Interfacial Dipole Layer Enables High-Performance Heterojunctions for Photoelectrochemical Water Splitting

TiO₂ has been widely used as an n-type overlayer, simultaneously serving as a protective layer for photocathodes. However, the photovoltage generated from a TiO₂ junction with p-type absorbers, such as p-Si, Sb₂Se₃, SnS, and Cu₂O, is insufficient. We report a dipole reorientation strategy to overcome this limitation by inserting a polyethylenimine ethoxylated (PEIE) layer between a p-type absorber and TiO₂. Furthermore, we demonstrate that the PEIE dipole orientation can be rearranged by increasing the layer thickness, leading to an upward shift of the TiO₂ band edge. The magnitude of band shift induced by the dipole effect depends on the TiO₂ layer thickness. Using this approach, the onset potential was significantly improved to 0.5 V versus the reversible hydrogen electrode (V_RHE) in a p-Si/PEIE/TiO₂/Pt device. The versatility of the effective dipole reorientation strategy was demonstrated by application to a range of TiO₂-protected heterojunction photocathodes based on Sb₂Se₃, Cu₂O, and SnS.