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.