸收峰取决于材料。从BiVO的选择性光激发后在频谱形状的变化₄，可以确认的是，在BiVO激发的电子₄迅速转移到的SnO ₂后〜3个PS层，但是孔留在BiVO ₄，并进一步转移到的CoO _X在几皮秒内。结果，抑制了载流子的重组，并且在BiVO ₄ / SnO ₂和CoO _x / BiVO ₄上5 s时存活了2.4倍和3.6倍的大量载流子。/ SnO ₂分别与裸BiVO ₄相比。对于这样的皮秒快速有效的电荷分离，以前很好地提出的唯一的层内或层间电荷分离机制是不够的。因此，提出了这两种机制的协同效应，即跨异质结的带弯曲辅助电荷转移。CoO _x / BiVO ₄ / SnO ₂电极增强的PEC活性可以通过这种协同电荷分离动力学来合理解释。电荷载流子动力学的基本知识将有助于设计卓越的太阳能转换系统。