Improving the Efficiencies of Metal-Insulator-Semiconductor (MIS) Photoelectrodes with Interfacial Design

Abstract

Photoelectrochemical systems have the potential to sustainably convert solar energy into chemical energy by splitting water into its constitutive hydrogen and oxygen components. One common photoelectrode system involves the use of a metal-insulator-semiconductor (MIS) structure. An efficient MIS photoelectrode uses a semiconductor with an ideal band gap to absorb sunlight, a metallic catalyst with high electrocatalytic activity, and a stable insulator which prevents the degradation of the otherwise unstable semiconducting material. The amount of photovoltage generated by the system is highly dependent on the physical characteristics of the interface. Conventionally, photovoltage has been optimized by maximizing the “barrier height” (interfacial electric field) of the system, while minimizing the thickness of the insulator to reduce resistance. This approach requires the use of high work function metals for photoanodes and low work function metals for photocathodes in order to maximize the barrier height. This greatly limits the number of viable materials in MIS systems since the metal layers must also possess high electrocatalytic activity and stability. This work shows that the tuning of insulator thickness can be leveraged to allow systems that suffer from moderate barrier heights to achieve high photovoltages. This was done by designing insulators that minimize the semiconductor-to-metal interfacial flux of majority charge carriers (leading to recombination), without creating an overly large impediment for the flux of minority carriers needed to drive the reaction. The effect was demonstrated experimentally with a photoanode consisting of n-type silicon, a hafnium oxide insulator, and a Ni electrocatalyst. This result shows that metals with non-ideal work function properties but optimal electrocatalytic activity can still achieve high photovoltages and greatly increases the number of viable materials for use in MIS systems. Another strategy that has expanded the phase space of viable materials for MIS systems involves the introduction of bilayer metals. This approach uses one metal layer to set the barrier height of the system, and another metal layer to provide stable electrocatalytic sites. In this work bilayer metal MIS systems were used to examine (1) how insulator thickness tuning affects various barrier height systems, and (2) the potential additive benefits of a design approach that combines insulator thickness tuning with bilayer metals. This was demonstrated experimentally using pSi-HfO2-Al-Pt and pSi-HfO2-Ti-Pt photocathode systems. The relevant interfacial mechanisms were captured using a comprehensive model that can be used to predict MIS performance.