Influence of Compactness and Crystallinity on Hole Transfer Ability of Nickel Oxide Layer in Modal Coupling Regime

This study investigates how structural properties influence the hole migration behavior of nickel oxide (NiO_x) by evaluating its photoelectrochemical performance. Two fabrication methods were employed for preparing NiO_x layers. One is a sol-gel wet process, which allows wide-area coating without the need for specialized equipment. The other is atomic layer deposition (ALD), which enables uniform and conformal coverage on 3D surfaces with the compact oxide layer by leveraging a self-limiting, layer-by-layer growth mechanism. Our findings reveal that both crystallinity and compactness significantly affect the hole transport capability of NiO_x layers. We further integrated the NiO_x layer as a hole transport layer in an Au-NPs/NiO_x/Pt-film photocathode operating under a modal coupling regime. This photocathode exhibited cathodic photocurrent at wavelengths below 800 nm, with a maximum incident photon-to-current efficiency of 0.16% at 600 nm. These results demonstrate that not only the hole injection process but also hole migration properties of the NiO_x layer influence the charge separation under modal coupling conditions.