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

Abstract

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.