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

TiO 2 has been widely used as an n-type overlayer, simultaneously serving as a protective layer for photocathodes. However, the photovoltage generated from a TiO 2 junction with p-type absorbers, such as p-Si, Sb 2 Se 3, SnS, and Cu 2 O, 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 TiO 2 . Furthermore, we demonstrate that the PEIE dipole orientation can be rearranged by increasing the layer thickness, leading to an upward shift of the TiO 2 band edge. The magnitude of band shift induced by the dipole effect depends on the TiO 2 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/TiO 2 /Pt device. The versatility of the effective dipole reorientation strategy was demonstrated by application to a range of TiO 2 -protected heterojunction photocathodes based on Sb 2 Se 3, Cu 2 O, and SnS.