(766c) Maximizing the Efficiencies of Metal-Insulator-Semiconductor (MIS) Photoelectrodes By Controlling the Flux of Charge Carriers with Interfacial Design

Photoelectrochemical systems have the potential to sustainably convert solar energy into chemical energy by splitting water into its hydrogen and oxygen components. One common photoelectrode system involves the use of a metal-insulator-semiconductor (MIS) junction. An efficient MIS photoelectrode uses a semiconductor with an ideal band gap ideal 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, which are required to maximize the barrier heights. This greatly limits the number of viable materials in MIS systems since the metal layers must also possess high electrocatalytic activity and stability.

In this work we show that by tuning the thickness of the insulator even systems that suffer from moderate barrier heights can achieve high photovoltages¹. This was done by designing insulators that minimize the interfacial flux of majority charge carriers (leading to recombination), without creating an overly large impediment for minority carriers to drive the reaction. This result shows that metals with non-ideal work function properties but optimal electrocatalytic activity can still achieve high photovoltages, and greatly improves the number of viable materials for use in MIS systems. Another strategy that has expanded the material phase space in MIS systems involves the introduction of bilayer metals^2–4. 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 we show the additive benefits of a design approach that combines insulator thickness tuning with the bilayer metals, which further improves photovoltage and expands the set of useful materials. 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.

1. Quinn, J., Hemmerling, J. & Linic, S. Maximizing Solar Water Splitting Performance by Nanoscopic Control of the Charge Carrier Fluxes across Semiconductor–Electrocatalyst Junctions. ACS Catal. 8545–8552 (2018).
2. Esposito, D. V., Levin, I., Moffat, T. P. & Talin, A. A. H2 evolution at Si-based metal–insulator–semiconductor photoelectrodes enhanced by inversion channel charge collection and H spillover. Nat. Mater. 12, 562–568 (2013).
3. Digdaya, I. A., Adhyaksa, G. W. P., Trześniewski, B. J., Garnett, E. C. & Smith, W. A. Interfacial engineering of metal-insulator-semiconductor junctions for efficient and stable photoelectrochemical water oxidation. Nat. Commun. 8, 15968 (2017).
4. Digdaya, I. A., Trześniewski, B. J., Adhyaksa, G. W. P., Garnett, E. C. & Smith, W. A. General Considerations for Improving Photovoltage in Metal–Insulator–Semiconductor Photoanodes. J. Phys. Chem. C 122, 5462–5471 (2018).