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Metal–insulator–semiconductor (MIS) photo‐electrocatalysts offer a pathway to stable and efficient solar water splitting. Initially motivated as a strategy to protect the underlying semiconductor photoabsorber from harsh operating conditions, the thickness of the insulator layer in MIS systems has recently been shown to be a critical design parameter which can be tuned to optimize the photovoltage. This study analyzes the underlying mechanism by which the thickness of the insulator layer impacts the performance of MIS photo‐electrocatalysts. A concrete example of an Ir/HfO2/n‐Si MIS system is investigated for the oxygen evolution reaction. The results of combined experiments and modeling suggest that the insulator thickness affects the photovoltage i) favorably by controlling the flux of charge carriers from the semiconductor to the metal electrocatalyst and ii) adversely by introducing nonidealities such as surface defect states which limit the generated photovoltage. It is important to quantify these different mechanisms and suggest avenues for addressing these nonidealities to enable the rational design of MIS systems that can approach the fundamental photovoltage limits. The analysis described in this contribution as well as the strategy toward optimizing the photovoltage are generalizable to other MIS systems.

Protective insulating layers between a semiconductor and an electrocatalyst enable otherwise unstable semiconductors to be used in photocatalytic water splitting. It is generally argued that in these systems the metal electrocatalyst must have work function properties that set a high inherent barrier height between the semiconductor and electrocatalyst and that the insulating layer should be as thin as possible. In this study we show that, for systems which suffer from inherently low barrier heights, the photovoltage can be significantly improved by tuning the thickness of the insulating layer. We demonstrate this in a case study of a system consisting of n-type silicon, a hafnium oxide protective layer (thickness 0–3 nm), and a Ni electrocatalyst. By optimizing the protective layer thickness, we observe increased efficiencies for photocatalytic oxygen evolution with a thick Ni electrocatalyst supported on n-Si. Our findings open avenues for the use of inexpensive electrocatalysts with favorable electrocatalytic and optical properties but poor work function characteristics.

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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.