Guidelines for Optimizing the Performance of Metal–Insulator–Semiconductor (MIS) Photoelectrocatalytic Systems by Tuning the Insulator Thickness

Layered metal–insulator–semiconductor (MIS) materials represent a promising platform for photoelectrocatalytic chemical transformations including solar water splitting. The introduction of insulators in these materials was motivated by the need to improve the stability of many inherently unstable semiconductors with desirable optical properties. Recently, it has been demonstrated that insulators, in addition to improving the stability, can also improve performance. Specifically, it was shown that the generated photovoltage of some MIS systems is highly affected by optimizing insulator thickness. In this study, we quantify the extent to which insulator tuning can be used to optimize photovoltage. This is shown experimentally by comparing the performance of two systems with inherently different barrier heights but, otherwise, identical optical and electrocatalytic properties for the hydrogen evolution reaction (HER). These photocathodes contained a p-type Si light absorber, an HfO2 insulator, and a layered Ti–Pt or Al–Pt electrocatalyst. A comprehensive model was developed to illuminate the underlying processes governing performance and guide the design of MIS systems.