(316b) Impact of Interfacial Properties on Carrier Transport and Photoelectrode Performance in Metal-Insulator-Semiconductor Structures

Photoelectrochemical (PEC) cells are of significant interest for their potential to convert solar energy into chemical bonds, thereby enabling the storage of solar energy in forms that can be transported and used on demand. A critical challenge that PEC systems face is the design and fabrication of stable photoelectrodes that can effectively absorb light and catalyze the reaction. This challenge has led many researchers to investigate metal-insulator-semiconductor (MIS) structures as photoelectrodes because the metal catalyst has lower kinetic overpotentials than the bare semiconductor surface at the same current density and the ultrathin insulator layer helps to protect the semiconductor from corrosion by the electrolyte. The ultrathin insulator also serves as a barrier for which charge carriers must tunnel through, and previous modeling work has shown how the insulator thickness can be tuned in order to enable selective tunneling of the desired charge carrier. This selective carrier tunneling helps to minimize carrier recombination in the metal and, thereby, enable higher current densities. These models, however, fail to explain how selective carrier tunneling fundamentally impacts the MIS photovoltage and subsequent fuel-formation rates as well as the effects of interfacial non-idealities. Continuum modeling is uniquely suited to elucidate the effects of carrier tunneling and non-idealities on MIS performance and to identify interfacial properties beyond the insulator thickness that can be leveraged for improved solar-to-fuel (STC) conversion.

This talk will present recent continuum modeling efforts to simulate an MIS photoelectrode used for PEC water splitting. Our simulations have identified how properties of the insulator modulate carrier tunneling rates and how these rates impact the MIS photovoltage. Specifically, tuning the insulator properties affects the probability of tunneling, which alters the carrier concentrations at the semiconductor surface. Increasing the carrier concentrations at the semiconductor surface increases the photovoltage (i.e., qausi-Fermi level splitting) by reducing the magnitude of the electric field and by increasing the quasi-Fermi level of the minority carrier. Our model has revealed optimal properties of the insulator that promote carrier buildup at the semiconductor surface and, thereby, enable high MIS performance. These predicted optimal properties of the insulator are a large bandgap (resulting in a high band offset with the semiconductor), a high dielectric constant, and a thickness of ~ 1.2 nm. Insulating materials that exhibit a large band offset with the semiconductor are predicted to lead to high MIS performances by creating an appreciable interfacial tunneling resistance that promotes carrier buildup at the semiconductor surface, and the moderate insulator thickness allows for unimpeded tunneling of the desired carrier. Increasing the thickness of these insulators beyond their optimal value results in hindered carrier tunneling and a very large concentration of carriers at the semiconductor surface. This extremely large carrier concentration causes a significant number of charges to be trapped at defect sites at the semiconductor-insulator interface, which results in a substantial drop in potential across the insulator that reduces the MIS photovoltage. Understanding how other interfacial properties can be engineered in order to mitigate the potential drop across the insulator is critically important for designing MIS photoelectrodes that can reach its theoretical maximum performance. The results of this study provide great insight into the mechanisms for which interfacial transport affects photoelectrode performance and provides optimal interfacial properties that tune carrier transport. Such an understanding is vital for the development of next-generation MIS photoelectrodes that enable highly efficient STC energy conversion.