Understanding Photovoltage Enhancement in Metal-Insulator Semiconductor Photoelectrodes with Metal Nanoparticles

A metal-insulator-semiconductor (MIS) structure holds great potential to promote photoelectrochemical (PEC) reactions, such as water splitting and CO₂ reduction, for the storage of solar energy in chemical bonds. The semiconductor absorbs photons, creating electron-hole pairs; the insulator facilitates charge separation; and the metal collects the desired charge and facilitates its use in the electrochemical reaction. Despite these attractive features, MIS photoelectrodes are significantly limited by their photovoltage, a combination of the voltage generated from photon absorption minus the potential drop across the insulator. Herein, we use multiscale continuum modeling of the carrier, electrolyte, and interfacial transport to identify strategies for mitigating the deleterious potential drop across the insulator and enabling high MIS photovoltages. To this end, we model Ni/SiO₂/n-Si photoanodes that employ a planar Ni film or Ni nanoparticles (np-MIS) and validate both models using experimental polarization curves and photovoltage measurements from the literature. The simulations reveal that the insulator potential drop is lower and hence achieves higher photovoltages for np-MIS structures than MIS structures because the electrolyte screens charge trapped at defect states between the semiconductor and the insulator. This electrolyte charge screening phenomenon can be further leveraged by using low loadings or small nanoparticles, which not only minimize the interfacial potential drop but also improve the photocurrent by enabling more light absorption. These insights contribute to the optimization of the np-MIS structures for sustainable energy conversion.