Understanding Photovoltage in Insulator-Protected Water-Splitting Half-Cells

Abstract

In the pursuit of a photosynthetic and efficient water splitting device, efforts have focused on optimizing specific aspects of the whole: catalysis, electronic conductivity, cathode and anode stability, and kinetics among other aspects. Improvement in one aspect can, however, often require fundamental tradeoffs affecting others. The photovoltage required to achieve a current density of interest for a water splitting device is an especially important metric. High solar-to-hydrogen efficiency is the ultimate goal of water splitting research, but reporting efficiency in a half cell can be misleading. This report investigates the photovoltage in insulator-protected water splitting cells using ALD-TiO2 protected silicon anodes as a case study and looks in depth at how it is correctly determined from typical electrochemical analysis and how this relates to the underlying solid-state carrier transport. Finally, the photovoltage at 10 mA/cm2 operating current referenced to the thermodynamic potentials is reviewed from several leading research reports, allowing for a direct comparison of cell performance.