Titanium Oxide Crystallization and Interface Defect Passivation for High Performance Insulator-Protected Schottky Junction MIS Photoanodes

Abstract

Atomic layer deposited (ALD) TiO2 protection layers may allow for the development of both highly efficient and stable photoanodes for solar fuel synthesis; however, the very different conductivities and photovoltages reported for TiO2-protected silicon anodes prepared using similar ALD conditions indicate that mechanisms that set these key properties are, as yet, poorly understood. In this report, we study hydrogen-containing annealing treatments and find that postcatalyst-deposition anneals at intermediate temperatures reproducibly yield decreased oxide/silicon interface trap densities and high photovoltage. A previously reported insulator thickness-dependent photovoltage loss in metal-insulator-semiconductor Schottky junction photoanodes is suppressed. This occurs simultaneously with TiO2 crystallization and an increase in its dielectric constant. At small insulator thickness, a record for a Schottky junction photoanode of 623 mV photovoltage is achieved, yielding a photocurrent turn-on at 0.92 V vs NHE or -0.303 V with respect to the thermodynamic potential for water oxidation.

Quantifying Losses and Assessing the Photovoltage Limits in Metal–Insulator–Semiconductor Water Splitting Systems

“…Finally, we note that several MIS water splitting systems have achieved over 600 mV of photovoltage through a combination of using relatively high doped Si and using high‐quality oxides with minimal surface states 30,45,53. As shown by the black region in Figure 6, even an ideal system with optimized insulator thickness may still fall significantly short of the photovoltage limits.…”

“…Therefore, strategies to remove nonidealities, such as by annealing the samples to restructure the interfaces, are important to optimize the photovoltage. Previous work has demonstrated the utility of annealing TiO 2 based MIS systems to improve photovoltage,45,47 although the improvements were not quantified in the context of directly influencing the ideality factor and barrier height. As a proof of concept, we have performed a 30 min, 300 °C forming gas anneal on a sample with 2.0 nm of HfO 2 , followed by depositing the Ir catalyst.…”

A Flexible Web‐Based Approach to Modeling Tandem Photocatalytic Devices

“…The exact efficiency values of these hybrid experimental/theory tandem devices mentioned in this PEC water splitting example should not be taken as precise predictions of actual tandem device efficiencies due to the number of assumptions in parameters made in this analysis. Additionally, researchers are continually improving these photoabsorbers, which will continually make the experimental results look more promising . However, the overall trends are where the true value in this analysis lies.…”

Interfacial engineering of metal-insulator-semiconductor junctions for efficient and stable photoelectrochemical water oxidation

“…This means that any collected minority charge carriers can be readily transferred to the electrolyte through the catalyst to drive an electrochemical reaction instead of flowing through the external circuit as in PV cells. This simple route allows the ability to use thin metals (that is, 1–10 nm) as the charge collector as well as the carrier conduction mediator22232425. The thin metal film allows the reaction to occur throughout the whole surface, and thus enabling the photogenerated current to flow perpendicular to the metal film.…”