Tailoring the Hydrogen Diffusion in Polycrystalline WO₃ Thin Films by a p–n Heterojunction

This study examines the lateral diffusion of hydrogen in tungsten trioxide (WO3) thin films with a nickel oxide (NiO) top layer. It focuses on the impact of the depletion region formed at the NiO/WO3 p–n heterojunction on the diffusion process. This depletion region influences diffusion by acting as a barrier to hydrogen movement. It effectively reduces the thickness in WO3 available for diffusion and increases the diffusion velocity due to the interplay with the concentration-dependent diffusion coefficient in polycrystalline WO3. Our in situ measurement technique allows for the detailed study of lateral hydrogen diffusion by inducing a concentration gradient in the layer plane. This method demonstrates by a direct comparison that diffusion is faster in the WO3/NiO layer structure compared to the pristine WO3 structure. This research demonstrates the technological potential of manipulating and tuning diffusion processes in electrochromic materials by incorporating them in layered structures and paves the way for more advanced applications.