ZnTiN₂ as an Electron-Selective, Protective Layer on Si Photocathodes

Photoelectrochemical production of fuels requires photoelectrodes that efficiently convert sunlight to electrochemical energy by producing photovoltage and photocurrent and maintain this ability over time under a variety of pH, illumination, and applied bias conditions. Work in the photovoltaic community has demonstrated that interfaces with high charge carrier selectivity provide high photovoltages. This offers a co-design opportunity to create semiconductor photoelectrodes with contact layers that are both carrier-selective and offer protection from degradation in aqueous solutions. In this work, we explored the ternary nitride ZnTiN₂ as an electron-selective, protective layer for Si-based photocathodes. We demonstrated that ZnTiN₂ formed a heterojunction with p-type Si that facilitated electron movement toward the ZnTiN₂ surface for light-driven reduction reactions. Across a variety of electrolyte conditions, ZnTiN₂/Si produced an open circuit voltage of ca. 400 mV vs the solution potential, while bare Si produced 220-480 mV vs the solution potential depending on conditions. ZnTiN₂ was also shown to protect Si over 72 h at open circuit in the dark in 0.1 M KHCO₃ aqueous solution at pH 10.5, with a 2.4% loss in open circuit voltage compared to a 17% loss for unprotected Si. A protective effect was also observed under illumination during methyl viologen reduction at pH 3.5 for 21 h, with a 2.5% loss in open circuit voltage observed for ZnTiN₂/Si compared to a 25% loss in open circuit voltage for unprotected Si under the same conditions. Elemental characterization revealed the presence of oxides on the surface of ZnTiN₂ that are consistent with the Pourbaix diagram after photoelectrochemical operation; these oxides appeared to support durability without hindering charge carrier extraction to drive electrochemical work. This work highlights the promise of ZnTiN₂ for durable photoelectrochemical applications.